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Ankle sprains are one of the most common sports injuries worldwide. It is estimated that 10,000 people are injured daily in the United States1, of which sports-related injuries account for 15%-45%2. The medical costs associated with treating ankle sprains in the United States amount to $4.2 billion annually3,4,5. Chronic foot instability is a common problem following ankle sprains and occurs in approximately 74% of ankle sprains6, including ankle or subtalar instability. However, due to the similar clinical symptoms and signs, it is difficult for medical staff to distinguish whether chronic ankle instability is also accompanied by chronic subtalar joint instability in the clinic, and as a result, chronic subtalar instability can be easily missed. Therefore, the true incidence of chronic ankle-subtalar complex joint (ASCJ) instability (a specific type of chronic foot instability that includes both chronic ankle instability and chronic subtalar instability) may be higher than reported7,8,9. If left untreated, chronic ankle-subtalar complex joint instability can cause repeated ankle sprains, leading to a vicious circle of ankle sprains and chronic ankle-subtalar complex instability. Long-term chronic ankle-subtalar complex instability can lead to degeneration of the ASCJ and post-traumatic osteoarthritis, which can affect the adjacent joints in severe cases10. For these diseases, the current clinical treatment is mainly conservative, in addition to surgical treatment methods such as ligament repair and ligament reconstruction11,12.
ASCJ is the core structure of the foot and maintains the balance of the body during movement13. Extensive research has been conducted on the structure of the ankle joint and the subtalar joint separately14,15,16,17. However, research on the whole ankle-subtalar joint is rare. About one-quarter of the cases of ankle injury are associated with subtalar joint injury18. Due to the complex injury mechanism of ASCJ instability, there is no consensus on diagnosing and treating it in the clinical setting. Considering the current situation of ankle injuries in the clinic, a more scientific method is needed to study the ankle and subtalar joint as a whole, thereby providing a new understanding for studying foot diseases.
Since the anatomical structure of the mouse hindfoot at the musculoskeletal level is comparable to that of the human foot19, in several studies, mouse models for foot/ankle research have already been implemented10,19. Chang et al.19 successfully developed three different mouse models of ankle osteoarthritis. Inspired by the successful establishment of ankle instability in the mouse model, we established a mouse model for ankle-subtalar complex instability, hypothesizing that the transection of the partial ligaments in the mouse hindfoot would result in mechanical instability of the ASCJ, which would lead to post-traumatic osteoarthritis (PTOA) of the ASCJ. The ASCJ instability animal model could be used for the treatment of both ankle instability and subtalar instability, which is more in line with the actual clinical situation than the currently used simple ankle instability model7,8,9,19. To test this hypothesis, two mouse models of ligament transection-induced instability of the ASCJ were designed. The results for the sensory-motor function-the balance beam test, footprint analysis, and thermal nociception assessment-were used to evaluate the feasibility of the model, and micro-computed tomography (CT) and histological staining were used to evaluate the damage and degeneration of the mouse articular cartilage. The successful establishment of a mouse model of ASCJ instability not only provides a new understanding for studying foot diseases but also provides a valuable reference for clinical research on the injury-related mechanisms, provides better treatment options for ankle sprains, and is helpful for further studies on the disease.