Cruciate ligament injury: diagnostic difficulties in the presence of meniscal injury.

During knee arthrography, the appearance of a mass within the intercondylar notch on the flexed lateral stress radiograph usually indicates a torn cruciate ligament. This mass may also be produced by a torn or peripherally separated meniscus, usually the lateral, that displaces during flexion to lie adjacent to and to obscure the outline of the cruciates. Recognition of this phenomenon is important, both in avoiding the erroneous diagnosis of a cruciate tear and in alerting the arthrographer to the possibility of a peripheral meniscal separation when the ligaments are obscured by a mass.