The sacrum is a bone located in the lower part of the human spine, forming a connection between the vertebral column and the pelvis. This singular bone plays a significant role in supporting the upper body and facilitating various movements.
Location and Structure
The sacrum is situated at the base of the vertebral column, nestled between the two hip bones, known as the ilium. This triangular-shaped bone forms a strong foundation for the spine and is curved. It develops from the fusion of five individual vertebrae, typically designated S1 through S5, a process that usually completes by early adulthood.
Its surface features several openings, or foramina, which serve as passageways for nerves and blood vessels branching from the spinal cord. A central channel, the sacral canal, runs through its length, housing sacral nerve roots and spinal meninges.
The sacrum connects superiorly with the fifth lumbar vertebra (L5), forming the lumbosacral joint. Laterally, it articulates with the ilium on both sides, creating the sacroiliac (SI) joints, which are robust yet allow for some limited movement. Inferiorly, the apex of the sacrum connects with the coccyx, or tailbone, completing the lower spinal structure.
Key Functions
A primary function of the sacrum is its role as a weight-bearing bone, supporting the weight of the entire body. It transfers weight from the upper body and spinal column down to the lower limbs, facilitating upright posture and bipedal locomotion. This load distribution is essential for maintaining balance during standing and walking.
The sacrum also forms the posterior wall of the pelvic girdle, providing structural integrity to the entire pelvic ring. This stability is essential for supporting abdominal organs and for the attachment of powerful muscles involved in leg movement. The sacrum’s shape and structure contribute to the pelvis’s overall stability, allowing it to support the body’s weight and facilitate movement.
The sacrum offers protection to the nerves of the sacral plexus, which emerge through its foramina. These nerves control sensation and movement in the lower limbs and pelvic region. The bone also serves as an attachment site for numerous muscles and ligaments that stabilize the pelvis and spine, including the piriformis, gluteus maximus, and various sacroiliac ligaments.
Common Related Issues
Problems involving the sacrum can lead to discomfort and functional limitations. One common issue is sacroiliac (SI) joint dysfunction, characterized by pain from inflammation or improper movement within the joints connecting the sacrum to the hip bones. This pain often presents in the lower back, buttocks, or radiates down the leg, sometimes mimicking sciatica. The pain may worsen with standing, sitting for long periods, or activities like climbing stairs.
Sacral fractures, though less common than other spinal fractures, can occur due to high-impact trauma, such as falls or vehicle accidents, or from stress in individuals with weakened bones. These fractures can cause severe localized pain in the lower back, buttocks, or hips, worsening with weight-bearing activities. Such injuries may affect nerve function, leading to symptoms like numbness, tingling, or weakness in the legs.
Nerve impingement, such as sciatica, can originate from issues related to the sacrum or the nerves passing through it. This condition involves compression or irritation of the sciatic nerve, leading to pain, numbness, or tingling sensations that travel from the lower back down the leg.