Knee X-rays

1.  Know your knee anatomy

Take a look at the main anatomical landmarks on the standard knee x-rays below.

Remember that the knees of younger children will look different, as the patella and ossification centres form.

2. Look for an effusion

There are two fat pads in the knee

• the suprapatellar fat pad
• the prefemoral fat pad

Make sure they are next to each other. Soft tissue density in between the two fat pads indicates an effusion – this is only reliably seen on the lateral view (see images below).

It is sometimes helpful to rotate the PACS view so you are looking at the knee in the horizontal plane, in the same way the image is taken.  Your eyes are much more adept at picking up an effusion or even a fat/fluid level (lipohaemarthrosis) that way.

3. Look at the main bones

Check for fractures in the fibular head, femur and tibia.

4. Check the tibio-femoral alignment

Draw a line along the margin of the lateral femoral condyle. The tibia should be within 0.5 cm of this line, otherwise, it suggests a tibial plateau fracture.

5. Look at the tibial plateau

Fractures most commonly occur on the lateral tibial plateau.

Check for a tibial plateau avulsion from the lateral edge (Segond fracture)

Tibial plateau fractures in children are exceedingly rare and require a marked degree of axial force. They are more likely to get a Salter-Harris V.

6. Look at the intercondylar eminence

A fracture here is most common in adolescents following hyperextension of the knee. It’s an avulsion fracture at the tibial attachment of the ACL.

7. Look for patellar tendon disruption

The patellar tendon goes from the inferior pole of the patella to the tibial tuberosity. Its length should be the same as the patellar length +/- 20%. If it’s too long then think of a patellar tendon rupture. This is the Insall-Salvatti ratio and should ideally be measured with the knee flexed at 30 degrees.

8. Look for a patellar fracture

Bipartite patellas are common. It is a congenital condition that occurs when the patella is made of two bones instead of a single bone. Normally the two bones would fuse together as the child grows but in bipartite patella, they remain as two separate bones. The edges appear well corticated as compared to in a fracture. See an example below.

Most patella fractures are transverse, but they can be vertical.

Consider a skyline view. This gives a clearer view of the patella in cases of clinically suspected patella fracture where the AP and laterals look ok. It gives a good view of the space between the patella and the femur. See a normal skyline view below.

9. Remember the fabella…

This is a normal variant and not a floating fracture! It’s normal sesamoid bone that lies in the posterior knee.

Selected References

Interpreting x-rays of the knee join – YouTube video

Knee radiograph: an approach. Radiopaedia

Trauma x-ray, Radiology Masterclass