MXPA99004003A - Orthopedic implant - Google Patents

Abstract

An orthopedic implant comprising a thermoplastic polymer or a composite comprising, in one embodiment, polyetheretherketone reinforced with 10%by volume of glass fibers, with an elastic modulus approximating the elastic modulus of bone. A porous coating is formed on the implant surface by creating a roughness thereon, by coating the surface with hydroxyapatite or by embedding a biocompatible material (18) such as titanium in the surface. A two piece embodiment of the implant is joined and locked together, after the opposite ends of each piece (10, 12) are inserted in the medullary canal, using an interlocking mechanism comprising a fluted protrusion (16) on one piece and a corresponding fluted cavity (14) in the other piece with the fluted portions being complementarily tapered.

Description

R () VVO 98/19617 A61B 17/72 ORTHOPEDIC IMPLANT EXHIBITION OF THE GOVERNMENT INTEREST This invention was made - with government support, according to Contract No. N00039-95-C-0001 granted by the Department of the Navy. The moan has certain rights in the invention. BACKGROUND OF THE INVENTION This invention relates to orthopedic implants (bones), which are used to replace a lost or diseased portion of the bone. Several conditions can lead to bone loss, including trauma, arthritic diseases, tumors, musculoskeletal defects, and the replacement of a failed implant. An intramedullary implant is usually used in bones - long bones (for example the femur and the humerus), and is inserted into the medullary canal, which goes through the diaphysis (axis) of the bone and it is filled with bone marrow. A long bone implant is one of two different types of intramedullary implants, the other being categorized as joint replacements. Replacement implants of joints (for example a hip or roller implant) have a much more complicated geometry than those of long bone, rod-type replacement. Both types of implants have shown similar modes of failure in clinical studies. The intramedullary implants that are currently used are, in general, made of metal, using an alloy of either titanium (ti) or cobalt-chromium (Co-Cr). Joint replacement implants are made primarily from a cobalt-chromium alloy containing molybdenum, which is added to improve the wear-resistant properties of the material, an important consideration when the implant is used to replace articulated surfaces. . Long bone replacement implants are most commonly manufactured from Ti, either in its pure state commercially or as an alloy with aluminum and vanadium. These materials have been experimentally and clinically proven to be biocompatible. This is not completely understood in biochemical form, but the tissue of the bone grows and binds to the Ti surface more easily than to other materials. This property allows the Ti to help in fixing the implant in the bone, an extremely important part of the operation that directly affects the duration of the event.It is important for the success of the implant that this implant remains stationary, so that the bone tissue can begin to grow around it. It achieves initial stabilization through the use of cement -the bone applied during surgery, which acts as a filler between the bone and the implant.The interfacial space is filled with cement to stabilize the implant and inhibit movement. Bone cement is a thermosetting particulate composite polymer, named polymethyl methacrylate (PMMA). Stabilization for a prolonged period of the implant in the bone is achieved by having a porous coating on the surface of the implant The porous coating is either added or molded over «The implant surface Ti or hydroxyapatite (HA) are two materials with good biocompatibility and / or factors of biostimulation, commonly used to create this porous coating. The Ti is sintered on the metal implant (for example Ti) in any of a mesh of corrugated wire or a random arrangement of particles. The HA is applied to the surface of the implant using plasma spray techniques. The surface coating should have pores large enough to allow bone cells to travel through it and to create a strong intertwined fixation by reconnecting with adjacent bone tissue through the mesh. This method of fixation depends on the connection of the bone tissue to hold the implant in place. If the bone tissue does not grow rapidly or is not strong enough, the implant does not fully stabilize and micro-movements may occur. Problems with current implant designs arise from the difference in mechanical properties between the materials used in the implant system and the bone itself. The Ti alloy has an elastic modulus equal to 110.3 Gpa and the Co-Cr alloy has an elastic modulus equal to 210.3 Gpa. In comparison with the cortical bone module, equal to approximately 13.8 Gpa, these metallic implants are at least eight times more rigid. This large gradient causes the protection of the voltage to through the interface of the implant and bone, where the implant supports and absorbs most of the load and leaves the bone virtually inactive and without tension. As noted in Wolff's law, the bone needs to be cycled tense to survive and remains strong enough to support the body. The protected bone, without tension, around a metal implant, begins to resorb and form cavities between the implant and the bone. These cavities weaken the fixation and allow micromovements of the implant in the bone, eventually producing waste from local wear. The microscopic waste foreign to the wear of the body in the surrounding tissue will cause a defense mechanism of the body and cause infectious reactions. The loosening of the implant