US20070225595A1 - Hybrid navigation system for tracking the position of body tissue - Google Patents

Abstract

A hybrid surgical navigation system for tracking the position of body tissue. The system includes a marker shaped to be mounted subcutaneously to the tissue the position of which is to be tracked. A tracker located above skin level is placed near the marker. A first navigation system integral with the marker and tracker generate data indicating the position of the marker relative to the tracker. A second navigation monitors the position of the tracker. The tracker also includes a transmitter for transmitting data generated by the first navigation system. Based on the data generated by the first navigation system representative of the position of the marker relative to the tracker and the data from the second navigation system representative of the position of the tracker, the position of the marker and tissue to which it is attached is determined.

Description

FIELD OF THE INVENTION
• This invention relates generally to a surgical navigation system used to track the position of body tissue. More particularly, this invention relates to a surgical navigation for tracking body tissue that does not expose the tissue to excessive trauma and that can be used in the presence of ferromagnetic objects.
BACKGROUND OF THE INVENTION
• Surgical navigation systems are increasingly used as aids in surgical procedures. Generally, a surgical navigation includes a tracker, a localizer and a processor. The tracker is attached to an instrument or section of tissue the position of which is to be tracked. The localizer, relative to the tracker, is static. One or more transmitters are contained in either the tracker or localizer. The other of localizer or the tracker contains one or more complementary receives able to detect the energy emitted by the transmitters. It is known to construct surgical navigation systems out of transmitter receiver pairs wherein the transmitted energy is photonic energy, (visible light, UV and/or IR), sonic energy, electromagnetic energy or RF energy. The processor receives signals from the receiver(s) indicating the strength of the energy emitted from the transmitter(s) or other position/orientation dependent characteristic. Based on these signals, the processor generates data representative of the position and orientation of the tracker relative to the localizer. By inference, this leads to the position and orientation of the body tissue or instrument to which the localizer is attached. Often this information is presented on a display connected to the processor.
• There are a number of reasons why, in a surgical procedure, it is desirable to track the position of body tissue. In an orthopedic surgical procedure, for example, it is desirable to track the position of hard tissue, bone. This tracking is often performed as part of a procedure to replace a joint such as knee, hip or shoulder. Prior to the replacement of the original joint, it is desirable to track the motion of the bones connected by the joint. For example, in a knee replacement procedure, the surgeon will want to know the relative position and range of motion of the below the knee tibia to the above the knee femur. During the actual joint replacement process, this information helps the surgeon fit the replacement joint to the bone so that, post procedure the patient's bones are properly aligned relative to each other and the bones have the appropriate range of motion.
• In other surgical procedures, it is useful to know the position of the patient's tissue in order to assist in the placement and/or control of a surgical instrument at or near the surgical site. In this type of procedure, the system tracks the location of the patient's tissue and the surgical instrument. Based on these data, the processor generates a map that indicates the position of the surgical instrument relative to the tissue or an adjacent surgical site. The surgeon, by reference to this map, properly positions the instrument to accomplish the desired surgical task. Some surgical navigation systems are integrated with the units that regulate the actuation of the surgical instrument. Some versions of these integrated systems are constructed so that based on the map data indicating the position of the surgical instrument relative to the tissue the actuation of the instrument is regulated.
• As mentioned above, the tracker-localizer pair of a surgical navigation system exchanges one of a specific form of energy. Many currently available surgical navigation systems are designed so that their trackers emit and localizers receive photonic energy such as infra red light. These systems often typically require trackers that are relatively large in size, surface areas of 4 cm²or more.
• When a tracker is attached to tissue, it must firmly be attached to tissue site it is intended to track. This is because, if the tracker moves relative to the tissue, the system may not generate signals that accurately represent tissue position. Currently, in order to track the position of bone with an IR tracker, the following protocol is employed. A hole is drilled in the bone. A post is fitted into the hole so it is firmly attached to the bone. Often, in order to accomplish this latter intermediate goal, it is necessary to secure the post to the bone so it extends through the opposed sides of the bone. Once the post is firmly secured in place, the tracker is mounted to an exposed end of the post. Having to so mount the tracker to the bone appreciably adds to the trauma to which the patient is exposed when required to undergo a surgical procedure. This is especially true when, in order to prevent the post from moving, it is necessary to extend the post through the bone.
• Moreover, in this type of arrangement, the post and tracker sub-assembly typically extend10 cm or more above the patient. Given the rather large size of the tracker, this sub assembly, while serving as an important aid in surgery, also functions as an obstruction the operating personnel have to take care to avoid.
• To avoid the above discussed disadvantages of conventional IR surgical navigation systems, there has recently been proposed a system that relies on electromagnetic navigation. This system relies on relatively small fiducial markers designed to be implanted in the bone subcutaneously. Given the relatively small size of these markers, when fitted to the bone there is no need fit them through the bone. Thus, use of these markers is expected to result in less trauma to the patient and reduced clutter adjacent the surgical site. These markers are intended to exchange EM signals with complementary localizers located adjacent the patient.
• While the above proposed system offers some benefits, there are some limitations associated with its use. Specifically, the system introduces into the operating room localizers with relatively large antennas, coils.. These structural members are used to both inductively transfer energy to and exchange signals with the components internal to the fiducial markers.
• Moreover, the signals exchanged between the fiducial markers and the complementary coils are electromagnetic signals. Thus, the strength and direction of the signals are affected by the presence of ferromagnetic materials in the path between the coils and markers. To ensure a surgical navigation system of this variety generates data that accurately indicates the positions of the fiducial markers, and the bones to which they are attached, it is necessary to ensure that space between the coils and markers are free of ferromagnetic materials or other objects that can distort the transmission of the EM energy. This may mean, for example, that instruments formed with ferromagnetic materials should not be introduced into the space during the tracking process. Such instruments include, but are not limited to, powered surgical tools with energized stators. This requirement limits the utility of this system.
SUMMARY OF THE INVENTION
• This invention is related to a new and useful hybrid surgical navigation system for tracking the position of body tissue. The system and method of this invention are designed to minimize trauma to the tissue it is used to track and can be used without appreciably limiting the introduction of ferromagnetic devices into the surgical field.
• The hybrid surgical navigation system of this invention includes two independent navigation systems. A first navigation system includes a tracker head designed to be loosely fitted over the body adjacent where the internal tissue the position of which is to be tracked. In one version of the invention, the tracker contains one or more transmitters that emit energy that can pass into the tissue without distortion. In one form of the invention, the transmitters emit EM or RF energy.
• The first navigation system also includes a tissue marker positioned subcutaneously at the tissue to be tracked. The tissue marker is formed with a structural member(s) that hold(s) the marker to the tissue so the two move in unison. Internal to the tissue marker are transducers. The transducers are sensitive to the energy emitted by the tracker head transmitters. Also internal to the tissue marker is a transmitter that outputs signals representative of the strengths of the signals detected by the transducers.
• The second navigation system of the hybrid system of this invention is located wholly outside of the patient. The second system includes a localizer. In one embodiment of the invention, the localizer contains transducers sensitive to IR light. The second navigation system also contains a number of IR transmitting LEDs. These LEDs are mounted to the tracker head.
• The hybrid navigation system of this invention also includes a processor. The processor receives as input data representative of the signals measured by the tissue mount. Based on these data, the processor determines the location and orientation of the tissue mount relative to the tracker head. The processor also receives input data signals representative of the light sensed by the localizer. Based on these data, the processor determines the location and orientation of the tracker head relative to the localizer. Based on these intermediate-generated data, the processor using transformation algorithms, generates data indicating the position and orientation of the tissue mount and, therefore, the tissue, relative to the localizer. These data are then provided to the surgical personnel.
• In the system of this invention, the tracker head and tissue marker are separated by distances of15 cm or less. Therefore, the signals exchanged between the tracker head and tissue marker are of relatively low strength. This makes it possible to provide a tracker head that is small in size. Consequently, only a minimal incision is needed to fit the marker. Further, only low strength energy needs to be transmitted transcutaneously, through the body. These features collectively minimize the trauma to which the patient's body is exposed when the system and method of this invention is employed.
• Still another feature of the low power requirement of this invention is that a battery is typically all that is required to power the tracker. Therefore, the need to introduce an addition power cord near the patient is eliminated
• Further there is only a relatively small space between the tissue marker and the tracker. This means a ferromagnetic object may be placed relatively close to the components of this system without adversely affecting the accuracy of the tracker position and orientation data generated by the system.
BRIEF DESCRIPTION OF THE DRAWINGS
• This invention is pointed out with particularity in the claims. The above and further features and benefits of the system and method of this invention are better understood by reference to the following Detailed Description taken in conjunction with the accompanying drawings in which:
• FIG. 1 depicts a hybrid surgical navigation system of this invention;
• FIG. 2 is a cross sectional view of the relationship between tissue tracked by the system and the components of the system;
• FIG. 3 is a perspective and partially broken away view of a bone marker of this invention;
• FIG. 4 is a perspective view of a tracker of this invention;
• FIG. 5 is a block and schematic view of the components internal to the tracker;
• FIG. 6 is a diagrammatic view of how the bone marker and tracker of this invention of this invention are fitted to the body in order to track the position and orientation of the body;
• FIG. 7 is a flow chart of the overall, basic steps executed by the system of this invention to determine position and orientation of a section of patient tissue;
• FIGS. 7A, 7B,7C and7D, are flow charts of the more detailed process step executed by the system when performing the overall process ofFIG. 7;
• FIG. 8 is a block and schematic diagram of the components disposed in an alternative tracker of this invention;
• FIG. 9 is a perspective view of the structure of an alternative tracker of this invention;
• FIG. 10 is a partial cross sectional view depicting how the tracker ofFIG. 9 may be fitted to the body;
