1. What do you think the diagnosis is?
This is good history and examination for a partial rupture of the patella tendon. He is the right age and the signs and symptoms all point to this. Sadly this diagnosis is commonly missed as he can still weight bear and extend at the knee (but not against gravity). If this injury is missed it is highly likely to convert to a full rupture following discharge. This will be bad for your medico-legal society and much worse for the patient.

2. How would you confirm it?
Ultrasound is the best method of making this diagnosis radiologically. Our patient showed a 50% rupture of the PT. Remember that with USS your patient has a built in comparator, the other knee!

3. How should it be treated?
This is controversial with some surgeons advocating surgery for partial ruptures, others not. Our patient was treated in a cricket pad splint and followed up in the fracture clinic. Initial management will probably involve 6/52 immobilisation before active therapy.

For more information see:-
http://www.emedicine.com/Orthoped/topic246.htm
http://www.wheelessonline.com/ortho/patellar_tendon_avulsion