is irreversible without intervention and finally leads to a revision operation. A patient may suffer only two or three additional procedures, before the bone becomes too weak and osteoporotic to support another replacement and is considered non-functional. COMPENDIUM OF THE INVENTION The isolastic bone and implant system of the invention minimizes, if not eliminated, the effect of Tension protection created by a metal implant, thus leading to a longer life of the implant in the body. In one embodiment, the thermoplastic polymer-with an elastic modulus, which approximates the bone module, is used for the implant. Since the bone is a natural composite material consisting of a matrix with organic and inorganic substances, the compounds are also an excellent selection of materials for use in implants, specifically in those situations where the material properties have a great impact on the success of the implant, such as the replacement of a hip. Thus, a second embodiment comprises a composite that includes a thermoplastic polymer and a reinforcing material, this compound having an elastic modulus that approximates the elastic modulus of the bone. The compound preferably comprises polyether ether ketone (PEEK), a temperature-controlled thermoplastic substance, preferably containing 10% by volume of cut E glass fibers, which result in a material having approximately the same stiffness as bone and, for the both, in a significant improvement with respect to the problem of protection of tensions. The final stage is the application or shaping of a porous coating on the surface of the implant, to create the porous environment for growth into the bone. The coating may comprise the hydroxyapatite applied to the surface, a roughness formed on the surface or a biocompatible material, such as titanium, embedded within the surface. A two-piece-of-an-intramedullary implant is joined and locked together, after the opposite ends of each piece are inserted into the medullary canal, using an interlocking mechanism comprising a grooved projection on a part and a corresponding grooved cavity on the part. the other piece, with the fluted portions being tapered in complementary fashion. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates the orientation parameters in the SMC Micromechanical Model for a software program (instructions) of Composite Materials; Figure 2 illustrates a three-dimensional finite element (FEM) model of a prosthesis with bone cement, and cortical bone, with an extracortical bone bridge; Figure 3 is a projection of the voltage gradients through different layer of materials for metal implants and composites, produced from the FEM; Figure 4 is a schematic representation of the internal injection molding assembly j Figure 5 is a photograph of a PEEK / 10% glass preform, longitudinally sectioned, randomly selected, - Figure 6 consists of Figures 6a and eb are , respectively, an image showing the relatively aligned (obscured) fibers, which are predominantly placed in the 1-2 plane; and an example of fibers pointed in the plane, and thus not chosen for the measurement of the orientation; Figure 7 illustrates the implant of the invention with the coincident grooves and the inclined grooves; Figure 8 consists of Figures 8a and 8b, showing, respectively, the coiling of the Ti coil around (in a section of) the implant; and one amplified image of the Ti embedded in the surface of the PEEK; Figure 9 illustrates a cross section of the epoxy impulse test (amplified 2 times), which represents the bone tissue, surrounding an implant of the PEEK, which does not have a Ti wire coil embedded in the surface of the implant; Figure 10 is a projection of load displacement v for four external impulse tests for the epoxy sections, representing the bone tissue, surrounding the PEEK implant having a Ti wire coil embedded within it. the surface of the implant; Figure 11 illustrates a cross section of the external pulse of the epoxy (amplified 2 times), which represents the bone tissue, which surrounds the PEEK implant with the Ti wire coil embedded therein; and Figure 12 illustrates the PEEK implant with the Ti wire coil embedded (2X amplified) after being driven completely through the epoxy. DETAILED DESCRIPTION OF THE INVENTION The first step in producing the implant of the invention was to develop a material having mechanical properties of a magnitude similar to those of bone. A number of polymers having such properties include some polymers that may be sufficiently rigid for use as an implant without the need for reinforcing fibers, for example Poly-X ™ self-reinforced polymers, manufactured by Maxdem Inc., of San. Dimas, California. A key aspect of the invention is to use a material such as an implant, having an elastic modulus that approximates the elastic modulus of the bone. For a composite implant, a high temperature thermoplastic polymer, polyether ether ketone (PEEK), was chosen as the resin or matrix, material for its relatively somparable strength, high firmness and previously recorded biocompatibility, with cells from human tissue. The glass fibers E were selected as the reinforcing material for their strength. Although carbon fibers can be used in the PEEK, the glass fibers were chosen because the material is more cost effective, and injection molding of the proferies (described below) is easier because the glass fibers are less abrasive than those of the carbon. More importantly, glass fibers are transparent to radiation therapy and do not create shadows or interfere with postoperative treatment. The properties of these constituent materials are listed in Table 1.

