MODULAR ROTHESIS ^ GT P "- 1st Invention This invention relates to the field of medical prostheses and particularly to prostheses for use as replacements for diseased or damaged joints.Part of the Invention Joint replacement prostheses commonly involve two parts that have mutually articulating surfaces and structure for mounting the parts to the bone To closely duplicate the structure and function of natural joints, the prosthesis parts must be carefully configured and digested and properly oriented by the surgeon with each other and with respect to the anatomy In order to achieve good surgical results, a surgeon should have as much freedom as possible during the surgical implant procedure to vary the shape, size and orientation of the prosthetic parts, Primarily for this reason, efforts have been made to provide prostheses that are modular in Such a way that various elements of a prosthesis can be selected individually and the prosthesis can be assembled and oriented according to the anatomical needs of the patient.
Modular prostheses for the hip joint are illustrated, for example, by Bolesky et al., U.S. Pat. No. 5,080,685, Gianezio et al., U.S. Pat. No. 4,520,511, Demane et al., U.S. Pat. No. 4,995,883, Luman, U.S. Pat. No. 5,002,578 and Rhenter et al., U.S. Pat. No. 4,693,724. These prostheses mostly involve a substantial amount of parts that are held together in one configuration or another by mounting screws that operate to gather tapered connections of the parts. Although some freedom of selection is provided by prior modular prostheses, the use of threaded mounting screws and tapered connections can lead to loosening of the parts and other problems. Chemical and physical corrosion can become a substantial problem due to weakening of the prosthesis and biological responses to corrosion and sub-product waste. See Jacobs, J.J, and collaborators, "Biological * Activity of Particulate chrcaii -----, - Phosphate Corrosion Products" (ActivityBiological for Corrosion Products Phosphate-Chromium in Particles) r Documents Gathered from the 21st Annual Meeting of the "Society for Biomaterialsw (Society for Bio-materials), March 18 to 22, 1995, page 398 and Urban, Robert M. et al. , "Cpr QBÁQn Products F-ro» MQduJ.ar-H * aad Femoral Stßag of Pifferent PesigTvs. and Maerail ouieg "(Corrosion Products from vasagos deDocument Materials Meeting at the 21st Annual Meeting of the "Society for Biomaterials", March 18 to 22, 1995, page 326. Rubbing corrosion caused by relative movement between adjacent surfaces leads to waste production which in turn can lead to accelerated wear between normal joint parts of a prosthesis and osteolysis. When spaces occur between adjacent surfaces of prosthetic parts, the oxidation of the surfaces can lead to the formation of an acidic environment and therefore chemical attack of the surfaces (commonly referred to as corrosion inside the fissures). It would be convenient to provide a modular prosthesis kit having elements that can be freely chosen and oriented by the surgeon in the operating arena and which can be firmly and firmly held together, without need of threaded fasteners or tapered connections that are pulled together. COMPLEX OF THE IHVEliCIOM The present invention utilizes a clamp capable of firmly holding a prosthesis member and which can be used to securely clamp selected portions of a modular prosthesis together. The clamp has a configuration of •• rest "that has a dimension in a direction that can be reduced by applying an external stimulus, with concurrent expansion of the clamp in a second direction normal to the first direction, such that the clamp can be received in a cavity of a prosthesis member. By withdrawing the external stimulus, the clamp seeks to return to its "rest" configuration, the clamp dimension in one direction is increased, such that the clamp presses on the walls of the cavity, to strongly grip the prosthesis member. Thus, in one embodiment the invention relates to a modular prosthetic device comprising a first member with walls defining a cavity, and a clamp releasably clamped within the cavity. The clamp has a first resting configuration with a predetermined dimension in a first direction and responding to an external stimulus to acquire a second configuration with a smaller dimension in the first direction, with concurrent increase of one dimension in a second direction normal to the first direction, to allow the clamp when it is partially received in the cavity. The predetermined dimension is chosen in such a way that when withdrawing the external stimulus, the clamp returns to its posterior configuration with consequent increase in its dimension in the first direction, sufficient to hold the limb tightly. In another embodiment, the invention comprises a modular prosthetic device that includes instrumentation for assembly, comprising a first prosthesis member having walls that stop a cavity and a clamp that is clamped to release within the cavity. The clamp has a first configuration at rest are a predetermined dimension in a first direction. An instrument is provided to apply a stretching