EP0696186A1

EP0696186A1 - Bone prosthesis and method of implanting the same

Info

Publication number: EP0696186A1
Application number: EP94911576A
Authority: EP
Inventors: Robert Poss; Tim Doolin; Doug Wentz; G. Kris Kumar
Original assignee: Zimmer Inc; Brigham and Womens Hospital Inc
Current assignee: Zimmer Inc; Brigham and Womens Hospital Inc
Priority date: 1993-03-12
Filing date: 1994-03-14
Publication date: 1996-02-14
Also published as: AU687800B2; AU6406494A; WO1994020046A1; JPH09501064A; EP0696186A4; CA2156740A1

Abstract

A prosthetic implant (100) including a pre-applied mantle (280) of substantial thickness disposed over a substantial portion of the outer surface of a stem (160), is disclosed. The stem (160) is provided with longitudinal channels (360) which allow ease of future extraction from the femur. The mantle (280) is sized and shaped to closely approximate the size and shape of the cavity (34) in which it is implanted to minimize the amount of cement (26) used to anchor the prosthesis (100). The mantle (280) may be applied to the stem (160) by injection molding alone or in combination with initially plasma spraying a thin film onto the stem (160).

Description

Bone Prosthesis and Method of Implanting the Same

I. Background of the Invention

Cross Reference to Related Applications

This application is a continuation-in-part of U.S. Application No. 08/030,722, filed March 12, 1993.

A. Field of the Invention

This invention relates generally to bone implants, and more specifically to a bone prosthesis having a preapplied cement mantle.

B. Related Art

A prosthesis is an artificial replacement of a limb, bone or other part of the body. A prothesis attempts to emulate the function of the replaced body part. The present invention is directed to those prostheses which are implanted into a bone of a body, human or otherwise.

Implantation of a new prosthesis into a bone may be required in several instances. One example is where it is impossible for a badly arthritic joint to be repaired. In this situation, the damaged portions of the joint are sectioned (cut) and removed. Another example is where removal of bone tissue occurs due to cancer or disease. In both examples, the prosthesis is implanted to compensate for the loss in length of the bone and/or joint mobility. In a further example, a new prosthesis may be required where problems with an existing prosthesis develop, such as loosening which creates subsequent movement of the prosthesis within the bone. Moreover, the body's rejection of an existing prosthesis may necessitate replacement. Replacement of an existing prosthesis is performed during revision surgery.

Generally, bone-implanted prostheses may be implanted into a bone by mechanical means such as (1) screwing the implant into the bone, or (2) drilling a cavity into the bone, inserting the prosthesis into the drilled cavity and anchoring the prosthesis to the bone cavity with an adhesive or cement.

The present invention is directed to the latter means of implantation.

A critical factor in the successful implantation of a prosthesis is the strength and stability of the means by which the prosthesis is anchored to the bone and the means by which the cement is anchored to the prosthesis. A stronger, more stable anchoring means increases the probability of a successful implantation. Various methods of anchoring prostheses into bone have been the subject of much recent research and development and form the basis for the present invention. In the human body, the femur (or thigh bone) is the bone located between the pelvis and the tibia. In order to facilitate explanation, the following discussion utilizes the femur as a bone suitable for a prosthesis implantation. However, it is noted that the present invention is directed to bone implants generally as opposed to the more narrow field of femoral bone implants.

Because destruction of the upper femur is a common result of falling or advanced arthritis, most femoral prostheses are utilized in what is commonly known as a hip replacement. In a hip replacement, the upper portion of the femur is replaced with a femoral prosthesis. Typically, in a hip replacement a femoral prosthesis consists of a body and an acetabular cup.

The body is provided with a stem, a neck and a ball. The stem is anchored into the femur while the acetabular cup is anchored into the hip joint space in the pelvis. The ball fits into the acetabular cup.

