Transcript of the Video
I would like to show you a patient from some years ago who had patellar instability , evident by the 'J sign ' which is descriptive of an abnormal movement of the patella during bending and straightening the leg. When the knee is bent or flexed, the patella engages properly in the groove of the underlying femur . But as the knee goes into terminal extension - fully straightened - the patella flips out - or subluxes - over the edge of the groove to the outer side.
This patient was dislocating frequently. Investigation by MRI showed that he had underlying trochlear dysplasia - an abnormality of the upper end of the groove - which was giving rise to these high forces on the patella as his knee moved into extension. In these circumstances the vastus lateralis part of the quads muscles then actively forces the patella laterally, until it subluxes. The J-sign in this patient indicated that high biomechanical forces were involved - if the instability were tackled via an MFPL reconstruction - or medial patello-femoral ligament - reconstruction alone, one could be fairly sure that the reconstruction could not stand up to those forces in the long run. I trust that you can see from the video that I was not able to hold back the patella.
Today - and this is in 2016 - I would probably also have looked to see if the patient had abnormal femoral ante-torsion, but this was back in 2006, and in those days a torsion - or twisting - of the long bones of the leg was not well appreciated by me as a possible contributory factor.
I corrected the underlying anatomical problem via a trochleoplasty procedure, with re-insertion of the native MPFL (a procedure we used to do regularly in those days). However, I need to stress that a patient with a J-sign who is non symptomatic should naturally not be treated.