WO1995013757A1

WO1995013757A1 - Modular prosthesis with shape memory alloy elements

Info

Publication number: WO1995013757A1
Application number: PCT/US1994/012943
Authority: WO
Inventors: Jason Dean Harry; Richard Wayne Woods; Charles Scott Harrison
Original assignee: Kirschner Medical Corporation
Priority date: 1993-11-18
Filing date: 1994-11-16
Publication date: 1995-05-26

Abstract

A modular femoral stem section (10) includes a proximal section (12) and a distal section (14). A locking screw (30) extends through the proximal section and into the distal section to secure the sections together. A plurality of securing screws (64) or pegs (74) secure an acetabular cup shell (60) to the pelvic bone. The locking screws and securing screws or pegs are made of a shape memory alloy (SMA). The screws or pegs are deformed when below a first predetermined temperature and are disposed to secure the proximal and distal sections together or to secure the cup shell to the pelvic bone while in the deformed state. Thereafter, the screws or pegs are heated above a second predetermined temperature to undergo a constrained recovery. In further embodiments, the prosthesis may include additional elements made of SMA which are shrunk fitted in place on the prosthesis.

Description

MODULAR PROSTHESIS WITH SHAPE MEMORY ALLOY ELEMENTS

BACKGROUND OF THE INVENTION

Field of Invention

The present invention is directed to a modular orthopaedic implant prosthesis, and more particularly, to a hip prosthesis utilizing elements made of a shape memory alloy. Description of the Prior Art

Joint prostheses of various types are well known in the art. In general, prostheses are surgically implanted in patients to secure one bone to another as replacements for damaged joints. A hip prosthesis is implanted to replace the joint which connects the leg bone or femur to the pelvic bone. The prosthesis replaces the hip joint. Femoral hip stem prostheses generally include a femoral stem section having a neck upon which a joint ball may be disposed. The prosthesis further includes an acetabular cup shell shaped to fit within the hip joint socket of the pelvic bone and a liner disposed within and adjacent to the concave surface of the cup shell. The joint ball is disposed so as to pivot within the liner, allowing the femur to move normally relative to the hip.

A femoral hip stem prosthesis may be modular. It may be assembled from more than one section, with the sections joined together to form a complete prosthesis. Sections of the same overall shape and function but having different dimensions are interchangeable with each other. Thus, the practitioner may construct a femoral hip stem prosthesis of appropriate dimensions for a given patient by picking sections of a desired size and joining them together. For example, the femoral stem section may include separate proximal and distal sections which are joined together by a locking screw extending through holes in the sections. The proximal section may include a neck upon which a separate femoral head is disposed to serve as a joint ball. The acetabular cup shell also is manufactured as a separate item.

The proximal and distal sections, the femoral head and the acetabular cup are manufactured in a plurality of sizes, for example, the proximal and distal sections would be manufactured in different lengths and diameters and the femoral head and cup shell would have different diameters. However, the locations at which the various components are joined together would have a uniform size and shape such that any distal section could be joined to any proximal section, and any femoral head could be disposed on the neck section of any proximal section. A cup shell and liner of appropriate diameter would be manufactured for each femoral head. Accordingly, the prosthesis may be assembled by a practitioner so as to have dimensions which are appropriate for a given patient by selecting proximal and distal sections, a femoral head, a cup shell and liner having desired dimensions.

In the known and commonly used method of implanting the prosthesis in a patient, the acetabular cup shell is disposed in the hip socket, and secured to the pelvic bone, for example, by the use of a plurality of fastening elements such as pegs, pins or screws. These fastening elements are driven into the bone through holes in the cup shell. A liner is placed in the acetabular cup shell and secured thereto. A proximal and distal section having appropriate dimensions are selected and secured together by a locking screw to form the femoral stem section. A femoral head is disposed on the neck of the proximal section. The femoral head is disposed within the concave liner, and the femoral stem section is forcibly disposed within the hollow interior of the patients femur. The femoral stem section may be secured to the femur by shape fitting alone or by other conventional means such as cementing in place.

Modular femoral stem prostheses assembled and implanted in this traditional manner suffer from certain drawbacks. The use of pins or screws to secure the acetabular cup shell to the pelvic bone allows for instability of the interface between the cup shell and the securing element heads, at the location of the holes. The instability results in micro-motion between the securing elements and the cup shell at the interface and creates frictional wear of the securing element and cup shell, further resulting in the creation of metallic debris. Further, the locking screw which joins the modular components of the femoral stem, for example, the above-described proximal and distal sections, may loosen over time and thus back out of the holes within which it is disposed. Thus, undesirable relative movement or total separation of the proximal and distal sections may result.

