A study of the sonic resonance of the femoral part of HIP endoprosthesis

Abstract

Mechanical resonance is the property of a mechanical system that responds to the oscillations coming from the outside of the observed system. The response amplitude of the system is the highest when the frequency of the oscillations matches the system's natural frequency (its resonance frequency). It is known that resonance can cause swaying motions and even catastrophic failure in improperly constructed structures. The phenomenon is known as resonance disaster. Sonic energy enters the system through excitation and is dissipated through damping. Damping can be internal (within the material) or external (mounting of an object). Six retrieved stems of hip endoprostheses were studied. For each of them sonograms were made, showing very distinctive and narrow resonance curves with one major resonance peak followed by several higher harmonic peaks. Simulation of endoprostheses standing waves was also performed resulting in the demonstration of various standing wave modes depending on the observed frequency. Results clearly show that, due to the geometry and the used material, the observed endoprostheses have very distinctive sonic resonance characteristics. The resonance is excited by the sound coming from outside (or inside) of the human body. The energy of resonance movement of the endoprosthesis is dissipated through the endoprosthesis-bone interface. A long-term exposure to the resonance oscillations might contribute to other causes of aseptic loosening of endoprostheses.