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Lateral ankle sprains (LASs) are among the most common injuries incurred during participation in sport and physical activity, and it is estimated that up to 40% of individuals who experience a first-time LAS will develop chronic ankle instability (CAI). Chronic ankle instability is characterized by a patient's being more than 12 months removed from the initial LAS and exhibiting a propensity for recurrent ankle sprains, frequent episodes or perceptions of the ankle giving way, and persistent symptoms such as pain, swelling, limited motion, weakness, and diminished self-reported function. We present an updated model of CAI that aims to synthesize the current understanding of its causes and serves as a framework for the clinical assessment and rehabilitation of patients with LASs or CAI. Our goal was to describe how primary injury to the lateral ankle ligaments from an acute LAS may lead to a collection of interrelated pathomechanical, sensory-perceptual, and motor-behavioral impairments that influence a patient's clinical outcome. With an underpinning of the biopsychosocial model, the concepts of self-organization and perception-action cycles derived from dynamic systems theory and a patient-specific neurosignature, stemming from the Melzack neuromatrix of pain theory, are used to describe these interrelationships.

The earliest investigators of CAI described 2 subgroups: those with patient-reported ankle symptoms and those with abnormal physical findings.36 Freeman37 subsequently termed these ankle symptoms functional instability. He defined functional instability as the tendency for the foot to give way. Thus, 2 main subgroups of CAI became widely accepted: those with mechanical instability and those with functional ankle instability.

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Although the definition of mechanical instability is universally accepted as pathologic ligamentous laxity about the ankle-joint complex,4,12,18,19 no universally approved definition of functional ankle instability exists.38 Evans et al31 described functional instability as a subjective complaint of weakness; this description was expanded by Lentell et al16 to include pain and the perception that the ankle felt less functional than the other ankle and less functional than before the injury. Tropp et al39 distinguished functional instability from mechanical instability by defining it as joint motion that did not necessarily exceed normal physiologic limits but that was beyond voluntary control. Other proposed characteristics of functional ankle instability have included perceived or actual giving way of the ankle (or both)10,16,40 and other characteristics previously associated with CAI in general: pain and swelling30 and recurrent sprain.41,42 That is, various definitions of functional ankle instability have been used to determine eligibility criteria when recruiting participants to studies. Such differences in inclusion criteria may explain the inconsistent findings from CAI research to date.

Hertel4 proposed a model involving mechanical and functional instability that is widely accepted. In this model, mechanical and functional instability are not mutually exclusive but part of a continuum, and recurrent sprain occurs when both conditions are present. Mechanical instability is thought to result from various anatomic changes that may exist in isolation or in combination. These changes are proposed to lead to insufficiencies that predispose the person to further episodes of instability. Functional instability is proposed to result from functional insufficiencies such as impaired proprioceptive and neuromuscular control.4 When mechanical and functional insufficiencies are present, recurrent sprain results. However, anecdotally, participants have reported residual feelings of instability and ankle laxity after ankle sprain but have not reinjured their ankles. This has led to a potential evolution of the Hertel model, separating recurrent sprain from the presence of both instabilities. Thus, our first objective was to propose a refinement of the CAI model proposed by Hertel. The new model expands the number of subgroups from 3 to at least 7 and examines the fit of available ankle data to both models.

We hypothesized, based on preliminary evidence, that impairments may vary among the subgroups of the proposed model.34,39,40,45 For example, previous researchers34,39,48 demonstrated that postural stability was impaired in participants with functional instability, whether or not mechanical instability was present. In contrast, peroneal reaction times after an inversion perturbation were longer in participants with functional instability without mechanical instability than in those with mechanical instability alone.45 Thus, depending on the impairment, participants with mechanical and functional instability may perform differently than those with only functional instability. Further exploration of all subgroups in the proposed model may reveal unique sets of impairments characterizing that subgroup.

To analyze relationships between test performance and group, data from only 1 ankle of each participant were included. Therefore, for participants with bilateral instability, we analyzed data from the more severely affected ankle, that is, the ankle that could be classified in several subgroups. In cases of bilateral instability with the same classification for both ankles, the ankle with the lower CAIT score was used.

Of the original 175 participants, 52 were excluded, either because they had fully recovered from an ankle sprain (n = 15) or because they were healthy uninjured participants who did not meet the inclusion criteria (n = 37). This resulted in 81 participants with CAI and 42 external controls. Of the participants with CAI, 45 had unilateral CAI and 36 had bilateral CAI. The test ankle for 15 of the participants with bilateral instability was selected on the basis of CAIT score because of similar classification for both ankles.

Exploring possible associations of different subgroups with different impairments indicates some potential trends. For example, all groups with perceived instability had greater impairment on the foot-lifts test of balance than the hypomobility group. The presence of recurrent sprain appeared to make it more likely that a person could not balance on demipointe when compared with subgroups without recurrent sprain. Finally, perceived instability alone or in combination appeared to be associated with a longer time to recover from an inversion perturbation than was demonstrated by either external controls or participants with ankle hypomobility.

Perceived instability appears to lead to the same degree of impairment as recurrent sprain in some tests, whereas the presence of hypomobility may modulate some effects. Chronic ankle instability has often been defined as the presence of recurrent sprain,4 with or without perceived instability.54,58,59 However, 52% of participants in the current study had perceived instability without recurrent sprain. It may be that the feeling of instability is the most prominent factor and is associated with increased severity of some functional impairments, activity limitations, and participation restrictions.

This further exploration indicated that single-leg balance was impaired in participants with perceived ankle instability but not in those with mechanical instability or in external controls.34,39,40 Tropp et al39 found increased postural sway, measured using stabilometry, in participants with both perceived ankle instability and recurrent sprain compared with external controls. Both Konradsen and Ravn48 and Ryan34 found differences in postural sway between external controls and participants with either perceived ankle instability alone or perceived ankle instability in combination with mechanical instability. These studies and the current study provide preliminary evidence that perceived instability, when present in any combination, is associated with the ability to balance on a flat foot.

Recovery from an inversion perturbation has also been studied by measuring peroneal reaction times rather than the protocol we used. Peroneal latency times were similar for external controls and participants with either mechanical instability alone61 or mechanical instability combined with recurrent sprain.45 In contrast, latency times were longer for participants with perceived ankle instability combined with either recurrent sprain or mechanical instability than for external controls.40,45,48 Again, the perceived instability appears to mediate the impairment.

In contrast, in an investigation58 of ankle motion during functional tasks, patterns of motion were altered in participants with mechanical instability combined with perceived instability but not in either those with perceived instability alone or those who had recovered. In this study, mechanical instability appeared to be a mediating factor. Repeated episodes of giving way or recurrent sprain were inclusion criteria, so it would have been illuminating to divide both the mechanical and functional instability groups into those with and without recurrent sprain.

Taken together, these findings provide preliminary evidence for different associations or levels of impairment with different subgroups. However, although the new model provides preliminary evidence for the proposed subgroups within CAI, it does not resolve all issues. A single test of mechanical instability may not comprehensively detect pathologic laxity of all ankle ligaments. For example, Hubbard et al62 found no correlation among various tests of mechanical instability in participants with CAI. The tests investigated were fibular position using fluoroscopy, instrumented ankle laxity in 3 directions, and talar hypomobility. Yet in a related study, anterior and inversion laxity were the factors most predictive of CAI group membership,5 and, therefore, we suggest that at least these 2 directions be considered when assessing mechanical instability. 17dc91bb1f