The sacrum is a large, triangular bone situated at the base of the spine, forming a foundational component of the pelvic girdle. It is formed by the fusion of five separate vertebrae that solidify into a single structure in early adulthood. The sacral ala, translating from Latin as “wing,” refers to the large, flattened, wing-like projections extending laterally from the superior part of the sacrum. This broad, paired structure is fundamental to the pelvis, providing a stable platform for the transfer of body weight and influencing the stability of the lower axial skeleton.
Anatomical Location and Structure
The sacral ala is located on the superior-lateral aspect of the sacrum, positioned immediately below the last lumbar vertebra (L5). Each ala is a large, triangular surface that forms the widest part of the sacrum, known as the base. Its structure is composed of the fused transverse processes and the costal elements of the first sacral segment (S1), rather than the central vertebral body.
A defining feature of the ala is the auricular surface, a rough, ear-shaped area on its lateral side. This specialized surface is lined with cartilage and is the point of articulation with the ilium, the large upper bone of the hip. The irregular contours of the auricular surface, with its ridges and grooves, contribute significantly to the mechanical interlocking of the joint, allowing the ala to withstand considerable forces transmitted from the spine.
The ala also serves as an attachment site for several muscles and ligaments. The ventral ramus of the L5 nerve root travels directly across the anterior surface of the ala, placing it in a vulnerable position. The posterior, non-articulating part of the ala features the sacral tuberosity, which anchors some of the strongest ligaments in the body.
Role in Sacroiliac Joint Stability and Load Transfer
The primary functional role of the sacral ala is to form the Sacroiliac Joint (SIJ), the mechanical junction between the spine and the lower extremities. This joint transmits the weight of the upper body from the vertebral column into the pelvis and legs. The design of the ala and the SIJ prioritizes stability over mobility.
Joint stability is achieved through two mechanisms: form closure and force closure. Form closure refers to the inherent stability provided by the bony architecture, where the wedged shape of the sacrum fits tightly between the two iliac bones. The irregular auricular surface of the ala enhances this form closure by physically interlocking with the corresponding surface of the ilium.
Force closure is provided by the dense network of surrounding ligaments that actively compress and stabilize the joint. The interosseous sacroiliac ligament, located behind the auricular surface, serves as a primary restraint against shear forces. The posterior and anterior sacroiliac ligaments also tightly bind the ala to the ilium, limiting movement.
The small amount of movement that occurs at the sacral ala is described as nutation and counternutation, which are slight anterior and posterior tilting motions. These motions are necessary for absorbing shock and accommodating forces generated during walking and standing. The strong ligaments and the wedged shape of the ala work together to prevent excessive movement, ensuring the spine’s load is safely transferred through the pelvis.
Clinical Significance and Associated Injuries
The anatomical position and function of the sacral ala make it susceptible to specific types of injuries. Sacral fractures most commonly occur through the ala, an area classified as Zone 1 in the Denis classification system. These fractures can result from high-energy trauma, such as motor vehicle accidents, or from low-energy forces.
A frequent presentation in older adults is the sacral insufficiency fracture, which involves the ala lateral to the neural foramina. These stress fractures occur in osteoporotic bone, often without significant trauma, and are a common cause of debilitating lower back and buttock pain. Diagnosis can be challenging because they are often difficult to visualize on standard X-rays.
The proximity of the L5 nerve root, which runs directly over the sacral ala, is a major clinical concern during fractures or surgical stabilization. Displacement of a sacral ala fracture can injure this nerve, potentially causing motor deficits such as foot drop. Orthopedic surgeons frequently use the robust bone of the ala as a secure anchor point for screw placement during pelvic ring stabilization procedures after severe trauma.
The SI joint itself can be a source of pain, known as SI joint dysfunction. This condition often presents as lower back or buttock pain and is related to micro-instability or inflammation at the articulation point between the ala and the ilium.