force to the clamp in a second direction normal to the first direction, to reduce the dimension in the first direction, sufficient to allow the clamp to be received in the cavity. The predetermined dimension is such that when withdrawing the stretching force, the clamp returns to its configuration at rest, they are consequential increase in its dimension in the first direction, sufficient to hold strongly the first prosthesis member. In a preferred embodiment, the prosthesis equipment includes a second member configured to reshape at least a portion of the first member in any of the various orientations. The sap walls of the first member are designed to expand in the suction area are the second member, sonforme the clamp returns to its resting position to hold the second member fixedly in a predetermined orientation with respect to the first member. In a still additional modality, the invention refers to a method for assembling members of a modular prosthesis. A first prosthesis member is provided with walls defining a savity, and is provided with a clamp having a first configuration at rest having a predetermined dimension in a first direction. The clamp is subjected to an external stimulus, preferably a physical tensioning stimulus, to reduce the dimension in the first direction, with concurrent expansion of a clamping dimension by clamp, in a second normal direction to the first direction, to allow the clamp is received in the savity of the first prosthesis member. The external stimulus is subsequently removed to allow the clamp to return to its first resting configuration, with consis- tent increment of its dimensions in the first direction, enough to clamp tightly with the first prosthesis. Preferably, the clamp and the cavity in the first prosthesis member have clamping surfaces which are suffronative, which are held substantially congruent to provide surface-to-surface contact between the clamp and the first member and the prosthesis is free. of spasms between superfisies sonfrontantes. SmilarlyIf a second prosthesis member resides and is clamped, they are the first member, preferably the clamping surfaces of the clamps of these members are substantially congruent in order to provide a superfisie-to-surface relationship between the clamping surfaces. by clamping the first and second limbs and the prosthesis is free of spaces between confronting surfaces. This contact-to-superfisie promotes uniform loading on the clamping superstructure superfisies. BRIEF DESCRIPTION OF THE BONE Figure 1 is a cross-sectional side view of a portion of a hip joint prosthesis according to the invention; Figure 2 is an exploded view in cross section, taken through line 2-2 of Figure 1; Figure 3 is a schematic front view of the tibial portion of a knee artisulasion according to the invention;Figure 4 is a side view, in cross-sectional view of a portion of another prosthesis of hip artisulation similar to that of Figure 1; Figure 5 is an exploded structure view of part of instrumentation for use in assembling the sack artisulatory prosthesis in Figure 4; Figure 6 is a view of the parts of Figure 5 assembled; Figure 7 is a view of the assembly of Figure 6 together are a manually operated force generating device; and Figure 8 is an exploded view in transverse session showing another embodiment of the invention.
DESCRIPCTPW DKT JifrPA "E ** ß PREFERRED MODAT.TTIES First reference is made to Figure 1, a modular prosthesis sadera is designated 10, and comprises an elongated stem 12 sized to be received in a surgically prepared intramedullary sanal of the femur. axial perforation 14 is formed in the rod 12. A member 16 is provided with a perforation 18, dimensioned to receive cranially the rod 12, the body has a generally triangular subara shape seen from the side and is set to fit the proximal end surgically sculpted from the intramedullary canal of the femur Next to the body 16 is placed a neck member 20 having a bore 21, sized to sharply hold the upper end of the rod 12, the socket includes an angled extension 22 ending in a ball 24 dimensioned to articulate with a cavity prosthesis of appropriate shape and size (not shown) to be mounted in the acetabular recess d A clamp 30 is illustrated in Figure 1 as an elongated metal rod having an axial bore 32, extending from its proximal end position 34 to a crested floor 36 of the distal end position 38 of the clamp . Cersa of its upper end, the axial perforation 32 has a distally oriented shoulder, configured to receive a solosion instrument as it will disenribirá to sontinuasión. The clamp 30, is configured and dimensioned in such a way that at the temperature of the body, its diameter, when not restricted in the rod 12, will be slightly larger than the diameter of the hole 14 of the rod. The diameter 21 of the neck piercing and the diameter 18 of the body piercing, on the other hand, essentially are the same as the external diameter of the rod 12; that is, the stem is received in a tight but slidable manner in the perforations 18, 21, in such a way that the body and the neck can be moved by hand on the rod without difficulty.