• FIG. 11 is a cross sectional view of a second alternative tracker of this invention;
• FIG. 12 is a perspective view of an alternative marker of this invention;
• FIGS. 13A and 13B, when assembled together, form a flow chart of the frequency shifting process performed by the system of this invention in order to operate when spurious electromagnetic waves are present;
• FIG. 14 is a flow chart of the process steps executed by the system of this invention to determine at which range of frequencies the electromagnetic signals should be emitted in order to determine the position and orientation of the tracked bone marker.
• FIG. 15 is a plot of the levels of the ambient EM signals that may be measured in the process ofFIG. 14;
• FIG. 16 is a block diagram of how the navigation system of this invention is networked to other devices in the operating room in which the system is used, including both corded and cordless power tools;
• FIGS. 17A and 17B collectively form a flow chart of the process steps executed by the EM navigation system of this invention to adjust for the presence of device-generated EM signals in the ambient environment;
• FIG. 18 is a perspective view of another alternative marker of this invention;
• FIG. 19 is a plan view illustrating how EM sensors, such as magnetoresisitive sensors are mounted on a flex circuit according to this invention;
• FIG. 20 is a perspective view illustrating how the distal end of the assembly ofFIG. 19 is shaped so that the sensors are able to monitor the EM signals present along the three mutually orthogonal axes;
• FIG. 21 is a perspective view of an alternative tracker and marker assembly of this invention;
• FIG. 22 is a plan view of alternative ankle for the tracker and marker ofFIG. 21;
• FIG. 23 depicts another alternative tracker and marker assembly of this invention;
DETAILED DESCRIPTIONI. Basic System
• FIGS. 1 and 2 provide an overall view of the components of thesurgical navigation system30 of this invention.System30 includes atracker32 that is loosely fitted over the tissue, here thetibia34, the position of which is to be tracked. Attached to thetibia34, below theskin36, is abone marker38.Tracker32 andbone marker38 contain complementary components of a first navigation system that generate data indicating the position of the bone marker relative to the tracker. Generally, this process occurs by one of thetracker head32 orbone marker38 emitting energy; components internal to the other of thebone marker38 ortracker32 sense the strength of the emitted energy.
• Alocalizer40 often spaced 1 m or more fromtracker head32 is also part of thesystem30. Internal to thetracker head32 orlocalizer40 are components the actively or passively broadcast energy to the other of the localizer or the tracker. Sensors internal to thelocalizer40 ortracker32 to which the energy is broadcast generate signals representative of the strength of the received energy. Generally, this sub-assembly is the second navigation system.
• The sensor signals generated by both the first and second tracking systems are forwarded to aprocessor44, (shown as phantom rectangle inFIG. 1,) also part of thehybrid system30. Based on the sensor signals,processor44 generates data indicating the position and orientation of thebone marker38. By extension, these data indicate the position and orientation of thebone34. Often these data are presented to the surgical personnel as an image on adisplay46 also part ofsystem30.
• As best inFIG. 3,bone marker38 of this invention includes ahead50 from which astem52 extends.Head50 generally has a circular cross sectional profile so as to give the head a generally cylindrical shape. The head is however further formed to have to have two diametricallyopposed flats54, (one flat shown). Theflats54 extend downwardly from the top of thehead50. In the illustrated version of theinvention flats54 project approximately 50% along the total length of thehead50.Flats54 functions as insertion and removal features for thebone marker38. When an insertion tool is employed to fit themarker38 or a removal tool is employed to extract the marker, members integral with these tools press against theflats54 to facilitate marker insertion/retraction, (tools not illustrated).
• Stem52 is shaped to hold the bone marker to the section of the bone to which the marker is fitted. Thestem52 is shaped to havedistal end tip56 that is generally in the shape of four-sided pyramid. (“Distal” it should be understood means toward the surgical site/away from the surgeon. “Proximal” means away from the surgical site/towards the surgeon.) Between themarker head50 andtip56, stem52 has abase58. The base is formed to have four concave walls60 (twowalls60 shown) each of which is aligned with one of the sides of thetip56. Between each pair of inwardlycurved walls60 there is corner wall62 (threecorner walls62 shown). Eachcorner wall62 is generally flat. However, stem52 is further shaped so that a number of spaced apart inwardlycurved grooves64 extend laterally across eachcorner wall62.
• Internal tobone marker head50 is atransducer66.Transducer66 is part of the first tracker system.Transducer66 is capable of emitting or sensing energy that can be transmitted through tissue without distortion. In some versions of the invention,transducer66 is capable of either emitting or sensing electromagnetic energy or RF energy.
• In some versions of theinvention marker38 has an overall length from the top of thehead50 to the stemdistal end tip56 of 30 mm or less and, more preferably, 20 mm or less.Head50 has a length 20 mm or less and more preferred 15 mm or less.Head50, the largest diameter portion of the marker has a diameter of 14 mm or less and, more preferably, 7 mm or less.
• As seen inFIG. 4,tracker32 includes ahousing70. In some versions of the invention,housing70 is formed from non-magnetic material. Autoclave sterilizable, reusable versions of the housing are formed from metal. Potential metals are aluminum, titanium and300 series stainless steel. In some versions of the invention,housing70 is formed from plastic or ceramic.Housing70 has aplanar base71, seen diagrammatically inFIG. 6. Fourside walls72 extend perpendicularly from the base71 atop panel74 is extends across the top of theside walls72 so thathousing70 forms a sealed enclosure.Top panel74 is generally planar.
• Tracker32 ofFIG. 4 is further formed so that theside walls72 on the left and right sides of thebase71. Twofingers77, each with the cross sectional profile of a truncated triangle, project outwardly from eachside wall72 to the adjacent base edge. Eachfinger77 is located at one end of the associatedside wall72. Eachfinger77 thus extends the enclosed sealed space inside thetracker housing70. Asingle LED122 is mounted to each of the two inclined sections of eachfinger77.LEDs122, as discussed below, are part of the second navigation system that forms the hybrid system of this invention. Generallytracker housing70 has a maximum length 20.0 cm or less, in more preferred versions, 10.0 cm or less, a width 12 cm or less, in more preferred versions, 9.0 cm or less and a depth of 8.0 cm or less and, in more preferred versions 4.0 cm or less.
• Thetracker32 is further formed so that integral withbase71 are twoopposed tabs78. Eachtab78 extends outwardly from the base edge from which a pair offingers77 extends. When thetracker32 is mounted to the patient two approximately parallel straps or bandages79 (FIG. 6) are placed over the patient around the location at which the tracker is to be placed. The straps/bandages79 are formed withpockets80, for receiving thetracker tabs78. The seating of thetabs78 in strap/bandage pockets thus holds the tracker to the patient.
• Top panel76 is however further formed to define tworibs78 each of which extends laterally across an opposed end of the top panel.Ribs78 have a generally triangular cross sectional profile.
• FIGS. 5 and 6 illustrate the active components oftracker32 andbone marker38. Specifically, internal to thetracker32 are twotransmitter assemblies82 and84. The transmitter assemblies are positioned so that a first assembly,transmitter assembly82, is located at one end of the housing and the second assembly,transmitter assembly84, is located at the opposed end of the housing. Eachtransmitter assembly82 and84, includes a first coil capable of transmitting an EM signal in the X direction a second coil capable of transmitting an EM signal in the Y direction and a third coil capable of transmitting a signal in the Z direction. InFIG. 6, the individual X-, Y- and Z-coils oftransmitter assembly82 are represented byarrows86a,88aand90a,respectively. In some preferred versions of the invention, the coils are mutually orthogonal and centered on a common point. In some versions of the invention it is contemplated the coils are wound around a common square block.
• Precision, voltage-controlled, bi-polarcurrent sources106 disposed in thetracker32 generate the signals that are applied to the transmitter coils. InFIG. 5,tracker32 is shown as having six separatecurrent sources106. Threecurrent sources106 apply separate signals to thecoils86a,88aand90aoftransmitter assembly82. The remaining threecurrent sources106 each apply signals to theseparate coils86b,88band90boftransmitter assembly84.
• While not illustrated, the individualcurrent sources106 each includes a feedback circuit to ensure that the DC current running through the associated coil is, as close as possible, zero. This DC current damping is needed to ensure the magnetic fields emitted by the coils are of as precisely controlled strengths as possible.
• Amicroprocessor108 asserts the signals that causecurrent sources106 to independently output signals at different current levels. In some versions of the invention,microprocessor108 is a digital signal processor. One such digital signal processor is the fixed point digital signal processor No. TM320VC5502 available from Texas Instruments of Dallas, Tex. Current source command signals are output byprocessor108 as digital signals. These signals are output over abus110 to a digital to analog converter (DAC)112 with six output ports. TheDAC112, based on instructions that comprise each command generated by themicroprocessor108, generates an AC level signal to the input pin of eachcurrent source106.
• Transducer66 is the active component internal to thebone marker38. The transducer consists of threecoils92,94 and96 sensitive to electromagnetic energy. Ideally, the transducer coils are mutually orthogonal and centered on a common point. Thus, like the transmitter assembly coils, coils92,94 and96 may be wound around a common square block. A set of conductors connectcoils92,94 and96 to the inside of thetracker housing70. InFIG. 6, the conductors are represented by asingle line98. In some versions of the invention, there are three conductors; a common ground and a single conductor connected to each coil. In alternative versions of the invention there are six conductors; a pair of conductors connects eachcoil92,94 and96 to thetracker32.
• As also seen inFIG. 5, the individual coils92,94 and96 of the transducer assembly are attached to separate fixedgain amplifiers116. The output signal from each fixedgain amplifier116 is applied to avariable gain amplifier118. The gain of eachamplifier118 is set by a control signal asserted by themicroprocessor108.Microprocessor108, it should be understood, is able to set the individual gains of theamplifiers118 independently from each other. In one version of the invention, fixedgain amplifiers116 each have a gain of 10,000; eachvariable gain amplifier118 can be set to have a gain of 1, 10 or 100. In an alternative version of the invention, the fixed gain amplifiers each have a gain of 1,000; eachamplifier118 can be set to have a gain of between 1 to 1,000 that is set in single step increments.
• The output signals produced by the variable gain amplifiers are applied to an analog todigital converter120. The digitized representations of the amplified versions of the signals measured across the transducer coils92,94 and96 are supplied from theADC120 to themicroprocessor108.
• As mentioned above,tracker32 also contains components of the second navigation system. InFIG. 5 this is represented by a singleIR emitting LED122. The application of a voltage across theLEDs122 is controlled by aFET124 tied to the LED cathode. Aload resistor126 is tied between the anode ofLED122 and ground.Microcontroller108 asserts the gate signal toFET124 to regulate the actuation of theLED122.
• InFIG. 5 anEEPROM128 is also shown connected tomicroprocessor108 overbus110. TheEEPROM128 stores both the operating instructions executed by themicroprocessor108 as well as some of the intermediate data generated by the microprocessor.
• Also integral with the tracker is awireless transceiver130.Transceiver130 exchanges signals withsystem processor44. Often the signals emitted bytransceiver132 are directed to a complementary transceiver in the localizer40 (transceiver132 shown as a phantom block inFIG. 1). In some versions of the invention,transceiver130 exchanges RF signals. In still other versions of the invention,transceiver130 exchanges visible, UV or IR signals. The exact type of signals exchanged by thetransceiver130 and the complementary external transceiver are not relevant to the structure of this invention.
