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The acetabular component is the focus of new testing methods dedicated to the development of improved total hip prostheses. Test methods had previously been developed to assess the performance of the femoral component in both the static and dynamic modes. These methods were designed in accordance with standards generated by such organizations as ASTM, ISO, and by research institutions. However, acetabular components were generally tested in the static mode to evaluate engagement strength and torsional resistance of the liner. While these tests are important and give a measure of the mechanical integrity of the device, they do not fully address clinical performance.
From retrieved components, it appears that a loose or failing locking mechanism allows torsional forces to transfer through the liner causing liner abrasion, burnishing, and in some cases degradation of the locking mechanism itself. Success of the device may be further compromised by the biological response to debris created by micromotion between the mating components. This study was undertaken to assess the rotational stability of the acetabular liner in a dynamic mode. Several acetabular component designs were torsionally fatigued at ±2.5 Nm at a rate of 3–5 hertz for 10 million cycles. The test components exhibited liner micromotion ranging from 0.059° to 2.89°, and damage to the nonarticulating interface and locking mechanism of tested liners was visually identified.
This method for dynamic testing of acetabular components provides an understanding of the performance of the device under torsional fatigue conditions. It demonstrates the importance of a rotationally stable liner, as damage to the nonarticulating interface of tested liners is similar to damage reported in the literature for retrieved components. Further, it determines the endurance of the liner locking mechanism when subjected to dynamic torsional forces.
total hip arthroplasty, acetabular, micromotion, wear, polyethylene
Senior Research Engineer, Orthopaedic Research, Smith & Nephew Orthopaedics, Memphis, Tennessee
Senior Research Technician, Orthopaedic Research, Smith & Nephew Orthopaedics, Memphis, Tennessee