Background of the Invention Prostheses for replacement of joints commonly involve two parts having mutually articulating surfaces, and structure for mounting the parts to bone. To duplicate closely the structure and function of natural joints, the prostheses parts must be carefully shaped and sized, and must be properly oriented by the surgeon with respect to each other and with respect to the anatomy of the patient.

To achieve good surgical results, a surgeon should have as much freedom as possible during the surgical implantation procedure to vary the shape, size and orientation of prosthesis parts. Mainly for this reason, efforts have been made to provide prostheses that are modular in form so that various elements of a prosthesis can be individually selected and the prosthesis can be assembled and oriented according to the anatomical needs of the patient.

Modular prostheses for the hip joint are shown, for example, in Boleski et al., U. S. patent 5,080,685, Gianezio et al., U. S. patent 4,520,511, Demane et al., U. S. patent 4,995,883, Luman, U. S. patent 5,002,578 and Rhenter et al., U. S. patent 4,693,724. Such prostheses for the most part involve a substantial number of parts that are held together in one configuration or another by means of mounting screws which operate to draw together tapered connections of the parts. Although some freedom of selection is provided by previous modular prostheses, the use of threaded mounting screws and tapered connections can lead to loosening of the parts and to other problems. Physical and chemical corrosion can become substantial problems due to weakening of the prosthesis and to biologic responses to corrosion debris and byproducts. See Jacobs, J. J. et al., Biological Activity of Particulate Chromium-Phosphate Corrosion Products.

Collected Papers of the 21st Annual Meeting of the Society for Biomaterials, March 18- 22,1995, p. 398, and Urban, Robert M., et al., Corrosion Products From Modular-Head Femoral Stems of Different Designs and Material Couples, Collected Papers of the 21st

Annual Meeting of the Society for Biomaterials, March 18-22,1995, p. 326. Fretting corrosion caused by relative motion between adjoining surfaces leads to the production of debris which in turn may lead to accelerated wear between normally articulating joint parts of a prosthesis and to osteolysis. When gaps occur between adjacent surfaces of prosthesis parts, oxidation of the surfaces may lead to formation of an acidic environment and hence to chemical attack of the surfaces (commonly referred to as crevice corrosion).

It would be desirable to provide a modular prosthesis kit having elements that can be freely chosen and oriented by the surgeon in the operating arena and that can be strongly and firmly fastened to one another without the need for screw fasteners or tapered connections that are drawn together.

Summary of the Invention The present invention makes use of a clamp capable of firmly clamping to a prosthesis member and that may be used to firmly clamp together selected parts of a modular prosthesis. The clamp has a"rest"configuration having a dimension in one direction that can be reduced by applying to it physical stress, with concurrent expansion of the clamp in a second direction normal to the first direction, so that the clamp may be received in a cavity of a prosthesis member. Upon release of the applied physical stress, the clamp seeks to return toward its"rest"configuration, the clamp dimension in the one direction increasing so that the clamp presses upon the cavity walls to strongly clamp to the prosthesis member.

Thus, in one embodiment the invention relates to a modular prosthesis kit comprising a first member having walls defining a cavity, and a clamp releasably clampable within said cavity. The clamp has a rest configuration having a predetermined dimension in a first direction and being responsive to applied physical stress to assume a second configuration having a lesser dimension in said first direction with concurrent increase of a dimension in a second direction normal to the first direction to permit the clamp to be at least partially received in the cavity. The predetermined dimension is so chosen that upon release of the applied physical stress, the clamp returns

toward its rest configuration with consequent increase in its dimension in the first direction sufficient to strongly clamp to said member.

In another embodiment, the invention comprises a modular prosthesis kit that includes instrumentation for assembly, comprising a first prosthesis member having walls defining a cavity and a clamp releasably clampable within said cavity. The clamp has a first, rest configuration having a predetermined dimension in a first direction. An instrument is provided for applying a stretching force to said clamp in a second direction normal to said first direction to reduce said dimension in the first direction enough to permit said clamp to be received in said cavity. The predetermined dimension is such that upon removal of the stretching force, the clamp returns toward its rest configuration with consequent increase in its dimension in the first direction sufficient to strongly clamp to said first prosthesis member. In a preferred embodiment, the prosthesis kit includes a second member configured to snugly receive at least a portion of the first member in any of several orientations. The cavity walls of the first member are configured to expand into clamping contact with the second member as the clamp returns toward its rest configuration to fixedly support the second member in a predetermined orientation with respect to the first member.

