Primary stability of acetabular reinforcement implants in revision surgery.

It was the purpose of this biomechanical in vitro study to characterize the initial mechanical stability of 3 different acetabular reinforcement prostheses as a function of implant design and bone stock conditions. Müller and Ganz rings and Burch-Schneider cage (Protek, Münsingen, CH) were fixed using 3 screws in normal acetabuli and in acetabuli with 5 different simulated conditions of segmental bone-stock defects. A servohydraulic testing machine (Instron, Canton, USA) was used for the inquiry. Preparations were tested to determine axial stability (2354 N). Three electromagnetic displacement transducers (Micro-Epsilon, Ortenburg, D) were placed in the 3 main quadrants of the acetabular rim to detect the micromotion of the implant. The amount of micromotion depended on the size of the defect and contact area of the prosthesis. All of the implants were stable (< 123 microns) in all quadrants of normal acetabuli and in case of ectasis, protrusio, and ventral defects. Displacement of more than 200 microns was observed at the ilium with the Muller implant in acetabuli with cranial defect (p < 0.05) and with both rings in acetabuli with dorsal defect. The Burch-Schneider cage was stable in all conditions, but displacement of more than 350 microns was observed in acetabula with pseudoarthrosis. The 3 reinforcement implants showed low displacement rates in most of the acetabular bony defects. The experimental data suggests that careful preoperative evaluation and intraoperative assessment to match bone defects and reinforcement implants are of paramount importance to achieve good stability.