Patellar Fractures

For Medical Students

What It Shows

A patellar fracture breaks the bone that transmits the extensor mechanism across the knee. 2.4.1 covers the patellar anatomy.

The radiograph shows the fracture line, its displacement and whether the fragments are separated. 5.3.2 covers the lateral view.

It matters because the clinical question is whether the extensor mechanism still works, and the radiograph answers a different question. 4.7.8 covers the extension lag.

How It Is Acquired

Obtain a lateral view, which shows displacement and separation best. 5.3.2 covers the lateral view.

Obtain a skyline view where a vertical or marginal fracture is suspected. 5.3.3 covers the skyline view.

Assess the patellar height, since a displaced fracture or a tendon rupture changes it. 5.12.2 covers patella alta.

Distinguish a fracture from a bipartite patella, which has a corticated margin rather than a sharp edge. 5.15.7 covers developmental variants.

Assess the extensor mechanism clinically, because the imaging cannot. 4.7.1 covers active motion.

What Normal Looks Like

A continuous cortical outline on the lateral and skyline views.

A patella of normal height for the flexion angle. 5.12.2 covers patella alta.

A bipartite fragment, where present, with smooth corticated margins at the superolateral corner. 5.15.7 covers developmental variants.

Clear soft tissue planes around the tendons. 5.5.10 covers the tendons.

What Abnormal Looks Like

A transverse fracture line with separation of the fragments. 5.3.2 covers the lateral view.

A vertical or marginal fracture, best seen on the skyline view. 5.3.3 covers the skyline view.

A high riding patella with a soft tissue gap below it, which indicates a patellar tendon rupture rather than a fracture. 5.12.2 covers patella alta.

A comminuted patella with multiple fragments.

MRCS Level

How It Is Acquired

In a viva the expected answer separates the radiographic finding from the functional assessment within the first sentence.

The expected points are the lateral, the skyline, the patellar height, the bipartite distinction, and the clinical extensor assessment.

Say that displacement and extensor function are two different questions. 4.7.1 covers active motion.

Say that an undisplaced fracture with an intact mechanism can be managed non-operatively. 4.7.8 covers the extension lag.

Say that patellar height change suggests tendon rather than bone. 5.12.2 covers patella alta.

What Abnormal Looks Like

The reasoning expected is that the patella is held by the quadriceps above and the patellar tendon below, so separation of the fragments means the retinaculum has failed as well. 2.4.5 covers the patellar surgical relevance.

An undisplaced fracture implies intact retinacular fibres, which is why displacement rather than the fracture line predicts function. 3.12.4 covers the trochlear function.

A bipartite patella is a failure of ossification rather than of bone, which is why its margins are corticated. 2.4.2 covers the patellar ossification.

Patellar height changes when the tendon fails, so a change in height redirects the diagnosis from the bone to the tendon. 5.5.10 covers the tendons.

How Good It Is

The radiograph identifies a patellar fracture reliably and predicts extensor function only through displacement.

Its best contribution is displacement and comminution, which decide the fixation. 5.3.2 covers the lateral view.

It is limited for the marginal and vertical patterns without a skyline, and for articular surface involvement. 5.3.3 covers the skyline view.

No figures appear for accuracy on this page, under decision 028, and no displacement thresholds appear either: the operative thresholds are published quantities and this environment cannot check them.

FRCS Level

How Good It Is

At decision level the radiograph supplies the fixation plan and the examination supplies the indication.

A displaced transverse fracture with a functional deficit is the standard operative picture. 4.7.8 covers the extension lag.

Comminution changes the fixation strategy, so fragment number matters more than the overall displacement. 5.3.2 covers the lateral view.

An articular surface step needs cross sectional assessment, which is often omitted for the patella. 5.16.1 covers the fracture assessment.

An intact extensor mechanism with an undisplaced fracture can be managed without surgery regardless of the radiographic appearance. 4.7.1 covers active motion.

Where It Misleads

The commonest error is confusing a bipartite patella with a fracture. 5.15.7 covers developmental variants.

A vertical fracture is missed because no skyline view was obtained. 5.3.3 covers the skyline view.

A tendon rupture is read as a fracture because the patella is displaced. 5.12.2 covers patella alta.

The articular surface is not assessed. 5.16.1 covers the fracture assessment.

Operative indication is taken from the radiograph rather than from the examination. 4.7.8 covers the extension lag.

Fellowship Level

Where It Misleads

At fellowship level the patella is the knee's most radiographically obvious bone and its articular surface the least assessed.

Articular step and comminution are what determine later patellofemoral arthritis, and neither is measured well on plain films or routinely imaged cross sectionally. 5.12.7 covers patellofemoral arthritis.

The functional outcome depends on extensor mechanism restoration, which no imaging measures, so the radiographic result and the clinical result diverge regularly. 4.21.7 covers return to sport testing.

Fixation constructs are assessed radiographically for position and not for the articular reduction they achieved, which is the parameter that predicts the compartment. 5.17.5 covers cartilage procedures.

A bipartite patella can become symptomatic, so the distinction from a fracture is not always the end of the question. 5.15.7 covers developmental variants.

The useful discipline is to assess the articular surface deliberately and to treat extensor function as a separate measurement made clinically. 4.7.1 covers active motion.