MRI of normal anterior cruciate ligament (ACL) and reconstructed ACL: comparison of when the knee is extended with when the knee is flexed.

The purpose of this study was to evaluate, using MRI, the morphology of normal anterior cruciate ligament (ACL) and ACL grafts when the knee was extended compared with when the knee was flexed. Eighteen normal controls and 22 ACL graft patients were studied. Spin-echo (SE) T1-weighted images (TR 330 ms/TE 15 ms, NEX 1) were obtained with a slice thickness of 3 mm. Oblique sagittal images parallel to the ACL were obtained at various flexed angles of the knee joint. In 12 of the 18 normal controls the ACL appeared convex toward the posterior side when the knee was extended and gradually became straight when the knee was flexed. In 15 of the 22 ACL graft patients the grafts appeared straight when the knee was extended and became convex toward the anterior side when the knee was flexed. It is concluded that the morphological changes seen on MR images of ACL grafts from when the knee is extended to when the knee is flexed are different from those in the normal ACL.