Sacrum

The sacrum is a large, triangular bone, situated in the lower part of the vertebral column and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones.

The sacrum is formed by the fusion of five sacral vertebrae (S1-S5)

Its upper part or base articulates with the last lumbar vertebra, its apex with the coccyx. It is curved upon itself and placed very obliquely, its base projecting forward and forming the prominent sacrovertebral angle when articulated with the last lumbar vertebra; its central part is projected backward, so as to give increased capacity to the pelvic cavity.

The sacrum has several key anatomical features:

1. Sacral Canal: This canal runs through the center of the sacrum and is a continuation of the vertebral canal. It houses the sacral nerves.

2. Sacral Foramina: There are four pairs of anterior and posterior sacral foramina on each side, which allow the passage of the sacral nerves and blood vessels.

3. Sacral Promontory: The anterior projecting edge of the first sacral vertebra (S1) that forms part of the pelvic brim.

4. Alae: The lateral wings of the sacrum that articulate with the ilium of the pelvis, forming the sacroiliac joints.

5. Median Sacral Crest: A ridge formed by the fused spinous processes of the sacral vertebrae, located on the posterior surface.

6. Intermediate and Lateral Sacral Crests: These are formed by the fusion of the articular and transverse processes, respectively, and run parallel to the median sacral crest.

7. Sacral Hiatus: An opening at the inferior end of the sacral canal, formed by the failure of the laminae of the fifth sacral vertebra to meet in the midline.

The sacrum plays a crucial role in providing stability to the pelvis and supporting the weight of the upper body. It also serves as an attachment point for various ligaments and muscles, contributing to the stability and movement of the pelvis and lower back