JOINT PROSTHESES

Abstract: A joint prosthesis comprising at least a first and a second load carrying member, the first load carrying member being substantially more shock absorbing and resilient than the second load carrying member.

Description: JOINT PROSTHESES The present invention relates to apparatus and methods for joint prosthesis surgery generally. Joint prostheses are well known in the art. Generally joint prostheses include a metal portion, typically constructed of steel or titanium, which articu¬ lates with a bony portion of the body. Non-articulating portions of the prosthesis are generally fixedly attached to tissue or bone. For example, a hip joint prosthesis of the art generally includes a metallic femoral head which articulates with a portion of the hip bone, and a metal¬ lic stem which is fixedly attached to the femur. Several problems are associated with prostheses of the art, for example, due to the mismatch between material properties of the prosthesis and bone. The contact between metal and bone may cause fretting wear of the bone. The difference in coefficient of thermal expan¬ sion between metal and bone may cause discomfort to the patient, especially during weather changes. The metallic prosthesis provides virtually no shock absorption or damping. It is known that a bone grows or regenerates according to the stress which it must bear. The metal prosthesis generally bears a much larger portion of weight than the surrounding bone. The reduced stress on the surrounding bone may tend to contribute to degenera¬ tion and recession of the bone, and to create an unde¬ sirable gap between the bone and the prosthesis. In order to overcome the aforementioned prob¬ lems, a great variety of prostheses with resilient por¬ tions have been proposed and developed. The following U.S. Patents are believed to be representative of the art: 5,522,904. 5,5-A,184, 5,514,182, 5,507,836, 5,507,833, 5,507.830. 5.507.823, 5,507,820, 5,507.818. 5,507,814, 5,491.882. 5.489.311, 5.458.651, 5,458.643, 5,448,489, 5,425,779, 5,415,662. 5.405.411, 5,405,403, 5,389.107. 5,387,244. 5.376.125, 5.376,064, 5.370,699, 5.358,525, 5.344.459, 5.336,268, 5.330,534, 5.326,376, 5.316,550, 5,314,494, 5.314,493. 5,314,478, 5.290,314, 5,282,868. 5,222.985. 5.217.499, 5,217,498, 5,201 ,882, 5,201,881 , 5,197.989. 5.197,987. 5.181,925, 5.171,276. 5.156,631. 5,151.521. 5.147.406. 5,146,933. 5.133.763. 5.116.374, 5.108.451. 5.108,446, 5.080,677. 5.049.393. 5,04l,l40, 5,019.107. 5.002.581. 4,997.447. 4.963.154, 4,963.153. 4,955,919. 4,955.912. 4,950,298, 4.938.773. 4.938,771, 4.936,856, 4,919,678, 4,919.674, 4.908,035. 4,904,269, 4,888,020, 4,822,365, 4,813.962, 4,808,186, 4,795.474, 4,795.470, 4.715.859. 4.664.668. 4.662,889, 4.661.112, 4.570,270, 4,344,193 and 3,875,594. The present invention seeks to provide improved joint prostheses which, inter alia, help overcome the above mentioned problems of the prior art. The prostheses provide shock absorption, damp¬ ing and resiliency. Portions of the prostheses which interface with human tissue are preferably constructed of resilient materials which are compatible with human bone or tissue, such as certain types of polyurethane. Certain portions of the prostheses may be constructed of compos¬ ite materials whose mechanical or physical properties may be optimized, such as to match properties of the local human bone or tissue. By matching properties of the local bone or tissue, the prosthesis behaves mechanically, structurally and thermally in a manner similar to the local bone or tissue, which helps make the prosthesis more efficient and comfortable. An important feature of the prostheses is that they help distribute stresses optimally, thereby stimu¬ lating regeneration of bone. The present invention is applicable for any joint in which there is free movement, known in technical terms as a true diarthrosis. True diarthroses include: 1. Gliding joints, known as arthrodias, in which the surfaces of the joint are flat, such as in the carpal bones ; 2. Hinge joints, known as ginglymi , such as the knee or elbow; 3. Condyloid joints, known as condylarthroses , which allow flexion, extension and lateral movement, but no rotation, such as the wrist, and saddle-shaped joints which allow the same type of movement as condyloid joints, but are generally stronger, such as the carpo- metacarpal joint of the thumb; 4. Ball and socket joints, known as enarthroses, such as the hip and the shoulder; and 5. Pivot joints, known as trochoides , which only allow rotation, such as the radio-ulnar joints. The present invention will be described in detail hereinbelow with respect to a prosthesis for an enarthrosis, such as the hip joint, and to a prosthesis for a ginglymus , such as the knee joint. It is appreciat¬ ed, however, that a prosthesis for any true diarthrosis is in the scope of the present invention. In a radical departure from the prior art, and in accordance with one embodiment of the present inven¬ tion, a hip joint prosthesis is provided which includes an artificial femoral head which is not fixedly attached to the femur, but rather articulates with both the femur and the acetabulum. The artificial femoral head is gener¬ ally spherical and may absorb shocks, provide damping and/or be resilient. A separate, artificial femoral head is easier to insert than the prostheses of the prior art which have a stem. In addition, the artificial femoral head may be provided with delimiting rails or grooves which serve to define and limit the movement paths of the femur with respect to the body, if required, and dislocation of the joint is substantially prevented. The delimiting rails or grooves may also serve as bumpers which damp and cushion the femoral head at the limits of its articulation. The prostheses of the present invention may also be provided with passageways for fluid, such as synovial fluid. Fluid present in these passageways helps to lubricate the prosthesis and provides viscous damping. Since the prostheses of the present invention are resilient, they geometrically adapt themselves to changes in static and dynamic forces borne by the joint. In the case of the hip joint prosthesis of the present invention, for example, normal raising of the thigh does not apply substantial forces on the hip joint, and the resilient hip joint prosthesis allows the freedom of movement of a ball and socket joint with substantially no deformation nor obstruction to movement due to friction between the prosthesis and human tissue or bone, or between adjacent regions of the prosthesis, such as between the artificial femoral head and an artificial socket . In contrast, when the person is standing, the static force of the weight of the person on the hip joint causes the resilient prosthesis to deform somewhat, i.e. , to be squashed a certain amount. This deformation pro¬ vides a relatively larger area for supporting the weight on the joint, thereby reducing pressure on the joint. The deformation also increases the friction force between the prosthesis and human tissue or bone, or between adjacent regions of the prosthesis, such as between the artificial femoral head and an artificial socket. The increased friction is beneficial because it does not hinder the stationary person; on the contrary, the increased fric¬ tion increases stability of the person. The resiliency of the prosthesis is also bene¬ ficial during sudden slips or falls. The dynamic and/or static forces due to the sudden movement tend to deform or squash the resilient prosthesis. As described above, the deformation reduces pressure on the joint, reduces danger of the prosthesis detaching from the bone, and increases friction which helps provide stability during the slip or fall. There is thus provided in accordance with a preferred embodiment of the present invention, a joint prosthesis including at least a first and a second load carrying member, the first load carrying member being substantially more shock absorbing and resilient than the second load carrying member. In accordance with a preferred embodiment of the present invention, at least one of the load carrying members is characterized in having at least one of strength and elasticity generally similar to that of human cartilage. There is also provided in accordance with a preferred embodiment of the present invention, a joint prosthesis including a plurality of alternating adjacent portions of substantially rigid and substantially resil¬ ient materials. There is also provided in accordance with a preferred embodiment of the present invention, a joint prosthesis including a plurality of alternating adjacent first and second portions, the first portion having a substantially rigid configuration and the second portion having a substantially resilient configuration. Preferably at least one of the first and the second portions is generally omega shaped. The joint prosthesis may include at least one portion compatible with human tissue. The joint prosthesis may have at least one hollow portion. Preferably, any of the joint prostheses in¬ cludes at least one delimiting rail or groove. Preferably, any of the joint prostheses in- eludes at least one passageway for a fluid. There is also provided in accordance with a preferred embodiment of the present invention, a hip joint prosthesis including an artificial, spherical femoral head which is adapted to articulate with an acetabulum and an upper portion of a thigh. There is also provided in accordance with a preferred embodiment of the present invention, a hip joint prosthesis including an artificial, self-articulat¬ ing femoral head, the head being attachable to at least one of an acetabulum and an upper portion of a thigh. Preferably, the hip joint prosthesis includes an artificial femoral head which is shock absorbing, provides damping and/or is substantially