30° of knee flexion means the knee is bent 30 degrees from full straight (0°), representing a mild bend. It is measured with a goniometer and is useful for assessing range of motion during injury recovery or joint exams. Limited or painful flexion at 30° can suggest joint effusion, meniscal injury, or early arthritis and helps guide rehabilitation and treatment planning.

The AP (anteroposterior) knee radiograph is centered at the knee joint (about 1/2 inch below the patellar apex). The central ray is usually perpendicular (0°) to the film. Adjustments by body habitus: angle about 5° caudal when ASIS-to-tabletop distance is <19 cm, and about 5° cephalad when that distance is >24 cm.

Normal knee range of motion is full extension (0°) to about 135° of flexion. Typical everyday activities need less — walking uses ~60°, climbing stairs ~80–90°, sitting requires ~90°, and deep squatting may approach 120–140°. Small individual variation is common; loss of extension or flexion beyond these ranges may indicate injury or arthritis and merits evaluation.

A PA flexion view of the knee is a posterior–anterior X‑ray taken with the knee bent (about 45°), usually weight‑bearing, with the X‑ray beam angled slightly downward. This projection—commonly called the Rosenberg view—better visualizes the posterior tibiofemoral joint and patellofemoral surfaces, improving detection of joint‑space narrowing, early osteoarthritis, and cartilage or meniscal changes.

Yes. Arthritis (osteoarthritis or rheumatoid) can reduce knee flexion by causing cartilage loss, inflammation, swelling, pain, osteophytes and joint deformity, and muscle weakness, all limiting bending and range of motion. Stiffness—especially after rest—further impairs flexion. Treatments such as physiotherapy and strengthening, weight loss, pain relief, injections or surgery can improve pain and mobility and sometimes restore range of motion.

The varus stress test at 30° assesses the lateral (fibular) collateral ligament. With the knee flexed ~30°, the examiner stabilizes the thigh and applies a varus (adduction) force to the lower leg, stressing the lateral joint. Increased lateral joint opening or pain compared with the contralateral side indicates LCL injury or instability; laxity severity helps grade the sprain.