THE BRAKE-DRUM EFFECT AND CONTACT STRESS IN TOTAL HIP ARTHROPLASTY

Introduction: We hypothesise that the fixation method of the acetabular component influences stress transmission to the host bone in vivo . We believe that the frequency of appearance of radiolucent lines at the prosthetic rim is directly related to the brake drum effect whereby compressive forces at the dome of a semi-rigid body leads to tensile forces at the rim. Method: A series of miniature pressure transducers were manufactured and positioned at the prosthetic/ bone interface of an acetabular component of a total hip arthroplasty (THA) in a jig designed to replicated the loading conditions of a THA in vivo . The transducers were arranged in a series of five concentric rings spaced from the centre of the acetabular dome to the prosthetic rim. A total of six transducers was used. Three separate experiments were performed: 1. a polyethylene component alone to act as a control. 2. a polyethylene component surrounded by a cement mantle and 3. a polyethylene component surrounded by a metal-back. A separate jig was constructed to provide a cement mantle of the same thickness as the metal back. The stress transmitted to the host bone was measured in each case. Results: The results indicated that successively less stress was transmitted when changing from controls to cemented then to metal-backed cups. Both cemented and uncemented cups demonstrated at the very least absence of compressive forces at the prosthetic rim and in some cases tensile forces, indicating that the brake drum effect is likely.