US20090259259A1 - Helical wound mechanically interlocking mating guide and advancement structure - Google Patents

Abstract

A structure including a pair of helically wound interlocking forms located on a cylindrical closure for an open headed medical implant and in a receiver between arms of the implant respectively. The interlocking forms each include overlapping gripping elements that engage mating elements during assembly to prevent radial splaying of arms of the implant.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application is a continuation of U.S. patent application Ser. No. 10/831,919, filed Apr. 26, 2004 which was a continuation from U.S. patent application Ser. No. 10/236,123, filed Sep. 6, 2002, now U.S. Pat. No. 6,726,689 and incorporated by reference herein.
BACKGROUND OF THE INVENTION
• The present invention is directed to a structure for use in interlocking together two elements and, in particular, to a structure for joining together parts of a medical implant. The structure includes a first interlocking form on a closure and a mating second interlocking form on a receiver. The closure is operably rotated into the receiver. The first and second interlocking forms are both helically wound so that the first interlocking form advances relative to the second interlocking form, when the closure with the first interlocking form is inserted in the receiver and rotated. At least one of the first or second interlocking forms includes a projection that overlaps and radially locks with the other interlocking form when the two forms are mated.
• Medical implants present a number of problems to both surgeons installing implants and to engineers designing them. It is always desirable to have an implant that is strong and unlikely to fail or break during usage. It is also desirable for the implant to be as small and lightweight as possible so that it is less intrusive on the patient. These are normally conflicting goals, and often difficult to resolve.
• One particular type of implant presents special problems. In particular, spinal bone screws, hooks, etc. are used in many types of back surgery for repair of injury, disease or congenital defect. For example, spinal bone screws of this type are designed to have one end that inserts threadably into a vertebra and a head at an opposite end thereof. The head is designed to receive a rod or rod-like member in a channel in the head which rod is then both captured in the channel and locked in the head to prevent relative movement between the various elements subsequent to installation.
• There are two different major types of bone screws and similar devices which are classified as closed headed and open headed. While the closed headed devices are highly effective at capturing and securing a rod, since the rod is threaded through an opening in the head, it is very difficult during surgery to thread the rod through the heads. This is because there are many heads and the rod is curved or the heads do not align. Consequently, the more screw heads that the rod must pass through, the more difficult it is to thread the rod into them.
• The second type of head is an open head wherein a channel is formed in the head and the rod is simply laid in an open channel. The channel is then closed with a closure member. The open headed bone screws and related devices are much easier to use and in some situations must be used instead of the closed headed devices.
• While the open headed devices are often necessary and often preferred for usage, there is a significant problem associated with them. In particular, the open headed devices conventionally have two upstanding arms that are on opposite sides of a channel that receives the rod member. The top of the channel is closed by a closure member after the rod member is placed in the channel. The closure can be of a slide in type, but such are not easy to use. Threaded nuts are sometimes used that go around the outside of the arms. Such nuts prevent splaying of the arms, but nuts substantially increase the size and profile of the implant which is not desirable. Many open headed implants are closed by plugs that screw into threads between the arms, because such have a low profile. However, threaded plugs have encountered problems also in that they produce radially outward forces that lead to splaying of the arms or at least do not prevent splaying that in turn loosens the implant. In particular, in order to lock the rod member in place, a significant force must be exerted on the relatively small plug or screw. The forces are required to provide enough torque to insure that the rod member is clamped or locked in place relative to the bone screw, so that the rod does not move axially or rotationally therein. This typically requires torques on the order of 100 inch-pounds.
• Because open headed implants such as bone screws, hooks and the like are relatively small, the arms that extend upwardly at the head can be easily bent by radially outward directed forces due to the application of substantial forces required to lock the rod member. Historically, early closures were simple plugs that were threaded with V-shaped threads and which screwed into mating threads on the inside of each of the arms. But, as noted above, conventionally V-shaped threaded plugs tend to splay or push the arms radially outward upon the application of a significant amount of torque, which ends up bending the arms sufficiently to allow the threads to loosen or disengage and the closure to fail. To counter this, various engineering techniques were applied to allow the head to resist the spreading force. For example, the arms were significantly strengthened by increasing the width of the arms by many times. This had the unfortunate effect of substantially increasing the weight and the profile of the implant, which was undesirable.
• Many prior art devices have also attempted to provide outside rings or some other type of structure that goes about the outside of the arms to better hold the arms in place while the center plug is installed and thereafter. This additional structure may cause the locking strength of the plug against the rod to be reduced which is undesirable, especially when the additional structure is partly located beneath the plug. Also, the additional elements are unfavorable from a point of view of implants, since it is typically desirable to maintain the number of parts associated with the implants at a minimum and, as noted above, the profile as minimal as possible.
• Other designers have attempted to resolve the splaying problem by providing a closure with a pair of opposed radially extending wedges or flanges that have mating structure in the arms of the implant. Such devices serve as a closure and do somewhat resist splaying of the arms, but are often very difficult to use. In particular, the rods normally have some curvature as the rods are bent to follow the curvature of the spine and normally bow relative to the bottom of the bone screw channel that receives such a rod. The rod thus fills much of the channel and must be “unbent” to rest on the bottom of the channel and be held securely in place. Therefore, the rod is preferably compressed by the plug and unbent by advancement of the plug into the channel in order to assume that the plug will securely hold the rod and that the rod and plug will not loosen when post assembly forces are placed on the rod. Because it takes substantial force to unbend the rod, it is difficult to both place the plug fully in the channel and rotate it for locking while also trying to line up the wedges with the mating structure. It is much easier to align the plug mating structure with the mating structure of the arms at the top of the arms and then rotate the plug so as to screw the plug into a plug receiver to advance the plug toward the rod. In this way the plug starts applying significant force against the rod only after parts of the mating structure have at least partly joined at which time torque can be applied without having to worry about alignment. It is noted that where wedges are used, the cross section of the structure changes therealong so that the device “locks up” and cannot turn further after only a small amount of turning, normally ninety degrees.
• Consequently, a lightweight and low profile closure plug was desired that resists splaying or spreading of the arms while not requiring significant increases in the size of the screw or plug heads and not requiring additional elements that encircle the arms to hold the arms in place.
