For a lateral knee X-ray, about 20–30° of knee flexion is typically recommended. This degree of flexion helps superimpose the femoral condyles, open the femorotibial joint space, and provide a true lateral profile of the patella and trochlea. Position the patient laterally with the affected knee flexed and the central X-ray beam directed through the joint.

Normal knee range of motion is typically from full extension (0°) to about 135° of flexion, with healthy adults often achieving roughly 120–140°. Everyday tasks require less—walking ~60°, climbing stairs ~80–90°, sitting ~90°, and deep squats ~120–140°. Flexion notably below these ranges may indicate injury, arthritis, or stiffness and could warrant assessment.

Patient lies on the affected side (or supine with the opposite leg elevated), the knee flexed about 30–45° to place the femoral condyles superimposed. Position the limb so the patella is lateral/in profile. Center the x‑ray beam perpendicular to the knee joint (at the level of the joint space), include distal femur and proximal tibia, and remove clothing/metal and immobilize to avoid motion.

For a standard lateral projection of the knee the limb is typically flexed about 20–30 degrees. This slight flexion relaxes the quadriceps, helps superimpose the femoral condyles, and opens the femorotibial joint space for optimal visualization. Positioning may be adjusted slightly for patient comfort, suspected injury, or to better show the patella or specific joint spaces.

To calculate knee flexion angle, have the patient supine with the hip neutral. Palpate and mark the greater trochanter, lateral femoral epicondyle and lateral malleolus. Align the goniometer axis over the lateral epicondyle, stationary arm toward the greater trochanter and moving arm toward the lateral malleolus. Ask the patient to flex the knee fully, then read the angle (0° = full extension).

Criteria for a proper lateral knee radiograph: patient positioned laterally with knee flexed about 20–30°; femoral condyles and posterior condylar outlines superimposed (no rotation); patella in profile; femorotibial and patellofemoral joint spaces visible; distal femur and proximal tibia included; soft tissues and fat pads assessed; appropriate exposure and alignment; use a horizontal beam for trauma or non–weight-bearing cases.