Modern Hip Replacements

Hip replacement is one of the most successful and reliable orthopedic operations currently performed and aims to provide pain relief and improved hip function in individuals who have developed conditions such as osteoarthritis, rheumatoid arthritis or hip fracture, for example.

One of the pioneers of the artificial hip joint used in modern hip replacement surgery is Sir John Charnley who worked on the implant at Wrightington Hospital, to provide a design that almost completely replaced all other implant designs by the 1970s.

Charnley’s design essentially comprises three parts including the one-piece femoral stem and head (made of stainless steel); the polyethylene acetabular component; and the PMMA (acrylic) bone cement.

Previously, the acetabular component had been made of teflon, but implants made of this material were found to fail within two years of the replacement procedure. The material was eventually replaced by Ultra High Molecular Weight Polyethylene or UHMWPE in 1962, after a German salesman showed a sample of polyethylene to Charnley’s machinist.

The hip replacement system on which modern implants are based is known of as the Low Friction Arthroplasty and is lubricated with synovial fluid. The femoral head is small, at 7/8" or 22.2 mm and was chosen because Charnley thought it would cause less friction on interaction with the acetabular cup, therefore minimizing wear and tear. However, the smaller femoral head was in fact more likely to dislocate after surgery and alternative, much larger heads such as the Mueller prosthesis were eventually developed. With these implants, dislocation was reduced, but wear and tear would eventually also lead to failure with these designs.

A hip stem that was developed in the UK around the same time as the Charnley device was the Exeter hip stem, which has a stem geometry that differs slightly to the Charnley implant. Both devices were very durable in the long-term and are still widely used in modern hip replacement techniques.