Hi Craggy8,
Sorry to hear of your ongoing knee niggle. Do you have full knee extension and flexion (straighten and bend), compared to the unaffected leg? Do you have any knee swelling?
A number of things can cause back of the knee pain - the first one is mild swelling. As you bend the knee the skin at the front of the knee will become tight, squeezing fluid to the back, causing a wedging effect.
As we tend to sit or hold the knee in a lightly bent position fluid will collect behind the knee. It does not need to be obviously swollen for this effect to occur.
If there is fluid present, it becomes thicker the longer it is sitting there, making it harder to displace and acting as a bigger wedge when you wish to bend the knee.
Another problem that can occur is a Baker's cyst, where part of the internal sealed bag inside the knee protrudes a little into the back of the knee. The internal lubricating fluid can seep into this protruding piece and also cause a wedge. An ultrasound scan can confirm the presence of a Baker's cyst.
Another less common problem can be scar tissue post surgery, restricting range of movement. If this is the case you will need to see your surgeon again.
I have not discussed your knee cartilage (meniscii), as you would likely report knee give way, locking and / or painful clicking if this were to be the case.
Let me know more symptom details and I will try to assist further.
Cheers,
MrPhysio+