Shape memory alloys (SMA) are well-known and commonly used in a variety of applications, including medical devices. In general, SMA's exist in a martensitic state below a first temperature and an austenitic state above a second temperature. Although well-known in a variety of contexts, SMA used for its shape memory properties in the specific context of orthopaedic devices is very limited. SUMMARY OF THE INVENTION

The present invention is directed to a prosthesis in which one or more of the elements are made from a shape memory alloy. In a preferred form of the invention, the prosthesis is a modular stem prosthesis which includes a modular femoral stem section, joint ball and acetabular cup shell. The femoral stem section includes a proximal section and a distal section which are shaped to fit together. A locking screw extends through the proximal section and into the distal section to secure the sections together. The locking screw is made of a shape memory alloy which exists in a martensitic state below a first temperature and an austenitic state above a second temperature. The screw is formed in the austenitic state to have an original shape or dimensions.

The locking screw is cooled to the martensitic state and deformed from its original shape or dimension while in the martensitic state, for example, by being stretched in length. The screw is disposed to secure the proximal and distal sections together while in the martensitic state, and thereafter, the screw is restored to the austenitic state by increasing its temperature above the second temperature. The screw undergoes a constrained recovery, that is, the screw attempts to recover its original dimensions but is blocked from making a full recovery due to its engagement with the proximal and distal sections within which it is disposed. Thus, a compressive load is created and acts upon the proximal and distal sections. For example, the screw may undergo a constrained recovery by contracting, thereby drawing and locking the proximal and distal sections together. Accordingly, loosening of the locking screw is prevented, further precluding unwanted relative movement or separation of the proximal and distal sections.

In a further embodiment, the acetabular cup shell is secured to the pelvic bone by the use of pegs or screws which are disposed through holes in the cup shell. The pegs or screws also are made out of a shape memory alloy which has been deformed while in the martensitic state, and after disposition to secure the cup shell to the pelvic bone, are restored to the austenitic state by raising the temperature above the second predetermined temperature. The resultant constrained restoration of the pegs or screws press fits them to the cup shell at the holes to prevent micro-motion therebetween. Thus, frictional wear and the resultant undesirable creation of small particles of metal are eliminated.

In a further embodiment, the prosthesis includes a modular head, a locking ring for the modular head and a distal sleeve, with the ring and/or sleeve made of SMA. The ring and sleeve are locked in position by the compressive forces created during the constrained recovery of the SMA.

In a further embodiment, the prosthesis includes an SMA collar disposed about the proximal section, thereby transforming the prosthesis from collarless to collared. The collar is locked in place by the compressive forces created during the constrained recovery of the SMA.

In a further embodiment, the prosthesis includes a modular proximal spacer made of SMA disposed about the femoral stem section. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a femoral stem section of a modular hip stem prosthesis according to a first embodiment of the invention and including proximal and distal sections.

Figure 2 shows a complete modular hip stem prosthesis including the femoral stem section of Figure 1, a joint ball disposed on the femoral stem section, a liner and an acetabular cup shell. Figure 3 shows a femoral stem section according to a second embodiment of the invention.

Figure 4a shows the acetabular cup shell of Figure 2 having fixation screws disposed through the holes thereof according to a third embodiment of the invention.

Figure 4b shows an acetabular cup shell of Figure 2 having modular pegs disposed through the holes thereof according to a fourth embodiment of the invention.

Figures 5a-c show a femoral stem prosthesis according to a fifth embodiment of the invention including a locking ring and a distal sleeve which are made of SMA.

Figures 6a-b show a femoral stem prosthesis according to a sixth embodiment of the invention including a modular collar made of SMA.

Figures 7a-b show a femoral stem prosthesis according to a seventh embodiment of the invention including a modular spacer made of SMA.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to Figures 1 and 2, a modular femoral hip stem prosthesis according to the present invention is shown. Prosthesis 1 includes femoral stem section 10, joint ball 40, liner 50 and acetabular cup shell 60, with the latter three elements shown only in Figure 2. Femoral stem section 10 includes proximal femoral section 12 and distal femoral section 14. (The terms proximal and distal refer to the location relative to the hip joint, and upon implantation, the proximal section would be disposed upwardly of the distal section.)

Proximal section 12 includes recess 16 formed at its lower end. Recess 16 is tapered radially outwardly in the downward direction. Longitudinal bore 18 extends through proximal section 12 to the top of recess 16. Bore 18 includes widened area 19 at the top. Proximal section 12 further includes neck section 20 extending at an angle to the axis of longitudinal bore 18. Neck section 20 includes neck 22.

