BACKGROUND ART A hip joint endoprosthesis is known from Polish patent PL 111200, with a ball mounted on a stem neck via a plastic pad in the form of a ring separating the ball from the stem and functioning as a thrust bearing during rotation around the metal neck of the stem.

German patent DE2618763 discloses a hip joint endoprosthesis having a ball mounted on a stem neck via a plastic pad in the form of a cap separating the ball from the stem neck and dampening the pressure of the ball on the neck, which allows the use of a ball made of ceramic.

Publications RU2198624 (C2) and DE 4441695 (Al) present constructions utilizing materials causing an increase of the elasticity of the endoprosthesis.

One of the common problems in the use of known endoprostheses is the use of metal elements touching bone.

The implantable alloys used to make endoprosthesis stems and the outer surface of the cup result, in certain groups of patients, in allergic reactions causing postoperative complications.

The contact of bodily liquids and tissues with metals causes their corrosion and penetration of metal ions to the body, which often results in inflammatory states and metalosis.

The metal parts of the prostheses cause postoperative complications and toxic and allergic reactions in tissues surrounding the prostheses, as well in tissues of parenchymatous organs, resulting in side effects, which are often not recognized as related to the prosthesis.

The excessive stiffness caused by a large difference between the

Young's modulus of the metals forming the typical prostheses and that of the bone is found to induce many remedial operations necessitated by loosening of the cup in the hip joint or of the stem in the bone marrow canal, as well as by hip bone fractures.

The cup of endoprostheses often utilizes an insert in the frictional junction made of UHMWPE polyethylene (ultra high molecular weight polyethylene).

The use of a low-friction material such as a high-molecular-weight polyethylene causes complications related to its relatively low abrasion resistance, and the wear products of the polyethylene cup are carried by bodily liquids outside the joint, causing inflammatory states.

The frictional elements of the ball-cup joint are known to be made of ceramic, which are biotolerable and highly resistant to abrasion, therefore the amount of wear products is minimal, the wear products are better tolerated by the organism and the durability of the kinematic joint of the endoprosthesis increases.

DISCLOSURE OF THE INVENTION

The object of the invention is to provide a joint endoprosthesis without at least one of the disadvantages of the prior art endoprostheses.

The object of the invention is a joint endoprosthesis comprising a cup, a ball and a stem, wherein the contact of bones with the endoprosthesis occurs via a layer of plastic, wherein the cup and the stem have a layered structure providing mechanical durability and elasticity approximate to the elasticity of the bones. The layer of plastic contacting the bones provides biotoleration of the endoprosthesis, which facilitates the avoidance of allergic reactions, transfer to the body of ions of metals which may form the internal layers of the endoprosthesis, as well as protects the internal layers from corrosion. Moreover, the layered structure providing elasticity of the endoprosthesis approximate to the elasticity of bones facilitates the dampening of static and dynamic loads carried to the kinematic joint of the endoprosthesis. Therefore, the endoprosthesis is elastic, and therefore the incidence of loosening of the cup or stem, as well as resulting bone fractures, are avoided.

The plastic can be polyethylene.

The joint endoprosthesis according to the invention is particularly useful as a hip joint endoprosthesis, wherein large loads occur.

The elasticity of the cup and of the stem can be lower than the elasticity of bones.

The elasticity of the cup and of the stem expressed in terms of a Young's modulus value can be from 50% to 200% of the value of Young's modulus of the bones.

The elasticity of the cup and of the stem expressed in terms of a Young's modulus value can be lower than 50.

The cup can be made of ceramic and plastic.

The ball is made of ceramic and plastic or only of ceramic.

The stem can be made of plastic and metal or metal alloys.

The ball can be mounted in the cup and a conical seat of the ball can be mounted on a conical part of the stem.

The cup, the ball and the stem are selected from a series covering the gender, age, height, body weight and/or physical condition of the patient. The cup can be made of ceramic covered by a layer of plastic, having a thickness and external shape depending on the series. The thickness of the plastic layer covering the ceramic of the cup can be from 2 to 15 mm. The ceramic of the cup can be a corundum ceramic having a wall thickness from 2 to 6 mm. The stem may comprise a core made of metal or metal alloys covered by a layer of plastic having a thickness and external shape depending on the series. The layer of plastic covering the core of the stem may have a thickness of at least 0.1 mm. The ball may have the shape of a spherical shell made of corundum ceramic with a wall thickness from 2 to 6 mm. The ball may have an internal part filled with a plastic, in which a conical seat is formed for mounting the stem.

The external extended surface of the cup and the extended surface of the stem can be configured to allow the ingrowth of bone tissue, and they may have a porous structure, comprise Velcro-like hooks or a partially moulded woven or non-woven fabric. This provides durable connection of the prosthesis with bones.

The layers of plastic can be formed in moulding forms, which provides for the repeatability of their shape and dimensions, as well as the repeatability of the mechanical and physicochemical parameters of the component parts of the endoprosthesis.

The layers of plastic comprise carbon nanotubes and/or carbon fibbers, which facilitates the local directional shaping of elasticity of these layers.

The layers of plastic may comprise silver nanoparticles, which provide bactericidal properties.

