Finite element simulation on posterior tibial tendinopathy: Load transfer alteration and implications to the onset of pes planus.BACKGROUND
 Posterior tibial tendinopathy is a challenging foot condition resulting in pes planus, which is difficult to diagnose in the early stage.  Prior to the deformity, abnormal internal load transfer and soft tissue attenuation are anticipated.  The objective of this study was to investigate the internal load transfer and strain of the ligaments with posterior tibial tendinopathy, and the implications to pes planus and other deformities.


 A three-dimensional finite element model of the foot and ankle was reconstructed from magnetic resonance images of a 28-year-old normal female.  Thirty bones, plantar fascia, ligaments and tendons were reconstructed.  With the gait analysis data of the model subject, walking stance was simulated.  The onset of posterior tibial tendinopathy was resembled by unloading the tibialis posterior and compared to the normal condition.


 The load transfer of the joints at the proximal medial column was weaken by posterior tibial tendinopathy, which was compromised by the increase along the lateral column and the intercuneiforms during late stance.  Besides, the plantar tarsometatarsal and cuboideonavicular ligaments were consistently over-stretched during stance.  Particularly, the maximum tensile strain of the plantar tarsometatarsal ligament was about 3-fold higher than normal at initial push-off.


 Posterior tibial tendinopathy altered load transfer of the medial column and unbalanced the load between the proximal and distal side of the medial longitudinal arch.  Posterior tibial tendinopathy also stretched the midfoot plantar ligaments that jeopardized midfoot stability, and attenuated the transverse arch.  All these factors potentially contributed to the progress of pes planus and other foot deformities.
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