Preferably, the clamp in its rest position is elongated in the second direction and the cavity has inner walls similarly shaped to receive the clamp, the clamp having a distal end dimensioned to be partially received in the cavity. Force may be exerted on the distal end of the clamp directed inwardly of the cavity. As the walls of the cavity engage the clamp at any point along its length to resist insertion of the clamp, continued force on the distal end of the clamp places in tension that portion of the clamp between its distal end and the point of engagement with the cavity, causing that portion to elongate with a concurrent reduction in the width of the clamp at the point of engagement by the cavity walls. This width reduction, in turn, enables the clamp to move further inwardly of the cavity. The force that is exerted on the distal end of the clamp is resisted by an essentially equal force in the opposite direction applied to the cavity, the latter force being transmitted to the clamp along its length where it is engaged by the cavity. In this

embodiment, instrumentation preferably is provided to engage the distal end of the clamp and the first prosthesis member.

In yet a further embodiment, the invention relates to a method for assembling members of a modular prosthesis. A first prosthesis member is provided with walls defining a cavity, and a clamp is provided having a first, rest configuration having a predetermined dimension in a first direction. The clamp is subjected to physical tensioning to expand a clamp dimension in a second direction normal to the first direction, with concurrent reduction of the clamp dimension in the first direction to enable the clamp to be received in the cavity. This is desirably accomplished by applying force directed inwardly of the cavity to a distal end of the clamp received in the cavity, as described above. The tensioning force is then withdrawn to allow the clamp to return toward its first, rest configuration with consequent increase in its dimension in the first direction sufficient to strongly clamp to said first prosthesis member.

In a preferred embodiment, the clamp and the cavity of the first prosthesis member have confronting clamping surfaces that, when clamped, are substantially congruent so as to provide surface-to-surface contact between the clamp and first member, and the prosthesis is substantially free of gaps between confronting surfaces.

Similarly, if a second prosthesis member receives and becomes clamped to the first member, preferably the clamping surfaces of these members are substantially congruent so as to provide surface-to-surface contact between the clamping surfaces of the first and second members, and the prosthesis is substantially free of gaps between confronting surfaces. Such surface-to-surface contact promotes uniform loading along the clamping surfaces.

Brief Description of the Drawing Figure 1 is a side view, in partial cross-section, of a portion of a hip joint prosthesis in accordance with the invention; Figure 2 is a cross-sectional, broken away view taken across line 2-2 of Figure 1; Figure 3 is a schematic front view of the tibial portion of a knee joint in accordance with the invention;

Figure 4 is a side view, in partial cross-section, of a portion of another hip joint prosthesis similar to that of Figure 1; Figure 5 is an exploded assembly view of parts of instrumentation for use in the assembly of the hip joint prosthesis of Figure 4; Figure 6 is a view of the parts of Figure 5 as assembled; Figure 7 is a view of the assembly of Figure 6 together with a manually operated force generating device; Figure 8 is a cross-sectional, broken away view showing another embodiment of the invention; Figures 9A, 9B and 9C are side views of the stem, body, and clamping member of a preferred hip joint prosthesis of the invention; Figure 10 is a side view, in partial cross-section, of a portion of a hip joint prosthesis of which elements are shown in Figures 9A, B, and C; Figure 11 is a side view of a modified stem of a hip joint; Figure 12 is a schematic top view of the prosthesis of Figure 10, showing positional adjustment of the body element with respect to the stem; Figures 13A, 13B and 13C are cross-sectional views showing different stages in the assembly of a clamping member and instrumentation; Figure 14 is a view in partial cross-section and partially broken away of a prosthesis of the invention during a step in its assembly; Figure 15 is a perspective view of instrumentation useful in the assembly of a prosthesis of the invention; Figure 16 is a schematic view of other instrumentation that can be used in the assembly of a prosthesis of the invention; and Figure 17 is a schematic view of a modified instrument that can be used in the assembly of a prosthesis of the invention.

Detailed Description of the Preferred Embodiments With reference first to Figure 1, a modular hip prosthesis is designated 10, and comprises an elongated stem 12 sized to be received in a surgically prepared intramedullary canal of the femur. Axial bore 14 is formed in the stem 12. A body

member 16 is provided with a bore 18 sized to closely receive the stem 12, the body having a generally triangular shape when viewed from the side and configured to fit the surgically sculpted proximal end of the intramedullary canal of the femur. Proximally of the body 16 is positioned a neck member 20 having a bore 21 sized to closely receive the upper end of the stem 12, the neck including an angled extension 22 terminating in a ball 24 sized to articulate with an appropriately sized and shaped socket prosthesis (not shown) to be mounted in the acetabular recess of the pelvis.

A clamp 30 is shown in Figure 1 as an elongated metal rod having an axial bore 32 that extends from its proximal end portion 34 to a floor 36 short of the distal end portion 38 of the clamp. Near its upper end, the axial bore 32 has a distally facing body fashioned to receive a placement instrument, as will be described below.

