Common pitfalls in magnetic resonance imaging of the knee

Magnetic resonance examinations of 200 knees were studied retrospectively to determine the frequency with which normal ligamentous and tendinous structures mimicked pathological changes in the meniscus or elsewhere in the knee. Although the course of the transverse geniculate ligament simulated a tear of the anterior horn of the lateral meniscus on forty-four (22 per cent) of the sagittal magnetic-resonance scans, no tear of the lateral meniscus was found in the nine patients in this group who had an arthroscopic examination. The normal anatomy of the meniscofemoral ligament varies greatly. The ligament of Humphry, the anterior branch of the meniscofemoral ligament, was visualized on sixty-six (33 per cent) of the magnetic resonance scans. The ligament of Wrisberg, the posterior branch of the mensicofemoral ligament, was identified on sixty-five (32.5 per cent) of the scans. On sagittal images, these ligamentous branches can be mistaken for osteochondral or meniscal fragments. However, a loose body was found in only one of the thirty-two patients who had an arthroscopic examination after the ligament of Humphrey or of Wrisberg was seen on the magnetic resonance image. The bursa of the popliteus tendon simulated a tear of the posterior horn of the lateral meniscus in fifty-five (27.5 per cent) of the studies, although an actual tear of the posterior horn was found in only two of the eleven patients who had an arthroscopic examination. The higher signal intensity, due to either bursae or fatty tissue, or both, that is normally present in and about the collateral ligaments of the knee should not be mistaken for ligamentous tear or hemorrhage. This study showed that normal anatomical structures in the knee are a common and predictable source of potential false-positive diagnoses of internal derangements.