Inflammation of the pes anserine bursa
The pes anserine bursa is a sac (lined space) under the attachments of the pes anserine tendons on the inner aspect of the knee.
If it becomes inflamed (bursitis) it can cause pain on the medial side of the knee, with swelling and sometimes snapping.
Risk Factors for pes anserine tendinitis and bursitis
Angular (valgus) stress on this part of the knee may trigger such an inflammation.
Valgus knee deformity, alone or in association with collateral instability, seems to be a risk factor for PATB." [pes anserine tendinitis/bursitis]