A Comparison of Tibial Bone Coverage Amongst Six Modern Total Knee Arthroplasties: An MRI Analysis With Optimal Component Rotation

AIM Tibial component design has be been scrutinized in a number of studies in an attempt to improve tibial coverage in total knee arthroplasty. However, very few have controlled for both component rotation and resultant changes to posterolateral tibial tray overhang and posteromedial underhang. We hypothesize that asymmetrical tibial components can provide greater coverage than symmetrical trays without increasing overhang. METHODS The 6 most commonly used tibial trays on the Australian Joint Registry (2009) were superimposed on MRI slices of normal knees to assess tibial component overhang, underhang and percent coverage. Rotational alignment in this analysis was based upon the line joining the junciton of the medial and middle 1/3 of the patellar tendon and the PCL insertion. RESULTS The popliteus tendon was on average 1 mm from the posterior tibial cortex. Only 28.2% of all tibial trays showed optimal posterolateral fit and 48.8% were oversized enough to cause popliteus impingement. NexGen symmetric tray had the largest number of optimally fitting trays on the posterolateral corner (33.7%, the difference was significant against the Genesis II and Triathlon only). The asymmetric Genesis II had the largest percentage of overhang greater than 1 mm. All 6 tray designs had over 80% tibial bone coverage. The Genesis II had the greatest amount of coverage at 88% (paired t test, p CONCLUSION Asymmetric trays in the analysis appear to offer improved bone coverage at the expense of tray overhang when compared to symmetric tray designs thus rejecting our hypothesis.