• It is noted that the tendency of the open headed bone screw to splay is a result of the geometry or contour of the threads typically employed in such devices. In the past, most bone screw head receptacles and screw plugs have employed V-shaped threads. V-threads have leading and trailing sides oriented at angles to the screw axis. Thus, torque on the plug is translated to the bone screw head at least partially in an axial direction, tending to push or splay the arms of the bone screw head outward in a radial direction. This in turn spreads the internally threaded receptacle away from the thread axis so as to loosen the plug in the receptacle.
• The radial expansion problem of V-threads has been recognized in various types of threaded joints. To overcome this problem, so-called “buttress” threadforms were developed. In a buttress thread, the trailing or thrust surface is oriented perpendicular to the thread axis, while the leading or clearance surface remains angled. This theoretically results in a neutral radial reaction of a threaded receptacle to torque on the threaded member received.
• Development of threadforms proceeded from buttress threadforms which in theory have a neutral radial effect on the screw receptacle to reverse angled threadforms which theoretically positively draw the threads of the receptacle radially inward toward the thread axis when the plug is torqued. In a reverse angle threadform, the trailing side of the external thread is angled toward the thread axis instead of away from the thread axis, as in conventional V-threads. While buttress and reverse threadforms reduce the tendency to splay, the arms can still be bent outward by forces acting on the implant and the threads can be bent by forces exerted during installation. Therefore, while certain threadforms may not exert radial forces during installation, at most such threadforms provide an interference or frictional fit and do not positively lock the arms in place relative to the closure plug.
• Finally, it is noted that plugs of this type that use threadforms are often cross threaded. That is, as the surgeon tries to start the threaded plug into the threaded receiver, the thread on the plug is inadvertently started in the wrong turn or pass of the thread on the receiver. This problem especially occurs because the parts are very small and hard to handle. When cross threading occurs, the plug will often screw part way in the receiver and then “lock up” so that the surgeon is led to believe that the plug is properly set. However, the rod is not tight and the implant fails to function properly. Therefore, it is also desirable to have a closure that resists crossthreading in the receiver.
SUMMARY OF THE INVENTION
• A non threaded guide and advancement structure is provided for securing a set screw, plug or closure in a receiver. Preferably the receiver is a rod receiving channel in an open headed bone screw, hook or other medical implant wherein the channel has an open top and is located between two spaced arms of the implant.
• The guide and advancement structure has a first part or interlocking form located on the closure and a second part or interlocking form that is located on the interior of the receiving channel.
• Both parts of the guide and advancement structure are spirally or more preferably helically wound and extend about the closure and receiving channel for at least one complete 360° pass or turn. Preferably, both parts include multiple turns such as 2 to 4 complete 360° rotations about the helixes formed by the parts. The helixes formed by the parts are coaxial with the closure when the closure is fully received in or being rotated into the receiving channel between the arms.
• One major distinguishing feature of the guide and advancement structure is that each of the parts include elements that mechanically interlock with the opposite part as the closure is rotated and thereby advanced into the receiving channel toward the bottom of the channel and into engagement with a rod received in the channel.
• Each part of the guide and advancement structure preferably has a generally constant and uniform cross section, when viewed in any cross sectional plane fully passing through the axis of rotation of the closure during insertion, with such uniform cross section extending along substantially the entire length of the interlocking form. It is noted that at opposite ends of each interlocking form, the form must be feathered or the like and so the cross section does change some at such locations, while retaining part of the overall shape. In particular, the outer surfaces of each interlocking form remain sufficiently uniform to allow interlocking forms to be rotated together and slide tangentially with respect to each other through one or more complete turns of the closure relative to the receiving channel. Each part may be continuous from near a bottom of the closure or receiving channel to the top thereof respectively. In certain circumstances one or both parts may be partly discontinuous, while retaining an overall helical configuration with a generally uniform cross sectional shape. When the interlocking form has multiple sections dues to being discontinuous, each of the sections has a substantially unfair cross section along substantially the entire length thereof.
• In order to provide an interlocking structure, the parts of the structure include helical wound projections or interlocking forms that extend radially outward from the closure and radially inward from the receiving channel. The interlocking forms may be of many different shapes when viewed in crossection with respect to a plane passing through the axis of rotation of the plug during insertion. In general, the interlocking forms increase in axial aligned width or have a depression at a location spaced radially outward from where the interlocking form attaches to a respective closure or receiving channel, either upward (that is, parallel to the axis of rotation of the closure in the direction from which the closure comes or initially starts) or downward or in both directions. This produces a first mating element that is in the form of a protrusion, bump, ridge, elevation or depression on the interlocking form that has a gripping or overlapping portion. The opposite interlocking form has a second mating element with a gripping or overlapping portion that generally surrounds or passes around at least part of the first mating element in such a way that the two are radially mechanically locked together when the closure is advanced into the receiving channel.
• Therefore, in accordance with the invention a mating and advancement structure is provided for joining two devices, that are preferably medical implants and especially are an open headed implant that includes a rod receiving channel and a closure for closing the receiving channel after the rod is received therein. The mating and advancement structure includes a pair of mateable and helical wound interlocking forms with a first interlocking form located on an outer surface of the closure and a second interlocking form located on an inner surface of the receiving channel or receiver. The first and second interlocking forms are startable so as to mate and thereafter rotatable relative to each other about a common axis so as to provide for advancement of the closure into the receiver during assembly when the closure interlocking form is rotated into the receiver interlocking form. The first and second interlocking forms have a helical wound projection that extends radially from the closure and the receiver respectively. Each interlocking form projection has a base that is attached to the closure or receiver respectively and preferably includes multiple turns that may each be continuous or partially discontinuous with constant or uniform cross-sectional shape. The interlocking forms have substantial axial width near an outer end thereof that prevents or resists misalignment of the interlocking form during initial engagement and rotation thereof.
• After assembly, in some embodiments each turn of each projection generally snugly engages turns of the other projection on either side thereof. In other embodiments there must be sufficient tolerances for the parts to slide tangentially, so that when thrust surfaces of the interlocking forms are very close during tightening, some gap occurs on the leading side of the closure interlocking form. In such a case the portions of the interlocking forms on the thrust side thereof lock together and prevent radial splaying. Located radially spaced from where the base of each projection is attached to either the closure or receiver respectively, is an axially extending (that is extending in the direction of the axis of rotation of the plug or vertically) extension or depression. The opposite or mating interlocking form has elements that wrap around or into such extensions or depressions of the other interlocking form. That is, the forms axially interdigitate with each other and block radial movement or expansion. In this way and in combination with the interlocking forms preferably being snug relative to each other with sufficient clearance to allow rotation, the interlocking forms, once assembled or mated lock to prevent radially slipping or sliding relative to each other, even if the base of one or both is bent relative to the device upon which it is mounted. It is possible that the cross section of the projection (in a plane that passes through the plug axis of rotation of the plug) of each section of each turn or pass of the interlocking form be the same, although this is not required in all embodiments. For example, part of the interlocking form may be missing in the region between opposed arms when assembly is complete as this area is not required to hold the arms together.
