Analysis of Bone Strength and Bone Turnover Markers in Ankylosing Spondylitis with Radiological Hip Involvement

BACKGROUND: Limited clinical data are available on bone loss in ankylosing spondylitis (AS) patients with hip involvement, especially for bone strength. The purpose of this study was to analyze bone strength and bone turnover markers in AS patients with hip involvement. MATERIAL/METHODS: The stiffness index (SI) calculated by quantitative ultrasound (QUS) was used to compare the bone strength between patients with AS with radiographic hip involvement (RHI-AS, BASRI-hip ≥2) and those without radiographic hip involvement (WORHI-AS, BASRI-hip ≤1). The Spearman correlation test was used to evaluate the association between SI and bone turnover markers [TP1NP, OC, β-CTx, 25(OH)VD3, and PTH]. RESULTS: RHI-AS (BASRI-hip ≥2) patients accounted for 52.2% (177/339) of all patients. There was no significant difference in most of the basic clinical features between RHI-AS and WORHI-AS patients, except for age and BMI. After adjusting for confounding factors (age and BMI), the stiffness index (SI) of RHI-AS patients was significantly lower than that of WORHI-AS patients (OR(adj)=0.982, 95% CI(adj)=0.968~0.997, P(adj)=0.017). The Z scores calculated by SI were lower in RHI-AS patients (OR(adj)=0.802, 95% CI(adj)=0.679~0.949, P(adj)=0.01). Among the 5 bone turnover markers in the RHI-AS patients, only 25(OH)VD3 had a correlation with SI (rho=0.279, P=0.001). CONCLUSIONS: AS patients have lower bone strength once the disease progresses to include radiologic hip involvement. Treatment of vitamin D deficiency may be an effective way to improve bone strength in AS patients with hip involvement.