Hemispherical acetabulum
This invention relates to a hemispherical acetabulum. More particularly, this invention relates to a hemispherical acetabulum having a fixing pin which projects from the outside surface of the acetabulum. 
     Heretofore, various types of acetabulum constructions have been known for implantation in a pelvis. For example, European Patent Application No. 0211619 describes an acetabulum having a fixing pin which projects from the outside surface of the acetabulum at an angle of 45°. Recently, an implantation technique has been developed for acetabula of this kind in which the operating surgeon first prepares the pelvis operatively and then introduces the pin of the acetabulum into a bore in the pelvis prepared therefor with the remainder of the acetabulum being offset, for example about 180°. Thereafter, the surgeon twists the acetabulum into the correct position such that the pin serves as a rotation axis enabling the hemispherical acetabulum to be &#34;guided&#34; into its correct position. 
     It is an object of this invention to facilitate and simplify the procedure for implanting a hemispherical acetabulum having an angularly disposed fixing pin thereof. 
     Briefly, the invention provides a hemispherical acetabulum comprising a hemispherical cup having a cavity for receiving a femoral head, a fixing pin secured to the cup and projecting from the cup at an angle of 45° relative to an equatorial plane of the cup (or the polar axis of the cup) and a sleeve which is rotatably mounted on the pin and which has a closed end enclosing the pin. 
     The sleeve and pin form a cylindrical journal bearing to facilitate twisting of the cup into position into a pelvis. In this respect, the sleeve forms a bearing in which the twisting movement &#34;runs&#34; much more easily than in a bore in a bone. As such, the sleeve facilitates accurate maintenance of a predetermined angle of twisting. 
     The fixing pin may be made of plastic. In this case, the sleeve which is usually made of metal also provides a screening effect to preclude direct contact between the plastic pin and the bone tissue. 
     These and other objects and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawing wherein: 
     The FIGURE diagrammatically illustrates a cross-sectional view taken in a meridian plane which contains the pin axis of an acetabulum constructed in accordance with the invention. 
    
    
     Referring to the drawing, the hemispherical acetabulum is constructed to be inserted into a pelvis 1 and comprises a plastic cup of hemispherical shape having a cavity 3 for receiving a femoral head (not shown). In addition, the acetabulum has a multilayer wire mesh 4 covering the cup 2 on the outer surface with at least one layer embedded in the cup in known fashion. 
     The acetabulum also has a plastic fixing pin 6 secured to the cup 2 and projecting through the wire mesh 4 on an angle of 45° relative to the polar axis 5 of the cup 2. In similar fashion, the pin 6 is also disposed at an of angle 45° relative to an equitorial plane of the cup 2. The pin 6 may be secured to the cup, for example by friction welding and may also be made of metal. 
     The acetabulum also has a metal sleeve 7 rotatably mounted on the pin 6 with a closed end enclosing the pin 6. The sleeve 7 is mounted on the pin 6 in spaced relation to the cup 2 and to the wire mesh 4 so as to be freely rotatable on the pin 6. The connection between the pin 6 and the sleeve 7 is by means of a snap fastening 8 in which the pin 6 engages when pushed into the sleeve 7. The generated surfaces of the pin 6 and the sleeve 7 form a cylindrical journal bearing 10. 
     As indicated, the outer surface of the sleeve 7 is structured, for example with grooves 9, for fixing in the pelvis 1. 
     In order to use the acetabulum, the surgeon prepares the pelvis 1 to receive the acetabulum. During this time, a bore is made in the pelvis for receiving the sleeve 7. Thereafter, the acetabulum is inserted into the pelvis 1 such that the sleeve 7 is seated in the prepared bore. The acetabulum may then be twisted about the axis of the sleeve 7 and, thus, the pin 6 so as to be brought into proper position. 
     The invention thus provides an acetabulum having a fixing pin which projects from the outside surface of the acetabulum which can be implanted in a facile and simple manner. In addition, the invention provides an acetabulum which can be twisted into position in a prepared pelvis without causing damage to the fixing pin, particularly, when made of plastic and without causing a reaming action in the bore prepared for reception of the sleeve.