Measurement related indicators of hallux valgus by Mimics software based on foot weight-bearing CT images

Objective To establish a new method for measuring the angle of pollex valgus by using Mimics image application software based on full weight-bearing position CT scan. Methods From July 2016 to March 2017, the data of 30 cases (36 feet) of hallux valgus in the Department of Orthopedics of the First Affiliated Hospital of Kunming Medical University were analyzed prospectively. Based on the foot weight-bearing position CT image application Mimics, the three-dimensional model of the hallux valgus was reconstructed. Using the software fitting function, the analysis of the proximal phalanx axis was finished automatically. The actual articular surface of the distal metatarsal bone was reconstructed, and corrected it with the proximal phalanx and the three-dimensional model of the first metatarsal. The usage of the above mentioned anatomic parameters were the fundamental of accurate hallux valgus angle(HVA), intermetatarsal angle(IMA), distal metatarsal articular angle(DMAA)measurement. Four testers used the method and measured 30 cases of hallux valgus by X-ray measuring, and the results were followed with statistical and Bland-Altman analysis. Results The reliability of the four measurement doctors using the conventional X-ray and CT modeling reconstruction methods were respectively analyzed, the intraclass correlation coefficients were 0.89 and 0.91. The consistency of four measurement doctors was reliable. The mean value of HVA, IMA measured by conventional X-ray measurement method were 29.10°±10.04°, 13.98°±4.38°, and the mean value of HVA measured by CT modeling method was 30.02°±10.62°, 13.83°±4.29°. The results were not statistically different (all P values>0.05). The mean value of DMAA measured by conventional X-ray measurement method was 13.98°±4.38°, and the mean value of DMAA measured by CT modeling method was 13.83°±4.29°. The results were statistically different (P<0.01). The Bland-Altman results showed that there was no difference in the consistency of the two measurement methods for IMA and HVA while the measurement results of DMAA were quite different. Conclusions The measurement of hallux valgus HVA, IMA, DMAA by Mimics software based on foot weight-bearing CT images can accurately generate the side lines of the above mentioned three angles. Thus, the angle measurement can be realized accurately and the error caused by conventional X-ray measurement method can be reduced, so as to reconstruct the distal articular surface of the first metatarsal bone. Then, for the measurement of DMAA, the accuracy and reliability were superior to the conventional X-ray measurement method. Therefore, it is believed that this method can make up for the shortcomings of the conventional X-ray measurement method and reduce the measurement error. It is a feasible, reliable and accurate measurement method, which provides a reliable basis for precise individualized treatment of hallux valgus, and has certain clinical value. Key words: Hallux valgus; Tomography, X-ray computed; Imaging, three-dimensional; Intermetatarsal angle; Hallux valgus angle; Distal metatarsal articular angle; Weight-bearing position