Patella Articulation: Knee Anatomy, Biomechanics & Clinical Relevance

The patella (kneecap) is the largest sesamoid bone in the body, embedded in the quadriceps tendon. It articulates with the femoral trochlea (trochlear groove) forming the patellofemoral joint (PFJ).

Articular Geometry

• Proximal patella: Base (superior), articulates with superior trochlea at full extension.
• Distal patella: Apex (inferior), articulates with distal trochlea at ~90° flexion.
• Medial & lateral facets: Medial facet larger; lateral facet higher (prevents lateral subluxation).
• Odd facet: Medialmost, contacts trochlea only in deep flexion.

Biomechanical Functions

1. Increases quadriceps moment arm – Improves extension torque by 30–50% (fulcrum effect).
2. Centralizes quadriceps force – Distributes load across trochlear cartilage.
3. Protects anterior knee – Shields femoral condyles from direct trauma.
4. Enhances lubrication – Compression-relaxation cycles pump synovial fluid.

Clinical Correlates

• Patellofemoral pain syndrome (PFPS) – Maltracking, lateral tilt, chondromalacia.
• Patellar instability – Subluxation/dislocation; trochlear dysplasia, high-riding patella (patella alta), TT-TG distance.
• Patellar fracture – Direct trauma or indirect (quadriceps contraction); transverse, comminuted, sleeve.
• Osteochondritis dissecans – Subchondral bone necrosis.
• Patellar tendinopathy – "Jumper's knee"; proximal tendon insertion.

Imaging

• Sunrise/Merchant view (30–45° flexion) – PFJ congruence, tilt, subluxation.
• CT – TT-TG distance, trochlear dysplasia metrics.
• MRI – Cartilage, bone marrow edema, tendon pathology.