**4. New horizons in bearing surfaces**

202 Recent Advances in Arthroplasty

femoral heads were introduced as a stronger alternative to Al2O3, but zirconia can undergo phase transformation and weakening, leading to unpredictable outcomes in vivo. Another strategy to reduce polyethylene wear is to modify the surface of zirconium alloy femoral heads. This avoids brittle failure, but the wear reduction, surface hardness, and scratch resistance of oxidized zirconium are less favorable than Al2O3, and oxidized zirconium cannot be used in ceramic-ceramic articulations. An alternative ceramic material was fabricated from silicon nitride (Si3N4) powder. Mechanical testing showed high-flexural strength, fracture toughness, Weibull modulus, and resistance to hydrothermal degradation. The Si3N4 cups produced low wear rates when tested with either CoCr or Si3N4 femoral heads. The Si3N4 showed a near 100% theoretical material density (3.20 g/cm3) and a uniform microstructure of fine, elongated grains (average grain width, 1.5 μm). The flexural strength was 923 ± 70 MPa, with a Weibull modulus of

The Si3N4-Si3N4 and Si3N4-CoCr bearings produced wear volumes of 0.2 mm3 and 0.18 mm3, respectively, at 1 million cycles. These data were extrapolated to 10 million cycle wear because previous hip simulator studies have shown that hard-on-hard bearing wear follows a characteristic biphasic pattern with higher wear in the early, run-in period to about 0.5 million cycles, followed by a lower, steady state linear wear pattern that can be extrapolated to predict long-term wear. A consistent wear rate, that is, the gradient of the linear regression trend is considered more significant than the actual magnitude of wear at any point in time. Accordingly, we calculated the volumetric wear rate from the slope of the linear regression curve during steady state wear; thus, at 10 million cycles, the predicted wear for Si3N4-Si3N4 and Si3N4-CoCr bearings was calculated to be 0.65 mm3 and 0.47 mm3, respectively. For THA bearings of the same diameter, the clinical wear volume of CoCrpolyethylene THA is 62 mm3, and the experimental wear volumes of CoCr-CoCr and Al2O3- Al2O3 measured in other hip simulators are 6.5mm3 and between 0.35 and 0.6 mm3, respectively. After wear testing, Si3N4 surfaces remained smooth and glossy, without

Modern Si3N4 is used in aerospace bearings and in high-temperature applications such as diesel engines. The Si3N4-Si3N4 pairings have a friction coefficient of 0.001 compared to 0.08 for Al2O3-Al2O3. The CoCr-Si3N4 articulations produced low wear rates in this study because of the favorable material hardness, tensile strength, fracture toughness, and lubricity of Si3N4 and the close match in elastic modulii between the bearing materials. Because Si3N4 has a higher fracture toughness than Al2O3, it is more resistant to the initiation and propagation of microcracks that can lead to brittle failure. The fracture toughness and strength of Si3N4 also result in superior damage resistance, that is, the ability of the material to retain strength after contact damage. Practically, this means that Si3N4 can withstand high

It has been postulated that hard bearings in THA could produce a greater number of smaller-sized wear particles, thereby increasing the exposed surface area of bioreactivity in vivo. Wear particles were characterized in this study, and those findings have been reported separately. Because wear between dissimilar mating surfaces originates from the softer, sacrificial material, Si3N4-CoCr couplings should generate predominantly CoCr metal particles. Assuming so, the volume of metal wear from Si3N4-CoCr calculated in this study was an order of magnitude lower than the wear of retrieved CoCr-CoCr THA bearings. Furthermore, particle size analysis has shown a mean equivalent circular diameter (ECD) of Si3N4 wear particles of 0.7 ± 0.6 μm in this study. This figure is comparable to ultrahigh-

19, and a fracture toughness of 10 ± 1 MPa.m1/2.

loads even if contact stress-related damage were to occur.

scratches or damage.

Metal on polyethylene, metal on metal, and ceramic on ceramic bearings will continue as the dominant bearing materials for total hip arthroplasty because of their excellent track record, resistance to damage, and ease of manufacture and use. Clinically, most bearing combinations consist of cobalt-chromium (CoCr) alloy or ceramic femoral heads articulating against highly cross-linked ultra-high-molecular weight polyethylene acetabular inserts. Alumina on alumina ceramic and CoCr on CoCr articulations are used in younger and more active patients. Each bearing combination has advantages and potential disadvantages. Second-generation HXPE materials are now available. Ceramic composite materials with high fracture toughness are under clinical investigation, as are ceramic on CoCr articulations. Outside this main area of bearing development, research efforts have been under way for some 20 years with two quite different classes of materials. The two classes of materials are carbon fiber polymeric composites and elastomeric materials.
