The other possibility is a stress fracture of the pubic ramus where the pain which is initially felt in the groin area can radiate to the knee, in which case a knee x ray would reveal nothing. More common in women, but a possibility in men.
Due to injury joined half way through Day 2 and did all of Day 3. Spent night in hotel in Wantage which was just as well given problems with hot water at Wantage leisure centre. As ever XNRG did all they could make the event enjoyable as possible in spite of all the problems. Looking forward to Pilgrims' next February.