The use of an assembly comprising a ceramic head and a metal femoral rod as a femoral prosthesis is known, one end of the rod being implanted in the femur and the other being nested in a blind recess cut into in the ceramic head. This assembly in general is associated with a natural or artificial acetabular cup supporting the rotation of the ceramic head in order to form an internal prosthesis for the femoral joint.
The blind recess made in the ceramic head (the female part) is customarily of truncated cone shape with a circular cross section and slight conicity; the male part of the femoral metal rod which is to be nested in it has practically the same angle at the apex, so that assembly by nesting the parts together is done without play between the two pieces, the relative rotation of the head on the rod being prevented by various means such as jointing by force, use of adhesives, wedging, brazing, etc.
Because the angle at the apex of the cone is slight, it quite often occurs that during the placement of the two pieces by the physician their alignment is not totally correct; thus instead of having a contact surface between the faces of the female and male pieces, the male piece is actually only in contact with some points on the surface of the recess of the female piece. For the same reasons it also happens that the male piece is not nested in place over all of the bearing surface provided in the recess of the female piece; for example, the apex of the male piece might be incorrectly supported by a point on the surface of the recess which is not located in the bottom of the recess and, at the same time, the contact surface of the male piece might be incorrectly supported on a point of the recess close to the periphery of the ceramic head.
Sometimes the male metal cone and the ceramic recess also have faults in their geometry which cause the contact between the two pieces to occur only at some points so that their placement is not correct.
All of these problems, associated with the placing of the male and female pieces by means of nesting them together, by definition mean that the mechanical stresses due to exterior stress and sustained by the ceramic head are transmitted from the male piece to the femur by means of only a few contact points between the male piece and the recess of the female piece, instead of by means of a contact surface. Thus the stresses supported by the ceramic head are considerably increased at these points and are the cause of a very serious increase of a risk of breaking of the ceramic head. These breaks usually occur in the anterior area, rather than the deep area, of the recess of the ceramic head.