[Primary stability of 2 acetabulum roof cups and an acetabulum reinforcement cup. Effect of osseous defects].

The aim of this experimental study was to analyse the effects of implant design and bone stock defects on the primary stability of three different acetabular components. Fresh frozen human pelves were employed for the investigation. Müller and Ganz rings and a Burch-Schneider cage (Protek, Münsingen, Switzerland) were fixed with screws in 6 normal acetabula using standard techniques, and in 30 acetabula previously prepared with 5 different segmental bone stock defects. A servohydraulic testing machine (Instron, Canton, USA) was used for the investigation. Three electromagnetic displacement transducers (Micro-Epsilon, Ortenburg, Germany) were placed in the three main quadrants of the acetabular rim to detect implant micromotion, which reflects stability. Displacement was recorded during 20 consecutive cycles under loads of up to 2354 N. All implants were stable (< 150 microns) in all quadrants of normal acetabula and also in those with ectatic, protrusive and ventral defects. There was no statistically significant difference in the results between Müller and Ganz rings. Displacement of more than 150 microns was observed in acetabula with cranial or dorsal defects. The cage was stable under all defect conditions. The reinforcement implants showed low displacement rates in most of the acetabular bony defects. Stability is a function of the area of surface contact between prosthesis and bone.