The clamp 30 is shaped and sized such that at body temperature, its diameter, when not constrained in the stem 12, will be slightly larger than the diameter of the bore 14 of the stem. The diameter 21 of the neck bore and the diameter 18 of the body bore, on the other hand, are essentially the same as the outer diameter of the stem 12; that is, the stem is snugly but slidably received in the bores 18,21 so that the body and the neck can be moved by hand upon the stem without difficulty.

The clamp 30, before installation in the bore 14 of the stem, first must be altered so that its diameter is slightly less than the bore diameter of the stem. This is accomplished by physically stretching the clamp in its long or axial direction to cause the diameter of the clamp to shrink sufficiently to enable the clamp to be inserted in the bore 14. Although the clamp may be made from various metals as described below, a preferred metal is a shape memory alloy such as nitinol, in its superelastic state in which applied stress results in a reversible martensitic phase transition. When a nitinol clamp 30 is stretched as described above, and providing that its temperature is maintained substantially above its austenite finish temperature (the temperature at which the alloy is completely in its austenitic form), a transition from the austenite phase to the martensite phase occurs. This is known as stress induced martensite formation and is the basis for the phenomenon known as pseudoelasticity or superelasticity. The shape memory alloy will remain at least partially in the martensite phase as long as the external stress is

maintained. Upon release of the stress, however, the clamp 30 will return to the austenite phase and toward its original shape and size. Because the clamp is constrained within the dimensions of the stem bore 14, however, it will not be able to completely resume its original shape and size. As a result, the clamp 30 will exert a continuous force against the bore 14 of the stem 12. That is, when externally applied stress is released, the clamp tends to return toward a configuration which may be referred to as a"rest"configuration.

The rest configuration of the clamp has a transverse dimension (the diameter in the case of a rod having a circular cross-section) that is slightly larger than the transverse dimension of the stem bore, and as a result the clamp pushes outwardly strongly upon the stem bore and becomes firmly clamped in the stem bore.

As shown in Figure 1, the walls 42 of the clamp have outer surfaces 44 that engage and push outwardly upon the bore 14. When appropriately in place, the outer wall 44 of the clamp pushes outwardly upon the surface of the stem bore 14, and the walls of the stem, in turn, are forced outwardly into contact with the inner surface 26 of the body 16 and also with the inner surface 28 of the neck 20.

Preferably, the outer surface of the clamp 30 is generally cylindrical and makes substantial surface-to-surface contact with the surface of the bore 14. Moreover, the stem wall is sufficiently flexible as to enable the outer wall of the stem to expand into contact with the bores of both the body and the shoulder, even when these bores are slightly different in diameter. A feature of a preferred embodiment of the invention is that the clamped surfaces-that is, the confronting surfaces of the clamp and first member, and the confronting surfaces of the first and second members-mate in surface-to-surface contact to fairly uniformly distribute the compressive forces over the clamped surfaces and preferably to avoid gaps between confronting surfaces. As used herein, a"gap"is the thin void space formed between slightly spaced confronting surfaces of a prosthesis when assembled, as, for example, the space formed between an elongated, smooth- walled rod having threads at one end and the bore receiving the rod. If the clamp is a cylinder having a circular cross-section and the cavity is a circular bore, the compressive clamping force exerted by the clamp against the walls of the bore would be primarily radial and substantially uniform along the length of the clamp. One may vary as desired

the concentration of compressive forces between the clamp (and between prosthesis members) by varying the shapes of the clamping surfaces. For example, if the cross- sections of the clamp and recess were oval rather than circular, one would expect the compressive clamping force to be somewhat greater in the longer transverse dimension than in the shorter transverse dimension.

The invention in another embodiment is shown in Figure 2, in which the ball 24 is firmly mounted to the angled neck extension 22. The ball 24 and the neck member 20 (from which extends the angled extension 22) generally will be assembled as a subunit, and the subunit will then be assembled with the body and stem as mentioned above.

As shown 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 shown in Figures 1 and 2, and opens into the bore 28. The distal end of the angled neck has a tapered head 54 that is received within a tapered bore 60 formed in the ball 24. In this embodiment, the angled neck 22 functions not only as a part of the prosthesis but also as the clamp. To positively and firmly connect the ball 24 to the angled neck, one first elongates the angled neck in the manner described above in connection with the clamp 30. Upon elongation of the angled neck 22 sufficient to enable the head 54 to be snugly received in the ball, the stretching force imparted by the instrument is withdrawn, and the neck 22 returns toward its original,"rest"configuration, the outer wall of the head 54 bearing outwardly against the confining walls of the bore 60 to firmly clamp the ball to the angled neck. Referring to Figure 1, it will be noted that the bore 50 is fully accessible through its open end prior to mounting of the neck 20 upon the stem 12. It may also be noted that the clamp and the cavity, although circular in cross-section and making mutual surface-to-surface contact, are tapered rather than cylindrical, illustrating how the shape of the clamp and cavity may be varied.