• Preferably the present invention provides such an interlocking form for use in a medical implant closure which resists splaying tendencies of arms of a receiver. In one embodiment the interlocking form of the present invention provides a compound or “non-linear” surface on a trailing face, thrust face or flank of the interlocking form.
• The interlocking form located on the closure in one embodiment is helically wound about a cylindrical outer surface of the closure and has an inner radius or root, and an outer radius or crest that remain constant over substantially the entire length of the interlocking form. The receiver has a mating or similar shaped interlocking form wound about the interior thereof. In this embodiment the interlocking form has leading or clearance surfaces and trailing or thrust surfaces, referenced to the direction of axial movement of the form when rotated into one another.
• The structure also includes an internal helical wound interlocking form located on an internal surface of a receiver member and having an outer root and an inner crest. The internal interlocking form has thrust surfaces which are oriented in such a direction so as to be engaged by the thrust surfaces of the external interlocking form of a member engaged therewith.
• In the interlocking forms of this series of embodiments, the thrust surfaces are “non-linear” or compound. That is, the thrust surfaces have a non-linear appearance when represented in cross section. The purpose for the non-linear or compound surface is to provide a portion of the thrust surface which is oriented in such a direction as to resist a tendency of the receiver to expand when tightening torque is applied to rotate the interlocking forms into a mating relationship. As applied to a closure for an open headed bone implant screw, the non-linear or compound surfaces of the interlocking forms resist splaying tendencies of the arms of the head. The objective of the interlocking form is not necessarily to generate a radially inwardly directed force on the receptacle in tightening the fastener (although this may occur in some embodiments), but importantly to resist and prevent outward forces generated by engagement of the closure with the closure receptacle or by other forces applied to the components joined by the closure and closure receptacle. It is noted that the present invention requires that only a portion of the thrust surfaces of a closure be so configured as to face toward the closure axis and only a portion of thrust surfaces of a closure receptacle face away from the axis.
• While the axial extension or depression in one series is located on the thrust or trailing surface, it is also possible for such to be located on the opposite or leading surface or both.
• In this series of embodiments, a section of the interlocking form at the crest, that is located radially outward of the root, is enlarged in cross sectional area to create a gripping, locking or stopping surface that resists slippage or sliding in a radial direction relative to an opposed interlocking form. In a complementary manner, a section of the interlocking form between the root and the crest and that is radially spaced from the root is enlarged in cross sectional area to create a gripping, locking or stopping surface that engages a like surface of the opposite interlocking form. The enlarged sections of the inner and outer interlocking forms are created, in practice, by cutting, molding, machining or the like grooves or channels or the like into a radially inward portion of the thrust surface of the external interlocking form and mating grooves or channels into a radially outward portion of the thrust surface of the internal interlocking form. Such grooves or channels may be formed by specially shaped taps and dies, cutting elements or by other suitable manufacturing processes and technologies, including molding.
• The interlocking forms of the present invention may be implemented in a variety of configurations of non-linear, compound, or complex trailing and/or leading surfaces. The nomenclature used to describe variations in the interlocking forms of the present invention is especially referenced to the external interlocking forms located on a closure, with complementary or similar shapes applied to the internal interlocking forms on a receiver. In an axial shoulder interlocking form of the present invention, a somewhat squared gripping shoulder is formed on an outer periphery of the external interlocking forms and an inner gripping surface on the internal interlocking forms. The axial shoulder interlocking form results in complementary cylindrical surfaces on the external and internal interlocking forms which mutually engage when the fastener or closure is rotated into a closure receptacle.
• In an axial extending bead interlocking form, the external interlocking form is provided with a rounded peripheral bead or lateral lip which projects in an axial direction along the interlocking form crest and a complementary rounded concave channel in the internal interlocking form. The reverse occurs with the internal interlocking form.
• In a radial bead interlocking form, a rounded bead enlargement is formed on the radially outward periphery at the crest of the external interlocking form, while the internal interlocking form is formed in a complementary manner to receive the radial bead interlocking form.
• A scalloped or scooped interlocking form is, in effect, a reciprocal of the axial bead interlocking form and has a rounded channel or groove located along the thrust surface of the external interlocking form, with a complementary rounded convex bead shape formed associated with the internal interlocking form.
• A variation of the axial bead interlocking form is a medial bead embodiment. In the medial bead interlocking form, a bead projects from a base thrust surface of an external interlocking form in an axial direction at a location medially between the root and crest of the interlocking form. In a complementary medial bead internal interlocking form, an axial groove is formed in a base thrust surface between the root and crest. In a medial groove interlocking form, an axial groove is formed in a base thrust surface of the external interlocking form medially between the root and crest, while the internal interlocking form has an axial bead located medially between the root and crest.
• Variations in the above described interlocking forms are envisioned with respect to relative extensions or enlargements and depressions or depth of grooves of the various interlocking forms. In some variations, the opposite interlocking forms have the same but reversed and inverted cross section, whereas in others the cross section of the paired interlocking forms is different. It is noted that many other configurations of interlocking forms with non-linear, compound or complex thrust surfaces are envisioned, which would be encompassed by the present invention.
• The interlocking forms of the present invention find particularly advantageous application in various types of bone implant devices, although the inventive interlocking forms are not limited to such use. The interlocking forms also have advantages in reducing misalignment problems of cross-interlocking and misinterlocking of interlocking forms when the opposed interlocking forms are joined and rotated which is commonly encountered in such devices when threads of various types are used.
OBJECTS AND ADVANTAGES OF THE INVENTION
