ESPE Abstracts

Background: The need to understand the relationship between bone, muscle, and fat within the bone-muscle unit has recently gained great prominence. Although high resolution (HR) MRI is a non-invasive imaging modality that can provide this information, there is limited expertise in children and young people.

Objectives: To establish MRI-based normative data for bone, bone marrow adiposity and muscle adiposity in children and young people.

Methods: 3T MRI of distal femur and proximal tibia was performed in healthy subjects aged between 8-29 years to assess trabecular microarchitecture (both sites), cortical bone geometry (femur); muscle area and adiposity (muscle fat fraction, MFF) was assessed at distal femur and ¹H-MRS of lumbar spine (L3) was performed to assess bone marrow adiposity (bone marrow fat fraction, BMFF).

Results: Of the 140 participants, 60 (M:F, 32:28) with a median age of 13yrs (range, 8.1, 23.8) for males and 13.6yrs (8.4, 22.3) for females were scanned at proximal tibia; 80 participants (M: F, 46:34) with a median age of 14yrs (8, 29) and 17.6yrs (9.3, 28) for males and females, respectively, were scanned at the femur. No sex differences were observed in trabecular microarchitecture but there was an inverse association of apparent trabecular bone volume fraction (app BV/TV) with age at tibia in both sexes (r,-0.58 P<0.005) and at femur in females (r,-0.37, P=0.03). Compared to the femur, the tibial scans revealed a higher appBV/TV (0.595 vs 0.550, P<0.005), higher trabecular numbers (appTbN) (1.988 vs 1.853mm^-1, P<0.005) and lower trabecular separation (appTbSp)(0.200 vs 0.239mm, P<0.005). Cortical area and endosteal and periosteal circumferences showed a positive association to age (r,0.49, P<0.005). Although cortical parameters tended to be higher in males than females, the difference did not reach statistical significance. Median BMFF was similar in females and males at 22.3% (9.4, 41.5) and 25.8% (4, 47.1) respectively (NS). Muscle area showed a positive association with age (r,0.76 P<0.005) which persisted when adjusted for height (b,0.39, P=0.001). After accounting for age and height, BMFF showed an independent association with weight (b,0.46, P=0.02). MFF was 3.9% (0.6, 9.9) and 4.5% (0.5, 9.7) in males and females, respectively (NS).

Summary: In addition to providing further insight into the normal relationships between bone, fat, and muscle, these novel data also emphasize the place of MRI as a non-invasive tool for the integrated assessment of musculoskeletal health in the young person.