An in-vitro evaluation of vibration as a diagnostic tool in the loosening of the femoral component of total hip replacement.

Introduction: Traditional methods of diagnosing aseptic loosening of total hip arthroplasties, such as radiography, scintigraphy and arthrography are unreliable. In 1989, Rosenstein et al at Oxford suggested that it was possible to detect loosening at an early stage using a vibration technique. However, the models they tested were probably very loose and would have been detectable by plain radiography, thus obviating the need for vibration testing. Aim: To find out if vibration technology is able to detect early loosening. Methods: Several models of loosening were created in-vitro. These included: 1. Late loosening hereby macromovement is present between bone and implant 2. Early mechanical loosening whereby the interlock between cement and trabecular bone is lost 3. The presence of soft tissue at the bone-cement interface 4. Early stem-cement debonding due to fatigue fracture of the cement mantle Each of those models was submitted to vibration testing and the output signal was analysed using the following methods: 1. Frequency (amplitude) response 2. Spectral analysis using the Fourier Transform Results: For late loosening, the vibration technique had an excellent diagnostic accuracy with a sensitivity and a specificity of 100% (19 specimens). However only 3 out of 8 specimens showing early mechanical loosening were correctly diagnosed (sensitivity 37.5%). The presence of a layer of soft tissue at the bone-cement interface was vibrationally undetectable provided there was no associated mechanical instability. Finally, fatigue fracture of cement was also vibrationally undetectable in the absence of instability at the bone-cement interface. Conclusions: The vibration technique can diagnose late loosening with a high degree of accuracy. However, this is likely to be clinically and radiographically apparent without the need to resort to vibration testing. Results for early loosening have been less satisfactory and further work may be required to improve the sensitivity of the test.