Investigation of socket reactions from transtibial prosthesis malalignment

Abstract

The effects of transtibial prosthesis malalignment on transtibial prosthetic socket reactions were measured in terms of axial force, sagittal moment and coronal moment. In addition, complementary study of amputee perceptions of their alignment and balance was undertaken. Mathematical predictive models of prosthetic alignment were created. Alignment perturbations of angulation and translation were induced from the baseline of a nominally "aligned" prosthesis as established by an experienced prosthetist. The Prosthesis Alignment Instrument, (PAI) was used to measure and affect sagittal and coronal changes in angular (+-3o and +-6o) and translational (+-5mm and +-10mm) alignment. An integral force transducer measured axial force, sagittal moment and coronal moment socket reactions. A wireless data acquisition system transmitted 100Hz PAI socket reaction data from consecutive steps, typically 15 meters at a time. Seventeen randomized alignment conditions were recorded with each of 11 subjects for a total of 187 trials. Trial identifiers, time, total axial force, sagittal socket reaction moment and coronal socket reaction moment were recorded to computer files at a sampling rate of 100Hz. Analysis by repeated measures ANOVA showed that socket malalignment had very significant effect on socket reactions in both the sagittal and coronal planes (p<0.000l, two-sided) with a maximum coefficient of determination (r2) of 0.9261. Subject perceptions were analyzed for Sensitivity and Specificity with regard to alignment perturbation and at least one of the Visual Analog scales was associated significantly with each malalignment (Fisher's Exact Test, p<0.05, two-sided). Discrete non-linear algebraic modeling of alignment was possible with prediction (r2) ranging from 0.8998 for coronal translations, and 0.9179 for coronal angulations to 0.8446 for sagittal angulation and 0.8498 for sagittal translations. Mean absolute prediction errors of models derived equated to only 1.13o of angulation and 1.96 mm of translation. With improved understanding of socket reactions as they relate to alignment it is hoped that future work will be able to establish objective criteria for optimal transtibial prosthesis alignment.