• All the components internal to thetracker32 are powered by abattery134 also internal to the tracker.Battery134 is shown connected to avoltage regulator136. One constant voltage connection, the connection that supplies a current to energize themicrocontroller108 is shown. To minimize complexity of the drawings the remaining power supply connections are not shown.
II. Operation of the Basic System
• Operation ofsystem30 of this invention is now described by initial reference to the flow chart ofFIG. 7. In a joint replacement procedure, the system is used to track the position and range of movement of bone prior to the procedure and during the procedure. In this type of procedure, a device known as a cutting guide135 (shown as a block disposed above the leg ofFIG. 6) is typically fitted to the bone adjacent to where the joint is to be replaced. Cuttingguide135 guides the cut of a saw that is employed to remove the joint to be replaced. The cuttingguide135 is used in this procedure to ensure that the bone left in place after the remove is properly shaped to receive the components forming the artificial joint.
• Initially, as represented bystep148,system30 is set-up for use. Oncesystem30 is set up for use, a measurement is made of the position and orientation of themarker38 and, by extension, the tissue to which the marker is attached,step150. Once the marker/tissue position and orientation information are generated, in astep152, this information is presented to the surgeon,step154. Typically, this information is presented ondisplay46.Steps152 and154 are repeatedly executed throughout the surgical procedure in order to provide real time information regarding the position and orientation of the tissue to which themarker38 is attached.
• Occasionally during the procedure,system30, in astep156, checks the accuracy of the position and orientation determination made instep152. For example in some versions of the invention, steps152 and154 are typically performed at a frequency of between 10 to 100 Hz and, more often, between 25 to 75 Hz. Step156 is performed at frequency of between 0.5 Hz to 5 Hz. Step150 represents the decision made by eithersystem processor44 ortracker microcontroller108 to determine which one ofsteps152 or156 is to be performed. Alternatively, in some versions of this invention, when the check ofstep156 is performed, this step is performed essentially simultaneously with, instead of as a substitute for, the measurements ofstep152.
• Steps150,152,154 and156 are repetitively preformed throughout the time it is necessary for the surgical personnel to be provided with tissue position and orientation data. Eventually, there is point in the surgical procedure at which it is no longer necessary to monitor the position and orientation of the tissue, step157 ofFIG. 7. Once the procedure is at this point, in astep158,system30 is removed from the patient.
• The individual steps that comprise the system set upstep148 are now described by reference toFIG. 7A. In astep160, an insertion tool is used to mount thebone marker38 to the bone,tibia32. Generally, themarker38 is positioned a distance from the cuttingguide135 greater than the length of thetracker34. In one method of fitting thebone marker38, after the marker is positioned, the insertion tool applies an impacting force to themarker head50. The force generated by the insertion tool drives the marker stem52 into the underlying bone cortical layer.
• During the actual insertion process, only the cortical material subtended by themarker stem52 is driven away from the stem, towards the center of the bone. The adjacent cortical material, the material in the space outward of theconcave walls60, remains static. This bone material abuts the stem inwardlycurved walls60 to prevent the rotation of thestem52. Immediately after the insertion, the material compressed outwardly away from the stem expands back towards its initial position. This material seats instem grooves64 to prevent longitudinal movement of thestem52. Thus, collectively, the bone material that abutsstem walls60 and that seats ingrooves64, block thestem54 and, therefore the whole of themarker38, from movement.
• In astep162, thetracker32 is positioned between thebone marker38 and the cuttingguide135. Bandages/straps79 are fitted aroundtracker tabs78 to hold thetracker32 to the leg. For reasons that will be apparent below, this invention does not require thetracker32 to be securely attached to the leg. In astep164,wire cable98 that extends frommarker head50 is passed through the soft tissue and skin and connected to thetracker housing70. This physical attachment connects transducer coils92,94 and96 toamplifiers116.
• Prior to the generation of EM energy by thetracker32, instep166,microcontroller108 asserts the control signals to establish the gains of theindividual amplifiers118. The variables that affect the gain settings include: the distance between the tracker and the bone marker; drive current; number of signal transmitting and receiving elements (windings); and ambient noise; geometries and sensitive surface areas of both the transmitter and receiver.
• Once the system is configured for operation,step148 is completed,steps151,152 and154 can be executed. The sub-steps that formstep152 are now described by reference toFIGS. 7B and 7C. The actual generation of data to indicate tissue position and orientation begins, instep168, with the simultaneous emission of EM signals bycoils86a,88a,and90aoftransmitter assembly82. Step168 is executed bymicroprocessor108 asserting signals throughDAC112 to the appropriatecurrent sources106 that cause AC currents to be applied tocoils86a,88a,and90a.In many versions of the invention, the signals have frequencies between 100 and 1,000 Hz. The signals applied to coils86a,88a,and90aare at different frequencies. In some preferred versions of the invention, the signals are at a harmonics of a base frequency. In some versions of the invention, one of the frequencies may even be applied at the base frequency. The Applicants' Assignee's U.S. Patent ApplicationSystem and Method for Electromagnetic Navigation in the Vicinity of a Metal Object,U.S. patent Application Ser. No. 11/123,985, U.S. Pat. Pub. No. ______, now U.S. Pat No. ______, the contents of which are now incorporated herein by reference, describe the reasons why it is desirable to apply signals having the above relationship to thecoils86a,88a,and90a.The signals applied simultaneously to thecoils86a,88a,and90aare at cumulative power level of 5 Watts or less and, in more preferred versions of the invention, 0.5 Watts or less. Thus, the signal applied to a single coil is generally at 1.67 Watts or less and more preferably 0.17 Watts or less.
• As part ofstep168, it should be understood the phases of the signals thecurrent sources106 apply to coils86a,88aand90aare also regulated.
• Simultaneously with, or as near as simultaneously as possible withstep168, in astep170,microcontroller108 also causes theLEDs122 of the second navigation system to emit photonic energy detectable bylocalizer40.
• The current flow through transmitter coils86a,88a,and90aresult in the emission of EM waves by the coils. As a consequence of the emittance of the EM energy bytransmitter assembly82, potentials simultaneously develop across the threetransducer coils92,94 and96,step172. As part ofstep172, these signals are therefore forward throughcable98 to fixedgain amplifiers116. Step174 represents the amplification of these signals. Specifically these signals are first amplified by the fixedgain amplifiers116. Then, also part ofstep174, the output signals fromamplifiers116 are subjected to variable gain amplification byamplifiers118. In astep176 the amplified signals from the transducer coils9294 and96 are digitized byADC120 and applied tomicrocontroller108.
• Then, in astep178,microcontroller108 deconstructs each of the signals produced by the transducer coils92,94 and96. Specifically, themicroprocessor108 employs a Fast Fourier Transformation (FFT) to break down each coil signal into the components formed as a consequence of the simultaneous emission of three magnetic fields bytransmitter assembly82. Each signal is broken down into the amplitude and phase components for each of the three EM waves that contributed to the generation of the signal.
• In astep180,microcontroller108, throughtransceiver130, transmits packets tosystem processor44 that contain data defining the components of the transducer assembly signals.
• The IR light emitted byLEDs122 instep170 is detected by the receivers internal to thelocalizer40,step184. Digitized signals representative of the direction and strength of the received light are, in astep186 applied tosystem processor44.
• Based on the digitized data from the localizer receivers received instep187,system processor44 generates data that describes the position and orientation of thetracker34 relative to thelocalizer40. These data include a translation vector {right arrow over (x)}_L→Tthat represents the position of thetracker34 relative to the localizer. A rotational matrix R_L→Tthat represents the rotation of x-, y- and z-axes of thetracker34 relative to the x-, y- and z-axes of the localizer is also generated.
• In astep188, based on the deconstructed transducer assembly data, system processor generates data that describes the position and orientation of thebone marker38 relative to thetracker32. These data include a translation vector {right arrow over (x)}_T→Mrepresentative of the position of thebone marker38 relative to thetracker32. These data also include rotational matrix R_T→Mwhich represents the rotation of the x-, y- and z-axes of thebone marker38 relative to the x-, y- and z-axes of thetracker32.
• Based on the data generated insteps187 and188, system processor, instep190 generates data representative the position and orientation of thebone marker38 relative to the localizer. These data are vector {right arrow over (x)}_L→M, which represents the position of the bone marker relative to the tracker and rotational matrix R_L→M, the rotation of the x-, y- and z-axes of thebone marker38 relative to the x-, y- and z-axes of thelocalizer40. Instep190, these data are calculated according to the following formulas:
  {right arrow over (x)}_L→M={right arrow over (x)}_L→T+R_L→T·{right arrow over (x)}_T→M  (1)
  and
  R_L→M=R_L→T·R_T→M  (2)
  The above equations assume thetransmitter assembly82 is located exactly at the center location of the tracker, at the position and with the orientation specified {right arrow over (x)}_Tand R_T. In actuality, there are offsets between the locations and orientations of thetracker assemblies82 and84 and the position and orientation of the tracker as determined instep186. These offsets are determined at time of manufacture. Therefore,step190 includes the execution of intermediate processing steps employing variations ofEquations 1 and 2 above to account for these offsets. In these steps, the translational vector and translation rotation matrix data are determined based on the data regarding tracker position at time of manufacture.
• Oncestep190 is executed,processor44 contains data that indicates the position of thebone marker38 relative to thelocalizer40.Processor44 is therefore able to, instep154, present an image on thedisplay44 that indicates the position of thebone marker38 and by extension, the section of bone in which the marker is mounted.
• In a knee replacement procedure, while not illustrated, it should be understood another bone marker is used to track the position of the femur. At the start of the procedure, the bone marker position and orientation data from both markers is used to determine the positions of the two bones relative to each other and the range of motion of the two bones. These data are then used by the surgeon to ensure that the components forming the implant are properly positioned so that, post-procedure, the bones will be in the proper positions and the patient has the appropriate range of movement.
• It should further be appreciated that, while the steps168-190 are shown as occurring sequentially, this is for simplicity of illustration only. In practice some of these steps are preformed simultaneously. For example in some versions of the invention, whiletracker microcontroller108 is performing the deconstruction of the signals fromcoils92,94 and96,step178,system processor44 can simultaneously be determining the position and orientation of the tracker,step187.
• Step156 ofFIG. 7 is performed to verify that the marker/tissue position and orientation data provided by thesystem30 is accurate. One reason these data may be inaccurate is that, if a ferromagnetic object is in very close proximity to the components of the system, the object can adversely affecting the accuracy of the measurements made by the system. A ferromagnetic object can have such an affect because, owing to the nature of the object it diverts the magnetic fields emitted by thetransmitter assemblies82 and84. This, in turn, causestransducer assembly66 to output signals that cannot be used to properly determine position and orientation of thebone marker38. As discussed below, another source of interference can be EM signals generated by other equipment used to perform the surgical procedure.
