Wear of bipolar hip prostheses

A method was developed to take radiographs showing the inner articulation of bipolar hip prostheses. By this method, wear was measured in 68 hips whose inner head diameter was 22 mm. Average annual wear rate was 0.17 mm. Osteolysis was observed in 25 hips (37%) and there was no difference between the annual wear rate of hips with and without osteolysis. Studying 19 retrieved prostheses, abrasion of the rim was deeper in hips with osteolysis than those without it. Wear rate of the inner articulation in bipolar hip prosthesis is much larger than that in Charnley's prosthesis, as linear penetration into the articulation surface reduces the motion range of the inner articulation and this increases impingement and advances rim abrasion.