Bone density in juvenile idiopathic arthritis (JIA) is largely normal whereas geometric parameters of bone are abnormal. The most prominent changes are a reduction in muscle cross sectional area (CSA) and muscle force. The aim of this study was to assess the evolution of these changes throughout the course of the disease.
Twenty-five JIA patients were assessed by peripheral quantitative computed tomography longitudinally with a median of 48 months between measurements. At the non-dominant forearm, parameters of bone density and geometry as well as muscle CSA were recorded. The strength-strain index (SSI) as an indicator of bone strength was determined.
Muscle CSA improved from a median Z-score of -1.94 to -1.10 at follow-up. Cortical thickness increased from -1.55 to -0.97 whereas marrow area remained enlarged at 0.96 vs 1.05. Cortical density remained normal at 0.34 vs 0.69 and trabecular density improved from -0.75 to -0.36. The SSI increased from -0.79 to -0.13.
JIA patients show some improvement in muscle CSA and an increase in cortical thickness. The marrow area remains enlarged but by increasing the cortical thickness, area and diameter, bone strength increases. These geometric adaptations, for the first time shown in this study, nevertheless represent a disturbance in skeletal development. In addition to efficient disease control, training modalities to improve muscle strength and subsequent bone development have to be included in therapeutic approaches.