Does metatarsus primus elevatus really exist in hallux rigidus? A weightbearing CT case-control study

Background Elevated first metatarsal, Metatarsus primus elevatus (MPE), has been a topic of controversy. Recent studies have supported a significantly elevated first metatarsal in hallux rigidus on weight-bearing radiographs (WBR). However, conventional radiographs have limitations for accurate measurement. Our objective was to comparatively assess MPE and other variables which can affect the spatial relationship of the forefoot in the HR group compared to controls using weight-bearing CT (WBCT). Methods In this single-center, retrospective, case-control study, 25 patients (30 feet) with symptomatic HR and 30 controls were selected. WBCT parameters were measured by two independent investigators. Inter-observer reliabilities were assessed using intra-class correlation coefficients (ICCs). MPE was evaluated by measuring the direct distance between 1st and 2nd metatarsals. Independent t tests were performed to compare the two groups. A threshold of MPE to diagnose HR was calculated using the Receiver Operating Characteristic (ROC) curve. Results HR groups had increased hallux valgus angle (HVA) (8.52 degrees in control vs 11.98 degrees in HR) and MPE (2.92 vs 5.09 mm), decreased 1st metatarsal declination angle (21.09 degrees vs 19.07 degrees) 1st/2nd metatarsal declination ratio (87.45 vs 79.71) indicating elevated first metatarsal compared to controls. Dorsal translation of the first metatarsal at the first tarsometatarsal joint was observed in 21 (70%) patients of the HR group when defined as a step-off or discontinuation of the curvature along the first TMT joint. The threshold of MPE for diagnosis of HR was 4.19 mm with 77% sensitivity and 77% specificity. Conclusion Our results confirmed a significantly elevated first metatarsal in the HR group compared to controls on WBCT. A MPE greater than 4.19 mm was found to be diagnostic for symptomatic HR. Significant number of patients in the HR group (70%) had dorsal translation of the first metatarsal at the first TMT joint which can contribute to increased MPE.

Automated summary

This study used weight-bearing CT to compare variables between the HR group and the control group. The HR group showed a significantly elevated first metatarsal and MPE greater than 4.19 mm was found to be diagnostic for HR. A significant number of patients in the HR group had dorsal translation of the first metatarsal at the first TMT joint, which may contribute to increased MPE.