An imaged-based three-dimensional study of first metatarsal protrusion distance in women with and without hallux valgus

Background: First metatarsal protrusion distance (MPD) has been commonly studied as a characteristic of hallux valgus deformity. To date, the majority of investigations have used radiographic methods, with most reporting first metatarsal (ray) protrusion to be associated with deformity. As an alternative, this study used a three-dimensional (3-D) image acquisition and data analysis method to quantify MPD. Methods: Magnetic resonance images were acquired in weightbearing on 29 women (19 with hallux valgus; 10 controls). After the 3-D images were reconstructed into virtual bone models, two examiners measured MPD in relation to the navicular. In addition to a reliability analysis, a t test assessed for group differences in demographics, foot posture (hallux valgus, intermetatarsal angles), and MPD. Results: Group demographics were not different, while measures of hallux valgus and intermetatarsal angles were different (P ≤ 0.01) between groups. The measurement of MPD was highly reliable (ICC ≥ 0.99; SEM ≤ 0.78 mm). Metatarsal protrusion averaged approximately –2.0 mm in both groups. There was no statistical group difference (P = 0.89) in MPD. Conclusions: The reconstructed image datasets captured the 3-D spatial relationship of the anatomy. Measurements of MPD were reliable. The first ray measured 2 mm shorter than the second ray in both the hallux valgus and control groups. Though unexpected, this result may prompt future study of the pathokinematics associated with hallux valgus that include the quantification of metatarsal protrusion with 3-D methods, instead of relying solely on single-plane radiograph reports.