CT and MRI measurement of the femoral anterior tangent for the rotational reference axis of femoral prostheses

This study aims to evaluate the value of the femoral anterior tangent (FAT) as a reference axis for the rotation of femoral prostheses on computed tomography (CT) and magnetic resonance imaging (MRI). We retrospectively included 66 patients (83 knees) with primary knee osteoarthritis. FAT, clinical transepicondylar axis (cTEA), surgical transepicondylar axis (sTEA), anterior-posterior axis (APA), perpendicular axis to the posterior-anterior axis of the femoral condyles (pAPA), and posterior condylar axis (PCA) were measured on CT and MRI. The rotation angles of FAT, pAPA, and PCA relative to cTEA and sTEA were measured on the same layer. Patients were further subgrouped based on Kellgren-Lawrence (K-L) grade and gender to assess the variance of different rotational reference axes. The mean values of FAT/cTEA, FAT/sTEA, and pAPA/cTEA measured on CT were significantly lower, indicating greater internal rotation, than those measured on MRI (P < 0.05). There were no statistical differences between FAT/sTEA, FAT/cTEA, pAPA/cTEA, and PCA/cTEA among genders (P > 0.05). The CT measurement of pAPA/cTEA in K-L grade 3 knees was −1.1° ± 3.7°, significantly higher than in K-L grade 4 knees (P = 0.036). FAT/cTEA, FAT/sTEA, and PCA/cTEA demonstrated higher stability in various subgroups of knee osteoarthritis. The rotational angles of femoral prostheses measured by CT for FAT and other reference axes show increased internal rotation compared to MRI measurements. Both measurement methods provide relatively stable values for FAT measurements. Clinicians should be cautious of the numerical differences between MRI and CT measurements in clinical applications and should consider modality-specificreference values for pre-operative planning.