Chronic anterior cruciate ligament (ACL) tear might be difficult to diagnose on MRI. Indirect signs might be a typical meniscal or cartilage lesion, or a spontaneous anterior drawer visualized by a decreased angle of the posterior cruciate ligament (PCL).
A 27-year-old former ballet dancer was admitted to the emergency department for a locked left knee, without never having experienced previous symptoms of giving way or locking. The MRI performed revealed a medial meniscus bucket handle tear, without traumatic bone marrow oedema or ligament injury. The PCL angle was 130°. A former MRI of her left knee performed 1 year previously to investigate on the recurrent catching of her left knee showed a grade III medial meniscal tear of the posterior horn, and buckling of the PCL angle of 100°, as a sign of chronic ACL rupture. During arthroscopy and medial meniscal repair, the ACL showed complete loss of tension, and was therefore reconstructed simultaneously to enable proper meniscal healing.
Chronic ACL insuffiency is a major risk factor for subsequent medial meniscus tear, especially bucket handle tear. The locked knee might unable proper pre-operative clinical examination. The preoperative MRI therefore being the only possibility to diagnose concomitant ligamentous injury. This is the first case reported in literature showing, that a positive PCL angle sign might be falsely negative due to a locked medial meniscus bucket handle tear.

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