OBJECTIVES--To compare serum markers of bone formation with the urinary excretion of pyridinium crosslinks (PYR) as a possible measure of bone and cartilage degradation which would detect changes in bone metabolism in patients with ankylosing spondylitis (AS) and to relate them to influences of inflammatory disease activity, and to treatment. METHODS--In 62 patients with AS, serum osteocalcin, alkaline phosphatase (ALP), and skeletal ALP isoenzyme levels were evaluated concurrently in comparison with urinary excretion of pyridinium cross links and were compared with values in 50 healthy controls. RESULTS--Osteocalcin concentrations in AS patients were in the middle normal range (3.5 (SD 1.2) ng/ml) and did not differ significantly from those in control subjects (4.2 (1.3) ng/ml); the same was true for ALP and skeletal ALP isoenzyme fraction (AS: ALP 149 (50.3) U/l, skeletal ALP 12.8 (4.1) micrograms/l; controls: ALP 133 (25.2) U/l, skeletal ALP 11.9 (4.3) micrograms/l). The urinary levels of PYR in AS (51.2 (25.2) nmol PYR/mmol creatinine) were significantly increased compared with controls (33.9 (12.4) nmol PYR/mmol creatinine (p < 0.001)). In the AS group there was a positive correlation between urinary excretion of PYR and inflammatory disease activity (erythrocyte sedimentation rate (ESR)) (r = 0.6, p < 0.0001) and C reactive protein (CRP) (r = 0.3, p = 0.02), but no significant correlation was found with ESR, CRP, and markers of bone formation. CONCLUSIONS--Bone metabolism in patients with AS is characterised by normal bone formation and enhanced cartilage/bone degradation, suggesting that impaired bone turnover is pronounced in active disease. The results clearly indicate that this comparison can be used to demonstrate impairment of cartilage/bone metabolism which correlates with disease activity. The data obtained further emphasise the importance of measuring both serum variables and urinary excretion of PYR crosslinks to obtain adequate evaluation of cartilage/bone metabolism in patients with AS.
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