Hip Bearings

Alexander P. Sah

In total hip replacement surgery, both the acetabular (socket) and femoral (ball) articulating surfaces are replaced with artificial materials including metal, ceramic and/or polymeric components. Wiles performed first total hip replacement in 1938 in United Kingdom using a metal-on-metal combination.  This was further developed by surgeons like Ring & McKee during the 1950s & 1960s. During the same period, Dr Ban Saw of Burma treated femoral neck fractures with hand-made ivory components The most significant development in the evolution of hip replacement bearing surfaces was the introduction of low friction arthroplasty by Sir John Charnley in 1958, using metal on high-density polyethylene. The principles proposed by him remain relatively unchallenged until recently, despite rapid evolution of many of the other aspects of hip replacement surgery.

With metal component fixation to the bone, either with or without cement, exceptionally durable and reliable, the focus on preventing failure in hip replacement has been on the bearing surfaces.  Currently, the bearing surface which has proved to produce the most consistent results in hip replacement is the combination of femoral head made of cobalt chrome alloy articulating on an acetabular component liner made from ultra-high molecular weight polyethylene (UHMWPE).  

Metal-on-highly crosslinked polyethylene

Ultra high molecular weight polyethylene (UHMWPE) was first used as a bearing surface in the hip in 1958. Early wear problems were likely related to sterilization, material properties, implant geometry and limited shelf life. The wear particles generated by metal on UHMWPE articulation can cause an inflammatory response which can lead to localized bone loss which can cause prosthesis loosening.

This wear rate has been substantially reduced by cross-linking of polyethylene using gamma radiation and thermal treatment which increased oxidation resistance of UHMWPE as well as decreased abrasive and adhesive wear rates. In 1998, highly-crosslinked polyethylene was used for the first time clinically and marked a significant improvement to UHMWPE bearing surface. Clinical follow-up results as well as simulator studies (10–22 years) have shown minimal wear of the highly crosslinked UHMWPE acetabular components during an expected clinical life span. 

While this has been the gold standard bearing, due to infrequent wear and rare consequences of osteolysis (localized bone weakening) or implant loosening, alternative bearing surfaces have been explored. These bearing surfaces include ceramic-polyethylene, metal–metal as well as ceramic–ceramic articulations and have demonstrated lower friction rates and significantly lower wear rates as compared to widely used metal on polyethylene surfaces.

Ceramic on crosslinked polyethylene

Use of a ceramic head on crosslinked polyethylene liners is gradually gaining wider acceptance.  Clinical as well as laboratory data have shown that wear rates for ceramic on polyethylene bearings may be significantly less than metal on polyethylene bearings with studies showing 10%–50% lesser wear. Possible advantages of ceramic femoral heads over metal femoral heads articulating with crosslinked polyethylene include a lower coefficient of friction, better joint lubrication, ceramics are chemically inert with no metal ion issues;, and ceramic heads have lower susceptibility to surface scratching.  Disdvantages have been reported to include ceramic head fracture, hip squeaking, and cost.

Metal on metal bearings

With the prior resurgence of the metal-on-metal hip resurfacing in the 2000’s, the potential advantages of the procedure were tempered by the complications due to the metal bearings.  Elevated blood levels of Cobalt and Chromium ions leading to concern about higher cancer risk as well as occurrence of pseudotumours.  This is an area of intense concern with some designs having been withdrawn from market (ASR prosthesis from DePuy) due to high failure rates.

Because of these concerns and reports of high failure rates, the use of metal-on-metal bearing surfaces has been abandoned largely over the last couple of years. Even in total hip replacements without a metal-on-metal articulation, there have been rare reports of corrosion at the trunnion of the stem to femoral head junction leading to coining of a new term called “trunnionosis.”  This complication is rare, and remains under investigation.

In conclusion, the modern bearing surfaces are durable and reliable, and expected to have a clinical lifespan of over 20 years in most cases.  Alternative bearings continued to be investigated to determine if there are advantages for the very young or very active patients.  Any improvements would be in addition to the already very successful articulations available.

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