A Novel Indicator of Plantar Fasciitis from Ultrasound Examination: A Cross-Sectional Cohort Study

Background/Objectives: Plantar fasciitis and Achilles tendinopathy are clinically linked conditions that share numerous biomechanical predispositions; however, the sonographic morphological correlations between the plantar fascia (PF) and Achilles tendon (AT) remain underutilized in routine diagnostic algorithms. The primary objective of this study was to investigate the concurrent relationship between the ultrasound morphometrics of the PF and AT in patients with plantar fasciitis. The secondary objective was an exploratory assessment to evaluate whether AT measurements can serve as an auxiliary diagnostic tool. Methods: A cross-sectional diagnostic study was conducted, evaluating 45 adults with clinically and ultrasonographically confirmed unilateral plantar fasciitis. The evaluation encompassed 45 affected feet and 45 contralateral unaffected control feet (internal controls). The main outcome measures included ultrasonographic PF thickness, AT thickness, and AT width. Statistical evaluations utilized the Wilcoxon signed-rank test for paired comparisons, Spearman correlations, Generalized Estimating Equations (GEE) multivariable logistic regression to account for paired-limb data clustering, and receiver operating characteristic (ROC) curves. Results: AT thickness and width correlated moderately, yet significantly, with PF thickness (rs = 0.49 and 0.59, p < 0.001). ROC analysis demonstrated that an AT thickness >0.43 cm achieved 86.0% sensitivity, 48.1% specificity, and an area under the curve (AUC) of 0.71. Furthermore, an AT width >1.48 cm yielded 85.2% specificity and an 88.9% positive predictive value. GEE multivariable logistic regression adjusted for age and gender and accounting for within-subject clustering revealed that an AT thickness >0.43 cm was significantly associated with a 6.99-fold increase in the odds of having concurrent plantar fasciitis (Adjusted OR 6.99, 95% CI: 2.05–23.89, p = 0.002). Conclusions: Ultrasonic AT thickness and width exhibit a moderate, significant morphological association with PF thickness. An AT thickness >0.43 cm serves as a valuable supplementary diagnostic indicator associated with plantar fasciitis, underscoring the clinical importance of evaluating the entire enthesis complex concurrently.