Significance of fiber orientation in the interosseous sacroiliac ligament: An anatomical and histological study of the implications of its mechanical adaptation

The sacroiliac joint allows limited but crucial movement for transmitting loads from the spine to the pelvis. Identifying anatomical structures that adapt to this motion may improve imaging-based diagnosis of sacroiliac joint dysfunction. The interosseous sacroiliac ligament (ISIL), a key stabilizer connecting the sacrum and ilium, has unclear adaptive responses to mechanical stress. This study aimed to investigate the ISIL's gross morphology, fiber orientation, and histological features. A total of 11 hemipelves from eight donors were examined, six for gross anatomy and five for histology. Fiber orientation was evaluated using micro-computed tomography. The distribution of fiber angles was compared between the superior and inferior ISIL regions, ranging from +90° (vertical) to -90°, using paired t-tests with statistical parametric mapping. The ISIL displayed a consistent fibrous architecture, with predominantly vertical fibers in the inferior region and a higher frequency of horizontal fibers in the superior region. Orientation analysis revealed more vertical fibers (70°-80°) inferiorly and more horizontal fibers (-50° to -30°) superiorly (p < 0.05). Histology confirmed these findings and revealed a synovial bursa-like structure between the ISIL and the posteroinferior sacrum. These findings suggest that the ISIL exhibits region-specific fiber alignment, with vertical fibers inferiorly and horizontal fibers superiorly, along with a synovial bursa-like structure adjacent to the sacrum. These patterns likely reflect mechanical adaptation to sacroiliac joint motion and may guide imaging-based assessment of sacroiliac joint dysfunction. Our suggestion requires validation based on the direct mechanical data through biomechanical and clinical research, and the limitation of quantitative analysis being 2D and having a small sample size need to be addressed in future studies incorporating in vivo 3D imaging.