CA2208796A1

CA2208796A1 - Prosthesis with variable fit and strain distribution

Info

Publication number: CA2208796A1
Application number: CA002208796A
Authority: CA
Inventors: Jorge A. Ochoa; Michael J. O'neil
Original assignee: Jorge A. Ochoa; Michael J. O'neil; Johnson & Johnson Professional, Inc.
Current assignee: Johnson and Johnson Professional Inc
Priority date: 1996-06-28
Filing date: 1997-06-25
Publication date: 1997-12-28
Also published as: EP0815809A2; JPH10137333A; DE69728001T2; EP0815809A3; DE69728001D1; US5935172A; EP0815809B1

Abstract

A joint prosthesis has a first part, including a body formed of metal or the like which constitutes a permanent overall shape and structural component of the prosthesis, such as a shell, plate or stem, and a plurality of negative surface features in the body, such as through-slots, deep grooves, tunnels or pits, or valleys defined between projecting fingers or flutes. A second part attaches to and extends the body to provide both a fit and change the initial stiffness. The second part provides a time-evolving structural coupling, such that the prosthesis initially fits the patient's remnant bone to provide rigid fixation, while the mechanical properties shift with time in vivo to change its contact or loading characteristics. In one embodiment a femoral stem joint prosthesis is modular and the first, or structural component accommodates bio-absorbable second components of varying geometries and dimensions which fit a range of bore sizes, and achieve different stiffnesses or strengths affecting load or strain distribution.

Description

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PROSTHESIS WlTH VARIABLE FIT
AND STRAIN DISTRIBUTION

BACKGROUND OF THE INVENTION

3 The invention relates to implantable bone prostheses, and more particularly to joint 4 prostheses that attach to bone and have properties, such as fit and fill, which change after implantation.

7 Joint prostheses are well known in the art and have long been used to replace natural 8 joints, in~lu-ling knees, hips, shoulders, and elbows. Such prostheses may include a projecting 9 part, such as a stem configured to mount in or attach to the remaining natural bone and secure the prosthesis. The size, shape and materials of a joint prosthesis are critical to ensuring proper 11 fit within a patient's body, and may also affect the extent of bone growth into and surrounding 12 the prosthecic, both of which contribute to fixation of the prosthesis within the patient.

14 Earlier versions of joint prostheses have relied extensively upon bone cements to fix a prosth~-cic within natural bone.

17 Cements provide the high degree of initial fixation necessary for healing following 18 surgery but result in a very stiff overall structure, are prone to loosening with time, and can 19 provoke tissue reactions or systemic responses. Nonetheless they remain widely used, although in current practice they may be applied over smaller regions than before, or be used in 21 conjunction with other modes of fixation. Over the longer term, fixation now also relies on the 22 provision of textured regions, and regions which enhance bone growth by providing a structure 23 or framework of porous material, with or without coatings of bone-growth enhancement 24 materials, such as hydroxyapatite or calcium oxide materials and various organic growth promoters, to bring about secure coupling by intergrowth of new bone material. In some 26 constructions, a bio-absorbable material is also used to fill irregularities, allowing pores or CA 02208796 1997-06-2~

cavities in a permanent metal body to open up and become filled in coordination with ong,oing 2 regeneration and ingrowth of the surrounding bone.

4 With these more complex constructions, resorption of a coating or filler, and its S replacement by bone ingrowth can cause the properties of the prosthesis (e.g., stiffness) to 6 change over time. For example, a particular joint prosthesis, such as a hip stem, may have a 7 stiffness which is acceptable at the time of implantation, but which becomes either too flexible 8 or too stiff subsequent to implantation as a result of bone ingrowth and dissolution of material.
9 Conversely, engineering an implant with properties at levels which are desirable for the long term may result in a prosthesis which has prope,lies that are not suitable at the time of implant.

12 Some attempts have been made to improve the fit and fill characteristics of joint 13 prostheses during healing by regulating the timing of bone ingrowth, generally by selection of 14 pore size and coating solubility.
16 U.S. Patent No. 4,713,076 discloses a bone implant coating formed of a calcium-based 17 filling material and a binding agent, both of which are bio-absorbable. The filling m~ l is 18 in the form of porous spherical particles with a diameter of about 10-200 micrometers and a pore 19 volume of 25-80%. The ultrastructure of the coating is stated to promote rapid ingrowth of bone into the coating.

