The sacral ala are the wing-like structures on either side of the sacrum, a large, triangular bone at the base of the spine that fits between the two halves of the pelvis. The term “ala” is Latin for “wing,” describing these broad projections extending from the upper and side portions of the sacrum. Their structure provides a foundation for the connection between the spinal column and the pelvic girdle.
Anatomical Location and Structure
The sacrum is composed of five vertebrae that fuse during development, and the ala forms from parts of the first sacral vertebra (S1). Located at the base of the lumbar spine, the sacrum connects with the last lumbar vertebra (L5) above and the coccyx (tailbone) below. The alae are the large, triangular surfaces that project laterally from the main body of the sacrum and are the widest and strongest parts of this bone.
A primary feature of the sacral ala is its articulation with the iliac bones of the pelvis. This connection point is the sacroiliac (SI) joint, a robust joint that provides pelvic stability. The surface of the ala that meets the ilium is concave and has small openings, or foramina, that allow for the passage of sacral nerves and blood vessels from the spinal canal out to the lower body.
Functional Role in the Body
The primary function of the sacral ala is its role in forming the sacroiliac (SI) joint. This joint transfers weight and forces from the upper body through the spine and into the pelvic girdle and lower limbs. This weight-bearing function is necessary for activities like walking, standing, and sitting upright. The structure and strength of the alae allow them to withstand considerable pressure and motion.
The sacral ala also serves as an attachment point for several ligaments and muscles. Ligaments such as the iliolumbar and lumbosacral ligaments attach to the ala, providing reinforcement to the lumbosacral junction. Muscles including the psoas major and iliacus, which are involved in hip flexion and stability, also have fibers that attach to this area. These connections contribute to the stability and mobility of the pelvis and lower back.
Common Injuries and Conditions
Due to its role in weight-bearing and force transmission, the sacral ala is susceptible to fractures. These injuries often result from high-energy trauma, such as car accidents or significant falls, particularly in younger individuals. In older adults with osteoporosis, these fractures can occur from lower-impact events due to decreased bone density.
Symptoms of a sacral ala fracture include pain in the lower back, buttocks, hips, or groin. This pain worsens with movement, and individuals may find it difficult to walk or bear weight. Visible swelling and bruising may be present in the lower back region. If nearby sacral nerves are affected, it can lead to symptoms like numbness, tingling, or weakness in the legs.
Another condition involving this structure is sacroiliac (SI) joint dysfunction. This condition arises when the SI joint becomes a source of pain due to abnormal motion, inflammation, or arthritis. The pain is very similar to that of a fracture, manifesting in the low back and buttock region, making diagnosis challenging without medical evaluation. Repetitive microtrauma from athletic activities is a common cause of SI joint injury.