CA1258555A

CA1258555A - Implant with resorbable stem

Info

Publication number: CA1258555A
Application number: CA000487891A
Authority: CA
Inventors: Stanley L. Kampner
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-09-06
Filing date: 1985-07-31
Publication date: 1989-08-22
Also published as: JPH0458985B2; ATE40519T1; JPS6194650A; EP0176711A1; DE3568004D1; EP0176711B1; US4990161A

Abstract

IMPLANT WITH RESORBABLE STEM

ABSTRACT OF THE DISCLOSURE
This invention relates to a biodegradable an-chor for a permanent implant of a bone joint, e.g. hip, shoulder, knee or finger. The biodegradable anchor is an elongated member which has an exterior surface which tightly engages a cavity in the bone and is substantially immovable within the cavity upon implant. A non-resorb-able means of securing the anchor to the permanent im-plant is also included.

Description

1~58555 00369-3fKKKKK3 IMPLANT WITH RESO~BABLE STEM

The prosthetic replacement of joints, ~uch as the ~houlder, hip, knee, ankle, and wrist has evolved over the years from early, relatively crude models to current pros~heses which closely duplicate functions and motions of a natural joint. As a result, prosthetic joints have provided patients with increasing comfort, freedom of motion and an ability to lead nearly normal lives.
Although there have been problems with exces-sive wear between components of pxosthetic devices which move with re~pect to each other, by and large the fixa-tion of the prosthetic ~omponents to the patient'~ bone ~tructure did and continues to represent the greatest difficulty. Early attempts to solve this problem by use of what were thought to be frictional locks between the implants ,and the bone and by use of tissue compati-ble acrylic cement were not successful.

In ~emented implants the cement initially acts as a grout to "form-fit" the implant to the bone.
It then "cold-cures" to a hard material which mechanic-ally fixes itself to the bone by interdigitating into . the bone trabeculae. This ability of the cement to fix a metal or plastic implant component ~ecurely to bone is the main factor which has so greatly improved the 6tatus of joint replacement ~ver the prior ~tate of merely implanting the component into bone and hoping that it actually stayed ~ecurely in place. Most of the 1~5855s components implanted without cement were not securely fixed and ultimately came loose in the bone with subse-quent pain and failure of the procedure. Thus, cement fixation of implants gives excellent short-term results;
however, in younger, heavier, or more active individuals, the bond between bone and cement eventually broke down.
The ~esult was a loosening or separation between the cement-bone implant interface which placed the device back into a category similar to implanted devices prior to use of cement-fixation, i.e., the implant was not securely fixed to the bone and pain which indicated a failure of the procedure was the end-result.
As a consequence of increasing numbers of failures with cemented devices, alternative methods of fixation of the implant were sought. One alternative, which is currently the state of the art for implant fixation, is to coat the surface of the implant with a porous material to allow the patient's bone to grow into the pores, thereby biologically fixing the implant to the bone. This appears to be the ideal method of implant fixation. The patient's own tissue now holds the implant and the latter has become a permanent part of the bone, thus obviating the problem of implant loos-ening.
Another problem encountered with joint implants is an abnormal stress transference from the implant to -the bone. The ideal stress transference of load to the bone is the normal, anatomical transference. To approx-imate it, the implant material should have mechanical properties similar to those o~ the bone and should re-place only the destroyed joint surface. This would 5 place no implant material, or only a minimal amount of implant material, within (intramedullary) the bone.
This is most difficult to do with joints having porous implant surfaces because they req~ire immediate rigid fixation for a ~ufficient time period to assure at least six to twelve wee~s in growth time. If the device is ~ ( ~
125~3555 not held rigidly, there will be micro-motion occurring at the implant-bone surface which results in a fibrous tissue in-growth rather than the necessary securely-fi~ed bony in-growth. The currently most common method of holding the implant rigidly in the bone is by pro-viding the implant with a stem which "press-fits" into the intramedullary cavity of the bone, e.g., the femur, or if no such cavity is present, by anchoring the im-plant to the bone, e.g., the pel~is, with a threaded anchor bolt. Such a press-fit of the stem into the shaft of the bone holds the device rigidly and allows an adequate bone in-growth for secure fixation. For the surgeon it also provides the desired proper anatom-ical placement of the implant in the bone in a repro-ducible manner.
The short coming of the aforesaid approach isthat loading of the bone is no longer physiologic.
Instead of being loaded primarily at the end of the bone near the joint surface as in the normal situation, the bone becomes loaded more distally in the shaft where the stem of the implant is fixed to the bone. The re-sult is an abnormal transference of stress which by-passes or "unloads" the end or joint surface portion of the bone, with a subsequent resorption of that bone.
This leads to a weakening of bone in that area over a period of years, thus creating the potential for frac-ture or disappearance of the bone that previously held the implant securely. The result is again a loosening of the implant within the bone with all the adverse consequences previously mentioned.
For implants, such as an acetabular cup of a hip prosthesis, which are vertically held in place with a screw, the non-physiologic transference of stresses is less pronounced because the location and orientation of the anchoring bolt can be selected to minimize non-natural load transference stresses. Nevertheless, at the very least the presence of the anchoring bolt in 125~SS~.

