I am focusing this discussion primarily on patients who are patellar dislocators, although I touch on procedures that also relate to people who have patellar instability and are subluxators but who do not necessarily experience full-blown dislocation.
Patients with patellar dislocation really fall into two groups
If you have a completely normal knee and you fall over and injure it and your patella becomes unstable as a result, that’s a relatively straightforward issue to resolve. The damage is likely to be to the inner patellar support structures, the most important of which is the medial patello-femoral ligament. There is a fairly new ligament procedure that treat such traumatic instability with, and that is medial patellofemoral ligament (MPFL) reconstruction. MPFL reconstruction is the 'new ACL' of knee surgery and lots of different techniques have been designed. We are now on 'technique mark 4' and it seems to work well. I will discuss this in the next section of this part of the course.
Then there is this other group of people that have patellar instability and who basically have abnormal anatomy – they have a flat trochlea ( V-shaped groove on the femur where the patella runs), or their patellar tendon is too and their tibial tubercle is too lateral. The anatomical abnormalities can be described as -
I will go into these two groups of patient in a bit more detail as we move forward in the course.
When assessing a knee we always get four different X-ray views. One of them, the skyline view of the patello-femoral joint (a special X-ray where you look down on the knee) does not give you as much information as we once thought. We all now tend to rely on the lateral knee X-ray more than anything else. Then we send the patients for an MRI scan and try and find answers to the following questions : “Is the trochlea dysplastic?”, that is “Is it too flat, or is it humped?” or “Is it truly ‘V’shaped as it should be?”. We also look at the quality of the cartilage, and we look critically at the tibial tubercle transfer distance (TTTD). The magic figure for this is 20mm. If a patient's TTTD is less than 20 mm then they will tend to do well either with conservative management or if that fails, with a soft tissue MPFL reconstruction. If that figure is greater than 20mm and the patients has failed conservative management then those patients do tend to do well with surgery. Surgery for someone who is skeletally mature is some form of bone realignment procedure, very occasionally adding in an MPFL reconstruction, but usually the bony procedure is enough. What will determine which bony procedure depends really on where most of the deformity is.
The French surgeon, Henri Dejour, was the one who worked out that this lateral X-ray is the key to evaluation. He came up with this sign called the ‘crossing sign’ (or Dejour’s sign). His son, David Dejour, is now one of the top knee surgeons in Lyon and France.