Knee fractures on X‑ray

Written by the X-Ray Atlas editorial team from the sources listed below · Medical review pending · Updated 24 October 2026 · Level: Intermediate

Most acutely injured knees have soft-tissue rather than bony injuries, and the Ottawa knee rule helps decide who needs an X-ray at all. When a film is taken, the important findings are often indirect: a fat–fluid level that betrays an intra-articular fracture, a tiny avulsion that signals a torn cruciate ligament, or a patella that sits too high. This guide covers when to X-ray, the views, and how to read them.

1. Who needs an X‑ray? The Ottawa knee rule

The Ottawa knee rule was developed in Canada to reduce unnecessary knee radiographs in adults with acute knee injury. A knee X-ray is indicated if any one of the following is present:

The rule is highly sensitive for clinically important fractures, so a patient with none of these criteria is very unlikely to have one. It does not apply well to patients with altered consciousness, multiple injuries or reduced sensation, and it says nothing about ligament or meniscal injury, which are clinical and MRI diagnoses.

2. Views and how to look

The standard series is an AP and a lateral. In trauma the lateral should be taken with a horizontal beam (cross-table), with the patient supine, so that fluid levels in the joint are visible. Additional views include obliques (for subtle plateau fractures), a skyline view of the patella, and a tunnel view for the intercondylar notch.

Read in order:

1. Soft tissues on the lateral: the suprapatellar bursa (effusion? fat–fluid level?), the patellar and quadriceps tendons.
2. Alignment: is the femur centred on the tibia on both views? Is the patella at a normal height?
3. Tibial plateaux: trace the medial and lateral articular surfaces; compare the joint spaces; look for depression or a split.
4. Femoral condyles, the fibular head and the patella.
5. The margins: small avulsion fragments at the lateral tibia, tibial spines, fibular head and patellar poles.

3. Effusion and lipohaemarthrosis

On the lateral, an effusion distends the suprapatellar bursa, seen as a soft-tissue density above the patella between the quadriceps tendon and the anterior femur.

A lipohaemarthrosis is a fat–fluid level in the suprapatellar bursa on a horizontal-beam lateral. The less dense fat (from bone marrow) floats on top of the blood. Marrow fat can only enter the joint through a fracture that communicates with the joint, so a lipohaemarthrosis means an intra-articular fracture, even if no fracture line is visible. The commonest culprit is a tibial plateau fracture. A vertical-beam lateral cannot show the level, which is why the horizontal beam matters.

4. Tibial plateau fractures

Tibial plateau fractures usually involve the lateral plateau, typically from a valgus force (the classic "bumper" injury of a pedestrian struck by a car), and in older people from a simple fall. They range from a split (a vertical line through the plateau), through depression of the articular surface, to complex bicondylar injuries.

• Look for a step or loss of the smooth line of the plateau on the AP, an increased density (impaction) beneath the surface, and widening of the lateral tibial margin.
• A depressed fragment may be visible only as a subtle area of sclerosis.
• The Schatzker classification (I–VI) runs from a lateral split (I) through lateral split-depression (II) and pure depression (III) to medial plateau (IV), bicondylar (V) and fractures with metaphyseal-diaphyseal dissociation (VI).
• CT is routinely used to measure depression and plan surgery.

Associated injuries include meniscal and cruciate tears and, with high-energy injuries, compartment syndrome and popliteal vessel injury.

5. Avulsion fractures: Segond and friends

Fragment	Where	What it signals
Segond fracture	A small, vertical sliver of bone just below the lateral tibial plateau on the AP	Avulsion by the lateral capsule or anterolateral ligament. Strongly associated with ACL tear (in about 75–100% of cases in reported series) and meniscal injury. Treat it as an ACL injury until MRI proves otherwise.
Reverse Segond	Medial tibial plateau	Associated with PCL and medial meniscal injury
Tibial spine (eminence) avulsion	Intercondylar eminence	ACL avulsion, especially in children and adolescents
Arcuate fragment	Fibular head (styloid)	Posterolateral corner injury, often with cruciate tears
Posterior tibial avulsion	Posterior tibia, on the lateral	PCL avulsion

6. Patella and extensor mechanism

• Patellar fractures follow a direct blow or a violent quadriceps contraction. Transverse fractures are common and the fragments may be pulled apart; stellate (comminuted) fractures follow direct impact. Vertical fractures may be visible only on the skyline view.
• Patellar dislocation is almost always lateral and has usually reduced by the time of the film. Look for osteochondral fragments in the joint and avulsions from the medial patellar margin.
• Patellar tendon rupture gives a high-riding patella (patella alta). The Insall-Salvati ratio (patellar tendon length divided by patellar length, on the lateral) is normally about 0.8–1.2; above about 1.2 suggests alta and below about 0.8 baja. Thresholds vary slightly between sources.
• Quadriceps tendon rupture, typically in older adults, may give a low patella (baja), suprapatellar soft-tissue swelling and loss of the quadriceps tendon outline.

Extensor mechanism ruptures are clinical diagnoses (the patient cannot straight-leg raise) and ultrasound or MRI confirm them; the X-ray provides supporting clues.

7. Pitfalls and mimics

• Bipartite patella: an accessory ossification centre, typically at the superolateral corner, with smooth, corticated margins, often bilateral. A fracture has sharp, irregular, non-corticated edges and is usually transverse.
• Fabella: a sesamoid in the lateral head of gastrocnemius, behind the knee on the lateral view. Normal.
• Vertical-beam laterals cannot show a lipohaemarthrosis.
• Osgood-Schlatter disease in adolescents (fragmentation of the tibial tuberosity with soft-tissue swelling) is a clinical diagnosis; the ossification centre can look irregular in normal children.
• A normal knee film does not exclude ligament, meniscal or cartilage injury, or a spontaneously reduced knee dislocation.
• Hip pathology can present as knee pain, particularly in children (slipped upper femoral epiphysis). Examine the hip.

Red flags: act now

What to focus on, by role

Role	Priorities
Medical student	The Ottawa knee rule, lipohaemarthrosis, and bipartite patella vs fracture.
Nurse / ENP	Apply the Ottawa knee rule. Recognise effusion and lipohaemarthrosis. Check distal pulses.
Junior doctor	Find the fracture behind a lipohaemarthrosis, recognise the Segond, and never miss a reduced knee dislocation.
Radiographer	Horizontal-beam lateral in trauma; add obliques or a skyline when indicated.
Advanced / FRCR	Schatzker typing, avulsion patterns of the posterolateral corner and cruciates, and patellar height.

Practise on real lower limb X-rays →

References

1. Stiell IG, et al. Implementation of the Ottawa knee rule for the use of radiography in acute knee injuries. JAMA 1997;278:2075–9. See also MDCalc: mdcalc.com
2. Stiell IG, et al. Prospective validation of a decision rule for the use of radiography in acute knee injuries. JAMA 1996;275:611–5.
3. Radiopaedia. Segond fracture. radiopaedia.org
4. Radiopaedia. Lipohaemarthrosis. radiopaedia.org
5. Royal College of Radiologists. iRefer. irefer.org.uk

For education only. Not a substitute for a formal report, senior review or your local policy. Never use this page to make decisions about a real patient.