Materials in Hip Surgery: Ceramics

Alumina

Alumina ceramic, or aluminum oxide (Al ₂ O ₃ ), is the major oxide ceramic used globally over the past 50-year history in joint replacement. Alumina is an extremely hard, monophasic polycrystalline material, which is chemically and thermodynamically stable, making it essentially bio-inert and corrosion resistant. Because of its hardness, it can be polished to a highly smooth surface finish, providing wear resistance that exceeds that of other bearing materials available for joint replacement.

For maximum performance, alumina must be processed properly with high density, high purity, and small grains. The manufacturing process is important for ceramic implant success, because abnormal shapes or sizes of ceramic grains or impurities within the ceramic material increase the risk of fracture, which proceeds as slow crack propagation along grain boundaries and at impurities ( Fig. 6.2 ). Ceramic bearing surfaces can wear through microfracturing on the scale of individual grain; the result is grain pullout from the surface. With the microfracturing mechanism, wear rate increases with increased grain size, as pullout of a single grain produces a larger surface defect for a larger-grain-sized alumina than it does for a finer-grain-sized alumina ( Fig. 6.3 ). Therefore, ceramics with small grain sizes are advantageous.

Marked improvements have been made in manufacturing of ceramic orthopedic implants to produce dense alumina with fine, uniform grain sizes. Modern alumina joint replacements are also hot isostatically pressed (HIPed alumina), which has greater wear resistance than non-HIPed alumina. The grain size of high-purity alumina ceramics has been reduced from approximately 40 µm in the 1970s to below 3 µm currently.

Three generations of pure alumina femoral heads have been commercially introduced since 1980 by CeramTec AG (Plochingen, Germany) using the general trade name BIOLOX: BIOLOX alumina (1980–1987); improved BIOLOX alumina (1988–1994), and BIOLOX forte (currently in use). With each subsequent generation, the mechanical strength and properties of the alumina material was improved. BIOLOX forte has increased bending strength compared to the first-generation alumina (650 MPa vs. 400 MPa, respectively), and increased density (3.98 g/m ³ vs. 3.94 g/m ³ , respectively; Table 6.1 ). To ensure safety, ceramic femoral heads undergo proof testing of 100% of implants prior to commercial sale. Proof testing, which is used to determine if a component will withstand maximum loads above physiologic levels, has significantly improved the reliability of alumina components.

Zirconia

Another well-known structural ceramic that was used for a bearing material in total hip joint replacement was zirconia, or zirconium oxide (ZrO ₂ ). Unlike pure alumina, zirconia is called polymorphic because it can exist in 3 metamorphs or phases: cubic, monoclinic, and tetragonal . Zirconia implant grades were alloyed with yttria or magnesia; the majority of zirconia that was used in femoral heads for THAs was ytrria-stabilized zirconia polycrystal ceramic manufactured to include predominantly the tetragonal phase. Tetragonal zirconia can easily undergo a phase transformation to the monoclinic phase, which results in a small volume increase that creates a localized compressive stress field that opposes further crack growth. This transformation toughening effect provided zirconia ceramics with a significant increase in strength. However, the tetragonal to monoclinic phase transformation at the surface of zirconia femoral heads led to an increase in surface roughness and catastrophic wear of the opposing bearing surface of ultra-high-molecular-weight polyethylene (UHMWPE) acetabular components in THAs.

The transformation back to the more stable room temperature monoclinic phase is accelerated by both thermal and mechanical energy. Zirconia is sensitive to low temperature degradation, which occurs after prolonged exposure to water vapor at intermediate temperatures (~30°C to 300°C), and thus steam sterilization procedures used at some hospitals could have contributed to increased monoclinic phase content of zirconia femoral heads and subsequent poor clinical results. Zirconia heads were shown through retrieval analysis to have transformed in vivo, presumably from both the heat and the mechanical energy generated at the bearing surfaces as patients went about their daily activities. Zirconia was considered for biomedical implants as early as 1969, but the instability of this material in vivo led implant companies to remove zirconia femoral heads from the orthopedic market in the early 2000s. Certain manufacturers of zirconia heads were identified as having higher failure rates, which led to implant recalls that likely played a role in the demise of zirconia heads.