Concern about metal on metal hip prostheses: reflections from the work of Sir John Charnley

The recent recall of a metal on metal (MoM) hip prosthesis by Depuy Orthopaedics has caused concern about the long term viability of this type of joint replacement. The ASR XL Acetabular Hip System and DePuy ASR Hip Resurfacing System were withdrawn from the market in 2010 because of high failure rates1 caused by inflammatory soft tissue reactions to metallic wear debris. Such wear debris causes cell necrosis in tissues surrounding the joint, which is not seen in other types of prosthesis, and results in early failure of the prosthesis. Of further concern are the raised levels of chromium and cobalt ions in patients with MoM prostheses, the long term effects of which are largely unknown.

One of the first people to highlight the importance and implications of prosthetic wear was Sir John Charnley. His earliest hip prostheses in the 1950s consisted of a stainless steel femoral head articulating with a Teflon acetabular cup, which showed high failure rates as a result of excessive wear of the Teflon component. However, Charnley correctly concluded that the main cause of prosthetic failure was not direct wear of the femoral head through the acetabular cup, but rather the tissue reaction to wear debris causing loosening of the prosthesis from underlying bone. We are now met with a similar situation in certain MoM prostheses.

Charnley’s early failures with Teflon did not deter him from pursuing the role of plastics in hip prostheses, and in 1962 he created the Charnley hip prosthesis, a high density polyethylene cup articulating with a stainless steel femoral head. He used polyethylene to mimic the low frictional properties of articular cartilage, thus reducing frictional torque to periprosthetic bone, an important contributor to aseptic loosening. He also found polyethylene to be more resistant to wear, and, by reducing the diameter of the femoral head component, he was able to reduce the amount of wear debris.2 In doing so, he dramatically increased the longevity of his prostheses.

Long term follow-up of Charnley hip prostheses has revealed a survival rate of 85% at 25 years3 and 78% at 30 years.4 Indeed, a 70 year old patient attends our unit with a right hip prosthesis implanted by Charnley himself in 1969. The patient underwent the procedure at the age of 28 because of hip degeneration secondary to her rheumatoid arthritis. More than 40 years later, there is protrusio acetabuli and wear of the acetabular component, but it remains fully functional to this day (figure⇓).

Charnley’s work highlighted the importance of prosthetic wear and in vivo tissue reactions to wear debris. Although his prostheses carry good long term prognosis, wear of the polyethylene component is still the weakest point of this type of joint replacement. With current work on more wear resistant polyethylene materials, perhaps future hip replacement will steer away from MoM prostheses to follow the path Charnley took 60 years ago.