The sacrum is a triangular-shaped bone located at the base of the spine, playing a fundamental role in the human skeletal system. It is a central component of the pelvis, providing a sturdy connection between the spine and the lower limbs, supporting upright posture and movement.
Location and Structure
The sacrum is positioned at the lower end of the vertebral column, situated between the two ilium bones. This bone forms the posterior aspect of the pelvis. Its inverted triangular shape results from the fusion of five individual vertebrae (S1 through S5), a process that typically begins in the mid-teens and completes by the early to mid-twenties.
The uppermost part of the sacrum, the base, articulates with the fifth lumbar vertebra (L5), forming the lumbosacral joint. On its lateral sides, the sacrum connects with the ilium bones at strong, L-shaped sacroiliac (SI) joints. These joints are reinforced by robust ligaments, contributing to pelvic girdle stability.
The anterior surface of the sacrum features four pairs of sacral foramina, which allow for the passage of sacral nerves and blood vessels. The posterior surface is rougher and includes the median sacral crest, formed by the fused spinous processes of the sacral vertebrae. Internally, the sacrum contains the sacral canal, a continuation of the vertebral canal, housing the lower portions of the spinal cord’s nerve fibers. The bottom tip of the sacrum, the apex, connects with the coccyx.
Essential Functions
The sacrum provides structural support for the entire upper body, acting as a sturdy foundation for the spine. This support helps maintain upright posture, whether standing or sitting.
The sacrum is instrumental in transmitting weight and forces from the upper body through the pelvis to the lower limbs. This weight-bearing capacity allows for efficient transfer of gravitational forces during activities like walking, running, and lifting. The sacrum, along with the coccyx, also forms the posterior wall of the pelvic cavity, offering protection for internal organs such as the bladder and rectum.
The sacrum protects the nerves of the lower spinal cord. The sacral foramina serve as conduits for sacral nerves that control functions in the lower body, including sensation and movement in the legs and feet, as well as bladder and bowel control. Its strong articulations, particularly the sacroiliac joints, contribute to the stability and limited movement of the pelvic girdle.
Common Issues
Sacroiliac (SI) joint dysfunction is a common issue, involving pain originating from these joints. This dysfunction can arise from either too much movement (hypermobility) or too little movement (hypomobility), often resulting in pain in the lower back, buttocks, and sometimes radiating down the leg. Inflammation of these joints, known as sacroiliitis, can be caused by injury, arthritis, or the stress of pregnancy.
Sacral fractures can occur due to high-energy trauma, such as car accidents or falls. They can also be insufficiency fractures, which happen when bone quality is weakened, often due to conditions like osteoporosis, making the bone susceptible to breaking even without significant trauma. Symptoms of a sacral fracture include severe pain in the lower back, buttock, or hip, and walking can become difficult.
Nerve compression, particularly sciatica, can involve the sacrum. Sciatica refers to pain that radiates along the sciatic nerve, formed by nerve roots originating from the lower lumbar and sacral spine (L4 to S3). Compression or irritation of these nerve roots, often due to conditions like a herniated disc or narrowing of the spinal canal, can cause sharp, shooting pain, numbness, or weakness in the buttock and down the leg.