The Bow Tie Shape of the Anterior Cruciate Ligament as Visualized by High-Resolution Magnetic Resonance Imaging

Abstract

Background: Although numerous studies have examined the anatomic characteristics of the anterior cruciate ligament (ACL), its actual shape remains unclear. Purpose: To determine the average shape of the ACL by analyzing its cross section through the use of high-resolution magnetic resonance imaging (MRI) data. Study Design: Descriptive laboratory study. Methods: The study included 96 MRIs, conducted using a 3.0-T magnet, to analyze the shape of the ACL. Three-dimensional, curved multiplanar reconstruction was used to obtain cross sections at 7 points (femoral insertion; midsubstance 1, 2, 3, 4, and 5 from the femoral side to the tibial side; and tibial insertion). The width and thickness of cross sections were measured by 2 independent observers, and the ratio of width to thickness was calculated to determine the proportions of each cross section. The 7 cross sections were accumulated and standardized to generate an average model through the use of image analysis software developed by the authors. Results: The mean ± SD width (femoral insertion, 17.02 ± 2.17 mm; tibial insertion, 17.33 ± 2.03 mm) and thickness (femoral insertion, 11.03 ± 1.75 mm; tibial insertion, 10.09 ± 1.70 mm) of both insertions were significantly larger than those of midsubstance 4 (width, 9.99 ± 1.87 mm; thickness, 6.53 ± 1.25 mm) ( P < .001). The mean ratios of width to thickness of the 7 cross sections from femoral insertion to tibial insertion were 1.57 ± 0.23, 3.36 ± 0.57, 3.07 ± 0.81, 2.18 ± 0.54, 1.56 ± 0.32, 2.16 ± 0.48, and 1.75 ± 0.28, respectively. The shape of the cross section at midsubstance 4 was an oval isthmus, which was the most narrow and well-balanced shape. It was transformed into a wide band at midsubstance 1 and 5. The shape of the femoral insertion was semicircular, with its anterior side slightly straight and its posterior side convex. The tibial insertion was kidney bean–shaped. Conclusion: On 3.0-T MRI, the ACL has a “bow tie” shape, including an oval isthmus, with a semicircular femoral insertion and kidney bean–shaped tibial insertion. Clinical Relevance: The measurement method will allow surgeons to quantitatively diagnose partial injuries of the ACL using a noninvasive system in actual patients.