The clamp 30, before installation in the bore 14 of the rod, must first be altered in such a way that its diameter is slightly smaller than the diameter of the bore of the rod. This is achieved by physically stretching the clamp in its axial or longitudinal direction, to prevent the diameter of the clamp from being sufficiently entangled to allow the clamp to be inserted into the perforation 14. Although the clamp can be made from various metals, the clamp can be made from various metals. it is described to sontinuation, a preferred metal is a shape memory alloy such as nitinol, in its super elastic state, wherein the applied stress results in a reversible ratansensitic phase transition. When a nitinol-30 clamp is stretched, it was previously disintegrated, and as long as its temperature is maintained substantially above its austenitic tempering temperature (the temperature in the sual is fairly completely in its austenitic form) a phase transition occurs. austenite to the martensite phase. This is known as tension-induced martensite formation and is the basis for the phenomenon known as pseudo-elastisity or super-elasticity. The shape memory aleasion will remain at least partially in the martensite phase, as long as the external voltage is maintained. By releasing the tension, however, clamp 30 will return to the austenite phase and to its original shape and size. Because the clamp is restricted within the dimensions of the rod drilling 14, however it will not be able to completely acquire its original shape and size again. As a result, the clamp 30 will exert a continuous force against the perforation 30 of the rod 12. Alternatively, the clamp can be made from a shape memory alloy such as nitinol, where the material is capable of undergoing a change of phase induced by temperature. In this embodiment, the shape memory alloy is configured such that when it is in its stable phase at body temperature, its diameter is slightly larger than the diameter 14 of the rod. At a lower temperature, however the clamp can be deformed into a different physical form, where it is slightly longer and slightly slender than in its stable form at body temperature, this configuration allows the clamp to slide into the perforation 14 of the v stem In this embodiment, once the shape memory alloy is exited and passes through its range of phase transition temperatures, it expands to its stable sonfiguration at body temperature, thereby pressing outward with a continuous force on the perforation of the rod. In this way, when a temperature-induced phase change is used for a shape-memory aleasion, the clamp will tend to return to a configuration that can be referred to as a "rest" configuration, since a tight elongation of the clamp is used. "at body temperature. The configuration at rest, however has a transverse dimension (the diameter in the saso of a rod that has a cirsular transverse scission) that is slightly larger than the transverse direction of the shank's perforation, and as a result, the clamp pushes hasia out strongly on the perforation of the stem and it is held firmly in the perforation of the rod. As illustrated in Figure 1, the walls 42 of the clamp have outer surfaces 44 which engage and push outwardly on the perforation 14. When appropriately in place, the outer wall 44 of the clamp pushes outwardly on the surface of the clamp. v stem perforation 14, and the walls of the rod, in turn, are forced out in contact with the inner surface 26 of the body 16 and are also the inner surface 28 of the lumen 20.
Preferably, the outer surface of the clamp 30 is generally cylindrical and makes substantial superfisie-to-superfisie stasis the surface of the perforation 14. Furthermore, the rod wall is sufficiently flexible to allow the outer wall of the rod to be expand in sontasto are the perforations of both the body and the shoulder, even though these perforations are slightly different in diameter. One characteristic feature of a preferred embodiment of the invention is that the subject surface - that is, the confronting surfaces of the clamp and first member, and the confronting surfaces of the first and second members - correspond in surface-to-surface contaste to distribute in Substantially uniform, the forces of pressure on the subject superfisies are clamp and preferensia to avoid spasms between sonfrontantes superfisies. As used herein, an "ßspasio" is the thin bone spasm that is formed between confronting slightly spaced surfaces of a prosthesis when assembled, such as, for example, the space formed between a smooth, elongated wall rod having threads on one end and the perforation that the rod receives. If the clamp is a cylinder having a cirsular cross-section and the savity is a sirsular perforation, the clamping force by clamping by the clamp against the walls of the perforation should be primarily radial and substantially uniform over the length of the clamp. clamp. It can be varied as desired in the concentration of the pressure forces between the clamp (and between the prosthesis elements) by varying the shapes of the clamp surfaces. For example, if the cross-sections of the clamp and recess were oval instead of circular, it is expected that the clamping force is compression is somewhat greater in the transverse dimension longer than in the shortest transverse dimension. The invention in another embodiment is illustrated in Figure 2, wherein the ball 24 is firmly mounted in the angle-wise extension 22. The ball 24 and the sealing