Experimental analysis of standard, asymmetric and constraint liners for total hip replacement: Investigation of range of motion and stability against dislocation

Aim: Recurrent dislocation after total hip replacement is a severe complication, which requires specific treatment and implants. The purpose of the present study was to compare a constraint liner with an elevated rim and standard liner regarding their range of motion and dislocation stability. Method: With a test device, range of motion until impingement (ROMImp) and dislocation (ROMLux) were experimentally analyzed using the above-mentioned insert types of a commercial total hip system. On the basis of movement combinations associated with dislocation, the ROM was determined. Further measuring parameter was the resisting moment against subluxation of the femoral head. Results: The constraint liners showed clear restriction of the movements "internal rotation combined with 90degrees flexion and 0degrees adduction" and "external rotation with 10degrees extension and 15degrees adduction" of up to 20degrees compared to the neutral liner. ROMImp was only decreased by about 8degrees with the elevated-rim liner. The constraint liners revealed the highest resisting moments in subluxation, however, at adequate orientation in the acetabular cup the elevated-rim liners provided a higher ROMLux of UP to 12degrees. Both designs were superior to the neutral liner at retroversion and steep cup position regarding resisting moment and ROMLux. Conclusion: In case of insufficient soft tissue tension the use of constraint liners may increase the dislocation stability, however, in contrast to elevated-rim liners the impingement-free movement interval is clearly reduced. Thus, material damage and high shear stress in the bone interface can result. Therefore, constraint liners should only be used in exceptional cases.