With reference to Figure 3, a tibial tray component is shown generally as 70 and comprises a stem 72 adapted to be received in the surgically prepared intramedullary canal of the tibia in a known fashion. The stem terminates upwardly in a metal tray 74 which in turn supports a bearing insert 76 of high molecular weight polyethylene or the like. The latter is adapted to articulate with the condyles at the distal end of the femur, or

with the condyles of a prosthetic femoral implant, all in a known fashion. Near the upper end of the stem is positioned a shoulder 78 which fits in the surgically prepared upper end of the tibial intramedullary canal, and serves to support the upper end of the stem.

A clamp such as that described above is shown at 80 in Figure 3. It is desirably cylindrical in cross section, having a diameter at body temperature that is slightly greater than the diameter of a bore 82 formed axial within the stem 72. The clamp 80 may be inserted by the same method described in connection with the clamp 30 of Figure 1.

When the stretching force is withdrawn, the clamp returns toward its"rest"configuration and its walls press outwardly against the walls of the stem 72, causing the latter in turn to clamp strongly to the walls of the bore 84 of the shoulder member 78.

A slightly modified hip joint prosthesis is depicted in Fig. 4 as 100, the prosthesis having a stem 112 adapted for insertion in the intramedullary canal of the femur. An axial bore 114 is formed in the stem, and the walls of the stem near its proximal end may have longitudinal slots 116 formed therein, the slots ending in round holes 118 to avoid stress concentration areas. The slots 116 enable the wall of the stem to expand more easily, and are spaced evenly about the circumference of the stem. Four slots may be employed. A body 120 is provided with an internal bore 122 sized to snugly receive the stem, the body bearing a ball 124 similar to ball 24 of Figure 1. The upper or proximal end of the body 120 extends slightly beyond the proximal end 126 of the stem.

Within the stem is received a hollow, tubular clamp 130 similar to the clamp 30 shown in Figure 1. Clamp 130 has a proximal, externally threaded end portion 132 that extends beyond the proximal end 126 of the stem but is yet preferably retained in the proximal end portion of the body bore 122, all as shown in Figure 4.

Figures 5-7 depict instrumentation for applying tensile stress to the clamp typified as 130 in Figure 4 of the drawing. Shown at 140 is a tubular gripping tool having an open distal end portion 142 that is internally threaded to receive the external threads of the proximal end portion 132 of the clamp. Square threads preferably are used. An aperture 144 is formed in the gripping tool 140 proximal of its distal end portion 142. An elongated pushing rod 150 is received in the hollow clamp, and has a distal end 152 shaped to engage the confronting distal end wall 134 of the clamp in

surface-to-surface contact. The proximal end 154 of the pushing rod is accessible through the aperture 144, as shown best in Figure 6, and has a recessed end surface 156.

Note that the proximal end wall 146 of the tubular gripping tool similarly has a recessed surface 148 facing the recessed end surface 156 of the rod.

Figure 7 depicts the assembly of Figure 6 in association with a manually operated plier-like force-generating device 170, the device having handles 172, oppositely facing nose portions 174 receivable in the aperture in the gripping tool, and a pivot 176 positioned to provide substantial mechanical advantage to the nose portions. Nose portions 174 bear against the respective recessed surfaces of the push rod and gripping tool as shown in Figure 7; squeezing of the handles together results in the application of substantial force to the rod 150, causing the clamp 130 to elongate slightly but sufficiently to enable the clamp to be inserted in the bore of the stem. A tooth and pawl mechanism 178 of known design and commonly used with surgical instruments is provided at the ends of the handles to hold them together and thus maintain the stem in its stressed, elongated configuration. Various other devices capable of delivering substantial force to stretch the clamp may be employed using any of a number of mechanical, pneumatic, and hydraulic means.

In use, referring again to Figures 4 through 7, a push rod or shaft 150 is inserted in an appropriate clamp 130, and the proximal end of the clamp is screwed onto the end of the gripping tool 140 to form the assembly shown in Figure 6. The nose portions 174 of the force-generating device 170 are inserted through the aperture 144 into contact with the respective recessed surfaces of the push rod and gripping tool, and the handles are squeezed toward each other and locked by the mechanism 178, thus holding the clamp in its elongated configuration. Body 120 is received over the stem, and is positioned where desired along the stem by the surgeon during the implantation procedure. Once the stem and body have been properly oriented with respect to each other, and the body has been suitably impacted by the surgeon into the intramedullary canal, the clamp is inserted into the stem bore. Mechanism 178 is then released, resulting in the release of pressure of the nose elements against the push rod and gripping tool. As the clamp 130 expands toward its rest configuration, it bears with substantial force against the walls of the stem, forcing

these walls into tight contact with the walls of the bore formed in the body. The gripping tool, of course, is then removed, and the open proximal end of the clamp is capped appropriately if desired.