22 U.S. Patent No. S,258,034 discloses a hip stem prosthesis that provides controlled bone 23 ingrowth. A bone ingrowth-promoting coating is provided at a proximal part of the prosthesis 24 to promote rapid bone ingrowth at that region of the prosthesis. The distal end of the prosthesis, where rapid bone ingrowth is not desired, includes an absorbable coating or sleeve. The 26 thickness of the absorbable coating or sleeve is designed to be absorbed over the period of time 27 during which bone ingrowth at the proximal end of the femoral stem would normally take place.

29 U.S. Patent No. 5,007,931 discloses a femoral stem implant in which longitudinal channels are made in the stem to reduce its section modulus, and elongated strips of a porous ' CA 02208796 1997-06-2~

material are bonded only to the floors of the channels keeping destr~lctive bending stresses low 2 and avoiding notching. The stem is shaped for cementless fixation in a bore prepared in the 3 femur.

S Despite advances made thus far in the design of joint prostheses, there remains a need 6 for a joint prosthesis in which the mechanical and physical properties of the prosthesis stay 7 appropriate in different phases of the post-implantation period.

9 It is thus an object of the invention to provide a joint prosthesis in which mechanical and physical plopelLies, including shape, size, stiffness, strength and density, vary over time in a 11 controlled fashion after implantation.

13 A further object is to provide a joint prosthesis having physical and mechanical properties 14 that evolve from a first plurality of properties which are advantageous at the time of implantation to a second plurality of properties which are advantageous at a later time.

17 A further object is to provide a joint prosthesis that possesses short term stiffness to 18 promote good fixation, and evolves long term bending flexibility to improve its performance and 19 useful life.
21 It is yet another object to provide a joint prosthesis that is modular in the sense that 22 adjustm~ntc and modifications to the prosthesis are readily made by a surgeon in the operating 23 room environment by interch~nging or modifying components to tailor the device to a specific 24 patient.
26 The attainment of these and other objects of the invention will be apparent to those skilled 27 in the art upon reading the disclosure that follows.

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SUMMARY OF THE INVENTION

3 The present invention achieves one or more of the foregoing objects by providing a joint 4 prosthesis having a first part, including a body formed of metal or the like which is permanent and which constitutes the essential overall shape and structural component of the prosthesis, such 6 as a shelf, plate or stem. The body has a plurality of negative surface features such as deep 7 grooves, tunnels or pits, or valleys or slots defined between or within projecting walls, wings, 8 fingers or flutes. A second part, which is non-permanent, is attached to the body, and provides 9 both a fit, and a time-evolving structural coupling tO surrounding bone, such that the prosthesis initially fits the patient's remnant bone, while the mechanical properties evolve and shift with 11 time in vivo to change the position or distribution of its coupling to bone along its length.

13 In one aspect of the invention, illustrated by a femoral stem, the negative surface features 14 formed in the body or structural component take the form of axially extending grooves made, for example, at the distal end of the metallic structural component or at a position intermediate 16 the proximal and distal ends of the metallic structural component. The negative surface features 17 can also include one or more transversely oriented grooves formed in the met~llic structural 18 co,.,ponent, or one or more bores formed in the metallic structural component. The negative 19 surface features may also be defined by axial flutes in a beam-like structure, or by slots which separate the body into prongs or fingers. Preferably, the second part is a bio-absorbable 21 component which is adhered to, frictionally engaged within or mechanically interlocked with the 22 negative surface features and which thereby augments the structural body, for example, by 23 increasing its size and/or bending stiffness, and forms a compound unitized pros~hesic. The 24 second part extends at least partially beyond or outside of the structural body to provide initial points of contact against surrounding bone in at least one region thus providing an initially rigid 26 fixation.

28 In one embodiment, the joint prosthesis is a modular set and the structural component 29 has formed therein one or more negative surface features that accommodate different ones of a s~ ct~ble set of bio-absorbable inserts or attachments of varying geometries and dimensions.

