Reliability of a novel in vivo optical strain assessment of carbon fiber custom dynamic orthoses.

BACKGROUND: Ankle-foot orthoses (AFO) design, including geometry and material selection, is tailored to meet specific clinical objectives. Understanding AFO mechanical behavior may provide insight into the effect of design considerations on the functional benefits to AFO users. Mechanical characteristics of AFOs are primarily assessed using strain gauges or computational modeling, both of which have limitations. OBJECTIVES: Determine the test-retest reliability of weight-bearing carbon fiber dynamic AFO (CDO) surface strain measurements obtained using digital image correlation (DIC). STUDY DESIGN: Prospective, randomized, cross-sectional reliability study. METHODS: CDO displacement and surface strain were measured at three strut locations using DIC while 30 participants deflected the CDO into dorsiflexion at three time points. Test-retest reliability of strain measurements was assessed by intraclass correlation coefficient (ICC), minimum detectable change (MDC), and percentage MDC values. RESULTS: Minimum detectable change and ICC values were between 0.005%‒0.010% and 0.14‒0.76, respectively. Intraclass correlation coefficient were generally higher at more distal strut locations and larger ankle dorsiflexion angles, with a mean ICC across locations of 0.55. Proximal strain was 31%‒59% lower than strain in middle and distal regions, respectively. Lower absolute strain values in the proximal location resulted in higher average relative measurement error (12.2%) compared to the distal and middle locations (4.4% and 6.5%, respectively). CONCLUSIONS: Three-dimensional digital image correlation can provide reliable estimates of CDO surface strain, supporting its use for evaluating orthosis mechanical behavior. Better understanding of in vivo strain behavior across the CDO surface may assist with future device design and prescription to improve functional outcomes.