resilient. Preferably, the femoral head has at least one hollow portion. Further in accordance with a preferred embodi¬ ment of the present invention, the hip joint prosthesis also includes an artificial acetabulum attachable to an innominate bone, the artificial femoral head articulating with the artificial acetabulum. Still further in accordance with a preferred embodiment of the present invention, the hip joint pros¬ thesis also includes an artificial femoral socket attach¬ able to a femur, the artificial femoral head articulating with the artificial femoral socket. Further in accordance with a preferred embodi¬ ment of the present invention, the artificial femoral head includes a device for substantially preventing dislocation of the artificial femoral head from the artificial acetabulum, the artificial femoral socket, or both. Preferably the device for substantially pre¬ venting dislocation is shock absorbing or resilient. Further in accordance with a preferred embodi¬ ment of the present invention, the artificial femoral head includes at least one delimiting rail or groove. The delimiting groove may have a different geometrical shape than that of the rail. This permits providing various predetermined ranges and paths of motion. Still further in accordance with a preferred embodiment of the present invention, the artificial femoral head has at least one passageway for a fluid. Additionally in accordance with a preferred embodiment of the present invention, the hip joint pros¬ thesis includes a sleeve which envelops at least one portion of the prosthesis and which is attachable to at least one of a portion of an innominate bone and a thigh. Preferably the sleeve includes a relatively high strength fabric . Further in accordance with a preferred embodi¬ ment of the present invention. the artificial femoral socket is adapted to fit substantially snugly with at least one upper portion of a femur. Still further in accordance with a preferred embodiment of the present invention, the artificial femoral head includes a plurality of alternating adjacent portions of substantially rigid and substantially resil¬ ient materials . Additionally in accordance with a preferred embodiment of the present invention, the hip joint pros¬ thesis further includes a stem insertable into a femur. Further in accordance with a preferred embodi¬ ment of the present invention, the hip joint prosthesis includes an outer layer attachable to a femur, the outer layer including a material compatible with human tissue. There is also provided in accordance with a preferred embodiment of the present invention, a knee joint prosthesis including a femoral portion and a tibial portion, the femoral portion being attachable to a femur and the tibial portion being attachable to a tibia, the femoral portion articulating with the tibial portion. wherein at least one of the femoral portion and the tibial portion is shock absorbing, provides damping or is substantially resilient. In accordance with a preferred embodiment of the present invention, the knee joint prosthesis includes a device operative to limit motion of the tibia with respect to the femur. Preferably, the device operative to limit motion of the tibia with respect to the femur, is shock absorbin . Additionally in accordance with a preferred embodiment of the present invention, the femoral portion is generally convex and the tibial portion is generally concave . Alternatively in accordance with a preferred embodiment of the present invention, the femoral portion is generally convex and the tibial portion is generally convex . Further in accordance with a preferred embodi¬ ment of the present invention, the knee joint prosthesis includes at least one roller element, the femoral portion articulating with the tibial portion via the at least one roller element. Preferably, the knee joint prosthesis has at least one fluid passageway. There is also provided in accordance with a preferred embodiment of the present invention, a bone fastener including a plurality of alternating adjacent portions of substantially rigid and substantially resil¬ ient materials. There is also provided in accordance with a preferred embodiment of the present invention, a bone fastener including a plurality of alternating adjacent first and second portions, the first portion having a substantially rigid configuration and the second portion having a substantially resilient configuration. There is also provided in accordance with a preferred embodiment of the present invention, a method of incision of a ligament including forming a substan¬ tially wave-like incision in the ligament. There is also provided in accordance with a preferred embodiment of the present invention, a method of insertion of a hip joint prosthesis including: fixedly attaching a first joint element to an upper portion of a femur, the first joint element fitting substantially snugly with the upper portion of the femur; fixedly attaching a second joint element to a portion of an innominate bone; and inserting an artificial femoral head intermedi¬ ate the first and the second joint elements, such that the artificial femoral head articulates with at least one of the first and the second joint elements. Preferably, one or more natural or artificial ligaments may be used to strengthen the hip joint. There is also provided in accordance with a preferred embodiment of the present invention, a method for limiting a range of movement of a hip joint includ¬ ing: implanting a hip joint prosthesis comprising at least one delimiting rail and at least one delimiting groove, the at least one delimiting rail articulating with the at least one delimiting groove, such that the at least one delimiting groove limits articulation of the at least one delimiting rail therein. The present invention will be understood and appreciated from the following detailed description, taken in conjunction with the drawings in which: Fig. 1 is a simplified illustration of a human hip joint ; Fig. 2 is a simplified illustration of a hip joint prosthesis, constructed and operative in accordance with a preferred embodiment of the present invention; Fig. 3 is a partially sectional illustration of the hip joint prosthesis of Fig. 2; Fig. 4 is a partially sectional illustration of the hip joint prosthesis of Fig. 2, wherein the femur is rotated laterally; Fig. 5 is a partially sectional illustration of the hip joint prosthesis of Fig. 2, wherein the femur is rotated rearwardly; Fig. 6 is a simplified pictorial illustration of an artificial femoral head of the hip joint prosthesis of Fig. 2, the femoral head comprising two delimiting rails ; Figs. 7A and 7B are simplified illustrations of an artificial femoral head, constructed and operative in accordance with another preferred embodiment of the present invention, articulating with artificial and natural acetabula respectively, and wherein the artifi¬ cial femoral head has no delimiting rails; Figs. 8A and 8B are simplified illustrations of an artificial femoral head fixedly attached to a stem, constructed and operative in accordance with yet another preferred embodiment of the present invention, articulat¬ ing with artificial and natural acetabula respectively, and wherein the artificial femoral head has no delimiting rails ; Fig. 8C is a simplified illustration of an artificial, self-articulating femoral head fixedly at¬ tached to a stem and to an acetabulum, constructed and operative in accordance with a preferred embodiment of the present invention; Fig. 9A is a simplified pictorial illustration of an artificial femoral head and an artificial acetabu¬ lum, constructed and operative in accordance with still another preferred embodiment of the present invention, and wherein the artificial femoral head has one delimit¬ ing rail which articulates with the generally elliptical- ly shaped acetabulum; Fig. 9B is a simplified sectional illustration of the femoral head of Fig. 9A , taken along lines 9B - 9B in Fig. 9A; Fig. 9C is a simplified pictorial illustration of a flexible and stable bone connector, constructed and operative in accordance with a preferred embodiment of the present invention; Figs. 9 n and 9E are simplified, sectional illustrations of a flexible and stable bone connector stem of a hip joint prosthesis, constructed and operative in accordance with a preferred embodiment of the present invention, before and after deployment, respectively; Figs. 9F and 9 G &re simplified, sectional illustrations of a flexible and stable bone connector stem of a hip joint prosthesis, constructed and operative in accordance with another preferred embodiment of the present invention, before and after deployment, respec¬ tively; Fig. 10 is a simplified sectional illustration of a non-hollow artificial femoral head, constructed and operative in accordance with a preferred embodiment of the present invention; Fig. 11 is a simplified sectional illustration of an artificial femoral head, constructed and operative in accordance with another preferred embodiment of the present invention, and wherein the femoral head comprises a plurality of hollow portions; Fig. 12 is a simplified sectional illustration of an artificial femoral head, constructed and operative in accordance with yet another preferred embodiment of the present invention, and wherein the femoral head comprises a plurality of hollow portions filled with a fluid; Fig. 13 is a simplified sectional illustration of an artificial femoral head, constructed and operative in accordance with still another preferred embodiment of the present invention, and wherein the femoral head comprises a plurality of portions, each portion not necessarily having the same mechanical or physical properties ; Fig. 14A is a simplified sectional illustration of an artificial femoral head, constructed and operative in