Evaluation of the longitudinal arch of the foot with hallux valgus using a newly developed two-dimensional coordinate system.

The purposes of the present study were to establish a method for the evaluation of the longitudinal arch, using a two-dimensional coordinate system, and to clarify the main point of flattening of the foot in patients with hallux valgus. Measurements on a weight-bearing lateral radiograph of the foot were performed in 186 feet of 110 women with symptomatic hallux valgus and in 93 normal feet of 72 women. A two-dimensional coordinate system was used to perform the measurements. In this coordinate system, the line connecting the lowest point of the first metatarsal head and the lowest point (O) of the calcaneus is fixed as the x-axis, and the line that is perpendicular to the x-axis and intersects the x-axis at point O is fixed as the y-axis. The mean values of the y-coordinates at the points of the medial longitudinal arch of the foot in the patients with hallux valgus were significantly lower than those in the normal subjects (P < 0.001). The values were noticeably lower at points on the navicular. However, no change was observed in the posterior talus body. The new coordinate system indicated that the main points of flattening of the foot in patients with hallux valgus were the plantar flexion of the talus and the sinking of the navicular.