Femoral Component Axial Rotation in the Gap-Balancing Approach to Total Knee Arthroplasty: Measurement by Computed Tomography.

Background: Rotational malalignment of total knee arthroplasty (TKA) is a potential cause for revision surgery; therefore, it is important to have valid criteria for evaluation of normal component rotational alignment. Because computed tomography (CT) is considered the most accurate method to assess the rotational alignment of prosthetic components, the objectives in this study were define the femoral component (FC) rotation by measuring the posterior condylar angle (PCA) and the condylar twist angle (CTA) in a patient population that underwent gap-balancing TKA; determine the reliability of the FC rotation by using these measurements; evaluate the inter-relationship between the PCA and CTA; and finally evaluate the frequency and agreement in identification of the medial epicondyle sulcus (MES).

Methods And Results: In this retrospective study, 2 radiologists examined 50 CT scans. Mean PCA values of -2.26° and -2.56° (internal rotation) and CTA values of -5.54° and -6.28° (internal rotation) were attained by 2 observers with a higher interobserver concordance for the PCA. Both measurements were considered to be reliable. There was moderate interobserver agreement for MES identification, with the MES present in 64% and 78% of patients, as identified by 2 observers.

Conclusion: Mean FC rotation values as evaluated by PCA were -2.26° and -2.56° and as evaluated by CTA were -5.54° and -6.28°. PCA and CTA measurement by CT is reliable; however, the use of PCA is preferable because of the higher observer concordance. PCA can be inferred by subtracting 3° or 4° from the CTA. MES was identified in 64% and 78% of patients, with only moderate interobserver agreement.