**5. Ceramics**

Ceramics were popularised in the 1970's as bearing materials. Ceramics consist of metallic elements such as aluminium, zirconium and silicon covalently and or ionically bound with non-metallic elements. The main advantage of ceramic bearings is the reduction of wear debris in the periprosthetic space, which can precipitate the osteolysis cascade and aseptic loosening [34] associated with metal-on-metal and metal-on-PE bearing couples. Oxide ceramics used in total joint arthroplasty (TJA) are chemically inert after binding to oxygen, resulting in excellent biocompatibility. Ceramics also possess a low surface roughness and high hardness, possessing the highest modulus of elasticity of any other biomaterial used in TJA.

Unfortunately, the trade-off of high hardness is ceramic bearing brittleness and subsequent catastrophic failure. The first ceramic bearings used in THAs in 1971 were marred by catastrophic failure due to acute debonding at the implant-cement interface of cemented ceramic sockets resulting in aseptic loosening and implant fracture. In more modern systems this has been corrected with the addition of metal backed acetabular shells to ceramic liners [35]. The primary mode of failure, in more modern systems is now edge loading due to implant mispositioning, resulting in stripe wear. This process disrupts the oxide layer, reducing fracture toughness and increasing surface roughness. Microscopic flaws, introduced during the manufacturing process or machining such as

notches, pores and scratches can also result in stress concentration, propagation of cracks and subsequent abrupt failure [34].

Ceramics are fabricated by mixing fine ceramic powder and water together and compressing the mixture into casts of the desired final shape. The mixture is then sintered in a kiln to bond the particles together and to increase density before being polished. The resulting organised crystalline microstructure and mechanical characteristics are subsequently determined by the grain size, porosity, crystallinity and density together with the implant design. Ceramics can be classified as non-oxides, oxides and composites, with oxide ceramics the material of choice in THA.

With the increased uptake of ceramic-on-ceramic bearings globally, audible squeaking arose as a new complication, with an incidence between 0 and 24.6% [36]. There have been a number of purported mechanisms described accounting for this complication. Suggested risk factors for COC squeak have included increased stripe wear and disruption of film-fluid lubrication, edge loading due to malposition of the acetabular cup, increased body mass index and femoral stem design geometry, among many others [37].

#### **5.1 Aluminium oxide**

Aluminium oxide (Al2O3) was developed as a biomaterial in the 1960's, making it a well characterised biomaterial. Modern alumina is processed using hot isostatic pressing, a process which reduces inclusions, grain size and grain boundaries, increasing hardness and increasing scratch resistance. Alumina also possesses a very low coefficient of friction due to a low surface roughness, resulting from its low grain size. This excellent tribological performance is further compounded by alumina' s high wettability and resulting film-fluid lubrication, which reduces in vitro wear. Retrieval studies have demonstrated alumina-on-alumina wear rates of a few micros per year [38]. Biologically the typical response to alumina wear debris is fibrocytic with no giant cell formation and little induction of macrophages, reducing osteolysis [35]. The estimated lifetime risk of catastrophic failure of alumina femoral heads is estimated to be 0.004% [38].

#### **5.2 Oxidised zirconium (zirconia)**

Oxidised zirconium (ZrO2) is a ceramic composite bearing which was introduced as a means to reduce the catastrophic failure rates associated with alumina heads, while still also retaining the desirable wear characteristics of smaller femoral heads on polyethylene [34]. Pure zirconia is not used as a bearing material as it undergoes phase transformation between its three crystalline arrangements (monoclinic, cubic, tetragonal). This can result in volume and shape changes that increase the susceptibility to fracture. As a result, pure zirconia, requires stabilisation through a process known as transformation toughening. Zirconia can be stabilised with CaO, Y2O3 or MgO which controls phase transformations.

Zirconia toughened alumina (ZTA), is a composite which consists of zirconia dispersed in an alumina matrix (**Figure 4**). This modification resulted in the improved strength, fracture toughness and tensile strength compared to aluminium oxide [34]. ZTA can be strengthened further by the addition of Cr2O3 and SrO, which prevents crack propagation.
