Does Distal Metatarsal Articular Angle Reflect A Real Joint Deformity in Hallux Valgus?

Abstract

Category: Bunion; Midfoot/Forefoot Introduction/Purpose: The Distal Metatarsal Articular Angle (DMAA) has long been described as a valgus increase of the distal articular surface of the first metatarsal in Hallux Valgus (HV) deformity. Since then, several studies have reported a poor reliability of this measurement and some authors currently claim that DMAA is misinterpreted as just the rounded shape of the lateral part of the first metatarsal head reflecting pronation of the first ray and could also be biased by the first metatarsal plantarflexion angle. Our study aimed to compare the DMAA in HV and control populations after correcting, with a dedicated software, pronation and plantarflexion of the first metatarsal. We hypothesized that after correction, DMAA will be higher in the HV population, especially in juvenile cases. Methods: We performed a retrospective case-control study including 36 HV and 20 control feet. Patients under 15 or with surgery antecedent were excluded. DMAA1 was measured as initially described on X-rays by the angle between the distal articular surface and the longitudinal axis of the first metatarsal. DMAA2 was measured on Weight Bearing Computed Tomography (WBCT) without any corrections. DMAA3 was measured after correction of the first metatarsal plantarflexion in the sagittal plane. DMAA4 was measured after correction of the pronation of the first ray relative to the ground in the coronal plane using the alpha-angle. And DMAA5 after both corrections. Corrections in the coronal and sagittal planes were performed along the axis of the first metatarsal. Normality was assessed using Shapiro-Wilk tests. Comparisons were made using Student tests for normal variables and Mann-Whitney for non-normals. Correlations between age and angles were assessed by the Pearson correlation coefficient. Results: HV and Control groups were comparable on BMI (p=0.69), Age (p=0.58) and Gender (p=0.27). DMAA1 (25.9°+/-7.3 vs 7.6°+/-4.2; p<0.01), DMAA2 (19.1°+/-7.1 vs 3.3°+/-2.4; p<0.01), DMAA3 (16.1°+/-6.2 vs 2.9°+/-2.4; p<0.01), DMAA4 (14.4°+/-5.7 vs 2.6°+/-2.5; p<0.01) and DMAA5 (11.9°+/-4.9 vs 3.3°+/-2.9; p<0.01) were significantly higher in the HV group than in the Control group. Significant decreases in angles were present between DMAA1 and DMAA2 (Δ=-6.9; CI95[-8.6;-5.1]; p<0.01), DMAA2 and DMAA3 (Δ=-3; CI95[-4.1;-1.9]; p<0.01), DMAA2 and DMAA4 (Δ=-4.7; CI95%[-6.3;-3.1]; p<0.01), DMAA2 and DMAA5 (Δ=-7.2 ;CI95%[-8.8;-5.6]; p<0.01) and between DMAA3 and DMAA4 (Δ=-1.7 ;CI95%[-2.9;-0.5]; p<0.01) in the HV group. No significant correlation was found between the 5 different DMAA values and the age in the HV group (respectively ρ=0.1,p=0.55 ; ρ=0.31,p=0.07 ; ρ=0.19,p=0.28 ; ρ=0.1,p=0.56 ; ρ=0.04,p=0.83). Conclusion: Although overestimated with the 2-dimensional DMAA assessment, the valgus increase of the distal articular surface of the first metatarsal was present in the HV deformity, even after correction of pronation and plantarflexion of the first ray. DMAA overestimation was close to 14 degrees on X-rays and 7 degrees on WBCT without any correction and pronation of the first metatarsal seemed to play a more important role on this overestimation than plantarflexion. Age did not seem to influence this deformity. Increase of valgus of the distal articular surface of the first metatarsal should be considered in HV correction surgical planning.