Biomechanical study of adult acquired flatfoot for intervention

Abstract

Adult-acquired flatfoot (AAF) is a common foot deformity characterized by medial longitudinal arch collapse, excessive hindfoot eversion, and pain and intolerance, which was associated with a variety of lower limb symptoms, such as patellofemoral pain, ankle pain, plantar fasciitis, and lower back pain. AAF individuals lack a stable arch support that attenuates shock absorption, gait efficiency and plantar foot fatigue. Posterior tibial tendon dysfunction (PTTD) is the common cause of AAF. Understanding the compensatory mechanism led by PTTD could provide guidance for conservative and surgical treatments. Meanwhile, orthotic and surgical treatments are normally prescribed depending on the AAF stages. Customized foot orthoses (i.e., arch support) have been used clinically to provide stability and relieve pain in early-stage. In stages III and IV, mid/hindfoot arthrodesis are recommended to treat severe foot deformity. These salvage treatments unavoidably impair mid/hindfoot mobility and may lead to foot pain and joint degeneration over time. Clinicians and researchers endeavor to optimize procedures to correct the foot misalignment and alleviate pain, whilst the procedures are normally empirical. Existing literature targeted on the muscle and joint biomechanics of AAF and may not adequately address the internal loading distribution essential to understand and improve the biomechanical performance of treatments with the existing foot models. Therefore, the overall purposes of the study were to investigate the internal biomechanical characteristics of the flatfoot and identify the effects of orthotic and surgical treatments by adopting the musculoskeletal multibody and finite element foot models. The biomechanical study evaluated the effects of tibialis posterior (TP) weakness and foot orthosis on the lower limb mechanics of AAF. Subsequently, we investigated the performance of customized foot orthosis (different arch support heights) and mid/hindfoot arthrodesis. The first study examined the impacts of TP weakness on lower limb mechanics in individuals with foot orthosis via gait analysis and musculoskeletal modelling. The results indicated that TP weakness increased the ankle joint force in the superior-inferior direction but decreased in the anterior-posterior direction. The flexor hallucis longus and flexor digitorum longus forces increased with the decreased TP strength. Foot orthosis significantly reduced the second peak knee force, peak ankle force, and most muscle forces. We believed that TP weakening might cause compensatory muscle activation and attenuated joint load. The orthosis could correct aberrant muscle and joint mechanics in flatfoot individuals with TP weakness. The second study investigated the internal foot biomechanics by reconstructing a muscle-driven foot-orthosis finite element (FE) model from a volunteer with flexible AAF. The model enabled a three-dimensional representation of the plantar fascia and its interactions with surrounding osteotendinous structures. The volunteer walked in foot orthosis with different arch heights (low, normal, and high). Muscle forces during stance were estimated by the multibody model and then applied to drive a FE foot model. The foot FE model was validated by comparing the predicted foot pressures with measurements. The results indicated that peak foot pressures decreased as the arch support height increased. However, peak pressures of midfoot increased during all simulated instants. Meanwhile, the plantar fascia loading decreased by 5% to 15.4% in proximal regions but increased for foot orthosis with high arch support in the middle and distal regions. Although arch support could decrease the peak foot pressure and plantar fascia loading, the excessive arch height may induce high midfoot pressure and loadings at the central portion of the plantar fascia. The third study evaluated the influence of mid/hindfoot arthrodeses on the internal foot loading with the developed foot FE. The severe flatfoot model was developed from the flexible flatfoot through the attenuation of ligaments and the unloading of the posterior tibial muscle. The simulations of five mid/hindfoot arthrodeses (subtalar, talonavicular, calcaneocuboid, double, and triple arthrodeses) and a control condition (no arthrodesis) were performed simultaneously in the detailed foot multibody dynamics model and FE model. Muscle forces calculated by a detailed multi-segment foot model and ground reaction force were used to drive the FE model of the foot. The internal foot loadings among the control and those arthrodeses conditions were compared. The results indicated that the navicular heights in double and triple arthrodeses were higher than that in other procedures, while the subtalar arthrodesis had the smallest values. Compared to the control condition, five mid/hindfoot arthrodeses reduced the peak plantar fascia stress. However, double and triple arthrodeses increased the peak medial cuneo-navicular joint contact pressures and peak foot pressures as well as the metatarsal bones stresses. The proposed computational platform can predict the biomechanical factors that lead to the postoperative symptoms in patients with flatfoot, thus assisting the optimization of the surgical procedures. In conclusion, this study adopted musculoskeletal multibody and FE models to investigate the lower limb mechanics and internal foot mechanics of AFF. This study identified the compensatory muscle activation and attenuation of the joint load caused by TP weakness that might be restored by the orthosis. Meanwhile, a muscle-driven flatfoot FE model was developed and validated. The proposed model was adopted to investigate the effects of orthosis insoles and five mid/hindfoot arthrodeses on the internal foot mechanics of AAF. The developed foot models could provide a simulation platform that can perform pathomechanics analysis, orthotic insole optimization and preoperative analysis for surgical treatment for the flatfoot.