• FIG. 7D illustrates the sub steps that formstep156. Thus, step156 starts with astep196 in whichmicrocontroller108 asserts control signals that cause thecurrent sources106 to whichtransmitter assembly84 coils86b,88band90bare connected to output drive signals. Again, it should be understood thatstep196 may be performed as an alternative to step168. In versions of the invention, whensteps152 and156 are preformed simultaneously, steps168 and196 are therefore preformed simultaneously.
• After the execution ofstep196, processing steps similar tosteps170,172,174,176,178,180,184,186,188 and190 are performed with regard to the EM waves emitted bytransmitter assembly84 and measured bytransducer assembly66. Collectively, these steps are represented asstep198, the generation of translation vector {right arrow over (x)}_L→M^2NDTRANSof the distance from the localizer to the bone marker and R_L→M^2NDTRANS, the rotational matrix that represents the rotation of the bone marker relative to the localizer as based on the signals emitted bytransmitter assembly84.
• Thus, after the execution ofstep190 and the near simultaneous execution ofstep198,system processor44 contains the following data, vector {right arrow over (x)}_L→M^1STTRANSof the distance from the localizer to the bone marker and R_L→M^1STTRANS, the rotational matrix that represents the rotation of the bone marker relative to the localizer as based on the signals emitted bytransmitter assembly82 and {right arrow over (x)}_L→M^2NDTRANSand R_L→M^2NDTRANS. Then in astep202processor44 determines if the two translation vectors are substantially equal and/or if the two rotational matrixes are substantially equal.
• If the evaluation(s) test(s) true ofstep202 tests true, thansystem processor44 recognizes the environment in one in which no significant ferromagnetic devices or objects are present,step204.Steps157,150,152 and154 are then repetitively reexcuted until thenext time step156 is reexcuted.
• Alternatively, instep202 it may be determined that vectors {right arrow over (x)}_L→M^1STTRANSand {right arrow over (x)}_L→M^2NDTRANSare not equal and/or rotational matrixes R_L→M^1STTRANSand R_L→M^2NDTRANSare not equal. One or both of these conditioning existing is interpreted bysystem processor44 as an indication that a ferromagnetic object is adversely affecting the magnetic waves that are being sensed by thetransducer assembly66. Therefore, in astep206,system processor44 asserts an alarm regarding this environmental state. The assertion of this alarm provides notice to operating room personnel that a ferromagnetic object has been introduced into the environment at a location that is adversely affecting the ability ofsystem30 to track thebone marker38. The steps necessary to remove the ferromagnetic object can be taken. Alternatively, as discussed below with respect toFIGS. 13A and 13B,system30 undergoes a frequency shifting process to find frequencies which thetransmitter assemblies82 and84 can emit EM waves that are not adversely affected by the ambient EM signals.
• Returning toFIG. 7, it is understood that eventually there is a point in the surgical procedure at which, instep157 it is no longer necessary to monitor marker/tissue position and orientation. At thistime step158 is performed in order to disconnect the system from the patient. First, thetracker32 is removed. Then, also as part ofstep158, thebone marker38 is removed. This task is accomplished by initially rotating themarker38. The rotation of themarker38 results in the bone material seated adjacent the marker stemwall60 rotating with the marker. This material thus breaks free of the surrounding bone. Simultaneously, the rotation of the marker results in thestem54 rotating to the position where the stem is free of the material seated ingrooves64. Thus, once themarker38 is so rotated, the removal is completed by the relatively easy task of the simply longitudinal pulling of the marker away from the bore in which thestem54 is seated.
• System30 of this invention used to track tissue location using a relativesmall bone marker38. Consequently, only a relatively small incision is required to mount this marker. Thus,system30 of this invention provides a means to track tissue without requiring the mounting of a large structural member to the tissue. Thus, this invention eliminates the trauma to the tissue associated with the mounting of such a device.
• Thesystem30 is further designed so thattracker32 is mounted to the patient immediately above the skin. Given that thetracker34 is of relatively small size, the tracker does not function as a large obstacle around which the surgical personnel need to maneuver.
• Still another feature ofsystem30 is that thetransmitter assemblies82 and84 are typically spaced less than 25 cm from thetransducer66 and more often 15 cm or less. One benefit of this arrangement is that only relatively low powered EM waves need to be transmitted from thetransmitters82 and84 to thetransducer66. The lower power of these waves essentially eliminates the possibility they will damage the tissue through which they are transmitted.
• A further feature of this invention is that relatively small amounts of power are required to energize the components internal to both thetracker32 and themarker38. Often these components collectively require an instantaneous power of 10 Watts or less of power and, in more preferred versions of theinvention 2 Watts or less. Thus, the power required to supply these components can be provided by a batter attached to thetracker32. This eliminates the need to introduce a power cord into the surgical field in order to monitor the position and orientation of the tissue.
• Another advantage of the proximity of the transmitter assemblies to the transducer assembly ofsystem30 is that it results in there being a relatively small space around the transmitter and transducer assemblies in which the presence ferromagnetic object can adversely affect the measurement of the EM waves. Thus, ferromagnetic surgical instruments can be placed relatively close to the tracker, within a distance as close as 10 cm and sometimes a distance as close as 5 cm, without adversely affecting the operation ofsystem30.
• The above utility ofsystem30 is further understood by reference toFIG. 6. Here it can be seen that thebone marker38 is spaced a distance from the area where the surgical procedure is to be performed by a distance greater than the length of thetracker32. This distance, the tracker length, defines the space in which the introduction of the ferromagnetic objects could affect the measurements made by thetransducer assembly66. Thetracker38 thus acts a guide block representative of the minimum distance ferromagnetic surgical devices and objects should be placed away from thebone marker38 in order to ensuresystem30 properly tracks the marker.
• During periods of time the transmitted EM waves are adversely affected by either other objects or other EM waves, for example, those emitted by a powered surgical tool,system30 determines if such interference is present. Thus,system30 of this invention is further designed to provide an indication if the ambient conditions inhibit the accurate generation of tissue tracking data.
• Tracker32 is designed so that the transmittingLEDs122 of the second navigation system are located on opposed sides of thefingers77. Thus, in substantially most orientations of thetracker32 relative to the localizer, sufficient photonic energy will be emitted by thetracker LEDs122 that will be received by the localizer to ensureprocessor44 has sufficient data to determine the position and orientation of thetracker32 relative to thelocalizer40.
• Still another feature ofsystem30 of this invention is thatbone marker38 is designed so that, upon insertion into the bone, it remains locked in position. Then, when it is time to remove the marker, once the marker is rotated from the locked position, minimal force is required to complete the removal process.
• The foregoing is directed to one specific version ofsystem30 of this invention and one specific procedure in which the system is used. Variations in both the constructions of the invention and its method and methods of use are possible.
• For example, thesystem30 can be further be used to assist in the tracking of the location of a surgical instrument relative to a surgical site. In these uses of thesystem30, prior to the actual procedure, the tissue at the surgical site is mapped. Data describing the map are loaded intosystem processor30. At the start of the procedure,bone marker30 is positioned at a precisely known mapped body position.
• During the procedure, the system monitors the position and orientation of thebone marker38 so as to, by extension, determine the position and orientation of the surgical site. Simultaneously,localizer40 is used to monitor the position and orientation of a surgical instrument. Based on these data,system processor44 is able to generate data indicating the position and orientation of the instrument relative to the surgical site.
III. First Alternative Tracker
• Further, constructions of the system may vary from what is described above.FIG. 8 illustrates in block diagram of an alternative components that may be provided in a tracker32a.In this version of the invention,tracker32 has threesignal generators222,224 and226 internal also internal totracker32 energize the coils oftransmitter assemblies82 and84. The signal generators222-226 are configured to be energized simultaneously. Each signal generator222-226 emits a signal at a constant frequency. Collectively, the signals emitted by signal generators222-226 are emitted at different frequencies.
• The signals output by theindividual signal generators222,224 and226 are amplified byseparate amplifiers228,230 and232, respectively. The output signals fromamplifiers228,230 and232 are applied to three input ports of a 2:1multiplexer234. A first set of output ports of themultiplexer234 are tied to the threecoils86a,88aand90aoftransmitter assembly82. The second set of three output ports ofmultiplexer234 are tied to the threecoils86b,88band90boftransmitter assembly84.
• Amicrocontroller236, again internal to thetracker32, both controls the emission of energy from thetransmitter assemblies82 and84 and is the component that initially monitors the energy detected bybone marker transducer66.
• Specifically,microcontroller236 asserts a control signal to multiplexer234 to tie the signals from the signal from the generator-amplifier pairs to eithertransmitter assembly82 ortransmitter assembly84.Microcontroller236 also asserts individual control signals toamplifiers118. Thus,microcontroller236 regulates the gain of the individual signals applied to the transmitter assembly coils.
• Microcontroller236 receives as input signals the signals emitted by transducer assembly coils92,94 and96. Prior to being input to themicrocontroller236 these signals are individually amplified byamplifiers116 and118 and digitized byADC120.
• The alternative tracker ofFIG. 8 also contains the previously describedLEDs122,transceiver130battery134 and related components.
• In this version of the invention, when it is time to executestep168, the emission of EM energy from thetransmitter assembly82,microcontroller236 first asserts a control signal to themultiplexer234 to tie the multiplexer inputs to theindividual coils86a,88aand90a.The gains of theindividual amplifiers228,230 and232 are set individually. Thus, coils86a,88band90aemit EM waves of appropriate strength.
• When the check procedure ofsteps196 and198 are to be performed,microcontroller236 asserts a control signal to multiplexer to cause the output drive signals to be applied tocoils86b,88band90boftransmitter assembly84.
• FIG. 9 is a perspective view of analternative tracker242 of this invention.Tracker242 has abody244 that is generally rectangular.Tracker body244 is further formed to have along the top surface thereof tworibs246 that have a generally triangular cross sectional profile. Eachrib246 extends laterally across one end of the tracker. TheLEDs122 are mounted toribs246 such that there are two LEDs on the side of each rib.
• Tracker242 is held to the body portion adjacent theunderling bone marker38 by a shapedstrap252 seen inFIG. 10.Strap252 is formed from a plastic and is C-shaped so that it can be compression fitted around the body portion.Foam padding254 around the inner surface ofstrap252 provides a cushion between the strap andunderlying skin36. The outer surface of the strap is formed with a rectangular shaped outwardly extendingweb256.Web256 defines an opening (not identified) in which thetracker242 is seated.
• It should likewise be appreciated that in alternative versions of the invention, transducer assembly may have transducer elements other than coils. In some versions of the invention, the EM sensitive devices may be Hall sensors or magneto-resistive sensors.
