Experimental investigation of threaded cup performance during insertion in total hip replacement

Cementless threaded cups are often used in Europe on the acetabular side of a total hip arthroplasty (THA), with some products achieving excellent clinical results. Titanium and titanium alloys are nowadays accepted as the materials of choice for these implants. In contrast, however, there is no consensus about the ideal thread design. This in vitro and human cadaver study aimed to compare the insertion torques of commercially available threaded cups during insertion. The aims were (1) to see whether the clinical experience of surgeons using these cups specifically related to insertion torque could be reproduced in the experiment and (2) to assess whether or not the cup size and thread design has a statistically significant influence on the insertion torque. Using a biaxial testing machine the cups were screwed into pre-machined polyurethane blocks and the insertion torque was recorded by an appropriate sensor. Clear differences were seen between the four different designs. A variance analysis using statistical software showed that both the cup size (p=0.003) and the thread design (p=0.0009) significantly affect the insertion torque. The human cadaver study allowing a right-left comparison of two different cups within the same specimen confirmed the insertion torque data of the experimental study. In addition, the trends observed during the course of the insertion agreed with the clinical experience of the orthopaedic surgeons consulted as part of this study. (Hip International 2004; 14: 11-7).