In hip replacements, procεdurally, the surgeon first exposes the joint socket. An acetabular cup is placed in the pelvis after reaming the acetabulum. Next, in the femur the surgeon removes all diseased or damaged tissues and cleans or hollows out a cone-like femoral cavity using a broach and reamer. The stem is inserted and anchored into the femoral cavity. A conventional femoral stem may be anchored to the bone in a variety of ways including: a mechanical interlock; a coating; polymer cement; or a combination of these. Each of these methods are well known in the art. U.S.

Patent No. 4,994,759 to Mallory et al. discloses an example of a screw and coating combination. U.S. Patent No. 4,983, 183 to Horowitz discloses an example of a combination of a coating and cement to anchor a prosthesis. An example of using a coating to anchor a prosthetic stem is disclosed in U.S. Patent No. 4,718,912 to Crowinshield.

Two types of coatings are conventionally used: porous arid hydroxyapatite. Hydroxyapatite is a common mixture of calcium and phosphate. Synthetic hydroxyapatite is a polycrystalline apatite ceramic that is dense, nonporous, nonresorbable, and inert. It bonds strongly to and integrates with living bone.

With either type, the coating is applied to the outside surface of the stem. After the coating is applied, the stem is press-fit into the femoral cavity. The porous coating comes into direct contact with the femur. Because the coating is porous, it contains interstices into which the bone can easily regenerate and grow. Such bone ingrowth into the stem enhances the stability and strength of the stem-to-bone interface, thereby anchoring the prosthesis into the bone. A stronger anchor increases the probability of a successfully implanted, stable femoral prosthesis.

Polymer cement used in bone implants commonly consists of polymethyl methacrylate (PMMA), although other compositions of polymer cement with a lower modulus of elasticity are also used. Polymer cement is also referred to by those skilled in the art as acrylic cement. Polymer cement is typically stored in powder form as two separate components which are mixed to a pourable, paste-like consistency to activate the cement characteristics. The cement typically hardens within approximately 10 minutes after mixing. When anchoring a prosthesis using polymer cement, the surgeon mixes enough cement to ensure a quantity sufficient to fill the cavity and coat the stem. This mixing step takes approximately two minutes. After mixing, the surgeon places the cement into the femoral cavity using a cement gun, being careful to avoid the sides of the femoral cavity to prevent trapping air. The prosthesis is coated with cement, pressed into the femoral cavity, and held until the cement hardens. Any excess cement displaced by insertion of the stem is wiped away.

However, the standard method of using cement to anchor a prosthesis has several disadvantages. First, before the cement hardens air pockets can be introduced into the cement which threaten the structural integrity of the cement and thus, the bond between the stem-to-cement interface and the bone- to-cement interface.

Further, if the operating room personnel touch the prosthetic implant anytime before insertion, foreign matter such as grease, blood and unmixed cement powder can be deposited on the implant causing the bond between the cement and the stem to be broken, resulting in a loosening or toggle effect of the stem within the femoral cavity.

Another disadvantage of the polymer cement anchoring method is the possibility of stem-to-bone contact. As described above, the polymer cement is mixed and poured into the femoral cavity; the stem is coated with polymer cement and inserted into the cavity, displacing the poured cement. During stem placement, the stem can be pushed too far in one direction within the cavity thereby completely displacing the cement in that area and causing a portion of the stem to come into direct contact with the bone. Left uncorrected, the cement will set and the stem will be permanently fixed in direct contact with the bone. This direct contact can cause a less than optimal distribution of cement thickness and lead to cement fracture and paniculate debris. When this happens, the cement must be broken and removed and the stem must be replaced during revision surgery. This increases the cost of the implant procedure and can result in additional loss of bone material critical to successful placement and stability of the prosthesis.

A further disadvantage of polymer cement anchoring is that cement shrinks approximately 4% in volume during curing. When this shrinkage occurs in the femoral cavity after the stem is inserted, the integrity of the interface between the cement and the cavity and between the cement and the stem is at risk.

Another disadvantage of polymer cement anchoring is that upon mixing of the bone cement, an exothermic reaction takes place. The resulting elevated temperature caused by cement curing within the femoral cavity can cause death of the surrounding body tissue.