It may be particularly valuable to utilize the stem of the prosthesis of Figure 1 itself as the clamp, eliminating the clamp 30. Here, the proximal end of the stem may be internally threaded to receive the distal threaded end of an externally threaded gripping tool similar to that shown at 140 in Figure 6. The gripping tool and push rod may be longer than that shown in the drawing to allow placement of the neck and body over the gripping tool prior to threading the gripping tool onto the threaded end of the stem.

By appropriately configuring the gripping tool 140, one may loosely position the neck 20 and body 16 on the gripping tool prior to use of the device to elongate the proximal portion of the stem. The push rod 150 is placed in the bore of the stem, and the gripping tool is threaded onto the stem. Once the stem 12 has been elongated by operation of the force generating device and appropriately positioned in the femoral cavity, the neck and body may be brought down over the nose portions and around the stem and positioned as desired within the intramedullary canal.

Desirably, the various parts of the prostheses of the invention that are clamped together are made of metal such as stainless steel, cobalt chrome alloys, titanium alloys or the like as are commonly employed for prostheses manufacture. The clamp, similarly, may be made of a shape memory alloy or of any metal that exhibits an initial proportional relationship between stress and strain (in the range of validity of Hooke's law). Various metals and metal alloys satisfy this requirement, including stainless steel.

The ratio of the lateral or transverse strain to the longitudinal or axial strain, commonly referred to as Poisson's ratio, can range from 0.2 to 0.5, depending on the material and its condition. Poisson's ratio for stainless steel, for example, is about 0.28.

The clamps according to the invention preferably are made of a shape memory alloy such as nitinol. Nitinol exhibits a Poisson's ratio of about 0.3, but this ratio significantly increases up to approximately 0.5 or more when the shape memory alloy is stretched beyond its initial elastic limit; that is, when the formation of stress-induced martensite begins to occur. Nitinol is a pseudoelastic material, that is, a material that

exhibits superelasticity at room temperature. A number of shape memory alloys are known to exhibit the superelastic/pseudoelastic recovery characteristic, and these are generally characterized by their ability, at room or body temperature, to be deformed from an austenitic crystal structure to a stressed-induced martensitic structure, returning to the austenitic state when the stress is removed. The alternate crystal structures give the alloy superelastic or pseudoelastic properties.

Nitinol clamps of the type referred to above in connection with Figures 1 and 3 can readily be elongated up to 8% or more through the use of instruments such as that shown in Figure 4. Using nitinol with an assumed Poisson's ratio of 0.3, if a clamp such as that shown in Figure 6 is elongated 8%, it would be expected to shrink about 2.4% in diameter. If the initial diameter of a clamp were in the neighborhood of 1/2 inch, the decrease in diameter would be on the order of 0.012 inches. Since tooling tolerances for the internal bores of stems and other prosthesis parts can easily be held within 0.002 inches, a change of 0.012 inches in the clamp diameter allows substantial room for design variations in size. It is generally preferred that the diameter of the stem bore, however, be only very slightly greater than the outer diameter of the clamp when the clamp is longitudinally stretched to an elongation of, for example, 8%.

A surgeon may select the desired sizes of the stem, body and head, and can assemble the same 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 is mounted as described above to the neck 120. The femoral prosthesis without the clamp 130 is then assembled. Assembly may take place away from the patient if the desired dimensions and respective angles of the prosthesis parts are known with accuracy ahead of time, as by measurement or by use of trial prosthesis parts. The prosthesis itself can be assembled in the intramedullary canal of the patient, with the correct orientations of the parts noted. Referring to Figures 4 through 7, once the parts have been arranged and oriented as desired in the intramedullary canal, a clamp 130 is tensioned to reduce its diameter through use of the gripping tool 140, the pushrod 150 and the force generating device 170, and is then gently placed in the bore of the stem. When tension on the clamp is withdrawn, the clamp expands immediately toward its larger diameter"rest"

configuration, thereby clamping itself to the stem and clamping the stem 112 to the body 120. It will be noted that the resulting prosthesis desirably has no threaded fastenings to come loose. While tension is maintained on the clamp, the body 120 may be positioned independently in axial and rotational directions on the stem as the surgeon may deem appropriate for the particular patient. In the same manner in which assembly was carried out, disassembly can be afforded by reversing the steps.

Similarly in connection with the prosthesis of Figure 3, once the shoulder 78 and stem 72 have been mounted in the distal end of the tibia as desired and oriented with respect to one another, the clamp 80 may be inserted in the bore 82 and permitted to expand toward its"rest"configuration. This, in turn, forces the walls of the stem outwardly and to contact with the bore 84 of the shoulder 78 to lock the stem and shoulder together.

