Experimental investigations of the insertion and deformation behavior of press-fit and threaded acetabular cups for total hip replacement

Background. The deformation behavior of threaded and press-fit acetabular cups in correlation with the applied torques and forces to insert the cups has not been widely investigated. The aim of this experimental study was to analyze the deformation behavior of threaded (BICON-PLUS) and press-fit (EP FIT PLUS and Metal shell) acetabular cup designs during insertion and extraction with regard to the possibility of cup failure. Methods. The experiments were carried out using artificial bone and human acetabular bone. The torque needed to insert the threaded cups and the force applied to the press-fit cups was measured. The force and torque were applied manually by a surgeon using common surgical instruments. The strain of the cups was assessed by tangentially and radially applied strain gauges during the insertion process, the implanted state, and extraction. These measurements were used to calculate the change in diameter of the acetabular cups and the strains at the notches of the threaded acetabular cup. Results. The results showed that maximum strains and applied torques and force occurred during cup insertion. In the case of the threaded cup, an average maximum insertion torque of 114 Nm was measured using bone substitute, whereas only 47 Nm was assessed using human acetabulum. A maximum change in diameter of 8 mu m was calculated during the implanted state for the threaded cup. The impact forces for both press-fit cups ranged from approximately 1.0 to 8.9 kN. The change in diameter was 8 mu m for the Metal shell and 4 mu m for the EP FIT PLUS. In all cases, no permanent cup deformation was detected. Conclusions. The observed deformations suggest that implant failure of the analyzed acetabular cups is unlikely during insertion, the implanted state, or extraction under normal conditions.