accordance with another preferred embodiment of the present invention, and wherein the femoral head comprises a protruding delimiting bumper; Fig. 14B is a simplified sectional illustration of an artificial femoral head, constructed and operative in accordance with yet another preferred embodiment of the present invention, and wherein the femoral head comprises a thin, resilient outer shell and a resilient core ; Fig. 15 is a simplified pictorial illustration of an artificial femoral head, constructed and operative in accordance with a preferred embodiment of the present invention, and including delimiting grooves; Fig. 16 is a simplified sectional illustration of an artificial femoral head, constructed and operative in accordance with another preferred embodiment of the present invention, and wherein the femoral head has fluid passageways ; Fig. 17 is a simplified sectional illustration of an artificial femoral head, constructed and operative in accordance with yet another preferred embodiment of the present invention; Figs. l8A and lδB are simplified pictorial and sectional illustrations respectively of an artificial femoral head, constructed and operative in accordance with another preferred embodiment of the present inven¬ tion, and wherein the femoral head comprises a plurality of alternating adjacent portions of substantially rigid and substantially resilient materials, Fig. l8B being taken along lines lδB - lδB in Fig. lδA; Figs. l8C - lδF are simplified pictorial illus¬ trations of alternative constructions of a femoral head including a plurality of alternating adjacent portions of substantially rigid and substantially resilient materi¬ als ; Fig. lδG is a simplified pictorial illustration of an artificial acetabulum, constructed and operative in accordance with a preferred embodiment of the present invention ; Figs. l8H, l8l and l8J are simplified illustra¬ tions of installing the artificial acetabulum of Fig. l8G into a natural acetabulum, in accordance with a preferred embodiment of the present invention; Figs. lδK and lδL are simplified sectional illustrations of two artificial acetabula, constructed and operative in accordance with two preferred embodi¬ ments of the present invention; Figs. 19A - 19C are simplified pictorial illus¬ trations of a method of incision of ligaments, such as prior to insertion of a hip joint prosthesis, in accord¬ ance with a preferred embodiment of the present inven¬ tion ; Fig. 20 is a simplified pictorial illustration of a sleeve for joining a femoral head with the innomi¬ nate bone, constructed and operative in accordance with a preferred embodiment of the present invention; Fig. 21 is a simplified sectional illustration of an expandable artificial femoral head, constructed and operative in accordance with a preferred embodiment of the present invention; Fig. 22 is a simplified illustration of a human knee joint; Figs. 23 and 24 are respective simplified side and front view illustrations of a knee joint prosthesis, constructed and operative in accordance with a preferred embodiment of the present invention; Fig. 25 is a simplified illustration of a femoral portion of the knee joint prosthesis of Figs. 23 and 24; Fig. 26 is a simplified illustration of a tibial portion of the knee joint prosthesis of Figs. 23 and 24; Figs. 27 and 28 are respective simplified side and front view illustrations of a knee joint prosthesis, constructed and operative in accordance with another preferred embodiment of the present invention; Figs. 29 and 30 are respective simplified side and front view illustrations of a knee joint prosthesis, constructed and operative in accordance with yet another preferred embodiment of the present invention; Figs. 31 and 32 are respective simplified side and front view illustrations of a knee joint prosthesis, constructed and operative in accordance with still anoth¬ er preferred embodiment of the present invention; Fig- 33 is a simplified illustration of a roller element included in the knee joint prosthesis of Figs. 31 and 32; Fig. 34 is a simplified, partially sectional illustration of a bone fastener, constructed and opera¬ tive in accordance with a preferred embodiment of the present invention; and Fig- 35 i a simplified, partially sectional illustration of a vertebra replacement, constructed and operative in accordance with a preferred embodiment of the present invention. The present invention will now be described in detail with respect to a prosthesis for an enarthrosis, an example being the hip joint, and to a prosthesis for a ginglymus , an example being the knee joint. It is appre¬ ciated, however, that a prosthesis for any true diarthro¬ sis is in the scope of the present invention. For a better understanding of a hip joint prosthesis, a basic description of the human hip joint is presented here with reference to Fig. 1, which illus¬ trates the hip of the right side of the body. The hip joint is a ball and socket joint, the ball being the femoral head (head of the thigh bone) which articulates with the acetabulum of the innominate bone, known in non¬ technical terms as the socket of the hip bone. The innominate bone in the area of the hip joint is made of three portions: the upper portion is called the ilium, the middle portion is called the pubis and the lower portion is called the ischium. The femoral head is connected to the innominate bone by a plurality of ligaments. The ligaments shown in Fig. 1 are the ilio- femoral ligaments and the pubo-femoral ligament. There is also an ischio-femoral ligament, not seen in Fig. 1. The femoral head articulates with a fibrous rim of the ace¬ tabulum called the cotyloid ligament. Generally a hip joint replacement of the prior art involves replacing the natural femoral head with a metallic artificial femoral head which is fixedly at¬ tached to a stem. The stem is generally inserted in the femur and the femoral head articulates with the acetabu¬ lum, if still intact, or some other depression, artifi¬ cial or natural, in the innominate bone. Some or all of the ilio-femoral , pubo-femoral and ischio-femoral liga¬ ments may be removed to provide access to the femoral head and acetabulum. Reference is now made to Figs. 2 and 3 which illustrate a hip joint prosthesis 10, constructed and operative in accordance with a preferred embodiment of the present invention. In a radical departure from the prior art, hip joint prosthesis 10 comprises a femoral head 12 which is not fixedly attached to the femur, but rather is capable of articulating with both the thigh and the innominate bone. In accordance with a preferred embodiment of the present invention, femoral head 12 may be constructed of a rigid material compatible with human tissue, for example, a metal such as stainless steel, or a structural plastic. In accordance with another preferred embodiment of the present invention, artificial femoral head 12 is constructed of a material which is shock absorbing, and additionally or alternatively provides damping, and additionally or alternatively is resilient. An example of such a material is polyurethane or synthetic rubber. A resilient artificial femoral head, unlike the prior art, yields upon application of forces and substan¬ tially returns to its original shape after such forces are removed . Artificial femoral head 12 is preferably, although not necessarily, generally spherical in shape. In accordance with a preferred embodiment of the present invention, and as shown in Fig. 3. femoral head 12 has a hollow core 14. Hollow core 14, inter alia, adds to the resiliency and shock absorbing characteristics of femoral head 12. In accordance with a preferred embodiment of the present invention, hip joint prosthesis 10 also comprises an artificial acetabulum 16 which is preferably fixedly attached to the innominate bone via an acetabulum interface 18. Artificial femoral head 12 articulates with artificial acetabulum 16. Additionally in accordance with a preferred embodiment of the present invention, hip joint prosthesis 10 comprises an artificial femoral socket 20 which is preferably fixedly attached to the femur via a stem 22 . Alternatively, socket 20 may be attached to the femur without a stem, for example, by bonding. Artificial femoral head 12 articulates with artificial femoral socket 20. Socket 20 is shaped to facilitate this articu¬ lation, such as being generally concave. In addition, socket 20 is preferably shaped to overlap, or snugly fit, the upper portion of the natural femur. The generally concave, overlapping shape of artificial femoral socket 20 helps distribute stresses optimally on the femur, thereby stimulating regeneration of bone. Socket 20 may overlap and "hug" the upper portion of the femur on the outside surface of the femur. Alternatively or additionally, socket 20 may be config¬ ured to fit snugly into an inner surface of the femur. It is important to note that socket 20 serves two general tasks, as described above. The first task is articulation with artificial femoral head 12. The second task is fitting snugly with the femur and distributing stresses evenly thereon. It is appreciated that in accordance with another preferred embodiment of the present invention, socket 20 may comprise two separate portions each gener¬ ally dedicated to serving one of the above described tasks. A first portion 20A, generally concave in shape, may be generally dedicated to articulation with artifi¬ cial femoral head 12. A second portion 20B, generally shaped as a "crown" to hug and snugly fit circumferen¬ tially around and on top of the femur, may be generally dedicated to distributing stresses evenly on the femur. This is true of any of the sockets described herein with reference to any of the embodiments of the present inven¬ tion. The "crown", i.e. , second portion 20B, greatly changes the loading conditions of