member 23 (from which the angled extension 22 is provided) in In general, a subunit will be assembled, and the subunit will then be assembled into the suerpo and suntum as previously mentioned. As illustrated in Figure 2, the angled extension 22 has an internal bore 50 that is open at one end and is closed at its other end 52. The bore 50 extends downwardly and laterally as illustrated in Figures 1 and 2 and opens to the perforation 28. The distal end of the angled neck has a tapered head 54 which is received within a tapered bore 60 formed in the ball 24. In this embodiment, the angled neck 22 does not only function as a part of the neck. of the prosthesis but also the clamp. To positively and firmly set the ball 24 to the angle at an angle, first the angle is lengthened in the form previously unscrewed in connection are the clamp 30. When lengthening the pitch at an angle 22, enough to allow the head 54 it resists tightly in the ball, the stretching force imparted by the instrument is removed, and the neck 22 returns to its original "rest" configuration, the outer wall of the head 54 leans outward against the remaining walls of the perforation 60, to firmly hold the ball to the angle floor. With reference to Figure 1, it will be noted that the perforation 50 is fully accessible through its open end before mounting the neck 20 on the rod 12. It can also be noted that the clamp and the savity, although sirculares in cross section and that They make mutual contact surface-to-euperfisie, are tapered instead of cylindrical, illustrating how the shape of the clamp and the savity can be varied. With referensia to Figure 3, a tibia tray somponent is generally illustrated as 70 and comprises a rod 72, adapted to be received in the surgically prepared intramedullary canal of the tibia, in a sonoside form. The rod ends up in a metal tray which in turn holds a support insert 76 of high molecular weight polyethylene or the like. The latter is adapted to articulate with the condyles at the far end of the femur, or are the sonobs of a prosthetic femoral implant, all in a sonoidal form. Cersa of the upper end of the rod is solosado a shoulder 78 which engages in the surgically prepared upper end of the intramedullary sanal of the tibia and serves to support the upper end of the rod. A clamp such as is previously disclosed, is illustrated at 80 in Figure 3. Conveniently it is cross-sectional silicon, having a diameter at body temperature, which is slightly larger than the diameter of an axially formed bore 82 within the rod. 72. The clamp 80 can be inserted by the same method as described in connection with the clamp 30 in Figure 1. When pulling forces are removed, the clamp returns to its "rest" configuration and its walls press down to the walls of the clamp. shank 72, providing that the latter in turn clamps firmly to the walls of the perforation 84 of the shoulder member 78.
A prosthesis for artisulasión de sadera slightly modified is illuted in Figure 4 as 100, the prosthesis has a rod 112 adapted for insertion into the intramedullary sanal of the femur. An axial perforation 114 is formed in the rod, and the walls of the stem sersa of its proximal end, may have longitudinal grooves 116 formed therein, the grooves end in round holes 118, to avoid areas of concentration of tension. The grooves 116 allow the wall of the rod to expand more easily, and are evenly spaced relative to the circumference of the stem. Four slots can be used. A body 120 is provided with an internal bore 122, sized to densely receive the rod, the body supports a ball 124 similar to the ball 24 of FIG. 1. The upper or proximal end of the body 120 extends slightly beyond the proximal end 126 of the stem. A hollow tubular clamp 130 similar to clamp 30 shown in Figure 1 is received within the rod. The clamp 130 has a proximal, externally-threaded end portion 132 that extends beyond the proximal end 126 of the rod, but which nevertheless preferably is retained in the proximal end portion of the body 122 puncture, all as illuted in Figure 4.
Figures 5 to 7 show inmentation for aplying tensile ss to the typified clamp somo 130 in the drawing. Shown at 140, there is a tubular section tool having an open distal end portion 142, which is internally threaded to receive the external threads of the proximal end portion 132 of the clamp. Preferably, supersized rossas are used. An aperture 144 is formed in the holding tool 140, proximate to its distal end portion 142. An elongated push rod 150 resides in the bony clamp and has a distal end 152 configured to engage the confronting remote end wall 134 of the clamp in contasto surface-to-superfisie. The proximal end 154 of the push rod is accessible through the opening 144 as best illuted in Figure 6, and has a recessed end surface 156. It should be noted that the proximal end wall 146 of the holding tool tubular, similar has a superfamily are recess 148 facing the recessed end surface 156 of the rod. Figure 7 shows the cture of Figure 6 in association with a manually operated clamp force-generated device 170, the device having handles 172, facing opposite nose portions 174, which are received in the opening of the tool. and a pivot 176 positioned to provide substantial mechanical advantage to the nose portions. Nose portions 174 rest against respective recessed surfaces of the push rod and the clamping tool as illuted in Figure 1, compressing the handles together resulting in substantial force aplication to the rod 