Do prosthetists compensate for improper foot stiffness by adjusting alignment?

Assess how prosthetic foot stiffness influences prosthetists' alignment decisions.

A double-masked, order-randomized, repeated measures study was conducted at Northeastern University. Seven certified prosthetists and seven individuals with limb loss (patients) were randomly paired. Each patient walked on five randomized prosthetic foot stiffnesses; pairs were masked to stiffness and instructed not to communicate. Prosthetists aligned each foot using only the distal adapter. Primary outcomes included sagittal plane ankle alignment, rollover shape, and stiffness preferences.

Five pairs showed increasing ankle dorsiflexion with increased stiffness; two showed the opposite. Linear mixed-effects modeling found no significant relationship between stiffness and alignment (β = 0.29°/category, R² = 0.07, p = 0.103). Excluding one outlier pair revealed a significant positive relationship (β = 0.41°/category, R² = 0.14, p = 0.037). No rollover shape trend was observed. Prosthetists and patients found multiple stiffness categories acceptable; only two of seven pairs agreed on a preferred stiffness.

Prosthetists demonstrated substantial between- and within-prosthetist variability in alignment responses to foot stiffness, with no consistent compensatory strategy across pairs. Combined with limited clinical stiffness trialing, this unpredictability may contribute to suboptimal prescriptions and underscores the need for improved stiffness selection tools.