As well as a value of the difference between head and cup radii, clearance can be expressed as a ratio to head diameter. There is an optimal clearance associated with each head diameter. Although low clearances work well in laboratory conditions, there may be an issue in the clinical environment. Factors such as bone density, implant position and post surgery may all effect the ability of the bearing to generate a fluid film. With low clearances, there is reduced tolerance for correct function in less than perfect implantation or patient conditions. As a Metal-on-Metal bearing is not in continuous motion, it operates in a mixed lubrication regime and its longevity is linked to its ability to generate and sustain a fluid film. Laboratory evidence confirms the BHR generates fluid film lubrication. Small clearances increase friction and may cause micro motion in the cup. This may hamper bony ingrowth resulting in impaired fixation.

The Stribeck Curve is a graphical representation of the measured frictional forces occurring in a bearing. From the shape of the curve, deductions can be made concerning the lubrication operating conditions of the bearing. Results of friction testing of the BHR are shown below in Graph A. The friction tests suggest boundary lubrication pre-testing but at 1 million cycles, a mixed lubrication regime was evident. By 2 million cycles, the classical Stribeck curve had formed indicating a considerable contribution from fluid film, which continued to be evident at 3 millioncycles.

Changes in Friction and Lubrication during a 3 Million-cycle weat test on a CoCrMo/CoCrMo Hip Re-surfacing Device.

Unsworth, K Vassiliou, APD Elfick, SC Scholes Centre for Biomedical Engineering, University of Durham, England.