prosthesis 10, de¬ creases stresses exerted on the femur by stem 22 , and creates a new and healthier stress distribution on the surrounding bone and tissue. With the support of second portion 20B, stem 22 may have a much smaller section throughout and particularly at its neck. Stem 22 preferably includes a core 24 and an outer layer 26, as seen in Figs. 2 and 3- Outer layer 26 preferably includes one or more ridges 28 , which, inter alia, help distribute stresses and help fasten stem 22 to the femur. Core 24 of stem 22 , artificial acetabulum 16 and artificial femoral socket 20 are preferably con¬ structed of a rigid material, for example, stainless steel or a structural plastic. Alternatively, the rigid material may be a composite material, such as a lay-up of graphite fibers, which may be constructed to have mechan¬ ical or physical properties, such as modulus of elastici¬ ty or coefficient of thermal expansion, equivalent to that of the local human bone. Stem 22 provides excellent three-dimensional anchorage to the bone, and induces three-dimensional loading stress conditions as close as possible to the natural conditions. The improved stress field distribu¬ tion at the interface between the prosthesis and the bone helps prevent lysis. The stress field set up by the prosthesis inside the bone helps induce regeneration and strengthening of the bone. Acetabulum interface 18 and outer layer 26 of stem 22 are preferably made of a resilient material compatible with human tissue, such as polyurethane, which helps distribute stresses optimally, thereby stimulating regeneration of bone. In accordance with a preferred embodiment of the present invention, acetabulum interface 18 and outer layer 26 of stem 22 are constructed of a material, such as polyurethane, which has one or more mechanical and/or physical properties substantially similar to human cartilage. Hip joint prosthesis 10 may include a device for facilitating removal as is known in the art, such as a threaded boss or hole (both not shown) . It may sometimes be desired to limit the number of degrees of freedom of the hip joint or the range of a particular degree of freedom of movement of the femur with respect to the acetabulum, depending on the needs of the patient. It may also be desired to provide safeguards to substantially prevent dislocation of the joint. In accordance with a preferred embodiment of the present invention, apparatus is provided to achieve these goals, as is now described. Reference is now made additionally to Figs. 4 - 6. In accordance with a preferred embodiment of the present invention, artificial femoral head 12 is provided with an upper delimiting rail 30 and a lower delimiting rail 32. Upper delimiting rail 30 slides in a channel 3 in artificial acetabulum 16. Channel 34 is oriented generally forwards and rearwards with respect to the human body. As seen in Fig. 5. upper delimiting rail 30 slides forwards in channel 3 when the femur is moved backwards. It is appreciated that upper delimiting rail 30 slides backwards in channel 34 when the femur is moved forwards . Lower rail 32 slides in a channel 36 in artifi¬ cial femoral socket 20. As seen in Fig. 4, as the femur is moved laterally away from the body, lower rail 32 slides in channel 3 of socket 20 and butts thereagainst. Upper delimiting rail 30 is similarly constrained to slide in channel 34. Constraining the travel of rails 30 and 32 in channels 3 and 36, respectively, substantially prevents overtravel of the femur and substantially pre¬ vents dislocation of artificial femoral head 12 from artificial acetabulum 16 and artificial femoral socket 20. Moreover, since femoral head 12 is preferably con¬ structed of a shock absorbing or resilient material, the butting of rails 30 and 32 against channels 34 and 36, respectively, is substantially cushioned and damped. Variations of hip joint prosthesis 10 are possible within the scope of the present invention. Reference is now made to Figs. 7A and 7B which illustrate an artificial femoral head 40, constructed and operative in accordance with another preferred embodiment of the present invention. Artificial femoral head 40 may be similar in construction and operation to artificial femoral head 12 of Figs. 2 - 6. Femoral head 40 differs from femoral head 12 in that femoral head 40 has no delimiting rails. In Fig. 7A, femoral head 40 is shown articulating with an artificial acetabulum 42. In Fig. 7B , femoral head 40 is shown articulating with a natural acetabulum 44. Reference is now made to Figs. 8A and 8B which illustrate an artificial femoral head 46 fixedly attached to a crown 48, constructed and operative in accordance with yet another preferred embodiment of the present invention. Femoral head 46 and crown 48 may be construct¬ ed of a resilient material, such as polyurethane. Crown 48 may be attached to an upper portion of the thigh. Alternatively, as shown in Fig. 8A, crown 48 may be attached to a stem 49- Stem 49 may include a core 49A and an outer layer 49B. Core 9A may be of solid construction, and additionally or alternatively, may include at least one hollow portion. Outer layer 49B may be constructed of a material with properties similar to human cartilage. In Fig. 8A , femoral head 46 articulates with an artificial acetabulum 50. Femoral head 46 may have an upper delimiting rail (not shown) which articulates with a corresponding groove (not shown) in artificial acetabu¬ lum 50. Alternatively, femoral head 46 may have a delim¬ iting groove with which articulates a corresponding rail in artificial acetabulum 50. In Fig. 8B , femoral head 46 articulates with a natural acetabulum 52. Femoral head 46 may include a hollow portion (not shown), as described hereinabove for artificial femoral head 12 with respect to Figs. 2 - 6. Alternatively, in accordance with another preferred embodiment of the present invention, artificial femoral head 46 may be fixedly attached to artificial acetabulum 0. In such an embodiment, artificial femoral head 46 may articulate with crown 48. Reference is now made to Fig. 8C which illus¬ trates an artificial, self-articulating femoral head 55 fixedly attached to a stem 56 and to an artificial ace¬ tabulum 57. constructed and operative in accordance with a preferred embodiment of the present invention. Femoral head 55 is preferably constructed of a resilient materi¬ al, such as polyurethane. Artificial acetabulum 57 m y have any suitable shape, typically being generally shell- shaped or spherical. Articulation of the thigh with the innominate bone is not achieved by articulation of femoral head 55 with artificial acetabulum 57. but rather is achieved by the self-articulation of femoral head 55- "Self-articula¬ tion" is defined as the ability of femoral head 55 to permit rotary and translatory motion of the thigh with respect to the innominate bone due to the resilient and elastic properties and configuration of femoral head 55- Stem 56 may comprise an outer layer 58 which may have properties similar to human cartilage. Reference is now made to Figs. 9A and 9B which illustrate an artificial femoral head 60 and an artifi¬ cial acetabulum 62, constructed and operative in accord¬ ance with still another preferred embodiment of the present invention. Artificial femoral head 60 has one delimiting rail 64 which articulates with a generally elliptically shaped recess 66 in artificial acetabulum 62. It is appreciated that this type of rail may be employed in any of the other artificial femoral heads described herein, either as an upper rail or a lower rail or both. Artificial femoral head 60 may be provided with one or more fluid passageways 67, as seen in Fig. 9A, for flow therethrough of a fluid (not shown) , such as synovi¬ al fluid. It is appreciated that any of the artificial femoral heads of the present invention may be provided with fluid passageways. Fluid passageways 67 help lubri¬ cate artificial femoral head 60, and provide damping. Reference is now made to Fig. 9C which illus¬ trates a flexible and stable bone connector 450, con¬ structed and operative in accordance with a preferred embodiment of the present invention. Connector 450 pref¬ erably includes a hollow, generally tubular portion 4 and a bone interface portion 54. Interface portion 454 is preferably shaped to snugly fit the inner geometry of the bone into which it is placed. The inner geometry of the bone may be determined by such methods as a computer tomography, and interface portion 54 may then be ma¬ chined accordingly. In accordance with a preferred embodiment of the present invention, interface portion 45 has a fluted shape with a plurality of protruding fins 56. The fluted shape of interface portion 454 and the hollowness of tubular portion 4 2 promote bone development and growth after implanting the prosthesis. Connector 4 0 may be fashioned in a variety of configurations, such as straight, curved, cylindrical or tapered, for example. A known problem associated with the repair of broken bones and with the insertion of stems of femoral prostheses into femurs, is that the bone may have a curvature which changes along the length thereof in three dimensions. It is difficult to match the curvature of the stem of the prosthesis to the natural curvature of the bone. In practice, usually a set of standardized connect¬ ing pins or prostheses are used and the closest matching prosthesis is selected and further machined or filed in the operating theater to match the measured natural curvature of the femur. Even with this method, gaps are almost inevitable between the prosthesis and the inner bone tissue. It is a particular feature of the present invention that connector 50 is sufficiently flexible so that it can be inserted into a bored portion of the bone, such as a femur, and deform to adapt to the changing curvature of the bone, thereby