Abstract
Objectives
We identified an unusual pattern of backside deformation on polyethylene (PE) inserts of contemporary total knee replacements (TKRs). The PE backside's margins were inferiorly deformed in TKRs with NexGen central-locking trays. This backside deformation was significantly associated with tray debonding. Furthermore, recent studies have shown high rate of tray debonding in PS NexGen TKRs. Subsequently, a field safety notice was issued regarding the performance of this particular device combination and the Option tray has been withdrawn from use. Therefore, we hypothesised that the backside deformation of PS inserts may be greater than that of CR inserts.
Design and Methods
At our national implant retrieval centre, we used peer-reviewed techniques to analyse changes in the bearing wear rate and backside surface deformation of NexGen PE inserts using coordinate measuring machines [N=84 (CR-43 and PS-41) TKRs with non-augmented-trays]. Multiple regression was used to determine which variable had the greatest influence on backside deformation. The amount of cement cover on trays was quantified as a %of the total surface using Image-J software.
Results
The median (IQR) bearing wear rate of the PS PEs [14(8-22) mm3/year] was not significantly different(p=0.154) to that of the CR PEs [18(8-27)mm3/year]. The median (IQR) backside deformation of the PS inserts [294(239-361) µm] was significantly greater (p<0.001) than that of the CR inserts [212(158-258)µm]. Multiple regression modelling showed that duration in-vivo (p=0.037), central-clearance between insert and tray (p<0.001) and constraint (p=0.003) were significantly associated with PE backside-deformation. 38(93%) PS and 31(72%) of CR trays exhibited ≤10% of cement cover. Non-contacting profilometry and microscopy revealed marked pitting and abrasive changes to the superior surface of the tray.
Conclusion
This explant study showed the PE backside deformation was significantly higher in PS than in CR inserts and this may be one explanation for the unsatisfactory clinical performance reported with this device combination.
