Alternative Bearing Surfaces

Alternative Bearing Surfaces

Alternative bearing surfaces have shown tremendous potential in extending the durability of hip replacement surgery. The alternative bearing concepts involve metal on metal or ceramic on ceramic surfaces.

1) Metal Bearings:
Some metal on metal designs were first used in the 1960's. Although quality control, and material engineering were far from today's advanced technology, some of these basic principles of metal on metal articulations showed good results. The primary drawback of those early designs involved component fixation to the bone, and not the metal on metal bearing.

Polyethylene articulations had dominated the hip joint replacement industry for several decades. As the data on polyethylene wear continued to accumulate, renewed interest in on metal on metal bearings progressed.

Current metal bearings are made of cobalt chrome, which have shown dramatically lower "wear" rates over time than the traditional polyethylene. In addition, the particles of wear produced by metal on metal have also been demonstrated to have a lower degree of an inflammatory reaction compared to polyethylene. These lower wear rates in hip simulators have also encouraged larger head diameters on the femoral component to thirty-six millimeters or occasionally greater. These larger head sizes improve range of motion with a lower risk for hip dislocation.

Concerns regarding metal on metal implants include the unknown potential long-term affects of a higher blood serum level of some of the ion particles (typically described in parts per million). No distinct problems have been found that are the result of these higher levels of trace metals in the blood. The rare patient may have a metal sensitivity issue that could make a metal on metal implant less advisable.

There is current optimism in the orthopaedic community that alternative bearing designs will allow implants to last well into the second decade.

2) Ceramic Bearings:
Ceramic as a bearing surface has been in use in various forms for ten to twenty years. Variable formulations of ceramic composites, which are now manufactured, have improved the durability and safety. Current surfaces are extremely smooth, providing very low wear rates. The ceramic material by nature, is very hard and scratch resistant. It also has favorable lubrication qualities, which reduce friction.

The main drawback in the history of ceramic components involves the brittle nature. It has been more prone to fracture and is less forgiving. The number of options in femoral head sizes is also less in ceramic than in metal on metal designs. It typically also is the most expensive of the alternative bearing options.

Polyethylene as a wearing surface has a long track record over three decades. It has been identified as a main method of failure during the 70's, 80's and 90's. Sterilization techniques have changed to improve the wear of the polyethylene material. Current sterilization techniques involve an inert environment without oxygen, typically including a very small, radiation dose. This has changed the material "cross linking", which has more favorable wear characteristics. These new, sterilization techniques will definitely improve the wear of polyethylene as a bearing surface when compared to the earlier generations of design.

John G. Mayer, M.D.

-JGM