Introduction: An insufficient range of motion (ROM) can lead to prosthetic impingement causing dislocation of a total hip replacement. The objective of this study was to analyze the influence of the wear coupling on ROM and dislocation stability.

Material and Methods: By means of an experimental test device, a total hip system (Alloclassic) with four different insert materials, standard ultra-high-molecular-weight-polyethylene (UHMW-PE), highly cross-linked-polyethylene (XL-PE), aluminium-oxide-ceramic and cobalt-chromium, was investigated concerning ROM and stability against dislocation. The tests were carried out under dry conditions as well as after lubrication of the articulating surfaces with fetal calf serum. In a supplementary test procedure, the force vector-induced dislocation, i.e. dislocation without previous prosthetic impingement, was analyzed.

Results: No significant differences in the ROM until impingement(ROMImp)weredeterminedbetweenthe UHMW-PE and XL-PE inserts. The overall ROMImp of ceramic and metal inserts was approximately 5° less than with PE because no plastic deformation of the rim surface occurred. There was no significant difference in the maximum resisting moment prior to dislocation between the metal-on-polyethylene couples, whereas ceramic-on-ceramic showed the lowest moments and metal-on-metal the highest. Generally, slightly decreased moments for subluxation were determined after lubrication of the sliding surfaces for all couples. In a proper cup position (45° inclination and 15° ante-version) varying the wear coupling had a minor impact on the ROM until dislocation (ROMLux). However, in a poor implant position, ceramic-on-ceramic revealed a clear decrease in the ROMLux of approximately 40° after lubrication of the articulating surfaces. In general, metal-on-metal provided the highest ROMLux. The force vector-induced dislocation provided similar results for the different wear couples.

Conclusion: The study underlines the importance of optimized implant orientation and the impact of the wear couple used on ROM and dislocation stability. Recurrent impingement with subsequent release of wear particles has to be considered for all wear couples. However, ceramic-on-ceramic couples should be used in optimal implant position to avoid impingement and dislocation.

Aims: Femoral component malalignment is known to lead to revision after Total Knee Arthroplasty (TKA). Little is known about the change of intraarticular stress distribution and the influence to compartmental instability due to malrotation. Moreover the influence of inlay design has never been characterized on a cadaver model. Methods: Eight human cadavers were selected. A holder was developed and constructed that allowed the femoral component to rotate along the femoral axis. After performing TKA (Scorpio) with a fixed and a mobile bearing PE inlay the specimens were tested in a servo-hydraulic machine (MTS). A Tekscan film device was installed between the inlay and the femoral component. Static and dynamic measurements with a constant quadriceps ligament force of 800 N were performed between 5°and 90° of knee flexion. The rotation of femoral component was changed between −15 and +15° in 5° incremental steps. Results: Malrotation causes a stress increase in the medial and lateral compartment. Internal malrotation of the femoral component causes higher stresses medially than laterally. A difference between the mobile and fixed bearing design was also observed. Conclusions: Internal femoral malrotation must be avoided because of higher stresses in the medial femoro-tibial compartment and instability in the lateral one.

Aims: To clarify whether joint loading after meniscus repair causes increased pressure on the femoral condyle and is responsible for early cartilage damage. Methods: In sixteen human cadaver knees a bucket handle tear was created at the posterior horn of the medial and lateral meniscus. The lesion was repaired using two biodegradable implants (either Stingerª, Arrow¨, Dartª or Meniscal Screwª) on each occasion. Loading was compared to intact menisci and menisci after suture repair using 2/0 Ethibond. The specimens were mounted into specially designed jigs, taking all degree of freedom of the knee joint into account, and þxed to a material testing machine (Bionix 858 MTS). Constant loading of 350 N was performed during knee motion of 0¡Ð90¡ of ßexion. The meniscofemoral pressure was measured using the Tekscansystem. All þxation techniques were tested þve times. Results: Increased joint loading at the posterior horn occurred with increased ßexion angle of 0¡, 30¡, 60¡ and 90¡ of knee ßexion in the medial and lateral compartment (p< 0.05). No signiþcant increase in joint loading was noticed after meniscus repair with biodegradable implants. Conclusions: Biodegradable implants do not cause higher meniscofemoral joint loading due to meniscus implants in the posterior horn and resulting cartilage damage at the femoral condyle is unlikely.