Evaluation of Hallux Valgus Using Rotational Moment of Midfoot Measured by a Three-dimensional Foot Scanner: a Cross-sectional Observational Study

The mechanism of hallux valgus (HV) development has not been fully clarified, and a new eval-uation method is required. We aimed to establish a method for calculating the rotational moment of the midfoot (RMM) by developing a three-dimensional foot scanner, and to reveal features of HV using this method. A smartphone was used to capture images and analyze regions of the foot in 592 participants. We focused on fea -ture points such as the great toe-first metatarsal head-heel (GFH) angle as the HV angle, the navicular bone, and the centerline of the foot. Navicular-moment-arm (NMA) was defined as the distance between the navicular bone and the centerline of the foot. RMM was calculated from NMA and body weight. The mean values of RMM were 12.3 and 9.4 Nm for male and female participants, respectively. With pronation of the midfoot, the rotational moment and the load on the midfoot increased because of the increase in navicular adduction and axis of bone distance (ABD). ABD and RMM increased with increasing GFH angle quartiles in male and fe -male participants. In particular, ABD and RMM were significantly higher at the fourth quartile of GFH angle. Transverse arch width and height were identified as predictors of GFH angle, both showing high contribution. The navicular bone associated with NMA is controlled mainly by the posterior tibial muscle. Dysfunction of the posterior tibial muscle causes an increase in NMA, leading to an increase in the first and second metatarsal (M1-M2) angle. Therefore, increases in RMM and NMA cause the first metatarsal to pronate and rotate, induc-ing an increase in the M1-M2 angle. The rotational moment applied to the navicular bone affects the adjacent medial and intermediate cuneiform bones. Particularly, the adduction motion of the navicular bone causes the medial cuneiform bone to rotate in conjunction, possibly inducing relaxation of the tarsometatarsal joint and leading to metatarsus primus varus. Measurement of the foot skeletal structure using a smartphone has the po-tential of widespread use.