Another less immediate disadvantage of the polymer cement anchoring method is the difficulty of revision surgery. In certain unfortunate situations, it becomes necessary to perform revision surgery to remove the prosthesis. Factors triggering the need for revision surgery include bone infection, cement failure or stem failure. When a conventional stem and polymer cement are used, revision surgery to remove the stem and cement is difficult.

One perceived solution to the above problems is to bond a preapplied cement mantle to the stem prior to the implantation operation. This solution was developed by Pratt et. al and is discussed in U.S. Patent No. 4,283,799

(the Pratt patent), the disclosure of which is herein incorporated in its entirety by reference. The inventor of the present invention is a named co-inventor of the invention of the Pratt patent.

The Pratt patent discloses a prothesis having a preapplied cement mantle ranging in thickness between 3-5 millimeters and a prepared bone cavity covered by a second quantity of cured cement. After the cement covering the cavity has cured, a third quantity of cement is introduced into the bone cavity to bond the mantle to the cavity-covering cement. Accordingly, the Pratt patent still requires that a large amount of cement be mixed and placed within the femoral cavity to set the prosthesis. Moreover, the chances of foreign matter being introduced into the cavity are greatly increased by requiring two layers of cement between the mantle and the femoral cavity. Further, the Pratt patent doubies the duration of time that the cavity tissue is exposed to elevated temperatures because the cement covering the cavity must cure before the third quantity of bonding cement is introduced and allowed to cure.

U.S. Patent No. 5,133,771 to Duncan et al. (the Duncan patent) (the disclosure of which is herein incorporated in its entirety by reference) discloses the incorporation of a mantle of substantial thickness on a temporary prosthesis stem prior to insertion. However, the Duncan patent uses the actual femoral cavity as a mold for the mantle. Therefore, the problem of air pocket formation or homogeneity in the cement mantle and bonding between the stem and the cement persists because the unavoidable steps of pouring a large quantity of cement into the cavity and inserting the stem remain unchanged from the conventional operating room procedure. U.S. Patent No. 4,491,987 to Park (the Park patent) (the disclosure of which is herein incorporated in its entirety by reference) discloses the application of a polymer mantle on a stem in a nonsurgical environment. However, the problem of difficulty in prosthesis removal in revision surgery persists. Prior to the development of the present invention there was no device for simultaneously providing a strong stem-to-cement and cement-to-bone bond, providing homogenous cement anchoring by eliminating air pockets, providing reduced cement shrinkage, providing decreased chance for death of bodily tissue from the exothermic reaction of the bone cement, providing an extremely close fit between the prosthesis and a femoral cavity, and providing ease of prosthesis extraction during revision surgery. The present invention provides these features by mating a thick cement mantle to a femoral stem under highly controlled conditions in a factory during manufacturing of the stem. While mating a thick cement mantle to a femoral stem, consideration must also be given to the possibility of future events such as infection, death of the bone tissue, breaking of the femur due to falling, or the like which necessitate the extraction of the prosthesis and cement for replacement. Because of these considerations, it is advantageous to also configure the present invention to allow ease of extraction of the prosthesis during revision surgery. This is accomplished by providing longitudinal channels, grooves, or apertures along the stem and providing a cement mantle which fills the channels, grooves or apertures while providing a smooth outer surface. The channels allow greater ease of extraction of the stem from the femur. Accordingly, it is an object of the present invention to provide a prosthesis with a cement mantle which is homogenous, without the air pockets inherent in operating room cement mixing and pouring operations. The optimal thickness of the mantle assures that only a small percentage of the entire volume of cement applied during surgery and occupying the femoral cavity has a chance to develop air pockets and that the cement will see increased pressurization into the bone interstices because of the bulk of the prosthesis-polymer construction.

A further object of the present invention is that the coating on the prosthetic stem can be made with the cross-sectional area of the polymer coating greater comprising at least 30% and as much as 40% or greater of the total cross-sectional area of the prosthesis. Therefore, the thick mantle-stem combination occupies nearly the entire femoral cavity and there is a large reduction in the volume of cement which must be poured into the cavity causing less volumetric shrinkage of the poured cement and less heat from reacting bone cement.

