August 2017

Frontal plane kinematics and risk of ankle sprain

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Research suggests that a simple step-down task has a strong relationship with frontal plane ankle kinematics during walking and jump landing, and may be one method of screening or assessing for increased inversion—and, in turn, increased risk of future ankle sprain—in clinical settings.

By Luke Donovan, PhD, ATC; and Mark A. Feger, PhD, ATC

Lateral ankle sprains (LAS) have been consistently reported over numerous decades as the most common musculoskeletal injury associated with physical activity.1-10 Although individual epidemiological studies3,4,7-10 have estimated various LAS rates across hetero­- geneous populations, a recent meta-analysis has synthesized these results and delineated the incidence rates across sport, sex, and age.2 The primary findings of this meta-analysis were that indoor court sports accounted for the highest incidence rate (7 ankle sprains per 1000 exposures), and that incidence rates are significantly higher in female athletes (13.6 ankle sprains per 1000 exposures) than male athletes (6.94 ankle sprains per 1000 exposures). The final key component of the meta-analysis was that there are also differences between age groups regardless of sport and sex: Children younger than 12 years had 2.85 ankle sprains per 1000 exposures; adolescents aged between 12 and 17 years had 1.94 ankle sprains per 1000 exposures; and adults 18 years and older had .72 ankle sprains per 1000 exposures.

It is well established that anywhere between 20% and 75% of people who sustain a LAS, depending on their sport, will develop chronic ankle instability (CAI).7-11 CAI can be defined by residual symptoms—episodes of giving way or recurrent ankle sprains—lasting longer than one year after an initial sprain. Individuals with CAI have been reported to have decreased proprioception,12-15 neuromuscular control,16,17 dorsiflexion range of motion (ROM),18-20 and ankle strength,21,22 as well as altered gait patterns17,19,23-27 compared with individuals who have no ankle sprain history.

Prevention and rehabilitation requires screening for risk factors prior to lateral ankle sprains and identifying functional insufficiencies following such injuries.

Given the high incidence rates of LAS and CAI, it should not be surprising that there is a large financial burden associated with these injuries. A recent study28 estimated that on average, patients within the US will spend approximately $1000 to treat one acute ankle sprain over the course of one year. When including long-term consequences of LAS, such as development of osteoarthritis,29 repetitive sprains,11 decreased physical activity,30 and time off from employment,31 the annual societal costs in the US have been conservatively estimated by the International Ankle Consortium to be approximately $6.2 billion.31

Because of the high rates and costs associated with LAS, it is imperative that our emphasis as researchers and clinicians be on prevention and rehabilitation strategies to reduce the rate of ankle sprains, as well as on restoring normal function when such injuries do occur to prevent development of CAI. However, to implement effective prevention and rehabilitation strategies, clinicians must be able to screen for risk factors prior to LAS and identify functional insufficiencies following such injuries.

Risk factors and evaluation tools

Figure 1. Commonly reported risk factors of lateral ankle sprains are decreased neuromuscular control during static and dynamic tasks and decreased dorsiflexion range of motion. Valid and reliability clinical tools to screen for these risk factors are the Balance Error Scoring System (A1-A6), Star Excursion Balance Test (B1-B3), and the weight-bearing lunge test (C).

Over the years, numerous studies and systematic reviews11,32-41 have identified risk factors associated with LAS and the development of CAI. The most widely accepted risk factor for sustaining a LAS is a previous history of ankle sprain.37 Despite the abundance of literature on this topic, however, there has been no clear consensus regarding other LAS risk factors. Inconsistencies in reported findings are most likely due to differences between studies with regard to study design (including dependent variables and patient populations), small sample sizes, and relatively short follow-up times.

However, some emerging themes can be surmised from the available literature; evidence supports high body mass index (BMI)32,33,41 and decreases in dynamic stability,33,42 postural control,32,33,42 ankle dorsiflexion ROM,33,42 and ankle strength40,41 as potential risk factors. All of these risk factors can be accurately and reliably measured in a clinical environment. Specifically, the Star Excursion Balance Test (SEBT), Balance Error Scoring System (BESS), weightbearing lunge test (WBLT), and hand-held dynamometers (HHD) can be used to assess an individual’s dynamic balance (SEBT), static postural control (BESS), dorsiflexion ROM (WBLT), and ankle strength (HHD), either during screening or as part of an impairment-based rehabilitation program43,44 following injury (Figure 1).