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These second parts mecil~llically interfit ~4ith the stmctural component, and may come in a range 2 of sizes or shapes such as wedges, or plugs, caps or sleeves of different diameters to fit a range 3 of femoral hollow sizes. They also may be formed of materials having different resorbabilities, 4 stiffness~s or strengths for affecting load or strain distribution. This enables a surgeon to 5 assemble or modify during the course of a surgical procedure a configuration and size of a 6 prosthesis that is best for a given patient at the time of implant. Over time, and after 7 implantation, the properties of the implant change as the second component is resorbed and the 8 overall mechanical properties of the compound prosthesis and its attachments evolve.

In an illustrative implementation, a hip prosthesis has a femoral stem with metal stem 11 body and a biocompatible and bio-absorbable second component installed at a distal end of the 12 stem. The configuration and dirnensions of the bio-absorbable component are such that it has 13 a diameter that is greater than the nominal diameter of the first component of the hip stem, and 14 fits a bore made by one of the standard size femoral canal straight or tapered bone reamers. The 15 second component provides a distally tight fit at the time of implantation and augments the stem 16 to provide a relatively rigid structure. However, after implantation, the distal end bio-absorbable 17 co~l-pollent resorbs, increasing distal flexibility. During this post-implantation phase, new bone 18 growth concurrently provides a more rigid attachment at the proximal end. The less rigid shaft 19 thereby shifts its load and strain distribution so that an increasing portion of the load is applied 20 at the femoral metaphysis and is carried by the intermediate portion of the natural femur. This 21 results in a more natural loading of the femur with a strain distribution that counteracts bone 22 atrophy and resorption. Thus, the modular construction largely avoids bone stress shiplding.

24 The bio-absorbable co~ onent itself need not have ~~esses, and may take various forms such as a coating, wedge, plug, cap, sleeve or insert. In general it serves both to provide initial 26 fit, and to control or guide the speed and extent of bone ingrowth. These two functions may be 27 achieved with a modular component. Alternatively, separate isolated bio-absorbable portions, 28 having the same or different resorption and growth characteristics, may implement both or 29 prim~nly a single one of these functions. For achieving initial fit and fixation, the second component may include projections such as blade-like fins oriented along a direction of insertion, CA 02208796 1997-06-2~

that, upon insertion, pro~ide an irrotational "scratch fit" against surrounding bone. It may have 2 one or more wing-like projections for anchoring or positioning the body. Furthermore, the bio-3 absorbable component need not be a separate modular piece, but may include a mass of irregular 4 topology that extends into and through recesses or tunnels in the first, structural component, and S projects outward therefrom to provide the initial external fit and a subsequent dissolution path 6 for natural bone ingrowth. In this case the mass may be molded in situ, for example by using 7 a deformable or viscous organic polymer, with a degree of cross-linking or hardening tailored 8 to achieve the desired resorption time. Similarly, the second component may include coatings 9 at one or more regions of the prosthesis, positioned to either enhance an initial interference fit, or promote later bone growth, together with separate, modular attachments.

12 Advantageously by providing a bio-absorbable and a metal component together in a 13 compound or modular prosthesis, applicant separately and independently tailors the initial and 14 ultimate values of fixation strength, strain transfer and bending stiffness of the prosthesis at two or more positions along the device. This new architecture achieves a high degree of initial 16 fixation and strength without sacrificing the mechanical characteristics required for long term 17 bone growth and compatibility.

21 Th,ese and other features of the invention will be understood from the description below 22 together with illustrative examples and drawings, wherein:

24 FIGURE 1 shows a two-part modular prosthesis in accordance with the present invention;
26 FIGURES 2A-2C show sections through the prosthesis of FIGURE 1;

28 FIGURES 3, 3A and 3B show another embodiment of a prosthesis in accordance with 29 the present invention;

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FIGURES 4, ~A and ~B show another prosthesis;
3 FIGURE 5 shows representative bending stiffness of the prosthesis of FIGURES 1 or 3;
4 and 6 FIGURE 6 is a conceptual graph illustrating the form of evolving stress distribution in 7 bone.

