The present study investigated the changes in length of the ATFL and CFL in-vivo during the positions of plantarflexion, dorsiflexion, supination and pronation using a combined MR and dual-orthogonal fluoroscopy. The results showed that the ATFL is significantly more elongated at plantarflexion than at dorsiflexion. Further, the ATFL is also significantly more elongated at supination than at pronation. Conversely, the CFL is more elongated with dorsiflexion than with plantarflexion and was also found to be more elongated at pronation than at supination. Therefore, these data also demonstrated the reciprocal function between the two ligaments. While one shortens, the other one elongates and vice versa.
Several cadaveric studies investigated the length of lateral ankle ligaments. Milner et al. measured length of osteoligamentous preparations of collateral ligaments in 40 ankle specimens and reported the length of the ATFL and CFL to be 13.0 ± 3.9 mm and 19.5 ± 3.9 mm respectively. Siegler et al. reported the average lengths of ATFL and CFL measured in osteoligamentous preparations of 20 cadaveric ankles. In their study, the average ATFL length was 17.8 ± 3.1 mm and the average CFL length was 27.7 ± 3.3 mm. Burks and Morgan also measured the length of lateral ankle ligaments in their sample of 39 cadaveric ankles. They reported the average length of the ATFL and CFL as 24.8 mm and 35.8 mm respectively. The in-vivo length of the ATFL and CFL measured in this study in a neutral position compares favorably with the results of Milner et al. and Siegler et al. who measured the distance between the ligament attachment sites. In this study the average length of the ATFL was 15.8 ± 2.9 mm and the average length of the CFL was 27.7 ± 2.7 mm in the neutral position. The study of Burks and Morgan measured the length of the longest fibers, while our study used centroids of insertion areas, which might explain why their values were larger. Burks and Morgan also showed that the ATFL and the CFL have adjacent attachment sites on the anterior edge of the fibula 8–10 mm from the distal tip and that with the foot in neutral position, the CFL forms an angle of about 130 degrees with the fibula whereas the ATFL was slightly less than 90 degrees anteriorly. It was therefore theorized that in dorsiflexion, the CFL assumes a course parallel to the axis of the fibula thereby functioning as a collateral ligament. In plantarflexion, the orientation of ATFL fibers assumes this position and may be expected to function as the main collateral ligament.[5, 16] Since the majority of sprains occur during plantarflexion, [4, 17] the ATFL would therefore be the first ligament to suffer disruption in this type of injury, as shown in several reports.[4, 18, 19] This notion was supported by numerous in-vitro studies which investigated the changes in ATFL and CFL strain in response to loading in various positions. Colville et al. measured strain in lateral ankle ligaments in 10 cadaveric ankles while applying inversion/eversion and internal/external rotational forces during the flexion-extension arc of motion. They demonstrated increased strain in the ATFL with increasing plantarflexion, inversion and internal rotation. Strain in the CFL was found to increase in dorsiflexion and inversion. Ozeki et al. investigated strain changes in central fibers of the lateral ankle ligaments in 12 cadaveric specimens during plantarflexion-dorsiflexion. Their results showed that ATFL was taut in plantarflexion and the CFL in dorsiflexion. The length change of each ligament during the arc of motion was less than 1.6 mm. Renström et al., Rasmussen[21, 22] and Bahr et al. also found that the ATFL acts as a primary restraint in inversion and plantarflexion, whereas the CFL tension was increased mainly in inversion and dorsiflexion. The lowest maximum load-to-failure of ATFL among the lateral ankle ligaments also explains why ATFL is often the first ligament to rupture during an inversion injury to the ankle. These cadaveric studies have provided fundamental knowledge for the understanding of function of these ligaments and injury mechanisms. However, the in-vivo behavior of the lateral ankle ligaments during the range of motion has not been well studied.
Data in this study demonstrate that the ATFL lengthens more in plantarflexion and supination (combination of plantarflexion, inversion and internal rotation) whereas the CFL elongates more in dorsiflexion and pronation (combination of dorsiflexion, eversion and external rotation). These data compare favorably with the results of previous cadaveric studies and suggest that under excessive loading conditions the ATFL might be more vulnerable in plantarflexion and supination while the CFL might be more susceptible to injury in dorsiflexion and pronation.
Certain limitations of this study should be noted. The ankles were studied during non-weightbearing motion and the subjects were asked to actively position their ankles into the extreme positions of plantarflexion, dorsiflexion, supination, and pronation. These ranges of motion are most likely smaller than what would be measured if the foot was passively moved and held in position. Additionally, the volunteers may not have applied the supination pronation motion in the same manner. However, we tried to minimize the potential inconsistencies by instructing the subjects about the target positions and visually verifying them during the scanning. It should be noted that this study did not investigate motion of the ligaments, but estimated the changes in length based on the distances between the centroids of the digitized attachment sites. The methodology has not been validated to measure in-vivo motion of the ligaments. Further, this study only included male subjects since it has been shown that male and female ankles and feet differ in several anthropometric characteristics.[24, 25] Therefore, in the future, ligament function should be investigated in female subjects. Finally, only four positions were studied (full plantarflexion, dorsiflexion, supination and pronation).
In conclusion, the results of this study contribute to the small pool of data on in-vivo behavior of the lateral ankle ligaments. We noted that the ATFL seems to elongate more during plantarflexion and supination whereas the CFL increases in length with dorsiflexion and pronation. Concurrently, these data also demonstrated the reciprocal function of the two ligaments. While one shortens, the other one elongates. The different elongation of the ATFL and CFL during the same motion arc suggests that under excessive loading conditions the ATFL might be more vulnerable in plantarflexion and supination while the CFL might be more susceptible to injury in dorsiflexion and pronation. Furthermore, in the case of surgical reconstruction the grafts used to reconstruct the two ligaments may need to be tensioned at different positions of the ankle in order to reproduce their natural in vivo function. In the future it will be possible to apply this technique to study the ligament function after various types of injury and to evaluate the effectiveness of operative or conservative treatment in restoring normal ligament behavior in vivo. Furthermore, dynamic motion of the ankle should be studied.