helping to solve the aforementioned problem. Connector 0 is preferably constructed of a material which provides flexibility to permit insertion into the bone, while at the same time providing sufficient structural stability once connector 450 is in place. A suitable material is one having a non¬ linear, "half-bell-shaped" stress-strain relationship, for example, a plastic such as polyurethane. The material may be reinforced with fibers, whose density and orienta¬ tion may be selected in accordance with a particular engineering requirement. Reference is now made to Figs. 9 n and 9E which illustrate a flexible and stable bone connector stem 460 of a hip joint prosthesis, constructed and operative in accordance with a preferred embodiment of the present invention. Stem 460 may be readily employed in any of the femoral prostheses of the present invention. Stem 460 is preferably constructed similarly to connector 450, and preferably includes a fluted elongated portion 462 having a hollow portion 464 and a plurality of protruding fins 466. A force transfer element 468, such as a wire, rod or cable, with a plurality of bulges 470 is preferably disposed in hollow portion 464. Element 468 may be made of any suitable stiff, biocompatible material, such as DYNEEMA. Stem 460 is preferably inserted into the femur in the orientation shown in Fig. S O . After insertion, element 468 is then moved generally in the direction of an arrow 472, thereby causing bulges 70 to deform fluted portion 462, as seen in Fig. 9A and fix stem 460 firmly in the femur. Reference is now made to Figs. 9F and 9G which illustrate a flexible and stable bone connector stem 480 of a hip joint prosthesis, constructed and operative in accordance with another preferred embodiment of the present invention. Stem 480 may also be readily employed in any of the femoral prostheses of the present inven¬ tion . Stem 480 preferably includes a fluted elongated portion 482 in which is disposed a sleeve 484 having a plurality of bulges 486. Disposed inside sleeve 484 is hollow shaft 488. Sleeve 484 is arranged for sliding, axial motion with respect to fluted portion 482 and shaft 488. As describe hereinabove for fluted portion 462, fluted portion 482 is sufficiently flexible so that it can be inserted into a bored portion of a femur and deform to adapt to the changing curvature of the femur. Stem 480 is preferably inserted into the femur in the orientation shown in Fig. 9F. After insertion, sleeve 484 is then moved generally in the direction of an arrow 490, thereby causing bulges 486 to deform fluted portion 482, as seen in Fig. 9 n , and fix stem 480 firmly in the femur. Artificial femoral head 12 shown in Figs. 2 6, has a hollow core 14. Reference is now made to Fig. 10 which illustrates a non-hollow artificial femoral head 70, constructed and operative in accordance with a pre¬ ferred embodiment of the present invention. Artificial femoral head 70 may include an upper rail 72 and a lower rail 74. Reference is now made to Fig. 11 which illus¬ trates an artificial femoral head 80, constructed and operative in accordance with another preferred embodiment of the present invention. Femoral head 80 comprises a plurality of hollow portions 82. Reference is now made to Fig. 12 which illus¬ trates an artificial femoral head 90, constructed and operative in accordance with yet another preferred embod- iment of the present invention. Femoral head 90 comprises a plurality of hollow portions 92 filled with a fluid, such as synovial fluid. Additionally or alternatively, one or more fluid passageways 94 may be provided. The fluid in portions 92 or passageways 94 may enhance the shock absorbing and damping characteristics of femoral head 90. Reference is now made to Fig. 13 which illus¬ trates an artificial femoral head 100, constructed and operative in accordance with still another preferred embodiment of the present invention. Femoral head 100 comprises a plurality of portions 102, each portion 102 not necessarily having the same mechanical or physical properties. Portions 102 may be used to enhance, to optimize or to customize the shock absorbing and damping characteristics of femoral head 100. Reference is now made to Fig. l4A which illus¬ trates an artificial femoral head 110, constructed and operative in accordance with another preferred embodiment of the present invention. Femoral head 110 comprises a protruding delimiting bumper 112 instead of a delimiting rail . Reference is now made to Fig. 14B which illus¬ trates an artificial femoral head 113. constructed and operative in accordance with yet another preferred embod¬ iment of the present invention. Femoral head 113 com¬ prises a thin, resilient outer shell 114 and a resilient core 115. Shell 114 may be constructed of DYNEEMA high performance polyethylene fibers, commercially available from DSM, Netherlands. DYNEEMA, particularly in the form of a woven fabric, provides a combination of high strength with excellent shock absorbing and damping characteristics, as well as being biocompatible. Core 115 may also be made of DYNEEMA with properties engineered to meet requirements such as strength or resilience, for example, and may be impregnated with other materials, such as a resin . Reference is now made to Fig. 15 which illus¬ trates an artificial femoral head 120, constructed and operative in accordance with a preferred embodiment of the present invention. Femoral head 120 includes delimit¬ ing grooves 122 which articulate with corresponding rails (not shown) in an artificial acetabulum and an artificial socket (not shown) . Reference is now made to Fig. 16 which illus¬ trates an artificial femoral head 130, constructed and operative in accordance with another preferred embodiment of the present invention. Femoral head 130 has fluid passageways 132 which allow flow therethrough of a fluid, preferably synovial fluid, the natural lubrication fluid of the human body. Fluid passageways 13 may be config¬ ured in a variety of orientations, configurations and sizes. Alternatively or additionally, passageways 132 may be provided in an artificial femoral socket 134 or an artificial acetabulum 136. Fluid flowing in fluid passageways 132 may help lubricate femoral head 130. The presence of fluid in fluid passageways 132 may also enhance the shock absorb¬ ing and damping characteristics of femoral head 130. As mentioned above, the ligaments connecting the femur and the innominate bone may be removed in the prior art, before placement of a hip joint prosthesis. This is unfortunate because these ligaments are amongst the strongest ligaments in the body. These ligaments strengthen the joint and help prevent dislocation. Pre¬ serving some or all of the ligaments is therefore desira¬ ble. Reference is now made to Fig. 17 which illus¬ trates an artificial femoral head l4θ, constructed and operative in accordance with yet another preferred embod¬ iment of the present invention. Artificial femoral head 140 is preferably relatively small in size, thereby helping to reduce the need for tampering with some of the hip joint ligaments. In this embodiment, femoral head l4θ articulates with an artificial acetabulum 142 and an artificial socket 144. Socket 144 may be attached direct¬ ly to the femur without a stem, such as by bonding or via a crown (not shown) similar to the crown-shaped portion 20B described hereinabove with reference to Figs. 2 and 3- By eliminating the stem, the need for tampering with or drilling into the femur may also be eliminated. There may be no need to remove the entire natural femoral head, but rather a portion thereof may be preserved. Preserving part of the femur may simplify the surgical operation and may preserve most of the strength of the bone . The need for tampering with some of the liga¬ ments may also be reduced. Indeed, the intact ligaments themselves act together with the prosthesis of Fig. 17, because they tend to keep femoral head 140 properly installed. In addition, the embodiment of Fig. 17 may be more easily and quickly implanted than prostheses having stems . Other embodiments of the present invention which address the problem of preserving the hip joint ligaments are described hereinbelow with respect to Figs. 19A - 21. Reference is now made to Figs. lδA and lδB which illustrate an artificial femoral head 150, con¬ structed and operative in accordance with another pre¬ ferred embodiment of the present invention. Femoral head 150 comprises a plurality of alternating adjacent por¬ tions 15 and 154 of substantially rigid and substantial¬ ly resilient materials respectively. The rigid material is preferably a composite material and the resilient material is preferably polyurethane. In accordance with a preferred embodiment of the present invention, femoral head 150 is attached to an artificial acetabulum I56. The material composition and the geometry of the portions 1 and 1 may be optimized to provide the desired rigidity and resiliency. In this manner, femoral head 150 may be constructed as a non-linear spring with multiple spring constants. Femoral head 150 may have different rigidity and resiliency for forward-backward motion as opposed to lateral motion. For example, as seen in Fig. lδB, adja¬ cent portions 152 and 154 are generally omega-shaped. Such a shape permits relatively easy swinging of the femur forwards and backwards with respect to the body, while at the same time constraining the swinging range to prevent overtravel of the femur. The resiliency of femo¬ ral head 1 0 damps the motion of the femur at the limits of its swing. The omega shape is stiffer in the lateral direction, thus limiting lateral motion of the femur with respect to the body. It is appreciated that femoral head 150 may be alternatively constructed to allow greater freedom of motion laterally than forwards and backwards. Reference