11 FIGURE 1 shows a side view of a hip prosthesis 100 according to a replesel-tative 12 embodiment of the present invention. Prosthesis 100 includes an elongated body or structural 13 part 110 and a distal cap or fitting part 120. The body 110 is adapted for insertion into the 14 hollow center of a femur, and extends from a metaphyseal region 114 shown at the top in the drawing to a distal end 116, with the stem 115 in the intermediate portion being generally 16 tapered along two planes in a known manner for fitting within the femur. The structural body 17 110 is formed of bio-compatible metals or alloys. Exemplary metals include cobalt and titanium, 18 while exemplary alloys include Ti6AI4V or CoCrMo. In the illustrated embodiment, the distal 19 end 116 has the form of four separate elongated fingers 116a through 116d, shown in cross-section in FIGURE 2A. Recause of their relatively small cross section, these fingers 16a 21 through 116d are capable of a relatively high degree of flexion or bending along their length in 22 a manner similar to but of lesser magnitude than a clothespin. It will be understood that 23 FIGURE 1 is intended to show a generic external contour, which might be more rounded, more 24 angular or otherwise correspond to known prostheses, and that as to the distal end, the FIGURE
is a schem~ic ~eplesentation only. That is, a person skilled in the art will understand that the 26 actual contour of the fingers, particularly at their junction with the solid mid portion 115 of the 27 stem, is to be configured to avoid the generation of stress cracks and provide a long and useful 28 fatigue life of the prosthesis. In general however it will be noted that the shape of the distal end 29 of the prosthesis involves deep grooves or entire through-slots 117 which separate the body of the stem into smaller more bendable beam-like structural portions.

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The second component of the illustr~ted prosthesis is the dista] end cap 120, one 2 embodiment of which is shown in FIGURE 1, and illustrated in cross section in FIGURE 2B.
3 Unlike the structural component 110, the end cap 120 is formed of a temporary material such 4 as polylactic acid (PLA) or polydixanone (PDS), which is both bio-absorbable when implanted in the body, and is configured to extend beyond or protrude from the metal component of the 6 distal stem portion. Suitable bio-absorbable materials include polyamino acids, polyacetates, 7 polyglycolates, poly (p-dioxanone), co-condensates thereof, copolymers thereof, gelatin, 8 collagen, and calcium phosphate-based materials. As illustrated, the cap 120 is an elongated 9 sleeve which fits around the fingers 116a through 116d, and fills the gaps therebetween, resulting in a solid but compound, i.e., two-material, tip structure. The sleeve has a generally 11 cylindrical portion 127 which fits around the stem, and a plurality of cross-bars 123; 125 or 12 spoke-like members positioned to slide into the slots 117 of the body 110. The members 123, 13 125 may be dimensioned thicker than the slots 117, and may have a Shore D hardness selected 14 such that by wedging into the slots 117, the members are placed under compression to couple the fingers 116 together. This rigidifies the distal end 116 at the time of implantation. Thus, 16 by filling the gaps between fingers, the resultant structure has a much higher bending stiffness 17 than the metal component alone, while it attains a precise outer contour for enhanced fit to a size 18 defined by the bio-absorbable component 120. Thus, both the diameter and the bending stiffness 19 are augmented by the bio-absorbable component, and, significantly both of these structural properties decrease as component 120 is resorbed.

22 Of course, depending on the particular configuration of the gaps or negative surface 23 features of the elongated body and the end cap, only a portion of the gaps or voids may be 24 filled. In other embodiments the gaps or voids are substantially filled, and in yet other embodiments the gaps and voids are entirely filled. The bio-absorbable component can also 26 protrude from the external surface of the femoral stem by a distance of about .001 inches to .250 27 inches, and more preferably by a distance of about .004 inches to .12 inches.

29 As shown generally in FIGURE 1, the bio-absorbable component 120 of this embodiment fits like a sock or elongated cap over the distal end of the prosthesis and contains elongated CA 02208796 1997-06-2~

ridges 121 oriented alon~ the direction of insertion in the fem~lr. The ~ery end portion 122 2 lacks these relief features and instead provides a gently bulbous rounded surface for tightly 3 fitting against a prepared bore formed in the femur. As further shown in FIGURE 2B, the cross 4 section of end sleeve 120 constitutes a segmented body configured to extend into the gaps or 5 grooves 117 of the metal component, and firmly interlock with and fill that component to thus 6 structurally augment it and provide an overall solid cross section of the prosthesis when the two 7 components are assembled together. Slight protrusions or indentations 124 may also be provided 8 internally to firmly locl~ the bio-absorbable component in place on the metal shaft.

