**3.4.4 Microseparation and wear particles**

Ceramic materials may have better biocompatibility than metal alloys, but the relative size, shape, number, reactivity, and local versus systemic distribution of the wear particles have not been fully determined. Hatton et al. investigated the tissues from ten noncemented COC THAs undergoing revision surgery. The tissues from the femoral and acetabular regions demonstrated the presence of intracellular particle agglomerates and mixed pathology, with areas that had no obvious pathology and areas that were relatively rich in macrophages, and over half of the tissues had 60% of necrosis. A bimodal size range of ceramic wear debris was observed, with particles as small as 5 to 90 nm (mean 24 nm) and as large as 0.05 to 3.2 µm (mean 430 nm).(Hatton 2002a) These two types of ceramic wear debris are generated by two different wear mechanisms in vivo, with very small wear particles being generated under normal articulating conditions and larger particles being generated under microseparation conditions. After total hip arthroplasty, the femoral head and the acetabular insert can separate up to 2 mm during the swing phase of a normal gait cycle. When a load is applied at heel strike in the stance phase, the femoral head moves vertically to relocate in the cup. With the geometry of a typical COC prosthesis, separations of only 2 mm will allow the femoral head to contact the rim acetabular liner, resulting in changes in the wear performance of the bearing couple (stripe wear). (Stewart 2001, 2003)

Fig. 11. A ceramic head and acetabular liner from a patient with "squeaking" hip with evidence of impingement and "stripe wear.

It is hoped that improvements in the manufacturing of ceramics and ceramic components will minimize or eliminate mechanical problems such as fracture and accelerated wear.