the bone weakens the latter and is undesirable for that reason alone.
Thus, if a stem placed down the medullary cavity of the bone (for a correct alignment of the im-plant and for its immediate, rigid fixation to allowbony in-growth fixation) produces an abnormal stress distribution, it would appear obvious to utilize an implant without a stem. In such a case the implant would essentially only resurface the destroyed articular surface. This is more readily done in joints, such as the knee, elbow, or ankle, than in others, such as the hip, shoulder or wrist. However, even if a stemless implant is feasible, its immediate rigid fixation is not as secure as if the implant were anchored with a stem, or an anchor bolt. When the stem functions to correctly align an anchor or an anchor bolt in its cor-rect position until bone in-growth is compl~te, an al-ternative mechanism i~ necessary therefore to accomplish the functions of the stem. One such mechanism could be a transcortical fixation of the implant such as multiple screws. This, however, makes it more difficult for the surgeon to correctly and reproducibly position and align the implant.

As the foxegoing discussion demonstrated, some types of stem or ~crew mechanisms are highly desir-able even if not absolutely essential to correct align-ment and ade~uate anchoring of the implant while bony in-growth occurs at the porous implant sur~aces. The most ideal biomechanical solution, from the standpoint of physiologic transference of stresses across the joint to the bone, would be to implant a stem or an anchor bolt which would disappear once it has ~erved its func-tion. This is accomplished in accordance with the pre-~ent invention by providing a composite implant havingan anchoring device, e.g., a stem or an anchor screw, -12~85s5 made of a biode~radable, resorbable material and a func-tional joint component defining the articulating surface of the implant made of a permanent, non-resorbable ma-terial, e.g., metal. Selected portions of the implant surface in contact with the bone are porous to limit and direct the bone in-growth to areas of natural load and stress transference.
Once in-growth has occurred and the implant is directly fixed to the bone it is implanted into, the stem is no longer required and it can be permitted to resorb, i.e., disappear. The resorption of the stem mat~rial can be programmed to take place over whatever length of time is necessary, i.e., 6 weeks, 8 weeks, 12 weeks, 6 months, 1 year or more, depending on how much time the particular joint or bone involved takes to a~e-quately fix the implant to the bone.
Following the resorption of the stem or anchor bolt, the only remaining permanent portion of the im-plant in the ~one is that portion closest to the joint surface. The biomechanical end-result is that the bone holding the implant is stressed primarily adjacent to the surface as in the normal physiologic situation.
There will thus be a near normal transference of stresses to the bone and no unloading of stresses on the bone ~5 immediately beneath the implant surface. In essence, the stressed bone will be as close to normal as possi-ble, almost as if no prosthetic joint device had been inserted.
Thus the present invention retains all the advantages of utilizing a stem or anchoring bolt during a prosthetic joint implantation, yet, it eliminates the drawbacks of permanently implanted stems and bolts.
The anchor stems or bolts can be fabricated from a variety of resorbable, biodegradable materials including certain ceramics, such as calcium hydroxyapa-tite or tricalcium phosphatei polymers such as polyesters of glycolic acid or lactic acid, and polyamides of ~-( 6 ~;~5~5S :~
amino acids; and unmodified or modified natural polymerssuch as gelatin or starch.
The use of biodegradable or resorbable mate-rials as such is known. For example, U.S. Patent No.
4,356,572 to Guillemin et al. discloses use of a biode-gradable bone prosthesis or implant made of a coherent material comprising calcium carbonate in crystalline form. The prosthesis or implant disclosed therein is a filler or replacement part for bone substance.
U.S. Patent 3,899,556 to Heide et al. discloses a process for producing an implantable, porous, c~ramic bone substitution or bone connection which has open pores of substantially regular or uniform size, distri-bution and disposition. The process suggested by Heide et al. includes producing a frame which corresponds approximately to the pores and pore connections of the desired finished product, filling the frame with a phy-siologically suitable and biologically compatible cast-able or pourable ceramic mass and after at least a par-tial hardening of the ceramic mass, dismantling or re-mo~ing the frame, whereby a finished raw product or ceramic material is obtained.
U.S. Patent No. 4,195,365 to Jarcho et al.
discloses the use of a ceramic material, i.e. whitlockite to fill a void in a bone. A defect or void in the bone is filled with this ceramic either as a shaped body or in particulate form.
U.S. Patent No. 4,344,190 to Lee et al. sug gests the use of a biodegradable plug with a hip implant.
The reamed out medullary cavity is filled with cement and the biodegradable plastic plug forms a mold for the cement.
U.S. Patent No. 4,202,055 to Reiner et al.
discloses a non-porous outercoating for am implantable prosthesis wherein the coating is composed of a calcium phosphate which is bioactive and at least one polymer which is mechanically and chemically stable in the body.

