Knee X-rays

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1.  Know your knee anatomy

See the anatomical landmarks on the diagrams below.

landmarks2
From wikiradiography.net
From https://www.wikiradiography.net/
From wikiradiography.net

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

childknee
From thesebonesofmine.wordpress.com

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.

fatpads
Case courtesy of Dr Jeremy Jones, Radiopaedia.org, rID: 29039
kneeeffusion2
Case courtesy of Dr Henry Knipe, Radiopaedia.org, rID: 32559

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.

tibiofemoral alignment
Case courtesy of Dr Jeremy Jones, Radiopaedia.org, rID: 29039

5. Looks at the tibial plateaus

These most commonly happen on the lateral tibial plateau.

tibialplateau

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

segond
From orthopaedicsone.com

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.

tibial eminence
Case courtesy of Gerry Gardner, Radiopaedia.org, rID: 13915

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.

insall-salvatti
Case courtesy of Dr Wael Nemattalla, Radiopaedia.org, rID: 10329

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.

bipartate
Case courtesy of Radiopaedia.org, rID: 11236

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

Patella_fracture

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.

skyline
From wikiradiography.com

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.

fabella
Case courtesy of Dr David Cuete, Radiopaedia.org, rID: 27428

Selected References

Interpreting x-rays of the knee join – YouTube video

Knee radiograph: an approach. Radiopaedia

Trauma x-ray, Radiology Masterclass

About the authors

  • Tessa Davis is a Consultant in Paediatric Emergency Medicine at the Royal London Hospital and a Senior Lecturer at Queen Mary University of London.

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17 thoughts on “Knee X-rays”

  1. Carlos Alberto Giglio

    I am professor of Radiology of the physiotherapy course at a Brazilian university and I would like to know if I can use the imaging exams (with the said quote from the site and author) of the site to set up online courses

    Grateful for the attention

    Cordially

    Prof. Dr Carlos Alberto Giglio

  2. Dear Tessa,

    Lots of information here. Thanks very much! I just want you to know your public-spirited gift of information is much appreciated.

    Regards,

    Ellen