Weightbearing CT Analysis of Hallux Rigidus: Does Metatarsus Primus Elevatus Really Exist?

CATEGORY: Midfoot/Forefoot INTRODUCTION/PURPOSE: The etiology of hallux rigidus has been unknown. Metatarsus primus elevates (MPE), elevated first metatarsal has been controversial. Recent studies have supported significantly elevated first metatarsal in hallux rigidus patients. Bouaicha reported MPE greater than 5 mm could be a predictive factor of hallux rigidus. Lateral weightbearing radiographs has been used to evaluate MPE, However, there are limitations of conventional radiography including variation in X-ray projection angle and foot position and superimposition of metatarsals. Cheung assessed foot alignment utilizing 3D reconstructions from WBCT and concluded that hallux rigidus patients had increased MPE. Our objective was to assess MPE and anatomical characteristics in foot alignment of hallux rigidus patients compared to a control group using WBCT. This is the first study measuring MPE on WBCT. METHODS: This is a single-center, retrospective study from prospectively collected data. 20 patients with hallux rigidus and WBCT data were enrolled from October 2014 to December 2020. As a control group, 20 patients with various foot and ankle pathologies were selected. Measured WBCT parameters included 1st TMT joint version, HVA, IMA, DMAA, 1st and 2nd metatarsal lengths, Foot width, Sesamoid station and rotation angle, 1st-5th Metatarsal Angle, Metatarsus adductus angle, 2nd cuneiform-2nd metatarsal angle, Talus-1st Metatarsal Angel, 1st Metatarsal-Proximal Phalanx Angle, 1st and 2nd metatarsal declination angles and ratio, and MPE. MPE was measured as a direct distance between 1st and 2nd metatarsals using a line tangential to the first metatarsal and another perpendicular line at the metadiaphyseal junction to reach second metatarsal on parasagittal view. A Cut-off value of MPE was calculated using receiver operating characteristic curve. Two investigators independently assessed each WBCT. RESULTS: Mean age was 43.3 in control group (45% male, 55% female) and 55.9 in HR group (60% male, 40% female). Significant differences were found in several facets of foot anatomy between HR and control groups including HVA (7.57 in control vs 14.05 in HR), DMAA (3.89 vs 8.06), forefoot width (92.96 vs 95.47), 1st MT declination angle (20.17 vs 17.82), 1st/2nd MT declination ratio (83.52 vs 76.02), and MPE (3.24 mm vs 5.40 mm). MPE was significantly higher in hallux rigidus group in all three parasagittal views (unmodified, parallel to 1st metatarsal and 2nd metatarsal). Dorsal subluxation/translation of the first metatarsal was observed at 1st TMT joint in the parasagittal view of WBCT in 9 (45%) patients of hallux rigidus group suggesting sagittal instability. No patient in control group had dorsal subluxation/translation. A cut-off value of MPE was 4.56 mm with 80% sensitivity and 90% specificity. CONCLUSION: To evaluate MPE on WBCT, we used a new direct measurement on parasagittal views. We found a significant difference in MPE in HR. Our WBCT results are consistent with other studies using conventional radiographs. A cut-off value of WBCT MPE for diagnosis of HR was 4.56 mm in our cohort. Considering 45% patients of the HR group had dorsal subluxation/translation of first metatarsal at 1st TMT and increased HVA, Hallux rigidus may be associated with first ray instability predominantly in sagittal plane with resultant MPE with varying degree of combined coronal plane instability resulting in increased HVA.