The theory of early loosening of hip prostheses.

The issue of prosthetic loosening is currently a matter of debate, particularly with regard to the timing and nature of the precipitating events. The theory presented here postulates that loosening begins at an early stage due to either insufficient initial fixation or early loss of fixation. The loosened prosthetic component is then affected by varying degrees of mechanical stress associated with normal daily activity, which differs according to patient characteristics (body weight and level of physical activity) and the components used (prosthetic design, positioning, friction, and wear). This theory of early loosening can explain without supplementary ad hoc assumptions the rapid early prosthetic migration detected by roentgen stereophotogrammetry, the development of focal osteolysis and wear granulomas, the phenomenon known as stress-shielding, and, to a great extent, the epidemiology of clinical failure.