The present invention relates to an acetabular prosthesis for hip arthroplasty.

An acetabular prosthesis, also known as acetabular cup prosthesis, is used in orthopedic surgery for the full replacement of the hip by coupling with a femoral stem.

The hip joint is constituted by two articulation surfaces: the acetabulum of the iliac bone (concave surface) and the head of the femur (convex surface). This joint has great mobility and withstands very high loads, and for this very reason it is also a joint that is often subject to degenerative pathologies, such as arthrosis, rheumatoid arthritis, cephalic necrosis, and traumas and dislocations. In all these situations a surgical procedure for replacement of the hip joint with an artificial joint may be necessary.

Currently commercially available hip prostheses are constituted by a femoral stem made of metallic material, on which the femoral head made of ceramic or metal is fitted. The acetabular prosthesis, commonly known as cotyloid element, is inserted in the acetabular cavity. The cotyloid element is constituted by a metallic component known as metal-back, which usually is hemispherical and inside which an insert is arranged which will be articulated to the femoral head.

The metal-back of a cotyloid element can be implanted with a press- fit method or with a cemented method, by using acrylic cement. Cotyloid elements implanted with the press-fit method of the known type are inserted by forcing in the appropriately prepared cavity, giving the joint so-called primary stability. The metal-back is commonly made of titanium alloy and has such an external surface as to facilitate bone regrowth and stabilize the prosthesis in the medium-long term, giving the joint so-called secondary stability.

It is known that excellent primary stability is indispensable in order to achieve excellent secondary stability, i.e., excellent osteointegration. Cotyloid elements implanted with the cemented method instead have such an external surface as to facilitate the adhesion of the cement, which will be interposed between the prosthetic component and the bone.

The insert, arranged within the metal-back, can be made of ceramic, metal or polyethylene. The latter is a low-cost material but can give rise to dislocations or wear depending on whether it is coupled to heads having a small or large diameter respectively.

It is known from the literature that one of the main complications of the replacement of the hip joint with an artificial joint is dislocation, which occurs more if heads having a small diameter are used.

Biomechanical studies have demonstrated that dislocation can be reduced by using heads having a large diameter, but these heads, if coupled to inserts made of polyethylene, create a very high rate of wear of said polyethylene. The presence of polyethylene debris causes a foreign body reaction which leads to removal of the peri-prosthetic bone tissue (osteolysis), consequent aseptic mobilization and failure of the implant. It is in fact known that aseptic mobilization is one of the main causes of failure of acetabular prostheses of the hip.

In order to overcome the failures described above, dual mobility acetabular prostheses were conceived over 30 years ago.

A dual mobility acetabular prosthesis of a known type consists of a metal-back and a polyethylene insert which rotates freely in the metal-back. Said insert has, in the equatorial part, an opening in which a femoral head made of metal or ceramic, having small dimensions, generally 22 or 28 mm in diameter, is to be inserted by forcing.

It is possible, therefore, to distinguish two joints with the same center of motion: a small joint, constituted by the femoral head and by the lower part of the polyethylene insert, and a large joint, constituted by the polyethylene insert and by the metal-back, wherein the polyethylene insert acts as if it were a head with a large diameter. From a kinematic standpoint, these components behave as follows: in the presence of small movements, the small joint (femoral head/interior of the polyethylene insert) moves. Once the motion of the small joint is used fully, dislocation does not occur because impingement, instead of causing dislocation, moves the entire insert within the metal-back, allowing to perform broader movements. Since broad movements are rarely necessary, the wear of the polyethylene is also minimized.

These acetabular prostheses of the known type, even with dual mobility, are not free from drawbacks, which include the fact that excellent primary stability is not always ensured.

The aim of the present invention is to provide an acetabular prosthesis that overcomes the limitations of the background art, improving its primary and secondary stability.

Within this aim, an object of the present invention is to provide an acetabular prosthesis that ensures higher stability, even in the case of a press-fit method, which minimizes wear phenomena and therefore extends the survival of the system.

Another object of the present invention is to provide an acetabular prosthesis that prevents the escape of the insert from the metal-back.

Another object of the invention is to provide an acetabular prosthesis that prevents the escape of the prosthetic femoral head from the insert.

A further object of the invention is to provide an acetabular prosthesis that is capable of giving the greatest assurances of reliability and safety in use.

Another object of the invention is to provide an acetabular prosthesis that is easy to provide and economically competitive if compared with the background art.

This aim and these and other objects that will become better apparent hereinafter are achieved by an acetabular prosthesis for hip arthroplasty, comprising a metal-back in which a cavity is formed which is adapted to accommodate an insert, characterized in that said cavity comprises a first region having a substantially hemispherical internal surface from which a second region extends which has a substantially cylindrical internal surface, the height of said second region being comprised in the interval between 1 and 5 millimeters and being adapted to help to retain said insert in said cavity.

Further characteristics and advantages of the invention will become better apparent from the description of a preferred but not exclusive embodiment of an acetabular prosthesis, illustrated by way of nonlimiting example with the aid of the accompanying drawings, wherein:

Figure 1 is a perspective view of an embodiment of a metal-back of an acetabular prosthesis, according to the invention;

Figure 2 is a top plan view of the metal-back of Figure 1 , according to the invention;

Figure 3 is a front elevation view of the metal-back of Figure 1, according to the invention;

Figure 4 is a sectional view of the metal-back shown in Figure 2, taken along the line IV-IV;

Figure 5 is an enlarged- scale view of a portion of Figure 4;

Figure 6 is a front elevation view of the metal-back of Figure 1, according to the invention, provided with an outer layer of porous metal;

Figure 7 is a perspective view of an embodiment of an insert of an acetabular prosthesis, according to the invention;

Figure 8 is a top plan view of the insert of Figure 7, according to the invention;

Figure 9 is a front elevation view of the insert of Figure 7, according to the invention;

Figure 10 is a sectional view of the insert shown in Figure 8, taken along the line X-X. With reference to the figures, the acetabular prosthesis, generally- designated by the reference numeral 1 , comprises a metal-back 3, in which a cavity 5 adapted to accommodate an insert 7 is formed.