~25~35SS

Reiner et al. do not contemplate use of a screw or a bolt of any type.
U.S. Patent No. 4,192,021 to Deibig et al.
discloses the use of a mixture of calcium phosphates and polymers which are biodegrada~le as an anchoring material.
U.S. Patent No. 3,929,971 to Roy discloses synthetic materials useful as biomaterials having a microstructure substantially corresponding to the micro-structure of porous carbonate skeletal material of ma-rine life and made up of hydroxyapatite or whitlockite.
U.S. Patent No. 3,787,900 to McGee discloses use of discrete microcr~stals of a calcium phosphate compound together with the discrete microcrystals of refractory compound, such as the mineral spinel, alumi-num phosphate, or aluminum oxide, as a prosthesis ma-terial useful for artificial bones or teeth.
U.S. Patent No. 3,905,047 to Long discloses use of particles such as alumina and calcium pyrophos-phate in a unitary body prosthesis.
U.K. Patent No. 2 072 514 B discloses orthope-dic implants composed of biologically inactive biocom-patible structural materials and a bioactive control component. The components consist of a layer of braz-ing or silver solder and the structural material can betitanium and/or titanium alloy and/or cobalt chrome molybdenum alloy. The control component produces a chemical or electrical effect to promote or inhibit bone growth in the region of the prosthetic implant.
The implant includes an elongated structural member which is inserted into bone material and the bioactive f material is inserted in the region of the inserted end.
The device assists or replaces mechanical bone function and the structural member remains in place after the bioactive material has been degraded.
Offenlegungsschrifft De 29 47 875-A discloses granular, corpuscular or chip-like aggregates of organic 8 ~L258555 materials which dis~olve in the human body over a period of time and are mixed with a monomer in a reaction-in-hibiting protective jacket, and a powdery second compo-nent which is reactive wi~h the monomer and ~urther S standard additives to form a cement composition useful in prosthesis.
Rowever, the prior art nowhere suggests to employ biodegradable materials for temporarily fixing bone implants until bony in-growth into a porous surface of the implant has been achieved.

In the drawings:
Fig. 1 is a ~chematic, front elevational view through a human hip joint, illustrating the pelvis and the upper portion of the ~emur, and shows a prosthetic hip constructed in accordance with the present invention immediately following insertion;
Fig. 2 is an enlarged cross-sectional view of the acetabular component of the prosthetic hip illu-~trated in Fig. l;
Fig. 3 is a front elevational view of the righthand femoral component of the prosthetic hip illu-strated in Fig. l;
Fig. 4 is a schematic, front elevational view through a human hip joint, illustrating an alternate embodiment of a prosthetic hip constructed in accordance with the present invention;
Fig. 5 is a schematic, front elevational view of a human knee joint, illustrating the lower portion of the femur and the upper portion of the tibia, and shows a prosthetic knee constructed in accordance with the present invention immediately following insertion;
Fig. 6 is a lateral side elevation of the prosthetic knee illustrated in Fig. 5; and Fig. 7 is a schematic, front ~levation through a human ~houlder, illustrating the scapula and the upper portion of the humerus and ~hows a prosthetic ~houlder ~ f^

9 125~3555 constructed in accordance with the present invention immediately following insertion of the prosthetic shoul-der.

