The sacrum is a large, triangular bone at the base of the spine that serves as the structural bridge between your upper body and your legs. Formed by the fusion of five vertebrae during adolescence and early adulthood, it sits wedged between the two halves of the pelvis and bears the full weight of everything above it. Despite its unglamorous location, the sacrum is involved in a surprising range of medical issues, from chronic low back pain and bladder control to rare tumors and forensic identification of skeletal remains.
Where It Is and What It Does
The sacrum sits just below the lumbar spine and just above the coccyx (tailbone). On either side, it connects to the ilium, the large wing-shaped bone of the pelvis, through the sacroiliac joints. These joints are the critical link in a load-bearing chain: every time you stand, walk, or lift something, the forces generated in your trunk pass through the sacrum and into your pelvis and legs.1PubMed Central. Biomechanics of the Sacroiliac Joint: Anatomy, Function, Biomechanics, Sexual Dimorphism, and Causes of Pain Think of the sacrum as a keystone in a Roman arch: it is wider at the top and narrower at the bottom, and the bones on either side press inward to lock it in place.
Despite handling enormous forces, the sacroiliac joints barely move. A systematic review of movement studies found that the range of motion at these joints tops out at roughly two degrees and can be as small as a hundredth of a degree.2PubMed. Movement of the sacroiliac joint: Anatomy, systematic review, and biomechanical considerations That near-immobility is by design. Stability, not flexibility, is the sacrum’s job. The muscles and ligaments surrounding it form a complex web that keeps the joint locked in place while still allowing just enough give to absorb shock during walking and running.3PubMed. A biomechanical model on muscle forces in the transfer of spinal load to the pelvis and legs
How Walking Upright Shaped the Sacrum
The human sacrum looks dramatically different from that of our closest primate relatives, and the differences trace directly to bipedalism. In apes, the sacrum is narrow and incorporates fewer vertebrae. In humans, the last two original lumbar vertebrae have been permanently absorbed into the sacrum, and the bony wings on either side (the alae) are far wider, creating a broader and more stable joint surface with the pelvis.4PubMed. Evolution of the sacrum in hominoids This widening is what allows the sacrum to support the full weight of an upright trunk and keep the body balanced over two legs rather than four.
Studies comparing newborn and adult pelvises show that the sacrum’s orientation changes during growth: it curves, sinks deeper between the iliac blades like a wedge, and tilts backward, shifting the trunk’s center of gravity into a position that makes bipedal walking efficient. The angle of sacral incidence, a measurement of how the sacrum sits relative to the hip joints, increases during development and is tightly linked with the curve of the lower back. Fossil pelves from australopithecines and early Homo erectus show sacral incidence values within the modern human range, suggesting this adaptation was in place well before modern humans appeared.5PubMed. How is sagittal balance acquired during bipedal gait acquisition? Comparison of neonatal and adult pelves in three dimensions
The sacrum has also been studied in non-human mammals. Research on carnivores found that sacral shape evolved in response to species-specific locomotor demands, with the joint surfaces where the sacrum attaches to the rest of the skeleton changing most to resist the particular stresses of each animal’s movement style. Different carnivore families arrived at different sacral shapes to solve the same functional problem of transmitting force efficiently.
Sacroiliac Joint Pain
If you have chronic low back pain, there is a reasonable chance the sacroiliac joint is involved. Estimates suggest that sacroiliac joint dysfunction accounts for somewhere between 15 and 30 percent of chronic, non-nerve-related low back pain, making it one of the most common and most underdiagnosed causes.6PubMed. Sacroiliac joint pain: a comprehensive review of epidemiology, diagnosis and treatment People with sacroiliac pain tend to experience it on one side, typically below the beltline, and they are more likely than people with other types of back pain to recall a specific event that triggered it, such as a fall or a difficult delivery.