• Therefore, objects of the present invention include: providing an improved closure for an open headed lightweight and low profile medical implant wherein the implant has a pair of spaced arms and the closure closes between the arms; providing such a closure which includes a pair of opposed interlocking forms and which resists tendencies of the arms to splay or separate during insertion of the closure, to thereby reduce the likelihood of failure of the implant and closure system during use; providing such a closure which can be installed at comparatively high torques to thereby secure the closure in the receiver channel and in certain embodiments to also lock a rod member in the open head of the implant where the closure engages and is urged against the rod by rotation in a receiver channel of the remainder of the implant; providing an interlocking form for such a closure which resists tendencies of parts of the channel receiver to expand radially outward in response to high torque applied to the closure; providing such an interlocking form in which the respective thrust surfaces of mating internal and external interlocking forms are “non-linear”, compound, or complex to provide only a portion of each trailing or leading surface which is oriented in such a direction as to resist the splaying or expanding tendencies of parts of the receiving channel; providing such an interlocking form wherein the interlocking form has a base that is secured to a member and the interlocking form extends radially outward from the base with an axial extension starting at or radially spaced from the base and further wherein the interlocking form has an extension or depression that extends in an axial direction relative to an axis of rotation of the interlocking form and which mates with the opposite interlocking form so as to grip or hold such extension or depression and yet further wherein opposed interlocking forms are rotatable relative to each other during assembly, but are preferably sufficiently snug or located sufficiently near to one another to prevent one interlocking member to slide radially past another when torque is applied thereto or when forces act on the implant; providing embodiments of such an interlocking form having an enlarged radial cross section wherein the enlargement is spaced radially outward of a root of the external interlocking form and a complementary enlarged cross section spaced radially inward of a root of the internal interlocking form; providing embodiments of such an interlocking form having a first groove or channel formed in a surface inward of a periphery of an external interlocking form and a complementary second groove or channel formed in a surface inward of a periphery of an internal interlocking form so that the paired interlocking forms overlap and radially lock together upon assembly; providing embodiments of such an interlocking form in which the enlarged peripheries and grooves of the external and internal interlocking form have or form angularly defined or axially extending shoulders; providing embodiments of such an interlocking form in which the enlarged peripheries of the external and internal interlocking form have or form arcuately defined or rounded shoulders; providing such interlocking forms having a generally uniform cross section along a substantial length thereof; providing such interlocking forms that rotate relative to each other at least one full turn upon assembly; providing such interlocking forms which reduce the likelihood of cross-interlocking or misinterlocking problems of members during initial joining; providing such interlocking forms which can be formed relatively economically using appropriate metal forming technologies; and providing interlocking forms, particularly for implant and bone fixation hardware, which are economical to manufacture, which are secure and efficient in use, and which are particularly well adapted for their intended usage.
• Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.
• The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a perspective view of a closure for an open headed bone screw that has a helical wound gripping interlocking form in accordance with the present invention mounted thereon.
• FIG. 2 is a side elevational view of the closure.
• FIG. 3 is a side elevational view at a reduced scale and illustrates an interlocking form of the closure mated with and installed in a companion interlocking form on an open headed bone screw to capture a fixation rod within a head of the bone screw and with the head of the bone screw partially broken away to illustrate detail thereof.
• FIG. 4 is an enlarged fragmentary side elevational view of the bone screw head with the closure installed therein, the closure and bone screw head incorporating the interlocking form according to the present invention with portions broken away to show detail thereof.
• FIG. 5 is a view similar toFIG. 4 and illustrates details of first modified bone screw and closure showing a medial bead embodiment of an interlocking form of the present invention.
• FIG. 6 is view similar toFIG. 4 and illustrates details of a second modified bone screw and closure showing an axial aligned shoulder embodiment of an interlocking form of the present invention.
• FIG. 7 is a view similar toFIG. 4 and illustrates details of a third modified bone screw and closure showing an axial bead embodiment of an interlocking form of the present invention.
• FIG. 8 is a view similar toFIG. 4 and illustrates details of a fourth modified bone screw and closure showing a shallow axial bead embodiment of an interlocking form of the present invention.
• FIG. 9 is a view similar toFIG. 4 and illustrates details of a fifth modified bone screw and closure showing a radial bead embodiment of an interlocking form of the present invention.
• FIG. 10 is a view similar toFIG. 4 and illustrates details of a sixth modified bone screw and closure showing a scalloped depression or scooped embodiment of an interlocking form of the present invention.
• FIG. 11 is a fragmentary cross sectional view of a seventh modified bone screw and closure, similar to the embodiment inFIG. 10, showing a pair of interlocking forms in accordance with the present invention.
• FIG. 12 is a fragmentary cross sectional view of an eighth modified embodiment of a bone screw and closure showing a pair of interlocking forms in accordance with the invention.
• FIG. 13 is a fragmentary cross sectional view of an ninth modified embodiment of a bone screw and closure showing a pair of interlocking forms in accordance with the invention.
• FIG. 14 is a fragmentary cross sectional view of an tenth modified embodiment of a bone screw and closure showing a pair of interlocking forms in accordance with the invention.
DETAILED DESCRIPTION OF THE INVENTION
• As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure.
• Referring to the drawings in more detail, the reference numeral1 generally designates a gripping interlocking form arrangement incorporating a non-linear or compound surface which embodies the present invention. The interlocking form arrangement1 includes anexternal interlocking form2 andinternal interlocking form3 which have respective thrust surfaces or load flanks4 and5 (FIG. 4) and which are used as pairs. The interlocking form arrangement1 may be used on any of a number of interlocking formed devices, such as an implanted bone fixation system8 (FIG. 3), including a receiver or open headedimplant member10 which receives a closure or closure member11 (FIGS. 1 and 2) to secure afixation member12 therein. In the interlocking form arrangement1 of the present embodiment, the thrust surfaces4 and5 are non-linear or compound in such a manner as to resist tendencies of thereceptacle10 to splay or expand when the closure member11 is rotated therein.
• The illustratedimplant member10 is also referred to as an open headed bone screw and includes aU-shaped implant head15 and a threadedshank16. Thehead15 has a pair of spaced apartarms18 forming a rod receiving channel19. Thearms18 are radially inwardly tapped with theinternal interlocking form3 that is discontinuous between sides to receive the closure member11. The illustratedshank16 tapers to a point (not shown) and is externally threaded and adapted to be received in a bone, such as a vertebra, to anchor therod12 to such a bone.
