Water on the knee: cysts, bursae, and recesses.

Many synovial-lined cavities occur around the knee, and MR imaging is capable of demonstrating the precise anatomic relationships of these spaces. Knowledge of normal anatomy, however, is necessary to accurately assess abnormal, fluid-filled structures. This article reviews the MR imaging appearance of normal and pathologic bursae and joint recesses, and the appearance of common cystic masses. Distinguishing features of several noncystic lesions occurring around the knee also are discussed because these can mimic bursae, recesses, and cysts.