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# Patellar fracture

A patellar fracture is a break of the kneecap; when the pieces separate, the knee may no longer be able to straighten.

Updated 2026-10-29

## Structures involved

Tap one to see what it is and open it in 3D.

- [Patella](https://durgical.com/en/anatomy/patella/)
- [Patellar cartilage](https://durgical.com/en/anatomy/articular-cartilage-of-patella/)
- [Medial retinaculum](https://durgical.com/en/anatomy/medial-patellar-retinaculum/)
- [Lateral retinaculum](https://durgical.com/en/anatomy/lateral-patellar-retinaculum/)
- [Rectus femoris](https://durgical.com/en/anatomy/rectus-femoris-muscle/)
- [Vastus medialis](https://durgical.com/en/anatomy/vastus-medialis-muscle/)
- [Vastus lateralis](https://durgical.com/en/anatomy/vastus-lateralis-muscle/)
- [Vastus intermedius](https://durgical.com/en/anatomy/vastus-intermedius-muscle/)
- [Prepatellar bursa](https://durgical.com/en/anatomy/subcutaneous-prepatellar-bursa/)
- [Joint capsule](https://durgical.com/en/anatomy/articular-capsule-of-knee-joint/)

## In the clinical exam

- [Ottawa ankle and knee rules](https://durgical.com/en/clinical-exam/ottawa-ankle-and-knee-rules/)

## What it is

A break of the kneecap (patella). Displaced fractures can disrupt the extensor mechanism and leave the patient unable to straighten the knee.

## Who gets it

An uncommon fracture overall; most often seen in adults after a fall onto the knee or a dashboard impact in road accidents.

## Causes

- Direct blow: fall onto the front of the knee, dashboard injury
- Indirect: violent quadriceps contraction against a flexing knee, causing a transverse fracture

## Risk factors

- Falls in older adults
- Road traffic accidents
- Osteoporosis
- Previous knee surgery using the patella (e.g. some ligament reconstructions or knee replacement)

## Pathophysiology

The patella is a sesamoid bone in the [quadriceps tendon](https://durgical.com/en/anatomy/tendon-of-quadriceps-femoris-muscle/) that increases the lever arm of the extensor mechanism. A transverse fracture with separation of the fragments, especially if the medial and lateral retinacula also tear, breaks the extensor mechanism so active knee extension is lost.

## Classification

- Non-displaced or displaced (criteria vary; step-off and fragment gap)
- Pattern: transverse, vertical, comminuted (stellate), marginal, pole (sleeve in children)
- Open or closed

## Symptoms and signs

- Pain and swelling at the front of the knee
- Haemarthrosis
- Inability to straighten the knee or lift the straight leg
- Palpable gap in the kneecap
- Bruising over the knee

## Red flags

- Open wound over the knee (open fracture or joint penetration)
- Loss of active extension (extensor mechanism disruption)

## Diagnosis

**Clinical**
Straight-leg raise and active extension test the extensor mechanism; a palpable gap supports displacement. Aspiration of haemarthrosis with local anaesthetic can help the exam.

**Tests**
Usually none; pre-operative tests if surgery is planned.

**Imaging**
AP, lateral and axial (sunrise) knee X-rays; the lateral shows displacement. CT for complex fractures. A bipartite [patella](https://durgical.com/en/anatomy/patella/) (usually upper-outer corner, smooth edges, often both sides) can mimic a fracture.

## Differential diagnosis

- Bipartite patella
- Quadriceps tendon rupture
- [Patellar tendon](https://durgical.com/en/anatomy/patellar-ligament/) rupture
- Patellar dislocation
- Tibial plateau fracture
- Prepatellar bursitis

## Goal of treatment

Restore a smooth joint surface and an intact extensor mechanism so the patient can actively straighten the knee, with early motion to avoid stiffness.

## Treatment

**Acute**
Initial splinting with the knee straight, analgesia and ice.

**Conservative**
Non-displaced fractures with an intact extensor mechanism: knee immobiliser or brace in extension, then progressive flexion.

**Surgical**
Displaced fractures or loss of active extension: open reduction and internal fixation (e.g. tension-band wiring or screws) with retinacular repair; partial patellectomy for severe comminution.

**Rehabilitation**
Physiotherapy for range of motion and quadriceps strengthening.

## Complications

- Post-traumatic patellofemoral arthritis
- Knee stiffness and weak extension
- Symptomatic hardware needing removal
- Loss of fixation or non-union
- Infection
- Deep vein thrombosis

## Prognosis

Most patients regain good function. Comminuted fractures and joint-surface step-offs raise the risk of anterior knee pain and arthritis.

## Test yourself

Think of the answer, then tap the question to see it.

## Which bedside test assesses the extensor mechanism in a suspected patellar fracture?

The straight-leg raise: the patient tries to lift the leg with the knee fully straight. Inability suggests extensor mechanism disruption.

## Which X-ray view best shows displacement of a patellar fracture?

The lateral view of the knee.

## What normal variant can be mistaken for a patellar fracture?

Bipartite patella: a smooth, rounded fragment usually at the upper-outer corner, often seen on both sides.

## When is surgery usually indicated for a patellar fracture?

When the fracture is displaced, the joint surface has a step, or the patient cannot actively extend the knee.

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## Related conditions

- [Patellar Dislocation](https://durgical.com/en/conditions/patellar-dislocation/)
- [Knee Osteoarthritis](https://durgical.com/en/conditions/knee-osteoarthritis/)
- [Deep vein thrombosis](https://durgical.com/en/conditions/deep-vein-thrombosis/)
- [Tibial Plateau Fracture](https://durgical.com/en/conditions/tibial-plateau-fracture/)
- [Prepatellar Bursitis](https://durgical.com/en/conditions/prepatellar-bursitis/)
- [Hip dislocation](https://durgical.com/en/conditions/hip-dislocation/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Anatomy, Bony Pelvis and Lower Limb, Knee Patella](https://www.ncbi.nlm.nih.gov/books/NBK519534/) (StatPearls (NCBI Bookshelf), 2023).

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Source: https://durgical.com/en/conditions/patellar-fracture/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/conditions/patellar-fracture/