Loosening of the anterolateral capsule affects the stability after arthroscopic lateral ankle ligament repair of chronic ankle instability

Purpose Arthroscopic anterior talofibular ligament (ATFL) repair for chronic lateral ankle instability (CLAI) has been widely performed. The recurrence of the instability after the surgery sometimes occurs, which may cause the development of osteoarthritis. Therefore, it is important to elucidate the factors of the recurrence. This study aimed to evaluate the loosening of the capsule in the MRI and whether it affected clinical outcomes or not in arthroscopic ATFL repair. Materials and Methods Thirty-eight ankles in 35 patients with CLAI treated by arthroscopic lateral ligament repair were included. The capsule protrusion area defined as the area that protruded ATFL laterally from the line connecting the fibula and talus attachment on MRI was measured. Capsule protrusion area in ankles with or without CLAI was compared and the relationships between it and clinical outcomes were assessed. Results The capsule protrusion area in the CLAI group (74.2 ± 36.4 mm 2 ) was significantly larger than that in the control (25.5 ± 14.3 mm 2 ) ( p < 0.01). The capsule protrusion area in the poor remnant group (93.8 ± 36.4 mm 2 ) was significantly larger than that in the excellent (53.2 ± 40.3 mm 2 ) ( p < 0.05). The capsule protrusion area in the patients with recurrent instability (99.8 ± 35.2 mm 2 ) was significantly larger than that without recurrent instability (62.4 ± 30.9 mm 2 ) ( p < 0.01). Clinical scores in the recurrent group were significantly lower than those in the non-recurrent group ( p < 0.05). Conclusions Capsule loosening would be one of the causes of the recurrence of instability after arthroscopic lateral ankle ligament repair. Evaluation of the capsule protrusion area on MRI is helpful to choose appropriate surgical procedures for CLAI patients. Level of evidence 4.