Table 1 ELASTIC MODULE FOR THE RESIN AND COMPOSITE FIBER MATERIALS In order to predict the properties of PEEK with glass fibers, a software package (named SMC Misromechanics Model for Composite Materials, was developed to determine the thermoelastic properties of glass fiber reinforced composite materials. constituent properties of the resin and the reinforcement phases, BU composition, the asbestos relasion of the fibers, and the degree of orientation of the reinforcement through the resin, to calculate the longitudinal modulus of the composite material. The longitudinal module for the created implant substrate was selected to be slightly smaller than that of the bone in anticipation of the additional rigidity and strengthening of the biocompatible metal porous surface, which is to be added later in the manufacturing process. Commercial compositions services typically provide fiber volumes of 10%, 20% and 30% for both glass and graphite fibers. These are the compositions used in the SMC program to determine the properties of the material. Instead of relying solely on commercial data published for these materials, the program was used to predict "the properties of the same compositions, but with various fiber orientations.Two parameters are used to describe the orientations of the fibers. Dessribe the orientation of flat fibers in the plane 1-2, and Fa defines the axial orientation relative to axis 3 (Figure 1) .The program was executed to find the range of modules for each composition from a completely random distribution of the fibers to a relatively aligned distribution. The values "are listed in Table 2. Table 2 PRODUCTION OF THE SMC PAKA PROGRAM COMPOUNDS OF PEEK A PEEK composition with 10% glass fibers had a predicted module range of 6.05 Gpa up to 10.54 Gpa for completely randomized and completely aligned fiber orientations, respectively. Note that both values are even lower than the bone modulus of 1.70 Gpa. The lowest volume ratio (10%) of available graphite fibers predicted a range of modules with an upper limit that was always greater than the bone module. For the thus, the composition of PEEK with 10% glass fibers was chosen as the substrate material for the composite implant. The properties determined by SMC were then used in the development of a finite element model (FEM) created using the Elementary Analysis Software. The model was made to study the induced stresses surrounding a bone replacement in vivo and to bridge the differences between a metallic implant and one composite. The model contains an implant, bone cement, cortical bone, and a layer of extracortical bone, in a three-dimensional arrangement, shown in Figure 2. The tensions resulting from a bending moment are studied, since they have a greater significance. than the stresses that result from axial and torsional loads. A comparison was made between the stresses resulting from a Ti implant and 10% glass-PEEK.The bending moment applied to the FEM produced longitudinal stresses through the implant and the bone, the most critical stress case considering the Tension protection The magnitudes of the tensions, in a section where the bridge of the extracortical bone is the thickest, were projected in Figure 3. The gradients of these stresses radially to the outside, through the extra-toric bone, cortical bone, bone cement and implant layers, are evident in the graph. While the tension that the composite implant carries is much lower than that for the metal implant, the tensions in the cement and the bone layers are greater for the composite implant than for the Ti. This is a direct result of the coincidence of the elastic modulus of the compound to that of the bone. The composite implant does not carry as much load as the metal implant, allowing the bone to absorb more of the pleated load. Therefore, the layer of cortisal bone carries more load when the composite implant is used, theoretically confirming that an implant with properties closer to those of the bone, leads to the elimination of the tension protection effect, evident with the metal implants of major modules. Prototypes of the composite implant are obtained using an injection and pressure molding system, shown in Figure 4, developed specifically for this project. The set consists of a deposit (1), where the material- sits and warms to its molten state, are a channel (2) that is opened and closed by a two-way valve, which connects the tank to the mold (3), the suals are held together tightly. The velocity of the process was controlled by the pressure applied by means of a piston (4) to the material in the tank. The material was released when the valve was opened and pushed at the end of the mold in the direction of a long axis of the part. Tank and mold temperatures are controlled individually by a set of four heaters each. The condition of the system initially uses thermoplastic materials of lower temperature. This allows observations of the process test and the discovery of any necessary modifications that are to be made, before the injection of the high temperature PEEK. The molded parts use ultra-high molecular weight polyethylene, acrylic, polycarbonate and filled polysarbonate are glass fibers, which progressively cure the tool at higher temperatures. The heshas modifications were made to improve the density of the parts that are produced, which include the addition of bleeding holes in the mold, to allow the spreading of the air bags and the adjustment of the pressures of insion and retro-pressures maintained in the piece. "Throughout this test period, the high pressure at the valve opening, followed by a lower pressure, is unselected. During the cooling of the part, its density increases Small pellets of 10% PEEK filled with glass, "They were heated to 360 ° C to reach their molten state. Since the PEEK is a highly viscous material, even in its molten form, the injection pressure was adjusted to 5250 g / cm2. A retro-pressure of 2100 kg / sm2 was maintained while the part cooled from 232 to 135 ° C. High in-sial pressure resulted in the highest injection velocity within the constraints of the system, and retro-pressure forced air balls residuals to escape. The production rate is sufficiently low due to the delay time in the heating _? system cooling every day, the limited amount of material used from filling the tank and the manual assembly and disassembly of the mold to obtain each individual part.