Although the clamp of the invention has been described in terms of a hollow rod with one open end and one closed end, it should be understood that a variety of clamp configurations may be employed. Also, while it is desired that the outer surface of the clamp and the inner surfaces of the bore or bores within which the clamp is received be smooth and regular so as to make good surface-to-surface contact, the outer surface of the clamp may, in fact, be ridged or roughened or longitudinally fluted or otherwise configured, as desired.

Moreover, as noted above, the clamps of the invention need not be round in cross section nor must 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 others. For example, with reference to Figure 1, the transverse dimension of the clamp may be greater near the top of the clamp where the stem portion that is clamped bears also against the bore of the body or vice versa.

The clamp desirably is hollow or tubular in design. Referring to Figure 8, head 54 of the neck extension 22 may be formed with a thimble-shaped clamp 180 having an outwardly flared skirt 182 at its open end. When the head 54 with clamp 180 attached is forced into the bore 60 of the ball (leaving a gap 62 between the end of the clamp 180 and the floor of the bore 60), the rim of the opening 60 encounters the skirt 182 and

forces the walls of the clamp to elongate. Upon release of the pressure forcing the head 54 into the opening 60, the walls of the clamp increase slightly in thickness, wedging the ball onto the head 54 and sealing the opening 60. The interface 61 between the head 54 and the clamp 180 is also sealed.

In a preferred embodiment, the confronting walls of the clamp and cavity may be so configured that any slippage between the clamp and the cavity results in the clamp being urged more deeply into the cavity. For example, the confronting walls of the clamp or cavity or both may be configured to have circumferential shoulders or tapered surfaces or other shapes, that coact to preferentially urge the clamp to move or"walk"in one direction rather than the opposite direction upon repeated slippage between the confronting surfaces. With reference to Figure 4, for example, the diameters of the clamp 130 and the bore in the stem 112 may be slightly greater near the distal end of the stem 112 than near the proximal end so that any movement or"walking"of the clamp due to repeated slippage of the clamp and the stem bore urges the clamp distally within the stem, drawing the widened threaded shoulder at the proximal end of the clamp into contact with the proximal end 126 of the stem.

Figures 9A and 9B depict a modified form of a hip prosthesis of the invention..

Stem 200 has an axially extending bore 202 through a portion of its length and open at its proximal end, the walls of the bore including a pair of diametrically opposed slots 204.

Each slot is formed between two small circular holes 206 which serve to release stress at the ends of the slots. A body 210 is provided with a circular bore 212 which snugly receives the proximal portion of the stem, and the body has a neck portion of the type shown in Figure 1. The proximal end of the stem has a reduced diameter, externally threaded portion 208. When the stem is received in the body, as shown in Figure 10, the threaded proximal end of the stem is of a diameter less than that of the bore 212 of the body, enabling the internally threaded end of a tubular instrument (shown at 216 in Figure 14) to be threaded onto the end of the stem. The tubular instrument is designed with an outer diameter enabling it to pass within the bore 212 of the body.

Referring to Figures 9C, 10 and 14, an elongated, hollow clamp 220, preferably of nitinol or other similar superelastic alloy, is received within the stem bore 202. The

clamp may be of the type shown at 42 in Figure 1, that is, with a closed distal end, or the clamp may be open throughout its length as shown in Figure 9C. In order to elongate the clamp and thereby reduce its transverse dimension sufficiently to enable it to be received in the stem bore 202, the distal end of the clamp (shown at 222 in Figure 9C, and 38 in Figure 1) must be subjected to force tending to stretch or elongate the clamp. Figures 6 and 7 in general show how the proximal end of the clamp may be threaded and gripped by a stretching instrument while a push rod 150 engages the closed end of the clamp to push against it; that is, tension is applied to the proximal and distal ends of the clamp.

With respect to Figure 8, on the other hand, the tapered head 54 is moved axially inwardly of the bore formed in the articulating ball, drawing with it the thimble-shaped clamp 180 and causing the clamp to elongate as the interaction between the clamp and the bore restrains axial movement of the clamp. This feature also is employed in the embodiments of Figures 9-17.

Turning now to Figure 14, a portion of a suitable instrument for forcing the clamp into the stem bore is shown at 216, this portion being tubular and having an internally threaded distal end 218 adapted to thread securely onto the threaded upper end 208 of the stem 200. Sliding axially within the tubular member 216 is a shaft or push rod 224 which, at its distal end, securely engages the distal end 222 of the clamp. From Figure 14, it will be understood that as the shaft 224 is forced distally, that is, downwardly in this figure, with respect to the stem 200, that portion of the clamp 220 within the stem will be subject to tensile stress. The distal end 222 of the clamp 220 tapers inwardly slightly at its end so that this end can readily be received within the rim 214 of the bore formed in the stem 200. In this manner, the distal end 222 of the clamp can be received partially within the proximal end of the bore without deforming the clamp. Thereafter, the force of the shaft 224 pushing distally on the distal end 222 of the clamp forces this end of the clamp inwardly of the bore 202. The outer walls of the clamp come into contact with the inner walls of the bore as insertion of the clamp proceeds, and this contact retards insertion of the clamp. However, application of continued force upon the distal end of the clamp causes the clamp to elongate at the points where its walls come into contact with walls of the bore, that is, where the clamp becomes"stuck"in the bore,

with the result that the clamp continues to proceed inwardly of the bore. When nitinol is used as the clamp, and cobalt chrome steel or titanium as the material from which the stem is made, insertion of the clamp within the bore proceeds smoothly and without substantial jerking due to incremental stops.

