Patella Anatomy and Clinical Relevance

Also written as kneecap, patellar sesamoid.

The patella is the largest sesamoid bone, embedded in the quadriceps tendon and articulating with the femur. Its position, blood supply and ligamentous attachments are key to knee stability and common injuries.

Overview

The patella is a triangular, flat bone that sits in the front of the knee joint, deep to the skin and subcutaneous tissue. It increases the lever arm of the quadriceps muscle, improving knee extension strength.

• Largest sesamoid bone in the body
• Develops from a cartilaginous nodule within the quadriceps tendon
• Articulates with the femoral trochlea

Location and structure

The patella lies between the quadriceps tendon superiorly and the patellar tendon inferiorly. Its anterior surface is convex and subcutaneous; the posterior surface is concave, matching the femoral trochlear groove.

• Height ≈ 5 cm, width ≈ 4 cm
• Base faces upward, apex points downward
• Two articular facets: medial and lateral

Relations

Anteriorly the patella is covered by skin, superficial fascia and the deep fascia of the thigh. Medially it is related to the medial retinaculum and the saphenous nerve; laterally to the lateral retinaculum and the iliotibial band.

• Quadriceps tendon attaches at the superior pole
• Patellar tendon attaches at the inferior pole to the tibial tuberosity
• Medial and lateral retinacula stabilize the bone

Blood supply

Vascularisation comes from the genicular anastomosis, mainly the medial and lateral inferior genicular arteries, with a peripatellar plexus supplying the anterior surface.

• Posterior surface receives branches from the genicular arteries
• Anterior surface supplied by a peripatellar plexus

Nerve supply

Sensory innervation is provided by the femoral nerve (via the saphenous branch) and the common peroneal nerve (via the lateral femoral cutaneous branch). Motor innervation is not applicable because the patella is a bone.

• Saphenous nerve supplies skin over the anterior patella
• Lateral femoral cutaneous nerve supplies lateral skin

Function

The patella acts as a fulcrum, increasing the quadriceps tendon’s angle of pull and thereby enhancing knee extension torque. It also protects the anterior knee joint.

• Improves quadriceps lever arm by ≈ 30 %
• Distributes compressive forces across the femoral trochlea
• Protects the knee joint capsule

Clinical relevance and injuries

Patellar disorders range from acute trauma to chronic overuse. In low‑resource settings, malnutrition, infection and delayed presentation increase morbidity.

• Plain knee X‑ray (AP, lateral, sunrise): Does the image show fracture lines, displacement or joint space narrowing? Positive fracture confirms need for immobilisation or surgery; normal X‑ray with persistent pain prompts MRI.
• Ultrasound of the knee: Is there an effusion or tendon rupture? Useful when X‑ray unavailable; a large effusion suggests inflammatory or infectious process.
• Joint aspiration and Gram stain: Is there purulent fluid? Positive Gram stain/culture confirms septic arthritis; if unavailable, treat empirically in high‑risk patients (HIV, diabetes).

• Patellar fracture – usually from direct blow; look for swelling, inability to extend, palpable defect
• Patellar dislocation – most often lateral; assess for pain, deformity, and neurovascular status
• Patellofemoral osteoarthritis – crepitus on flexion/extension, pain worsened by stairs
• Septic arthritis of the knee can involve the patella; consider TB or Staph aureus in HIV patients

Exam pearls

A focused knee exam can differentiate patellar pathology from other causes of anterior knee pain.

• Patellar apprehension test – positive when lateral translation of the patella elicits pain or fear of dislocation
• Patellar grind test – patient contracts quadriceps while examiner applies pressure; crepitus suggests patellofemoral arthritis
• Check for extension lag – inability to fully extend indicates quadriceps tendon rupture or severe fracture

Questions an examiner may ask

• A 22‑year‑old footballer presents with sudden lateral knee pain after a twist – what is your next step? — Perform the patellar apprehension test to confirm lateral dislocation before ordering imaging.
• A patient with HIV presents with knee swelling and fever – which investigation is essential and why? — Joint aspiration for Gram stain and culture; it identifies septic arthritis, a surgical emergency.
• You find a non‑displaced transverse patellar fracture on X‑ray – how would you manage it in a district hospital? — Immobilise the knee in extension with a splint, arrange referral for possible surgical fixation if displacement occurs.
• During a routine exam you note crepitus on patellar grind test – what does this indicate and what further assessment is needed? — Suggests patellofemoral osteoarthritis; obtain weight‑bearing AP and lateral X‑rays to grade joint space loss.
• A child presents with anterior knee pain after a fall, but X‑ray is unavailable – what bedside tool can help and what will you look for? — Use ultrasound to detect an effusion or cortical disruption; a large effusion may point to hemarthrosis or infection.
• Why is the saphenous nerve important when examining the patella? — It supplies sensation over the anterior knee; injury can cause numbness that mimics deeper joint pathology.
• In a patient with chronic knee pain, you suspect patellar maltracking – which clinical sign helps differentiate it from simple arthritis? — Positive patellar apprehension test with normal joint line tenderness suggests maltracking rather than arthritis.
• What contraindicates the use of NSAIDs in a patient with patellar fracture? — Active gastrointestinal ulcer or severe renal impairment; NSAIDs may worsen bleeding and impair bone healing.

When to seek help

• Severe knee swelling with fever – possible septic arthritis
• Inability to extend the knee after trauma – possible fracture or tendon rupture
• Rapidly worsening pain and deformity – risk of neurovascular compromise

Quick questions

Why does a patellar fracture impair knee extension?

The fracture disrupts the quadriceps tendon’s attachment, reducing the lever arm needed for effective extension.

What makes lateral patellar dislocation common in young athletes?

A shallow femoral trochlea, strong lateral pull of the quadriceps, and inadequate medial retinaculum allow the patella to slip laterally.

How does malnutrition affect patellar health?

Poor bone mineralisation predisposes to stress fractures and delayed healing after injury.