The prototype molded part is a preform of the final implant. Several additional machining and molding processes have to be performed to arrive at the final configuration. Tests were carried out to verify if the molded parts were consistent with those of the samples of the material supplied comersially. Explorations were made to make a visual evaluation of the density of the part. A cross section was photographed under amplification to measure the distribution of the fibers through the body of the part. Unlike the high production extrusion / somerial injection line, the material processed in the previous set remained molten for a much longer time, resulting in some oxidation. The characterization test was performed to confirm that the parts that are produced have retained the properties of the original material. The tension tests were carried out in a group of six moldings, randomly selected, machined to adapt an extensometer and have a caliber length of 2.54 cm. The results of the stress test were compared with those of the stress tests done in tension bars, commercially supplied, of different somatizations of the PEEK filled with glass. The commercial voltage bars were tested to measure the fault and final voltage resistance. The "internal" samples failed in the rosses used to adapt the machine and the final tensile strengths were never reached. The elastic modules, listed in Table 3, were comparable for all tests, which confirm the predictions of the SMC program and verify that the integrity of the material was preserved through the internal molding process.

Table 3 TEST RESULTS THAT CONFIRM THE PROPERTIES OF THE ORIGINAL MATERIAL Internal molded profiles Measured voltage resistance when threads fail * Standard ASTM test rods, commercially supplied.

We also evaluated the compound utterances of PEEK / 10% glass in a non-destructive way by means of the C-scan, to observe if there are gaps or air pockets in the parts that can interfere -finally they are the resistance of the part. The scans were calibrated to a sectioned part to see which signals correspond to the impurities and discontinuities in the material. The rest of the parts were explored non-destructively, and the results showed solid parts consistently _without significant defects. A random preform part was chosen and the cross section exposed the flow pattern of the injected molded material (see Figure 5). Images of the misro-polished transverse section were sampled at a 40-fold amplifisation, using an Optical microscope, which produces a clear image of the fibers.The image was then digitally imported into the NIH image software, to measure the angles of displaced axes of the fibers with respect to the horizon (longitudinal axis). The angles of the fibers were measured from the images taken along the center line of the part in a two-dimensional plane, spaced by approximately 3.49 mm. The measured angle of these images represents the position of the fiber in the 1-2 plane, as described with the SMC program (see Figure 1). Only the fibers that are predominantly placed in that plane were selected to be measured (Figure 6a).

