Abstract
Dislocation remains among the most common complications of, and reasons for, revision of both primary and revision total hip arthroplasties in the United States. Hence, there is great interest in maximising stability to prevent this complication.
Highly cross-linked polyethylene has allowed us to increase femoral head size, without a clinically important increase in wear. Head size has long been recognised to have a strong influence on the risk of dislocation postoperatively. As femoral head size increases, stability is augmented, secondary to a decrease in component-to-component impingement, which is theoretically eliminated at head sizes greater than 36mm in diameter (however osseous impingement can still occur). Larger head sizes also greatly increase the “jump distance” required for the head to dislocate (in an appropriately positioned cup) and eliminate the need for skirts. Level one studies support the use of larger diameter heads as decreasing the risk of dislocation following primary and revision THA.
Larger diameter heads do, however, have negatives with the most recent concern being larger forces imparted upon the trunnion, which may contribute to adverse local tissue reactions (ALTR) which have recently been reported in patients with a metal on polyethylene bearing. However, in the series by Cooper et al, 32mm was the most common head size identified with no head sizes >36mm in this series. This suggests that the cause of ALTR is probably multifactorial and while femoral head size may be a contributor, the trunnion itself may be more important including its diameter, length and modulus of elasticity as well as the specific finish of the taper. Finally, when larger femoral head sizes are used in smaller acetabular components, the result is a thinner polyethylene liner, which may increase the risk of liner breakage. Larger diameter bearings may also increase the risk of squeaking in ceramic on ceramic bearing couples.
Hence, the decision on femoral head size probably should include a balance between patient risk factors for instability and the risks of increased head size. Hence for revision procedures, and in primary cases where the risk of dislocation is known to be high, the risk of a larger femoral head is probably outweighed by the benefits of enhanced stability if a larger femoral head is utilised.
