Characteristics

• Largest sesamoid bone in the body. Forms part of the extensor mechanism of the knee and is held in place by the patellar tendon, quadriceps tendon and the adjacent retinaculae.

• Classified according to site and appearance – longitudinal, transverse, stellate, marginal, polar or osteochondral fractures.

• Look for ‘sleeve fractures’ in the paediatric population with distal pole injury on lateral radiograph often indicating a significant articular avulsion.

• Usually due to direct force, such as the knee striking the dashboard in a road-traffic accident or a heavy object falling on the knee.

• May also be caused by an indirect force, such as severe muscular contraction. This can also cause patellar tendon rupture, quadriceps tears or avulsion of the tibial tuberosity.

• The commonest fracture is the transverse type resulting from a powerful muscular contraction transmitted to the patella. This type is commonly displaced.

Clinical features

• Suspect from mechanism of injury history.

• Most cases show an inability to extend the knee although this may be preserved by the intact retinaculum.

• Bruising or abrasions, a palpable step at the site of tenderness, or proximal displacement of the patella.

• Beware of associated injuries such as a femoral neck/shaft fracture, femoral-head dislocation and PCL injury.

Radiological features

• AP and lateral are essential. The skyline view is inappropriate in the acute stage but can be helpful for delayed follow-up.

• The fracture is usually obvious. Look for an associated lipohaemarthrosis on the horizontal-beam lateral.

• Beware the congenital bipartite and multipartite patella; usually occur at the superolateral aspect of the patella. In these the fragments tend to be rounded and corticated as compared to the sharp non-sclerotic margins in a fracture.

• […]