Introduction Flatfoot deformity (pes planus) involves not only loss of the medial longitudinal arch of the foot, but is also characterized by forefoot abduction, hindfoot eversion, and possible Achilles tendon contracture. Most current cadaveric flatfoot models rely on substantial sectioning of ligaments, which simulates complete rupture, to create the flatfoot deformity. We are unaware of a model that has examined the effects of Achilles tendon contracture on the flat foot. The objectives of this study were to develop and validate a new cadaveric flatfoot model that includes selective ligament attenuation, rather than sectioning, and to determine if Achilles tendon overpull in the model is associated with increased pes planus severity.
Methods Ten fresh-frozen human cadaver feet were used for this IRB-approved study. A custom-designed acrylic loading frame was used to apply compressive forces to the tibia and fibula and tensile forces to the extrinsic tendons of the foot. The Fastrak® electromagnetic motion analysis system was used to measure the normal three-dimensional (3D) orientation of the bones of interest in the unloaded, loaded, and Achilles overpull conditions. A flatfoot model was created by attenuating ligaments involved in pes planus followed by cyclic axial loading with the extrinsic tendons still loaded. Flatfoot 3D bone orientations were acquired in the unloaded, loaded, and Achilles overpull conditions.
Results Changes seen between normal feet and the created flat feet were consistent with those seen in pes planus deformity. The first metatarsal dorsiflexed 4.89° ± 3.59° (p < .0001) and abducted 3.98° ± 3.30° (p < .0001) relative to the talus. The navicular abducted 3.02° ± 2.20° (p < .0001) relative to the talus. The calcaneus everted 4.01° ± 2.66° (p < .0001) relative to the tibia. The talus plantar flexed 2.00° ± 2.37° (p = .0002) and adducted 4.35° ± 2.95° (p < .0001). Achilles tendon overpull resulted in talar plantar flexion (2.17 ± 1.25°, p = .0001), first metatarsal-to-talus dorsiflexion (2.38 ± 1.19°, p = .0015), and navicular-to-talus abduction (0.96 ± 0.87°, p = .028).
Discussion Selective ligament attenuation followed by cyclic axial loading with tendons loaded can create a cadaveric flatfoot model that is consistent with the in vivo deformity. Longitudinal arch depression, hindfoot eversion, talonavicular joint abduction, forefoot abduction, and talar plantar flexion are all common features of pes planus and were all seen in the current model. This study also indicates that the Achilles tendon contracture seen in many patients with flat feet may contribute to the severity of the deformity, particularly in longitudinal arch depression and forefoot abduction.