S Referring first to Fig. 1 a total prosthetic hip 2 constructed in accordance with the present inven-tion comprises an acetabular component 4 fixed to a patient's pelvis 6 and a femoral component 8 attached to the patient'~ femur 10. ~ig. 1 and Fig. 2 illustrate the acetabular component which is defined broadly by an acetabular cup 12 connected to a biodegradable, resorb-able anchor bolt 14 threaded into the pelvis along the pectineal line at an inclination of approximately 20 relative to the longitudinal axis 16 of the femur.
The cup is constructed of an non-corroding, non-resorbable, high strength material such as a cobalt-chromium alloy or a titanium alloy. Disposed within the cup is a low ~riction liner or insert 18 which de-fines a ~pherically ~haped acetabular socket 20 that movably receives a ball 22 attached to the femoral im-plant 8. The cup includes a fixation hole or bore which is positioned so ~hat, upon implantation of the cup, the bore is aligned with the pectineal line of the patient's pelvis. The bore includes a recess which defines an inwardly facing shoulder that, upon implan-tation, is engaged by a set screw 42 having a threaded end 43 which extends into and engages the anchor bolt 14. The set screw is non-resorbable and acts as an interconnection between the resorbable anchor bolt 14 and the non-resorbable cup 12.
The cup also has an exterior porous surface area 50 which ~urrounds the bore and promotes bone in-growth in the area of load transfer between the pelvis and the cup. To achieve the desired bone in-growth, the pores are preferably of a size in the ran~e of between about 250-450~. The remainder of the exterior cup 1;~S~3555 1.0 surface is smoo-th to inhlhit bone in-growth -thereover. The exact dimensioning of the porous surface 50 is not critical.
To prevent any potential toxic effect of increased ion transfer as d result of the increased surface area about the porous surface 50, at least the porous surface area can be coated with a thin impervious layer, including methylmethacrylate cement, carbon, calcium hydroxyapatite and tricalcium phosphate or polyactic acid. Such coatings are made commercially according to methods which are unknown to applicant and which applicant believes are maintained as a trade secret.
Resorbable anchor bolt 14 has a first end with external thread 56 formed to cut into and firmly engage the bone structure. The opposite, free end of the anchor bolt includes an internal thread which is engaged by set screw 42.
The anchor bolt is fabricated from a resorbable material such as biodegradable ceramics, modified or unmodified natural polymers, such as starch or gelatin, polymers such as polyesters of glycolic acid or lactic acid and polyamides of ~ -amino acids, etc. The material is selected to resorb at a predetermined time. For most applications this will coincide with a time period sufficiert to allow mature bony in-growth about the cup for ade~uate mechanical fixation of the implant to the pelvis.
During implantation, the resorbable anchor bolt is aligned with the bone in the cup so that the set screw 42 can be tightened to firmly secure the cup to the anchor bolt and thereby, to the bone.
After implantation, the insert may be positioned so that an overhanging portion 49 protrudes beyond the cup as shown in Figure 1. This provides for a better seating of the associated ball over a wider range of motions and helps prevent accidental dislocations of the ball and -the socket.

12~355S

Referring now to Figs. 1, and 3, the construc-tion and implantation procedure for the femoral component 8 is discussed in detail. To enable the im-plantation of the hip prosthesis to the femoral compo-nent 8~ the head and neck of the femur 10 are initiallyresected to form an exterior femoral face 24 which is planar, generally perpendicular to a pectineal line and is at an angle of approximately 70~ to the longitudinal femur axis 16.
The femoral implant includes an intertrochan-teric body 28 with a non-resorbable male screw 98 de-pending therefrom. The intertrochanteric body is press-fit into a correspondingly shaped and appropriately dimensioned intertrochanteric, upwardly opening cavity 30 in the femur. A resorbable stem 32 extends downward-ly from the body into the medullary cavity 34 of the femur. The femoral component further has a neck 36 on its medial side which extends generally upwardly and in a medial direction and to which is mounted the ball 22 that cooperates with the socket 20 of the acetabular component. The resorbable stem 32 acts as a guide to ensure proper placement of the femoral component in the intertrochanteric body and keeps it there until bony in-growth has fixed the body to the femur.
The stem is fabricated from a biodegradable, resorbable material including polylactic acid, ceramics, etc. and is connected to the intertrochanteric body by a female thread 96 located on the stem which cooperates with a corresponding non-resorbable male screw 98 de-pending from the intertrochanteric body. Locating the female thread 96 in the stem section prevents the forma-tion of stress concentrations in the intertrochanteric body. The interconnection between intertrochanteric body and the stem typically is in the region of the stem where it straightens out in the lateral plane, although it may also be located at other sites along the stem.

