Evaluation of the two bundles of the anterior cruciate ligament with 1.5 tesla magnetic resonance imaging

Objectives: Studies on the anatomy of the anterior cruciate ligament (ACL) have shown that the normal ACL consists of two functional bundles named as anteromedial (AM) and posterolateral (PL) bundles. In this study, we evaluated the AM and PL bundles of the ACL using 1.5 tesla mag¬netic resonance imaging (MRI), which is routinely used in clinical practice. Methods: The study included 150 patients (96 females, 54 males; mean age 33.4±11.6 years; range 18 to 59 years) who did not have any signs of ACL insufficiency and whose knees were examined by MRI for other reasons. Standard magnetic resonance images (77 right, 73 left) were evaluated independently by an orthopedist and a radiologist in terms of distinguishable ACL bundles. The angle between the ACL (and each bundle) and the tibial plateau was measured on sagittal and coronal sections. Arthroscopic surgery was performed in 64 patients (42.7%) for pri¬mary diagnoses and arthroscopic and MRI findings were compared. Results: Magnetic resonance imaging showed an intact ACL in all the patients. The ACL was as¬sessed as a single bundle in the axial, coronal, and sagittal planes in 93 patients (62%). A double- bundle appearance was noted in 57 patients (38%), involving all three planes in 14 patients (9.3%), axial and coronal planes in 41 patients (27.3%), coronal and sagittal planes in one patient (0.7%), and only coronal plane in one patient (0.7%). On MRI sections showing a single bundle ACL, the mean angle between the ACL and the tibial plateau was found as 55.3° in the sagittal plane, and 70.3° in the coronal plane. On sections with a double-bundle appearance, the mean angles be-tween the AM bundle and the tibial plateau were 70.1° and 55.1° in the coronal and sagittal planes, respectively. The corresponding angles for the PL bundle were 81° and 53.5°. The incidence of double bundle ACL appearance in coronal, sagittal, and axial MRI sections was not influenced by sex and side (p>0.05). The number of bundles identified in each plane did not show a significant difference between the two observers (p>0.05). During arthroscopic surgery, both bundles were identified with normal integrity and function of the ACL in all the patients. Of these, MRI could depict a double-bundle appearance in one or more planes in only 42.2% of the patients. Conclusion: Even though standard 1.5 tesla MRI, routinely used in clinical practice, has a very high success rate in demonstrating the ACL, it can visualize the two-bundle structure only in about one-third of the patients.