is now made to Figs. lδC and lδD which show femoral heads l6θ and 162 respectively, with adjacent layers l6l and 163 of substantially rigid and substantially resilient materials, respectively, con¬ structed and operative in accordance with an alternative preferred embodiment of the present invention. As shown in Figs. lδE and lδF, femoral heads 160 and 162 may be provided with apertures 164 and 166 respectively, which may, for example, provide a passageway for synovial fluid, nerves, blood vessels, ligaments, tissues, elon¬ gated force transmitting members or prosthetic controls. Fluid in apertures 164 and 166 may enhance the damping of femoral heads 160 and 162 respectively. It is appreciated that the embodiments of Figs. lδA - lδF may be used as hinge or joint elements in other applications where it is desired to provide different rigidity or resiliency in different directions of motion. Reference is now made to Fig. lδG which illus¬ trates an artificial acetabulum 400, constructed and operative in accordance with a preferred embodiment of the present invention. Artificial acetabulum 400 preferably includes an interface 402 made of a resilient, cartilage-like material, and preferably has a generally triangular cut¬ out 4θ4. Acetabulum 400 preferably also includes an outer ridge 406 that "snap-fits" into the natural acetabulum socket, thereby substantially fixing artificial acetabu¬ lum 400 in the natural socket. The natural acetabulum may have to be drilled, cut or otherwise machined to ensure a proper snap fit so that acetabulum 400 is rigidly held in place. Ridge 406 may be continuous or may be formed of discrete portions that protrude into the natural acetabu¬ lum recesses. As seen in Fig. lδG, interface 402 may comprise a deformable, resilient flange 4θ8 with expand¬ able, accordion-like folds. Interface 402 may be one highly deformable piece, or may be slightly deformable, in which case it may fit into the natural acetabulum with a slight "click". It is a particular feature of the present invention that ridge 4θ6 provides shock absorption and positively locks interface 402 into a recess prepared in the natural socket, without any need for screws or adhe¬ sive. Interface 402 may comprise one or more layers. The large surface area of interface 402 provides a large load bearing and shock absorbing surface for a femoral head. Interface 402 may itself serve as an articulating surface for a femoral head, in which case the large surface area diminishes fretting and wearing of the articulating surfaces. Reference is now made to Figs. lδK and lδL which illustrate a cross section of interface 402 con¬ structed in accordance with two preferred embodiments of the present invention. In Fig. lδK, it is seen that interface 402 preferably includes a plurality of protru¬ sions 4l0 for locking interface 402 in recesses prepared in the natural socket. As seen in Figs. lδG and lδL, interface 402 may also include an "umbilical" protrusion 412 that is configured to fit the natural or restructured "umbilical" recess of the natural acetabulum. This allows reduction of machining of the innominate bone and leaves a stronger bone. Acetabulum 400 also preferably includes a locking piece 4l4 complementary shaped and sized to snugly fit into triangular cut-out 4θ4. Interface 402 together with locking piece 4l4 may be used as the artic¬ ulating portion of the prosthesis with the femur. Addi¬ tionally, there is preferably provided an articulation portion 4l6 which snaps together with a recess 4l8 formed in interface 402. Articulation portion 4l6 may be made of metal, composite material, cartilage-like material, polyurethane or DYNEEMA. Articulation portion 4l6 may alternatively be attached to interface 402 by means of a bayonet type of connection or simply a press fit without clicking. Articulating portion 4l6 makes artificial acetabulum 400 into one stable integral assembly which is easily assembled in and removed from the innominate bone. Reference is now made to Figs. lδH, lδl and lδJ which illustrate installing artificial acetabulum 400 into a natural acetabulum, in accordance with a preferred embodiment of the present invention. In Fig. lδH, an insertion tool 420 squeezes triangular cut-out 404 in¬ wardly in the direction of arrows 422, to allow insertion of interface 402 into the natural socket. Upon release of insertion tool 420, cut-out 4θ4 springs outwards much in the manner of a retaining ring, thereby pressing inter¬ face 402 firmly against and into the natural socket. Prongs of the insertion tool may be placed in prepared recesses or holes in interface 402. In Fig. lδl, locking piece 4l4 is inserted into cut-out 4θ4 , thereby completing the shape of acetabulum 400 and firmly locking interface 402 into the natural socket when an articulating head or an additional articu¬ lating surface is assembled therewith. In Fig. lδJ, articulation portion 4l6 is snapped together with inter¬ face 402, thereby making artificial acetabulum 400 into one integral assembly. As mentioned above, the ligaments connecting the femur and the innominate bone may be removed in the prior art, before placement of a hip joint prosthesis. Methods for preserving the hip joint ligaments, or rein¬ forcing or replacing them, are now described. Reference is now made to Figs. 19A - 19C which illustrate a method of incision of ligaments, such as prior to insertion of a hip joint prosthesis, in accord¬ ance with a preferred embodiment of the present inven¬ tion. A primary goal of the method of incision is to preserve the ligaments. Fig. 19A illustrates a hip joint 170 prior to incision. As seen in Fig. 19B, a wave-like incision 172 may be made, such as with a laser device, in any or all of the ilio-femoral , pubo-femoral and ischio-femoral ligaments. As seen in Fig. 19C, the cut ligaments allow ample room for placement of a hip joint prosthesis (not shown) . As is known in the art, ligaments generally contract after incision, impairing mending of the liga¬ ment tissue. The wave-like shape of incision 172 permits slightly shifting the ligaments so that there is suffi¬ cient contact or overlap of the ligaments even after contraction, thereby helping to promote stitching and mending of the ligament tissue. Reference is now made to Fig. 20 which illus¬ trates a sleeve 180 for joining a femoral head 182 with the innominate bone, constructed and operative in accord- ance with a preferred embodiment of the present inven¬ tion. Sleeve lδθ is preferably made of a high strength woven fabric, such as DYNEEMA, polyethylene, nylon or polyurethane. Sleeve lδO preferably comprises filaments with a high elasticity modulus in the longitudinal direc¬ tion of the ligaments, along an axis 184, and a circum¬ ferential stretching weave along an axis 186, generally perpendicular to axis 184, as seen in Fig. 20. Sleeve lδO also preferably includes an anchor¬ ing band lδδ for attaching sleeve 180 to the innominate bone. Anchoring band lδδ may be attached to the innomi¬ nate bone and sleeve 180 may be attached to the femoral head by any suitable means, such as bonding or with mechanical fasteners. Sleeve lδO may replace or assist the natural ligaments of the hip joint. Sleeve lβO may also help in mending of ligament tissue after surgery. All or portions of sleeve lδO may be constructed of a material, such as material used for dissolving sutures, which eventually dissolves after a predetermined period. The present invention also provides a hip joint prosthesis which may substantially reduce the need for tampering with the hip joint ligaments. Reference is now made to Fig. 21 which illustrates an expandable artifi¬ cial femoral head 190, constructed and operative in accordance with a preferred embodiment of the present invention. Femoral head 190 is preferably constructed of a resilient material, such as polyurethane. When expand¬ ed, femoral head 190 has substantially the same shape as femoral head 12, described hereinabove with reference to Figs. 2 and 3- In contrast to the prior art, femoral head 190, before expansion, may be inserted between the existing ligaments with minimum tampering thereof. Femoral head 190 may then be expanded to the desired shape. Femoral head 190 may be inflated by means of a fluid (not shown) introduced, for example, via a thin needle valve (not shown) . Alternatively, femoral head 190 may be expanded by introducing therein components of an expandable foam (not shown) , which expand inside femoral head 190. The present invention will now be described in detail with respect to a prosthesis for a ginglymus , namely the knee joint. For a better understanding of a knee joint prosthesis, a basic description of the human knee joint is presented here with reference to Fig. 22, which illustrates the knee of the right leg. The knee is a hinge comprising the internal and external condyles of the femur which articulate with the upper end of the tibia. The femoral condyles are separat¬ ed by a deep fossa. The upper end of the tibia comprises two tuberosities , the external of which articulates with the head of the fibula. The knee also comprises the trochlea of the femur (not shown in Fig. 22 ) which is located forward and upward of the condyles. The patella slides along the trochlea. The patella is shown pulled down in Fig. 22 in order to show some of the ligaments and cartilage which connect the femur, tibia, fibula and patella. These ligaments and cartilage include, inter alia, the external lateral ligament which connects the external femoral condyle to the fibula, the internal lateral ligament which connects the internal femoral condyle to the tibia, the ligamentum patellae to which is attached the patella, and the transverse ligament and internal semilunar carti¬ lage which are attached to the head of the tibia. The transverse ligament, internal semilunar