414 Minimum Clinically Important Differences in Quantitative Objective Markers of Physical Function in Patients Undergoing Lumbar Spine Surgery

INTRODUCTION: While patient-reported outcome measures (PROM) are widely used to assess outcomes, they are subjective tools with associated limitations. By assessing objective measures, mobile health technology such as wearable devices, can address these shortcomings. Although the association between objective activity and spine disease severity has been established, precise thresholds of their meaningful change in activity after surgery have not. METHODS: This prospective single-center study recruited patients aged 21 to 85 undergoing elective lumbar/thoracolumbar fusion for degenerative disease. PROMs were collected before and after surgery. At the same time, patients were passively monitored using Fitbit trackers, and data were extracted at the minute level. MCIDs were calculated using the anchor-based method with PROMIS pain interference as an anchor and tested for their ability to predict improvement in six-month outcomes using linear regression. RESULTS: We included 62 patients with a mean (SD) age of 59.6 (12) years and 53% female. MCIDs for daily “step count”, “active time”, and “time with more than 40 steps/min,” were defined. AUCs for optimal thresholds were 0.7, 0.66, and 0.67 respectively. A patient was considered “improved” if his daily step count, active time, and time with more than 40 steps/min remain within 1490 steps/day, 73 min/day and 7 min/day of their preoperative baseline respectively. Classifying our patients based on “time with more than 40 steps/min” and “active time” new cutoffs significantly improved six-month ODI improvement prediction compared to PROM alone. Moreover, the “active time” classification enhanced physical function and pain interference improvement prediction. CONCLUSIONS: Our study defined objective activity measures MCIDs and demonstrated their utility in predicting improvement in disability, pain, and physical function at 6 months after surgery.