Introduction
Lateral ankle sprains are a frequently occurring musculoskeletal injury in sports [
1]. Approximately 34% of individuals suffer from chronic ankle instability (CAI), which is characterized by a recurrent sprain, episodes of giving-way of the ankle joint, pain, swelling and decreased function [
2]. Functional treatment, such as muscle strength training and balance training, is a common strategy for treating CAI with good results, especially for those with grade I and II ligament injuries [
3]. Unfortunately, the effectiveness becomes significantly reduced for those with an initial severe ligament injury that develops into mechanical ankle instability (MAI). Approximately 20–40% of those with chronic MAI experience failed to the rehabilitation interventions and is recommended for surgical treatment [
4]. The poor effect of rehabilitation may be related to severe deformity of the joint after the ligament is completely ruptured, which may result in excessive muscle strength loss. The conventional muscle strength training programme that applies the same training intensity in each direction of each patient’s ankle joint might not be targeted for those with severe ligament injury and relatively more instability, thereby reducing the training effect. An optimized and more targeted strength training strategy for MAI patients is expected to improve the prognosis of rehabilitation and to reduce the probability of surgery. Compared with mild ankle sprains, these MAI patients may have special muscle strength characteristics and related factors. However, the characteristics and the predictors of muscle strength deficits in MAI patients have rarely been studied.
Most of the reported studies only investigated the overall characteristics of muscle strength in CAI patients without distinguishing functional or mechanical instability, which might be different due to the severity of ligament injury. Even in those studies, the characteristics of the muscle strength deficits in CAI were contradictory. Some have suggested that subjects with CAI showed eversion strength deficits [
5], while more recent research using isokinetic dynamometry revealed strength deficits in the invertor musculature [
6,
7]. The contradictory results might be due to the relatively small sample size, and the characteristics of the strength deficits in MAI patients are still unknown.
The factors related to ankle sprain recurrence in MAI patients include sex [
8], body mass index (BMI) [
9,
10], a history of previous ankle injuries [
11] and muscle strength deficits. In addition, a muscle strength deficit could be also closely related to the ligament injury pattern and concomitant injuries, which have rarely been studied. In terms of the ligament injury pattern, an isolated lesion of the anterior talofibular ligament (ATFL) occurs in approximately 65% of cases, while combined ruptures of the ATFL and calcaneofibular ligament (CFL) occur in approximately 20% [
12]. The isolated ATFL injury or the combined both ligament injuries may affect the degree of lateral instability, resulting in different characteristics of muscle strength. The osteochondral lesions (OCLs) and osteophytes are common concomitant injuries in severe MAI cases, which might also affect walking posture and muscle strength due to the increased joint pain.
The aim of the retrospective study was to explore the characteristics and the predictors of the muscle strength deficits in MAI patients, which could contribute to the development of an optimized muscle training strategy to improve the prognosis of rehabilitation. In the present study, the chronic MAI patients with an initial lateral ankle ligament rupture and postinjury duration of more than 6 months were included. All patients underwent Biodex isokinetic examination of the ankle joints. Then, the associations between the muscle strength deficit and the patients’ demographics, clinical features, ligament injury pattern and concomitant lesions were analysed.
Discussion
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 dorsiflexion [
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.
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