• Further, the locations of the components may vary from what has been described. In some versions of the invention, it may be desirable to place a transmitter assembly in thebone marker38 and one or more sensor assemblies in the tracker.
• Likewise, in other versions of the invention, the amount of processing of the EM sensor signals performed in the tracker may vary from what has been described. In some versions of the invention, there may be essentially no processing of the sensor signals in the tracker except what is needed to transmit them to thesystem processor44. In still other versions of the invention, these signals may be completely processed by the processing unit internal to the tracker. Thus,step188 is performed entirely by the tracker processor. Upon completion of this step, vector {right arrow over (x)}_T→Mand rotational matrix R_T→Mare transmitted by the tracker to the receiver internal to thelocalizer40.
IV. Second Alternative Tracker
• FIG. 11 illustrates still anothertracker260 of this invention. Tracker250 is formed to have ahousing262. Internal tohousing262 is ashell264 formed from a thin layer of ferromagnetic material.Shell264 is formed to havebase266 immediately below the top ofhousing262. Theshell264 also has side walls268 (two shown) that are formed integrally with thebase256 and extend downwardly from the perimeter of the base.Shell264 thus defines a void space that is directed toward the surface of the body section in which thebone marker38 is mounted.
• Internal to the shell void space is asubstrate270. Either the transmitting assembly or sensor assembly of the EM navigation system is suspended from the void space. InFIG. 11 an array of downwardly directedcoils272 represent the transmitter assembly or sensor assembly internal to the tracker. Each of the coils is wrapped around apost274 that extends downwardly from thesubstrate270.
• In the above described tracker of this invention, shell264 functions as shield that prevents ambient electromagnetic waves from entering the space between thetracker260 andbone marker38. This construction makes it possible to place ferromagnetic devices relatively close to the tracker250 without the concern that such devices could interfere with the accuracy of the EM based measurements of the position and orientation of the bone marker.
• FIG. 11 illustrates another alternative feature of this invention. There is no requirement that the transmitter (sensor) array comprises a set of emitters (transducers) arranged in three dimensions. In some versions of the invention the transmitter (sensor) array can include a plurality of emitters (transducers) that are arranged in a common dimension. Such construction may also be incorporated into the emitters (transducers) internal to the marker.
V. First Alternative Marker
• It should likewise be understood that the navigation system of this invention is not limited to systems used to tracker hard tissue, bone. In alternative versions of the invention, the marker is provided with features that allow it to be secured to soft tissues, such as muscles, ligaments, tendons and the diaphragm.FIG. 12 illustrates onesuch marker280. Heremarker280 has ahead282. Internal to the head is the transmitting assembly or sensor assembly. InFIG. 12,coil block284 represents the transmitter or sensor assembly.
• Twolegs286 formed of bendable metal or plastic extend downwardly frommarker head282.Marker280 is fitted to the soft tissue by inserting legs around the tissue adjacent the marker.Legs286 are pressed together so that the legs clamped around the tissue to hold the marker in position.
• Thus in procedures other than orthopedic surgical procedures, the system of this invention can be used to track the position and orientation of body tissue while only requiring the minimal invasion of the body and the minimal transmission of energy through the body.
VI. Em-Based Navigation Frequency Hopping
• Further, as now described by reference toFIGS. 13A and 13B,system30 of this invention is also configured to operate in a certain situations when ambient EM signals would otherwise adversely affect the accuracy of the marker/tissue position and orientation determinations. As discussed above with respect to step156 (FIG. 7) and steps196-204 (FIG. 7D),system30 determines whether or not the position and orientation data generated are accurate by comparing the determinations made based on signals generated by theseparate transmitter assemblies82 and84.
• In some situations, the position and orientation data are inaccurate because a nearby medical device, such as a power drill or other instrument with a motor, emits EM signals. Generally, the frequency at which this other device functions as an EM signal generator varies with its operating rate. Therefore, there is no exact way to automatically compensate for the presence of the EM signals emitted by the device.
• Accordingly, once, in step202 (FIG. 7D), it is determined that the position and orientation data generated by thesystem30 are inaccurate,system30 attempts to find a set of frequencies at which it can operate at which the ambient EM signals will not adversely affect the position and orientation determination. As seen byFIG. 13A, this process starts, instep290, with the resetting of the drive frequencies at which the transmitter assembly coils86a-90aand86b-90bare driven. The actual reset command it should be appreciated originates with thesystem processor44 and is transmitted through thetransceiver130 thetracker microcontroller108. For example, in one version of the invention, three coils oftransmitter assembly82 or84 are, for example, are normally driven at frequencies of 570 Hz, 630 Hz and 690 Hz. Instep290, these frequencies are reset to, respectively 870 Hz, 930 Hz and 990 Hz. Each frequency is adjusted by a constant offset.
• Then insteps291 and292 system performs an ambient EM noise check. This sub-process starts with the system, instep291, measuring the strength of the ambient EM energy detected by sensors (coils)92,94 and96. More particularly, the system measures the ambient EM energy present in the frequency spectrum in whichtransmitters82 and84 are set to operate. These measurements are performed by monitoring the voltage across eachcoil92,94 and96 during a period in which thecomplementary tracker transmitters82 and84 are both switched off.
• Instep292,processor44 compares the strength of the ambient EM energy to a threshold energy level. This threshold level depends on the sensitivity and noise properties of the sensors and the strength of the magnetic field generated by the transmitters. Generally, the transmitted signal to noise ratio needs to be at least 100:1 and, more preferably, 300:1 or more. In one system, with a 45 milliamp driving current through one coil of a 0.5 cubic inch transmitter, at 4 inches away from thetransmitter assembly82 or84, the magnetic field is as low as 6 milliGauss. In this environment, the threshold value is a maximum of 60 microGauss and, more preferably, 20 microGauss or less.
• If the comparison ofstep292 indicates the ambient EM energy level at the reset output frequency spectrum for the transmitters is less than the threshold level, the environment is considered to be one in which the ambient EM signals would most likely not affect the operation of thesystem30. Accordingly the system proceeds to perform a second sub-process to further ensure the subsequently generated tracker position/orientation data will be accurate. This sub-process starts with the below describedstep293. However, instep292 it may be determined that the ambient EM signals at the new transmitter frequency spectrum are above the threshold level. This means the ambient EM signals could adversely affect the generation of accurate position and orientation data. In this event,system30 performs a second frequency hop, frequency resetting, described below with respect to step301.
• Instep293, the system determines the position and orientation of the marker based on the detection of drive signals applied to the coils86a-90aof thefirst transmitter assembly82. In astep294, the system determines marker position and orientation of the marker based on the detection of drive signals applied to coils86b-90bof thesecond transmitter assembly84. Again, in some versions of the invention, steps292 and294 are performed simultaneously.
• Oncesteps292 and294 are executed, the marker position and orientation determinations based on the signals from the twodifferent transmitter assemblies82 and84 are compared,step296. Thus,step296 is similar to the determination ofstep202. If the position and orientation data are identical, then the EM signals interfering with EM signals generated bysystem30 within the frequency range is not interfering with the system's EM signals emitted at the second frequency range. Accordingly, as represented bystep298, the system continues to operate,steps152,154 and156 are again cyclically executed. However, at this time the tracker drives the transmitter assembly coils at the frequencies of the reset frequency range.
• Alternatively, from the evaluation ofstep296, it may be determined that system is still not accurately tracking the position and orientation of themarker38. In this situation, the system proceeds to the second resetting of the range of the drive signals applied to the transducers,step301. In the present example, the range of frequencies is downshifted by an amount two that they were up-shifted. Thus, the drive signals are applied to the individual transmitter coils at frequencies of 270 Hz, 330 Hz and 390 Hz.
• Oncestep301 is executed a measurement of ambient EM signal strength at the new frequency spectrum is made as represented bystep302. In astep303 the measured ambient EM signal strength is compared to the threshold value.Steps302 and303 are thus similar to, respectively, steps291 and292. If, as a result of the execution ofstep303, it is determined that significant EM signals are present at this second reset frequency spectrum, an alarm may be asserted,step312.
• However, instep303 if it is determined that strong EM signals that could potentially affect the position and orientation determinations are not present in the ambient environment, the system proceeds to execute a sub-process to determine if the EM signals transmitted within the new spectrum can be used to provide accurate position and orientation data. This sub-process starts withstep304.
• Instep304, the position and orientation of themarker38 is determined based on the second set or reset drive signals applied to coils86a-90aof thefirst transmitter assembly82. In astep306,system30 determines marker position and orientation of the marker based on the detection of drive signals applied to coils86b-90bof thesecond transmitter assembly84. Again, steps304 and306 may be performed simultaneously.
• Astep308 is then executed to compare the two marker position and orientation determinations. Thus,step308 is, likestep296, similar to step202. If the determinations are identical, then the system is generating transmitter assembly drive signals within a frequency range at which the ambient EM signals are not adversely affecting the measurements made bytransducer66. In this situation, as represented byFIG. 310, the system returns to normal operation. At this time though, themicrocontroller108 causes the transmitter assembly coils to be driven at frequencies within the second reset frequency range.
• However, the determination ofstep308 may test false. Thus, collectively as a result of the tests ofsteps202,296 and308 it is apparent that the ambient EM signals are within a very wide frequency range. In this situation, it may not be possible for the system to continue to accurately generate marker/tissue position data. Therefore,system30, in astep312, asserts an alarm indicating that the ambient EM signals have such characteristics that they system cannot operate at its transmitter assemblies at any frequencies in order to ensure the accurate generation of position and orientation data.
• It should be appreciated that the foregoing is only one potential frequency shifting protocol of this invention. In some versions of the invention,system30 may only make a single frequency shift to find a range of operating frequencies at which it can operate free from ambient EM signal interference. In other versions of the invention,system30 makes three or more shifts before determining the ambient EM signals are of such strength and/or within such a range that the system cannot provide accurate market and position and orientation data. The300 Hz shift up and down from the base frequency range should understood to be exemplary, not limiting.
• FIG. 14 illustrates an alternative process by whichsystem30 of this invention by whichsystem processor44 resets the frequencies at which tracker transmitters emit EM signals if there are significant ambient EM signals within the first operating spectrum. In astep320,system30, through the marker sensors (coils)92,94 and96 measures the ambient EM signals throughout the complete frequency spectrum in whichtransmitters82 and84 emit EM energy. This process may be accomplished by Fast Fourier Transformation or any other spectral analysis technique for determining the frequency of the emitted signals.
