Knee Joint Function: Movement, Stability, and Shock Absorption

The Knee: Largest and Most Complex Synovial Joint

The knee is a modified hinge joint enabling:

Primary Functions

• Support & Weight Bearing: Transmits body weight from femur to tibia; bears 1–3x body weight walking, 5–7x running/jumping.
• Flexion/Extension: 0° (full extension) to 135–150° (flexion); enables sitting, standing, walking, stairs, squatting.
• Slight Rotation: "Screw-home mechanism" – terminal 10–15° extension: tibia externally rotates ~10° on femur, locking knee (ligamentous tension). Flexion requires popliteus to unlock (internal rotation).
• Shock Absorption: Menisci (medial/lateral fibrocartilage) distribute load, increase congruity, absorb impact. Articular cartilage (hyaline) provides low-friction surface.
• Muscular Control: Quadriceps (extension), Hamstrings (flexion), Gastrocnemius (flexion), Popliteus (unlocking), Hip abductors/adductors (frontal plane stability).
• Ligamentous Stability: ACL/PCL (AP stability), MCL/LCL (varus/valgus), Patellofemoral ligament.
• Lubrication: Synovial fluid (hyaluronic acid, lubricin) from synovium reduces friction.

Patellofemoral Joint

Patella glides in femoral trochlea; increases quadriceps lever arm; subject to high compressive forces (3–7x body weight stair climbing).

Clinical Relevance

OA (cartilage wear), meniscal tears, ACL/PCL tears, patellofemoral pain, tendinopathies, bursitis. Maintenance: strength, flexibility, weight control, neuromuscular training.