The sacrum is a triangular-shaped bone located at the base of the spine, between the two hip bones. It is a key component of the pelvis and plays a crucial role in supporting the weight of the upper body and maintaining balance and stability during movement.
Structure
The sacrum is made up of five vertebral bodies that are fused together, forming a single bone. The top of the sacrum, also known as the base, articulates with the fifth lumbar vertebra (L5) of the lower back. The bottom of the sacrum, also known as the apex, projects downward and articulates with the coccyx (tailbone). The sacrum is wider at the base and tapers down to a point at the apex.
It is surrounded by a thin layer of bone called the cortex, which is thin enough to allow for the passage of blood vessels and nerves. The bone tissue within the sacrum is composed of trabecular bone, which is spongy and porous, and cortical bone, which is denser and harder. The trabecular bone is found at the base of the sacrum, while the cortical bone is found at the apex.
The sacrum is divided into three regions: the upper region (also known as the presacral region), the middle region (also known as the sacral region), and the lower region (also known as the sacrococcygeal region). The upper region is the part of the sacrum that articulates with the lumbar vertebrae, while the middle region is the part that is fused to the coccyx. The lower region is the part of the sacrum that extends beyond the coccyx.
It has several bony landmarks that are important for identifying its different regions and for performing various clinical procedures. These landmarks include the following:
- The sacral promontory: This is the most prominent bony protrusion at the base of the sacrum and marks the beginning of the upper region.
- The sacral canal: This is a hollow channel that runs through the middle of the sacrum and contains the sacral nerve roots.
- The sacral foramina: These are small openings on either side of the sacrum that allow the passage of nerves and blood vessels.
- The sacral cornua: These are small, bony protuberances located on either side of the sacrum at the base.
Location
The sacrum is located in the pelvis, at the base of the spine. It is situated between the two hip bones (ilium) and is posterior (behind) to the pubic bone. The sacrum is oriented vertically and is tilted slightly forward, with the base facing upward and the apex facing downward. The sacrum is held in place by the sacroiliac joint, which is a strong, fibrous joint that connects the sacrum to the ilium.
Function
The sacrum has several important functions in the body, including:
- Supporting the weight of the upper body: The sacrum is an important weight-bearing structure that helps to support the weight of the upper body. It acts as a foundation for the spine, allowing it to bear the load of the body’s weight and distribute it evenly across the pelvis.
- Providing stability and balance: The sacrum helps to maintain stability and balance during movement. It acts as a pivot point for the spine, allowing the upper body to twist and turn without losing its balance. The sacrum also helps to transfer the forces of movement from the lower body to the upper body and vice versa, allowing the body to move smoothly and efficiently.
- Protecting the spinal cord: The sacrum surrounds and protects the lower end of the spinal cord, which is a bundle of nerves that runs through the spinal column. The sacrum and the vertebrae above it form a protective bony enclosure called the spinal canal, which helps to shield the spinal cord from injury.
- Providing attachment points for muscles and ligaments: The sacrum has several bony protuberances called transverse processes, which provide attachment points for muscles and ligaments. These muscles and ligaments help to support the spine, maintain posture, and facilitate movement.
Clinical Significance
The sacrum is an important bone in the body, and any abnormalities or injuries to the sacrum can have significant consequences. Some common conditions that can affect the sacrum include:
- Sacral fractures: A sacral fracture is a break in the sacrum bone. These fractures can be caused by falls, car accidents, or other types of trauma.
- Sacroiliac joint dysfunction: The sacroiliac joint is the joint that connects the sacrum to the ilium. Dysfunction of this joint can cause pain in the lower back or pelvis, as well as difficulty moving or standing.
- Spondylolisthesis: This condition occurs when one of the vertebrae in the spine slips out of place, often as a result of a degenerative condition such as osteoarthritis. It can cause pain in the lower back or legs, as well as difficulty standing or walking. Treatment for spondylolisthesis may include physical therapy, pain medication, or surgery.
- Sacral dimples: A sacral dimple is a small indentation in the skin on the lower back, just above the crease of the buttocks. These dimples are usually benign and do not cause any problems. However, in some cases, they may be associated with underlying spinal abnormalities, such as a tethered spinal cord or spina bifida. In these cases, further evaluation and treatment may be necessary.
