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In Vivo Deterioration of Contemporary Tibial Baseplate Locking Mechansisms with Modular Total Knee Components

Gerard A. Engh, MD, Smain Lounici, MS, Anand Rao, BS, Matthew B. Collier, MS

To be published in The Journal of Bone and Joint Surgery

Abstract

Background: The results of recent studies documenting backside wear on retrieved polyethylene inserts call into question the stability of locking mechanisms of modular tibial components. Wear of the metal tibial baseplate suggests that the capture mechanisms of some modular fixed-bearing tibial components do not adequately restrict in vivo motion of the insert. The purposes of this study were 1) to present a method for evaluating locking-mechanism stability and 2) to investigate the stability of modular tibial components after an in vivo interval.

Methods: We measured anteroposterior and mediolateral motion between the polyethylene insert and tibial tray in a variety of modular total knee tibial components. A uniaxial mechanical testing machine was used to evaluate the stability in vitro of ten unimplanted components (control group), fifteen implants obtained at revision total knee arthroplasty surgery (revision group), and fifteen postmortem-retrieved devices (autopsy group). We applied loads along the anteroposterior and mediolateral axes of the tibial component and extrapolated the displacement that occurred between the baseplate and insert before mechanical resistance was encountered from the baseplate locking mechanism. From this we obtained an insert motion index, the magnitude of a two-dimensional vector that represented the total motion in the transverse plane.

Results:  Insert motion was recorded in all implant designs and in all groups. For the control group, the insert motion index was 64mm ± 13mm (mean ± standard error; range, 6mm to 157mm); for the revision group, it was 341mm ± 51mm (range, 104mm to 718mm); for the autopsy group, it was 380mm ± 45mm (range, 122mm to 657mm). The insert motion index for the control group was significantly less than for the revision (p = 0.001) and autopsy groups (p < 0.001).

Conclusions:  Motion between the polyethylene insert and metal baseplate in contemporary modular tibial designs increases after a period of in vivo loading.

Clinical Implications:  From a wear standpoint, the use of modular polyethylene tibial inserts is a compromise. Debris from backside wear secondary to modular insert motion combined with wear from the articular side might account for the increasing prevalence of osteolysis with the widespread use of modular components.

 

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