Anatomical studies show that the native ACL consists of two distinct functional bundles, termed the anteromedial (AM) and posterolateral (PL) bundles. The utility of using routine magnetic resonance imaging (MRI) to distinguish the individual bundles of the ACL has not been evaluated. The purpose of this study was to evaluate the intra- and inter-observer agreement for assessment of the AM and PL bundles using MRI in the axial, coronal, and sagittal viewing planes. We identified a series of patients seen in the senior author's clinic during a 16-month period. Images were independently evaluated in blinded fashion at two separate time points by a musculoskeletal radiologist and two orthopaedic residents. The AM bundle was detected in most planes of view with high frequency and reliability, while detection of the PL bundle was less frequent and had a lower associated reliability. Our results indicate that it is difficult to reliably detect both the AM and PL bundles using a low-field strength magnet with standard planes of view. It has been demonstrated that the ACL may be imaged effectively in planes that are based on the natural course of the ligament, and it is likely that this will also facilitate visualization of the individual AM and PL bundles. The use of additional oblique planes of view offers a potential approach for improved evaluation of the ACL, even with low field strength magnets. Future work in this area may assist in the pre-operative assessment of isolated AM or PL bundle injuries, facilitating a more anatomic approach to ACL reconstruction.

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Following anatomic double-bundle anterior cruciate ligament (ACL) reconstruction with hamstring tendon autografts, 38 consecutive patients were evaluated with high-speed three-dimensional computed tomography. Scans were performed within 3 days following surgery. The length and width of the reconstructed ACL footprint were measured on axial images. Then, 3D images were converted into 2D with radiologic density for measurement purposes. Tunnel orientation was measured on AP and lateral views. In the sagittal plane, the center of the anteromedial (AMB) and posterolateral bundle (PLB) tibial attachment positions was calculated as the ratio between the geometric insertion sites with respect to the sagittal diameter of the tibia. In addition, the length from the anterior tibial plateau to the retro-eminence ridge was measured; the relationship of this line with the centers of the AM and PL tunnels was then measured. The AP length of the reconstructed footprint was 17.1 mm ± 1.9 mm and the width 7.3 mm ± 1.2 m. The distance from retro-eminence ridge to center of AM tunnel was 18.8 mm ± 2.8 mm, and the distance from RER to center of PL tunnel was 8.7 mm ± 2.6 mm. The distance between tunnels center was 10.1 mm ± 1.7 mm. There were no significant differences between the intra- and inter-observer measurements. The bone bridge thickness was 2.1 mm ± 0.8 mm. In the sagittal plane, the centers of the tunnel apertures were located at 35.7% ± 6.7% and 53.7% ± 6.8% of the tibia diameter for the AMB and PLB, respectively. The surface areas of the tunnel apertures were 46.3 mm(2) ± 4.4 mm(2) and 36.3 mm(2) ± 4.0 mm(2) for the AM and PL tunnels, respectively. The total surface area occupied by both tunnels was 82.6 mm(2) ± 7.0 mm(2). In the coronal plane, tunnel orientation showed the AM tunnel was more vertical than the PL tunnel with a 10° divergence (14.8° vs. 24.1°). In the sagittal plane, both tunnels were almost parallel (29.9° and 25.4° for the AM and PL tunnels, respectively). When using anatomic aimers, the morphometric parameters of the reconstructed tibial footprint in terms of length and distances to the surrounding bony landmarks were similar to the native ACL tibial footprint. However, the native footprint width was not restored, and the surface area of the two tunnel apertures was in the lower range of the published values for the native footprint area.

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