Referring again to Figures 9-12 and 14, the prosthesis there shown includes the stem 200 as a first prosthesis member and the body 210 as a second prosthesis member.

In this embodiment, the clamp 220 is received within the cavity formed by the bore 202 in the first member, and the first member in turn is received within the bore 212 of the second member. Here, it is important that the walls of the first member-the stem 200- be capable of expanding outwardly into contact with the bore 212 as the clamp 220, returning toward its rest state, forcefully pushes radially outwardly upon the inner wall 202 of the stem. Axially elongated slots 204, generally two or four in number, may be spaced about the circumference of the stem, the slots terminating in circular bores 206 to avoid stress concentration at the slot ends. A similar concept is shown in Figure 11, in which a pair of diametrically opposed slots (of which one is shown as 205) extend proximally from stress relief holes 207 and then curve medially as shown at 209 where they meet, the slots thus defining a"tongue"shaped portion of the stem wall which can more easily be displaced outwardly.

A preferred clamp 220 is shown Figures 13A, B and C. As earlier mentioned, the distal end 222 of the clamp desirably is tapered inwardly slightly as shown, to enable it to enter the outer rim 214 at the proximal end of the stem bore 202. The push rod or shaft 224 terminates distally in a reduced diameter portion having a tapered end section 227 (Figure 13C), the walls of which converge distally. At the end of the section 227 is a button 228 of increased diameter. An expandable, annular collar 230 is formed with a substantially cylindrical outer surface 232 and a tapered inner bore 234 (Figure 13A) preferably matching the taper at the end section 227 of the shaft 224. The walls of the collar 230 may be provided with axially extending slots, as needed, such that when the tapered end portion 227 of the shaft 224 is forced axially in the distal direction with respect to the collar, the collar expands into clamping contact with the interior walls of the clamp at its distal end 222.

Referring to Figure 13A, when the collar 230 is positioned against the terminal button 228, the collar, as yet unexpanded, can slide through the inner diameter of the hollow clamp 220. To wedge the collar within the distal end of the clamp, one may utilize a block 236 of a hard material such as steel, the block having a bore 238 formed through its thickness. The bore 238 is of a diameter just large enough to admit the button 228, but not large enough to admit the collar 230. As shown in Figure 13B, the clamp is oriented with its distal end against the block 238 and with the button 228 extending into the bore. As the shaft 224 is forced distally (in the direction of the arrow F in Figure 13B), interaction between the tapered portions of the shaft and collar cause the collar to expand into rigid engagement with the inner walls of the clamp 230 near its distal end, this configuration being shown in Figure 13C. After insertion of the clamp in the stem, the shaft 224 may be left in its position, or may be removed by withdrawing the staff proximally. As the shaft is withdrawn from the position shown in Figure 13C to the position shown in Figure 13B, the walls of the collar are allowed to collapse inwardly, reducing the diameter of the collar and enabling the shaft 224 and collar 230 to be removed proximally as a unit.

Figure 12 illustrates how the body 210 of a prosthesis of the invention may be oriented with respect to the stem. Proper angular and axial orientation between the stem and body may be initially set by the surgeon, and these elements may be locked in place through a clamping element as described above. If it becomes necessary to reorient the stem and body, the clamp can again be put under tension to enable the body to be moved with respect to the stem.

Figure 15 shows a mechanical device that can be employed to appropriately elongate a clamp such as that shown at 30 in Figure 1 within the cavity of a prosthesis member. The instrument itself, designated 240, includes a tubular member 216 having an internally threaded distal end that is threaded to the proximal end of the clamp. The device 240 includes a rigid frame 244 having side walls 246 and end walls 248, the side and end walls being rigidly connected to each other. Near its distal end, the device has a pivot bar 250 that is guided by the side walls 246 and that is enabled a small amount of movement axially of the clamp 30. A pair of elongated compression bars 252 are

positioned within the framework 244, the compression bars having outer ends that can pivot against the proximal end wall 248 and the distal pivot bar 250, respectively. At their adjacent ends, the compression bars are pivotally attached to a yoke 254. The yoke has a threaded end which receives the threaded end of a screw drive handle 256, the end of the screw bearing on the adjacent side wall 246. As shown in Figure 15, the compression bars 252 extend at a slight angle to one another such that as the yoke 254 is forced to the left in this drawing by the turning of the handle 256, the compression bars will become more nearly in alignment with one another, forcing the pivot bar distally.

