Increased Achilles tendon force required to achieve heel-lift in cadaveric model of midfoot instability - A cadaveric study.

The progressive ligament instability associated with flatfoot deformity may result in altered foot biomechanics and represent a risk factor for the development of Achilles Tendinopathy (AT). The aim of this study was to determine whether an increased tendo-Achilles (TA) force was required to generate a set heel lift in a cadaveric flatfoot model. Thirteen fresh (previously frozen) cadavers, with no previous foot pathology and sectioned from the knee were mounted on a testing frame. The frame maintained a dorsiflexed ankle position, to simulate the terminal stance phase of gait, and allowed a constant axial load to be applied through the tibia. The heel was elevated 3 cm by a measured traction force through the TA to a pre-determined laser mark. The flatfoot model was created by sequential sectioning of the medial arch supporting structures, creating progressive midfoot instability and flatfoot deformity, from mild to severe. The force (N) required to generate heel lift was recorded at each stage of the development of the flatfoot. Initially, following Spring ligament and Tibialis Posterior sectioning, the force required to generate heel lift decreased. However, as the flatfoot deformity progressed the force required for heel lift increased. A severe flat foot deformity, characterised by medial ray destabilisation significantly increased the force required to generate a heel lift (5.1 ± 7.2 N, p = 0.02) as did sectioning of the (ii) short and long plantar ligaments (5.5 ± 8.7 N, p = 0.03). Progressive ligament failure associated with acquired flatfoot deformity may be a risk for AT overload due to increased TA force required for heel lift, resulting in TA overload symptoms. The paradoxical decrease in force required to generate heel lift with isolated SL sectioning has not been previously described. This biomechanical study raises the possibility that interventions that support the medial longitudinal arch, such as orthotics, may protect against TA overload.