• The illustrated closure member11 includes a plug, base section orbase22 and a break offhead section23 that breaks from the base22 at a preselected torque. It is foreseen that such a closure could be made without a breakoff head and other structure could be added for torquing or removing the base section. Furthermore, it is foreseen that such a base both captures the rod and locks the rod as in the embodiment illustrated inFIGS. 1 to 4 or, alternatively, that the base could just capture the rod and a set screw could be used in a threaded bore in the base to lock the rod in place. Thebase section22 is provided with theexternal interlocking form2 which is compatible with theinternal interlocking form3 of thebone screw head15. Both interlockingforms2 and3 are helically wound and rotatably mateable together through rotation or turning of the closure member11 about a central axis42 thereof. Thehead23 includes structure for positive engagement by an installation tool (not shown) to install the closure member11 in thebone screw member10. The structure that allows for installation of the illustrated break offhead23 includesfaces25 forming a hexagonal shape or “hex” head to receive a complementary hexagonally shaped installation driver or tool. Thehead23 also includes acentral bore26 and across bore slot27. The outer end of thehead23 is chamfered at28, and thebore26 is provided with an interior conical countersink at29. The region where thehead23 meets thebase22 is reduced in cross sectional thickness to form a weakened breakaway orfracture region30. Thebreakaway region30 is designed so that thehead23 separates from the base22 when a selected torque is applied by the installation tool, as is diagrammatically illustrated by breaking away of thehead23 inFIG. 3. Thebase22 is preferably provided with structure to facilitate removal of the base22 from theimplant head15, such as the illustrated removal bores32. Thebores32 may be formed by drilling from alower end surface34 of theplug22, since anupper end surface36 of theplug22 is normally not accessible for drilling thebores32 prior to break-off of thehead23. It is foreseen that many different types of removal devices or structures can be utilized with the base such as: axially aligned bores with hex, torx or other multifaceted cross-section, step down bores for engagement by an easy out, bores at the periphery or non axially aligned on the face of the base, bores with a left handed thread or the like. Further, the same structure used to torque the base on installation may be used to remove the base.
• Thebase22 is rotated into the receiving member of thebone screw head15 to clamp thefixation rod12 therein for any of a variety of surgical purposes. In general, therod12 is used to fix the position of a bone or portion of a bone, such as a plurality of vertebrae. Therod12 may be anchored relative to some vertebrae and, in turn, used to secure other vertebrae in desired positions or orientations or used to properly align a series of vertebrae. It is generally required that the union formed between thebone screw10, closure11 and therod12 be very tight or snug to avoid relative movement therebetween. Thefixation system8 preferably employs structure that positively engages and seats thehead15 and/or the base22 with respect to therod12, such as aconical set point38 formed on thebottom surface34 of the base22 which engages therod12. Thepoint38 positively “bites” into the surface of therod12 to help prevent rotational or axial movement of therod12 relative to thescrew10. Alternatively or in combination with apoint38, other structures may be used to positively engage theclosure plug22 with therod12, such as a sharp edged coaxial ring (not shown) having a V-shaped cross section formed on thelower surface34 of the base22 or point extending upwardly from the channel.
• The interlocking forms2 and3 are helical and are intended to advance the closure member11 linearly along the axis of rotation42 of the closure member11 and the interlocking forms2 and3 relative to another member as the closure member11 is rotated relative to thebone screw10. A spatial reference for such rotation and linear movement is along the axis42 (FIG. 4). The axis42 locates the coincident axes of the external or radially outward interlockingform2 of thebase22 and the internal or radially inward interlockingform3 of thehead15, when thebase22 is inserted into thehead15 by starting at the top of the interlocking form3 (top is up inFIG. 4) and rotated. Thebase22 has a basic cylindrical shape, and theexternal interlocking form2 includes aroot45 and acrest47 formed by cutting a helical wound channel of the desired cross section into the original surface of thebase22. Thecrest47 of theexternal interlocking form2 has a greater radius than theroot45. In a like manner, theinternal interlocking form3 of thehead15 of thescrew10 has a helical channel under cut thereinto, forming aroot49 andcrest51. Theroot49 of theinternal interlocking form3 has a greater radius than thecrest51.
• The thrust surfaces4 and5 respectively of the external andinternal interlocking forms2 and3 engage frictionally when thebase22 is rotated into thehead15. The thrust surfaces4 and5 are located on the trailing sides respectively of thecrests47 and51, as referenced to the tightening direction movement of the base22 into thehead15. In general, there is minimal contact between clearance surfaces or stabflanks53 and55 when thebase22 is rotated in a tightening direction into thescrew head15 to allow rotation. The clearance surfaces53 and55 may frictionally engage when thebase22 is rotated in a reverse direction to remove it from thescrew head15.
• Frictional engagement of the thrust surfaces4 and5 due to rotation causes thebase22 to be advanced linearly along the axis42 into thescrew head15. However, once the base22 “bottoms out” by contact of thelower surface34 or theset point38 with therod12 and therod12 is unbent and pushed downwardly as far as it will go into the channel or seat19, further rotation of the base22 cannot result in further linear movement of thebase22 within thehead15. The interlocking forms2 and3 thereafter are radially locked together and each turn or pass of theforms2 and3 is preferably sufficiently snug with respect to turns of the opposite interlocking form to prevent eitherform2 or3 from slipping or sliding radially past one another upon application of additional torque or with application of forces due to usage by the patient.
• The various compound, complex, or non-linear interlocking form arrangements of the present invention are intended to resist splaying tendencies of thearms18. In particular, eachthrust surface4 and5 of the interlocking forms2 and3 have a gripping, blocking orsplay resisting surface59 or60 respectively which is oriented in such a direction as to resist splaying of thearms18 of thescrew head15 when thebase22 is rotated to a high degree of torque. On theexternal interlocking form2, thesplay resisting surface59 is directed generally toward or faces the axis42. Conversely, on theinternal interlocking form3, thesplay resisting surface60 is directed generally away from or faces away from the axis42. Each of thesurfaces59 and60 in this manner wrap over or around the opposite and block substantial radially relative movement there between. It is especially noted that thesurfaces59 and60 are extensions of the interlocking forms2 and3 in an axial direction (that is parallel to the axis42 or up and down as seen inFIG. 4). This axial extension is spaced away from the juncture of the interlocking forms2 and3 with thebase22 andscrew10. It is foreseen that such an extension can take many shapes and configurations (some of which are shown herein) and may also functionally be depressions or grooves. In each case the paired interlocking forms, such asforms2 and3, overlap each other and are snug about each other so as to prevent substantial relative radial slippage or movement between them during and after assembly of the base22 into thebone screw10.
• FIG. 5 illustrate a non-linear or compound thrust surface interlockingform arrangement70 which is of a medial bead interlocking form type. The interlocking form arrangement70 a thrust surface4 located on aplug22 andinternal interlocking form3 withthrust surfaces5 within ahead15 of abone screw10. The thrust surfaces4 and5 are contoured to provide complementary, interacting,splay resisting surfaces59 and60 on the external andinternal interlocking forms2 and3 respectively. Theexternal interlocking form2 is provided with abead72 on the thrust surface4, and theinternal interlocking form3 is provided with a complementary channel or groove74 formed into thethrust surface5. The illustrated thrust surfaces4 and5 are substantially perpendicular to the axis42; however, such surfaces may alternatively be angled somewhat with respect to the axis42 so as to slope downward or upward as the surface extends radially outward.
• Thebead72 is located at a radius which is between or medial with respect to theroot45 andcrest47 of theexternal interlocking form2. Similarly, thegroove74 is located at a radius which is medial to theroot49 andcrest51 of theinternal interlocking form3. The illustratedbead72 andgroove74 are rounded and somewhat triangular in cross section. Alternatively, the bead andgroove72 and74 could be pointed and triangular, squared off, or semicircular. It should also be noted that the bead andgroove72 and74 could be replaced by a medial groove formed in theexternal interlocking form2 on the thrust surface4 and a medial bead formed on thethrust surface5 of theinternal interlocking form3. An inwardly facingsurface76 of thebead72 forms thesplay resisting surface59 thereof, while an outwardly facingsurface78 of thegroove74 forms the splay resisting surface of thegroove74. Engagement of thesplay resisting surfaces76 and78, respectively of thebead72 andgroove74, resists tendencies of thearms18 of thescrew head15 to splay when theclosure base22 is rotated into thehead15.