The ends of the compression bars that pivot on the end walls and pivot bar are suitably shaped so that the compression bars cannot be moved"over center"but will, instead, be limited to a configuration in which they are in substantial alignment with one another.

The pivot bar 250 contacts the proximal end of the push rod 150 such that when the device of Figure 15 is operated, the push rod is moved distally with respect to the clamp 30, all in the manner described above.

Another force generating device is shown in Figure 16. Here, the stem 200 is again shown with an externally threaded proximal end to which the internally threaded distal end of a tubular member 216 is attached. In this embodiment, a piston 260 having a piston rod 264 is positioned within a cylinder 262, the distal end of the piston rod 264 coming into contact with the proximal end of the shaft 224. In this embodiment, the clamp 266 has a closed distal end which is engaged by the distal end of the shaft 224.

On the other side of the piston 260, the cylinder defines a high pressure chamber 268. The hydraulic fluid used in this chamber preferably is a sterile saline solution or other biologically compatible liquid. Sterile saline is supplied through valve port 270, through one-way valve 272 and into a second high pressure chamber 274, the latter having a diameter much smaller than that of the diameter of the chamber 268. A piston rod 276, itself acting as a piston, is received within the chamber 274. Movement of the piston rod 276 distally forces sterile saline from the high pressure chamber 274 through a one-way valve 278 and into the high pressure chamber 268. Piston rod 276 is driven, in turn, by a piston 280 which axially moves in a low pressure chamber 282. The chamber 282, and the downstream side 284 of that chamber, are connected to sources of a gas

such as air or nitrogen under pressure through the use of an appropriate spool valve shown schematically at 286 in Figure 16.

During operation of the device of Figure 16, air or other gas under pressure is admitted to chamber 282 and withdrawn from chamber 284, causing piston 280 to descend distally and causing the much smaller diameter piston rod 276 to compress sterile saline in the second high pressure chamber 274. The high pressure sterile saline in turn enters chamber 268, forcing piston 260 and piston rod 264 downwardly into contact with the shaft 224. When the piston 280 has bottomed out within the chamber 284, the flow of air or other gas is reversed to draw the piston rod 276 proximally and to admit sterile saline to the high pressure chamber 274. The one-way valve 278 maintains pressure in chamber 268. The process is then repeated, with the piston 260 thus traveling distally in increments as the piston 280 moves back and forth. By appropriately sizing the cross-sectional areas of the pistons 280,276 and 260, low pressure air can be employed to generate very high compressive forces in the piston rod 264. The seals for piston 260 and piston rod 276 are high pressure seals that can withstand the substantial pressures that are generated. The fit between the piston rod 264 and the tube 214 need not be particularly tight, permitting air in the distal chamber 267 to readily escape. If desired, an escape port may be placed in the walls of this chamber.

A similar actuating device is shown in Figure 17 in which the stem 200 is attached at its upper end via threads to the tube 216. In a fashion similar to that depicted in Figure 16 (similar numbering being used to designate similar parts), a piston rod 264 has its distal end in contact with a staff (not shown) that is in turn received within a clamp of the type described above. The piston rod 264 extends downwardly through the tube 216 from a piston 260 within a high pressure cylinder 268, the latter containing, preferably, a biologically acceptable fluid such as sterile saline. An upper chamber 282 is provided with a one-way valve 290 for attachment to a high pressure source of gas such as nitrogen. A piston 292 operates in the cylinder 282, the piston 292 operating as a diaphragm to separate the gas side of the cylinder 282 from the sterile saline side as shown at 294. Upon actuation of this embodiment by supplying nitrogen gas, for example, under high pressure to the chamber 282, sterile saline under substantially the

same pressure in compartment 294 is forced through flow control valve 296 into cylinder 268, acting there to force piston 260 and its piston rod 264 distally.

A variety of force-generating devices can be employed in the present invention.

The clamp element, as noted above, is generally placed under substantial tensile stress, and the stress-producing elements desirably are so configured and arranged as to avoid harm to a patient or medical personnel in the event of a material failure of any of the elements. The devices of Figures 16 and 17 offer the advantage of using an incompressible liquid in the high pressure chambers adjacent the stem of the prosthesis.

Any material failure that causes even a slight increase in the volume afforded the liquid, as, for example, a slight increase in the dimensions of the high pressure chamber, can substantially completely release the pressure in that chamber. Moreover, by completely enclosing the clamp and activating shaft during assembly, as shown in the embodiments of Figures 15,16 and 17, danger from catastrophic failure of the shaft or clamp is largely avoided.

While a preferred embodiment of the present invention has been described, it should be understood that various changes, adaptations and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.