All artificial bearings create wear debris, just as the rubber wears off your tire going down the road. This wear debris is deposited in your body. If the load is small, you can usually tolerate it well for many years. Our goal is to use implants that generate the least quantity of wear debris as well as the type of debris that results in the least tissue reaction. The original Charnley bearing was stainless steel ball against plastic (polyethelene). This is no longer used. Modern alternative bearings have about 100X lower wear rates than the older cobalt chrome against standard polyethelene bearing.

Metal on Plastic

Metal on plastic are the most commonly used combinations. Plastic wear debris is deposited around the hip joint and may travel along lymph channels. But it does not leave the body. In young active people with standard plastic (polyethelene, SPE) liners, enough plastic debris is released to cause severe bone destruction in 30% of patients within 8 years of implantation. Newer cross-linked liners polyethelene (XLPE) have dramatically lowered this problem in small early studies out to 10 years. When ceramic balls are used against the plastic, wear debris seems to be further reduced.

Smaller bearing diameters produce less wear but result in higher risk of dislocation. Larger bearings were not possible with SPE for this reason. Because XLPE has better wear properties, larger heads are now being used. Although XLPE wears better, it is often more brittle. As the bearing size increases, the liner thickness decreases. More brittle XLPE may therefore be more subject to breakage rather than wear. Also, larger heads have recently been associated with trunion (where the head is attached to the stem) corrosion. Therefore, there is much disagreement about ideal bearing size because of competing problems: wear/breakage vs. instability.

Ceramic on Plastic

Ceramic on plastic has similar characteristics as above but probably cuts the wear rate in half.

For the standard 28mm bearing size dislocation risk is about 5% within 1 year, while it drops to 1% for 36mm bearings. By 10 years follow-up dislocation risk nearly doubles. About half of dislocations are recurrent and require revision surgery. The most common reason for revision hip surgery is instability.

Ceramic on Ceramic

Ceramic on ceramic bearing produces the least quantity and best tolerated wear debris of all bearings. Ceramic surfaces are brittle and can fracture, especially with impact activity. This is now exceedingly rare (less than 1:10,000). When these implants are placed in nonideal positions, they may exhibit a stipe wear pattern and emitting a loud squeak that can be head across the room. This can be very unpleasant and require revision surgery. This occurs in 1-2 % of cases, but can probably be resolved by better implant positioning and larger ceramic bearings.

The main problem with ceramic bearings is their size. The same instability problems exist with the standard 28mm bearing size. Larger sizes are now in use with a new stronger ceramic called Biolox. This will reduce the dislocation rate, but will require thinner ceramic socket liners. Time will tell if these will be equally fracture resistant as the thicker alumina ceramic liner that have 10 year data.

Metal on Metal

Metal on metal bearings made of cobalt chrome were first used in the U.S. when joint replacement began in the late 1960s. The component design and fixation techniques were primitive by today's standards. Further, the bearing manufacture was inconsistent and these devices were discontinued in the 1970s. Now with modern technology, bearing surfaces can be made optimally smooth and round and thus the wear is minimized.

We have learned that for optimal function, there needs to be less than 5 um residual roughness and a polar bearing arrangement with a 50-100um radial clearance. Cobalt chrome is the only metal that works. Trace amounts of molybdenum and Nickel are present in this alloy. There is still controversy about ideal metallurgy (cast vs. forged, high vs. low carbon content, heat treated or not) but the most commonly used is cast, high carbon non-heat-treated. M/M devices were reintroduced in Europe in 1988. There are now U.S. manufacturers as well as European firms manufacturing all-metal bearings. The reaction of our body against excess metal debris results in more soft tissue inflammation while plastic causes more bone destruction (osteolysis).

Metal bearings are so strong that very thin (4mm) socket components can safely be built without any risk of fracture. Also a bone ingrowth layer can be directly attached to this implant. Thin, strong, one-piece sockets allow reconstructing the hip joint with a natural bearing size, virtually eliminating hip instability, the most common complication of this surgery. In combination with similar thin femoral components, hip resurfacing is made possible.

Despite laboratory studies showing minimal wear, high wear states resulting in metalosis (excess metal in the tissues) have now been reported in patients. The incidence of this adverse wear failure (AWF) problem varies; in my experience it has been a cause of failure in 1% of cases at 10 years. In my experience, revision for AWF is no more difficult than revision for other failure modes. We have now learned the proper acetabular component positions to completely avoid this problem. It turns out that the problem is caused primarily by two factors, socket components that are designed very shallow as well as steep socket component inclination position. The combination of these problems results in edge wear releasing excess metal debris. (discussed more elsewhere)

Ceramic on Metal

Ceramic on metal has shown slightly lower metal wear in laboratory studies, but this has not been confirmed in the clinical studies.