Background: Repetitive trauma and overuse of the plantar aponeurosis are believed to be causal factors of plantar fasciitis. Therefore, it is important to know how an orthosis influences loading of the plantar aponeurosis. The aim of this study was to quantify strain in the plantar aponeurosis in cadaveric feet with the use of various combinations of orthotic wedges.
Methods: An in vitro test that simulated static stance was used to determine the loading characteristics of the plantar aponeurosis. A differential variable reluctance transducer was operatively implanted into the plantar aponeurosis of nine fresh-frozen cadaveric lower limbs. Each specimen was mounted in an electromechanical testing machine that applied an axial load of as much as 900 newtons to the tibia. Eight different combinations of test conditions, in which wedges (each with a 6-degree incline) were or were not positioned under the medial and lateral aspects of the forefoot and hindfoot, were evaluated, with the plantigrade foot used as a neutral control.
Results: Each of the test conditions that involved a wedge under the forefoot resulted in strain that was significantly different from that in the neutral control. A wedge under the lateral aspect of the forefoot decreased strain in the plantar aponeurosis, and a wedge under the medial aspect increased strain (p < 0.05). The test conditions that involved a wedge under the hindfoot but not under the forefoot resulted in strains that were not significantly different from those in the neutral control (p > 0.05).
Conclusions: A wedge under the lateral aspect of the forefoot transmits loads through the lateral support structures of the foot, locking the calcaneocuboid joint and decreasing strain in the plantar aponeurosis. A wedge under the medial aspect of the forefoot transmits loads through the medial support structures of the foot, which produces a truss-like action that increases strain in the plantar aponeurosis.