Biomechanical analysis of ankle sprain ‘copers’

Researchers are investigating why some people develop chronic ankle instabilty after a sprain while others seem to heal normally. Biomechanical differences between the two cohorts may offer clues to the mechanisms underlying CAI and enhance preventive efforts.

By Adam Rosen, MS, ATC, and Cathleen N. Brown, PhD, ATC

Ankle sprains are among the most common injuries in athletics,¹ with a previous history of lateral sprain being one of the strongest risk factors for repeated injury.² More than 50% of those with a prior history of lateral sprains develop chronic ankle instability (CAI),³ defined as self-reported episodes of instability or “rolling over” with decreased function at the joint.⁴ A number of factors are thought to contribute to CAI, including deficits in kinesthetic awareness and proprioception, weakness in or alterations of musculature, mech­anical laxity of the lateral ligaments, and deficits in motion patterns and neuromuscular control.⁴

Previous research has compared those who developed CAI after injury with individuals who never sustained a lateral ankle sprain. However, because half of those who sprain their ankle develop CAI, it may be important to observe the other half who have a similar history of ankle sprain but do not subsequently develop CAI.⁵ This line of investigation has used as a model in the anterior cruciate ligament (ACL) injury literature, in which individuals who return to activity following ligament injury and do not develop instability are termed “copers.”⁶ In CAI literature, ankle sprain copers have a history of sprain, but do not develop repetitive episodes of giving way and self-report higher levels of ankle joint function than those who do develop instability.⁵ Determining factors that enable some individuals to return to full activity following injury could help develop treatments targeting those at risk for CAI.

Several research groups are studying copers to determine if there are differences in kinematics, kinetics, electromyography (EMG) activity, and functional activity outcomes between the groups. A dose-response relationship in severity or significant group differences in these factors may provide evidence for a continuum of instability that copers have avoided. If differences are noted, characteristics that determine whether patients will become copers could be identified and utilized for rehabilitation or interventions to prevent CAI. Individuals with CAI are at greater risk for osteoarthritis at the ankle, and prevention of instability may help avoid that outcome.⁷

Kinematics

Several retrospective studies utilized coper groups in kinematic analysis and found differences in movement patterns between copers and unstable individuals that could influence injury at the ankle and other joints.^8-11 One study compared copers with two different CAI groups: one with mechanical instability (complaints of giving way coupled with lateral ankle ligamentous laxity or looseness) and one with functional ankle instability (complaints of giving way without ligamentous laxity or looseness). These three groups (copers, those with mechanical CAI, and those with functional CAI) were compared in walk, run, step down, drop jump and stop jump tasks.¹⁰

The authors reported that copers demonstrated movement differences at the ankle compared with the CAI group with mechanical laxity (looseness) in several tasks.¹⁰ Overall, copers demonstrated more “typical” movement patterns, unlike those in the mechanically unstable group, who seemed to adopt a pattern designed to avoid injury. Specifically, copers had more ankle sagittal plane displacement and less frontal plane displacement, were more plantar flexed at landing and at maximum, and were more inverted at landing in several tasks. The authors hypothesized the copers had maintained or returned to movements that the mechanically unstable group avoided due to risk of injury.¹⁰

Another study assessed foot movements during terminal swing, a common point for injury in the gait cycle.⁸ The copers had less foot external rotation, were more internally rotated, and were less everted than the CAI group during walking. During running, the copers had less foot external rotation and higher minimum metatarsal height than the CAI group.⁸ Walking and running with less foot external rotation and greater metatarsal height may decrease copers’ risk of “catching” their toe on the ground during gait, thus avoiding one mechanism for sprain.¹²

Variability of motion has also been proposed as a potential mechanism in the development of CAI. Too much variability may put individuals in risky positions that could elicit a sprain. Too little variability could lead to movement strategies that are ineffective for handling various movement situations and task and environmental constraints, leading to injury.^9,13

A previous study reported that copers had less variability in ankle frontal plane motion (inversion-eversion) than an unstable group during a stop jump.¹¹ In this case decreased variability may be beneficial to help control ankle motion and avoid approaching the edge of stability and a rollover incident.

