Strange knee pain

Hi everybody!
Yesterday I saw a patient having a strange pain that I can't understand. He is a male, 67 years old, retired, active person, no other physical problems. This pain is referred at the internal part of the right knee and appeared about 2 months ago for the first time. He describe his pain like a stab that hurts a lot for 10 seconds and than desappears completly. After the first time the pain occurred after 2 weeks. Than the pain went back again in a week and now it occurs 2-3 times a week. It seems no activity correlated becouse it occurred sometimes when he was walking, sometimes when he was sitting, one time when he was just waiting for the bus. Normal range of motion, strenght and muscle length. Manual tests for meniscus or ligaments injury negative.
I really can't understand what could be the cause of this pain... Anyone have got some ideas?
If it is not clear ask me more!

Thanks,

Franz

The pain in my knees is a real problem with people waiting for an answer.

Hi Franz! What exactly do u mean by pain being in the *internal* part of his knee? Is it deep pain located under the knee cap, pain within the join capsule, pain laterally, medially, at the back or the front? Any previous neck, back problems?

Monika