We hypothesise that the stiffness of the acetabular component influences the stresses transmitted to bone. Thus stress shielding or stress overload of the underlying host bone may be influenced by the choice of fixation method. In addition, we believe that the so called “brake drum effect” plays a significant role in the development of rim stresses and subsequent failure of fixation.

We have constructed a jig which allows the direct comparison, under controlled conditions, of contact stresses measured behind the acetabular component of polyethylene controls, uncemented metal backe cups and cemented all polyethylene cups, under physiological load. The design of the jig also allows measurement of stresses transmitted to the acetabular rim of the same three prostheses in order to confirm the presence and magnitude of the brake drum effect. The contact stresses are measured by miniature pressure transducers which are inserted through specially drilled holes in the shell of the jig so that the transducer is flush with the prosthesis under test. A total of 6 transducers are arranged in concentric circles radiating away from the prosthetic dome, so that contact stresses may be directly measured in various parts of the acetabular bed under conditions that reproduce as closely as possible the situation in a total hip prosthesis in vivo. Thus our method can be compared to other laboratory and theorectical techniques for investigation into stress transmission around acetabular components. The transducers were callibrated prior to each test to ensure parity of test results. The transducers were prestressed to ensure contact prior to each test. 6 polyethylene uncemented liners were tested alone in the jig to act as a control. In the same fashion, the same 6 polyethylene components were tested firstly in an uncemented, metal back acetabular component, and then as a cemented, all polyethylene component.

The results indicate that significantly less stress is transmitted to bone when metal back components are used as compared to cemented components and controls. The data confirms that the brake drum effect occurs in both cemented and uncemented prostheses, leading to at least the absence of compressive forces at the prosthetic rim and in some circumstances tensile forces.

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.