Are tensor-only gap measurements interchangeable with trial-based assessment? A paired intraoperative study in total...

Accurate intraoperative assessment of flexion and extension gaps is essential for appropriate polyethylene insert selection in total knee arthroplasty (TKA). Gap measurement using a tensor can be performed directly on bone cuts (tensor-only, TO) or with the femoral trial in situ (tensor-on-trial, TT). Whether these techniques give comparable measurements and whether differences can be explained by femoral component thickness remains unclear. This study compared TO and TT measurements and evaluated their ability to predict the final insert thickness.

This retrospective study included 250 primary TKAs. Extension and flexion gaps were measured using both TO and TT techniques under a standardized distraction force. Differences between measured gaps and the insert thickness were calculated. TO measurements were adjusted by subtracting femoral component thickness (TOadj). Paired comparisons were performed using the Wilcoxon signed-rank test.

TO measurements were consistently larger than TT measurements in both flexion and extension. The median difference between TO and final insert thickness ranged from 8 to 10 mm across compartments, whereas TT showed small errors with median differences of 1-2 mm. Wilcoxon analysis demonstrated significant differences between TT and TO across all conditions. Although TOadj measurements approximated TT values in flexion, significant differences persisted in extension and mean gap measurements.

TT measurements more accurately reflect functional knee gaps and better predict polyethylene insert thickness than TO measurements. Adjustment for femoral component thickness alone does not fully account for the discrepancies, particularly in extension. Reliance on tensor-only measurements may therefore lead to overestimation of insert thickness in TKA.