Introduction
Lateral ankle sprain is a common musculoskeletal injury in sports [
1]. Although most patients resume daily life after the primary sprain, about 34% of individuals can suffer from chronic ankle instability (CAI), which is characterized as recurrent sprain, episodes of giving-way of the ankle joint, pain, deficits of postural control and muscle strength [
1]. Treatment strategy includes conservative or operative solutions, mainly depending on the severity of ankle sprains, which are classified from grades I to III (mild, moderate, or severe) [
2]. Conservative treatment is more effective for CAI patients with Grade I and Grade II ligament injuries while treatment for chronic ankle instability (CAI) with grade III injuries is controversial [
3]. Surgery is commonly recommended to CAI patients, especially to those with combined intra-articular lesions (osteochondral lesions (OCLs), osteophyte, impingement, loose body, etc.) causing obstructive symptoms [
4‐
6]. However, considering the invasiveness and potential complications of the surgery, conservative treatment could be the first choice for CAI cases with isolated lateral ankle ligament but without intra-articular lesions.
Balance-training programs have been shown to be effective in improving postural control and muscle strength in CAI patients [
7] in the short term. However, some research suggests that the effectiveness of conservative treatment (such as subjective symptoms [
8] and eversion muscle strength [
9]) might be temporary, and about 21.4% of CAI patients still had re-sprains [
8] and the postural stability deficits at 6 months post-intervention [
9]. However, previous studies merely targeted some single treatment (such as resistance tubing [
8] and wobble board [
9]), and the follow-up period was only 6 months. In addition, those studies included CAI cases with varying degree of ligament injury but did not distinguish the CAI cases with grade III ligament injury. At present, there is limited evidence for long-term effectiveness and continuity of systematic rehabilitation training for CAI patients with grade III ligament injury.
In terms of the post-training functional evaluation, most of the studies focused on the subjective feeling instead of objective evaluation, thus, make it difficult to guide clinical practice. In fact, the foot pressure measures, such as center-of pressure (COP) excursion, time-to-boundary (TTB) and peak plantar analysis have been commonly used to identify the postural control deficits of those with CAI. CAI cases have significantly less time to make postural corrections to meet the stability demands [
10] and a significantly increased lateral loading [
11] compared with healthy controls. These objective parameters might be important predictors for the effectiveness and sustainability and help treatment decision making.
In the present study, 20 CAI patients with isolated lateral ankle ligament injury were included and accepted 3 months of supervised balance training and consecutive follow-up for 1 year. The purpose of the present study was to determine the mid-term effectiveness and the sustainability of a balance training program and preliminarily explore the risk factors of sprain recurrence in the CAI cases. We hypothesized that balance training would improve the muscle strength, foot pressure distribution and postural stability, but some improvements would be weakened with time. These results may help us optimize rehabilitation strategies to improve the effectiveness of balance training for the CAI cases with grade III ankle ligament injury.
Discussion
The most important finding of the present study was that the 3 months of supervised balance training significantly improved self-reported function, postural control, and muscle strength for CAI patients with isolated ligament injury and most CAI patients with grade III ligament injury (80%) had satisfactory outcomes. However, some improvements on postural control and muscle strength declined after 6 months and 1 year. Additional strength exercises for dorsiflexion and eversion should be supplemented from 6 months. The generalized joint hypermobility (gJHM) and the initial inversion strength weakness could be the potential risk factors for sprain recurrence after balance training.
According to our results, most CAI patients with grade III ligament injury had satisfactory outcomes after the balance training. Similar improvements in self-reported function questionnaire [
24,
25] and muscle strength [
26,
27] were also found, but none of them investigated the long-term effectiveness of balance training and distinguish the severity of the ligament injury. The results of the present study implicated that the balance training is still effective even for patients with severe ligament injuries in the long term (i.e., up to 1 year). Although the ligaments are completely disrupted in the acute phase and the joints are obviously unstable in the chronic phase, the feeling of instability is partly improved by increasing the muscle strength and the posture control. However, we found that the strength improvement in dorsiflexion and eversion disappeared at the 6 months follow-up and a similar situation occurs with 60° eversion muscle strength at 1 year. It could be inferred that balance training will provide a short-term muscle strength improvement and additional strength exercises for dorsiflexion and eversion from 6 months and inversion exercise from 1 year might enhance and maintain the effect of balance training again. Further studies are needed to improve the balance training program for CAI cases.
