Errors in visual estimation of flexion contractures during total knee arthroplasty

AIM: To quantify and reduce the errors in visual estimation of knee flexion contractures during total knee arthroplasty (TKA). METHODS: This study was divided into two parts: Quantification of error and reduction of error. To quantify error, 3 orthopedic surgeons visually estimated preoperative knee flexion contractures from lateral digital images of 23 patients prior to and after surgical draping. A repeated-measure analysis of variance was used to compare the estimated angles prior to and following the placement of the surgical drapes with the true knee angle measured with a long-arm goniometer. In an effort to reduce the error of visual estimation, a dual set of inclinometers was developed to improve intra-operative measurement of knee flexion contracture during TKA. A single surgeon performed 6 knee extension measurements with the device during 146 consecutive TKA cases. Three measurements were taken with the desired tibial liner trial thickness, and 3 were taken with a trial that was 2 mm thicker. An intraclass correlation coefficient (ICC) was calculated to assess the test-retest reliability for the 3 measurements taken with the desired liner thickness, and a paired t test was used to determine if the knee extension measurements differed when a thicker tibial trial liner was placed. RESULTS: The surgeons significantly overestimated flexion contractures in 23 TKAs prior to draping and significantly underestimated the contractures after draping (actual knee angle = 6.1° ± 6.4°, pre-drape estimate = 6.9° ± 6.8°, post-drape estimate = 4.3° ± 6.1°, P = 0.003). Following the development and application of the measurement devices, the measurements were highly reliable (ICC = 0.98), and the device indicated that 2.7° ± 2.2° of knee extension was lost with the insertion of a 2 mm thicker tibial liner. The device failed to detect a difference in knee extension angle with the insertion of the 2 mm thicker liner in 9/146 cases (6.2%). CONCLUSION: We determined the amount of error associated with visual estimation of knee flexion contractures, and developed a simple, reliable device and method to improve feedback related to sagittal alignment during TKA.