[Experimental analysis of neutral, asymmetric and constraint liners for total hip replacement: investigation of range of motion and protection against joint instability].
OBJECTIVE
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.


METHODS
With a test device, range of motion until impingement (ROM (Imp)) and dislocation (ROM (Lux)) 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 90 degrees flexion and 0 degrees adduction" and "external rotation with 10 degrees extension and 15 degrees adduction" of up to 20 degrees compared to the neutral liner. ROM (Imp) was only decreased by about 8 degrees 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 ROM (Lux) of up to 12 degrees. Both designs were superior to the neutral liner at retroversion and steep cup position regarding resisting moment and ROM (Lux).


CONCLUSIONS
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.
Orthopädische Klinik und Poliklinik, Universität Rostock. rainer.bader@med.uni-rostock.de
R
R
Bader
E
E
Steinhauser
R
R
Scholz
M
M
Simnacher
W
W
Mittelmeier
D019544:Equipment Failure Analysis / Q000295:instrumentation* / Q000379:methods*; D006621:Hip Joint / Q000503:physiopathology* / Q000601:surgery*; D006622:Hip Prosthesis / Q000009:adverse effects*; D006801:Humans; D007593:Joint Instability / Q000175:diagnosis / Q000503:physiopathology* / Q000517:prevention & control*; D011474:Prosthesis Design; D011475:Prosthesis Failure; D016059:Range of Motion, Articular