Even though there is some uncertainty regarding LAS risk factors, several prevention studies have shown that interventions targeting the sensorimotor system using balance training45,46 or bracing47,48 are successful at decreasing LAS incidence rates. However, when comparing the reduction in incidence rates between balance training and ankle bracing studies, it appears ankle bracing may be more effective. Bracing has been associated with significant reductions in ankle sprain rates in individuals with and without sprain history,47,48 whereas balance training appears to reduce risk only in patients with a history of previous sprain.45,46

One theory that could explain the differences in sprain reduction rates between the two interventions is that, in addition to improving neuromuscular control and muscle activation49-54 and unlike balance training,55,56 bracing can also improve stability and alignment of the ankle joint.47,48,54-57 The combination of improved stability, specifically frontal plane stability, and neuromuscular control may provide dual protection against sudden inversion perturbations during sport.

Although there is conflicting evidence as to whether joint laxity and ankle frontal plane kinematics are truly risk factors for LAS, the strong evidence supporting the use of ankle braces suggests that ankle biomechanics influence the occurrence and recurrence of LAS. Therefore, we believe frontal plane ankle kinematics should be a focus when determining prevention and rehabilitation strategies. Unfortunately, at this time, there is not a valid clinical biomechanical ankle assessment tool that can identify excessive frontal plane ankle kinematics.

Ankle kinematics and injury risk

As previously mentioned, there are no prospective cohort studies that link increased ankle inversion during functional activities to LAS rates.34 However, numerous studies have demonstrated that CAI patients have greater inversion during swing phase, initial contact, and loading phases of walking,17,24,27,58 jogging,25,26 stepping down,23 and jump landing59 compared with individuals with no history of LAS. Therefore, it is reasonable to hypothesize that excessive ankle inversion in the presence of other deficiencies may contribute to the high rates of initial and recurrent sprains.

Figure 2. There is a very strong correlation in ankle frontal plane kinematics (AFPK) between the aerial and initial contact phases of walking, step down, and jump landing. Furthermore, there are moderate to very strong correlations for AFPK among all three tasks. Although this has not been further evaluated, a step-down task may be capable of identifying increased AFPK across various tasks since it is the task is most related to walking and jump landing.

Furthermore, it has been shown that during movements that include both an aerial and landing phase, foot position prior to and at initial contact will dictate the trajectory of the loading response.15,60,61 Stated differently, if the foot lands in an inverted position during walking, unless corrected, the individual’s center of mass (COM) will continue to move laterally during the subsequent phase of the task. Under most conditions, the somatosensory system is able to recognize that improper frontal plane kinematic profile and either correct it or stabilize it through various neuromuscular feedback mechanisms, which allows the individual to maintain their COM within their base of support. However, in the event of a sudden perturbation or disruption of the somatosensory system, an individual who lands in excessive inversion may not be able to stop their COM from exceeding the lateral limit of their base of support, which results in the individual losing their balance or sustaining an injury. With this background in mind, it is easy to conceptualize the mechanisms underlying the reported efficacy of neuromuscular rehabilitation and prophylactic bracing for ankle sprain prevention.

Consider the following scenario: A CAI patient has impaired neuromuscular control and also demonstrates excessive inversion prior to and at ground contact when walking. The CAI patient will land with a more laterally projected COM than an uninjured individual and must consistently rely on an already comprised neuromuscular control system to prevent injury. Under ideal circumstances, this would not result in an episode of giving way or an ankle sprain, however, in the event of a sudden perturbation or momentary disruption of the somatosensory system, an injury could occur.

If this same CAI patient performed rehabilitation and improved their neuromuscular control, despite not changing their frontal plane ankle kinematics during gait, they would theoretically be more successful at controlling the COM trajectory in response to perturbations. However, they would still be at a disadvantage relative to an individual with normal neuromuscular function and normal frontal plane ankle kinematics.

We have learned that, to cause change with regard to functional tasks, we must prescribe targeted interventions specific to excessive inversion during a given task.

Now consider the same patient, who this time is using a lace-up ankle brace, which has been shown to both improve neuro­muscular control and reduce frontal plane ankle motion during activity.49-54,57 In this scenario, the brace may help keep the COM within the base of support by mechanically maintaining proper ankle alignment prior to landing, and may enhance the neuromuscular response during a perturbation.

