Role of the posterior tibial tendon in plantar pressure distribution across progressive stages of ligamentous insufficiency in a cadaveric flatfoot model.

BACKGROUND: Progressive collapsing foot deformity involves failure of both static and dynamic medial arch stabilizers. The relative timing and interaction between posterior tibial tendon dysfunction and ligamentous attenuation remain unclear. This study investigates whether the posterior tibial tendon can compensate for progressive ligament failure in distributing pressure under the foot in a cadaveric progressive collapsing foot deformity model at balanced stance. METHODS: Eight cadaveric lower limbs underwent sequential ligament transections replicating progressive collapsing foot deformity. Plantar pressure was recorded during balanced stance using TekScan sensors under both loaded and unloaded posterior tibial tendon conditions. Axial and tendon loads mimicked physiologic forces. Medial-lateral and hindfoot-forefoot pressure ratios were analyzed using Wilcoxon signed-rank tests. FINDINGS: Unloading the posterior tibial tendon significantly increased hindfoot-to-forefoot loading ratios and shifted the center of pressure posteriorly across all ligamentous transection stages (P < 0.05). Medial-to-lateral loading for the entire foot shifted laterally only after the deltoid ligament transection (P = 0.016). In intact feet and short and long plantar ligament conditions, posterior tibial tendon unloading caused a medial shift in hindfoot force (P = 0.039 and P = 0.016, respectively). INTERPRETATION: This study demonstrates the posterior tibial tendon plays a critical role in distributing plantar load and minimizing hindfoot overload through various stages of ligamentous failure. While posterior tibial tendon unloading consistently increased hindfoot loading, its influence on medial-lateral balance was variable. These findings support early interventions that preserve or augment posterior tibial tendon function to maintain biomechanical stability of the hindfoot.