Clinical Significance of Hip Separation in Metal on Polyethylene, Metal on Metal, and Ceramic on Ceramic THA Due to Resonant and Energy Dispersion Effects

Femoral head sliding within the acetabular cup does occur in THA. Since it is
known that impact conditions can lead to component ringing, the present study
investigates the vibrational response of variable bearing surface materials. A further understanding of the physical response resulting from impact during femoral head sliding may lead to valuable insight pertaining to THA failure.
Accelerometers were mounted to synthetic bones which had been implanted with hip prostheses. Data was collected from these sensors as a machine impacted the femoral head into the acetabular cup. These tests were carried out for metal-metal, ceramicceramic, and metal-polyethylene bearing surfaces at varying loads. Contrary to the non impact condition, when hip separation was simulated leading to impact conditions, the frequency magnitudes were significantly increased. The results from this study revealed that the amplitude and range of vibration is dramatically increased when the femoral head is separated from the acetabular cup under impact conditions. The ceramic-ceramic material experienced the largest magnitude of frequency. The vibration caused by the ringing of components in the THA system has the potential to damage both prosthetic components as well as the area of the bone in
contact with them. If the dominant frequencies seen during the testing for this study were to coincide with the resonant frequency of the bone at the implant/bone interface, then damage and degradation becomes much more likely, with the ultimate possibility of loosening and failure.