Another study used similar groups with a single leg jump landing.⁹ The authors reported that uninjured controls were more variable in knee rotation and hip flexion than copers and the CAI group. There were no differences in variability between copers and the CAI group at any lower extremity joint^.9 In this case decreased variability in copers and those with CAI may indicate lack of flexible responses to challenging landing situations. Further research is necessary to determine if increased or decreased variability is important to copers and those with CAI.

Electromyography

Previous studies have reported differences between the surface EMG activity of individuals with CAI and those with normal ankles, but few studies have included copers.^14-17 A recent study found that copers demonstrated significant decreases in reactive peroneus longus EMG while on a supinating platform designed to simulate an inversion ankle sprain mechanism compared with a functionally unstable group.¹⁸ The unstable group appeared to need a potentially compensatory strategy to protect the ankle during perturbed landings, while the coper group did not.

Kinetics

Few studies have measured differences in landing kinetics between copers and a CAI group. In the previously described study that utilized a variety of tasks,¹⁰ no significant differences were observed between coper and CAI groups; the mechanically unstable group, however, demonstrated slightly greater peak medial ground reaction forces than the coper group during the drop-jumps. Although small, those differences may eventually lead to degenerative changes in the ankle joint.

A study of the variability of landing kinetics between the same groups during a stop jump reported no significant differences between the copers and the unstable groups.¹¹ In both studies, there were no differences in landing forces or their variability between coper and CAI groups. Mitigating factors such as kinematics may be limiting differences in the ground reaction force variables measured.

Proprioception and balance

Individuals with CAI have demonstrated deficits in proprioception and balance compared with uninjured controls.¹⁹ It appears that copers also have altered proprioception and balance compared with uninjured control groups, but the effects of injury and the balance outcomes are not necessarily the same as in individuals with CAI. Current research has assessed copers’ proprioception and balance using joint position sense, dynamic postural stability measures, and traditional postural stability measures.

One study assessed active and passive joint position sense in uninjured controls, a CAI group, a coper group within two years of injury, and a coper group within three to five years of injury.²⁰ The authors reported the CAI group had a significantly lower mean active joint position sense error (greater error) than the uninjured controls and both coper groups at one position.²⁰ The authors attributed the similarities in error between uninjured controls and copers and overall low error to a potential coping mechanism that the CAI group did not display. The copers had normal proprioception, while the CAI group did not.²⁰

In a study involving single leg jump landings, copers demon­strated higher dynamic postural stability indices (DPSI) in the anterior-posterior, medial-lateral, and composite directions than uninjured controls.²¹ Using ground reaction force data, the time participants required to stabilize their forces during landing into small oscillations indicated that copers took longer, and were less stable than uninjured controls in those three directions.

The copers also took longer than individuals in a CAI group to stabilize in the medial-lateral direction.²¹ The authors asserted that increased anterior-posterior values in copers were attributable to the ligamentous damage of the initial sprain. Applying nonlinear dynamics theory suggests the copers, who are more flexible than individuals with CAI in their landing strategy, may increase oscillation frequency at the ankle in the frontal plane as a strategy to stay in balance, reflected by increased medial-lateral DPSI scores.²¹

A study on static balance reported that the coper group had greater medial-lateral center of pressure (COP) velocity and shorter time to boundary medial-lateral minima mean values than an uninjured control group, indicating worse balance.²² Fourteen other measures of COP, time to boundary, and COP-center of mass (COP-COM) moment arms were not statistically significantly different between copers and uninjured controls, indicating similar balance abilities.²²

Additionally, the coper group demonstrated lower COP medial-lateral velocity, anterior-posterior velocity, and anterior-posterior peak COP-COM and resultant mean COP-COM moment arm than the CAI group, indicating better balance than the unstable group. There appears to be a dose-response effect in this study, in which controls exhibit the best balance ability, followed by copers, who have some deficiencies, and finally the CAI group, which has the most deficiencies.²² These data provide evidence that copers have adapted, to some extent, and are not as functionally deficient as individuals with CAI, although they do not completely return to “normal” following injury.