The average angle of the displaced axis was 26.13 degrees with the interval extending from 0 to 93.92 degrees. Assuming that the same results can be seen in the plane that goes on the screen (Figure 6b), these images confirm that the orientation of the fiber can be classified as completely random. The values taken from the SMC data were further verified to "be accurate with the predicted module and the assumed orientation with the input." As shown in Figure 7, an implant of the invention comprises the first and second pieces 10, 12, each with a tip or end that fits into the intramedullary canal and extends from a wider body that has the porous coating, to support the growth of the bone-extracortical.Each tip is inserted into the medullary cavity at either end of the fractured bone in the diseased or damaged site deep enough to secure the ankle in the healthy bone The second piece has a cavity 14 to receive a member 16 which is probed in the first piece .The two pieces, which are tapered "in shape complementary for ease of alignment and assembly, they are joined by being -ahused together and locked by the pressure taper fit. The intramedullary implant is also designated as a means to resist rotation between the first and second pieces. In one embodiment, this comprises a six-groove intertrabado, to ensure that the implant is rotationally stable. Stretch marks are chosen to minimize the concentration of tension, while the intertrabado or fixation is maximized, but any wedge or groove element is acceptable to prevent rotation, such as grimaces and channels. In addition to using an adhesive to seal this connection, the matching grooves apply a positive lock during twisting. The six-flute co-incision design can be altered in consideration of the surgical procedure to implant these devices.The addition of more grooves will retain the strength and resistance to the torsional forces, while decreasing the angle between the two stripes. This decrease in angle will make it much easier for the two halves to correspond, when they secure each other during surgery, when time is of the essence.

The intertrawn flutes are molded on / within the ends of the proflerations and are designed to have a friction lock when they are tapered together, making the dimensions and accuracy of the molding very critical. Striations at the tips (the ends that are inserted into the medullary cavity of the bone) provide more surface area for the bone cement to fill and hold the implant in place. The final stage in obtaining the implant is to embed a firm Ti coil 18, inside the surface of the implant body, for example, by winding the coil around the implant and pressing it into the polymer after or during the application of heat. Titanium is used for its biocompatibility. The critical aspect of the design of this surface, apart from achieving a strong union of the Ti to the compound, is its porosity. In order to allow the necessary bone cells to "fit through the pores and create the desired mechanical intertracking, the pores, ie, the interstices between the exposed (non-incrusted) portions of the Ti wire coil, must be in the range of 150 to 200 μm, which may require that the wire coil be integined, that is, overlapped, to achieve it. The desired porosity can also be achieved by using the HA on the surface of the implant in. place of -Ti or, without Ti or HA, forming different surface roughnesses in the material that forms the implant. The surface roughness in any material increases the union of cells and can be created using a mold with a rough surface or by some treatment, for example chemical etching, sand treatment, or sandblasting, of the implant, after the molding process. The titanium coil can be embedded within the polymer by one of several methods, all the suals must be made in a vacuum or an inert gas atmosphere: 1) the coil is heated by an electrical resistance, while it is pressed inside of the polymer, - 2) the coil is preheated in an oven and then placed around the polymer, while it is pressed in place; 3) the coil is heated by induction in a high frequency RF field, while pressing on the polymer surface; 4) the coil and polymer surface are both heated by "a hot gas stream under pressure, while the coil is pressed into the polymer; and 5) the coil and the polymer are both blown at the point of intersection by an infrared beam, while the coil is wound around the polymeric implant and embedded. In each case, the coil should only be embedded in the polymer by 1/3 to 1/2 of its diameter, when the process is completed. Also, the mechanism of embedding the coil should not interfere with the heating method. The result is a process that leaves the Ti coil embedded in the PEEK surface by approximately half, as shown in the photographs in Figure 8. The outward impulse tests are performed to prove that the Ti coil it was safely embedded and mechanically locked on the surface of the PEEK implant. For each test, a section of the implant is placed in an epoxy, using a ratio of 33: 100"of the EPON Curing Agent V-40 to the EPON 826 Resin. The bone tissue, represented by the epoxy, surrounding the implant , which creates a mesánico intertrabado through and around the Ti coil.