• FIGS. 6 to 14 illustrate further variations in the paired interlocking forms of the present invention. In each case the base closure and bone screw, except as noted with respect to the interlocking forms, of the variations shown inFIGS. 6 to 14 are essentially the same as those shown inFIGS. 1 to 4, so only differing detail of the interlocking form structure will be described in detail and reference is made to the description given forFIGS. 1 to 4 for the remaining detail.
• InFIG. 6, a guide andadvancement structure80 includes theexternal interlocking form81 having an axially aligned shoulder or flange-like shaped configuration when view in cross section in a plane passing through an axis of rotation83. The interlockingform81 has athrust surface84 on abase85. Thestructure80 also has an internal interlocking form86 with athrust surface87 within thehead88 of abone screw89. The internal interlocking form86 has aroot90 and acrest91, while theexternal interlocking form81 includes aroot92 andcrest93. The thrust surface84 of theexternal interlocking form81 includes an axially oriented orcylindrical shoulder94 which forms asplay resisting surface95 thereof.
• Similarly, thethrust surface87 of the internal interlocking form86 includes a mating or complementary axially oriented orcylindrical shoulder97 which forms asplay resisting surface98. Engagement of thesplay resisting surfaces95 and98 resists tendencies of thearms99 of thehead88 to splay when the plug orbase85 is rotated into thehead88 and torqued tightly or at later times during usage. It is foreseen that a variation of the axial shoulder interlocking form would provide shoulders at inclined angles (not shown) to the axis42. The illustratedsplay resisting shoulder94 is formed by a rectangularcross section bead100 formed on thethrust surface84 of theexternal interlocking form81. Similarly,splay resisting shoulder97 is formed by a somewhat rectangularly cross section shaped bead orfoot portion101 adjacent agroove102 for receivingbead100 and formed in thethrust surface87 of the internal interlocking form86. The interlocking forms81 and86 have a general flange-like shape configuration when viewed in cross section that is also some what L-shaped with thebeads100 and101 forming feet of the flange shape that overlap and lock so as to prevent substantial radial movement of thearms99 of thebone screw89 relative to theclosure plug base85.
• FIGS. 7 and 8 illustrate further variations of the axialshoulder interlocking structure110 and130 respectively in the form of a rounded axial bead interlocking form111 shown inFIG. 7 and a shallow rounded axialbead interlocking form131 inFIG. 8. The rounded axial bead interlocking form111 includes arounded bead112 projecting in a direction parallel to anaxis113. Thebead112 is formed on athrust surface114 of anexternal interlocking form115 and a rounded groove116 is formed on athrust surface117 of aninternal interlocking form119. Thebead112 includes asplay resisting surface120, while the groove116 also includes asplay resisting surface122.
• In a similar manner, the shallow rounded axialbead interlocking form130 includes a shallowrounded bead131 formed on athrust surface133 of anexternal interlocking form134 and a shallowrounded groove135 formed on athrust surface136 of aninternal interlocking form137. Thebead131 includes asplay resisting surface140, and thegroove135 includes asplay resisting surface141. Thesurfaces140 and141 engage and abut to resist splaying or significant radial separation movement therebetween.
• FIG. 9 illustrates a radial bead embodiment of an implant150 having a guide andadvancement structure151. Thestructure151 includes a rounded external and bead interlockingform153 projecting radially from abase154 and forming acrest155. Thebead interlocking form153 has a pair ofsplay resisting surfaces158 facing generally toward anaxis156 of rotation of thebase154. A complementary grooveinternal interlocking form160 is part of a screw head161. Thehead interlocking form160 has a pair ofsplay resisting surfaces163 facing generally away from theaxis156. Thestructure151 has thesplay resisting surfaces158 and163 on thrust surfaces168 and169 respectively of the interlocking forms153 and160, as well as onclearance surfaces170 and171 thereof. The illustrated radial bead interlocking form150 is, in some ways, a double sided variation of the rounded axial bead interlocking form of an earlier embodiment.
• FIGS. 10 and 11 illustrate a scalloped or scooped embodiment structure180 including a pair ofcompound interlocking forms181 and182 according to the present invention. Theinterlocking form181 is scalloped and, in effect, an inversion of the shallow rounded bead interlocking form similar to that of an earlier embodiment. Theinterlocking form182 includes ashallow groove184 formed in athrust surface185 of theexternal interlocking form181 of abase187 and ashallow bead188 formed on athrust surface189 of theinterlocking form182 of ascrew head190. Thegroove184 has asplay resisting surface193 which cooperates with a complementarysplay resisting surface194 of thebead188.
• Illustrated inFIG. 12 is another guide andadvancement structure200 associated with areceiver member201 and a closure member, such as a plug,202 that is rotated into thereceiver member201. Thestructure200 includes afirst interlocking form205 and asecond interlocking form206 attached to theclosure member202 andreceiver member201 respectively.
• Thefirst interlocking form205 includes an arcuateupper surface207 with a gripping or lockingsection208. Thesecond interlocking form206 includes an arcuatelower surface209 with a gripping or lockingsection210. The interlocking forms205 and206 also have respective lower or leadingsurfaces214 and215 respectively that are sufficiently spaced to allow rotation about the axis thereof, but sufficiently close to be snug and not allow substantial movement of theforms205 and206 relative to each other in an axial direction without rotation.
• FIG. 13 shows an alternative flange shaped embodiment of a guide andadvancement structure230 in accordance with the invention. Thestructure230 is mounted on aclosure231 and areceiver232 so that interlockingforms233 and234, which are seen in cross section, are helically mounted on theclosure231 andreceiver232 respectively.
• Thefirst interlocking form233 is L or flange-shaped in cross section with a vertically or axially extendingfoot portion240 with agripping surface241. Thesecond interlocking form234 generally complements the first and is also L or flange shaped except that a foot243 thereof is much wider than thefoot portion240. The foot243 has a gripping orwraparound surface245 that abuts thesurface241 during assembly and resist radial movement between thereceiver232 and theclosure231.
• Shown inFIG. 14 is another embodiment of a guide andadvancement structure260 in accordance with the invention. Thestructure260 is utilized with areceiver261 and a closure or plug262. Thestructure260 has first and second interlocking forms263 and264. The first interlocking form has anelongate wall268 with acircular bead269 attached to an end thereof opposite theclosure262. Thebead269 has opposed grippingsurfaces270 and271. Thesecond interlocking form264 is shaped to mate with an generally surround thefirst interlocking form263 except sufficient clearance is provided to allow theclosure262 to be rotated and advanced into thereceiver263 by sliding tangentially, but not radially. Thesecond interlocking form264 has a circularcross section channel270 that receives thebead269 and a pair ofgripping surfaces273 and274 that engage and abut against the bead surfaces270 and271.
• It is foreseen in accordance with the invention that certain regions of the interlocking forms may be eased or removed to allow for easier use which still maintaining the primary objective of resisting radial movement between the closure plug and the opposed arms of the bone screw to prevent splaying of such arms.
• It is also seen in accordance with the invention that the axial aligned extension or depression on the described interlocking forms could in some cases be multiple in nature or formed by an undulating pattern.
• It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown.

