The difference in second - third metatarsal length is associated with the risk of dislocation or arthritic change in the second metatarsophalangeal joint in patients with hallux valgus.

Background: Chronic mechanical stress on the second metatarsal head (2 MH) can be a risk factor for osteoarthritis (OA) and dislocation of the second metatarsophalangeal joint (2MTPJ) in hallux valgus (HV). This study aimed to investigate the stress distribution of the 2 MH in HV patients and determine the factors associated with dislocation and OA of the 2MTPJ.

Methods: In total, 115 feet with HV were retrospectively reviewed and divided into two groups: those with subluxation or dislocation of the 2MTPJ (group D, 27 feet) and those without (group N, 88 feet). The control group (group C) included 33 feet without HV. Group N was divided into OA and non-OA (NOA) groups according to the presence or absence of OA of the 2MTPJ. The Hounsfield Unit (HU) value of the subchondral bone of the 2 MH was measured on sagittal slices of the preoperative computed tomography images and divided by the HU value of the navicular region (HU ratio). The relationship between the HU ratios and radiographic parameters was compared.

Results: The HU ratios were significantly higher in group N than those in groups C and D. In group N, the protrusion of the second metatarsal relative to the third and fourth metatarsals (M2-M3, M2-M4) was positively correlated with the dorsal and central HU ratios, respectively. Group D had a significantly larger HV angle (HVA) and M2-M3 than group N. HVA, the first-second intermetatarsal angle, M2-M3, M2-M4, and dorsal and central HU ratios were significantly larger in the OA group than in the NOA group. The cutoff value of M2-M3 was 5.5 mm when comparing groups D and N, and 4.4 mm when comparing the OA and NOA groups.

Conclusions: Severe HV and a longer M2-M3 and M2-M4 may be associated with a high risk of dislocation and OA of the 2MTPJ.

Level of evidence: Ⅲ.