Tests were done on an Instron machine, which performs a general compression of a cylinder. In order to prove that the results of the outward impulse tests represent the forces at the interface of the coil and the epoxy, instead of the PEEK and the epoxy, an initial test was made using an implant section without any embedded coils on the surface. The results showed that there is no binding of epoxy to PEEK. The implant was gently pushed out with a maximum load of 245 kilograms, creating a shear stress of 29.33 kg / cm2. Figure 9 is a cross-sectional image of the epoxy surrounding the part. It is obvious that there is no sorte or failure of the epoxy, which would have resulted if it joins are the PEEK. Then tests were done using implant sessions are the coil embedded in the surface. Three tests were done with a low displacement regime, applied at a constant of 1.27 mm / min. The twig of the maximum thrust out registered was on average 1536 kg, with an average maximum cutting tension of 159 kg / cm2. Test 4 was done with a higher displacement regime, applied to a constant of 25.4 cm / min. The maximum strength was approximately doubled, compared to the slower tests. The load curves vs. displacement for all tests, are shown in Figure 10. The consistency is obvious with tests 1-3 and a deeper tilt is shown for test 4 (this maximum force was not recorded quickly enough and was, so both, estimated from the final cutting stress of the epoxy, since the epoxy failed completely). Figure 11 is a cross-sectional image of the epoxy that surrounds part. The Ti trimmed from the epoxy and "which remained completely sealed in the PEEK, finally framed the epoxy." The mechanical intertrabado of the epoxy in the coil impelled this epoxy with it, as it is pushed through it. The amount of epoxy that remained attached to the prob coil, the mechanical intertramping of the material through the porous surface is extremely strong, and the push-out tests confirm that the Ti coil is locked mesanisably in the implant surface, considering that the greatest effort Damage to the interface of the implant / coil will be experienced when the implant is cut in vivo, these tests have proven that the Ti coil is essentially permanent on the surface. The amount of epoxy that remains in the coil as it is pushed shows that this type of porous surface is more than adequate in providing sufficient spasm for a material (ie the epoxy in the test and the left of the bone in vivo). to creser through it and create a mechanical intertrabado. Test 4 (performed with a higher displacement regime) simulates a scenario in the worst case of the force that the implant can suffer if the repaired bone experiences a greater impact. Clinically, the use of an implant that has an elastic modulus that approaches the elastic modulus of the bone, will have a great impact in the orthopedic industry. The advantages for a prolonged period of this new technology include a decrease in the amount of revision surgeries needed, reducing the rise in health care costs. The new implant will have a longer life of fatigue, which will better serve the population of younger patients, with a lower probability of resorting to pain and patient's surgery.

Claims (42)

