  Progressive collapsing foot deformity results from degeneration of the
ligaments and the posterior tibial tendon (PTT). Our understanding of the
relationship between the failures of them remains incomplete. We sought to
improve this understanding through computational modeling of force in these
soft tissues. The impact of PTT and ligament tears on force changes in the
remaining ligaments was investigated using a previously validated finite
element model of the foot. The ability of the PTT to restore foot alignment in
a collapsed foot was evaluated by increasing the PTT force in a foot with
attenuated ligaments and comparing the alignment angles to the intact foot.
Rupture of any one of the ligaments led to overloading the remaining ligaments,
except for the plantar naviculocuneiform, first plantar tarsometatarsal, and
spring ligaments, where removing one led to unloading the other two. The
attenuation of the plantar fascia, long plantar, short plantar, and spring
ligaments significantly overloaded the deltoid and talocalcaneal ligaments.
Isolated PTT rupture had no effect on foot alignment, but did increase the
force in the deltoid and spring ligaments. Increasing the force within the PTT
to 30% of body weight was effective at restoring foot alignment during quiet
stance, primarily through reducing hindfoot valgus and forefoot abduction as
opposed to improving arch height. The attenuation of any one ligament often
leads to overload of the remaining ligaments which may lead to progressive
degeneration. The PTT can maintain alignment in the collapsing foot, but at an
increased load which could lead to its injury. Early intervention, in the form
of ligament repair or reconstruction, might be used to prevent the progression
of deformity. Moreover, strengthening the PTT through therapeutic exercise
might improve its ability to restore foot alignment.
