Relationship of Plantar Fascia and Abductor Digiti Minimi: The Importance of a Dual-Design Study in Plantar Fasciitis

Background: Fascial thickening in plantar fasciitis is well established on ultrasound, whereas the abductor digiti minimi (AddV), the imaging target of Baxter neuropathy, remains poorly characterized. We quantified plantar fascia, AddV and posterior chain with a dual design: within-subject (affected vs. contralateral foot) and between-group (patients vs. controls). Methods: A cross-sectional study was conducted in 18 patients with plantar fasciitis and 20 healthy controls. Thickness, cross-sectional area (CSA), echo-intensity, pennation angle and 35 texture variables were analyzed with Wilcoxon signed-rank, Mann–Whitney U, Cohen’s d, Spearman’s ρ and Benjamini–Hochberg correction. Results: AddV CSA was lower in the affected than in the contralateral foot (median 85.8 vs. 99.2 mm2; p = 0.038, unadjusted; dz = −0.34), but did not differ from controls (92.9 ± 22.8 vs. 92.1 ± 22.5 mm2; p = 0.917). Within the affected foot, AddV CSA correlated inversely with fascia CSA (ρ = −0.62; p = 0.018, unadjusted). The fascia was thicker, larger and more hypoechoic in cases than in controls (thickness 4.99 vs. 3.07 mm; CSA 59.94 vs. 25.28 mm2; both p < 0.001, d > 2) and than contralaterally. Conclusions: Dual-design ultrasound suggests a small AddV asymmetry and an inverse AddV–fascia relationship that emerge only within subjects, even without overt Baxter neuropathy. These exploratory findings require confirmation before intrinsic-muscle assessment can be recommended in the sonographic evaluation of plantar fasciitis.