The advantage of the endoprosthesis according to the invention is that it provides a high mechanical durability and elasticity at the contact with the bones.

BRIEF DESCRIPTION OF DRAWINGS

The object of the invention is shown by means of embodiments in drawings, in which:

Fig. 1 shows a lateral section of the best embodiment of the joint endoprosthesis.

Fig. 2 shows an exploded view of the side cross-section of the best embodiment of the joint endoprosthesis.

Fig. 3 shows axonometric view of the cup of the endoprosthesis.

Fig. 4 shows axonometric view of the endoprosthesis. Fig. 1 shows a lateral section of a second embodiment of the joint endoprosthesis.

Fig. 2 shows an exploded view of the lateral section of the second embodiment of the joint endoprosthesis.

MODES FOR CARRYING OUT THE INVENTION

BEST MODE FOR CARRYING OUT THE INVENTION The elastic joint endoprosthesis according to the best embodiment, as shown in Figs. 1-4, comprises an acetabulum cup 1 , a ceramic ball 2 and a stem 3.

The cup 1 is made of a ceramic part 4 and comprises a spherical surface 5 for positioning the ball 2. The outer part of the ceramic cup 4 is covered by a layer 6 made of plastic, preferably polyethylene, having the form and thickness depending on the series.

The cover 6 of the cup 1 may, if necessary, comprise openings 7 for fixtures, preferably screws, facilitating the additional immobilization of the cup 1 in the pelvis bone. In such a case the ceramic part and the cover may be separate.

The ball 2 made of ceramic 9 and having a spherical surface 8 for positioning in the cup 1 has an internal seat 11 in the form of a truncated cone for seating the stem 3.

The stem 3 of the endoprosthesis has a core 12 made of metal, metal alloys or other materials providing appropriate transmission of static and dynamic loads occurring in a given endoprosthesis. The core 12 is covered by a layer 14 made of plastics, preferably polyethylene, having a thickness and an external shape depending on the series.

Thus, the cup 1 and the stem 3 have a layered structure, wherein the internal layer or layers provide mechanical durability and the external layer made of plastic provides elasticity. The elasticity of the cup 1 and the stem 3 is approximate to the elasticity of bone, i.e. it does not substantially differ from the elasticity of bone such as in case of the endoprostheses having the cup 1 and the stem 3 made only of hard metals such as titanium with a Young's modulus exceeding 100. Preferably, the elasticity of the cup 1 and the stem 3 is lower than the elasticity of the bone. Preferably, the elasticity of the cup 1 and of the stem 3 expressed by Young's modulus is approximate to the Young's modulus of bone, namely from 50% to 200% of the average value of Young's modulus of the bone, which for most bones varies from 1 to 20. Preferably, the elasticity of the cup 1 and the bone 3 expressed as a Young's modulus is lower than 50.

The plastic layers 6, 14 may comprise, as a whole or in parts, carbon nanotubes or carbon fibers, preferably aligned directionally, for the local patterning of the elasticity of these layers. The plastic layers 6, 14 may also comprise silver nanoparticles for bactericidal purposes. The plastic layers 6, 14 may further comprise other materials.

The extended surface 15 of the cup 1 and the extended surface 16 of the stem 3 are shaped in such a way as to allow bone ingrowth, as shown in Fig. 4- 5. They may have a porous form, a hairy form, a form of Velcro-like hooks, or may comprise partially moulded-in woven or non-woven fabric. The extended surface can be obtained by appropriately shaped forming moulds, mechanical working by means of numerically controlled machine tools, or connecting the surface with woven or non-woven fabric by means of a laser etc.

SECOND MODE FOR CARRYING OUT THE INVENTION The joint endoprosthesis according to the second embodiment, shown in

Figs. 5-6, comprises elements equivalent to elements of the embodiment shown in Fig. 1-4. The equivalent elements have been indicated with the same reference numbers.

The joint endoprosthesis comprises an acetabulum cup 1 , a ball 2 and a stem 3. The cup 1 has the shape of a spherical shell made of a ceramic 4, which forms a cap 5 for housing the ball 2. The external part of the shell is covered by a layer of plastic, preferably polyethylene 6, having a thickness of 2- 15 mm, depending on the series of the endoprosthesis. Openings 7 for fittings, preferably, screws are made in the polyethylene part, which facilitate the further immobilization of the cup in the pelvic bone if necessary. The ball 2 with surface 8 is a spherical shell made of ceramic 9, preferably corundum ceramic, having a wall thickness from 2 to 6 mm, filled with a layer of plastics 10, preferably polyethylene. It has an internal seat 11 having the shape of a truncated cone for mounting the stem 3.

The stem 3 of the endoprosthesis is made of metal or metal alloys 12, has a top part shaped like a truncated cone 13, making it possible to mount the ball 2 of the hip joint thereon. The stem is covered by a layer of plastic 14, preferably polyethylene having a thickness not smaller than 0.1 mm. The external shape of the stem 3 depends on the series. The endoprosthesis according to the invention is shaped depending on the series, wherein the series covers gender, age, height, body weight and/or physical condition of the patient. The endoprosthesis is useful for bone surgery of joint, in particular for implants of hip joints, knee joints, elbow joints, tarsal joints, acromioclavicular joints, as well as joints of the hand and foot.