Morphometric anatomical and CT study of the human adult sacroiliac region.

PURPOSE: To identify and describe the morphometry and CT features of the articular and extra-articular portions of the sacroiliac region. The resulting knowledge might help to avoid complications in sacroiliac joint (SIJ) fusion. METHODS: We analyzed 102 dry hemi-sacra, 80 ilia, and 10 intact pelves and assessed the pelvic computerized tomography (CT) scans of 90 patients, who underwent the examination for conditions not involving the pelvis. We assessed both the posterior aspect of sacrum with regard to the depressions located externally to the lateral sacral crest at the level of the proximal three sacral vertebrae and the posteroinferior aspect of ilium. Coronal and axial CT scans of the SIJ of patients were obtained and the joint space was measured. RESULTS: On each side, the sacrum exhibits three bone depressions, not described in anatomic textbooks or studies, facing the medial aspect of the posteroinferior ilium, not yet described in detail. Both structures are extra-articular portions situated posteriorly to the SIJ. Coronal CT scans of patients showing the first three sacral foramens and the interval between sacrum and ilium as a continuous space display only the S1 and S3 portions of SIJ, the intermediate portion being extra-articular. The S2 portion is visible on the most anterior coronal scan. Axial scans show articular and extra-articular portions and features improperly described as anatomic variations. CONCLUSIONS: Extra-articular portions of the sacroiliac region, not yet described exhaustively, have often been confused with SIJ. Coronal CT scans through the middle part of sacrum, the most used to evaluate degenerative and inflammatory conditions of SIJ, show articular and extra-articular portions of the region.