Osteoclastic activation is the principal mechanism leading to secondary osteoporosis in rheumatoid arthritis.

OBJECTIVE: To use clinical measures and biochemical markers of bone turnover to investigate mechanisms of generalized bone loss in early rheumatoid arthritis (RA).
METHODS: We studied 232 patients with RA of less than 2 years' duration and 72 healthy controls using serial dual x-ray absorptiometry scanning of lumbar spine and hips. Patients attended the clinic for clinical and laboratory assessment with storage of serum, urine, and plasma at each visit. Change in bone mineral density (BMD) was calculated for patients and controls and compared with baseline and mean serial values of bone markers over the same intervals. Serum was assayed for procollagen I carboxyterminal propeptide (PICP) and skeletal alkaline phosphatase (sALP); urine for pyridinoline and deoxypyridinoline corrected for creatinine; and plasma for interleukin 1 (IL-1) and IL-6.
RESULTS: Patients lost bone significantly faster than controls at all sites (p < 0.01 for all). At first visit patients had significantly lower PICP levels than controls (p < 0.05) and sALP correlated with initial BMD in both patients (p < 0.01, r > 0.35, all sites) and controls (p < 0.0001, r > 0.50, all sites). We rescanned 167 patients at one year and 121 patients at 2 years. Mean urinary pyridinoline and deoxypyridinoline levels correlated strongly with BMD change at all sites, were increased in patients with active disease (p < 0.005), and correlated closely with mean C-reactive protein (CRP) (p < 0.005, r > 0.41 for both).
CONCLUSION: This study suggests that osteoclastic activation, rather than suppression of bone formation, is the dominant process leading to bone loss in early RA. Although urinary pyridinoline and deoxypyridinoline were excellent markers of BMD change, CRP was found to be best overall. This provides a rational approach for selecting and treating patients with RA to reduce their established longterm risk of osteoporotic fracture.