Clinical Validation and Outcome Measures From Bend Ease: A Novel, Sensor-Based Digital Measurement Tool for Assessing At-Home Morning Stiffness and Spinal Range of Motion in Axial Spondyloarthritis

INTRODUCTION: To evaluate the accuracy, reliability, and usability of Bend Ease, a novel smartphone-based digital health technology (DHT), which objectively self-measures spinal range of motion (SRoM) and remotely assesses morning stiffness. METHODS: This phase 1 study involved healthy volunteers (HV) and patients with axial spondyloarthritis (axSpA). Participants used Bend Ease by placing a phone against their chest during a forward-flexion bend, and the application collected and processed accelerometry data to measure bend angle in both clinical and at-home settings. Bend Ease measurements were compared to the video-based method (gold standard) and functional ability questionnaires. Impaired bending for patients with axSpA relative to HV was most pronounced upon waking (65.3° versus 88.3°, P < 0.001), with increasing bend angle improvements observed for patients with axSpA at later time points (71.0° and 75.8° at 30 min and 1 h after waking, respectively). Waking bend angle correlated with self-reported morning stiffness and functional ability scores. A minimum clinically important difference in bend angle of 14 degrees was established for patients with axSpA, providing a benchmark for improvement. Bend Ease demonstrated robust test-retest reliability, and participants reported high usability. CONCLUSIONS: Bend Ease is an accurate, reliable, and user-friendly tool for assessing SRoM. As the first DHT to objectively evaluate morning stiffness upon waking, Bend Ease provides valuable assessments of spinal mobility when it is most impaired.