1. CLAIMS 1. An orthopedic implant, to replace "a lost or diseased portion of a bone, this implant comprises a thermoplastic polymer, which has an elastic modulus that approximates the elastic modulus of the bone 2. An orthopedic implant, to replace" a lost or diseased portion of a bone, this implant includes a sompuesto, which somprende: a thermoplastic polymer; and a material for reinforcing the polymer; in which the compound has an elastic modulus-that approximates the elastic module of the bone. 3. The implant, as defined in claim 2, wherein the polymer comprises polyether ether ketone (PEEK) 4. The implant, as defined in claim 3, wherein the reinforcing material comprises glass fibers. 5. The implant, as defined in claim 4, wherein the glass fibers appear between 5 and 35% of the implant in volume. 6. The implant, as defined in claim 4, wherein the glass fibers comprise 10% of the implant by volume. The implant, as defined in claim 3, wherein the reinforcing material comprises carbon fibers 8. The implant, as defined in claims 1, 2 or 6, further comprising a porous overlay on the implant. a superfisie of the implant. 9. The implant, as defined in claim 8, wherein the porous coating comprises the hydroxyapatite. The implant, as defined in claim 8, wherein the porous coating comprises a rough state formed on the surface of the implant. 11. The implant, as defined in claim 8, wherein the porous re-cover comprises a biocompatible material partially embedded within the surface of the implant. 12. The implant, as defined in claim 11, wherein the biocompatible material comprises the titanium. 13. The implant, as defined in claim 11, wherein the biosompatible material appears in the form of a coil 14. The implant, as defined in claim 13, wherein the coil is wound around of the implant 15. The implant, as defined in claim 14, wherein the coil is embedded at a depth between 1/3 and 1/2 of the diameter of the coil 16. The implant, as defined in FIG. claim 15, in which, after the coil is partially embedded in the surface of the implant, the interstices between the non-incrusted portions of the coil vary from 150 to 200 microns 17. The implant, as defined in claim 16, in which the coil comprises titanium. 18. The implant, as defined in claim 17, which also includes a first piece and a second piece, the first and second pieces are joined and locked together by an internal locking element, the sual includes: a member projecting on the first piece and a cavity in the second piece, to receive the projecting member. 19. The implant, as defined in claim 8, further comprising a first piece and a second piece, the first and second pieces are joined and locked together by an internal locking element, the sual includes: a projecting member on the first piece and a cavity in the second piece, to receive the projecting member. 20. The implant, as defined in claim 19, further comprising an element for resisting rotation between the first and second pieces. 21. The implant, as defined in claim 20, wherein the element resisting rotation comprises a groove on the projecting member and a corresponding fluted opening in the cavity. 22. The implant, as defined in claim 20, wherein the projecting member and the cavity are tapered in complementary fashion. 23. The implant, as defined in claim 22, wherein the end opposite the member projecting on the first part and the opposite end of the cavity in the second part are both scored. 24. An intramedullary implant, to replace "a lost or diseased portion of a bone, this implant comprises: a first part, comprising a first end, for insertion into the medullary savity of a bone, and a second end having a member that is projected on the same; and a second piece, comprising a first end for insertion into the medullary cavity of a bone and a second end, having a cavity formed to receive the member projecting on the second end of the first part 25. The intramedullary implant, as defined in claim 24, further comprising an element for resisting rotation between the first and second pieces. 26. The intramedullary implant, as defined in claim 25, wherein the rotation resisting member comprises a groove on the projecting member and a corresponding grooved opening within the pocket. 27. The intramedullary implant, as defined in claim 26, wherein the fluted member projecting from the first part and the fluted cavity of the second part are tapered in complementarity. 28. The intramedullary implant, as defined in claim 25-, wherein the first ends of the first and second pieces are scored. 29. The intramedullary implant, as defined in claim 25, further comprising a porous coating on a surface of the implant. 30. The intramedullary implant, as defined in claim 29, wherein the porous coating comprises the hydroxyapatite. 31. The intramedullary implant, as defined in claim 29, wherein the porous coating comprises a rough state, formed on the surface "of the implant 32. The intramedullary implant, as defined in claim 29, wherein the coating The porous composite comprises a biocompatible material, partially embedded in the surface of the implant 33. The intramedullary implant, as defined in claim 32, wherein the bicompatible material is titanium 34. The intramedullary implant, as defined in claim 33 , wherein the titanium comprises the shape of a serpentine 35. The intramedullary implant, as defined in claim 34, wherein the coil is embedded at "a depth between 1/3 and 1/2 of the diameter of the coil. 36. The intramedullary implant, as defined in claim 35, wherein, before being embedded, the coil is wound around the implant. 37. The intramedullary implant, as defined in claim 35, wherein, after the coil is implanted in the surface of the implant, the interstices between the non-incrusted portions of the coil vary from 150 to 200 microns. 38. The intramedullary implant, as defined in claims 25, 29, 32 d 37, which further comprises a thermoplastic polymer, having an elastic modulus z approaching the elastic modulus of the bone 39. The intramedullary implant, as defined in claims 25, 29, 32 or 37, which further comprises a fiber-reinforced polymer, in which this fiber-reinforced polymer has an elastic modulus that approximates the elastic modulus of the bone 40. The intramedullary implant, according to is defined in claim 39, wherein the polymer comprises polyether ether ketone (PEEK). 41. The intramedullary implant, as defined in claim 40, wherein the reinforcing fiber comprises the glass. 42. The intramedullary implant, as defined in claim 41, wherein the glass fibers comprise 1 10% of the implant by volume. "