- Yet another object of the present invention is that stem-to-bone contact is eliminated by providing a mantle which covers the entire stem within the femoral cavity.

A further object of the present invention is to configure a stem with longitudinal channels which allow ease of extraction of the prosthesis from the femur during revision surgery. A further object of the present invention is to provide a mantle with a modulus of elasticity which is lesser than that of the stem, but greater than that of the bone cement.

/ . Summary of the Invention

It is in view of the above problems that the present invention was developed. The invention is a prosthetic implant comprising a stem having a proximal end, a distal end, an outer surface and a preapplied mantle disposed over the entire portion of the outer surface of the stem. The mantle has an outermost surface that is preshaped to correspond approximately to the size and shape of a femoral cavity.

In one aspect of the invention, the mantle is preapplied to the stem of the prosthetic implant by injection molding.

In yet another aspect of the invention, a thin film of precoat is applied to the stem prior to injection molding the mantle. The precoat may be a polymer and may be applied by plasma spraying. Additives may be introduced into the mantle to enhance its flexibility over conventional mantles made from PMMA. The stem may have longitudinal channels which allow ease of extraction of the prosthesis during revision surgery.

Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. III. Brief Description of the Drawings

The present invention will be described with reference to the accompanying drawings in which:

Figure 1 is a sectional view showing a conventional femoral component of the prior art;

Figure 2 is a sectional view showing the femoral component of the present invention immediately after implantation in a patient;

Figure 3 is a detail view taken from square 3 of Figure 2;

Figure 4 is a sectional view taken through line 4-4 of Figure 2; Figure 5 is a side view of one embodiment of the prosthesis of the present invention;

Figure 6 is a front view of the prosthesis of Figure 5;

Figure 7 is a cross-section of Figure 5 cut along line 7-7; and

Figure 8 is a cross-section of Figure 5 cut along line 8-8.

IV. Detailed Description of the Preferred Embodiment

Referring now to the accompanying drawings in which like reference numbers indicate identical or functionally similar elements, Figure 1 illustrates a sectional view of a known implanted femoral prosthesis. An implant or prosthesis 10 having a stem 16 with a proximal end 18 and a distal end 20 is shown. A ball 12 and a neck 14 are disposed adjacent to proximal end 18.

Stem 16 functions as an anchor in femur 30 for the entire prosthesis and provides load-bearing and load-distributing ability. Neck 14 functions to provide a transition between stem 16 and ball 12, and provides a secure foundation on which ball 12 sits. Prosthesis 10 is shown implanted in the femur 30 of a body. Femur

30 is comprised of a calcar 22 or outer bone covering and a bone interior 24. A collar (not pictured) may be disposed between the neck and the stem and, if provided, would be seated over the calcar 22 and bone interior 24. Before prosthesis 10 is placed into femur 30, a broach and reamer (not shown) is utilized to hollow out a cavity 34 in calcar 22. A bone plug 31 is inserted and tightly fitted at the bottom of cavity 34. Polymer cement 26 is mixed and poured into the opening 36 of cavity 34. Prosthesis 10 is inserted through opening 36 into cavity 34 causing cement 26 to be displaced upwardly towards opening 36. Any excess cement 26 is wiped away leaving neck 14 exposed at proximal end 18. Bone plug 31 forces cement 26 to flow towards proximal end 18 of stem 16 by blocking cement 26 from flowing downwardly into the medullary canal of femur 30. Referring now to Figure 2 of the drawings, an implant or prosthesis

100 according to the present invention is shown. Prosthesis 100 comprises a stem 160, a neck 140 and a ball 120 similar to that shown in FIG. 1. A collar (not pictured) may be disposed between the neck and the stem and, if provided, would be seated over the calcar 22 and bone interior 24. The invention utilizes a load-bearing stem 160 which has sufficient strength and geometry to transfer stresses to a bone in which prosthesis 100 is utilized.

Stem 160 and neck 140 can be made of any conventionally used materials, such as cobalt-chromium alloy, titanium alloy, stainless steel, platinum metals, carbon fiber, composite material, and the like. If formed from a composite material, the material may include a plurality of carbon fibers or whiskers. Another possible composite material is a combination of metal alloy and high density polyethylene or polytetrafluoroethylene. The preferred stem material is the cobalt-chromium alloy.

