The human spinal column is divided into five distinct regions: cervical, thoracic, lumbar, sacral, and coccygeal. The first sacral segment, S1, is the uppermost portion of the triangular sacrum, located beneath the final lumbar vertebra. Unlike the 24 movable bones above it, S1 is part of a single, fused bone that anchors the spine to the pelvis. This position makes S1 a significant anatomical landmark, acting as a structural transition point.
Defining the First Sacral Segment (S1)
The sacrum is a large, inverted triangular bone located at the base of the spine, positioned like a wedge between the two hip bones (iliac crests). S1 is the largest and most superior of the five sacral segments (S1 through S5). The lateral parts of S1, called the alae, extend outward to form the sacroiliac joints, which connect the spine to the pelvis.
The body of S1 is robust and supports the weight of the upper body transmitted down the spinal column. The sacrum forms through a fusion process that begins with separate vertebrae during childhood and typically completes by early adulthood. This fusion results in a structurally stable, immobile foundation for the spine.
A key anatomical feature is the sacral promontory, the prominent anterior edge of the S1 vertebral body. This ledge projects forward into the pelvic cavity and serves as a landmark used by clinicians and anatomists. The promontory, along with other bony ridges, helps define the posterior boundary of the pelvic inlet.
The L5-S1 Junction
S1 connects directly to the fifth lumbar vertebra (L5) at the lumbosacral junction. This joint is the final mobile segment of the spine before the rigidity of the fused sacrum and pelvis. The lumbosacral junction is under immense mechanical stress because it manages the transfer of gravitational load from the trunk to the lower limbs.
The L5 and S1 segments are separated by the L5-S1 intervertebral disc, the lowest disc in the spine. This disc is typically thicker anteriorly, a shape that helps create the normal inward curve of the lower back (lumbar lordosis). The bony connection between L5 and S1 forms a distinct angle, often called the lumbosacral angle.
The junction is structurally complex, involving the intervertebral disc and a pair of facet joints that link the posterior elements of L5 and S1. This arrangement allows for necessary movements of the lower back, such as flexion and extension, before the spine meets the fixed pelvis. Because this flexible column meets a fixed base here, the L5-S1 segment is susceptible to wear and tear.
S1’s Role in Major Nerve Pathways
The S1 segment is functionally significant because the S1 spinal nerve root exits the spinal canal nearby. This nerve root is a major contributor to the formation of the sciatic nerve, the largest and longest nerve in the human body. The sciatic nerve is a thick bundle of fibers derived from the L4 through S3 spinal nerve roots.
The S1 nerve root primarily provides motor function to muscles in the lower leg and foot. It controls the muscles responsible for plantar flexion (pointing the foot and toes downward). S1 also provides sensory function to a narrow strip of skin running down the back of the leg and into the sole of the foot. Clinically, S1 function is often tested by checking the Achilles tendon reflex.