Claims (29)

1. A medical implant device comprising: a receiver member including a plurality of wall sections defining a longitudinal bore, wherein said wall sections have an inner threaded portion; and a closure member including a rearward end, a forward end, a substantially cylindrical body having a longitudinal axis and an outer threaded portion for threaded engagement with said inner threaded portion of said receiver member, wherein the outer thread portion includes a screw form for a given cross-section of thread through a plane which includes the longitudinal axis, the screw form comprising: a rearward-facing thread surface having a substantially straight sloped portion and at least two non-contiguous curve portions, such that a point on the rearward-facing thread surface at a root of the thread surface is farther from the forward end than a point on the rearward-facing thread surface at a crest of the thread surface when measured along a line parallel to the longitudinal axis; wherein a maximum thickness of the thread occurs at a point closer to the crest of the thread than the root of the thread.

2. The medical implant device ofclaim 1 wherein said receiver member also includes a transverse channel substantially perpendicular to said bore.

3. The medical implant device ofclaim 1 wherein said receiver member is a part of a bone fixation device.

4. The medical implant device ofclaim 3 wherein said bone fixation device is a bone screw.

5. The medical implant device ofclaim 3 wherein said bone fixation device is a spinal hook.

6. The medical implant device ofclaim 1 wherein said closure member is a set screw.

7. The medical implant device ofclaim 1 wherein the screw form further comprises: a forward-facing thread surface having a substantially straight sloped portion and at least two non-contiguous curve portions, such that a point on the forward-facing thread surface at a root of the forward facing thread surface is closer to the forward end than a point on the forward-facing thread surface at a crest of the forward facing thread surface when measured along a line parallel to the longitudinal axis.

8. The medical implant device ofclaim 1 wherein said outer threaded portion is configured as a helical spiral about said body, and wherein the thickness of said outer threaded portion at its crest varies along the helical spiral.

9. The medical implant device ofclaim 1 wherein said outer threaded portion is configured as a helical spiral about said body, and wherein the thickness of said outer threaded portion at its root varies along the helical spiral.

10. The medical implant device ofclaim 1 wherein said outer threaded portion is configured as a helical spiral about said body, and wherein a peak thickness of said outer threaded portion occurs crestward of said outer threaded portion's root, and wherein the thickness of said peak thickness varies along said helical spiral.

11. The medical implant device ofclaim 10 wherein said thickness of said peak thickness is thicker at a rearward portion of said helical spiral than at a forward portion of said helical spiral relative to the direction of advancement of said closure member when being inserted into said receiving member.

12. A medical implant device comprising: a receiver member including a plurality of noncontiguous wall sections defining a longitudinal bore, wherein said plurality of noncontiguous wall sections include a female threaded portion configured as a helical spiral about a center longitudinal axis of the bore; and a closure member including a substantially cylindrical body having a forward end, a rearward end, a longitudinal axis and a male threaded portion for interlocking engagement with said female threaded portion of said receiver member, wherein the male threaded portion includes a screw form for a given cross-section of thread through a plane parallel to the longitudinal axis, the screw form comprising: a rearward-facing thread surface having a substantially straight sloped portion and at least two non-contiguous curve portions, such that a point on the rearward-facing thread surface at a root of the thread is farther from the forward end than a point on the rearward-facing thread surface at a crest of the thread when measured along a line that is parallel to the longitudinal axis: wherein a maximum thickness of the thread occurs at a point closer to the crest of the thread than the root of the thread.