As further shown in FIGURE 1, the prosthesis 100 has an upper or metaphyseal fitting 11 region 130 which is configured to contact surrounding bone. Conventionally, this region is fitted 12 less precisely to the bone than the distal region, owing largely to the greater variation in size and 13 shape of the femoral metaphysis. This region is fastened by a cement layer to surrounding bone, 14 or is provided with a textured region and a coating to promote bone growth so that through this 15 growth process ultimately the prosthesis later becomes rigidly coupled in its top portion directly 16 to the femur.

18 FIGURE 2C illustrates a horizontal cross section through the prosthesis in the region 130.
19 As shown therein, the stem 110, illustratively a solid body, has an external coating 135 which 20 as illustrated in FIGURE 2C, is a thin shell or layer covering the bone contact and regrowth 21 region in this metaphyseal area. It will be understood that this region may also be textured and 22 have three dimensional relief features of a conventional kind to enhance trabecular bone growth 23 and promote the long-term formation of a shear-free and irrotational coupling. ~o far as relevant ~ 24 - hereto, any of the coating and texturing processes of the prior art may be applied to this region.
25 In general, the large diameter and greater contact area in this region allow a very strong 26 coupling to ultimately be achieved. However, as is well known in the art, when the healing 27 process results in the prosthesis being firmly attached in the region 130 and remaining firmly 28 attached at its distal end, the sharing of load between the prosthesis and surrounding bone can 29 result in the intermediate portions of bone bearing very little load and being subjected to very 30 little stress. This phenomena, known as bone stress shielding, is addressed in accordance with CA 02208796 1997-06-2~

a principal aspect of the present invention by the provision of the bio-absorbable component 120 2 at the distal end which over time disappears or is replaced by ne~v bone growth so that only the 3 structural component l lS remains. As noted above, the distal end structural component has a 4 low bending stiffness. For example, it may be subject to deflections of ten microns or more S when subjected to a normal load caused by movement of the body. Furthermore, the outer shell 6 127 in some embodiments is compounded to resorb faster than it is replaced by new bone 7 growth, so that once effective coupling has occurred in the metaphyseal region 130 the distal end 8 may become free. This assures that intermediate portions of the femur will bear a high 9 proportion of the load and that the bone will remain stressed in use. The long term loading of the femur effected after metaphyseal attachment in region 130 is therefore substantially similar 11 to that of the natural bone.

13 FIGURES 3A and 3B illustrate other stem/cap configurations in which a bio-absorbable 14 component 120' is configured to provide a high degree of initial fit and stability while allowing a metallic distal stem ~ortion of lesser bending stiffness to reside permanently implanted. As 16 shown in FIGURE 3A, the sleeve 120' may be a roughly cylindrical sleeve which fits around 17 a solid rod or shaft distal end portion of the metallic component. Like component 120 of 18 FIGURES 1 and 2, this illustrated sleeve 120' has an outer surface with a plurality of blade-like 19 protrusions 121' which extend radially outward, and may score the bone as the prosthesis is longitudinally inserted, to provide a secure grip against rotation of the implant. In this 21 embodiment, a second bio-absorbable component 140 is separately fitted over the end of the 22 sleeve 120', and has the shape of a bullet or plug having a fixed internal diameter matched to 23 the sleeve 120'. The second component 140 is one of a set in which a modular selection of 24 different size external diameters are configured for the different size bone reamers commonly used for prosthesis installation. The end block, bullet or plug 140 may be formed of a different 26 material than the sleeve 120', and may, for instance, have faster or slower resorption 27 characteristics, be provided with a loading of bone growth enhancement material, be made of 28 a softer material to provide fit without increasing bending stiffness, or may otherwise be of a 29 composition to specially tailor its resorption time and mechanical properties.

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While each of the above embodilllents h~s shown a separately-tltted distal component, 2 FIGURE 4 shows an embodiment wherein a bio-absorbable component 120'' is non-3 interchangeably interfitted with the stem body. In this construction, the metal stem shaft is a 4 tapered hollow cylindrical shell of undulating, polygonal or star-like shape, and the bio-5 absorbable component is interfltted or affixed to the stem in a more complex fashion to fill an 6 interior region of the stem, for example, by an in silu molding or casting process rather than by 7 assembly from modular components at the surgical site. As before, it extends outwardly to 8 provide the initial areas of contact with surro~ ding bone.

