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POS0126 UTILITY OF THE SUBCHONDRAL BONE ATTENUATION COEFFICIENT OF THE SACROILIAC MARGINS TO DIFFERENTIATE SPONDYLOARTHRITIS AND OSTEITIS CONDENSANS ILII

Differentiating ankylosing spondylitis (AS) from osteitis condensans ilii (OCI) remains challenging for clinicians.The aim of this study was to determine whether Subchondral Bone Attenuation Coefficient of the SacroIliac margins (SBAC-SI) is different in AS, OCI and diffuse idiopathic skeletal hyperostosis (DISH).A monocentric retrospective observational study was performed at a French University Hospital. Patients included were followed for AS, DISH or OCI and underwent CT scan including sacroiliac joint. Patients with tumor lesion of bone or a history of pelvic radiotherapy were excluded. AS and OCI patients were matched with a control of the same age and sex. All scans were acquired on the same CT-scan unit (Somatom 64 definition AS+, Siemens Healthineers, Erlangen, Germany), with a slice thickness of 0.625 mm. In the coronal oblique plane of the SIJ, three slices (anterior, middle and posterior) and four quadrants per joint were defined. Twenty-four identical circular regions of interest (ROIs) (30 mm2), 8 per slice, were manually placed separately subcortical to the SIJ, four on the sacral side and four on the iliac side. The distance between the circle of the ROI and the cortical bone was 2 to 3 mm. An overall score was obtained from the sum of all ROIs. For every ROI, an Attenuation Coefficient was measured and expressed in Hounsfield Unit. The total SBAC-SI score was the sum of the 24 ROI. The sacral and iliac SBAC-SI scores were the sum of the sacral or the iliac ROI.Thirty AS and AS controls, 31 DISH, 29 OCI and OCI controls were included. SBAC-SI score was 9727 (±2430) in the OCI group (p<0.001), 3563 (±1860) in the AS group, 3899 (±1937) in the DISH group, 4224 (±1693) in the AS control group and 5445 (±1205) in the OCI control group. A threshold of 7500 HU had the best discriminative value between OCI and AS (youden index: 0.89). In AS, disease duration is negatively associated with SBAC-SI (r: -0.623; p< 0.01) and HLA B27 is associated with lower SBAC-SI (6523 [5198; 7137]VS 2809 [1568; 3371]; p <0.001).SBAC-SI is significatively different between AS and OCI and could help to distinguish these two diseases.Figure 1.Distribution of the Subchondral Bone Attenuation Coefficients of the Sacroiliac Margins of the 24 ROI in the anterior (A), middle (B) and posterior (C) slices in the coronal oblique plane of the sacroiliac jointsNone declared