• As a consequence ofstep320,system processor44 generates an internal plot of the strength of the ambient EM signals throughout the spectrum,step322.FIG. 15 is a graphic representation of this plot. Here, the relative strengths of the ambient above noise level EM signals range from 0 to approximately 3.5 milliGauss. In this particular case, the noise generator is a surgical saw. Often, an EM signal generator adjacent the surgical site at which thesystem30 is used emits EM signals within a defined frequency range or ranges. InFIG. 15 this is represented by the fact that the ambient EM signals are above nominal noise levels at frequencies below 200 Hz and between approximately 650 and 850 Hz.
• Consequently, based on the data plot generated instep322, in astep324,system processor44 determines the frequency range (or ranges) at which potentially interfering ambient EM signals are present.
• Then, in astep326,system processor44 generates commands to cause the frequencies at which thetransmitter assemblies82 and84 emit signals to be reset to a range outside those at which the interfering EM signals are present. This step may be considered similar to step290 (FIG. 13A). The system may then perform steps similar to those described inFIGS. 13A and 13B to ensure that the transmission and complementary sensing of the signals within the reset frequency range will cause accurate tracker position and orientation data to be generated.
VII. Integrated Em Navigation and Device Operation
• As illustrated byFIG. 16, it is further possible to integratesystem30 of this invention with the other devices employed in the operating room. More particularly,system30 is integrated to operate in unison with the devices, such as the power tools that can emit potentially interfering EM signals. InFIG. 16system processor44 of this invention is connected to abus340 to which other devices are connected. These devices include acontrol console344.Console344 generates energization signals to corded powered tools such ashandpiece342. Such control consoles are discloses in the Applicants' Assignee's U.S. Patent No. U.S. Patent U.S. Pat. No. 6,017,354, INTEGRATED SYSTEM FOR POWERED SURGICAL TOOLS, issued Jan. 25, 2000 and its U.S. patent application Ser. No. 10/955,381, INTEGRATED SYSTEM FOR CONTROLLING PLURAL SURGICAL TOOLS, filed Sep. 30, 2004, U.S. Pat. Pub. No. ______ now U.S. Pat. No. ______, the contents of both documents now incorporated herein by reference. Still another device that may be connected tobus340 is a wireless/voice control head350. This type of device is capable of receiving spoken commands and/or commands entered through a wireless control pendant.Control head350, upon receipt of the command, converts it into a processor-executable instruction that can be processed bycontrol console344. In this manner,head350 allows the surgeon to enter spoken or touch command in order to regulate the operation ofhandpiece342. Onesuch control head350 is sold by the Applicants' Assignee under the trademark SIDNEE.
• Another processing unit that can be attached tobus340 is a personal computer or aserver352. Either unit can perform the data processing, including the data process described below with respect to the process ofFIG. 17. Providing this additional processing unit minimizes the data processing other units such asnavigation processor44 or the processor internal to controlconsole344 are required to perform.
• Bus340 is any suitable bus over which data and commands can be exchanged between multiple processing units. The bus may be any bus such as an IEEE-1394 Firewire bus or LAN.
• Cordless power tools are also connected tosystem30. One such power tool, acordless driver346 is depicted inFIG. 16. The Applicants' Assignee's U.S. Patent Application No. 60/694,592, POWERED SURGICAL TOOL WITH SEALED CONTROL MODULE, filed Jun. 28, 2005, U.S. Pat. Pub. No. ______, now U.S. Pat. No. ______, the contents of which are incorporated herein by reference, discloses how a cordless tool can, through atracker347 attached to the tool and thelocalizer40 transmit real time operating data tonavigation processor44.
• FIGS. 17A and 17B form a flow chart of the process steps executed by the integrated system of this invention. Step360 represents the initial step of waiting to see if device that generates EM signals is actuated. Such device could be the motor internal tocorded handpiece342 or the motor internal to thecordless driver346. Again, it is recognized by those skilled art that other surgical power tools not just motorized tools can be the source of EM signals. Such tools include, and are not limited to, RF ablation tools, laser and other light emitting tools, and devices that emit sonic or ultrasonic energy. Accordingly, the type of EM signal-emitting surgical device is not limited to one specific type of device.
• When instep360, the EM energy emitting device is actuated, a data packet describing the operation of the device is generated, step362. This data packet, in addition to containing data indicating that the device is actuated, describes the characteristics of the operation of the device. For example if the device is a motorized power tool, the data packet describes the speed of the motor. The unit of the integrated system that generates this data packet is a function of the specific type of device. For example, if the device is a corded tool,control console344 generates this data packet and places onbus340. If the device is cordless a processor internal to the device generates the packet and transmits the packet to the complementary transceiver132 (FIG. 1) internal to localizer40.
• Based on the describing the operation of the device, in astep364, the frequency range of EM signals generated by the device is determined. For example, for a motorized tool, there is generally a proportional relationship between motor shaft RPM and the frequency at which EM signals are emitted. The exact relationship can be determined by empirical analysis.Step364, depending on the configuration of the system, it should be understood, can be performed by either thenavigation system processor44 or the network (operating room) computer/server352.
• Then, in a step366 a determination is made whether or not the frequency range at which thenavigation system30 is emitting EM signals approximates the range at which the device is generating EM signals. In some versions of the invention, the processor that executesstep364 also executesstep366 and the below describedsteps370,376 and378. If, instep366, it is determined that the device is generating EM signals within a frequency range whole different from the range at whichsystem30 is generating EM navigation signals, operation of the navigation system continues as before,step368.
• Alternatively, instep366 it may be determined that the present EM frequency range at which thenavigation system30 is operating is at least partially overlaps or is relatively close to the frequency at which the powered device is emitting EM signals. If this condition exists, instep370 thenavigation system30 resets the frequency range at which the transmitter assemblies emit EM signals to themarker38. Again, the instructions to reset the operating range of the transmitter assemblies may be generated by any suitable processor such as thenavigation system processor44 or the operating room computer/server352.
• Navigation system30 continues to operate at the reset range of frequencies. Eventually, the EM generating device is deactivated,step374. When this happens the processor associated with the device, instep376 outputs a data packet reporting the device deactuation. In response to the receipt of this packet, the device responsible for establishing the operating frequencies of the navigation system, instep378, resets the navigation system so that it operates at its initial frequency range.
• Thus the integrated system of this invention is constructed so that, as soon as the device starts to generate potentially interfering EM signals, the frequency of these signals is determined. If, in fact, the device-generated EM signals could adversely affect the operating of thenavigation system30, the operation of the navigation system is reset so it outputs EM signals at a different frequency. This substantially reduces the possibility that the generation of EM signals by a surgical device in the vicinity of thenavigation system30 could result in thesystem30 generating potentially inaccurate marker position and orientation data.
VIII. Second Alternative Marker
• FIG. 18 illustrates still anothermarker390 of this invention.Marker390 has ahead392 from which astem394 extends.Head392 contains the previously described transducer66 (FIG. 3).Head392 is formed to define atopmost crown396.Crown396 has a surface that, from the top of the head curves outwardly toward the side of the head. Belowcrown396, the head is formed to define agroove398 that extends inwardly relative to the outer perimeter of the crown. Both thecrown396 and thegroove398 extend circumferentially aroundmarker head392.
• The above geometry is provided to facilitate the fitting of a removal tool to themarker390. Specifically it should be understood, that oncemarker390 is fitted to the bone, it may not be possible to visually detect the marker. Making a larger incision to facilitate marker removal reduces one of the advantages of this invention, that the marker can be fitted without exposing the patient's tissue to large amounts of trauma. The removal tool is therefore often attached to the marker by feel. The tool itself typically has a number of legs that are configured to be biased together around themarker head392. When the removal tool is, by tactile probing, fit over themarker head392, the legs when extending over thecrown396. Further insertion of the legs over the head result in the legs snapping intogroove398. This movement provides the surgeon with tactile and audible feedback that the removal tool is properly position over themarker head392.
• Stem394 ofmarker390 is formed to have a number ofbarbs402. More particularly, there are four longitudinal rows ofbarbs402. The barb rows, (two shown) are equangularly spaced apart. Upon insertion of themarker stem394 into the bone,barbs402 dig into the bone to hold themarker390 in position.
• Stem394 is further constructed so that immediately above the proximalmost barbs402 there areindentations404 that extend substantially inward of the barbs.Indentations404 thus define adjacent the proximal end of the stem394 a separation zone along axis of the stem represented inFIG. 18 by aphantom cylinder406. Structurally, the separation zone has, in comparison to the rest of the barb, a low tensile strength. For example, in some versions of the invention, the material forming the separation zone may separate if thestem394 is exposed to tensile force, a pulling force, of 50 pounds or more. In still other versions of the invention, the stem separates when exposed to a force of 80 pounds or more or 100 pounds or more. In practice, markers with stems with a separation zones that separate upon the application of different amounts are force are provided. At the start of the procedure, step160 (FIG. 7A) a marker with a stem separation zone that separates upon the application of a specific force is then fitted to the patient as a function of such variables as bone density, bone age and bone size.
• At the completion of the procedure in whichsystem30 is used,marker390 is removed. More particularly, the removal tool is attached to themarker head392. Force is then used to remove themarker390. This force may exceed the tensile strength of the separation zone of thestem394. In this situation, the material forming the separation zone separates. Thus, in this procedure, themarker head392 and proximal end of thestem394 are removed from the surgical site; the portion of thestem394 distal to the separation zone remains embedded in the bone.
• An advantage of the foregoing marker construction is that the removal of themarker390 does not expose the hard tissue, bone, surrounding the marker to appreciable trauma. Since thestem394 is formed from biocompatible material, leaving of the stem distal end section in the patient does not have any adverse affects.
IX. Alternative Marker Sensors
• It should likewise be appreciated that the transducer elements used to measure the signals transmitted through the body may vary from what has been described.
• For example, as an alternative to the coils, magnetoresistive devices may be used to measure the strength of EM signals. It is believed that these transducer elements are less affected by noise induced from such sources as thermal noise.