One unique aspect of the present invention is the marriage of a homogenous mantle 280, thereby providing maximum strength, to the outer surface of a state-of-the-art stem 160 under controlled conditions. The mantle 280 is formed by applying a polymer to prosthetic stem 160. The surface of stem 160 may be cleaned by acid etching or gritblasting prior to application of the polymer in order to provide a pristine surface for bonding. The polymer may be any conventional cement having a low modulus of elasticity. The preferred polymer used to manufacture mantle 280 is PMMA which is also used in making plexiglass, although other polymers having varying moduli of elasticity are also contemplated. Using a factory manufactured mantle 280 eliminates the need to combine large quantities of PMMA powder components in the operating room. The polymer mantle 280 is applied to a clean stem 160 by injection molding wherein stem 160 is disposed inside- a mold and the polymer is injected into the mold to surround and adhere to stem 160. Further, because the application of mantle 280 to stem 160 occurs under factory-controlled conditions, environmental factors such as temperature, pressure and position can all be controlled to ensure optimal bonding. It is possible to provide additives to the PMMA prior to application of the mantle 280 to the stem 160 to provide a mantle 280 having greater flexibility than a mantle 280 made solely from PMMA. Possible additives to mantle 280 include styrene and barium sulfide. Mantle 280 occupies a large sectional area relative to stem 160. Mantle 280 is constructed to closely approximate the diameter of cavity 34 such that when placed therein it deviates from the inner surface of cavity 34 by as little as possible, for example about 1.0 mm.

In the alternative, it is contemplated to plasma spray a thin film of PMMA to a clean prosthetic stem 160 prior to injection molding the polymer mantle 280. The plasma sprayed thin film provides a superior stem-to-cement bond and an improved surface for adherence by the injection molded mantle 280. It is contemplated that the thin film need not be formed from the same material as the material which is injection molded to form the mantle. One advantage of custom manufacturing prosthesis 100 is that all its dimensions may be calculated using finite element analysis to maximize the dimensions of prosthesis 100 along its length within a reshaped femoral cavity 34 while allowing for insertion of prosthesis 100 into cavity 34. The stem geometry is a critical aspect of the invention. An objective function is comprised of a linear combination of three critical stress parameters: maximum effective stress (i.e. von Mises stress) in the cement mantle 280, the maximum effective stress in the metal core, and the maximum stress difference (the degree of stress protection) in the proximal medial cortical bone. A parameter value (core diameter) that produces the minimum critical stress parameters (the "local minimum" of the objective function) can be determined for a series of prostheses models corresponding to each parametric variation.

A cobalt-chromium alloy model and a titanium alloy model can be used. The optimal solution occurs when no further improvement (reduction) in the objective function is possible. Among other results, these models provide quantitative relationships for the principal stresses in the cortical bone and the shear stresses at the prosthesis-bone interface as a function of the modulus of the prosthesis. The principal stresses in the cortical bone are normalized to the corresponding stresses predicted for the intact femur to evaluate the degree of stress protection as a function of the material properties of the metal core.

The stem 160 resulting from such analysis preferably will not have a circular cross-section because such a cross-section may not provide sufficient strength upon torsional loading. In addition, stem 160 preferably does not have sharp edges because such a geometry may cause mantle 280 to split, thereby interfering with its integrity. Therefore, stem 160 must be of a geometry which provides torsional strength while promoting the integrity of mantle 280.

The mantle 280 of prosthesis 100 will have a radial thickness of between 2.0 to 5.0 millimeters.

