Acoustic Emissions from Loaded and Unloaded Knees to Assess Joint Health in Patients with Juvenile Idiopathic Arthritis.

Objective We studied joint acoustical emissions in loaded and unloaded knees and investigated their characteristics as digital biomarkers for evaluating knee health status during the course of treatment in patients with juvenile idiopathic arthritis (JIA). Methods Knee acoustic emissions were recorded from 38 study participants including 20 subjects with JIA and 18 healthy controls. Ten of the subjects with JIA had a follow-up recording, 36 months after initial measurements. Each subject performed 10 repetitions of unloaded flexion/extension (FE) and multi-joint weighted movements involving knee and hip flexion/extension (squat) exercises. The recorded acoustical signals were divided into movement cycles and processed to extract 72 features, and a novel algorithm was developed to detect and exclude the windows with artifacts such as loose microphone contact or rubbing noise. Signal features for FE and squat exercises were down-selected based on three different criteria to train logistic regression classifiers, which were later used to output the knee health scores for all subjects using leave-one-subject-out cross-validation. The classifiers achieved a peak cycle-wise accuracy of 84% and a subject-wise accuracy of 92%. Finally, classifiers were trained with healthy and pre-treatment data, and were then used to predict the knee scores of post-treatment data for the same patients with JIA who had a follow-up recording. The knee health score represents the probability of having JIA in a subject (0 for healthy and 1 for arthritis). Results Knee health score outputs from the classifiers were always lower for the post-treatment cases compared to pre-treatment values, agreeing with the clinical findings that the treatment was successful for all subjects. Regarding loaded versus unloaded knee scores, the squats achieved a higher score on average compared to FEs for all groups. Conclusion In healthy subjects with smooth cartilage, the knee health scores of squat and FE were similar indicating that the vibrations from the friction of the articulating surfaces does not significantly change by the loaded state of the joint. However, in subjects with JIA, the scores of squats were higher than the scores of FEs, revealing that these two exercises contain different, possibly clinically relevant, information that could be used to further improve this novel assessment modality in JIA.