150, providing that the clamp 180 lengthen slightly, but enough to allow the clamp to insert into the bore of the rod. A catch and ratchet mesanism 178 design and somosely employed with surgical inments, is provided at the ends of the handles to hold them together and thus maintain the rod in its elongated, tensioned sonfiguration. Various other devices for supplying substantial force to tch the clamp can be employed using any one member of mechanical, pneumatic and hydraulic means. In use, again refer to Figures 4 to 7, a push rod 150 is inserted into an appropriate clamp 130, and the proximal end of the clamp is threaded onto the end of the clamping tool 140, to form the cture shown in Figure 6. The nose portions 174 of the force generating device 170 are inserted through the opening 144., in sontasto are the recessed recessed surfaces of the push rod and the clamping tool, and the handles are squeezed together and enslaved by the mesanism 178, thereby holding the clamp in its elongated configuration. The body 120 is received on the rod, and subarachnoid is desired on the rod by the surgeon, during the implant procedure. Once the rod and body have been properly oriented to each other, and the body has been conveniently impacted by the surgeon in the intramedullary canal, the clamp is inserted into the perforation of the stem. The mechanism 178 is then released, resulting in the release of pressure from the nose elements on the push rod and the clamping tool. As the clamp 130 expands into its resting configuration, it supports the substantial strength of the walls of the rod, forcing these walls into tight sontasto are the walls of the perforation formed in the body. The clamping tool of course is then removed and the open proximal end of the clamp is terminated at the end appropriately, if desired. It may be particularly valuable to use the stem of the prosthesis of Figure 1 itself as the clamp, by removing the clamp 30. Here, the proximal end of the rod can be internally threaded to receive the distal threaded end of an externally threaded clamping tool, similar to that shown at 40 in Figure 6. The clamping tool and the push rod may be longer than those shown in the drawing, to allow the neck and body to be thrown over the clamping tool, before threading the tool. Clamping on the threaded end of the rod. By properly configuring the clamping tool 140, the lumen 20 and the body 16 can be loosened in the clamping tool before using the device to lengthen the proximal position of the rod. The push rod 150 collapses in the bore of the rod, and the clamping tool is threaded on the rod. Once the rod 12 has been lengthened by tension of the force generator device and properly positioned in the femoral cavity, the neck and body can be lowered over the nose portions and around the rod and positioned as desired within the intramedullary canal. Conveniently, the various parts of the prostheses of the invention that are held together, are made of metal such as stainless steel, chrome cobalt alloys, titanium alloys or the like, as commonly used for prosthetic fabrication. The clamp can similarly be made from a memory with shape memory or from any metal that exhibits an initial proportional relation between stress and strain (in the range of validity of Hooke's law). Various metals and metal alloys meet this requirement, including stainless steel. The recession of lateral or transverse tension to the longitudinal or axial tension, somnly referred to as the Poisson's ratio, can be in the range of .2 to .5, depending on the material and its condition. The Poisson ratio for stainless steel for example is approximately 0.28. The clamps according to the invention are preferably made of a shape memory alloy such as nitinol. Nitinol exhibits a Poisson ratio of about 2.3, but this ratio increases significantly to about 0.5 or more, when the shape memory alloy is stretched beyond its limit inisial elastix; that is, when stress-induced martensite formation begins to occur. Nitinol is a pseudo-elastic material, that is, a material that exhibits super elasticity at room temperature. A number of shape memory alloys are shown to exhibit the super elastic / pseudo-elastic recovery characteristic and these are generally characterized by their ability, at room temperature or at body temperature, to be deformed from a austenitic glass stress. a stress-induced arthritic structure returning to the austenitic state suando tension is removed. The alternating crystal clastres give the pseudo-elastic or super-elastic properties of the alloy. Nitinol clamps of the type referred to above in connection are Figures 1 and 3 can easily elongate up to 8% or more through the use of instruments such as this is illustrated in Figure 4. Using nitinol is a Poisson ratio considered to be 0.3, if a clamp such as that shown in Figure 6 is extended by 8%, would be expected to shrink by approximately 2.4% in diameter. If the initial diameter of the clamp was in the vicinity of 1.27 cm (the decrease in diameter would be in the order of .3048 mm (.012")) since tool tolerances can be easily maintained for the internal perforations of rods and other prosthetic parts within + .0508 mm (± .002"), a .3048 mm (.012") sarabium in the diameter of the clamp allows for substantial spasm for several design sizes in size. The rod perforation however is only slightly larger than the outer diameter of the clamp, the clamp being stretched longitudinally to an elongation for example of 8% .A steward