Prosthesis 100 is implanted into femur 30 in the following manner. First, a femoral cavity 34 is created. Then, a bone plug 31 may be inserted at the bottom of cavity 34. Next, a small amount of cement 26 is poured into the cavity 34 and used to coat the mantle 280. Because the geometry of the mantle 280 provides a close fit between the cavity 34 and the mantle 280, only a small quantity of cement is required to bridge the gap. Stem 160 and mantle 280, which is adhered thereto, of prothesis 100 are inserted into cavity 34. Because of the close proximity between mantle 280 and the wall of cavity 34, mantle 280 pressurizes cement 26 causing it to flow upwardly and completely surround mantle 280. Bone plug 31 is tightly fitted within cavity 34 and forces cement 26 to flow towards proximal end 18 of stem 160 by blocking cement 26 from flowing downwardly into the medullary canal of femur 30. Referring now to Figure 3, a magnified view of a portion of Figure 2 is depicted. Specifically, mantle 280 is shown in contact with cement 26 which penetrates into interstices 32 in bone 24. The greatest distance between mantle 280 and bone 24 which is occupied by cement 26 is approximately 2.0 mm. As illustrated, the insertion of stem 160 into cavity 34 pressurizes cement 26, thereby pushing it into interstices 32 of the bone 24. Filling the interstices 32 with cement 26 is highly desirable in that a higher strength bond is formed between the femur 30 and cement 26 and air pocket formation due to unfilled interstices 32 is avoided.

Figure 4 illustrates the formation of cement 26 around mantle 280. The cross-sectional area of prosthesis 100 is defined as the combined the cross-sectional area of mantle 280 and stem 160. Mantle 280 occupies at least

30% an as much as 40% or greater of the total of the cross-sectional area of prosthesis 100. The stem 160 is provided with a plurality of longitudinal channels 360. The longitudinal channels 360 allow ease of extraction of the femoral component 100 from the femur by allowing a drill to be introduced into the channels to separate the stem 160 from the mantle 280 during revision surgery. Preferably, longitudinal channels 360 of stem 160 do not have sharp edges because such a geometry may interfere with the integrity of mantle 280 by causing mantle 280 to split. Thus, another aspect of the present invention is a merging of the best possible stem 160 with the best possible mantle 280 together with a means for removing prosthesis 100 without destroying the bone.

Prosthesis 100 is formed when mantle 280 is secured to stem 160. Mantle 280 is applied to stem 160 at a temperature which is greater than room temperature. In addition, stem 160 may be heated while mantle 280 is secured thereto. Applying mantle 280 to stem 160 before surgery has several advantages. First, a superior bond between mantle 280 and stem 160 is assured by using a controlled application of polymer to stem 160. Control is exercised over factors such as quantity, temperature, pressure, and time. The cooling of stem 160 and surrounding mantle 280 must be controlled to avoid nonuniform shrinking rates relative to stem 160 and mantle 280 as well as to intensify the mechanical adhesion between stem 160 and mantle 280.

Prosthesis 100 also has the relative advantage of decreased overall stiffness while being substantially femoral canal-filling. In the operating room theater prior to inserting the femoral component

100 into the femur 30, a small quantity of cement 26 is mixed and placed in femoral cavity 34. Then, prosthesis 100 is inserted into femoral cavity 34 as discussed above. Mantle 280 pressurizes mixed cement 26 so that it flows upwardly to form a thin layer of hardened cement 26 distributed throughout femoral cavity 34. The PMMA mantle 280 and the mixed cement 26 may be chemically the same composition or the PMMA mantle 280 may include additives as previously described. A chemically altered mantle 280 having additives will be more flexible than the layer of hardened mixed cement 26 lining femoral cavity 34 while still allowing for chemical bonding between mantle 280 (with additives) and mixed cement 26.

Shown in Figures 5-8 is a preferred embodiment of the invention. Figure 5 shows an embodiment of a prosthesis 50. Prosthesis 50 has a distal end 52 and a proximal end 54. In addition the prosthesis has a medial side 56 and a lateral side 58. In Figure 6, the anterior side 60 and the posterior side 62 of prosthesis 50 are identified. The component parts of a prosthesis, includes the stem 64, the neck 66 and the ball 68. The centerline of the neck 70 and the centerline of the stem 72 form an angle of approximately 45 degrees in the plane depicted in Figure 5. In the plane depicted in Figure 6, the angle between the stem centerline 72 and the neck centerline 70 is approximately 17.5 degrees. As previously described with reference to Figures 1-4, the invention relates generally to a preapplied mantel which, because of an object of the invention, occupies as much of the volume of the intramedullary canal as possible. Because the intramedullary canal and a typical stem may not taper to the same extent, the preapplied mantel may vary in thickness. In otherwords, the shape of the outermost surface is not strictly determined by the shape of the stem but rather, in large part a function of the shape of the canal in which the stem is inserted.