Regarding the plantar pressure evaluation, we found a significant change in the TTB related measures during single leg standing. TTB related measurements involved a spatiotemporal analysis of COP data points, which was a novel approach to assessing postural control deficit in single-limb stance [
10,
28]. It quantified the theoretical amount of time an individual had to use to make a postural correction to maintain postural stability. The results indicated that the grade III CAI patients needed longer time to reach the balance boundaries and had a lower risk to fall after the balance training. During walking, we found that the TPF of HM was more delayed and M2 came earlier after the balance training, which means a shorter duration of contact of the heel to central forefoot. The longer duration indicated a slowing down of weight transfer from heel strike to toe off, which was a sign of walking stability improvement [
20]. The results also showed that greater loading under the M2 and HM which indicates a medial shift of the COP so as to decrease the susceptibility of ankle sprain [
11]. However, improvements on stability during single leg standing and pressure distribution during walking was declined gradually over time.
The present study showed that the sprain recurrence patients had significantly higher Beighton scores, which indicated that the patients with gJHM might have poor rehabilitation effect for balance training. Previous studies [
29,
30] have reported generalized joint hypermobility was an intrinsic risk factors related to recurrent lateral ankle sprain, so the hypermobility of ankle joint structure might increase the risk of ankle injury during the rehabilitation process. Similar poor outcomes have been reported for surgery treatment, such as the modified Broström procedure [
16,
31]. In those cases, the repaired ligaments would eventually stretch and the patients need a longer period for lateral ligament recovery. However, the impact of gJHM on conservative treatment effect was rarely reported. In theory, the increased muscle strength brought by functional training can enhance joint stability. However, the joint capsule of these patients with gJHM may be compromised, so that the improvement of muscle strength is not enough to restore sufficient stability. This may be the reason for the relatively poor results of these patients. Considering the limited number of cases, the effect and the mechanism of joint hyper-relaxation on conservative training requires further controlled studies.
In this study, we also found the relatively weaker 60°/s inversion strength could be another risk factor of the failure. Several studies have shown that subjects with CAI exhibited strength deficits in their invertor musculature [
32‐
34]. It is suggested that the ankle joint invertors play an integral role in controlling the rate of calcaneal eversion as the body’s center of mass is displaced laterally beyond the base of support [
35]. The inversion strength weakness lead in uncontrolled weight transfer to the lateral side of the foot during single leg standing, thus increased frequency of ankle inversion episodes. For patients with inversion strength weakness, more specific and targeted rehabilitation needs to be done.
To our knowledge, this is the first comprehensive study to investigate the long-term effectiveness and its related factors of sprain occurrence for CAI patients with isolated grade III ligament injury. The strength of this study included a relatively long-term follow-up, a detailed systematic balance training protocol and a comprehensive evaluation of the patients and the objective assessment on the postural control improvement in terms of plantar pressure and TTB. It was important to note that although some improvements seem not to last over time, the rehabilitation for the severe type of CAI patients were still effective in the long term. Our results provided more evidence for the use of balance training in CAI cases and would help to optimize rehabilitation plans for this condition. Those with high-risk factors might be prompted to receive early and comprehensive treatment, including the consideration of surgery.
There were still some limitations of the present study. Firstly, the sample size was relatively small in the sprain recurrence group although overall participants enrolled in this study was calculated by the sample size, larger sample are needed to be included in the future research. Secondly, this study focused on patients with lateral ligament injury only while other ligament and syndesmosis injuries also impeded rehabilitation outcomes. So, our results were only suitable for the isolated lateral ankle ligament injury, the balance training for more complex injury type are needed in the future. Thirdly, the patients selected a self-chosen, comfortable walking speed to accomplish the walking task, which might affect the plantar pressure results.
Conclusion
For the grade III CAI patients with isolated lateral ankle ligament injury, the 3 months’ supervised balance training program significantly improved FAAM scores, foot pressure distribution, static postural control, and muscle strength. However, the effectiveness may be partial declined after 6 months and 1 year. Additional strength exercises for dorsiflexion and eversion should be supplemented from 6 months. The high Beighton score and the initial inversion muscle strength weakness might be the potential risk factors of sprain recurrence after balance training.
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