Distal femoral section 14 includes protrusion 26 extending upwardly from the top end thereof. Protrusion 26 is tapered radially inwardly in the upward direction so as to substantially correspond to tapered recess 16. Protrusion 26 includes screw-threaded longitudinally extending hole 28 disposed partially therethrough. Proximal section 12 and distal section 14 may be disposed in a substantially fully contacting relationship as shown in Figure 2, with bore 18 aligned with hole 28 to form longitudinal cylindrical hole 24. Locking screw 30 is disposed through bore 18 and extends into hole 28.

As shown in Figure 2, joint ball 40 such as a spherically shaped modular head would be disposed about neck 22 before implantation. After implantation, modular head 40 would fit within the concave surface of liner 50, which is disposed within the concave surface of cup shell 60. The method of implantation is discussed further below. Proximal section 12, distal section 14, modular head 40, liner 50 and acetabular cup shell 60 are made of conventional materials which are known for use in such applications, and would be manufactured in a conventional manner. For example, sections 12 and 14 may be made of titanium alloy, modular head 40 may be made of CoCrMo alloy, liner 50 may be made of ultra high molecular weight polyethylene, and cup shell 60 may be made of titanium alloy.

Femoral stem section 10 is modular such that proximal section 12 and distal section 14 each would be manufactured of various sizes, for example, with various lengths and diameters. The range of lengths and diameters runs from 50-300 mm and 10-24 mm, respectively. However, the size and shape of tapered protrusion 26 and tapered recess 16 would be uniform for all distal and proximal sections, and in a preferred embodiment would have a length of 30 mm, a major diameter of 12.5 mm, and a minor diameter of 9 mm. Accordingly, the sections would be interchangeable such that any proximal section 12 would fit with any distal section 14. A prosthesis may be fitted to an individual patient by selecting a distal section and proximal section of appropriate size and securing them to each other by use of screw 30. The overall length of bore 18 and hole 28 also would be uniform. The size of neck 22 would be uniform for all proximal sections 12, with modular heads 40 of different diameters fitting thereon. Liner 50 and cup shell 60 would be selected to fit an individual patient.

Locking screw 30 is manufactured of a shape memory alloy (SMA). SMA has the characteristic that when the alloy is cooled below a predetermined first temperature and subsequently deformed from its original shape by expanding or contracting when at or below this temperature, the alloy will return to its original shape or overall dimensions upon being raised above a second predetermined temperature. In particular, elements made from SMA exist in an austenitic state in a certain temperature range. When the temperature of the element is reduced below a first predetermined temperature, the SMA is transformed into a martensitic state and is more deformable in this state by contracting or expanding its dimensions, for example, the length. Thereafter, when the element is heated above a second predetermined temperature at which the SMA transforms back into the austenitic state, the SMA is "activated" and restored to its original shape or dimensions by either contracting or expanding. In the range of temperatures between the first and second predetermined temperatures, the SMA does not change states, for example, austenitic SMA which has been cooled from an original temperature in this range to below the first predetermined temperature so that the SMA transforms to the martensitic state, will remain in the martensitic state even if heated back to the original temperature. The SMA will not undergo transformation back to the austenitic state until heated above the second predetermined temperature.

For example, the first predetermined temperature may be 20°c and the second predetermined temperature may be 40°c. Locking screws 30 are manufactured in the austenitic state to have predetermined dimensions, such as a predetermined length, and are then cooled below the first predetermined temperature so as to be in the martensitic state. Thereafter, screws 30 are deformed, for example by expanding the length. For example, screws 30 may be manufactured to have an initial length of 43 mm when in the austenitic state and be deformed to have an expanded length of 44 mm when in the martensitic state. Since the second predetermined temperature is higher than normally encountered temperatures, locking screws 30 may be shipped to a site where they will be used, such as a hospital, and will remain in the martensitic state and retain the same length until needed for assembly of a prosthesis, assuming appropriate thermal protection is used.

Femoral stem section 10 is constructed by the practitioner before installation into a patient. The practitioner selects a proximal section 12 and a distal section 14 of appropriate size, and assembles the sections together by inserting tapered protrusion 26 into tapered recess 16 so as to have a close fit as shown in Figures 1 and 2. Locking screw 30 while still deformed in the martensitic state is inserted through bore 18 from the top end thereof and is screwed into screw-threaded hole 28 to secure proximal section 12 to distal section 14. The overall length of cylindrical hole 24 preferably is slightly larger than the length of screw 30 such that a small clearance remains between the distal end of screw 30 and the bottom of hole 28, even after locking screw 30 is tightened to secure the sections together.

