gms | German Medical Science

Objectives: Alumina ceramic bearings have been reported to reduce wear in total hip arthroplasty. There is an incidence of 2 to 20% of audible sounds reported with cementless stems, a condition that has been termed 'squeaking hip'. The aetiology of this, is associated with patient demographics, implant design, loading conditions and the natural vibration frequencies of the components. The femoral stem and its design have been found to have an important influence on the incidence and characteristics of squeaking. We hypothesised that the cement mantle might have a damping effect on the sound transmitted through the metal stem, and thus be partially responsible for the lower incidence of squeaking hip reported when hybrid hip arthroplasty is used.

Methods: To test a damping effect of cement on sound propagation an in-vitro model was designed. The study was conducted with two different stem designs: a polished tapered cemented and a non-cemented tapered flat stem. A pulse of sound in the audible range was propagated along the stem under different conditions of material interfaces. Measurements of sound attenuation were carried out on each of the following variables: stem design, cemented condition and stem measurement position. The measurement protocol consisted of excitation of the femoral stem and detection of a mechanical pulse by a piezoelectric device responsible for transduction of a mechanical vibration into an electrical pulse.

Results: Mechanical vibrations in the audible range (below 2000 Hz) was fully 'filtered' after cementing in femoral bone, with no audible components perceived. The magnitudes of the maximum amplitudes of the stem without cement, were in the range 10.8-11.8 V ( 0. 5 V), whereas the amplitudes for the same stems with cement decreased to 0.3-0.7 V ( 0.03 V), implying a pulse-attenuation efficiency larger than 97%. This involves an energy loss in pulse energy larger than 99.6 (0.05).

Conclusions: It has been shown that the increase in friction at a ceramic-bearing surface is not enough to generate squeaking. The frictional energy as a form of vibration has to be transmitted to a flexible stem, which amplifies the vibration by resonating, in order to produce an audible sound. Higher incidences of squeaking hip have been reported from series that combined ceramics bearing with cementless stems. Recently, some studies suggested a low incidence of squeaking with hybrid total hip arthroplasties , this coincides with our experience in 277 arthroplasties using the Exeter stem and modern cementing techniques with an incidence of 0.7%. We found that the cement had a remarkable effect on sound attenuation and strong energy absorption in cement mantle and bone.

In conclusion, we believe that the visco-elastic properties of cement can contribute to the dissipation of vibro-acoustic energy, thereby sound attenuation propagation and thus preventing hip hybrid prostheses from squeaking.