Morphological approaches for optimizing lateral knee radiographs in normal knees and knees with osteoarthritis

Accurate lateral knee radiographs are essential for assessing pathology and planning surgery. However, achieving adequate femoral condyle overlap is technically challenging because of individual variations in lower limb alignment. We analyzed the alignment-dependent bone morphology and proposed practical X-ray tube angles to optimize lateral imaging. Full-length lower limb radiographs of 212 normal and 191 knees with osteoarthritis (KOA) were examined. The lateral distal femoral angle (LDFA) and medial proximal tibial angle were measured to classify the alignment into varus, neutral, and valgus types. The LDFA increased with varus alignment in both the normal (89.1°, 88.0°, and 85.2°) and KOA knees (90.2°, 88.0°, and 85.0°). The joint line orientation consistently exhibited an apex-distal pattern. The distal femoral tangent angle (θ = 90° - LDFA) ranged from - 0.2° to 5.0°, providing reference targets for X-ray tube inclination. This alignment-based approach improved imaging reproducibility and diagnostic accuracy in both normal and KOA knees.