Patient Preference in the Selection of Prosthetic Joint Stiffness

Clinical prescription and tuning of prosthetic legs relies heavily on a trial-and-error paradigm. Patient preference, a formalized representation of what patients want provides an efficient method for identifying appropriate prosthesis mechanics. Unfortunately, a lack of the scientific and technological tools necessary to formally measure and rigorously understand preference has led to limitations in ascertaining its role in the prescription process. In this study, our objective was to understand how preference changes with walking speed and how preference corresponds to clinical metrics of gait performance. We characterized preferred stiffness of the Variable Stiffness Prosthetic Ankle Foot (VSPA Foot) across speeds in persons with below knee amputation. We then assessed metabolic expenditure and performance in the ten-meter walk test at several stiffness values around the preferred stiffness. Preferred stiffness did not correlate with body mass, and was lower at the self-selected treadmill walking speed than at either the fast or slow speeds. There was a significant increase in self-selected overground speed in the ten meter walk test at stiffness values at or above the preferred stiffness. Changes to prosthetic ankle stiffness within +/-30% of the preferred stiffness did not significantly affect metabolic expenditure. These results indicate that preferred stiffness may be associated with significant changes in self-selected overground walking speed (p$\lt $0.001), and may not be driven by metabolic optimality (p$\gt $0.93). This outcome motivates future work to identify other potential drivers of preference, including biomechanical correlates of gait, and highlights the potential value of formally incorporating patient preference into the clinical prescription process.