Abstract: Objective To observe the ultrasonic features of knee lesions in rheumatoid arthritis (RA) patients and to explore the relationship between the thickness of femoral condylar cartilage (FCC) and clinical features of RA. 
Methods The lateral, middle and medial thickness of FCC was measured by ultrasound in 70 patients with RA, 50 patients with knee osteoarthritis patients (KOA) and 30 healthy controls (HC). Body mass index (BMI), erythrocyte sedimentation rate (ESR), C-reaction protein (CRP), rheumatoid factors (RF), anti-cyclic citrullinated peptide antibody (anti-CCP antibody), and 28-joint disease activity score (DAS28) were recorded,and the differences in laboratory and ultrasonic indexes in each group were compared. The correlation between FCC thickness and clinical features was analyzed by Pearson correlation coefficient test. 
Results ①The BMI of KOA group was higher than that of RA group and HC group (P＜0.05). The thickness of FCC in RA group and KOA group was thinner than that in HC group (all P＜0.05). There was no significant difference in thickness of FCC between RA group and KOA group (P＞0.05). ②The ultrasonic lesions of knee joint in RA group were more diverse than those in KOA group. The positive rates of cartilage lesions, joint effusion and osteophytes in RA group were lower than those in KOA group(all P＜0.05). There was no significant difference in the positive rate of synovial hyperplasia and Baker′s cyst between two groups (P＞0.05). In addition, the RA group had blood flow signal in synovial membrane, bone erosion, loose body, enthesitis and peritendinitis. ③Age, BMI, ESR, CRP, anti-CCP antibody and DAS28 in RA knee swollen/tender group were higher than that in RA knee without swollen/tender group (all P＜0.01), and there was no significant difference in thickness of FCC between two groups (P＞0.05). The lateral thickness of FCC in RA group (≤50 years of age) was thinner than that in HC group (≤50 years of age) (P＜0.05). The lateral thickness of FCC in RA≥5 years group was thinner than that in RA≤2 years group (P＜0.05). ④FCC lateral thickness was negatively correlated with RA duration(r=-0.301, P=0.019). 
Conclusion Multiple ultrasound presentations of RA knee lesions may be helpful in the differential diagnosis of RA and KOA. FCC injury may occur in RA patients regardless of whether they have clinical symptoms in the knee. RA patients younger than 50 years of age have a thinning of the FCC lateral thickness. The FCC lateral thickness is thinner in RA patients with more than 5 years of duration. 

Key words: arthritis, rheumatoid, osteoarthritis, ultrasound, cartilage