When the knee is extended, the fibers of the medial collateral ligament are taunt and limit hyperextension. When the knee is flexed, the medial collateral ligament provides primary valgus stability. If the medial collateral ligament fails at 10 to 15 mm of joint opening, the cruciate ligaments become the primary restraint to valgus stress. The function of the medial collateral ligament is tested when valgus stress is placed on a partially flexed knee with the tibia in external rotation which allows for relaxation of the cruciates. When fluid collects in this bursa then bursitis may be suspected.
Patients typically report pain in the medial joint line in the absence of mechanical symptoms.
The diagnosis is suspected when the medial meniscus and medial collateral ligament are intact. Symptoms may be relieved with oral nonsteroidal medication or steroid injection into the bursa. Differential diagnosis to be considered includes ganglion cyst and meniscal cyst. Meniscal cyst should be considered whenever a meniscal tear is identified. Ganglion cysts are frequently septated.