can choose the desired sizes of the rod, body and groove and can assemble the clamps. During a surgical procedure, with reference to the femoral implant shown in Figure 4, an articulating ball 124 of the appropriate size is selected and assembled as previously disengaged at the neck. 120. The femoral prosthesis without the clamp 130 is then assembled. The assembly can be carried out outside the patient, if the desired dimensions and angles of the prosthetic parts are sonosed are pression and are antisipasion in time, such as by trial or by use of trial prosthesis parts. The prostheses themselves can be assembled in the intramedullary canal of the patient, with the corresponding orientations of the annotated parts. With reference to Figures 4 to 7, once the parts have been arranged and oriented as desired in the intermediate channel, a clamp 130 is tensioned to reduce its diameter through use of the clamping tool 140, the rod push 150 and the force generating device 170, and then generally colossus in the bore of the rod. When tension is removed in the clamp, the clamp expands immediately to its larger diameter "rest" configuration, thereby holding the stem and holding the stem 112 to the body 120. It will be noted that the resulting prosthesis is not conveniently it has loose fasteners that come loose. While the tension is maintained in the clamp, the body 120 can be positioned independently in axial and rotational directions in the rod as the surgeon may deem appropriate for the partisan pasture. In the same way in which the armed is savored, disarming can be achieved by reversing the stages. Similarly, in connection with the prosthesis of theFigure 3, once the shoulder 78 and the stem 72 have been mounted at the distal end of the tibia as desired and oriented to each other, the clamp 180 can be inserted into the bore 82 and allow it to expand toward its "rest" configuration " This in turn forces the walls of the rod outward and to count are the perforations 84 of the shoulder 68, to enslave the rod and the shoulder in a joint. Although the clamp of the invention has been broken down in terms of a bony rod are an open end and a serrated end, it will be understood that a variety of clamp shapes can be employed. If a sambium-shaped clamp is desired due to a phase change in aleasion, it is martensite-to-austenite-shaped memory, whereby a solid clamp may be preferred instead of a hollow clamp. Hollow structures are preferred even in this case, however, since the individual bone of the clamp provides a means for cooling the clamp in the case of requiring disassembly of a prosthesis. Also, while it is desired that the outer surface of the clamp and the inner surfaces of the perforation or perforations within the suals are received the clamp be smooth and regular to make good surface-to-surface contact, the outer surface of the clamp , in fact, it may be flanged or trimmed or longitudinally grooved or otherwise shaped as desired. Furthermore, sorao was previously noted, the clamps of the invention do not need to be round in transverse session nor should they have a uniform dimension transverse to the longitudinal axis. If desired, the outer surface of the clamp may have a greater transverse dimension in some areas than in another. For example, with reference to Figure 1, the transverse dimeneion of the clamp may be larger crest of the upper part of the clamp by sucking the portion of the clamped stem also against the perforation of the body or vice versa. The clamp can be hollow or tubular in design. With reference to Figure 8, the head 54 of the neck extension 22 can be formed with a thimble-shaped bracket 180 having an outwardly abutting skirt 182 at its open end. When the head 54 with the conformed clamp 180 is forced into the bore 60 of the ball 37 leaving a spacing 62 between the end of the clamp 180 and the floor of the bore 60, the edge of the opening 60 includes the skirt 182 and force the walls of the clamp to lengthen. By releasing the pressure that forces the saber 54 in the opening 60, the walls of the clamp are slightly inscremented in thickness, grabbing the ball on the head 54 and sealing the opening 60. The interface 61 between the head 54 and the clamp 180 also It is sealed. In a preferred embodiment, the sonnet walls of the clamp and cavity can be configured, such that any sliding between the clamp and cavity results in the clamp moving more deeply into the cavity. For example, the confronting walls of the clamp or cavity or both can be configured to have circumferential shoulders or tapered surfaces or other shapes, which act to preferentially move the clamp to move or "tighten" in a direction instead of the opposite direction. , suando there is repeated sliding between the superfisies sonfrontantes. With referensia to Figure 4, for example, the diameters of the clamp 130 of the bore in the stem 112 may be slightly larger than the distant end of the stem 112 that is crested from the proximal end, so that any movement or " Advance "of the clamp due to repeated sliding of the clamp and the rod perforation displaces the clamp distally inside the rod, directing the rossed shoulder in the proximal end of the clamp, in sontaste are the proximal end 126 of the stem. While a preferred embodiment of the present invention has been disclosed, it will be understood that various changes, adaptations and modifications may be practiced, without departing from the spirit of the invention and scope of the appended claims.