This study elucidated the morphological characteristics of the Lisfranc ligament and the plantar ligament in Japanese cadavers. To the best of our knowledge, there have been no detailed anatomical studies of these ligaments like the present study.
The classification based on differences in the Lisfranc ligament and the plantar ligament was Type I-a in 15 feet (37.5%), Type I-b in 4 feet (10%), Type II-a in 12 feet (30%), Type II-b in 3 feet (7.5%), Type III-a in 3 feet (7.5%), Type III-b in one foot (2.5%), and Type IV in 2 feet (5%). Previous anatomical studies reported that the Lisfranc ligament has a single fiber bundle [9], two fiber bundles (single fiber bundle in 73%, two fiber bundles in 27%) [11], and four fiber bundles (17 cases of one, 45 cases of two, 14 cases of four) [10], that the plantar ligament has varied directionality, and it divides into three directions depending on ligament morphology: linear in 32 cases, Y-shaped in 32 cases, V-shaped in 8 cases, and unclassified in 2 cases[10]. Therefore, no consensus has been obtained, and there are differences from the results of the present study. The reason for the differences was thought to be that it is difficult to distinguish between the Lisfranc ligament and the plantar ligament. The origin of the plantar ligament is defined as the C1 sublateral surface [9] or the C1 lateral surface [14], which is adjacent to the origin of the Lisfranc ligament. Therefore, it is possible that differences in the views of the Lisfranc ligament and the plantar ligament may occur between studies. In the present study, classification was performed depending on whether the origin of the Lisfranc ligament and the plantar ligament clearly differed from each other as a criterion.
Regarding sex differences, Type I-a was significantly more common in females than males. In previous studies, sex differences were not sufficiently investigated. It will be necessary to further investigate the cause for the sex difference in the future.
In the present study, the morphological features of the Lisfranc ligament and the plantar ligament were obvious. Although no statistical analysis was performed, the morphological characteristics of each type were also obvious. In previous studies of the Lisfranc ligament, it was reported that the fiber bundle length was 8.02 ± 1.5 mm [12], 9.17 ± 1.5 mm (6.6-10.95) [13], and 33.7 ± 0.8 mm (2.2–3.1) [14]. The fiber bundle width was 2.53 ± 0.61 mm [12], 5.21 ± 1.28 mm (3.75–7.55) [13], and 12.5 ± 2.8 mm (8.7–18.1) [14]. The bundle thickness was 5.4 ± 1.4 mm (3.1–8.1) [14], 6.9 ± 1.28 mm (5-9.1) [13], 6.96 ± 1.01 mm[12], and 7.68 ± 1.25 mm[15]. For the plantar ligament, the bundle thickness was 3.25 ± 0.97 mm [15]. Therefore, no consensus has been obtained, and there are differences from the results of the present study. The reason for the differences was thought to be that it is difficult to distinguish between the Lisfranc ligament and the plantar ligament. In addition, it was considered that there were differences in measurement methods and in the number of samples. Hirano et al. [10] used a caliper for fixed cadavers (N = 78), Kura et al. [14] used a caliper for fresh-frozen cadavers (N = 12), Johnson et al. [15] used calipers for fresh-frozen cadavers (N = 20), Castro et al. [13] used MRI for an in vivo study (N = 10), and Ablimit et al.[12] used MRI for an in vivo study (N = 60).
In comparisons of morphological features between the Lisfranc ligament and the plantar ligament, the Lisfranc ligament was significantly larger than the plantar ligament in total fiber bundle width, total fiber bundle thickness, and total fiber bundle angle. In previous studies, Kura et al. [14] found the thicker, more plantar ward ligament that they described as the Lisfranc ligament to be stronger than the thin dorsal ligament. De Palma et al. [9] found the interosseous ligament (Lisfranc ligament) to be the thickest compared with the dorsal and plantar ligaments. Therefore, the present study supported the previous study. Regarding total fiber bundle angle, in the biomechanical study using fresh-frozen cadavers, amputation of the Lisfranc ligament and the plantar ligament was necessary to cause instability of the Lisfranc joint (C1-M2 joint and second cuneiform-M2 joint) [11]. Therefore, both ligaments may stabilize the Lisfranc joint.
The limitation of this study is that only the morphological features of the Lisfranc ligament and the plantar ligament were examined using fixed cadavers. What the relationship is between Lisfranc joint injury and ligament type in vivo remains unknown. Therefore, an in vivo study using ultrasound examination is needed in the future.