( 12 1 ~ 58 5~5 Once the femoral implant is firmly fixed by bony in-growth in its proper position, the function of the stem section is over and it is eliminated by resorp-tion. The final result is a stemless femoral component that allows a more physiologic transference of stresses to the pro~imal femur as opposed to a stemmed implant where there is the potential for fixation distally, thereby potentially causing stress shielding and bone resorption proximally with an increased chance of me-chanical failure.
Turning now to the implantation procedure forthe femoral component, the head and neck of the femur are first resected to form the external femoral loading face 24 and ~he intertrochanteric cavity 30 is then shaped ~y removing soft (non-load bearing) bone tissue.
The entire intertrochanteric cavity is broached slightly undersize relative to the implant body to achieve a press-fit therewith.
Prior to implanting the stem is connected to the intertrochanteric body by screwing the body into the stem. Next, the femoral component is implanted by placing the stem into the medullary cavity and then pressing the intertrochanteric ~ody 28 into the inter-trochanteric cavity until the internal and external loading surfaces firmly engage the bone.
During the implantation process the eiongated stem acts as a guide t~ prevent an accidental misalign-ment of the intertrochanteric body during the insertion step, particularly when substantial forces must be ap-plied to overcome the press-fit between the body and the cavity. Once inserted, however, the stem has no significant function. In the design of the present invention with its resultant stemless implant, there are no distal stresses, as seen in the normal femur.
The absence of such stresses about the femoral cortex adjacent to the stem, coupled with the transfer of loads at the internal and external loading surfaces assures 13 :~LZ5~3555 that the femoral implant is top loaded in a manner analogous to the physiological loading of a femur in a healthy hip joint.
Referring now to Fig. 4, an al-ternate embodimen-t of a femoral implant in accordance with the present invention is illustrated. The alternate femoral implant is composed of a generally semicircular non-resorbable femoral head component 21 which is chosen to fit into cup 12 upon implant. Non-resorbable male screw 98 is attached to and protrudes from the generally planar surface of the femoral ~ead component 21. Porous surface 50 promotes bone in-growth. Resorbable stem 32 with internal thread 96 is engaged by non-resorbable screw 98. After resorption the bone will totally surround male screw 98.
Referring now to Fig. 5 a prosthetic knee 100 having a resorbable anchor and a resorbable stem is shown and illustrates a non-resorbable femoral component 102 fixed by a resorbable ~tem 32 to a patient's femur 104 and a tibial component 116 attached by a resorbable anchor 14 to the patient's tibia 108. Broadly speaking, the tibial component is defined by a tibial cup 110 which is connected to a resorbable anchor bolt 14 threaded into the tibia. The tibial cup is constructed of a non-resorbable, high strength material preferably a metal such as cobalt-chromium alloy or a titanium alloy, for example. Non-resorbable, externally threaded set screw 43 serves as an interconnection between the nonresorbable tibial component and the resorbable anchor bolt 14 in a manner similar to that of set screw 43 in the hip prosthesis. Cup 110 includes a bone for the set screw and an exterior porous surface area 50 which surrounds the bore and promotes bone in-growth in the area of load transfer between the tibia and the cup. To prevent potential toxic effect of increased ion transfer as a result of the increased surface area about the lZ5855~;