We acknowledge the above scenario is simplified and the mechanisms underlying CAI and LAS are more complex. However, as clinicians, we must be aware of the role of frontal plane ankle kinematics in LAS and CAI, and we should find ways to include ankle kinematics in screening and assessment of our patients.

Ankle kinematics after rehabilitation

Recently, we have developed an impairment-based rehabilitation strategy to improve deficits associated with CAI.44 This rehabilitation strategy relies on the ability of the clinician to assess, treat, and reassess deficits commonly associated with CAI: decreased ROM, strength, balance, and altered kinematics during functional tasks.

When developing this rehabilitation paradigm, we were able to provide clinicians with examples of valid low-cost tools to assess all of these deficits except for altered biomechanics during functional tasks. It is therefore not surprising that when we implemented the rehabilitation paradigm with the assess/treat/reassess framework, we were very effective at improving ROM, strength, and static and dynamic balance in patients with CAI,43 but did not make any meaningful improvements in gait biomechanics.56 These CAI patients reported significant improvements in self-reported function, but some also self-reported that functional deficits were still present, and we attributed this to their persistent alterations in frontal plane ankle kinematics during gait.

In hindsight, our underlying assumption was that if we improved functional parameters in the domains of ROM, strength, and balance, gait neuromechanics would improve as well. We have since realized that to cause change with regard to functional tasks, we must prescribe targeted interventions specific to excessive inversion during the task of interest.

This theory was tested with a small cohort of CAI patients that underwent only one week of gait retraining (without ROM, strength, or balance training), which by itself was associated with improvement in gait parameters and self-reported function.62 The patients within this study completed five gait retraining sessions on a treadmill over one week; each session lasted less than five minutes. During each session, the patients used a gait training device that was developed to work with a treadmill and cause medial resistance to the distal shank throughout the gait cycle.63 Patients were instructed to overcome the medial resistance by maintaining their feet shoulder width apart as they walked on the treadmill. After the intervention patients demonstrated a medial shift in their COP throughout the stance phase of gait and significant improvements in their self-reported function.62

The improvements in self-reported function were not as large as those associated with our comprehensive rehabilitation program, but the findings underscore the important role of walking biomechanics in the treatment of CAI. Now that research has shown gait retraining can improve walking biomechanics,62 it is imperative that we develop screening methods to identify the patients most likely to benefit from these interventions, so gait retraining can be effectively integrated into our assess/treat/reassess paradigm.

A biomechanical assessment tool

We set out to identify a method of accurately and reliably assessing functional biomechanics with regard to increased ankle inversion in clinical settings. We hypothesized that any task requiring a patient to transition from an aerial or flight phase to a loading response phase would require a neural circuit that would be similar regardless of the specific type of task (eg, walking, stepping down, landing). In simplest terms, there must be some processing of somatosensory information regarding foot and ankle alignment during the aerial phase and then some preprogrammed response to either maintain or correct frontal plane alignment prior to landing and during the loading response. In theory, if a movement pattern proved to be conserved across multiple functional tasks, we might be able to develop a single biomechanical assessment tool that would identify patients at risk for ankle sprain due to excessive inversion during functional tasks.

We have since demonstrated that the amount of frontal plane ankle inversion actually is conserved across walking and step-down and jump-landing tasks.64 Furthermore, we found that peak inversion during the step-down task had the greatest shared correlation with peak inversion during the other two tasks (Figure 2).64 We believe this is because the step-down task involves a heel-to-toe transition, similar to that associated with walking, and a vertically directed vector similar to that of jump landing.64

This finding suggests screening during a step-down task may be one method of assessing for increased inversion in clinical settings. Future research is needed to establish how to best integrate these findings into clinical practice, but clinicians should be aware that if excessive inversion is present during one task, it is likely present across various functional tasks.

Conclusion

Great progress has been made in identifying risk factors for LAS, clinical assessment tools for assessing functional limitations in CAI patients, and targeted interventions for these deficits. However, more research is still required. Emerging evidence has highlighted the role excessive inversion likely plays in recurrent ankle sprain risk and in self-reported function of patients with CAI. Clinicians should be aware that excessive inversion is a conserved movement pattern across functional tasks and that a simple step-down task has the strongest relationship to frontal plane ankle kinematics during walking and jump landing.

Luke Donovan, PhD, ATC, is an assistant professor in the Department of Kinesiology at the University of North Carolina at Charlotte. Mark A. Feger, PhD, ATC, is a medical student at the Virginia Commonwealth University School of Medicine in Richmond.

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