Shooting Knee Pain with Squats

Question:

I am currently working with a 30-year-old male ex paratrooper with a left knee injury. He has only the posterior horn of the medial meniscus remaining. He is showing early signs of osteo arthritis. All ligaments are in tact (this was all shown via MRI). He has no weight issues and is extremely fit apart from his knee. I have found that his glute minimus and medius are weak. Measuring around his VMO, there is a 1 1/2 inch diffrerence between legs. He has tight psoas and rectus femoris in the left leg and tight quadratus lumborum on the left side. He is also lordotic. He has no problems squatting with both legs; however, when he tries a single leg squat, he gets a shooting pain behind the patella tendon. This comes on in the first quarter of the squat and is so severe he cannot go down any further. This also causes inflamation for a couple of days after. Can you advise me on what this pain may be?

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