**3.4.9 Delta ceramic-on-alumina ceramic articulation in primary THA**

The newest generation of Ceramic-on-Ceramic bearings incorporate nanosized, yttriastabilized tetragonal zirconia particles producing an alumina matrix composite. This new bearing surface provided high survivorship.

The newest generation of ceramics, incorporating zirconia into the alumina matrix (marketed as Biolox delta by CeramTec, Plochingen, Germany). Nanosized, yttria-stabilized tetragonal zirconia particles improve the mechanical properties by preventing the initiation and propagation of cracks. Oxide additives produce platelet-like crystals that dissipate energy by deflecting cracks. The addition of chromium oxide further increases the strength and toughness by composite hardening. The final product is a mixture of roughly 75% aluminum oxide, 25% zirconia, and less than 1% chromium oxide and strontium oxide. The result is a composite ceramic with improved mechanical properties and reduced wear as shown on scanning electron microscopy (SEM) and a predicted longer lifespan. These improved mechanical properties reduce overall wear rates of 28-mm heads in a hip simulator wear test from 1.84 mm3 per million cycle to 0.16 mm3 per million cycle, comparing alumina-on-alumina versus alumina matrix composite-on-alumina matrix composite, respectively. (Lombardi 2010; Hamilton 2010)

### **3.4.10 Other advances**

There are some laboratory and very short-term clinical data to support the use of ceramic femoral heads articulating with metal acetabular components. These appear to produce less stripe wear with edge loading than metal-on-metal implants. It also appears that there is a reduction in metal ions in the patients at 6 months. Though there have been reports of early catastrophic failure if this pairing is reversed and a metal head is articulated with a ceramic acetabular component.

### **3.4.10.1 Fabrication and testing of silicon nitride bearings in total hip arthroplasty**

A concern with ceramic femoral heads is brittle catastrophic failure in vivo. Improvements in the quality and manufacturing of Al2O3 have not eliminated this risk. In the past, zirconia

The Bearing Surfaces in Total Hip Arthroplasty – Options, Material Characteristics and Selection 203

molecular-weight polyethylene (mean ECD, approximately 0.4 ± 0.2 μm) and Al2O3 (mean ECD, approximately 0.4 ± 0.4 μm) particles obtained from clinical retrievals. Furthermore, the morphology of Si3N4 wear particles in this study was similar to Al2O3 particles produced under similar testing conditions. For the same size and concentration, ceramic particles may be less bioreactive than those of polyethylene. Previous biocompatibility tests have shown that Si3N4 does not elicit adverse in vivo reactions; specifically, exposure to Si3N4 does not produce acute or chronic toxicity, mutagenicity, allergenicity, carcinogenicity, or localized

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

Carbon fiber ultra-high-molecular weight polyethylene (UHMWPE) composites were introduced in the early 1980s as tibial bearings.(Fruh 1998) However, the clinical performance was poor with many revisions. Load can be transferred only by compressive forces between the fibers and matrix, resulting in an inefficient reinforcement. Carbon fiberreinforced epoxy composites (CFRP) were introduced in Europe in the late 1980s as acetabular components with alumina femoral heads. Simulator studies reported a wear rate of 1 to 3 µm per million cycles. Canine hips with CFRP cups were implanted in six dogs for up to 5.5 years with no adverse consequences. A clinical study began in 1989, and 101 patients received cups of CFRP. In cases of revision, there were few particles in the tissue and the biologic response was benign. The wear of retrieved components was 6.1 to 6.3 µm/year. These wear rates are comparable to those seen with HXPE materials. Epoxy resins are thermoset materials and cannot be shaped other than by machining once they have "set" via chemical reaction. Many thermoplastic composites can be shaped by combinations of heat and pressure and "set" thereafter. Polyether ether ketone (PEEK) is a thermoplastic high performance polymer that is attractive as a composite matrix owing to its strength,

There has long been interest in developing bearing materials that exhibit friction and wear behavior similar to that of articular cartilage. Cartilage is an example of a compliant bearing that has a low modulus but is capable of large deformation without failure. The friction coefficient between cartilage surfaces in a synovial joint is less than 0.01. This low friction is

materials are carbon fiber polymeric composites and elastomeric materials.

tissue toxicity.(Bal 2009)

**4. New horizons in bearing surfaces** 

**4.1 Carbon fiber polymeric composites** 

toughness, stability, and biocompatibility.(Wang 1999)

**4.2 Compliant bearings** 

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

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 scratches or damage.

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

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 ultrahighmolecular-weight polyethylene (mean ECD, approximately 0.4 ± 0.2 μm) and Al2O3 (mean ECD, approximately 0.4 ± 0.4 μm) particles obtained from clinical retrievals. Furthermore, the morphology of Si3N4 wear particles in this study was similar to Al2O3 particles produced under similar testing conditions. For the same size and concentration, ceramic particles may be less bioreactive than those of polyethylene. Previous biocompatibility tests have shown that Si3N4 does not elicit adverse in vivo reactions; specifically, exposure to Si3N4 does not produce acute or chronic toxicity, mutagenicity, allergenicity, carcinogenicity, or localized tissue toxicity.(Bal 2009)