In order to lock distal section 14 to proximal section 12, the temperature of screw 30 is raised above the second predetermined temperature to transform the SMA back to the austenitic state. The shape memory property may be activated by placing the assembled femoral stem section 10 in warm water or by other means such as resistance heating or autoclaving. Screw 30 undergoes a constrained recovery, that is, the screw attempts to recover its length before being deformed but is blocked from making a full recovery due to its engagement with proximal section 12 and distal section 14 within which it is disposed. Thus, a compressive load is created and acts upon proximal section 12 and distal section 14, pulling proximal section 12 into even tighter contact with distal section 14, and locking or cinching the sections together. In one embodiment, upon activation, the screw will apply approximately 1500 lbs. of load on the sections.

By making use of the shape restoring properties of the SMA, the prior art problem of locking screw 30 potentially loosening and backing out of cylindrical hole 24 is reduced. Locking screw 30 is, in effect, self-locking in cylindrical hole 24. The exact lengths of screw 30 and cylindrical hole 24 are a design choice. It is preferred that the length of locking screw 30 be slightly less than the length of cylindrical hole 24 when screw 30 is disposed to secure proximal section 12 to distal section 14, that is, before activation of the shape memory effect.

In a preferred embodiment, locking screws 30 are made of alloys of Titanium and Nickel (TiNi), for example, the alloy sold under the name Nitinol. In particular, an alloy containing 50% titanium and 50% nickel is preferred. Such an alloy can be made to transform completely from the austenitic state to the martensitic state below 20 °c and will transform completely back to the austenitic state above 50-60°c, at which point the original length of unconstrained screws 30 will be recovered.

With reference to Figure 3, a second embodiment of the femoral stem section of a prosthesis according to the invention is shown. Femoral stem section 100 includes proximal section 112, distal section 114 and separate neck section 122. Proximal section 112 includes upper recess 116 and lower recess 119. Lower recess 119 is tapered radially outwardly in the downward direction. Upper recess 116 includes upper section 116a which is tapered radially outwardly in the upward direction and lower cylindrical-shaped section 116b. Ledge 117 is formed between lower section 116b and upper section 116a. Bore 118 extends through proximal section 112. Distal section 114 includes protrusion 126 extending upwardly from the top end thereof and tapered radially inwardly in the upward direction so as to substantially correspond to lower tapered recess 119. Screw-threaded hole 128 is formed in protrusion 126. Neck section 122 includes projection 122a which is tapered radially inwardly in the lower direction and cylindrical projection 122b which projects downwardly from the lower surface of projection 122a. Peripheral surface 123 is defined between projections 122a and 122b. Cylindrical projection 122b corresponds in shape and size to lower cylindrical-shaped section 116b, tapered projection 122a correspond in shape and size to upper tapered section 116a, and peripheral surface 123 corresponds in shape and size with ledge 117. Longitudinal bore 125 is formed through neck section 122. Neck section 122 further includes neck 124.

Femoral stem section 110 is assembled in a similar manner to section 10 disclosed in Figures 1 and 2. Neck section 122 is inserted within upper recess 116 of proximal section 112, with cylindrical projection 122b fitting within cylindrical- shaped section 116b, tapered projection 122a fitting within tapered recess 116a and peripheral surface 123 disposed on ledge 117. Protrusion 126 of distal section 114 is inserted within tapered recess 119 of proximal section 112. Bores 118 and 125 and hole 128 are aligned to form longitudinal cylindrical hole 127. As in the first embodiment, SMA locking screw 30, which is in the deformed martensitic state, is disposed through cylindrical hole 127 and tightened to secure the sections of femoral stem section 110 together. Thereafter, the shape memory effect is activated by heating locking screw 30 beyond the second predetermined temperature, transforming the SMA back to the austenitic state and causing locking screw 30 to attempt to contract back to its original length. That is, locking screw 30 undergoes a constrained recovery, and thereby applies a compressive load on proximal section 112, distal section 114, and neck section 122, pulling the sections into even tighter contact with each other, and locking the sections together.

In a preferred embodiment, as shown in Figure 3, head 30a of locking screw 30 can be manufactured to have an undersurface 30b which is mechanically textured or roughened. The roughened or textured surface increases the frictional holding power and further inhibits loosening when screw 30 undergoes recovery. The use of a roughened or textured surface, and the advantages obtained by such use, are not possible with prior art locking screws due to the particulate debris which are created during the torsional tightening.