Figures 5-8 depict a specific geometry of the present invention and illustrate a preferred embodiment of the invention. As shown, mantel 74 is disposed on the surface the stem 64. The preapplied mantel 74 has a thickness of approximate 3-4 mm. at the proximal end 76 of stem 64 on the medial side 56 of prosthesis 50. The location at which the mantel has a thickness of 3-4 mm. is designated by A on Figure 5. On the medial side 56 of prosthesis 50 the thickness of the mantel decreases to approximately 1.5 mm. There is a smooth transition from the mantel thickness at location A to the 1.5 mm. mantel thickness at location B. The thickness of mantel 74 on the medial side 56 of the prosthesis 50 remains relatively constant from location B to a location C, a point near the distal end 52 of prothesis 50. In otherwords, the thickness of the mantel between location B and C on the medial side 56 of prothesis 50 is a approximately constant and is approximately 1.5 mm.

On the lateral side 58 of prosthesis 50, the mantel 74 has a thickness of approximately 1.5 mm at location D, near the proximal end 76 of stem 64. It should be noted that there is a smooth transition 79 from the preapplied mantel 74 to the stem 64 at the proximal end 76 of stem 64. The thickness of the mantel 74 on the lateral side 58 of prosthesis 50 remains at a constant 1.5 mm from location D to location E a point which is approximately halfway down the length of stem 64. The mantel tapers from location E to the distal end 52 of prosthesis 50. However, stem 64 has a slightly greater taper, resulting in the thickness of the mantel increasing to approximately 2 mm. at location F. Turning to Figure 6, the thickness of the mantel on both the anterior side 60 and the posterior side 62 of prosthesis 50 is approximately 1.5 mm. and is approximately constant.

As readily seen in both Figures 5 and 6, the preapplied mantel may be particularly thick at the distal-most end of the prosthesis. This thick region has been designated with reference number 78.

Shown in phantom in Figures 5 and 6 is the walls of the intramedullary canal 80. There is a small space between the canal 80 and the outermost wall of the mantel 74. This space is ultimately filled in the operating room with a bone cement using the method previously described. The volume of the canal 80 which is ultimately filled with the bone cement which is mixed in the operating room is approximately 10%. In contrast, the volume of the canal 80 which is occupied by the preapplied mantel 74 is approximately 20-25%. Generally, the volume of the canal 80 which is occupied by the stem is approximately 65-70%.

Figure 7 shows a cross section of Figure 5, cut along line 7-7 and Figure 8 shows a cross-section of Figure 5, cut along line 8-8.

The foregoing description of the preferred embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obviously, many modifications and variations may be made in light of the above teachings. For example, although the invention is depicted in the drawings as inserted in the femur, it is contemplated that the invention may be utilized as a bone implant in any bone in the human or animal body. It is also anticipated that the mantal may have a different geometry than described above. For example, the outside geometry of the mantal may have a constant diameter while the stem tapers resulting in a mantal that increases in thickness in the distal direction.

Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.

Claims

We Claim:

1. A prosthesis, comprising: a stem having a proximal end, a distal end, and an outer surface, wherein the outer surface has a medial side and a lateral side; and a mantel, said mantel being disposed on the outer surface of said stem and wherein the thickness of the mantel on the medial side of said stem has a maximum thickness at the proximal end of said stem and wherein the thickness of the mantel varies from a maximum of 2.5 - 4.5 mm to a minimum of 1 - 2 mm.

2. The prosthesis of claim 1, wherein the thickness of the mantel decreases on the medial side of said stem along at least a portion of the length of said stem.

3. The prosthesis of claim 1, wherein the thickness of the mantel on the lateral side of said stem increases along at least a portion of the length of the stem in the distal direction.

