Background: With increasing ultrasound use, data for the normal amount of joint fluid and synovial appearances are needed.
Objective: To 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 methods: Prospective, 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° flexion, in extension and during provocation.
Results: One hundred twenty-seven volunteers (67 females), median age 10.9 years (percentile 2.5th-97.5th: 3.4-17.2), were included. With 30° 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.
Conclusion: The 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.
Keywords: Knee; Musculoskeletal ultrasound; Suprapatellar recess; Synovial fluid; Synovial membrane.
© 2025. The Author(s).