In vivo kinematics of functional ankle instability patients during the stance phase of walking

•Dual fluoroscopic imaging system can be used to study ankle instability.•The patients with functional ankle instability showed subtalar hypermobility.•Kinematic data correlated with the Cumberland Ankle Instability Tool scores. Previous studies showed functional ankle instability (FAI) patients have morphological ligamentous abnormality, despite having no apparent joint laxity. Whether tibiotalar and subtalar joints hypermobility exists in FAI patients during stance phase of walking, remains controversial. Ten unilateral FAI patients, ten unilateral lateral ankle sprain (LAS) copers and ten healthy controls were included. A dual fluoroscopy imaging system was utilized to capture the fluoroscopic images of tibiotalar and subtalar joint during the stance phase of walking. Kinematic data from six degrees of freedom were calculated utilizing a solid modeling software. The range of motion and joint excursions about six degrees of freedom were compared among the three groups. The correlations between range of motion and Cumberland Ankle Instability Tool (CAIT) scores were assessed utilizing the Spearman’s correlation coefficient (r). During the stance phase, the FAI patients and LAS copers showed larger tibiotalar anterior/posterior translation than the healthy controls (FAI patients, p = .013; LAS copers, p = .002). The FAI patients also showed significantly larger lateral/medial translation (p = .035) and inversion/eversion rotation (p = .003) of subtalar joints than healthy controls. By contrast, the subtalar joints of the LAS copers were not different from those of the healthy controls in the lateral/medial translation (p = .459) and inversion/eversion rotation (p = .091). CAIT scores were negatively correlated with range of motion. During the stance phase of walking, FAI patients showed significantly larger hypermobility of subtalar joints than healthy controls, contrary to the LAS copers. These findings justify the utilization of dual fluoroscopy imaging system to detect joint hypermobility in FAI patients. Treatment for FAI patients may require stabilization of the subtalar joint.