Chronic Ankle Joint Instability Induces Ankle Sensorimotor Dysfunction: A Controlled Laboratory Study

Background: Chronic ankle instability (CAI) is a clinical sequela that causes the recurrence of ankle sprain by inducing ankle sensorimotor dysfunction. Animal models of CAI have recently shown that ankle ligament injuries mimicking an ankle sprain result in chronic loss of ankle sensorimotor function. However, the underlying mechanisms determining the pathogenesis of CAI remain unclear. Hypothesis: Ankle instability after an ankle sprain leads to the degeneration of the mechanoreceptors, resulting in ankle sensorimotor dysfunction and the development of CAI. Study Design: Controlled laboratory study. Methods: Four-week-old male Wistar rats (N = 30) were divided into 2 groups: (1) the ankle joint instability (AJI) group with ankle instability induced by transecting the calcaneofibular ligament (n = 15) and (2) the sham group (n = 15). Ankle instability was assessed using the anterior drawer test and the talar tilt test at 4, 6, and 8 weeks after the operation (n = 5, for each group at each time point), and ankle sensorimotor function was assessed using behavioral tests, including ladder walking and balance beam tests, every 2 weeks during the postoperative period. Morphology and number of mechanoreceptors in the intact anterior talofibular ligament (ATFL) were histologically analyzed by immunofluorescence staining targeting the neurofilament medium chain and S100 proteins at 4, 6, and 8 weeks postoperatively (n = 5 per group). Sensory neurons that form mechanoreceptors were histologically analyzed using immunofluorescence staining targeting the mechanosensitive ion channel PIEZO2 at 8 weeks postoperatively (n = 5). Results: Ankle sensorimotor function decreased over time in the AJI group, exhibiting decreased ankle instability compared with the sham group ( P = .045). The number of mechanoreceptors in the ATFL was reduced ( P < .001) and PIEZO2 expression in the sensory neurons decreased ( P = .008) at 8 weeks postoperatively. The number of mechanoreceptors was negatively correlated with ankle sensorimotor dysfunction ( P < .001). Conclusion: The AJI model demonstrated degeneration of the mechanoreceptors in the ATFL and decreased mechanosensitivity of the sensory neurons, which may contribute to CAI. Clinical Relevance: Ankle instability causes degeneration of mechanoreceptors and decreases the mechanosensitivity of sensory neurons involved in the development of CAI. This finding emphasizes the importance of controlling ankle instability after ankle sprains to prevent recurrence and the onset of CAI.