| Objective: Ankylosing spondylitis (AS) is a chronic inflammatory disease with a main involvement of the spine and sacroiliac joints, which is associated with HLA-B27. There was a higher incidence of osteoporosis(OP) in AS, but the mechanisms were not entirely clear. The aim of this study is to investigate the mechanism of AS complicating OP through measurement bone mineral density (BMD) and the levels of bone metabolism index in serum bone morphogenetic protein-2 (BMP-2), interleukin-11 (IL-11), platelet derived growth factor (PDGF), bone alkaline phosphatase (BALP) and tartrate resistant acid phosphatase 5b (TRACP5b), and compared with the normal controls.Methods: Thirty AS cases and twenty normal controls were chosen, whose ages were 20 years or older. All the patients included were in accord with the American College of Rheumatology (ACR) 1984 revised classification criteria of ankylosing spondylitis. All patients were never continuously treated with glucocorticoid(GC) or disease modifying antirheumatic drugs (DMARDs). The BMD of lumbar, femoral neck, and greater trochanter of AS patients were measured through dual-energy X-ray absorptiometry (DXA) and the levels of serum BMP-2, IL-11, PDGF, BALP and TRACP5b were determined by avidin biotin peroxidase complex enzyme-linked immunosorbent assay (ABC-ELISA). The clinical data of AS patients were collected such as age, disease duration, erythrocyte sedimentation rate (ESR), c-reactive protein (CRP), platelet(PLT), levels of serum calcium (Ca), phosphorus (P) , alkaline phosphatase (ALKP),PTH, HLA-B27. The AS patients were compared with normal controls in indicators related to bone metabolism. The AS patients were divided into OP group and non-OP group according to the diagnostic criteria of OP, then they were compared with relative factors each other. The correlation between the above-mentioned indexes and clinic indexes such as age, disease duration, activity of disease, PTH was analyzed.All the data were analyzed with SPSS13.0 for Windows statistical software. Quantitative data were expressed as ( x±s). The t or t'test was adopted for comparison between groups. Linear correlation analysis was performed for correlationship. P value<0.05 was considered significant.Results: 1 The demographic details in AS: In the group of 30 patients with AS, the mean disease duration at presentation was (6.78±6.85)yr, with a mean age of (28.3±7.65)yr, and all were males. The HLA-B27 positive AS patients were 26 patients (86.67%). The group of 20 normal controls were males, with a mean age of (30.2±5.2) yr. There were no difference between the AS group and normal subjects in age, gender (P>0.05).2 The BMD condition in AS patients: The incidence of normal BMD, low BMD and OP were 16.67% (5 patients), 46.67% (14 patients) and 36.67% (11 patients) in 30 AS patients. The incidence of OP in lumbar, femoral neck and greater trochanter were 13.33%(4 patients), 0%(0 patients) and 26.7% (8 patients).3 The levels of index related to bone metabolism in serum: The levels of serum IL-11 in AS patients (112.29±88.9) pg/ml, was significantly lower(P<0.05) than normal controls (175.14±60.46) pg/ml. The levels of serum BMP-2, PDGF and TRACP5b in AS patients were(113.08±99.5)pg/ml, (44271.51±7077.98) pg/ml and (1.87±1.4) U/L. And the levels of normal controls were (60.98±48.27) pg/ml, (38506.74±6295.56) pg/ml and (0.96±0.17) U/L. Compared with the normal control, the level of serum BMP-2, PDGF and TRACP5b in AS were significantly higher (P<0.05). There was no significant difference between the levels of serum BALP in AS patients(28.44±14.56) U/L and the normal controls (27.77±6.0) U/L (P>0.05).4 AS patients were divided into two groups: OP group (11 cases) and non-OP group (19 cases) according to the complication with OP or not for further comparison. Compared with non-OP group, the levels of serum BALP, ALKP, PTH, BMP-2, PDGF and TRACP5b in OP group were lower. But there was no statistically significant (P>0.05) except BALP (P=0.017). Compared with non-OP group, the levels of serum IL-11 in OP group were higher, but the difference was not statistically significant (P >0.05). There was no statistically difference (P>0.05) between OP and non-OP group in age, disease duration, inflammatory indicators and the levels of Ca, P (P >0.05).5 According to correlation analysis: The level of BMP-2 was negative correlated with the level of PTH (r=-0.436, P=0.016). The level of serum PDGF was positively correlated with the level of PLT(r=0.4, P=0.028), but negatively correlated with the level of serum P(r=-0.432, P=0.017). The level of BALP was positively correlated with the level of ALKP (r=0.393, P=0.032). The femoral neck and greater trochanter BMD were positively correlated with the level of PDGF(r=0.464, 0.477; P=0.01, 0.008), and the lumbar BMD was negatively correlated with the level of IL-11(r=-0.516, P=0.003). The rest indexes were no linear correlation each other (P>0.05).Conclusions: 1 A great deal of bone loss was found in AS patients, and most of AS were complicated with low BMD or OP. The degree of bone loss was varying in different measurement sites. For a better overall evaluating the BMD, it was necessary to measure BMD in multiple sites and make a comprehensive analysis.2 The low BMD and OP of AS patients were related with the abnormality of indexes related to bone metabolism . It was increased both bone formation and bone resorption at the same time. Especially in bone formation, more pathologically partical excessive production of bone formation was shown.3 The AS patients complicated with OP was caused by multiple factors.
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