We investigated the static and cyclical strength of parallel and angulated locking plate screws using rigid polyurethane foam (0.32 g/cm³) and bovine cancellous bone blocks. Custom-made stainless steel plates with two conically threaded screw holes with different angulations (parallel, 10° and 20° divergent) and 5 mm self-tapping locking screws underwent pull-out and cyclical pull and bending tests. The bovine cancellous blocks were only subjected to static pull-out testing. We also performed finite element analysis for the static pull-out test of the parallel and 20° configurations. In both the foam model and the bovine cancellous bone we found the significantly highest pull-out force for the parallel constructs. In the finite element analysis there was a 47% more damage in the 20° divergent constructs than in the parallel configuration. Under cyclical loading, the mean number of cycles to failure was significantly higher for the parallel group, followed by the 10° and 20° divergent configurations.

In our laboratory setting we clearly showed the biomechanical disadvantage of a diverging locking screw angle under static and cyclical loading.

Aims: Malposition of the acetabular cup after THR leads to dislocations and impingement, reduces ROM and increases pressure and wear inside the cup. The use of the OrthoPilot? kinematic computer navigation technique may reduce the rate of cup malposition. Methods: After the registration of 3 pelvic bone landmarks, stereocameras record the motion of bone and instument- þxed infrared Ç ridgid bodies È. From these data the hip center, the position and direction of reamers and cup insertion instruments are calculated. Since 2001, 155 cementless plasma coated titanium pressþt cups were implanted by using this OrthoPilot¨technique. The intraop measured data of inclination and anteversion angles were compared to the angles from postop X-rays. Results: In 147 evaluated cases no speciþc complications occurred. After the þrst 35 cases minor primary technical problems could be solved. In the following cases the operation time was prolonged by 9 minutes only compared to conventional technique. The mean intra-operative value for inclination was 41¡, (29–48¡), for anteversion 15.9¡, (3–29¡). The evaluated data from the x-rays were 42¡ (34–50¡) for inclination and 10.9¡ (3–22¡) for anteversion. No dislocation occurred due to cup mal-position. Conclusions: By using kinematic navigation technique an improvement of the cup position can be shown clearly. The intended þnal cup position within the Ç safe zone È can be reached reliable. The technique is easy, safe, quick and inexpensive. The rate of postop dislocations might be reduced.