Abstract
Introduction
Recurrent instability after total hip arthroplasty remains a serious and somewhat frequent problem. Constrained implants have proven effective to manage instability. This has led to a liberal utilization of these devices. However, sporadic mechanical failures have been reported. This report analyzes the failures of a single constrained device at our institution.
Materials and Methods
Forty-three constrained implants (Stryker Constrained Liner™) in 34 patients were revised out of total 390 similar implants performed at our institution. There were 24 females and 10 males. Constrained implant was inserted at the first revision in 6 hips and after an average of three surgeries (1-6) in 37 hips. Seven different methods of constrained liner fixation were observed. Eight different theoretical failure mechanisms were identified: six are mechanical device failures at each of the implant interfaces, infection and catastrophic polyethylene wear being the other two.
Results
Average time to failure was 28.4 months (1-78). Several failure mechanisms were operating in most cases. The predominant mechanism was infection in 12 (28%), type I (failure of fixation to bone) in 11 (26%), type II (failure of mechanism holding the liner to shell) in 6 (14%), type III (failure of the bipolar holding mechanism) in one (2%), type IV (dislocation of bipolar) in 9 (21%), type V (dislocation of femoral prosthetic head from bipolar) in 3 (7%), and in 1 hip we could not identify the failure mechanism; There were no instances found of type VI failure (disengagement of the prosthetic head from trunion) or catastrophic polyethylene wear.
Conclusion
Constrained tripolar implants are complex devices with multiple interfaces. We have shown multiple mechanisms of failure of these devices. Avoiding technical errors can reduce the number of failures. However, judiciously restricting their use to salvage situations seems warranted.
