Many children with musculoskeletal disorders are at high risk of fracture, yet assessment of their bone mineral density (BMD) by dual energy X-ray absorptiometry (DXA) can be challenging if they have positioning limitations that create safety or technical concerns for standard scan sites such as the whole body or lumbar spine. The Official Pediatric Positions Statement from the International Society of Clinical Densitometry (ISCD) affirmed use of LDF scans in children who are non-ambulatory or if standard scan sites are not feasible or optimal.1 LDF DXA scans may be useful in monitoring bone health in children with musculoskeletal disorders who are at high risk of distal femur fractures. However, reference data are needed to interpret BMD of the LDF in children.

Henderson et al. published sex-specific reference ranges for LDF areal BMD (BMD) based on 256 children (54 % female; 83 % of European ancestry), ages 3 to 18 y.2 Scans were acquired on a pencil beam densitometer. Regression equations were provided for calculating BMD Z-scores. Subsequently, reference ranges published by Zemel et al. were based on a more diverse cohort of 821 children (52 % female; 45 % Black), ages 5 to 18 y, with scans acquired on a fan-beam densitometer.3 Sex-specific reference ranges were developed using contemporary statistical methods4 with separate reference ranges for children who identify as Black vs non-Black.

Prior reference ranges had two major limitations. First, children ages < 5 y were not included in the fan-beam norms. Second, for most other skeletal sites measured by DXA, children with shorter stature-for-age have lower BMD compared to their peers.5 BMD Z-scores adjusted for height-for-age Z-score (HAZ) provide an estimate of the degree to which a low BMD for age Z-score is influenced by short stature. Previous studies have not examined the association of height status with LDF BMD.

To addresses these gaps, we generated smoothed reference ranges for LDF BMD for ages 1 to 18 y. In addition, we examined the association of LDF BMD with HAZ and present a method of estimating the effect of shorter vs. taller stature on LDF BMD. We also examined sex and racial group differences in LDF BMD to understand this metric of bone health more fully.