cartilage and the anterior crucial ligament are attached to the spine (not shown in Fig. 22 ) of the tibia. The spine is a series of elevations on the head of the tibia opposite the fossa between the femoral condyles. Reference is now made to Figs. 23 and 24 which illustrate a knee prosthesis 200, constructed and opera¬ tive in accordance with a preferred embodiment of the present invention. Knee prosthesis 200 comprises an upper or femoral portion 202 and a lower or tibial portion 204, as seen in Figs. 23 and 24. Referring additionally to Fig. 25, it is seen that femoral portion 202 preferably comprises two pads 206 upon which rest the internal and external condyles of the femur, as seen in Fig. 24, and a trochlear portion 20δ which is intermediate the patella and the trochlea, as seen in Fig. 23- Pads 206 articulate with tibial portion 204. As seen in Fig. 25, femoral portion 202 also preferably has a socket 210, whose function is described hereinbelow with reference to tibial portion 204. Referring additionally to Fig. 26, it is seen that tibial portion 204 includes an artificial spine 212 which extends into socket 210. It is appreciated that when the knee flexes, socket 210 restricts the movement of spine 212 therein. Socket 210 and/or spine 212 are preferably constructed of a shock absorbing or resilient material, such that the movement of spine 212 is cush¬ ioned at the limits of travel in socket 210. As seen in Fig. 23. spine 212 may extend beyond socket 210 into the fossa of the femur. Tibial portion 204 also preferably has two depressions 214 with which pads 206 may articulate. Tibial portion 204 preferably includes a stem 216 for attachment to the tibia. Tibial portion 204 may have one or more hollow portions 2lδ to increase shock absorption, damping or resiliency. As seen in Fig. 23, a tibial cushion 220 may be placed intermediate tibial portion 204 and the head of the tibia, tibial cushion 220 preferably being construct¬ ed of a material compatible with human tissue, such as polyurethane. In accordance with a preferred embodiment of the present invention, femoral portion 202 may be constructed of a substantially rigid material, such as a composite material, and tibial portion 204 may be con¬ structed of a substantially resilient material, such as polyurethane . In accordance with another preferred embodiment of the present invention, femoral portion 202 may be constructed of a substantially resilient material and tibial portion 204 may be constructed of a substantially rigid material. In accordance with yet another preferred embod¬ iment of the present invention, femoral portion 202 may be constructed of a substantially resilient material and tibial portion 204 may be constructed of a substantially resilient material. It is appreciated that knee prosthesis 200 is operative to absorb static and dynamic shocks. It is a particular feature of the present invention that the resiliency of either femoral portion 202 or tibial portion 204 allows the configuration of the contact surfaces between portions 202 and 204 to change according to physical factors, such as load or motion. For example, when bearing loads directed downwards on the tibia, the contact area between portions 202 and 204 becomes relatively large, thereby increasing stability and decreasing pressure on the tibia. When the knee flexes, the contact area is relatively small, which facilitates motion of the tibia with respect to the femur. Femoral portion 202 and tibial portion 204 have different radii of curvature when not exposed to forces. The radii of curvature approach equality when bearing forces directed downwards on the tibia. Reference is now made to Figs. 27 and 2δ which illustrate a knee prosthesis 230, constructed and opera¬ tive in accordance with another preferred embodiment of the present invention. Knee prosthesis 230 comprises an upper or femoral portion 23 and a lower or tibial por¬ tion 234, as seen in Figs. 27 and 2δ. Tibial portion 234 is substantially identical to tibial portion 204 de¬ scribed hereinabove with reference to Figs. 23, 24 and 26. Femoral portion 232 preferably comprises two pads 236 upon which rest the internal and external con¬ dyles of the femur, as seen in Fig. 2δ, and a trochlear portion 23δ which is intermediate the patella and the trochlea, as seen in Fig. 27- Pads 236 articulate with tibial portion 23 . Pads 236 preferably have one or more hollow portions 240, and may have one or more fluid passageways 242, for permitting flow therethrough of synovial fluid, thereby providing lubrication and enhanc¬ ing the shock absorbing and damping characteristics of knee prosthesis 230. Femoral portion 23 articulates with tibial portion 234 of knee prosthesis 230 by sliding along the generally concave surface of tibial portion 234. Refer¬ ence is now made to Figs. 29 and 30 which illustrate a knee prosthesis 250, constructed and operative in accord¬ ance with another preferred embodiment of the present invention. Knee prosthesis 250 comprises an upper or femoral portion 252 and a lower or tibial portion 25 , as seen in Figs. 29 and 30. Femoral portion 252 is prefera¬ bly generally convex and articulates with tibial portion 254 by rolling along the generally convex surface of tibial portion 2 4. As described hereinabove with reference to Fig. 23, the resiliency of either femoral portion 2 2 or tibial portion 2 4 allows the configuration of the con¬ tact surfaces between portions 252 and 254 to change according to physical factors, such as load or motion. Reference is now made to Figs. 31 and 32 which illustrate a knee prosthesis 260, constructed and opera¬ tive in accordance with another preferred embodiment of the present invention. Knee prosthesis 260 comprises an upper or femoral portion 262 which articulates with a lower or tibial portion 264 by means of one or more roller elements 266, as seen in Figs. 31 and 32. Portions 262 and 264 may be substantially rigid and roller elements 266 may be substantially resilient. Conversely, portions 262 and 264 may be substantially resilient and roller elements 266 may be substantially rigid . Roller elements 266 may permit articulation of femoral portion 262 with tibial portion 264 by means of rolling, sliding, a combination of rolling and sliding, or rolling combined with a deformation of one or more of roller elements 266. Roller elements 266 may be formed in any shape which provides such rolling and sliding, such as being generally cylindrical in shape. An alternative shape is shown in Fig. 33- At least one fluid passageway 26δ may be pro¬ vided in each roller element 266 for passage therethrough of a fluid, such as synovial fluid, thereby providing lubrication and enhancing the shock absorbing and damping characteristics of knee joint prosthesis 260. Alternatively or additionally, each roller element 266 may have at least one hollow portion. Alter¬ natively or additionally, each roller element 266 may comprise a plurality of portions, each portion not neces¬ sarily having the same mechanical or physical properties. These portions may be used to enhance, to optimize or to customize the shock absorbing and damping characteristics of the roller element 266. Femoral portion 262 may be attached directly to the femoral condyles. Alternatively, as shown in Fig. 32, a femoral pad 270 may be placed intermediate femoral portion 262 and the femoral condyles. Femoral pad 270 may be constructed of a material with properties similar to human cartilage, such as polyurethane. Reference is now made to Fig. 3 which illus¬ trates a bone fastener 300 for fastening bone fractures, constructed and operative in accordance with a preferred embodiment of the present invention. 3δ Bone fastener 300 preferably includes a core 302 and an outer layer 304. Outer layer 304 preferably includes one or more ridges 306, which, inter alia, help distribute stresses and help fasten bone fastener 300 to a bone. Core 302 may be of solid or hollow construction. Bone fastener 3 0 may have any suitable cross sectional shape, such as circular or elliptical. In accordance with a preferred embodiment of the present invention, core 302 is preferably constructed of a rigid material, for example, stainless steel or a structural plastic. Alternatively, the rigid material may be a composite material, such as graphite fibers, which may be constructed to have mechanical or physical proper¬ ties, such as modulus of elasticity or coefficient of thermal expansion, equivalent to that of the local human bone . Outer layer 304 is preferably made of a resil¬ ient material compatible with human tissue, such as polyurethane, which helps distribute stresses optimally, thereby stimulating regeneration of bone. In accordance with a preferred embodiment of the present invention, outer layer 304 is constructed of a material, such as polyurethane, which has one or more mechanical and/or physical properties substantially similar to human carti¬ lage. Reference is now made to Fig. 35 which illus¬ trates a vertebra replacement 310, constructed and opera¬ tive in accordance with a preferred embodiment of the present invention. Vertebra replacement 310 preferably includes at least one inner member 312, at least one intermediate member 314 and at least one outer member 316. Inner member 312 is preferably constructed of a substantially resilient material and may have one or more hollow por¬ tions 318. Alternatively, portions 318 may be filled with a fluid, such as synovial fluid. Additionally or alterna- tively, a fluid passageway (not shown) may be provided for fluid flow therethrough, thereby providing lubrica¬ tion and enhancing the shock absorbing and damping char¬ acteristics of vertebra replacement 310. Intermediate portion 31 may be less flexible than inner member 312, and is preferably constructed of a rigid material, for example, stainless