Axial and torsional stiffness of pediatric prosthetic feet

Prosthetic stiffness likely affects the walking biomechanics of toddlers and children with leg amputations, but the actual stiffness values for prostheses are not reported by manufacturers or in standardized testing procedures.

We measured axial (k_A) and torsional (k_T) stiffness from four brands of pediatric prosthetic feet (Trulife, Kingsley Mfg. Co., TRS Incorporated, and College Park Industries) over a range of foot sizes.

We applied forces and torques onto prostheses with a materials testing machine that replicated those exhibited in vivo by using the kinetics measured from four non-amputee toddlers (2-3years) during walking.

Across brands, k_A averaged 35.2kN/m during heel loading, was more stiff during midfoot loading (121.8kN/m, P<0.001) and less stiff during forefoot loading (11.8kN/m, P=0.013). k_A was similar across brands with no statistically significant effect of prosthetic foot size, with the exception of the TRS feet. Plantarflexion torsional stiffness (k_T1), was not statistically different across brands. For every 1cm increase in foot size, k_T1 increased 0.16kN·m/rad (P<0.001). College Park prostheses had 4.54kN·m/rad lower dorsiflexion torsional stiffness (k_T2) (P<0.001) compared to other brands. For every 1cm increase in foot size, the k_T2 applied on the foot increased 0.63kN·m/rad.

The axial and torsional stiffness testing methods are reproducible and should be adopted by prosthetic foot manufacturers. Axial and torsional stiffness values of commercially available prosthetic feet should be publically reported to health practitioners to ensure evidence-based decisions and meet the specific needs of each patient with a leg amputation.