Diagnosis is tricky. No single physical exam test or piece of medical history can reliably identify the sacroiliac joint as the pain source. The pain referral patterns are highly variable: it can mimic hip pain, radiate into the groin, or feel like a deep ache in the buttock. The current approach uses a battery of three or more provocation tests (hands-on maneuvers that stress the joint in specific ways), and if those are positive, a diagnostic injection of local anesthetic into the joint under imaging guidance is used to confirm.7PubMed Central. A Review and Algorithm in the Diagnosis and Treatment of Sacroiliac Joint Pain Risk factors include leg length differences, older age, inflammatory arthritis, prior spinal surgery, pregnancy, and trauma.8PubMed. Sacroiliac joint dysfunction: pathophysiology, diagnosis, and treatment
Sacroiliitis and Inflammatory Disease
Sacroiliac joint pain from mechanical causes overlaps in location and symptoms with sacroiliitis, which is inflammation of the sacroiliac joint driven by an autoimmune or inflammatory process. Sacroiliitis is a hallmark feature of ankylosing spondylitis and other forms of spondyloarthritis. Because catching inflammatory back disease early can change the treatment plan dramatically, MRI of the sacroiliac joints has become a standard diagnostic tool.9PubMed Central. Imaging of sacroiliitis: Current status, limitations and pitfalls
In a study of patients with recent-onset inflammatory back pain, about a third showed signs of active inflammation on MRI, and roughly one in six already had structural changes in at least one sacroiliac joint.10PubMed Central. Magnetic resonance imaging changes of sacroiliac joints in patients with recent-onset inflammatory back pain: inter-reader reliability and prevalence of abnormalities The distinction between mechanical sacroiliac pain and inflammatory sacroiliitis matters because the treatments diverge sharply: mechanical dysfunction responds to physical therapy, injections, or joint stabilization procedures, while inflammatory sacroiliitis often requires disease-modifying drugs or biologics to slow the underlying autoimmune process.
The Sacral Nerve Plexus
The sacrum is not just a load-bearing bone. It houses the sacral spinal canal and a series of openings called sacral foramina through which major nerves exit the spine. The sacral plexus, formed by nerve roots from the fourth lumbar through the fourth sacral segments, gives rise to some of the body’s most important nerves: the sciatic nerve (the longest nerve in the body, running down the back of each leg), the pudendal nerve (which supplies the pelvic floor, genitals, and anal region), and the gluteal nerves (which power the muscles you use to stand up from a chair or climb stairs).11Surgery (Oxford). Anatomy of the lumbar and sacral plexuses and lower limb peripheral neuropathies
Because sacral nerves control bladder, bowel, and sexual function, any injury or disease affecting the sacrum can have consequences that extend far beyond back pain. Sacral fractures, tumors, or surgical procedures in the area can damage these nerves and lead to urinary incontinence, bowel dysfunction, or sexual problems. The sacral nerve roots also coordinate the reflexes involved in micturition (urination) and defecation, and age-related changes in these neural pathways may contribute to incontinence in older adults.12PubMed Central. Neurogenic mechanisms in bladder and bowel ageing
Sacral Neuromodulation
The fact that sacral nerves regulate bladder function has led to a treatment called sacral neuromodulation, in which a small device implanted near the sacral nerve roots delivers mild electrical pulses to restore more normal nerve signaling. Originally approved for overactive bladder and urge incontinence, sacral neuromodulation has since expanded to cover urinary urgency and frequency, non-obstructive urinary retention, and fecal incontinence.13Continence. Sacral neuromodulation for urinary incontinence Studies have shown that it reduces the number of daily voids, increases bladder capacity, and cuts down on leakage episodes, with effects lasting years after the device is implanted.14PubMed Central. Sacral neuromodulation in overactive bladder: a review and current perspectives Recent data report success rates as high as 93 percent at two-year follow-up for overactive bladder and urgency urinary incontinence.15Continence. Sacral neuromodulation for urinary incontinence
Caudal Epidural Anesthesia and Sacral Anatomy
At the very bottom of the sacrum is the sacral hiatus, a natural opening where the posterior wall of the sacral canal is incomplete. This opening is the entry point for caudal epidural anesthesia, a technique widely used in pediatric surgery, chronic pain management, and some obstetric procedures. A needle is inserted through the sacral hiatus to deliver anesthetic or steroid medication into the epidural space surrounding the lower spinal nerves.
