Knee-ultrasound: reference values for the amount of joint fluid and synovial appearances in healthy children and adolescents

BackgroundWith increasing ultrasound use, data for the normal amount of joint fluid and synovial appearances are needed.ObjectiveTo establish reference values for knee-ultrasound in healthy volunteers by age. To examine the association of joint fluid and synovial appearances with age, sex, and time spent in sports activities.Materials and methodsProspective, cross-sectional, including 3-17-year-old volunteers. Examinations were performed by one of two experienced radiologists after meticulous standardisation of the technique. Knees were examined in 30 degrees flexion, in extension and during provocation.ResultsOne hundred twenty-seven volunteers (67 females), median age 10.9 years (percentile 2.5th-97.5th: 3.4-17.2), were included. With 30 degrees knee flexion, the median amount of suprapatellar recess joint fluid (sagittal view) was 0.5 mm for females and 0.9 mm for males (P = 0.014), increasing to 1.9 mm and 2.4 mm (P = 0.031), respectively, during provocation. The amount of fluid during provocation increased with age for both sexes (P = 0.007 females; P = 0.012 males) and by time spent in sport-activities (P = 0.003). Further, there was a positive association between the presence of joint fluid speckles and sport-activities (P = 0.039). Median double-layered synovium was 1.5 mm in 3-6-year-olds, increasing to 2.1 mm in 7-9-year-olds and 2.0 mm in 10-17-year-olds (P = 0.024). No synovial Doppler signal was seen. Observers' agreement was excellent, with ICC values >= 0.9.ConclusionThe majority of healthy children have measurable knee joint fluid, more so when examined with a flexed knee and under provocation. Speckles within the joint fluid are seen in one-fourth, especially in males, and the presence of speckles and joint fluid amount are associated with sports-activity.