According to the results of the present study, there was significantly weaker muscle strength on the affected side in individuals with MAI. Greater muscle strength deficits were shown in plantar flexion than in dorsiflexion at a velocity of 60°/s, which was not influenced by the testing velocity. The female and isolated ATFL injuries were significantly related to a lower LSI in eversion and a greater eversion strength deficit.
The existence of an ankle muscle strength deficit on the affected side was similar to the results of other studies reporting deficits in dorsiflexion[19], plantar flexion[20], inversion[7] and eversion[7]. A prospective study[19] showed that a dorsiflexion muscle strength deficit was an intrinsic factor for individuals with inversion ankle sprains, and the individuals with ankle instability showed a weaker dorsiflexion muscle strength than the healthy individuals. Phillip A Gribble[20] also found that individuals with CAI exhibited significantly weaker plantar flexion strength in their injured limb than in their non-injured limb. However, for those studies, individuals with severe MAI with grade III ligament tears have not been isolated from among all CAI patients, which may limit the application of conclusions in the clinical practice. Our results indicated that MAI patients present a similar muscle deficit to CAI patients with weaker strength in all directions on the injured side.
General CAI strength training emphasizes dorsiflexion muscle strength to compensate for lateral ankle instability caused by the initial ligament rupture[21, 22]. On the other hand, the results of the present study showed that the LSI at 60°/s in plantar flexion was significantly lower than that in dorsiflexion (0.87 vs 0.98, p<0.001), indicating that the muscle strength deficit in plantar flexion was more serious than that in dorsiflexion for MAI patients. Gribble also found that patients with CAI showed a significant muscle strength deficit in plantar flexion but not in dorsiflexion[20]. The differences might result from the test indicators; Gribble researched the peak torque, while we researched the LSI. In addition, it is noteworthy that the testing velocity did not affect the muscle strength deficit of the mechanical instability individuals. A meta-analysis found that there were no differences between the <110°/s group and the >110°/s group in terms of concentric eversion strength[23]. Therefore, the muscle deficit tends to be more serious in plantar flexion and not related to velocity.
In terms of the predictors, a significantly lower LSI at 120°/s in eversion was evident for females than for males (0.82 vs 0.94, p=0.016), which indicated a greater ankle eversion strength deficit for the females. Hosea et al[24] also found that compared with male athletes, female athletes were at 25% increased risk of suffering a grade I ankle sprain. Compared with men, women showed a significantly increased rate ratio for ankle sprain of 1.83 (95% confidence interval, 1.52-2.20) [8]. Although not directly noted, the tendency of sprain recurrence in women could be related to the relatively weaker muscle strength. More serious eversion strength deficits may be the cause of higher sprain risk in the female athletes. Future research could focus on whether sex differences are activity-specific and thus related to training behaviours or whether the difference in risk is related to anatomical or physiological sex differences.
Interestingly, the patients with isolated ATFL injuries showed significantly lower LSI at 60°/s in eversion (0.86 vs 0.95, p=0.012) than the patients with combined CFL injuries, which might differ from what is generally assumed. A previous study showed that the CFL accounted for 50%–70% of complex ankle joint stability during inversion, especially in dorsoflexion[25]. As an important structure for maintaining ankle varus and subtalar joint stability, the injury of calcaneal and fibula ligaments will significantly increase the joint relaxation. Therefore, it seems that MAI individuals combined with a CFL injury have weaker eversion strength and more ankle laxity. The interesting results of the present study might be attributed to the additional CFL injury contributing to the compensated increase in eversion strength to account for the instability of inversion activities. The mechanism of this interesting finding of the present study needs further biomechanical or kinematic studies.
To our knowledge, this is the first study to evaluate the characteristics of individuals with MAI and to explore the potential predictors to the LSI. The most prominent strength of the present study is the relatively large sample size with complete and accurate information for all the demographics and clinical features. The ligament injury pattern and concomitant lesions were obtained from medical records and confirmed by intraoperative evaluation. The integrity of those factors could help us to better analyse the correlations with muscle strength deficits for MAI patients with initial severe ligament injuries.
There were still some limitations of the present study. Firstly, all the patients included were ready to undergo surgery and thus had complicated and uncontrollable treatment backgrounds prior to enrolment, limiting the applicability of the conclusion of the present study. Secondly, although this study has covered the patient's clinical features and injury data, there are still some factors (the number of sprains of the patient, daily activity, the previous rehabilitation, etc.) have not been analysed. Thirdly, the ligament injury pattern only incorporated ATFL and CFL injuries, the effect of other stabilizing structure (deltoid ligament, syndesmosis, etc.) need further study.