Sacrum

What’s the Buttock Bone

The buttock bone is a large, flat, triangular-shaped, irregular bone, also known as the sacral vertebra or sacral spine. Comprising five fused vertebrae (S1-S5), it is located at the base of the vertebral column or spine and connects the spine with the hip, contributing to hip stability.

Where Is the Buttock Bone Positioned

The buttock bone is situated at the base of the vertebral column or spine, between the right and left iliac bones of the hip, and beneath the last lumbar vertebra (L5).

Key Points

Roles

• Maintains the connection between the hip bones on the backside, thus providing support to the spine’s base.
• Encompasses and safeguards the spinal nerves of the lower back.
• Together with the hip bone, it forms the pelvic cavity, supporting and protecting the delicate organs of the excretory and reproductive systems.
• Assists in sustaining and transmitting the body weight when standing or sitting.
• Contributes to the stability of the hip.

Components and Structure

The buttock bone is a concave, uneven bone, resembling an inverted triangle. The broadest part, known as the base, is at the top, while the pointed end, or apex, is at the bottom. It also includes three surfaces – dorsal, lateral, and pelvic. Additionally, the bone has a hollow canal referred to as the sacral canal, which runs through its core.

Each part has numerous important bony features.

Distinctive Features

Base

The upper part of the buttock bone, beneath the spinal base, is called the base of the scarum. It is the broadest section of the bone. The first of the five fused sacral vertebrae, S1, is located here. S1, the largest vertebra, has concave superior articular facets that project posteromedially to articulate with the fifth lumbar vertebra (L5). On both sides of the base, there are large wing-like projections known as alae of the buttock bone or sacral ala. These alae articulate with the ilium bones of the pelvis, forming sacroiliac (SI) joints.

Apex

This is the pointed part of the buttock bone, directing downwards. The fifth sacral vertebra is located in this most inferior segment of the bone. The apex projects posteriorly, increasing the size of the pelvic cavity. This region features an oval facet for articulation with the coccyx.

Sacral Canal

The sacral canal is a hollow space running from the base to the apex of the buttock bone. Internally, it is a continuation of the vertebral canal, running along the core of the buttock bone and ending at the fourth sacral foramina, known as the sacral hiatus.

Despite being a continuation of the vertebral column, it does not contain the spinal cord, which ends around the second lumbar vertebrae (L2). Instead, it houses the cauda equina, a bundle of spinal nerve roots, and the filum terminale, a fibrous band of connective tissue.

Dorsal Surface

The dorsal surface of the buttock bone is rough and rugged due to the fusion of the sacral vertebrae, resulting in three bony ridges or crests: median, intermediate, and lateral.

The five sacral vertebrae fuse on the midline, forming a central ridge called the median sacral crest, created by the fusion of the spinous processes of the first three sacral vertebrae. The supraspinous ligament attaches here.

All the articular processes of the buttock bone, except the superior articulating process of S1 and the inferior articulating process of S5, fuse to form the intermediate sacral crests. The posterior sacroiliac ligaments are attached along this crest. The upper part of S1 articulates with the inferior articulating process of L5, and the lower part of S5, known as the sacral cornu, articulates with the coccyx.

The transverse processes of the five sacral bones fuse to form the lateral sacral crests, which serve as a point of attachment for the posterior sacroiliac and sacrotuberous ligament. The processes do not completely fuse, leaving four pairs of openings on either side, also known as posterior sacral foramina. The sacral nerve fibers enter and exit the sacral canal through these four pairs of sacral foramina.

Lateral Surface

The lateral surface of this bone is broad above but narrows into a thin edge as it descends. The upper part of the surface presents an ear-shaped area, the auricular surface, which remains covered with cartilage and articulates with the ilium. There is a rough prominence behind the auricular surface called the sacral tuberosity, containing three uneven depressions for the attachment of the posterior sacroiliac ligament. The sacrotuberous and sacrospinous ligaments, along with some fibers of the gluteus maximus, attach to the lower half of the lateral surface.

Pelvic Surface

The pelvic surface of the buttock bone is comparatively smoother than its dorsal surface and is marked by four transverse lines, remnants of the fused sacral intervertebral discs.

Anteriorly, there is an anterior bony projection known as the sacral promontory, marking the posterior margin of the pelvic inlet and continuous with the margin of the sacral ala. The front sides of the four pairs of sacral foramina are found on this surface.

Joints

1. Lumbosacral joint: The base of the buttock bone articulates with the fifth lumbar vertebrae (L5) superiorly through the L5/S1 intervertebral disc, forming this amphiarthrodial joint.
2. Sacrococcygeal joint:  Here, the apex of the bone articulates with the base of the coccyx to form another amphiarthrodial joint.
3. Sacroiliac joint: The sacral ala laterally articulates with the ilium of the pelvis, forming this synovial joint.

Ossification

Although the buttock bone starts as 4-6 sacral vertebrae, it transforms into a single bone as humans grow older. The fusion of these vertebrae forms a single bone, solidifying the overall shape of the buttock bone. The fusion does not occur simultaneously in all sacral vertebrae but begins with thefusion of S1 and S2.

The process usually commences during adolescence and is typically finished during the early to mid-twenties. This ossification process is believed to commence earlier in females compared to males.

Anatomical Alterations of the Sacrum

On occasion, the sacrum exhibits anatomical diversity, including variations in the number of vertebrae, its surface, and curvature.

• The most common anatomical variation of the bone is the diversity in the number of sacral vertebrae. The sacrum commonly consists of five fused vertebrae, but instances of four or six sacral vertebrae have also been recorded.
• Another anomaly of the sacrum is related to its surface and curvature. The curvature of the bone varies widely among individuals.
• In some individuals, the first and second sacral vertebrae do not fuse but instead remain separately articulated.

Sacrum in Females vs. in Males

The sacrum is sexually dimorphic, meaning it has a slightly different appearance in females and males. In females, the sacrum is broader and more backwardly curved than in males, thereby increasing the size of the pelvic cavity. This wider pelvic cavity in females aids in enduring pregnancy, provides more space for the developing fetus, and houses reproductive organs.

Muscle Connections

Various muscles of the lower limb and back have their origin or insertion on the sacrum.

Originating from the sacrum

1. Piriformis
2. Iliacus
3. Multifidus lumborum

Inserting on the sacrum

1. Coccygeus
2. Erector spinae

Nerves

As mentioned, the cauda equina, long sacral roots of spinal nerves, pass through the sacrum via the sacral canal.

These nerves enter the sacrum from the vertebral foramen of the lumbar vertebrae through the sacral canal. From there, they branch out and exit the bone through four pairs of sacral foramina or the sacral hiatus, present at the bottom end of the canal.

FAQs

Q.1. Is the sacrum part of the axial skeleton or the appendicular skeleton?

Ans. The sacrum is a component of the axial skeleton.

References

1. Sacrum – Kenhub.com
2. The Sacrum – Teachmeanatomy.info
3. Sacrum – Innerbody.com
4. Sacrum – Radiopaedia.org
5. Anatomy, Back, Sacral Vertebrae – Ncbi.nlm.nih.gov