Bone Density, Structure and Strength in Canadian Children and Youth with Juvenile Idiopathic Arthritis: The LEAP Study (Linking Exercise, Activity, and Pathophysiology in Canadian Children with Arthritis)

Authors:  CM Duffy, A Baxter-Jones, L Ward, H Macdonald, H McKay, M Erlandson, AM Huber, S Benseler, M Gibbon, J Guzman, LB Tucker and the LEAP Study Investigators.

Background/Purpose:   Children and youth with JIA have lower levels of physical activity (PA) than healthy children; lower levels of PA lead to bone mass deficits. The LEAP study is a prospective study, conducted at 12 centres in Canada, of children and youth with JIA, aimed at describing the trajectory of bone development and its relationship to disease factors and physical activity.  Here, we compare bone density, structure and strength parameters of children and youth with JIA at study enrollment to reference standards and identify differences between newly and previously diagnosed subjects.   

Methods:   Inclusion criteria were confirmed diagnosis of JIA, age 10-17 years, and informed consent. Patients had either newly diagnosed JIA (< 6 months since diagnosis) or previously diagnosed JIA (> 6 months after diagnosis).  Using a peripheral quantitative computed tomography (pQCT) scanner, a 2.3 mm slice was obtained at the mid-shaft of the tibia (50% site; proximal to the distal tibial end plate).  Total bone area (Tt.Ar, mm²), cortical density (Ct.BMD, mg/cm³) and polar strength-strain index (SSI_p, mm³) were calculated. Appropriate reference standards were used to calculate z-scores (ZTt.AR, ZCt.BMD and ZSSI_p) and described as mean z score ± SD. 

Results:   Data was available on 124 subjects (66.1 % female; mean age 13.3 ± 2.2 yrs); 48 newly diagnosed, 76 previously diagnosed.  JIA patients, overall, had significantly lower bone area and strain index – ZTt.AR -0.67 ± 0.09,  ZSSI_p -0.64 ± 0.09  but significantly greater Ct.BMD; ZCt.BMD 0.30 ± 0.10, than reference children.  Bone parameters for the different JIA groups are shown (Table).  There were no significant differences in bone area or strain index between the groups; however, the newly diagnosed JIA group had significantly higher cortical density. Numbers are the mean and standard deviation for age, sex and ethnicity adjusted Z-scores.

Conclusion:   Children with JIA had lower tibial bone area and polar strength-strain index than appropriately matched peers but had higher cortical density. While no differences in total area or strain index were found between JIA groups, the newly diagnosed group had greater cortical density. This higher cortical density may reflect lower bone turnover, as this has been observed in other inflammatory diseases of childhood.  However, it is also possible that these initial cross-sectional results suggest that cortical density may be compromised by disease over time.  Longitudinal analysis of this cohort will provide a better understanding of cortical density and other included measures over time.