The catch is that the sacral hiatus varies dramatically from person to person. Its shape, depth, and exact location relative to other bony landmarks differ enough that relying on a single surface landmark can lead to a failed block.16PubMed. Anatomy of the sacral hiatus and its clinical relevance in caudal epidural block When the hiatus is shallower than about three millimeters, the needle may not enter properly. Variations in the shape of the hiatus and defects in the bony wall of the sacral canal are common enough that imaging guidance or ultrasound is increasingly used to improve success rates.17PubMed Central. A Morphometric and Radiological Study of Sacral Hiatus in Human Adult Sacra and Its Clinical Relevance in Caudal Epidural Anaesthesia
Sacral Fractures
Sacral fractures fall into two broad categories. In younger people, they are usually the result of high-energy trauma such as car accidents or falls from height, and they often accompany pelvic ring injuries. In older adults, particularly postmenopausal women with osteoporosis, sacral insufficiency fractures can develop without any memorable injury at all. These stress fractures occur when normal forces act on weakened bone and are an easily overlooked cause of low back and buttock pain.18PubMed Central. Sacral insufficiency fracture, usually overlooked cause of lumbosacral pain
One study of patients with symptomatic vertebral compression fractures found sacral insufficiency fractures in about 3 percent of cases overall, but the rate climbed steeply with age: roughly 6.5 percent of patients over 80 had them, compared to under 2 percent of those younger than 80. Women outnumbered men by more than four to one.19PubMed Central. Osteoporotic sacral insufficiency fracture An easily neglected disease in elderly patients Standard X-rays frequently miss sacral insufficiency fractures because the sacrum is a complex three-dimensional shape that overlaps with bowel gas and pelvic structures on plain films. MRI or CT is usually needed to make the diagnosis.
For traumatic sacral fractures, multiple classification systems exist to describe the fracture pattern and guide treatment. Each pattern carries different risks for nerve damage, pelvic instability, and surgical complications.20PubMed Central. CT of Sacral Fractures: Classification Systems and Management
Lumbosacral Transitional Vertebrae
The boundary between the lumbar spine and the sacrum is not always where anatomy textbooks say it should be. In a substantial number of people, the lowest lumbar vertebra partially or fully fuses to the sacrum (sacralization), or the top sacral segment partially separates and takes on lumbar characteristics (lumbarization). These variants are collectively called lumbosacral transitional vertebrae, and they are far from rare: a systematic review put the average prevalence at about 15 percent, with individual studies reporting rates anywhere from roughly 4.5 to 36 percent.21PubMed Central. Prevalence, diagnosis, and clinical significance of lumbosacral transitional vertebrae: A systematic review and narrative analysis
The clinical headache with transitional vertebrae is twofold. First, they alter the biomechanics of the lower spine. The segment above the transitional vertebra takes on extra stress, and the abnormal articulation itself can become a pain generator. Low back pain in someone with a transitional vertebra may arise from the disc above, from the opposite-side facet joint, or from the anomalous joint between the enlarged transverse process and the sacrum.22PubMed Central. Lumbosacral transitional vertebrae: classification, imaging findings, and clinical relevance Second, transitional vertebrae create a counting problem: if a surgeon misidentifies which level is which, an operation can be performed at the wrong disc. Preoperative imaging needs to account for the variant to avoid this error.23PubMed Central. Sacral Dysmorphism and Lumbosacral Transitional Vertebrae (LSTV) Review
Sacral Tumors
Chordomas
The most common primary tumor of the sacrum is the chordoma, a slow-growing but malignant cancer thought to arise from leftover cells of the notochord, the embryonic structure that guides early spinal development. Sacral chordomas account for more than half of all sacral tumors.24PubMed Central. Surgical Management of Sacral Chordomas: Illustrative Cases and Current Management Paradigms They tend to grow silently for years before they are detected, partly because the sacrum has space for a mass to expand before it compresses anything critical.