13. The medical implant device ofclaim 12 wherein said receiver member also includes a transverse channel substantially perpendicular to said bore.

14. The medical implant device ofclaim 12 wherein said receiver member is a part of a bone fixation device.

15. The medical implant device ofclaim 14 wherein said bone fixation device is one selected from the group consisting of: a bone screw and a spinal hook.

16. The medical implant device ofclaim 12 wherein said closure member is a set screw.

17. The medical implant device ofclaim 12 wherein said male threaded portion is configured as a helical spiral about said body, and wherein thickness at said at least one point crestward of its root varies along the helical spiral.

18. The medical implant device of12 wherein said male threaded portion is configured as a helical spiral about said body, and wherein the thickness of said male threaded portion at its root and the thickness of said male threaded portion at its crest vary along the helical spiral.

19. The medical implant device ofclaim 12 wherein the screw form further comprises: a forward-facing thread surface having a substantially straight sloped portion and at least two non-contiguous curve portions, such that a point on the forward-facing thread surface at a root of the forward facing thread surface is closer to the forward end than a point on the forward-facing thread surface at a crest of the forward facing thread surface when measured along a line parallel to the longitudinal axis.

20. A noncontiguous receiver member and complementary closure member included in a medical implant device, comprising: said noncontiguous receiver member having a plurality of noncontiguous wall sections separated by a slot, said wall sections at least partially defining a longitudinal bore, wherein said plurality of noncontiguous wall sections include a female threaded portion that forms substantially a helical spiral about a center longitudinal axis of the bore; and said complementary closure member having a substantially cylindrical body portion and male threaded portion that forms substantially a helical spiral about a center longitudinal axis of the body portion, wherein the male threaded portion includes a screw form for a given cross-section of thread through the longitudinal axis, the screw form comprising: a rearward-facing thread surface having a substantially straight sloped portion and at least two non-contiguous curve portions, such that a point on the rearward-facing thread surface at a root of the thread is farther from a forward end of the closure member than a point on the rearward-facing thread surface at a crest of the thread to form a rearward peak of said male threaded portion on its trailing-edge surface at some point crestward of its root, and wherein a forward peak of said male threaded portion is provided on its leading-edge surface at some point crestward of its root; wherein the distance between the rearward peak and the forward peak provides a maximum thickness of the thread: wherein the maximum thickness of the thread occurs at a point closer to the crest of the thread than the root of the thread.

21. The noncontiguous receiver member and complementary closure member ofclaim 20 wherein said rearward peak occurs at the crest of said trailing-edge surface.

22. The noncontiguous receiver member and complementary closure member ofclaim 21 wherein said forward peak occurs at the crest of said leading-edge surface.

23. The noncontiguous receiver member and complementary closure member ofclaim 20 wherein said receiver member also includes a transverse channel substantially perpendicular to said longitudinal bore.

24. The noncontiguous receiver member and complementary closure member ofclaim 20 wherein said receiver member is a part of a bone fixation device.

25. The noncontiguous receiver member and complementary closure member ofclaim 24 wherein said bone fixation device is one selected from the group consisting of: a bone screw and a spinal hook.

26. The medical implant device ofclaim 20 wherein the screw form further comprises: a forward-facing thread surface having a substantially straight sloped portion and at least two non-contiguous curve portions, such that a point on the forward-facing thread surface at a root of the forward facing thread surface is closer to the forward end than a point on the forward-facing thread surface at a crest of the forward facing thread surface when measured along a line parallel to the longitudinal axis.

27. A medical implant device comprising: a means for receiving a closure means, the receiving means including a plurality of noncontiguous wall sections at least partially defining a longitudinal bore, wherein said wall sections have an inner threaded portion; and said closure means for engaging said plurality of noncontiguous wall sections, the closure means including a substantially cylindrical body having an outer threaded portion for threaded engagement with said inner threaded portion of said receiving means, wherein said outer threaded portion includes a screw form for a given cross-section of thread through a longitudinal axis of the closure means, the screw form comprising: a rearward-facing thread surface having a substantially straight sloped portion and at least two non-contiguous curve portions, such that a point on the rearward-facing thread surface at a root of the thread is farther from a forward end of the closure means than a point on the rearward-facing thread surface at a crest of the thread when measured along a line parallel to the longitudinal axis: wherein a maximum thickness of the thread occurs at a point closer to the crest of the thread than the root of the thread.

28. The medical implant device ofclaim 27 wherein the screw form further comprises: a forward-facing thread surface having a substantially straight sloped portion and at least two non-contiguous curve portions, such that a point on the forward-facing thread surface at a root of the forward facing thread surface is closer to the forward end than a point on the forward-facing thread surface at a crest of the forward facing thread surface when measured along a line parallel to the longitudinal axis.

29. A medical implant device comprising: a receiver member including a plurality of noncontiguous wall sections at least partially defining a longitudinal bore, wherein said plurality of noncontiguous wall sections include a female threaded portion arranged as a helical spiral about a center longitudinal axis of the bore; and a closure member including a substantially cylindrical body having a male threaded portion for interlocking engagement with said female threaded portion of said receiver member, wherein said male threaded portion includes a trailing edge having a root adjacent said body and having a crest at a point on said trailing edge that is furthest from a longitudinal axis centered in said cylindrical body when measured along a line perpendicular to said longitudinal axis, wherein said outer threaded portion includes a screw form for a given cross-section of thread through a plane which includes the longitudinal axis, the screw form comprising: a rearward-facing thread surface having a substantially straight sloped portion and at least two non-contiguous curve portions, such that a point on the rearward-facing thread surface at a root of the thread is farther from a forward end of the closure member than a point on the rearward-facing thread surface at a crest of the thread, and a forward-facing thread surface having a substantially straight sloped portion and at least two non-contiguous curve portions, such that a point on the forward-facing thread surface at a root of the forward facing thread surface is closer to the forward end than a point on the forward-facing thread surface at a crest of the forward facing thread surface when measured along a line parallel to the longitudinal axis: wherein a maximum thickness of the thread occurs at a point closer to the crest of the thread than the root of the thread.

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