The Sacrum & Coccyx
The sacral spine anatomy forms the base of the spinal column, connecting the spine to the pelvis and lower body. The sacrum is a large triangular bone formed by five fused vertebrae (S1–S5), wedged between the two hip bones (ilia) to create the posterior wall of the pelvis. Below it sits the coccyx — the tailbone — composed of three to five small fused vertebrae. Together, these structures distribute the weight of the entire upper body through the sacroiliac joints to the legs. Dr. Alok Sharan, MD, MHCDS — board-certified orthopedic spine surgeon and Castle Connolly Top Doctor in Edison, NJ — treats sacral spine conditions with his signature minimally invasive, awake approach.
Fused Vertebrae
Key Structures of the Sacral Spine
Sacrum (S1–S5) — five fused vertebrae forming a rigid triangular bone
Sacroiliac (SI) joints — two strong joints linking the sacrum to the pelvis on each side
Sacral foramina — openings on each side through which sacral nerve roots exit
Sacral canal — the continuation of the spinal canal housing the sacral nerve roots
Coccyx (tailbone) — 3–5 fused vertebrae providing muscle and ligament attachment
The sacrum is fully fused by early adulthood (typically age 18–25)
The Sacroiliac Joint
The sacroiliac (SI) joint is one of the largest joints in the body, formed by the articulation of the sacrum with the iliac bones of the pelvis on each side. Unlike most joints, it is designed primarily for stability rather than motion — it moves only a few degrees in normal function. An extensive network of ligaments and muscles surrounds the SI joint, making it exceptionally strong to withstand the forces of walking, running, and lifting.
Despite its stability, the SI joint can become a significant source of pain when ligaments are damaged, joint surfaces degenerate, or the joint becomes inflamed. Sacroiliac joint dysfunction is estimated to be the cause of 15–25% of all cases of lower back pain, yet it is frequently underdiagnosed. The pain typically presents in the lower back and buttock on one side, sometimes radiating into the thigh — mimicking sciatica.
The sacral nerve roots (S1–S4) that exit through the sacral foramina contribute to the sciatic nerve and supply the lower extremities. Compression or irritation of sacral nerve roots can cause similar symptoms to lumbar nerve root problems, requiring careful examination to distinguish between the two.
Common Sacral Spine Conditions
Sacral Spine Treatment at The Spine and Performance Institute
Dr. Alok Sharan is a NY Magazine Best Doctor with 20+ years of experience, fellowship-trained at NYU Hospital for Joint Diseases and Deputy Editor of Clinical Spine Surgery. With 201 five-star Google reviews and over 100 peer-reviewed publications, he brings world-class expertise in treating conditions of the lumbosacral junction and sacral spine at his practice in Edison, NJ — 35-37 Progress St., Suite B5, Edison, NJ 08820.
The L5–S1 level — the junction between the last lumbar vertebra and the sacrum — is the single most commonly treated surgical level in the spine. Dr. Sharan’s expertise here includes:
L5–S1 Lumbar Fusion — stabilizing the lumbosacral junction through his pioneering awake spinal fusion technique under regional anesthesia. Most patients are discharged the same day with a 2–4 week recovery versus the traditional 6–12 weeks.
ALIF (Anterior Lumbar Interbody Fusion) — an anterior approach to the L5–S1 disc space that allows large interbody implants and avoids posterior muscle disruption.
TLIF (Transforaminal Lumbar Interbody Fusion) — a posterior approach offering excellent access to the L5–S1 disc through a minimally invasive corridor.
Frequently Asked Questions About Sacral Spine Anatomy
Is the sacrum part of the spine or the pelvis?
The sacrum is both — it is the lowest segment of the vertebral column and simultaneously forms the posterior wall of the pelvis. The sacroiliac joints on each side connect the sacrum firmly to the iliac bones of the pelvis, creating a ring of bone that transmits forces between the spine and the legs.
What causes sacroiliac joint pain and how is it treated?
SI joint pain is commonly caused by ligament laxity (particularly after pregnancy), joint degeneration, trauma, or inflammatory arthritis. Initial treatment includes physical therapy, SI joint injections with corticosteroid, and activity modification. For persistent cases, radiofrequency ablation of the SI joint nerves or surgical SI joint fusion may be considered.
Can coccyx pain require surgery?
Most coccydynia (tailbone pain) resolves with conservative management — donut cushions, physical therapy, anti-inflammatory medications, and coccygeal injections. Surgical removal of the coccyx (coccygectomy) is considered only for rare refractory cases and is uncommon. Dr. Sharan will review your imaging and history to rule out other causes before considering any intervention.
What symptoms suggest a sacral nerve root problem?
Compression of sacral nerve roots can cause pain radiating into the buttock or down the back of the leg, weakness in the foot or ankle, numbness along the outer leg or sole, and in severe cases, bowel or bladder dysfunction. If you are experiencing these symptoms, contact Dr. Alok Sharan at (732) 898-3950 for prompt evaluation.
