Abstract: BACKGROUND: Many foreign studies have discussed the reliability of foot posture index, but few domestic studies have discussed the reliability of foot posture index applied to patients with flat feet, nor the application of foot posture index in three-dimensional (3D) printed insoles.  
OBJECTIVE: To explore the reliability of foot posture index in the foot posture of patients with flat foot, to predict the injury of foot and ankle posture function in patients with flat foot, and to provide a theoretical basis for the design of 3D printing personalized orthopedic insole.
METHODS:  Forty patients with flat feet aged 6-60 years were selected from the Department of Orthopedics, the Ninth People’s Hospital Affiliated to Shanghai Jiao Tong University School of Medicine from May to November 2020. These patients were divided into two groups according to age: 20 cases in group of children aged 6-18 years; 20 cases in group of adults aged 19-60 years. Two raters (rater 1 and rater 2) were trained by FPI-6 system, and then the foot posture was evaluated for the first time. The rater 1 was evaluated for the second time within seven days after the first evaluation. The intra group correlation coefficient was calculated to evaluate the test-retest reliability and inter tester reliability. The protocols were in accordance with Declaration of Helsinki and the relevant ethical requirements of Ninth People’s Hospital Affiliated to Shanghai Jiao Tong University School of Medicine (approval No. 2016-124-T73; approval date: 2016-10-26). All subjects and guardians read and signed the informed consent before participating.  
RESULTS AND CONCLUSION: (1) The test-retest reliability of foot posture index was higher than 0.75 except for the curvature of lateral malleolus, with high reliability. The ICC value of lateral malleolus was between 0.4 and 0.74, with moderate reliability (P < 0.05). The inter rater reliability and the total score of foot posture index showed that ICC was more than 0.75 except for the upper and lower curvature of the lateral malleolus, which was highly reliable, and the ICC value of the upper and lower curvature of the lateral malleolus was between 0.4 and 0.74, with medium reliability (P < 0.05). (2) The test-retest reliability and interrater reliability of adult foot posture index were moderate to highly reliable (P < 0.05). (3) The results show that foot posture index is moderate to highly reliable in adults and children with flatfoot. Foot posture index can be used to evaluate Chinese children and adults with flatfoot and grade the severity of flatfoot. The high reliability of foot posture index can provide a theoretical basis for the design of 3D printing insole and the evaluation of treatment effect.

Key words: foot posture index, flatfoot, children, adults, reliability, 3D printing, insole, orthosis