Stiffness-Tailored Composites in Lower-Limb Load-Bearing Devices: A Review

Abstract

Mechanical stiffness can be controlled at the device level with composite materials, especially carbon-fiber-reinforced polymers (CFRPs) and polyetheretherketone-based composites, which is a revolution in the design of lower-limb load-bearing devices. This review discusses the engineering of stiffness-tailoring strategies across three types of devices: ankle-foot orthoses (AFOs), prosthetic feet/running blades, and fracture fixation plates; validation of the engineering; and the translation of the engineering to clinical and biomechanical outcomes. In each of the three domains, a design challenge arises: the requirement to optimise the stiffness of the device to the mechanical requirements of the target tissue, loading regime and patient population, while maintaining its structural integrity. For AFOs, stiffness tuning refers to the compromise between maintaining push off power and stabilizing the joints, and an optimal stiffness window has been shown to result in a reduction of the metabolic cost of transport. In prosthetic feet and blades, the qualitative distinction of gait adaptations depends on the degree of regional stiffness difference between the hindfoot and forefoot, and energy-storage-and-return performance is related to walking speed and predictably depends on composite architecture. For the fixation of fractures, intentional stiffness reduction of the plate to the bone (e.g. CFRP, PEEK or functionally graded composites) helps reduce stress shielding and increases callus formation without compromising construct strength as required for clinical use. The review also highlights key gaps: there is no consensus in the stiffness measurement protocols across different stiffness device types, little evidence of longitudinal assessment of device adaptation and durability, and a lack of an evidence base for patient-specific optimization of stiffness. Future work involves computational stiffness-prescription tools, biodegradable composite fixation plates and easily accessible locally produced ESAR prosthetic feet in under-resourced environments.