Effect of bearing size on the long-term wear, wear debris, and ion levels of large diameter metal-on-metal hip replacements - An in vitro study

Hip resurfacing arthroplasty has become a popular alternative to conventional hip surgery. Surface replacements with bearing sizes of 55 mm (n = 5) and 39 mm (n = 5) were tested in a hip simulator for 15 million cycles (Mc). Wear debris was isolated from the serum lubricant and characterized by field emmitting gun scanning electorn microscopy, and ion levels were measured via inductively coupled plasma mass spectroscopy at intervals throughout the test. The 39 mm bearings showed significantly greater bedding in volumetric wear (2.58 mm³) compared with the 55 mm bearings (1.15 mm³). There was no significant difference between the steady state wear rates (1-15 Mc) between the two sizes (0.10 and 0.09 mm³/Mc, respectively); however, this parity only became clear after 7 Mc. The wear debris isolated was oval in morphology with a mean particle size of 28 nm and a range of 9-108 nm. The Co levels measured at 0.13 Mc were significantly greater than at 3.6 Mc for both bearing sizes (10926 ppb and 176 ppb, respectively). After 0.5 Mc, the Co levels from the 39 mm bearings were significantly higher than the 55 mm (11,007 vs. 1475 ppb). The wear results support previous findings showing that increasing the femoral head size decreased volumetric bedding in wear. The ion levels measured suggest both bearing sizes have similar initial wear rates; however, the 55 mm bearings reach steady state wear more rapidly.