Background/Purpose: Juvenile idiopathic arthritis (JIA) is among the most common chronic diseases of childhood.  Arthritis is a potentially disabling disease that can result in ongoing pain and inflammation. Although chronic intra-articular inflammation is common to both children and adults with chronic arthritis there are, apart from onset age, clinical characteristics that further distinguish chronic arthritis in pediatric and adult populations.  Collagen induced arthritis (CIA) is a model of inflammatory joint disease.  For the first time, this project demonstrates that juvenile and adult rats respond differently to the inflammation and pain resulting from CIA. 

Methods: Juvenile (5 wks old) and adult (13 wks old) male Wistar rats were immunized with an emulsion of bovine type II collagen and incomplete Freund’s adjuvant.  Naive juvenile and adult rats were included as controls. Paws were scored on a scale of 0 (normal paw) to 4 (disuse of paw).  Animals were monitored for weight changes.  Two weeks following development of arthritis, animals were euthanized and hindpaws were collected and imaged with micro-computed tomography (micro-CT), magnetic resonance imaging (MRI) and ultrasonography.  Images were evaluated qualitatively for the degree of soft tissue swelling, joint changes and bone destruction.

Results: Juvenile rats developed arthritis significantly earlier than their adult counterparts (p=0.0126) and experienced less erythema and edema of the affected paws (p<0.0001).  Micro-CT of affected hindpaws demonstrated that adult CIA rats exhibited widespread boney changes over the phalanges, metatarsals and tarsus while juveniles had more localized damage of phalanges and metatarsals.  MRI of hindpaws confirmed extensive edema and inflammatory insult in the adult CIA rats compared to their juvenile counterparts.  Ultrasound of distal phalanges of the juvenile CIA animals showed tissue edema not evident on inspection. 

Conclusion:   This research is the first to identify differences between juvenile and adult rats with CIA and provides evidence that the disease process may differ between the two age groups.  This model will allow for studying arthritis pathogenesis and treatment interventions in juvenile versus adult populations.