Patient and joint characteristics
During the study period, there were 46 non-systemic JIA patients without uveitis receiving regular follow-up at Dr Yang’s clinics，of which 11 were excluded due to shoulder, axial skeleton, or hip joint involvement. Thirty-five patients were enrolled in this study with a total of 62 visits. Among the 35 patients, 21 were girls and 14 were boys; 13 patients had oligoarthritis (11 patients had persistent oligoarthritis, and 2 had extended oligoarthritis), 15 patients had polyarthritis (4 patients had positive rheumatoid factor (RF) and 11 had negative RF), and 7 patients had enthesitis-related arthritis (ERA). There were 16 patients with one visit, 12 patients with 2 visits, 6 patients with 3 visits, and 1 patient with 4 visits. The mean age on the visiting day was 14.09 years old. The gender ratio of 62 patients on visits was 40:22 (Table 1). Fifteen patients on visits were in remission state (remission on medication), the other 47 patients on visits were in active state. Of patients in active state, 9 were evaluated in the first diagnosis/onset, while the others were followed regularly. As shown in Table 1, 29 patients on visits were treated by non-steroidal anti-inflammatory drugs (NSAIDs), 35 by disease-modifying anti-rheumatic drugs (DMARDs), and 35 by biologics. At each visit, a total of 1 to 12 involved active joints were scanned, which was depended on JIA subtypes and disease status at that time. Finally, a single joint with highest GSPD among all involved active joints was selected as the indicator joint. Therefore, 62 indicator joints were finally recruited for the analysis. Twenty-four joints were derived from JIA patients with oligoarthritis, 29 joints from JIA patients with polyarthritis, and 9 joints from JIA patients with ERA. Among these, 27 were knees, 18 were wrists, 8 joints were elbows, 6 were ankles, 2 were fingers, and 1 was a toe (Table 1).
Disease activity and physical function scores
JIA disease activity was shown as PGA score, while the physical function was presented as CHAQ-DI. The PGA score and CHAQ-DI of 62 visits were 18.77 ± 22.41 and 0.14 ± 0.88, respectively. The PGA score among the JIA subtypes showed no significant difference (F = 2.043, p = 0.139). As can be seen in Figure 2, the disease activity parameter PGA score had a positive correlation with the physical function parameter CHAQ-DI (rho = 0.692), indicating that the status of disease activity evaluated by a physician was consistent with the reported functional disability in patients with JIA.
The MSUS features of indicator joint in different subtypes of JIA
Effusion, synovial hypertrophy, and enthesopathy are main MSUS features of involved joints of JIA [14, 23]. Figure 3 summarized the presence of above 3 features in joints of different subtypes. Of the 62 indicator joints, all joints (29/29) of patients with polyarthritis were characterized by the presence of effusion and synovial hypertrophy. Of the 24 joints from patients with oligoarthritis, 21 (87.5%) and 19 (79.2%) joints were detected with effusion and synovial hypertrophy, respectively. Compared with patients with polyarthritis and patients with oligoarthritis, effusion and synovial hypertrophy were less seen in joints of patients with ERA, 7 of 9 (77.8%) and 6 of 9 (66.7%), respectively. However, enthesopathy was only seen in joints of patients with ERA (2/9) but not in joints of those with polyarthritis (0/29) and oligoarthritis (0/24).
The correlations between MSUS parameters of indicator joints and laboratory data
The mean values of GS, PD, and GSPD of the 62 indicator joints were 1.74 ± 0.89, 0.53 ± 0.82, and 2.27 ± 1.48. The details of 62 indicator joints were shown in the supplementary table. Since chronic inflammation usually leads to leukocytosis, anemia, thrombocytosis, and elevated ESR, CRP, C3, and C4 , laboratory tests including WBC, Hb, PLT, ESR, CRP, C3, and C4 are routinely performed at our clinics to provide another objective parameters for JIA evaluation. The data of the 62 visits showed WBC: 7.93 ± 2.19 × 103/μL, Hb: 12.62 ± 1.67 g/dL, PLT: 340 ± 94 × 103/μL, CRP: 0.53 ± 0.99 mg/dL ESR: 25.22 ± 19.63 mm/hr, C3: 119.3 ± 32.5 mg/dL, C4: 25.4 ± 18.0 mg/dL. We then analyzed the relationship between MSUS parameters (GS/PD/GSPD) and the above laboratory data. As shown in Table 2, GS was weakly correlated with WBC, PLT, C3, and C4. PD had a weak negative correlation with Hb and a weak positive correlation with C4. Moreover, GSPD had a weak positive correlation with WBC and C3, a weak negative correlation with Hb, while had a moderate positive correlation with C4. ESR and CRP, the two common inflammatory parameters, however, were not significantly correlated with GS, PD, or GSPD.
The correlations between MSUS/laboratory parameters and JIA disease status
In each visit, there were 10 objective parameters of one patient with JIA including GS, PD, and GSPD of the indicator joint and 7 laboratory data (WBC, Hb, PLT, ESR, CRP, C3, and C4). Their relationship with JIA disease status that included disease activity (PGA score) and physical function (CHAQ-DI) was further elucidated. The results showed PD, WBC, ESR, CRP, and C4 had a weak positive correlation with the PGA score, while GS and GSPD had a moderate positive correlation with the PGA score. On the other hand, GS, PD, GSPD, CRP, and C4 were weakly correlated with CHAQ-DI. The other parameters had no significant correlations with CHAR-DI (Table 3).