• As seen inFIG. 19, in this version of the invention twomagnetoresistive sensors410 and412 are mounted on acommon flex circuit416.Flex circuit416 has a generally elongated shape to, as discussed below, facilitate the seating ofsensors410 and412 in the marker.Conductors415 that provide energization signals tosensors410 and412 and that supply the signals from the sensors are also disposed on theflex circuit416.Sensors410 and412 are located near the distal end of theflex circuit416 and are equidistantly spaced apart from the longitudinal axis of the flex circuit. It is further observed that theflex circuit416 is formed to have aslot418 that extends rearwardly from the distal end of the circuit.Slot418 is located along the longitudinal axis of the flex circuit so that theindividual sensors410 and412 are on opposed sides of the slot.
• When the marker in whichsensor410 and412 are employed is assembled, the distal end of theflex circuit416 is folded aroundslot418 so that the two opposed sections are, as seen inFIG. 20, at 90° to each other. Eachsensor410 and412 includes two sensing assemblies (not illustrated) that are themselves at 90° to each other. Thus as result of the bending of the substrate supporting thesensors410 and412, there is at least one sensor assembly for measuring EM field signals along each of the x-, y- and z-axes. This is seen inFIG. 20 wherearrow420 throughsensor410 represents that internal to that sensor there is a sensor assembly capable of measuring EM signals along the x-axis; to the left and right in the drawing sheet.Arrow422, also throughsensor410, represents that the second sensing assembly ofsensor410 is capable of measuring EM signals along the z-axis; up and down in the drawing sheet.Arrow424, throughsensor412, represents that, due tosensor412 being at a 90° angle tosensor410,sensor412 has a sensing assembly sensitive to EM signals in the y-axis; in and out of the drawing sheet. A potting compound, a fixture close tolerances and/or angled brackets can be used to hold the distal end sections of theflex circuit416 to ensure that the sensor maintain the proper orientation relative to each other.
X. Integrated Tracker and Marker
• FIG. 21 depicts a tracker andmarker unit430 in which the sensor assembly ofFIGS. 15 and 16 may be fitted. It should be recognized thatunit430 may housing other EM sensitive transducers.Unit430 has a pyramidal-shapedhead432.LEDs122 used for tracking thehead432 are mounted to the outer surface of the head.Additional LEDs123, that are part of the transceiver unit130 (FIG. 5) are also mounted to the surface of thehead432.
• Astem434 projects below the flat distal end base ofunit430.Stem434 includes aleg436 that is immediately located below and is rigidly attached to theunit head432. Arigid foot440 forms the most distal portion ofstem434.Foot440 is connected toleg436 by aflexible ankle438.Threading442 is disposed around the distal end of thefoot440.Threading442 allows both the screw securement and screw removal of thefoot440 from the bone to whichunit430 is mounted.
• Ankle438 is shaped to allowfoot440 to bend relative to theleg436 and also to transmit rotational force, torque, from the leg to the foot. In some versions of the invention, thestem434 is formed from a single tube-shaped piece of metal. The wall sections of the stem material forming theleg436 andfoot440 are solid. The wall sections of the stem material forming the ankle are formed withslots444. Theslots444 provide theankle438 with its flexibility relative to the longitudinal axis of theleg436 while ensuring that the ankle is able to transfer torque to thefoot440. It is believed that thestem434 can be formed out of a nickel titanium allow such as the alloy marketed as NITINOL.
• A flexible sleeve, not illustrated, disposed in the sleeve provides insulation around the active components in the unit. These components include themagnetoresistive sensors410 and412, (FIG. 16). In addition, or alternatively, a biocompatible coating is disposed over theflex circuit416 andsensors410 and412. Also in this version of the invention, the transmitter assemblies that emit the EM signals may be disposed in theleg436 of theunit stem434. Thus, in most versions of this embodiment of the invention, the EM transmitters are 3.0 cm or less from the complementary receiving units and in, more preferred versions of the invention, this distance is 2.0 cm or less.
• Unit430 is used by screw securing thestem foot440 into the bone at the position and orientation of which is to be monitored. The pointeddistal end tip443 of thefoot70, as well as the threading442, facilitates this securement. In this version of the invention, the complementary EM signal transmitters and receiver are very close proximity. Consequently only relatively small strength signals, need to be transmitted between the transmitters and the receivers. In some versions of the invention the cumulative strength of the signals emitted simultaneously by the plural transmitters is 1.0 Watts or less (0.34 Watts or less from each transmitter) and more preferably, cumulatively, less than 0.5 Watts or less (0.17 Watts or less per transmitter.). Further the space in which these signals are transmitted is relatively small, the distance between these components. This means that there is only a relatively small space aroundunit430 in which neither ferromagnetic objects nor interfering EM signals should be restricted to ensure the accurate tracking of tissue position and orientation.
• In this version of the invention, the overall length of theunit430 from thedistal end foot440 to the proximal end ofhead432 is typically 15 cm or less and in more preferred versions of the invention 12 cm or less. Thus because of its size and the fact that there is no need to maintain theunit head432 stable relative to thefoot440 means that the presence ofunit430 does not serve as a significant physical obstacle adjacent the surgical site.
• It should be appreciated that the physical features of unitary tracker and marker assembly may vary from what has been described with respect toFIG. 21.FIG. 22, for example, illustrates an alternativeflexible ankle438aofstem434. Here,ankle438ais formed in the stem by providing ahelical cut448 around the ankle-forming section of the stem. Cut448 also is formed to define interlockingcastellations450. Thecastellations450 function as the torque transmitting members of theankle438a.
XI. Alternative Integrated Tracker and Marker
• An additional alternative tracker andmarker unit450 of this invention is now described by reference toFIG. 23.Unit450 has acylindrical head452.LEDs122 and123 used to for, respectively, tracking and data/command signal exchange are mounted to the outer surface of thehead452. An elongaterigid shaft454 extends downwardly from the base of thehead452.
• Amarker456 is located below the distal end ofshaft454.Marker456 has ahead458 in which the EMsensitive transducer66 orsensors410 and412 are located. Apointed stem460 is integral with and extends below themarker head458.Stem460 is formed with the geometric features needed to releaseably secure the marker to the tissue to be tracked. In the illustrated version of the invention, the nail like structure ofstem460 facilitates the driving of the stem into bone.
• Acable462 flexibly connects themarker head458 toshaft454. While not illustrated it should be understood that disposed withincable462 are the conductors used to for signal exchange with the marker transducer assembly.Cable462 extends from a location insideshaft454. More particularly,shaft454 is formed with a distal end bore466 (shown in phantom) dimensioned to accommodatemarker head458. Internal toshaft454 and proximal to the base of thebore466 there is a void space, not illustrated. This void space accommodates the slack portion ofcable462 when themarker head458 is seated inbore466.
• Shaft454 is further formed so that at the distal end thereof there are two diametricallyopposed slots468, (one shown). Eachslot468 extends rearwardly from the distal end of theshaft454 and opens intobore466.Marker head458 is formed with two diametrically opposed, outwardly extendingears470.Marker ears470 are dimensioned to slip fit intoshaft slots468.
• Unit450 is fitted to the patient by positioning themarker456 so that thehead458 is in shaft bore466 andears470 are seated inshaft slots468. Force is then applied throughhead452 to drive themarker stem460 into the bone. Oncemarker456 is secured in place, the shaft can be extended away from themarker head458. The head and shaft subassembly can then move relative to themarker456. As in the above described versions of the invention, the measurement of EM signals transmitted from the head and shaft to themarker456 make it possible to continuously monitor the position of the marker relative to thehead452.
• When it is time to removeunit450, the head and shaft are repositioned so thatmarker head458 seats in shaft bore466 andmarker ears470 reseat inslots468. Torque needed to remove the marker stem460 from the bone is applied from theshaft454 to the marker throughears470. Thus, this version of the invention reduces the effort required after the tracking process to recapture marker so it can be removed. Moreover,unit450 functions as its own marker insertion and removal tool.
• It should be appreciated that in versions of the invention that use magnetoresistive sensors, the output signals of these transducers are affected by their thermal state, temperature shifts. To reduce signal changes caused by these temperature shifts, it is believed best to apply a constant current to them. Moreover, the voltage across each sensor assembly, which is in the form of a Wheatstone bridge, should be monitored. The change in this voltage is a function of the temperature-shift induced changes in the sensitivity of the assembly. Based on the change in the voltage level across an individual sensor assembly, the gain in the output signals from the sensor assembly is adjusted. This gain adjustment compensates for the temperature-induced changes in the sensitivity of the assembly.
XII. Alternative Embodiments
• While in some versions of the invention, EM waves are the medium through which energy is transmitted through the body it should be appreciated that this exemplary, not limiting. In other versions of the invention, the energy exchange may be by RF waves. Still in other versions of the invention sonic or ultrasonic energy may be transmitted between the subcutaneous marker and the above skin level tracker.
• It should also be understood that the second navigation system, the system used to determine the position and orientation of thetracker32 relative to thelocalizer40, may rely on transmission of other forms of energy than IR energy to determine the position and orientation of the tracker. These alternative forms of energy include but are not limited to, sonic, ultrasonic, visible light, ultraviolet light, EM and RF energy.
• Further there is no requirement that in all versions of the inventions a wired connection exist between the tracker and the marker. In some versions of the invention, the marker may have its own battery or receive power inductively from the tracker. In these versions of the invention, the signals generated by the sensors internal to the marker are transmitted at RF wavelengths to a complementary receiver in the tracker.
• Likewise, in some versions of the invention, the tissue marker contains the components that emit energy. In these versions of the invention, the tracker contains the sensors that monitor the strength of the emitted energy.
• Also, both the frequency hopping processes of this invention and the integration of the EM producing devices to the EM navigation system are not limited to implementation in the disclosed hybrid navigation system. These features of the invention of this application can be integrated into a conventional unitary navigation system that relies on the measurement of EM signals to determine the position and orientation of a tracker. Again, the Applicant's incorporated by reference U.S. patent application Ser. No. 11/123,985 describe constructions of EM navigation system to which the above features of this invention can be integrated.
• It should be appreciated that in these navigation systems, the components in thetracker32 andlocalizer40 may be reversed. Thus, in some of these navigation systems, the tracker contains the EM signal emitting assemblies and the localizer contains the sensors used to measure the strength of the EM signals. In alternative versions of these systems, the localizer contains the EM signal emitting assemblies; the tracker contains the EM sensors.
• Further in alternative surgical navigation systems such as systems wherein the strength of RF, photonic or ultrasonic signals are monitored, the frequency shifting process of this invention can be employed to reduce the instances wherein ambient releases of energy adversely affect can adversely affect the determination of either tracker position and orientation and/or marker position and orientation. Thus, it should be understood that the frequency detecting and frequency shifting processes of this invention are not limited to implementation in navigation systems that monitor EM energy emissions.
• Therefore, it is an object of the appended claims to cover all such variations and modifications that come within the true spirit and scope of the invention.