FIGURE 4A illustrates a cross-section of the stem, showing the interconnection of the 11 bio-absorbable core and the structural shell. As before, a bullet-like end cap 150 similar to cap 12 140 may be provided for fit. This cap may be a separate item, or may be integrally formed with 13 the component 120".

As described above, the present invention advances the prior art engineering of bone 16 prostheses by providing a structure in which the distal stiffness and fit evolve over time and, 17 moreover, may follow different change function dictated by the physical structure of the stem, 18 and by the modulus and "solubility" of the bio-absorbable components. Representative forms 19 of the bending and axial stiffness are shown in FIGURES 5 and 6, respectively, at two points 20 in time, indicated by curves "B" (initial implantation) and "C" (after implantation). The post-21 implantation curve is taken to be a time weeks or months later when both substantial distal 22 component resorption and adequate metaphyseal regrowth have occurred. However, as noted 23 above, each of these latter processes may be separately and independently modified by adjusting 24 the compressibility and solubility of the non-metal portions and by use of appluyliate growth 25 promoters.

27 In both FIGURES 5 and 6, the stiffness of a representative natural femur is shown in 28 curve A. In each case, the stiffness of the prosthesis upon implantation (curve B) is generally 29 higher at all points along the prosthesis than that of the original bone. This is done simply to assure adequate overall strength in the immediate post-operative period while reducing the strain in the metaphyseal region to allow healing. However, time progress, as shown in curve C, the 2 stiffness distribution shifts markedly. In particular, the bending stiffness in the distal region may 3 be reduced to zero by employing a fast-resorbing polymer that completely disappears between 4 the stem and surrounding bone by the time proximal end growth occurs. Alternatively, with a S multi-finger embodiment as shown in FIGURE~ 1 and 2 and a slowly resorbed wedge 6 component, a low but non-zero bending stiffness may be assured in that region over a protracted 7 time. A range band around curve C indicates these design features.

9 It must be understood that the illustrated values are merely representative of an exemplary embodiment of the invention and that the values will vary considerably depending upon the 11 configuration and materials selected. However, in one configuration, resorption of the bio-12 absorbable component reduces bending stiffness of the distal region of the prosthesis to under 13 approximately 0. le+9 mm4 (N/mm2) .

As further shown in these Figures, the proximal stitfness may remain relatively constant, 16 with the only change being a result of degree of attachment in that region, so that curves B and 17 C are substantially identical. Alternatively, if stiffness is provided by filling or augmenting a 18 hollow shell in the metaphyseal region, the stiffness may decrease as the filler is resorbed, so 19 that the stiffness of the regrown bone with prosthesis body is comparable to that of the filled prosthesis, and after implantation, load becomes transferred primarily through bone. In this 21 case, each end of the prosthesis changes its actual stiffness to accommodate the burden taken up 22 or released by the regrowth of bone and the changes in the other end of the prosthesis.

24 The invention being thus described, its structure and operative methods of practice will be readily applied and adapted to diverse Known prosthesis constructions, and further variations 26 and modifications will occur to those skilled in the art. All such adaptations, variations and 27 modifications are considered to be within the scope of the invention, as set forth in the claims 28 appended hereto.

Claims

1. A joint prosthesis, comprising:
a metallic structural component adapted to fit within a cavity formed in existing bone;
a negative surface feature formed in the metallic structural component; and a bio-absorbable component filling at least some of the negative surface feature.

2. The joint prosthesis of claim 1, wherein the metallic structural component forms a part of a joint implant selected from the group consisting of a knee joint femoral component, a knee joint tibial component, a hip femoral stem, a hip acetabular shell, an elbow implant, a shoulder implant, and a vertebral implant.

3. The joint prosthesis of claim 1, wherein the metallic structural component includes a longitudinal axis and the negative surface includes a groove formed within the metallic structural component at a distal end of the metallic structural component, wherein the groove is substantially parallel to the longitudinal axis of the metallic structural component.

4. The joint prosthesis of claim 1, wherein the metallic structural component includes a longitudinal axis and the negative surface includes a groove formed within the metallic structural component at a position intermediate a proximal and a distal end of the metallic structural component.