porous surface 50, at least the porous xurface area can be coated with a thin impervious layer.
~eferring now to Figs. 5 and 6, the femoral implant includes a non-resorbable femoral component 102, having porous coating 50, and a non-resorbable male screw 98. A
resorbable stem 32 is attached to the femoral component with a female thread 96 on the resorbable stem which cooperates with a corresponding non-resorba~le male screw 98 extending from the femoral component. The resorbable stem acts as a guide to prevent an accidental misalignment during implantation, thereafter it holds rigidly to allow bony fixation into the porous coating. A component 112, preferably plastic, is interposed between the tibial component and the femoral component. After resorption, screw 98 and screw 43 will be surrounded by bone so that the implant will not become loose or rattle.
Referring to Fig. 7 wherein a schematic front elevation through a human shoulder of a prosthe-tic shoulder 114 having resorbable anchors and a resorbable stem in accordance with the present invention is shown immediately after insertion of the prosthetic shoulder and following resorption of the anchor devices. The prosthetic shoulder 114 comprises a glenoid component 117 and the acromial component 126 fixed to a patient's shoulder and a humeral component 118 attached to the patient's humerus 120. Broadly speaking, the glenoid component and the acromial component are defined by a cup 121 which is comprised of a plastic insert with a metal backing. The cup has a substantially semi-spherical configuration and is secured to the scapula with one or more non-resorbable set screws 42 which are threaded into resorbable anchor bolts 14. Disposed within the cup is a low friction socket that movably receives the humeral head component 118. Cup 121 includes an exterior porous surface area 50 which surrounds the bore through which the set screw extends to ~25~355S
promote in-growth in the area of load transfer between the shoulder and the cup. To achieve the desired bone in-growth, the pores are preferably of a size in the range between about ~50-450~. To prevent any potential toxic effect as the result of the increased surface area about the porous surface, at least the porous sur-face areas can be coated with a thin impervious layer of material, such as methylmethacrylate cement, carbon, calcium hydroxyapatite or tricalcium phosphate.
To enable the implantation of the shoulder prothesis to the humeral component, the head and the neck of the humerus are resected to the form humeral face 124 which is planar. A non-resorbable semi-circu-lar humeral head component 118 having porous surface area 50 with a male screw 98 depending th~refrom is provided. A resorbable stem 32 which has a correspond-ing female thread 96 located therein is attached to the humeral head component. The stem acts as a guide to prevent accidental misalignment during implantation.
Once the humeral implant is firmly fixed by bony in~
growth in its proper position, the fixation function of the stem is over and it is eliminated by resorption.
The final result is a stemless humeral component that allows a more physiological transference of stresses to the humerus.
With respect to all prosthetic implants of the present invention, there remains a permanent male thread or screw which protrudes into the bone after resorption of the anchoring device. After resorption of the anchor, the bone has grown back and surrounds the non-resorbable permanent anchor or screw so that the implant does not come loose or rattle. Although hip, shoulder and knee replacement has been described with particularity, it should be understood that the 3~ present invention is not so limited. Resorbable anchors of the present invention are applicable to all joints including wrist, finger, elbow, ankle, foot and toe.

1'~5~35S5 As will be apparent to those skilled in the art in light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Prosthetic implant for a bone joint comprising a component defining an operative portion of the joint, the component having an interface facing generally away from the joint and adapted to be place into intimate contact with a natural bone to which the component is to be fixed, a part of the interface being adapted to directly connect with the natural bone; an anchor connected to the component extending from the interface, adapted to be inserted in a cavity in the bone for orienting and immovably fixing the component to the bone immediately upon implantation, the anchor being constructed substantially entirely of a biodegradable material so that it resorbs after a period of time and thereby vacates the cavity to permit natural bone to grow back into the cavity.

2. An implant according to claim 1 including means for securing the anchor to the component.

3. An implant according to claim 1 wherein the securing means comprises a threaded hole in the anchor engaged by a set screw from the component.

4. An implant according to claim 1 wherein the anchor includes an exterior surface shaped to tightly engage a cavity defined by the bone and to be substan-tially immovably disposed within the cavity after being placed therein.

5. An implant according to claim 4 wherein the exterior surface defines a male thread.

6. An implant according to claim 4 wherein the exterior surface is substantially smooth.

7. An implant according to claim 4 wherein the exterior surface is shaped to tightly fit into a canal defined by a natural bone.

8. An implant according to claim 3 wherein the threaded hole is engageable with an externally threaded screw engaging a surface of the component.

9. An implant according to claim 1 wherein the biodegradable material is selected from the group consisting of polyesters of glycolic acid, polyesters of lactic acid, polyamides of -amino acids, unmodified polymers, modified polymers, calcium hydroxyapatite, tri-calcium phosphate or polylactic acid.

10. An implant according to claim 1 wherein the component is selected from an alloy of cobalt-chromium or titanium.

11. An implant according to claim 1 wherein the component is part of a prosthetic hip.

12. An implant according to claim 1 wherein the component is part of a prosthetic knee.

13. An implant according to claim 1 wherein the component is part of a prosthetic shoulder.

14. An implant according to claim 2 wherein the anchor is a threaded bolt and the securing means include a screw engaging the component and threaded into the bolt.

15. An implant according to claim 1 wherein the anchor is a stem having an exterior surface that is substantially smooth and wherein the securing means comprises a threaded hole in the stem and a screw engaging the component and threaded into the hole in the stem.

16. An implant according to claim 1 wherein the part of the interface is porous so that bony in-growth directly connects the component to the natural bone.