steel or a structural plastic. Alternatively, the rigid material may be a composite material, such as graphite fibers, which may be constructed to have mechanical or physical proper¬ ties, such as modulus of elasticity or coefficient of thermal expansion, equivalent to that of the local human bone . Outer members 314 are preferably made of a resilient material compatible with human tissue, such as polyurethane, which helps distribute stresses optimally. In accordance with a preferred embodiment of the present invention, outer members 3 4 may be constructed of a material, such as polyurethane, which has one or more mechanical and/or physical properties substantially similar to human cartilage. In accordance with a preferred embodiment of the present invention, geometrical data may be provided, such as by computerized tomography, and be used to prepare and select an optimal prosthesis or bone fastener prior to surgery. Data input, such as from the results of computerized tomography, may be used to match the geometry of the prosthesis or bone fastener to the needs of the patient. Either the prosthesis or the bone, or both, may be shaped, such as by computerized machining, using the geometrical data obtained. Alternatively, the geometry of a preformed, standard prosthesis may be used to reshape the bone to match the prosthesis. Alternatively, the geometrical data used to reshape the bone may be used generally to form the prosthesis in real time. It is appreciated that various features of the invention which are, for clarity, described in the con¬ texts of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, de¬ scribed in the context of a single embodiment may also be provided separately or in any suitable subcombination . It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention is defined only by the claims that follow:

Claims: CLAIMS 1. A joint prosthesis comprising at least a first and a second load carrying member, said first load carry¬ ing member being substantially more shock absorbing and resilient than said second load carrying member. 2. A joint prosthesis according to claim 1 and wherein at least one of said first and said second por¬ tions is generally omega shaped. 3. A joint prosthesis according to claim 1 or claim 2 and wherein at least one of said load carrying members is characterized in having at least one of strength and elasticity generally similar to that of human cartilage. 4. A hip joint prosthesis comprising a load carry¬ ing, spherical femoral head which is adapted to articu¬ late both with an acetabulum and with an upper portion of a thigh. 5. A hip joint prosthesis comprising an artifi¬ cial, self-articulating, load carrying femoral head, said head being attachable to at least one of an acetabulum and an upper portion of a thigh. 6. A joint prosthesis according to any of the preceding claims and wherein at least one of said load carrying members comprises at least one delimiting rail adapted for moving in a corresponding delimiter groove for controlling movement of said prosthesis. 7. A joint prosthesis according to any of the preceding claims and wherein at least one of said load carrying members has at least one delimiting groove and said prosthesis comprises at least one delimiter rail adapted for moving in said groove for controlling move¬ ment of said prosthesis. 8. A hip joint prosthesis comprising an artificial femoral head which articulates with either of an artifi¬ cial acetabulum and an artificial femoral socket, wherein said artificial femoral head comprises a device for substantially preventing dislocation of said artificial femoral head from either of said artificial acetabulum and said artificial femoral socket. 9- A hip joint prosthesis according to claim 8 and wherein said device for substantially preventing disloca¬ tion is shock absorbing. 10. A hip joint prosthesis according to either of claims 8 and 9 and wherein said femoral head has at least one passageway for a fluid. 11. A hip joint prosthesis according to any of claims 8 - 10 and comprising a sleeve which envelops at least one portion of said prosthesis and which is attach¬ able to at least one of a portion of an innominate bone and a thigh. 12. A hip joint prosthesis according to claim 11 and wherein said sleeve comprises a fabric. 13- A hip joint prosthesis according to claim 12 wherein said fabric comprises a material selected from the group consisting of DYNEEMA, polyethylene, nylon and polyurethane . 14. A hip joint prosthesis according to any of claims 8 - 13 and wherein said femoral head comprises a plurality of alternating adjacent portions of substan- tially rigid and substantially resilient materials. 15. A hip joint prosthesis according to any of claims 8 - 14 and further comprising a stem insertable into a femur. 16. A hip joint prosthesis according to any of claims 8 - 15 and comprising an outer layer attachable to a femur, said outer layer comprising a material compati¬ ble with human tissue. 17- A joint prosthesis comprising at least a first and a second load carrying member, wherein said at least first and second load carrying members are snap fit to each other. 18. A joint prosthesis comprising at least a first load carrying member, wherein said at least first load carrying member is snap fit to at least one of a natural or artificially formed socket in at least one natural or artificial bone element. 19- A crown-shaped prosthesis comprising an element which is configured to hug and snugly fit circumferen¬ tially around and on top of a bone and is operative to distribute stresses evenly on said bone. 20. A bone connector comprising an elongate member which is sufficiently flexible to be insertable into a bored portion of a bone and deform to adapt to a changing curvature of said bone, and is structurally stable after being fixed to said bone. 21. An artificial acetabulum comprising a shock absorbing and resilient interface having a cut-out por¬ tion and a ridge, said ridge being pressingly fit into a recess of an innominate bone. 22 . An artificial acetabulum according to claim 21 and comprising a deformable, resilient flange with ex¬ pandable, accordion-like folds. 23. An artificial acetabulum according to either of claims 21 and 22 and wherein said interface includes a load bearing and shock absorbing surface for a femoral head . 24. An artificial acetabulum according to any of claims 21 - 23 and comprising a locking piece complemen¬ tary shaped and sized to snugly fit into said cut-out. 25. An artificial acetabulum according to any of claims 21 - 24 and further comprising an articulation portion which snaps together with a recess formed in said interface . 26. A knee joint prosthesis comprising a femoral portion and a tibial portion, said femoral portion being attachable to a femur and said tibial portion being attachable to a tibia, said femoral portion articulating with said tibial portion, wherein at least one of said femoral portion and said tibial portion is load carrying and shock absorbing. 27. A knee joint prosthesis according to claim 26 and comprising a device operative to limit motion of the tibia with respect to the femur. 28. A knee joint prosthesis according to claim 27 and wherein said device operative to limit motion of the tibia with respect to the femur is shock absorbing. 29- A knee joint prosthesis according to any of claims 26 - 28 and wherein said femoral portion is gener¬ ally convex and said tibial portion is generally concave, as viewed looking upwards from the tibia. 30. A knee joint prosthesis according to any of claims 26 - 28 and wherein said femoral portion is gener¬ ally convex and said tibial portion is generally convex, as viewed looking upwards from the tibia. 31. A knee joint prosthesis according to any of claims 26 - 29 and comprising at least one roller ele¬ ment, said femoral portion articulating with said tibial portion via said at least one roller element. 32. A knee joint prosthesis according to any of claims 26 - 31 and having at least one fluid passageway. 33- A bone fastener comprising a plurality of alternating adjacent portions of substantially rigid and substantially resilient materials. 34. A bone fastener comprising a plurality of alternating adjacent first and second portions, said first portion having a substantially rigid configuration and said second portion having a substantially resilient configuration . 35- A method of incision of a ligament comprising forming a substantially wave-like incision in said liga¬ ment . 36. A method of insertion of a prosthesis for a hip joint comprising: fixedly attaching a first joint element to an upper portion of a femur, said first joint element fit- ting substantially snugly with said upper portion of the femur ; fixedly attaching a second joint element to a portion of an innominate bone; and inserting an artificial femoral head intermedi¬ ate said first and said second joint elements, such that said artificial femoral head articulates with at least one of said first and said second joint elements. 37- A method according to claim 36 and comprising using one or more natural ligaments to strengthen said hip joint . 38. A method according to claim 3 and comprising using a one or more artificial ligaments to strengthen said hip joint. 39- A method for limiting a range of movement of a hip joint comprising: implanting a hip joint prosthesis comprising at least one delimiting rail and at least one delimiting groove, said at least one delimiting rail articulating with said at least one delimiting groove, such that said at least one delimiting groove limits articulation of said at least one delimiting rail therein.

Inventors: STEINBERG AMIRAM, STEINBERG, AMIRAM
Applicants: LIMBER LTD, STEINBERG AMIRAM, LIMBER LTD., STEINBERG, AMIRAM
Application year: 1996
Main classification: A61F
IPC codes: A61F2