Claims (12)

1. A surgical navigation system comprising:

a subcutaneous marker including a geometric feature shaped to attach said marker to a section of tissue;

a tracker at least partially disposed above skin surface adjacent said marker, said tracker being moveable relative to said marker;

a first navigation assembly located in said marker and said tracker for generating signals indicating the position of said marker relative to said tracker;

a second navigation assembly contained in said tracker and in a localizer spaced from said tracker for generating signals indicating the position of said tracker to said localizer; and

a processor connected to said first navigation assembly for receiving data representative of the position of said marker relative to said tracker and to said second navigation assembly for receiving data representative of the position of said tracker to said localizer and configured to, based on the received data, generate data indicating the position of said marker relative to said localizer.

2. The system ofclaim 1, wherein said first navigation assembly includes:

a transmitter in one of said marker or said tracker for transmitting energy wirelessly; and

a transducer in the other one of said tracker or said marker for measuring the strength of the transmitted energy.

3. The system ofclaim 2, wherein the energy transmitted from said marker or said tracker to the other of said tracker or said marker is either EM or RF energy.

4. The system ofclaim 2, wherein the energy transmitted from the one of said marker or said tracker is transmitted at 5 Watts or less.

5. The system ofclaim 1, wherein said second navigation system transmits and receives one from the following types of energy: EM energy; RF energy; light energy; sonic; and ultrasonic energy.

6. A surgical tracker, said tracker comprising:

a transmitter for transmitting energy to a subcutaneous marker;

a receiver for receiving from the mark signals representing measurements of the transmitted energy received by the marker and

components of a remote navigation system that includes a localizer that allows the remote navigation system to determine the position of said tracker relative to the localizer; and

a transmitter connected to said receiver for wirelessly broadcasting to a remote receiver data representative of the signal measurements made by the marker.

7. A method for performing surgical navigation including the steps of:

attaching a tracker to body tissue or a surgical device;

providing a localizer, wherein at least one of the tracker or the localizer is moveable relative to the other of the localizer or the tracker;

transmitting energy at a select frequency from the tracker to the localizer or from the localizer to the tracker;

measuring the transmitted energy received by the localizer or the tracker;

based on the measurements of energy received by the localizer or the tracker, determining the position or orientation of the tracker;

determining if a device external to the surgical navigation system is emitting energy that can potentially adversely affect said determination of the position or orientation of the tracker; and

if a device external to the surgical navigation system is emitting energy that can potentially adversely affect said determination of the position or orientation of said tracker, in said step of transmitting energy, changing the frequency at which the energy is transmitted.

8. The method of performing surgical navigation ofclaim 7, wherein said step of determining if a device external to the surgical navigation system is emitting energy that can potentially adversely affect said determination of the position or orientation of the tracker is performed by monitoring the accuracy of said determination of the position or orientation of the tracker.

9. The method of performing surgical navigation ofclaim 7, wherein said step of determining if a device external to the surgical navigation system is emitting energy that can potentially adversely affect said determination of the position or orientation of the tracker is performed by monitoring whether or not the external device is actuated.

10. The method of performing surgical navigation ofclaim 7, wherein, in said step of transmitting energy, energy from one of the following types of energy is transmitted: electromagnetic; RF; sonic; ultrasonic; and photonic.

11. The method of performing surgical navigation ofclaim 7, wherein:

in said step of attaching a tracker, the tracker is fitted subcutaneously to tissue of a patient; and

in said step of providing a localizer, the localizer is disposed over the patient in the vicinity of the tracker.

12. The method of performing surgical navigation ofclaim 11, wherein:

said navigation system used to track the position and orientation of said tracker is a first navigation system;

the position or orientation of the localizer is determined; and

based on the determination of the position or orientation of the tracker relative to the localizer and the determination position or orientation of the localizer, the position or orientation of the tracker relative to a reference point is determined.

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