After assembly of femoral stem section 10 (or 110) is completed, modular head 40 is disposed about neck 22 (or 122), and prosthesis 1 is ready for implantation into a patient. Acetabular shell cup 60 is inserted into the hip joint socket in the pelvic bone and secured thereto. Liner 50 is inserted within the concave surface of shell cup 60 and secured thereto in a known manner. Modular head 40 is inserted within the concave surface of liner 50 to secure the femoral stem section within the pelvic bone so as to pivot. The femoral stem section is fitted into the femur and secured thereto in a known manner, for example, by press fitting or cementing, to complete the implantation of prosthesis 1.

The manner in which the shape memory effect may be used to secure and lock acetabular cup shell 60 to the pelvic bone is discussed with reference to Figure 4a. Cup shell 60 includes screw holes 62 disposed therethrough. Screw holes 62 include smaller diameter radially outer portion 62a which is essentially cylindrical, and larger diameter radially inner portion 62b which is essentially hemispheric in shape. Cup shell 60 includes inner ledge 62c which is defined between portions 62a and 62b. The diameter of holes 62 is slightly decreased at the location of ledge 62c so that it is less than the diameter at outer portion 62a. Fixation or securing screws 64 are disposed through holes 62, and include screw- threaded shank portion 64a, head 64b and indented region 64c formed between shank portion 64a and head 64b. Screws 64 are made of SMA, for example, the alloys identified above with respect to locking screw 30.

As described above with respect to locking screw 30, screws 64 are cooled below the temperature at which the SMA is transformed to the martensitic state and are then deformed. In a preferred embodiment, screws 64 are deformed by being extended in length, and remain in that state at normal room or environmental temperatures until the prosthesis is implanted by the practitioner. The diameter of shank portion 64a in the un-recovered state is slightly less than the diameter of ledge 62c. Cup 60 is inserted into the hip socket of the pelvic bone, and is secured to the bone by screws 64 which are inserted through holes 62 and driven into the pelvic bone.

After screws 64 are fully driven into the bone, they are heated above the second predetermined temperature and are transformed back to the austenitic state. Screws 64 undergo a constrained recovery, for example, by contracting in length,

and therefore expanding in diameter, causing screws 64 to be press fitted to cup shell 60 to fill any gaps formed between cup shell 60 and screws 64, in particular, at the location of heads 64b. By expanding the diameter of the screws in this manner, the interface between screw heads 64b and the surfaces of cup 60 with which they are in contact are stabilized to prevent micro-motion therebetween. Thus, frictional wear and the resultant undesirable creation of small particles of metal are eliminated. Shank portion 64a also contracts within the bone, improving the fixation of the screw to the bone, thus providing an additional mechanism for locking the screws in place. The diameters and lengths of the screws and holes are a design choice.

With reference to Figure 4b, a further embodiment of the invention is shown in which fixation screws 64 are replaced by modular pegs 74. Pegs 74 also are made of SMA, and include shank portion 74a, head 74b and indented region 74c. As with screws 64, the diameter of shank portion 74a in the un-recovered state is slightly less than the diameter of hole 62 at the location of ledge 62c. Pegs 74 are deformed so as to be extended when in the martensitic state. Pegs 74 are driven into the pelvic bone by the practitioner to secure cup shell 60. Pegs 74 are then heated back to the austenitic state and contracted, providing an additional mechanism for locking the screws in place.

With reference to Figures 5a-b, a further embodiment of the invention is shown in which additional components of the prosthesis may be made of SMA. Femoral stem section 10' includes proximal section 12' and distal section 14'. Proximal section 12' includes neck section 20' having tapered neck 22'. Modular femoral head 140 includes upper spherical or ball portion 140b and lower cylindrical portion or collar 140a. Tapered recess 141 extends through collar 140a and into ball portion 140b. Modular femoral head 140 is disposed on proximal section 12' with neck 22' fitting within tapered recess 141. Channel 142 is formed about the periphery of collar 140a. Circular locking ring 144 is disposed in channel 142. Femoral stem section 10' also includes circular groove 146a formed in distal section 14'. Distal sleeve 146 is disposed in groove 146.

Locking ring 144 is disposed in channel 142 and also serves to secure modular head 140 on neck 22'. Locking ring 144 is made of SMA, and is shrink- fitted onto head 140 to eliminate any gaps which may exist between head 140 and neck 22' at the location of channel 142. Thus, SMA locking ring 144 further locks head 140 in place on neck 22' and reduces the potential for entrapment of particles in the incongruities between tapered recess 141 and tapered neck 22', thereby reducing the potential for crevice corrosion. Distal sleeve 146 is disposed about distal section 14' near the lower end thereof by sliding into circular groove 146a. Distal sleeve 146 serves as a spacer to ensure a tight fit between femoral stem section 10' and the femur. Distal sleeve 146 also may be made of SMA and is shrink-fitted onto distal section 14' in the same manner as described above, providing an improved mechanical locking of sleeve 146 to distal section 14'.

