[Shock absorption by intermediate hip prostheses]

The bipolar prosthesis was developed in an attempt to alleviate acetabular wear of conventional metallic endoprostheses. The prosthesis was designed to achieve low-friction metal-on-polyethylene inner bearing motion while decreasing shear stress across the acetabular cartilage. Although good clinical results were obtained, the principle of a persistent inner mobility was contested and some authors have assigned the delay of cartilage erosion to the shock absorption capacity of the polyethylene. The present study was focused on assessing the vibration and shock damping effect of metal monoblock and bipolar head prostheses.

The transmission of the shock wave through these two types of prostheses was studied in vitro. The two heads were first tested in a rigid environment to eliminate all external parasite frequencies. The impact excitation was applied with a hammer connected to an oscillometer. A second accelerometer was connected to the prosthesis stem. Shock wave transmission was analyzed. Secondly, the head was tested with a system vibrating in the same order of frequencies as the skeleton, with and without pre-constraint. The frequency response functions were analyzed.

For the metallic prosthesis, the frequency of vibration recorded on the femoral stems was in a large frequency range from 0 Hz to 10 KHz. For the bipolar prosthesis, all the high frequencies of the shock wave were eliminated and only shock wave frequencies from 0 to 500 Hz were recorded. In an environmental system vibrating below 100 Hz, the metallic head did not express high frequencies of vibration. The coefficient of shock absorption was not significantly different for the two heads.

In the first rigid environment, the metallic head did not filter the high frequency components of the shock wave and the bipolar head tended to eliminate high frequency components due to the flattening effect of the impulse load by the polymer. But, the lower leg is not a rigid structure and the musculo-skeletal system vibrates in frequencies below 100 Hz. In similar conditions, the increased shock-absorbing effect of the polyethylene is far more difficult to observe. For the metallic prosthesis, the recorded frequency of vibration also belongs to a small domain of frequency, from 0 Hz to 100 Hz. The range of frequency is similar for the two types of prostheses.

Even though polyethylene is characterized by a more pronounced damping capacity than metallic materials, in experimental conditions simulating the vibratory characteristics of the human body, the introduction of a high-density polyethylene liner does not afford any additional shock-absorbing effect compared with a metallic head. It cannot explain cartilaginous sparing.