Background/Purpose

Patients with ankylosing spondylitis (AS) have high fracture risk. BMD, bone microarchitecture and strength determine fracture risk. However, in AS, DXA-based BMD measurements of the lumbar spine may be falsely normal due to the presence of syndesmophytes. Also, DXA cannot differentiate between trabecular and cortical bone. The effect of AS on bone microarchitecture and strength is also unknown. We assessed bone microarchitecture and strength in patients with AS and compared that with non-AS controls.

Methods

AS was defined by the modified New York criteria. Disease activity of AS was measured by BASDAI, mSASSS, serum ESR and CRP levels. Volumetric BMD (vBMD) and microarchitecture were measured using HRpQCT, and bone strength was estimated using finite element analysis (FEA). Multivariable linear regression was used to analyze the effect of AS on HRpQCT parameters. 

Results

There were 44 cases (82% Caucasian). The mean (+SD) age and BASDAI were 44+2 and 6.3+ 1.8 respectively. Median (IQ) disease duration was 20 (7.3-27.8 years). Twenty-three subjects had mSASSS >0. Four cases (9%) reported a history of fragility fracture. Use of TNF inhibitors (none), bisphosphonates (n=2) and corticosteroids (n=2) was negligible. Mean serum ESR, CRP and SAP levels were 22.0+23.6, 12.6 +15.6 and 96.2 +43.2 IU respectively. In multivariable linear regression models adjusted for age and gender, cases (n=44) had lower vBMD (trabecular, cortical and total), cortical thickness, BV/TV, bone stiffness and stress, and higher cortical porosity and trabecular separation at the radius (Table 1) when compared to non-AS controls (n=85). Tibial vBMD, BV/TV and cortical porosity were also abnormal in cases. But trabecular architecture at tibia was not different between cases and controls. 

Conclusion

This study documents abnormalities of bone structure and strength in patients with AS. Patients with AS had lower volumetric BMD and worse microarchitecture at the trabecular and cortical regions compared to controls. Bone stiffness and stress at the radius and tibia, as estimated by FEA, also tended to be lower in cases than controls. These abnormalities might partly explain the high fracture risk in patients with AS.

Table 1: Multivariable linear regression showing abnormal bone microarchitecture and strength in patients with AS (44 cases and 85 controls).

	Outcome variables	Exposure variable		Covariate 1		Covariate 2
		Cases vs. controls		Age		Gender
Site		Beta	p	Beta	p	Beta	p
Radius	Trabecular vBMD	-.232	.032	-.211	.048	.444	.000
	Cortical vBMD	-.294	.008	-.530	.000	-.143	.092
	Total vBMD	-.351	.003	-.394	.001	.128	.148
	Trabecular number	-.208	.071	-.057	.613	.328	.000
	Trabecular thickness	-.195	.068	-.310	.003	.421	.000
	Trabecular separation	.242	.035	.090	.422	-.318	.000
	BV/TV	-.231	.033	-.210	.049	.443	.000
	Cortical thickness	-.327	.003	-.604	.000	-.019	.822
	Cortical porosity	.215	.029	.661	.000	.298	.000
	Stiffness, K (kN/mm)	-.337	.004	-.254	.028	.136	.129
	Stress	-.337	.004	-.254	.028	.137	.129
Tibia	Trabecular vBMD	-.232	.044	-.103	.359	.328	.000
	Total vBMD	-.271	.019	-.403	.000	.136	.122
	Cortical vBMD	-.246	.019	-.636	.000	-.008	.918
	Trabecular number	-.153	.182	-.040	.722	.333	.000
	Trabecular thickness	-.122	.313	-.076	.521	.101	.276
	Trabecular separation	.213	.067	.050	.660	-.299	.001
	BV/TV	-.233	.043	-.104	.353	.329	.000
	Cortical thickness	-.176	.065	-.719	.000	-.191	.010
	Cortical porosity	.227	.025	.685	.000	.092	.233
	Stiffness, K (kN/mm)	-.209	.073	-.361	.002	.109	.222
	Stress	-.212	.068	-.363	.002	.108	.228

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/compromised-volumetric-bone-density-bone-microarchitecture-and-bone-strength-in-patients-with-ankylosing-spondylitis-high-resolution-peripheral-quantitative-computerized-tomography-hrpqct-based-st/