With reference to Figures 6a-b, a further embodiment of the invention is shown in which femoral stem section 10' includes modular collar 160 made of SMA. Proximal section 12' includes neck section 20'. Recessed channel 161 is formed in proximal section 12' at the base of neck section 20'. Channel 161 includes indented portions 162 formed therein on opposite sides of proximal section 12'. Collar 160 has an open oval shape, and includes projections 160a extending from the open ends. Collar 160 is fitted into channel 161 with projections 160a fitting within indented portions 162 to secure collar 160 in position, and thereby transform femoral stem section 10' from collarless to collared. Collar 160 is shrink-fitted on proximal section 12' by restoration to the austenitic state in the same manner as discussed above, to lock collar 160 in position.

With reference to Figures 7a-b, a further embodiment of the invention is shown in which femoral stem section 10' includes modular proximal spacer 200 made of SMA. Spacer 200 allows for augmentation and customizing of the femoral stem section. Stem section 10" includes collar 242 disposed at the base of the neck section. Spacer 200 is oval-shaped with an open center and is disposed about the lower portion of stem section 10" by sliding upwardly until it contacts collar 242. Spacer 200 is shrink-fitted on stem sections 10" in the same manner as discussed above with respect to sleeve 146, locking ring 144 and collar 160 in order to secure spacer 200 to femoral stem section 10". Although femoral stem section 10" is shown as comprising only one section, stem section 10" may be modular and include separate distal and proximal sections as shown in Figure 1.

Claims

1. A modular femoral stem prosthesis comprising: a proximal section; a distal section; a locking element disposed within said proximal section and said distal section to secure said proximal and distal sections to each other and prevent relative motion therebetween, said locking element made of a shape memory alloy which exists in a martensitic state below a first predetermined temperature and an austenitic state above a second temperature, said locking element having been formed in the austenitic state to have original dimensions and deformed from its original dimensions while in the martensitic state, wherein, said locking element may be restored to the austenitic state by increasing its temperature above the second predetermined temperature to provide a constrained recovery and lock the proximal and distal sections together.

2. The prosthesis recited in claim 1, said locking element comprising a locking screw, one of said sections comprising a tapered protrusion, the other of said sections including a recess having walls tapered so as to substantially correspond to said tapered protrusion, said tapered protrusion fitting within said recess, said locking screw extending through a bore formed through said other section and into a screw-threaded hole formed in said tapered protrusion to secure said sections.

3. The prosthesis recited in claim 2, said one section comprising said distal section, said other section comprising said proximal section, said proximal section further including a neck extending at an angle to the longitudinal axis of said hole.

4. The prosthesis recited in claim 3 further comprising a modular head disposed on said neck.

5. The prosthesis recited in claim 2 further comprising a neck section having a hole disposed therethrough, said locking screw also disposed through said hole in said neck section to secure said neck section to said proximal section, said neck section including a neck extending at an angle to the longitudinal axis of said hole through said neck section.

6. The prosthesis recited in claim 5 further comprising a modular head disposed on said neck.

7. The prosthesis recited in claim 1, said shaped memory alloy comprising an alloy of nickel-titanium.

8. The prosthesis recited in claim 1, said locking element comprising a locking screw formed in the austenitic state to have an original length and deformed by being stretched in length from the original length when in the martensitic state, said locking screw contracting in length when restored to the austenitic state.

9. The prosthesis recited in claim 1, said locking eleuent comprising a locking screw, said locking screw comprising a head having an undersurface which is textured.

10. The prosthesis recited in claim 1, said locking element comprising a locking screw, said locking screw comprising a head having an undersurface which is roughened.

11. A method of manufacturing a prosthesis for subsequent assembly and implantation in a patient, the method comprising the steps of: forming first and second sections, one of the sections including a bore extending therethrough, the other of the sections including a hole extending at least partially therethrough, the sections formed so as to allow the sections to fit together in substantial contact with each and with the bore and the hole aligned; forming a locking element made of a shape memory alloy which exists in a martensitic state below a first temperature and in an austenitic state above a second temperature, the locking element formed in the austenitic state with original dimensions which allows the locking element to be disposed within said bore and said hole when aligned and thereby secure said first and second sections in contact; cooling the locking element below the first temperature so as to transform the shape memory alloy to the martensitic state; and deforming the locking element when the shape memory alloy is in the martensitic state so as to have at least one dimension which is different from at least one of the original dimensions.

