Introduction: Recent studies suggested that trans-epicondylar axis (TEA) as the origin of collateral ligament was valuable axis for the parallel cut of the posterior condyle. An alternative landmark of the angle between the TEA and anterior trochlear line of the lateral and medial femoral condyles (trochleo-epicondylar angle) for determining the rotational positioning of the femoral component could be considered. We here report a simple radiographic view with a landmark of the anterior and posterior femoral condyle for determining the rotational alignment of the femoral component in TKA.

Subjects and methods: Our new radiograph presented an axial view of distal femur of a patient. The patient lay in the supine position and flexed the knee about 120 to 130 degrees. An x-ray beam was applied to the knee at the angle of 20 degrees to the ground surface. We measured the external rotational angle between posterior condylar (PC) line and clinical TEA that was condylar twist angle, and the internal rotational angle between the anterior trochlear line (AT line) and clinical TEA. This study involved 122 knees in 82 patients with osteoarthritis of the knee, an average age of 67.3 years. And we compared our measured angle with the angle from 3D reconstructed images with 3-dimensional helical CT system (n=35).

Results: The former angle was 5.7° ± 3.2° and the latter was −5.6° ± 2.9°. There was a variation by individual patients, the condylar twist angle was negative correlation with tibio-femoral angle. The internal rotation angle of the trochlear line and clinical TEA (trochleo-epicondylar angle) was 4.9°±2.1°. The tibio-femoral angle was positively correlated with the trochlear line angle. The trochlear line angle from 3D-CT was 5.4°±1.9°. The average of the difference between our view and the 3D-CT was 0.5°± 1.0°, R=0.87 with a Spearman’s rank test.

Discussion and conclusion: We improved the simple radiographic view in order to evaluate the TEA and PC line, and also the anterior trochlear line, for assessing the rotational alignment of the distal femur in total knee arthroplasty (TKA). We are able to measure and evaluate both angles and do double-checking the condylar twist angle and trochlear line angle. Our new radiographic technique is easy to measure the condylar twist angle, and the angle between AT line and clinical TEA (trochleo-epicondylar angle), simple and reliable, and may be an alternative method for the assessment of TEA of the femur in TKA as preoperative planning.