In any case, here are a few tips I have picked up (or had forced upon me!) that will hopefully make lateral knees a little easier.
Standard Lateral Knee (patient rolled onto side)
- Ask the patient to roll right up onto their side/hip - not merely roll their leg out
- Keep the other leg behind the one you are x-raying - I find rolling it over the top will more often than not make you over rotated HOWEVER
- If the patient is struggling to roll onto their side (often the case with large patients) have the bring their other leg over the top
- Try and feel for the epicondyles to see if they feel superimposed (not always possible to determine)
- 2 finger rule - I never really got this but people swear by it. I believe your 2nd and 3rd fingers should sit between the patella and the table when correctly positioned - can be difficult with large knees however
- Feel for the tibial spine, this should be running parallel to the table. If not, place a small sponge under the ankle to raise the lower leg and open the joint space.
- Angle up 5 degrees (again, seems to work most of the time but not fool proof)
- Slight bend in knee - should never be 45 degrees!
- The best tip I have learned is to look at the fibula
- If you can see too much fibula - the knee is over rotated - try lifting the knee up off the cassette slightly or rolling the patients hips back towards the table slightly
- If most of the fibula is behind the tibia, you are under rotated - this is when rolling the other leg over the top may be useful or simply rolling the patient slightly more onto their hip
- It is usually only a very small movement required to fix the rotation
Trauma or Horizontal Beam Lateral
- Should always be performed after trauma (regardless of whether the patient can roll for a lateral) to assess for fluid
- Have a slight bend in the knee if tolerable to open the joint space
- Can feel for the epicondyles to assess rotation but again can be difficult with large knees or swelling
- Use ankle as an indicator however remember that some people's feet turn out/in so that may be misleading
- Again, I find feeling for the tibial spine to be most accurate. Run your hand down the spine to ensure it is running parallel to the table
- As the beam is now going lateral to medial (as opposed to medial to lateral in a standard AP) angle down 5 degrees
- If you see too much fibula, the leg is under rotated. Turn the whole leg slightly inwards
- If you don't seen enough fibular, the leg is over rotated. Turn the whole leg slightly outwards.
Again, these are general rules, however I find them to be successful most of the time. You will always find that one shonky knee which you just can't seem to get right. Don't be afraid to concede defeat after a few attempts. For some reason, a new set of eyes and hands will annoying get it right when you continue to fail.
Remember to feel for landmarks to see if you are in the right position. These include the tibial spine, epicondyles and patella. For trauma or post surgical knees, try feeling the unaffected knee to get an idea of normal anatomy (patients also appreciate not poking their sore knee). This will also help get an idea of correct centring position as often swollen knees and be misleading.
Finally, if you image a lot of knee replacements, remember that the joint is artificially made and therefore a lot easier to show perfect superimposition of condyles and a large joint space. With knees suffering from bad OA, the joint space is often tiny and the condyles may be uneven to start with, making superimposition very difficult. Don't necessarily expect a normal knee to look like a TKR replacement knee.
Please let us know if you have come across any other tips for lateral knees!