Abstract: BACKGROUND: Three-point mechanics is an effective method for ankle foot orthosis correction and prevention of various foot diseases. At present, the clinical application research on 3D printing ankle foot orthosis has been widespread; however, there are relatively few reports on numerical simulation and finite element analysis involving three-point mechanical correction. There is a lack of relevant biomechanical experimental verification.  
OBJECTIVE: Three-point force was loaded to analyze the composite model of ankle foot orthosis and foot by finite element method, observing the effect of foot correction with ankle foot orthosis under three-point force intervention, verifying the effectiveness of three-point force and the reliability of ankle foot orthosis.
METHODS: A three-dimensional foot and ankle model of a healthy volunteer was constructed based on the medical image processing software Mimics. Rodin 4D and Geomagic reverse engineering software were used to optimize the models and design personalized ankle foot orthosis models. Solidworks software was utilized to turn the ankle model inside for 10° to simulate the foot varus disease. Static loading was carried out on the foot force application area by ANSYS software combined with the three-point mechanics principle. The deformation and stress changes of the foot and ankle tissues were analyzed when the human foot pain threshold was met. The display dynamics was used to further verify the effectiveness of the three-point force applied by the ankle foot orthosis.  
RESULTS AND CONCLUSION: (1) The personalized ankle foot orthosis designed in this paper had the effect of preventing and fixing foot and ankle varus. The ankle varus was 1.81 mm after being loaded with 1 N•m of varus when not wearing ankle foot orthosis, while it was only 0.44 mm after wearing ankle foot orthosis, the deformation rate was reduced by 75.7%, and the effect of preventing varus was significantly enhanced. (2) When only coronal correction was performed, the low calcaneal force would aggravate the varus angle of the front foot. After adjusting the correction force on the inside of the heel and above the medial malleolus, the varus angle of the front foot and the calcaneus position were improved; however, the medial phalangeal region of the foot still had different degrees of adduction and displacement, which would aggravate the adduction deformity of the patient’s front foot. (3) The correction effect of the coronal plane and horizontal plane was better than that of the single coronal plane. There was no adduction and displacement of the medial phalanges of the front foot and the varus angle of the front foot decreased under the force (25, 10, 10, 20 N) of the medial heel, the medial shaft of the first metatarsal, below the lateral malleolus and above the medial malleolus, and the valgus along the X-axis was corrected by 1.395 mm, the calcaneus valgus was corrected by 1.227 mm. The calcaneus varus angle was corrected from 10.21° to 7.25°, and the varus angle was improved by 28.9%. (4) The lateral plantar metatarsal load decreased, the medial plantar metatarsal load increased under the action of a two-plane three-point force, and the plantar bone stress was significantly improved after correction. Thus, the reliability of the three-point force principle was further verified. This study provides an important theoretical support for the implementation of ankle foot orthosis in the treatment of varus in clinical practice.

Key words: varus foot, three-point mechanics, ankle foot orthosis, 3D printing, biomechanics, finite element analysis, reverse engineering