Aims:Malpositioning of implant components plays a signiþcant role in instability of THR. Our aim was the determination of the inßuence of anteversion of the acetabular cup. Methods: The biomechanical study was performed on a model which enables different deþned implant positions. Rotation of the femoral stem are carried out in different anteversion positions of the acetabular cup with the hip joint in neutral position and in 90¡ ßexion, as well as inclination of the cup. The range of motion (ROM) is determined until impingement or dislocation is evident, as well as the recording of the resisting moment. Results: While the resisting moment shows minor deviation in several anteversion (AV) positions with the joint in neutral position, being almost independent from the inclination, a major difference is determined with minimal resisting moment with minor retroversion (RV) and with the hip joint in 90¡ ßexion (0,51 Nm/15¡ RV vs. 3,69 Nm/30¡ AV). Dislocation occurs very early due to low ROM at retroversion. Variation of inclination of 30¡ can only increase ROM until dislocation by 2,8¡. With same inclinations angles ROM is increased by 38,1¡ in the 30¡ AV. Conclusions: Correct anteversion of the acetabular component is a signiþcant factor in prevention of dislocation. Minor differences in anteversion are more important than inclination variation to improve stability of THR.

We studied the fixation of a cementless titanium femoral prosthesis partially coated with hydroxyapatite ceramic (HAC) 10.4 months after implantation. Histomorphological investigation showed extensive new bone formation between the HAC coating and the bone bed; morphometry showed bone contact indices of up to 91.60%. There were a number of resorption lacunae on the HAC coat with depths of up to 76.6 microns and widths of up to 453 microns. Our results confirmed that considerable bone remodelling had taken place and that the apatite-coated prosthesis had united with bone despite the lack of appreciable immediate press-fit. Hydroxyapatite particles which had been released did not appear to show any negative effects on the stability of the implant.