5. The joint prosthesis of claim 1, wherein the negative surface feature includes a through-slot formed within the metallic structural component.

6. The joint prosthesis of claim 5, wherein the through-slot is formed adjacent a distal end of the metallic structural component.

7. The joint prosthesis of claim 1, wherein the negative surface feature includes a bore formed in the metallic structural component.

8. The joint prosthesis of claim 1, wherein the structural component is a shell having a longitudinal axis and the negative surface feature includes a tunnel formed in the shell that is parallel to the longitudinal axis.

9. The joint prosthesis of claim 1, wherein the metallic structural component is substantially elongate and the negative surface feature is defined by a plurality of fingers at the distal end of the metallic structural component.

10. The joint prosthesis of claim 9, wherein two perpendicular slots are formed in the metallic structural component.

11. The joint prosthesis of claim 1, wherein the bio-absorbable component is formed from a material selected from the group consisting of polyamino acids, polyacetates, polyglycolates, poly (p-dioxanone) co-condensates thereof, copolymers thereof gelatin, collagen, and calcium phosphate-based materials.

12. The joint prosthesis of claim 1, wherein the bio-absorbable component is frictionally or compressively engaged with the negative surface feature.

13. The joint prosthesis of claim 1, wherein the negative surface feature includes internal structural features with which a portion of the bio-absorbable component mates in a mechanical interlocking arrangement.

14. The joint prosthesis of claim 11, wherein the bio-absorbable component has a textured outer surface.

15. The joint prosthesis of claim 14, wherein the textured outer surface of the bio-absorbable component includes a microscopic surface texture.

16. The joint prosthesis of claim 1, wherein the metallic structural component has textured, bone ingrowth surfaces formed in one or more portions of a bone engaging surface thereof.

17. The joint prosthesis of claim 1, wherein the bio-absorbable component is of dimensions sufficient to cause the metallic structural component to fit within the cavity formed in existing bone in an interference fit due to mechanical interference between bone and the bio-absorbable component.

18. The joint prosthesis of claim 1, wherein the negative surface feature is adapted to receive one of a plurality of bio-absorbable components having differing external geometries and dimensions.

19. The joint prosthesis of claim 1, wherein the metallic structure is a femoral stem and the negative surface feature includes a circumferentially oriented groove formed in an external surface of the femoral stem between the proximal and distal ends thereof, and the bio-absorbable component is a circumferentially oriented rib disposed in and protruding from the negative surface feature.

20. The joint prosthesis of claim 1, wherein the bio-absorbable component includes a rib formed within and protruding from the negative surface feature.

21. The joint prosthesis of claim 1, wherein the bio-absorbable component includes a distal cap that is adapted to fit over a distal end of the metallic structural component to engage and interlock with at least a portion of the negative surface feature.

22. The joint prosthesis of claim 1, wherein the negative surface feature includes at least one axially oriented elongate slot disposed in the distal end of the metallic structural component to form a femoral stem having a discontinuous distal end, and the bio-absorbable component is a distal cap that is adapted to engage and interlock with at least a portion of the negative surface feature fitting over the distal end of the metallic structural component to fill and strengthen the stem.

23. The joint prosthesis of claim 1, wherein the metallic structure includes a femoral stem, the negative surface feature includes a bore formed within the femoral stem, and the bio-absorbable component has a first end adapted to fit securely within the bore and a second end that protrudes from the bore to alter the geometrical profile of the femoral stem and to provide an external fitting surface.

24. The joint prosthesis of claim 22, wherein the bio-absorbable component protrudes from an external surface of the femoral stem by a distance of about .001 inches to .250 inches.

25. The joint prosthesis of claim 22, wherein the bio-absorbable component protrudes from an external surface of the femoral stem by a distance of about .004 inches to .12 inches.

26. The joint prosthesis of claim 1, where resorption of the bio-absorbable component reduces bending stiffness of the distal region of the prosthesis to under approximately 0.1e +9 mm4 (N/mm2).

27. A hip joint prosthesis, comprising:
a femoral stem adapted to fit within a cavity formed in a femur;
a plurality of negative surface features formed in the femoral stem; and a bio-absorbable component filling at least some of the plurality of negative surface features.