HIPCLINIC

'St George's Joints'

Welcome to the South West London Hip Clinic

What is the bearing surface ?

The bearing is the moving part of the joint, with the ball of the femoral component moves against the inner surface of the socket in. The original hip replacements had metal against polyethylene bearings, which generally functioned well in the low demand patient who had hip replacements in the early days. As the operation started to be done on more active patients it became apparent that the implants were loosening and bone was being eroded around the artificial hip. The artificial hip is subjected to millions of movement cycles over the years of its lifetime, so it is not surprising that it can wear out. We now know that this is because the wear debris produced a reaction in the tissues around the hip, which damaged the bone. In order to try and reduce wear A lot of work has gone into improving the bearing surfaces. New bearing surfaces have been developed and are now under long-term evaluation and improved polyethylenes have been produced which are highly resistant to wear.

Metal against metal bearings have extremely low wear rates and are thus potentially also ideal for hip replacement prostheses. They have been used since the early days of hip replacement, but were largely abandoned as the technology was not there to manufacture them accurately enough. There has recently been a great resurgence of interest in this bearing, led by the Birmingham and then other hip resurfacings. Wear rates are very low, but problems have been identified and overall the survivorship of this type of bearing is less good than with other types of bearing. Some patients seem to react to the metal debris, causing pain and sometimes even soft tissue damage around the joint. We now know these types of bearing need to be positioned extremely accurately to function well. We are not currently recommending these bearings, as they seem to provide no advantage over other bearing types, with potential for increased complications.

Ceramic bearing surfaces are extremely attractive for several reasons. Ceramics are extremely hard, very smooth and easily lubricated by the body. Ceramic on ceramic has an even lower wear rate than metal on metal and produces debris, which seems to be largely inactive in the body. Problems have been seen with fracture of the ceramic and some patients notice a squeaking from the joint. We have been tending to move away from these bearings.

Great improvements have been made in the performance of polyethylene. The wear rate is greatly reduced by cross linking the polyethylene chains and this can now be achieved without weakening the material. In the lab the wear rates are comparable with hard bearings. We now recommend a cross linked polyethylene acetabular component, either cemented or as part of a cementless socket.