Accuracy and reliability of distal femoral rotation measurements using the transepicondylar axis in total knee arthroplasty: A cadaver study

Introduction: The correct alignment of femoral components is necessary to achieve a successful total knee arthroplasty (TKA) and preventing complications such as pain, stiffness, loosening, instability; which itself, depends on making perfect cuts. The posterior distal femoral cut, is one of the most important factors. Yet, the best method of its measurement is controversial. Methods: In this study, 20 cadavers of femoral bones were used; on which, the distal femoral rotation (DFR) was measured manually by 8 orthopedic surgeons and also was calculated on the basis of the trans-epicondylar axis (TEA) by 2 orthopedic surgeons and 2 radiologists on CT scan image. Accuracy and reliability were assessed in both groups, and the risk of severe malrotation was determined according to an outlier defined as 3 and 5 degrees. Results: The mean DFR measured manually, was 1.8±2.1° and the one attained by CT scan images was reported 3.3±1.1°, projecting a remarkable difference between two groups. The Kappa reliability index was calculated about 0.81 in CT scan results and 0.64 in the other group. (k 0.7 was considered as valuable). With an outlier defined as 3°, from the normal distal femoral rotation based on posterior condylar axis (PCA), 3° external rotation; 12% of data were outlying in the manually of measured data, and there was no outlier data were reported in the other group. (With a statistically significant difference between two groups) Conclusion: According to the unreliability and noticeable risk of error in measuring DFR manually based on TEA, this study suggests using CT scan in the patient’s pre-op assessment to obtain the most accurate data.