12. The method recited in claim 11, wherein, the locking element is formed as a locking screw and the hole is screw-threaded.

13. The method recited in claim 12, the prosthesis comprising a modular femoral hip stem prosthesis, the first section comprising a tapered protrusion, the second section comprising a tapered recess substantially corresponding to the tapered protrusion such that the tapered protrusion may be inserted within the tapered recess to allow the sections to be secured in contact with each other.

14. The method recited in claim 13, the first and second sections being formed in a plurality of sizes, the protrusion and the recess being formed of a uniform size for all of the plurality of sections, wherein, any of the first sections may be interchangeably fit with any of the second sections by insertion of the tapered recess in the tapered protrusion and secured by the locking screw.

15. A method of assembling a prosthesis out of component parts, the prosthesis including first and second sections and a locking means for locking the first and second sections together, one of the sections including a bore extending therethrough, the other of the sections including a hole extending at least partially therethrough, the sections formed so as to allow the sections to fit together in substantial contact with the bore and the hole aligned, the locking means made of a shape memory alloy which exists in a martensitic state below a first temperature and in an austenitic state above a second temperature, the locking means having been formed in the austenitic state with original dimensions which allows the locking means to be disposed within the aligned bore and hole and thereby secure the first and second sections in contact, the locking means having been cooled after formation to below the first temperature so as to transform the shape memory alloy to the martensitic state, and deformed when the shape memory alloy is in the martensitic state so as to have at least one dimension which is different from the corresponding original dimension, the method comprising the steps of: fitting the first section into contact with the second section with the hole and bore aligned; disposing the locking means in the bore and hole to secure the first and second sections together; and raising the temperature of the locking means above the second temperature to transform the shape memory alloy to the austenitic state and thereby cause the at least one different dimension of the locking means to undergo a constrained recovery and to thereby apply a compressive load upon and lock the first and second sections together.

16. The method recited in claim 15, wherein, the locking means comprises a locking screw and the hole is screw-threaded, and wherein the step of disposing the locking means within the bore and hole to secure the first and second sections together comprises engaging the screw threads with the locking means.

17. The method recited in claim 16, wherein, the prosthesis comprises a modular femoral hip stem prosthesis, the first section comprises a tapered protrusion, the second section comprises a tapered recess substantially corresponding to the tapered protrusion, the method further comprising fitting the tapered protrusion within the recess.

18. The method recited in claim 17, wherein, the first and second sections having been formed in a plurality of sizes, the protrusion and the recess having been formed of a uniform size for all of the plurality of sections, and wherein, any of the first sections may be interchangeably fit with any of the second sections by inserting the tapered recess into the tapered protrusion and secured by the locking screw.

19. The method recited in claim 16, the locking screw having been formed in the austenitic state to have an original length and deformed in the martensitic state to have a stretched length, wherein, the locking screw undergoes a constrained recovery when transformed back to the austenitic state.

20. A modular stem prosthesis comprising: a femoral stem section; an acetabular cup shell disposed in contact with said femoral stem section, said cup shell including at least one hole disposed therethrough; and a securing element disposed through and protruding from said hole so as to be disposable in a pelvic bone to secure said cup shell to the bone, said locking element made from a shape memory alloy which exists in a martensitic state below a first temperature and in an austenitic state above a second temperature, said securing element having been formed to have original dimensions while in the austenitic state and deformed from at least one of its original dimensions while in the martensitic state, wherein, said securing element may be restored to the austenitic state by increasing its temperature above the second predetermined temperature to undergo a constrained recovery and thereby be press fitted to said cup shell.

21. The prosthesis recited in claim 20, said securing element comprising a modular peg.

22. The prosthesis recited in claim 20, said securing element comprising a fixation screw.

23. The prosthesis recited in claim 20, said at least one hole comprising a plurality of holes, said prosthesis comprising a plurality of securing elements, one said securing element disposed through each of said holes.

24. The prosthesis recited in claim 20, said shape memory alloy comprising an alloy of nickel and titanium.

25. In a modular hip stem prosthesis for implantation into a patient to join the patient's femur and pelvic bone, the prosthesis including an acetabular cup shell, the cup shell including at least one hole disposed therethrough, a method for manufacturing securing elements for use in securing the cup shell to the pelvic bone by being disposed through the holes and into the pelvic bone, the method comprising the steps of: forming the securing elements from a shape memory alloy which exists in a martensitic state below a first temperature and in an austenitic state above a second temperature, the securing elements formed to have original dimensions while in the austenitic state; cooling the securing elements below the first temperature to transform the alloy to the martensitic state; and deforming the securing elements from at least one of their original dimensions while in the martensitic state, wherein, the securing elements may be heated above the second temperature to transform the shape memory alloy back to the austenitic state and cause the securing elements to undergo a constrained recovery after being disposed into the pelvic bone.

