Background and objectives <p>Progressive Collapsing Foot Deformity (PCFD), a multifactorial disorder, involves the synergistic failure of dynamic restraints, primarily the posterior tibial tendon (PTT), and static restraints, chiefly the calcaneonavicular ligament (SL). The necessity of routinely reconstructing the SL complex in the early, flexible stages of the disease is a significant clinical debate.To clarify this, this study aimed to assess whether measurable macro-structural changes, specifically alterations in the ultrasonographic thickness of the supramedial fasciculus of the calcaneonavicular ligament, are already established in patients presenting with symptomatic, flexible PCFD compared with healthy controls.</p> Methods <p>An observational cross-sectional study (level of evidence: III, prognostic study) included 108 adult subjects: a symptomatic flexible PCFD group (n = 55) and a control group (n = 53). Exclusion criteria were strictly implemented to prevent the inclusion of advanced, rigid deformities, specifically excluding patients presenting with talar uncoverage greater than 50% on weight-bearing anteroposterior radiographs or a plantar sag indicative of severe collapse on lateral radiographs. The thickness of the calcaneonavicular ligament (SL) was the primary outcome measure measured using high-frequency ultrasound. Secondary measures included PTT thickness to confirm the pathological status of the experimental cohort.</p> Results <p>The mean thickness of the calcaneonavicular ligament showed no statistically significant difference between the control group (5.07 mm, SD = 0.116) and the PCFD group (4.81 mm, SD = 0.119) (<i>p</i> = 0.368). This demonstrates the structural integrity of the SL complex in the early, flexible stage of the deformity. In contrast, all measurements of the PTT demonstrated significant thickening in the experimental group, confirming established tendinosis (<i>p</i> &lt; 0.001).</p> Conclusions <p>In symptomatic, flexible Progressive Collapsing Foot Deformity dynamic tissue pathology (PTT tendinosis) is established, yet the structural thickness of the superomedial calcaneonavicular ligament remains stable. This supports a temporal pathomechanical sequence in which the dynamic restraint failure precedes the gross dimensional collapse of the primary static stabilizer. The preservation of SL structural stability in early disease stages argues against the routine, mandatory exploration and reconstruction of the medial soft tissues unless functional dynamic imaging reveals severe laxity.</p> Level of Evidence <p> III</p>