Transepicondylar Axis Accuracy in Computer Assisted Knee Surgery: a Comparison of the CT-based Measured Axis Versus the CAS-determined Axis

Rotational malalignment is recognized as one of the major reasons for knee pain after total knee arthroplasty (TKA). Although Computer Assisted Orthopaedic Surgery (CAOS) systems have been developed to enable more accurate and consistent alignment of implants, it is still unknown whether they significantly improve the accuracy of femoral rotational alignment as compared to conventional techniques. We evaluated the accuracy of the intraoperatively determined transepicondylar axis (TEA) with that obtained from postoperative CT-based measurement in 20 navigated TKA procedures. The intraoperatively determined axis was marked with tantalum (RSA) markers. Two observers measured the posterior condylar angle (PCA) on postoperative CT scans. The PCA measured using the intraoperatively determined axis showed an inter-observer correlation of 0.93. The intra-observer correlation, 0.96, was slightly better than when using the CT-based angle. The PCA had a range of -6 degrees (internal rotation) to 8 degrees (external rotation) with a mean of 3.6 degrees for observer 1 (SD = 4.02 degrees ) and 2.8 degrees for observer 2 (SD = 3.42 degrees ). The maximum difference between the two observers was 4 degrees . All knees had a patellar component inserted with good patellar tracking and no anterior knee pain. The mean postoperative flexion was 113 degrees (SD = 12.9 degrees ). The mean difference between the two epicondylar line angles was 3.1 degrees (SD = 5.37 degrees ), with the CT-based PCA being larger. During CT-free navigation in TKA, a systematic error of 3 degrees arose when determining the TEA. It is emphasized that the intraoperative epicondylar axis is different from the actual CT-based epicondylar axis.