Caliper measurements of femoral resection thickness in kinematic alignment total knee arthroplasty are accurate and reproducible.

Purpose: To resurface the patient's knee to the prearthritic state in caliper-verified kinematic alignment (KA) total knee arthroplasty, the thickness of femoral resections after adjusting for saw blade kerf and wear should equal the femoral component. The primary objective was to determine accuracy of caliper measurement by quantifying the systematic (i.e., bias) and random (i.e., precision) errors of both unworn and worn articular surfaces from varus and valgus knees. The secondary objective was to assess reproducibility. Methods: In this diagnostic test study, thickness of each femoral resection from the first patients with 20 varus knees and the first patients with 20 valgus knees was measured by the surgeon using a caliper with 10 mm wide jaws. Subsequently, an optical method established the gold standard thickness. From the difference between caliper and optical measurements, the mean error (bias) and the standard deviation of the error (precision) were computed. Three observers measured resection thickness in three trials on both worn and unworn surfaces for 12 patients with varus knees. Repeatability and reproducibility were assessed by computing the variance components from a two-factor analysis of variance and corresponding intraclass correlation coefficient (ICC) values. Results: For worn and unworn surfaces from both varus and valgus knees, the bias was negligible and the precision was less than or equal to the 0.5 mm resolution of the caliper. For worn and unworn surfaces from varus knees, ICC values were > 0.95 for both repeatability and reproducibility. Conclusions: The caliper used in this study is a sufficiently accurate tool for measuring resection thickness intraoperatively on worn and unworn femoral articular surfaces regardless of knee deformity. The error is less than the resolution of the caliper and the reproducibility is high thus giving surgeons confidence that the native limb and joint lines will be restored provided that the measured resection thickness after correcting for sawblade kerf and wear equals the femoral component thickness to within ±0.5 mm. Level of Evidence: N/A.