Strength

Limited studies have assessed strength in an ankle sprain coper population. In one previously mentioned study that also looked at joint position sense,²⁰ investigators measured isokinetic values for concentric and eccentric inversion-eversion ankle movements, comparing a CAI group to a group of copers two years postinjury and to another group of copers who were three to five years postinjury.

The two-year postinjury coper group had greater eccentric eversion peak torque normalized to body weight than the CAI group at 120°/s. The three-to-five years postinjury coper group had greater eccentric eversion peak torque/body weight than the CAI group at 30°/s.²⁰ The coper groups were not significantly different in any strength measure compared with uninjured controls. The authors suggested that maintaining or regaining strength in the evertors may contribute to coping ability.²⁰

Function and laxity

Several studies have demonstrated that individuals with CAI display greater laxity on instrumented talar tilt and/or anterior drawer tests than uninjured controls.^23,24 In one study, a coper group also demonstrated less anterior displacement and inversion rotation at the ankle compared to a CAI group.²³ There were no differences between the groups in initial treatment, immobilization, rehabilitation exercises or their length, number of sprains, or loss of time related to activity.²³

The authors proposed that increased laxity may be contributing to instability in the CAI group, and noted that, although the difference was not statistically significant, copers were more likely than individuals with CAI to have been immobilized, undergone more rehabilitation, and experienced longer return-to-play times.²³ These differences in treatment and resultant laxity may account for the instability.

Another study measured ankle joint stiffness and reported that both the coper and CAI groups had greater stiffness than uninjured controls.²¹ The authors were unsure why the difference might have occurred, but proposed that altered arthrokinematics, capsular adhesions, talar mobility, or muscle tone could have played a role.²¹

Ankle joint function may be assessed through self-report questionnaires on activities of sport or daily living or through functional tests including hopping or other movements. Some studies have found that copers rate their function significantly higher than individuals with CAI but the same as controls.^9,25 In one of those studies, the copers had significantly better function than a CAI subgroup with mechanical instability for activities of daily living, and significantly better function than CAI subgroups for sporting activities.

However, many functional tests fail to find differences between copers and those with CAI. A side-to-side hop test, a measure of the number of failed trials for the hop test, and comparing the involved to the uninvolved leg did not show any significant differences among individuals with CAI, copers, and uninjured controls.²⁵ In this case the authors noted that while copers rated their ankle function higher than those with CAI, there was no difference in actual ability to complete a task that challenged the ankle joint.²⁵ This suggests that perception of instability, and not actual functional ability, may be a driving factor of CAI.

Conclusion

Ankle instability researchers are beginning to utilize groups of ankle sprain copers in their studies to draw more effective comparisons and highlight subtle differences between those with CAI and uninjured controls. There appear to be some significant differences between copers and CAI groups, specifically with regard to motion patterns, muscle activity, proprioception, and laxity.

By identifying these differences in coper groups, we may be able to identify factors that help individuals avoid instability after injury. An understanding of such factors would allow clinicians to target at-risk individuals with specific rehabilitation interventions to decrease risk of CAI and its associated negative long-term consequences. Research in this area is still relatively new and continues to reveal surprising outcomes.

Adam Rosen, MS, ATC, is currently pursuing a doctorate in kinesiology at the University of Georgia in Athens. Cathleen Brown, PhD, ATC, is codirector of the Biomechanics Laboratory in the University of Georgia’s Department of Kinesiology and director of the Athletic Training Education Program.