Defining the Mechanical Properties of a Spring-hinged Ankle Foot Orthosis to Assess its Potential Use in Children With Spastic Cerebral Palsy

Abstract

A rigid ventral shelf ankle foot orthosis (AFO) may improve gait in children with spastic cerebral palsy (SCP) whose gait is characterized by excessive knee flexion in stance. However, these AFOs can also impede ankle range of motion (ROM) and thereby inhibit push-off power. A more spring-like AFO can enhance push-off and may potentially reduce walking energy cost. The recent development of an adjustable spring-hinged AFO now allows adjustment of AFO stiffness, enabling tuning toward optimal gait performance. This study aims to quantify the mechanical properties of this spring-hinged AFO for each of its springs and settings. Using an AFO stiffness tester, two AFO hinges and their accompanying springs were measured. The springs showed a stiffness range of 0.01-1.82 N · m · deg(-1). The moment-threshold increased with increasing stiffness (1.13-12.1 N · m), while ROM decreased (4.91-16.5°). Energy was returned by all springs (11.5-116.3 J). These results suggest that the two stiffest available springs should improve joint kinematics and enhance push-off in children with SCP walking with excessive knee flexion.