Accuracy of digital inclinometers for measuring knee extension during total knee arthroplasty: comparison with visual estimation and computer navigation

BACKGROUND: Accurate intra-operative assessment of knee extension angle is essential in total knee arthroplasty (TKA), as residual flexion contracture or hyperextension is associated with inferior post-operative function and increased implant wear. Visual estimation remains widely used, but prior research has demonstrated substantial inaccuracy. This study evaluated the accuracy and reliability of a digital inclinometer technique for measuring knee extension angle during TKA, using computer navigation as the reference standard. METHODS: A prospective comparative study was performed on 130 knees undergoing primary posterior-stabilized TKA. After trial implant insertion, knee extension was assessed using two methods: visual estimation and a digital inclinometer method utilizing the femoral chisel holder and anterior border of the tibial crest. All measurements were compared with OrthoPilot navigation. Accuracy was defined as measurements within ± 2° of navigation. Correlation, agreement, and intra- and inter-observer reliability were evaluated. RESULTS: = 0.799; both p < 0.001). Bland-Altman analysis demonstrated minimal bias and no proportional error for the inclinometer, whereas visual estimation showed wide limits of agreement. Intra- and inter-observer reliability for inclinometer measurements was excellent (ICCs of 0.91 and 0.93), superior to visual estimation. CONCLUSION: The digital inclinometer technique provides accurate and reproducible intra-operative knee extension measurements, outperforming visual estimation and approximating navigation-based assessment. This low-cost approach is feasible where navigation is unavailable. TRIAL REGISTRATION: Thai Clinical Trials Registry (TCTR20180828002). Registered on 28 August 2018. Prospectively registered.