Sacrum

Description

Centered in the posterior pelvis, the male sacrum sits inferior to L5 and posterior to the pelvic cavity, with the superior surface of S1 (the sacral base) forming the promontory at the anterior margin. L5 articulates with S1 across the L5 to S1 intervertebral disc, and the paired superior articular processes of S1 flank the sacral canal at the midline. Lateral to the base, the sacral alae broaden toward the auricular surfaces that meet the iliac bones at the sacroiliac joints. Posterior elements of the lumbar vertebrae, including spinous and transverse processes, frame the lumbosacral junction above. Orientation at the sacral base matters for both biomechanics and procedures. The lumbosacral angle, defined by the relationship of L5 to the S1 endplate and promontory, is a common teaching point when discussing sagittal balance and the shear forces that contribute to L5 spondylolysis and spondylolisthesis. A clear view of the S1 superior endplate and adjacent foramina also supports discussions of S1 nerve root symptoms and transforaminal epidural steroid injection trajectories, as well as sacral anatomy relevant to iliosacral screw fixation and sacroiliac joint fusion planning. Use this plate in gross anatomy labs to anchor the transition from lumbar vertebrae to fused sacral segments, and in orthopaedic, neurosurgical, and pain medicine teaching decks that cover the L5 to S1 disc, sacral promontory, and sacral foramina. It also fits well in textbook chapters on pelvic osteology, sagittal alignment, and lumbosacral pathology. Anatomical accuracy verified by SciePro's Medical Advisory Board.