According to the invention, the cavity 5 comprises a first region 50 provided with a substantially hemispherical internal surface 52 from which a second region 54 extends which has a substantially cylindrical internal surface 56, the height of the second region 54 being comprised in the interval between 1 and 5 millimeters and being adapted to contribute to the retention of the insert 7 inside the cavity 5.

Advantageously, the height of the second region 54 is comprised between 2 and 4 millimeters and is preferably substantially equal to 3 millimeters.

Advantageously, the metal-back 3 is made of a material such as chromium-cobalt, and preferably of the type of the chromium-cobalt- molybdenum alloy, using additive powder technology. Preferably, the metal-back 3 is provided by using exclusively additive powder technology. Starting from metal powders, a high-precision finished product is obtained advantageously, reducing considerably both production times and process waste material.

The use of additive technology with chromium-cobalt powder allows to obtain an external surface, designed to make contact with the bone, that does not require further finishing with traditional machines after its production, reducing both production times and the waste of material.

Moreover, the use of chromium-cobalt, with respect to other metals, offers the advantage that it can be worked easily in order to obtain inside the metal-back 3 a particularly smooth surface without any roughness, thus reducing the phenomenon of wear of the polyethylene of which the insert 7 is generally made.

Advantageously, furthermore, the metal-back 3 can comprise a layer of porous metal 30 which comprises a plurality of protruding elements 32. By means of additive powder technology it is in fact possible to obtain complex porous structures, also made of metallic material, preferably chromium-cobalt and even more preferably chromium-cobalt-molybdenum alloy.

Advantageously, the layer of porous metal 30 can have a three- dimensional porous structure with a thickness comprised between 0.2 and 1 millimeters; even more preferably, such thickness is substantially equal to 0.8 millimeters.

The external surface of the layer of porous metal 30 advantageously comprises a plurality of protruding cusps, which have a substantially triangular shape and protrude from the external surface of the metal-back 3 with an inclination preferably comprised between 3° and 20°, even more preferably substantially equal to 7°, with respect to directions that are tangential to the metal-back 3.

Advantageously, these cusps are particularly sharp and such as to ensure effective primary stability and consequently, also by virtue of a hydroxyapatite coating, excellent secondary stability.

Advantageously, the insert 7 has a cavity 9 adapted to accommodate a femoral head.

Advantageously, the cavity 9 of the insert 7 comprises a first region

90, which has a substantially hemispherical internal surface 92 from which at least one second region 94 protrudes which has a substantially cylindrical internal surface 96, adapted to contribute to the retention of the femoral head in the cavity 9.

The insert 7 is advantageously made of a material selected from the group constituted by ultra high molecular weight polyethylene (UHMWPE), crosslinked ultra high molecular weight polyethylene (UHMWPE), ultra high molecular weight polyethylene (UHMWPE) with the addition of an antioxidant, and crosslinked ultra high molecular weight polyethylene (UHMWPE) with the addition of an antioxidant. Preferably, the antioxidant comprises vitamin E or vitamin E derivatives.

The insert 7 is articulated to the metal-back 3, advantageously made of chromium-cobalt, and to the femoral head, made of metal or ceramic.

Advantageously, the internal surface 52, 56 of the metal-back 3 is polished to a mirror finish. In this manner, the wear phenomena that can occur following contact between the internal surfaces 52, 56 and the insert 7 are minimized.

Advantageously, the metal-back 3 comprises, at its peripheral edge, a radially protruding perimetric collar 36, which is also polished to a mirror finish. In this manner any damage to soft tissues located at the collar 36 are also minimized.

The operation of the acetabular prosthesis is clear and evident from what has been described.

In particular, the insertion of the prosthetic femoral head in the cavity

9 of the insert 7 can be performed by pressing. Once the prosthetic femoral head is in position in the insert 7, the substantially cylindrical internal surface 96 of the second region 94 of the insert 7 eliminates the risk that the prosthetic femoral head can escape from the cavity 9, acting in practice as its retention collar.

In practice it has been found that the acetabular prosthesis according to the present invention achieves the intended aim and objects, since it ensures exceptional primary and secondary stability.

Another advantage of the acetabular prosthesis according to the invention resides in that it has a metal-back that can be manufactured completely by using additive powder technology and therefore ensuring modest costs and simplicity in production.

A further advantage of the acetabular prosthesis according to the invention resides in that it is a dual mobility prosthesis and therefore is exceptionally durable and smooth. Another advantage of the acetabular prosthesis according to the invention resides in that the metal-back, by virtue of the use of chromium- cobalt or chromium- cobalt-molybdenum, has an internal surface that is absolutely smooth and without roughness, therefore capable of reducing drastically the phenomenon of wear of the polyethylene of which the insert is made.

The acetabular prosthesis thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the appended claims.

All the details may further be replaced with other technically equivalent elements.

In practice, the materials used, so long as they are compatible with the specific use, as well as the contingent shapes and dimensions, may be any according to the requirements.

The disclosures in Italian Patent Application No. 102016000026736

(UA2016A001662) from which this application claims priority are incorporated herein by reference.

Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.