The top part of S1 articulates with the inferior articulating process of L5, while the bottom part of S5, known as the sacral cornu, articulates with the coccyx. The posterior sacroiliac ligaments are attached along this crest. The supraspinous ligament attaches here.Īll the articular processes of the sacrum, except the superior articulating process of S1 and the inferior articulating process of S5, fuse to give rise to the intermediate sacral crests. It is formed by the fusion of the spinous processes of the first three sacral vertebrae. The five sacral vertebrae fuse on the midline of the dorsal surface, giving rise to a central ridge called the median sacral crest. The dorsal surface of the sacrum is coarse and rugged due to the fusion of the sacral vertebrae, which gives rise to three bony ridges or crests, median, intermediate, and lateral. Instead, it has the cauda equina, which is a bundle of spinal nerve roots, and the filum terminale, a fibrous band of connective tissue. Though it is a continuation of the vertebral column, it does not contain the spinal cord, as the cord ends around the second lumbar vertebrae (L2). Internally, it is a continuation of the vertebral canal that runs along the core of the sacrum and ends at the fourth sacral foramina, as the sacral hiatus. The sacral canal is a hollow space that runs from the base to the apex of the sacrum. This region features an oval facet for articulation with the coccyx. The apex projects posteriorly to increase the size of the pelvic cavity. The fifth sacral vertebra lies in this most inferior segment of the bone. It is the pointy part of the sacrum, directing downwards. These alae articulate with the ilium bones of the pelvis, forming sacroiliac (SI) joints. Both sides of the base bear a large wing-like projection known as an ala of sacrum or sacral ala. The S1 vertebra is the biggest one, having concave superior articular facets that project posteromedially to articulate with the fifth lumbar vertebra (L5). The first one of the five fused sacral vertebrae, S1, is located here. The top part of the sacrum, lying just below the spinal base, is referred to as the base of the scarum.
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