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Tribology and Wear of Metal-on-Metal Hip Prostheses: Influence of Cup Angle and Head Position

¹ Institute of Medical and Biological Engineering, School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom. E-mail address for J. Fisher: J.Fisher{at}leeds.ac.uk

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from DePuy International. In addition, one or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (DePuy International, Tissue Regenix, and BITECIC). Also, a commercial entity (DePuy International) paid or directed in any one year, or agreed to pay or direct, benefits in excess of $10,000 to a research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which one or more of the authors, or a member of his or her immediate family, is affiliated or associated.

Methods: Hip-joint simulator studies of metal-on-metal bearings were undertaken with cup angles of 45° and 55°. The femoral head was either aligned to the center of the cup or placed in a position of microlateralization. Wear was measured gravimetrically over 5 million cycles.

Results: A steep cup angle of 55° showed significantly higher long-term steady-state wear than a standard cup angle of 45° (p < 0.01). The difference was fivefold. Microlateralization of the head resulted in a fivefold increase in steady-state wear compared with a centralized head. The combination of a steep cup angle and a microlateralized head increased the steady-state wear rate by tenfold compared with a standard cup angle with a centralized head.

Conclusions: These studies support the hypothesis that both an increased cup angle and a lateral head position increase wear in metal-on-metal hip prostheses.

Clinical Relevance: These experimental studies support the recent clinical observations of increased wear with increased cup angle. Furthermore, the experimental studies indicate that the position and alignment of the femoral head may also be an important determinant of wear in vivo.

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