26. The method recited in claim 25, the securing elements formed to have an original length and deformed by extending the length.

27. The method recited in claim 25, the securing elements comprising modular securing pegs.

28. The method recited in claim 25, the securing elements comprising fixation locking screws.

29. In a modular hip stem prosthesis for implantation into a patient to j oin the patient's femur and pelvic bone, the prosthesis including an acetabular cup shell having at least one hole disposed therethrough, and at least one securing element made from a shape memory alloy which exists in a martensitic state below a first temperature and an austenitic state above a second temperature, the securing element having been formed in the austenitic state to have original dimensions, cooled below the first temperature to transform the alloy to the martensitic state and deformed from the original dimensions while in the martensitic state, a method for securing the cup shell to the pelvic bone, the method comprising the steps of: disposing the securing element through the hole and into the pelvic bone to secure the cup shell to the pelvic bone; and heating the securing element above the second temperature to transform the shape memory alloy back to the austenitic state and thereby causing the securing element to undergo a constrained recovery to press fit the securing element to the cup shell.

30. The method recited in claim 29, the securing element formed to have an original length and deformed by extending the length.

31. The method recited in claim 29, the securing element comprising a modular securing peg.

32. The method recited in claim 29, the securing element comprising a fixation locking screw.

33. In a modular hip stem prosthesis for implantation into a patient to join the patient's femur and pelvic bone, the prosthesis including a femoral stem section having a proximal section and a distal section, a method for manufacturing securing elements for use in securing the proximal section to the distal section, the method comprising the steps of: forming the securing elements from a shape memory alloy which exists in a martensitic state below a first temperature and in an austenitic state above a second temperature, the securing elements formed to have original dimensions while in the austenitic state; cooling the securing elements below the first temperature to transform the alloy to the martensitic state; and deforming the securing elements from their original dimensions while in the martensitic state, wherein, the securing elements may be heated above the second temperature to transform the shape memory alloy back to the austenitic state to cause the securing elements to undergo a constrained recovery after being disposed to secure the proximal section to the distal section to thereby lock said proximal section to said distal section.

34. The method recited in claim 33, said proximal section and said distal section having holes disposed therein, the securing elements comprising locking screws which are disposed in the holes to secure the proximal section to the distal section.

35. In an implantable orthopaedic device for implanting within the human body and having an implantable orthopaedic element and a fastener for connecting the orthopaedic element in its desired position, the improvement wherein: said fastener is made from a shape memory alloy and has an original shape, so that, when said fastener is cooled to its martensitic state and is subsequently deformed, said fastener will retain its deformed shape while in the martensitic state, but when said fastener is warmed to its austenitic state said fastener will return to its original shape; whereby, said fastener, when warmed to its austenitic state while disposed to connect the orthopaedic element in its desired position, undergoes a constrained recovery and locks said orthopaedic element into its proper position.

36. The improvement recited in claim 35, the device comprising a femoral stem prosthesis including a femoral stem section having a neck, said element comprising a femoral head disposed on said neck, said fastener comprising a locking ring disposed about said head.

37. In a prosthetic device for implanting within the human body, said device including a femoral stem section and at least one additional component disposed on said femoral stem section, the improvement wherein said additional component is made of a shape memory alloy which has an original shape in an austenitic state, and when cooled to its martensitic state and subsequently deformed, will retain its deformed shape while in the martensitic state, but when warmed back to its austenitic state will recover its original shape, whereby, said component when warmed back to its austenitic state while disposed on said femoral stem section is locked in place relative to said femoral stem section by undergoing constrained recovery.

38. The improvement recited in claim 37, said component comprising a distal sleeve disposed about a distal part of said femoral stem section, said distal sleeve deformed by expansion from its original shape and locked in place by undergoing constrained contraction.

39. The improvement recited in claim 37, said femoral stem section comprising a neck, said component comprising a collar disposed about said section at the location of said neck, said collar deformed by expansion from its original shape and locked in place by undergoing constrained contraction.

40. The improvement recited in claim 37, said femoral stem section comprising a neck, said component comprising a spacer disposed about said femoral stem section below said neck, said spacer deformed by expansion from its original shape and locked in place by undergoing constrained contraction.