Accuracy amidst errors: Evaluating a commercially available wearable sensor system and its associated calibration procedures for monitoring sagittal knee motion in patients undergoing total knee arthroplasty

Commercially available wearable sensors monitoring knee range of motion (ROM) are gaining traction in orthopaedics, but few studies validate against optical motion capture (MOCAP) in total knee arthroplasty (TKA) patients. Furthermore, wearable calibration is essential for accurate measurements, yet few investigations evaluate calibration and ROM accuracy. This study assessed one commercial wearable sensor system's calibration (goniometric versus MOCAP) and sagittal knee angle computation accuracy in TKA patients during activities.

Twenty TKA patients were recruited (5 lost to follow-up). Following a sensor tutorial (MotionSense, Stryker, Mahwah, NJ), participants self-applied sensors for pre-TKA data capture. TKA was then performed by one surgeon followed by identical post-TKA data captures. MOCAP and wearable sensor data were collected during activities. MOCAP sagittal knee angles (θ_MOCAP) were compared to two wearable sensor knee angles: 1) θ_CalGoni = goniometric calibration, 2) θ_CalMOCAP = MOCAP calibration. Two-way ANOVAs evaluated the impact of time (pre-TKA vs. post-TKA) and calibration type (goniometry vs. MOCAP) on calibration angles and wearable sensor error. Variance equality tests compared pre-TKA vs. post-TKA and goniometric vs. MOCAP calibration.

No significant differences were noted pre-TKA vs. post-TKA. Calibration angles differed significantly with goniometry yielding significantly more error than MOCAP. MOCAP calibration reduced error below clinically acceptable levels (<5°) during activities and with significantly less error variance.

MOCAP calibration significant improved accuracy of knee angle computations to acceptable levels (<5°). Accordingly, these wearables are suitable for continuous knee ROM monitoring after calibrating with correct angles, Future studies should investigate specific activities and sensor misplacement on angle measurements.