4. The prosthesis of claim 1, wherein said outer surface of said stem has an anterior side and a posterior side and wherein the thickness of the mantel along the entire length of said anterior side of said stem is substantially constant.

5. The prosthesis of claim 1, wherein said outer surface of said stem has an anterior side and a posterior side and wherein the thickness of the mantel along the entire length of said posterior side of said stem is substantially constant.

6. The prosthesis of claim 1, wherein the maximum thickness of the mantel on the medial side of said stem is between 3 - 4 mm.

7. The prosthesis of claim 1, wherein the mantel has a thickness of between 3 - 4 mm at the proximal end of the stem on the medial side of said stem surface and wherein said mantel tapers to a thickness of between 1.25 - 1.5 mm at the distal end of said stem on the medial side of said stem surface.

8. The prosthesis of claim 7, wherein the mantel has a thickness of approximately 4 mm at the proximal end of the stem on the medial side of the stem surface and wherein the mantel taper to a thickness of approximately 1.5 mm.

9. The prosthesis of claim 4, wherein the thickness of the mantel along the length of the stem on the anterior surface of said stem is approximately 1.5 mm.

10. The prosthesis of claim 5, wherein the thickness of the mantel along the length of the stem on the posterior surface of said stem is approximately 1.5 mm.

11. A prosthesis comprising: a stem having a proximal end and a distal end, wherein said stem has an outer surface; and a preapplied mantle, said mantle being secured to said outer surface of said stem, wherein said mantle is disposed on said outer surface of said stem, said mantle having a mantle surface which is sized and shaped to approximate the size of a cavity in which it is inserted.

12. A prosthesis according to claim 11, wherein said stem has longitudinal channels formed on said outer surface.

13. A prosthesis according to claim 11, wherein said stem comprises a metal alloy.

14. A prosthesis according to claim 11, wherein the thickness of said mantle increases from the proximal end of said stem to the distal end of said stem.

15. A prosthesis according to claim 11, wherein said mantle comprises polymethyl methacrylate.

16. A prosthesis according to claim 15, wherein said mantle further comprises styrene to increase the flexibility of said mantle.

17. A prosthesis according to claim 15, wherein said polymer mantle further comprises barium sulfide to increase the flexibility of said polymer mantle.

18. A prosthesis comprising: a stem having an outer surface; a thin film comprising a polymeric material disposed on said outer surface of said stem; and a thick polymer mantle disposed on said thin film prior to insertion of said stem within a cavity, said thick polymer mantel being substantially homogeneous and having an outermost surface which is sized and shaped to approximate the cavity in which it is inserted.

19. A prosthesis according to claim 18, wherein said stem has longitudinal channels formed on said outer surface.

20. A prosthesis according to claim 18, wherein said polymer mantle has a minimum thickness of at least 1.5 mm.

21. A prosthesis according to claim 18, wherein said polymer mantle comprises polymethyl methacrylate.

22. A prosthesis according to claim 21, wherein said polymer mantle further comprises styrene to increase the flexibility of said polymer mantle.

23. A prosthesis according to claim 21, wherein said polymer mantle further comprises barium sulfide to increase the flexibility of said polymer mantle.

24. A prosthesis according to claim 18, wherein said stem tapers along a portion of its length in the distal direction.

25. The prosthesis of claim 24, wherein the outer surface of the mantel is substantially circular in cross-section along a portion of the length of said stem and wherein the cross-section has substantially a constant diameter for said portion of the length of said stem.

26. The prosthesis of claim 24, wherein at least a portion of said mantel increases in thickness in the distal portion of said stem.

27. A prosthesis, comprising: a stem having a surface; a preapplied mantel disposed on said stem; and bone cement; wherein said stem; said preapplied mantel and said bone cement fill an intramedullary canal and wherein said stem occupies approximately 65-70% of a volume defined by the intramedullary canal, said preapplied mantel occupies approximately 20-25% of the volume of the intramedullary canal; and the bone cement occupies approximately 10% of the volume of the intramedullary canal.