Limited Accuracy of Tibial Stylus Measurements in Conventional Total Knee Arthroplasty

Background: Accurate tibial resection is important for component alignment and implant longevity in total knee arthroplasty (TKA). Despite the utilization of standardized instrumentation, discrepancies between predicted and actual resection depth persist. In the current study, the correlation between stylus-derived measurements and actual tibial resection thickness is analyzed. Methods: A total of 109 patients undergoing primary TKA using the Legion system (Smith & Nephew, Memphis, TN, USA) were included. The intraoperative resection depth was measured with a stylus at the selected tibial reference point and compared with the caliper-measured osteochondral thickness of the resected tibial fragment at the same location. Correlations were assessed using Spearman's rank correlation, while agreement was evaluated using Bland-Altman analysis. Results: The mean stylus-indicated resection depth was 4.24 ± 2.15 mm, whereas the mean caliper-measured osteochondral thickness at the same marked reference point was 5.75 ± 2.30 mm. After adjustment for the 1.37 mm saw blade kerf, the mean osteochondral resection thickness was 7.12 ± 2.30 mm. The mean discrepancy between planned and kerf-adjusted osteochondral resection was -2.85 (SD = 1.99 mm; 95% CI: -3.22 to -2.47 mm) indicating a tendency toward over-resection. The correlation between stylus-indicated depth and actual osteochondral resection thickness was moderate (ρ = 0.540, p < 0.001). Bland-Altman analysis showed wide 95% limits of agreement from -6.75 to 1.05 mm. Conclusions: Stylus-based predictions of tibial resection depth show limited accuracy. Stylus-based measurements should be interpreted with caution and complemented by intraoperative verification.