Biomechanics of longitudinal arch support mechanisms in foot orthoses and their effect on plantar aponeurosis strain

Objective. The purpose of this investigation was to quantify the longitudinal arch support properties of several types of foot orthosis. Design. An in vitro method that simulated ‘static stance” was used to determine arch support capabilities, with plantar aponeurosis strain implemented as the performance measure. Background. A longitudinal arch support mechanism of an orthosis resists depression of the footʹs arches by transferring a portion of the load to the medial structures of the foot. Since the plantar aponeurosis is in tension when the foot is loaded, a quantifiable decrease in strain should occur with an adequate orthotic arch control mechanism. Methods. A differential variable reluctance transducer was surgically implanted in the plantar aponeurosis of cadaveric donor limb feet (n= 7). Each specimen was mounted in an electromechanical test machine which applied a load of up to 900 N axially to the tibia. The test schedule was divided into seven test conditions: specimen barefoot; specimen with shoe and specimen with shoe and five different orthoses. Results. The University of California Biomechanics Laboratory Shoe Insert and two other foot orthoses significantly decreased the strain in the plantar aponeurosis compared to the barefoot control and were considered effective arch supports (P <0.05). The functional foot orthosis, stock orthosis, and test shoe did not effectively reduce plantar aponeurosis strain. Significant variations of time required to achieve the specified load levels were recorded among the test conditions, indicating the relative cushioning properties of the shoe/ orthosis systems. Conclusions. The patterns of plantar aponeurosis strain observed in cadaveric tests suggest that certain types of orthoses are more effective than others in the support of the footʹs longitudinal arches. It is suggested that to support the longitudinal arches of the foot effectively the medial surface contours of the orthosis must stabilize the apical bony structure of the footʹs arch.