Mechanical properties of a spring-hinged floor reaction orthosis

Abstract

Introduction: For children with cerebral palsy (CP) whose gait pattern is characterized by excessive knee flexion in midstance, a Floor Reaction Orthosis (FRO) is commonly prescribed. FROs are very stiff andaimto counteract excessivekneeflexion inmidstance, by shifting the ground reaction force anteriorly. Although an FRO is effective in this respect [1], it impedes plantar flexion in preswing, therebyobstructingpush-off power. A spring-like FROcouldpotentially be more beneficial, since it can store energy at the beginning of the stance phase, being returned in preswing, thereby creating ankle push-off power. Literature shows that energy cost of walking with a typical spring-like Ankle-Foot Orthosis can be minimized by selecting an optimal stiffness [2]. Recently, a new type of hinge with adjustable springswas designed, which can be integrated into an FRO. However, exact knowledge of themechanical properties of the hinge is lacking. This study aims to quantify the mechanical properties of this new hinge build within an FRO. Patients/materials and methods: One test FRO with an integrated 14mm Neuro Swing® ankle joint (Fior & Gentz, Germany) was constructed.Weusedfive springswithin thehinge,withdifferent stiffnesses. The spring’s mechanical properties were measured with BRUCE [3]. Each spring was fully compressed and released slowly (i.e. a few seconds for the whole range of motion (ROM)) three times, while the hinge angle and the exerted net moment were continuously measured [3]. ROM (deg) (i.e. elastic range), stiffness (Nm/deg) (i.e. slope of the linear fit of the relation between angle and net joint moment in the ROM), threshold (Nm) (i.e. exerted moment at the start of ROM), and hysteresis [%ESTOR] (i.e. storage (ESTOR) minus release (ERLS) of energy) were averaged for each spring. ESTOR was defined as the surface underneath the compression phase (upper line) and ERLS as the surface underneath the release phase (lower line). Results Discussion and conclusions: Our results indicate that the five springs seem to be useful in optimizing orthoses with respect to energy cost of walking in children with CP. Hysteresis, probably mainly caused by friction of the spring in the shaft of the hinge, is present in all springs, resulting in a loss of energy that potentially could be used for ankle push-off. Nevertheless, FROs with integrated springs might still be more beneficial compared to conventional FROs, in which no energy can be stored at all. However, it remains unknown whether the stiffness of the springs is sufficient to counteract knee flexion in stance. This will be subject of further research to investigate the effects of the mechanical properties of the springs on gait in CP.