Background: The results of recent studies documenting the backside wear of polyethylene inserts retrieved from total knee implants call into question the stability of the 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 intervalin vivo.
Methods: We measured the anteroposterior and mediolateral motion between the polyethylene insert and the tibial tray in a variety of modular total knee tibial components. A uniaxial mechanical testing machine was used to evaluate the stability of ten unimplanted components (control group), fifteen implants obtained from patients who were undergoing revision total knee arthroplasty (revision group), and fifteen devices retrieved post mortem (autopsy group). We applied loads along the anteroposterior and mediolateral axes of the tibial component and recorded the maximum insert displacement that occurred. From this value, we calculated an insert-motion index, the magnitude of a two-dimensional vector that represented the total motion in the transverse plane.
Results: For the control group, the mean insert-motion index was 64 ± 13 m (range, 6 to 157 m); for the revision group, it was 341 ± 51 m (range, 104 to 718 m); and for the autopsy group, it was 380 ± 45 m (range, 122 to 657 m). The insert-motion index for the control group was significantly lower than that for the revision group (p = 0.001) or autopsy group (p < 0.001).
Conclusions: Motion between the polyethylene insert and the metal baseplate in contemporary modular tibial designs increases after a period of in vivo loading.
Clinical Relevance: Although there are several advantages to the use of modular tibial components, these ad-vantages must be weighed against the disadvantage of backside wear debris secondary to motion of the modular insert. Debris from backside wear combined with wear from the articular side might account for the increasing prevalence of osteolysis since modular components have become widely used.