The comparison of femoral component rotational alignment with transepicondylar axis in mobile bearing TKA, CT-scan study.

OBJECTIVE: The tibial axis referencing method with a balanced tension flexion gap at 90 degrees knee flexion provides adequate femoral component rotation usually in external rotation, the trans-epicondylar line being parallel to the proximal tibial cut. The LCS mobile bearing TKA uses this technique to automatically determine the femoral component rotation with desired tension. The determination of the epicondyles may lead to some confusion. On the lateral side, the prominence of the lateral condyle makes it easy to define. However on the medial side, some surgeons use the prominent part of the medial epicondyle (well recognized on CT scan as the most proximal ridge that gives insertion to the superficial collateral ligament) and use the anatomical transepicondylar axis (aTEA). Other surgeons use the depression below called sulcus that defines the surgical transepicondylar axis (sTEA).

MATERIAL AND METHOD: The authors evaluated 40 clinically successful mobile bearing TKA in 33 patients. All the knees were performed by single surgeon and the rotational alignment of the femoral component was applied with balanced flexion gap technique. Post-op CT-scans were done in all knees with 2-mm interval and measurement of the different angles (between aTEA and the prosthetic posterior condylar line and between the sTEA and the prosthetic posterior condylar line) with the UTHSCSA Imagetool (IT) version 3 from the University of Texas Health Science Center at San Antonio.

RESULTS: The authors found that the mean femoral implant angle was in 2.39 degrees (SD = 2.80) of internal rotation with reference to the aTEA and in 1.34 degrees (SD = 1.57 degrees) of external rotation with reference to the sTEA when the medial sulcus was perfectly detected (nine knees, 22.5%). The angle between the aTEA and the sTEA was -3.98 degrees (SD = 1.05 degrees). No patella subluxation was identified. Nineteen or 47.5% of the femoral components were in internal or external femoral rotation of more than 3 degrees to the aTEA. When sTEA was detected, no knee was in internal or external rotation more than 3 degrees to sTEA.

CONCLUSION: The balanced flexion gap technique positions the femoral component in external rotation with the LCS TKA. Within 3 degrees to aTEA or sTEA, this technique produced femoral rotational angle closer to sTEA when the sulcus was detected and produced a wide range of different angles when compared to aTEA. However sTEA is not the consistent bony landmark. This technique is a reliable method to determine femoral rotational alignment.