The development of chronic ankle instability (CAI) is the primary residual deficit after ankle joint sprain. It has been proposed that CAI is characterized by two entities, namely, mechanical instability and functional instability. Each of these entities in turn is composed of various insufficiencies. Research of functional insufficiencies to date has shown large variances in results. One particular reason for this could be discrepancies in inclusion criteria and definitions between CAI, mechanical instability, and functional instability used in the literature. Thus, we endeavored to undertake a systematic investigation of those studies published in the area of CAI to identify if there is a large discrepancy in inclusion criteria across studies. Methods: A systematic search of the following databases was undertaken to identify relevant studies: Cochrane Central Register of Controlled Trials, PubMed, CINAHL, SportDiscus, PEDro, and AMED. Results: The results of this study indicate that there is a lack of consensus across studies regarding what actually constitutes ankle instability. Furthermore, it is evident that the majority of studies use very different inclusion criteria, which leads to a nonhomogenous population and to difficulties when comparing results across studies. Conclusions: Future studies should endeavor to be specific with regard to the exact inclusion criteria being used. Particular emphasis should be given to issues such as the number of previous ankle sprains reported by each subject and how often and during which activities episodes of "giving way" occur as well as the presence of concomitant symptoms such as pain and weakness. We recommend that authors use one of the validated tools for discriminating the severity of CAI. Furthermore, we have provided a list of operational definitions and key criteria to be specified when reporting on studies with CAI subjects. Recently published data from a large-scale systematic review have shown that the ankle joint is the second most commonly injured body site in sports, with ankle sprains being the most frequently encountered ankle joint injury (33). The incidence of ankle sprains is particularly high in sports such as rugby, soccer, volleyball, handball, and basketball (33). Of particular concern to the sports medicine, physiotherapeutic, and athletic training communities is the high reported incidence of residual symptoms after an initial ankle sprain incident. These residual symptoms, which include recurrent sprain, episodes of ankle joint "giving way," pain, swelling, and decreased function, have been termed chronic ankle instability (CAI). The reported incidence of CAI after initial ankle sprain varies, with some studies indicating that sporting populations involved in high-risk sports such as basketball can have incidence rates up to 80% (126). A more recent study by van Rijn et al. (121) reported that within a period of 3 yr, as much as 34% of the patients will report at least one resprain with many more reporting other continual residual symptoms. It has been suggested that joint motion beyond the normal physiological range of motion or mechanical instability (MI) cannot independently explain the ongoing residual difficulties experienced by a high percentage of patients after initial ankle sprain (18). Consequently, it has been proposed that some pathological process distinct from MI is present. This phenomenon of recurrent persistent symptoms in the absence of aberrant mechanical laxity has been termed functional instability (FI) of the ankle joint or functional ankle instability (FAI). Recently, Hertel (48) proposed a paradigm of CAI, whereby he outlines those mechanical and functional insufficiencies that are thought to contribute to the development of CAI. The functional insufficiencies include impaired proprioception, neuromuscular control, postural control, and strength deficits. In terms of functional insufficiencies, research to date has concentrated on two main areas. Those areas have been investigatory studies aimed at quantifying those specific functional insufficiencies that are thought to contribute to the development of CAI and interventional studies investigating treatment effects. Unfortunately, there has been a large discrepancy in results from studies investigating similar functional insufficiencies (18). One potential explanation lies in the fact that the criteria used to define FI vary greatly across studies; hence, different studies may have included cohorts with different characteristics. This point is highlighted by Konradsen (65), who suggested that "The disability designated FAI has no universally agreed upon definition…." Furthermore, Konradsen (65) indicated that "There are no requirements as to how often distortions need to be sustained, how long the disability has had to be present, at what level of activity injuries are sustained or at what degree of external action …." With these points in mind, we undertook to review the inclusion criteria used by studies published in the area of CAI investigating functional insufficiencies. We have used the paradigm proposed by Hertel (48) and classified studies into the various functional insufficiencies that contribute to the development of CAI. The particular aims of the present systematic investigation were to identify 1) the definition of ankle instability used by the authors; 2) the terminology used to classify subjects (e.g., CAI, MI, FI, or other); and 3) what specific inclusion criteria studies have used.
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