RESEARCH ARTICLEMorphological features of the anterior talofibular ligament by the number of fiber bundles
Introduction
Injury to the lateral ligaments of the ankle is one of the most frequent sports injuries in both competitive and recreational sports. Of them, roughly 66–85% are injuries to the anterior talofibular ligament (ATFL) alone (Fong et al., 2007, Sitler et al., 1994, Woods et al., 2003). About 20–40% are compound injuries to the ATFL and calcaneofibular ligament (CFL), while injury to the CFL alone or to the posterior talofibular ligament (PTFL) is reported to be very rare (Ferran and Maffulli, 2006). The morphological features of the lateral ligaments of the ankle are thought to be prominently involved in these injuries, and many anatomical studies of the ATFL in particular have been reported.
The main morphological features of the ATFL that have been investigated are the number of fiber bundles, fiber bundle length, fiber bundle width, and fiber bundle angle. Consistent findings have not been obtained for the differences in the number of fiber bundles. There are reports of the existence of types with one, two, and three fiber bundles (Fick, 1904, Milner and Soames, 1997, Ugurlu et al., 2010), as well as reports that types with three bundles do not exist (Burks and Morgan, 1994, Clanton et al., 2014, Delfaut et al., 2003, Neuschwander et al., 2013, Raheem and O’Brien, 2011, Taser et al., 2006, Wenny et al., 2015, Yildiz and Yalcin, 2013). There is also large variation among reports on fiber bundle length, from 13.0 cm to 24.8 cm (Burks and Morgan, 1994, Milner and Soames, 1998, Siegler et al., 1988, Raheem and O’Brien, 2011, Taser et al., 2006), and fiber bundle width, from 7.2 cm to 11.0 cm (Burks and Morgan, 1994, Milner and Soames, 1997, Siegler et al., 1988, Raheem and O’Brien, 2011, Taser et al., 2006, Ugurlu et al., 2010, Yildiz and Yalcin, 2013). The angle of the fiber bundle is reported to be 25.29° (Taser et al., 2006) against the horizontal plane, 47.86° (Taser et al., 2006) against the sagittal plane, and 44.8° (Burks and Morgan, 1994) against the coronal plane, although the number of reported investigations of these angles is small. One major reason for the above differences among reports in the morphological features of the ATFL is thought to be the small number of specimens, from 4 to 46 legs (Burks and Morgan, 1994, Clanton et al., 2014, Delfaut et al., 2003, Milner and Soames, 1997, Neuschwander et al., 2013, Raheem and O’Brien, 2011, Taser et al., 2006, Ugurlu et al., 2010, Wenny et al., 2015, Yildiz and Yalcin, 2013).
The ATFL is reported to serve a major function in ankle plantarflexion and inversion control (Ozeki et al., 2006). It is reported that, during plantarflexion, the superior fiber bundle tenses and the inferior fiber bundle relaxes, and that, during dorsiflexion, the superior fiber bundle relaxes and the inferior fiber bundle tenses (van den Bekerom et al., 2008). In these earlier studies, the ATFL was mainly of the the two-fiber bundle type, and the functions of the superior fiber bundle and of the inferior fiber bundle differed. In other earlier studies, however, the fiber bundle length and total fiber bundle width were the same even when the number of fiber bundles differed, and so it was thought that functional differences were unlikely (van den Bekerom et al., 2008). In all of these reports, however, the effects of the differences in ATFL number of fiber bundles on function remained at the level of speculation. There were no actual investigations. The fact that the inferior fiber bundle of the ATFL and the CFL cross at the fibular side attachment site also suggests the possibility that there is a relationship to the many CFL injuries that occur in conjunction with ATFL injuries, although this has not been fully investigated (van den Bekerom et al., 2008).
Therefore, an investigation of the morphological features of the ATFL and CFL is thought to require a large number of specimens. The aim of this study was to clarify, using a large number of specimens, the differences in morphological features (fiber bundle length, fiber bundle width, fiber bundle angle) according to differences in the number of ATFL fiber bundles, and to investigate the relationship between the ATFL and the CFL.
Section snippets
Cadavers
In this investigation, 81 legs from 43 Japanese cadavers (mean age at death, 77 ± 12 years; 51 sides from men, 30 from women; 41 right sides, 40 left sides) that had been transferred to alcohol after placement in 10% formalin were examined. None showed signs of previous major surgery around the ankle.
Methods
In the ATFL dissection procedure, isolated specimens of the lower leg were prepared by first cutting them off 10 cm above the knee. The skin, subcutaneous tissue, and crural fascia were then removed,
Classification by number of ATFL fiber bundles
The ligament type was Type I in 27 legs (33%), Type II-a in 14 legs (17%), Type II-b in 32 legs (40%), and Type III in 8 legs (10%) (Fig. 1). There were no significant differences between men and women and between left and right feet (Table 1).
Morphological features based on differences in the number of ATFL fiber bundles (Tables 2 and 3)
The fiber bundle length was significantly shorter (p < 0.05) in inferior fiber bundles than in superior fiber bundles in Type II-a and Type II-b. No significant differences were seen in comparisons of mean fiber bundle length between each type.
The fiber
Discussion
This study examined the morphological features (differences in fiber bundle length, fiber bundle width, and fiber bundle angle) according to differences in the number of ATFL fiber bundles and clarified the relationship between the ATFL and CFL. To the best of our knowledge, there have been no previous large-scale anatomical studies of morphological features that focused on the number of ATFL fiber bundles.
The classification based on differences in ATFL fiber bundles was Type I in 27 legs
Conclusions
In this study, the morphological features of the superior fiber bundle and the inferior fiber bundle differed significantly within each type. The morphological features of the inferior fiber bundle differed significantly between Type II-a and Type III. Consequently, it is possible that, in types with different numbers of either two or three fiber bundles, the ankle control function differs within and between types. In the future, biomechanical studies will be needed, with the present results as
Acknowledgements
The authors would like to acknowledge and thank those anonymous individuals who generously donated their bodies so that this study could be performed. This study was supported by a Research Activity Young B Grant from the Japan Society for the Promotion of Science (JSPS).
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