Treatment is surgical, and the stakes are high. Complete removal of the tumor in one piece (en bloc resection) is the strongest predictor of long-term survival, but the operation is complex because the tumor is often entangled with sacral nerves and pelvic organs. In one large series, median survival was 84 months and five-year disease-specific survival was 77 percent, but local recurrence occurred in 40 percent of cases and was strongly linked to having had prior incomplete resections. The functional toll of surgery is considerable: roughly three-quarters of patients in that series needed to self-catheterize, about a third required bowel retraining, and two-thirds reported sexual dysfunction.25Spine. The Surgical Management of Sacral Chordomas
Sacrococcygeal Teratomas in Newborns
At the other end of the age spectrum, sacrococcygeal teratoma is the most common tumor in newborns, arising at the junction of the sacrum and coccyx. These tumors are present at birth, sometimes detected on prenatal ultrasound, and can range from small cystic masses to enormous growths. They are classified by how much of the tumor is external versus internal (the Altman system), and most are benign. A meta-analysis covering over 1,300 patients found an overall mortality rate of about 5 percent, a recurrence rate of 9 percent, and wound complications in roughly a quarter of cases. Longer-term functional problems are not trivial, with urinary dysfunction in about 15 percent and bowel dysfunction in about 21 percent of cases.26PubMed. Surgical outcomes and complications of sacrococcygeal teratoma in neonates and infants: A systematic review and meta-analysis stratified by tumor characteristics and surgical approach One institutional series reported overall survival of 95 percent at a mean follow-up of 54 months, with the female-to-male ratio running about three to one.27PubMed Central. Sacrococcygeal Teratoma: Clinical Characteristics, Management, and Long-term Outcomes in a Prospective Study from a Tertiary Care Center
Sacral Dimples in Babies
Many newborns have a small pit or dimple in the skin over the sacrum, which understandably worries parents. The concern is that a sacral dimple might signal spinal dysraphism, an underlying defect in the spinal cord or its coverings. In practice, a simple sacral dimple by itself carries a very low risk of hidden spinal pathology. A retrospective study of 151 infants who had spinal ultrasound for a sacral dimple found that 80 percent had normal results; of those with abnormal findings on ultrasound, only about 5 percent had abnormalities confirmed on MRI, and just 1 percent (two infants) needed neurosurgical intervention.28PubMed. Screening for Spinal Dysraphisms in Newborns With Sacral Dimples
The risk changes when a sacral dimple is accompanied by other skin markers, such as a tuft of hair, a skin tag, a hemangioma, or a dermal sinus tract (a small tract leading deeper toward the spine). A dermal sinus in particular carries a higher risk of underlying spinal cord problems, and the presence of multiple skin markers at once significantly increases the likelihood that something abnormal is going on.29PubMed. Occult spinal dysraphisms in newborns with skin markers: role of ultrasonography and magnetic resonance imaging The general guideline is that a simple, shallow dimple within a couple of centimeters of the anus, with no other findings, rarely needs imaging. Dimples that are large, high on the back, or accompanied by other markers warrant ultrasound and potentially MRI.
Pressure Injuries Over the Sacrum
The sacrum is the most common site for pressure ulcers (bedsores) in bedridden or immobile patients, because when you lie on your back the sacrum is a prominent bony point with relatively little padding between bone and skin. Even short-term immobilization on a rigid surface can be a problem: one study found that spine-board immobilization produced tissue pressures over the sacrum high enough to risk tissue breakdown.30PubMed. Pain and tissue-interface pressures during spine-board immobilization Monitoring tissue oxygen levels at the sacrum in immobile patients showed that sacral tissue oxygen saturation hovered around 73 percent and declined slightly over several hours of immobility, though the drop did not reach statistical significance within a four-hour window.31PubMed. Examination of sacral tissue oxygen saturation among immobile patients Regular repositioning and pressure-redistributing surfaces remain the mainstays of prevention.
Forensic Identification From the Sacrum
When forensic scientists need to determine the sex of skeletal remains, the pelvis is the gold standard, but it is not always intact. The sacrum, as part of the pelvis, can serve as a backup. Male sacra tend to be longer from top to bottom, while female sacra tend to be relatively wider, producing a higher sacral index (the ratio of width to length). One morphometric study found that the sacral index alone had an area under the receiver operating characteristic curve of 0.994, meaning it distinguished male from female sacra with near-perfect accuracy in that sample.32PubMed Central. Determination of Sexual Dimorphism of the Human Sacrum Based on Receiver Operating Characteristic Curve Analysis of Morphometric Parameters
Other studies have been less optimistic. A geometric morphometric analysis of sacral shape found extensive overlap between males and females, achieving only 72 percent classification accuracy.33PubMed Central. Sex classification using the human sacrum: Geometric morphometrics versus conventional approaches A Spanish population study reached about 81 percent accuracy when combining multiple measurements.34PubMed. Sex determination in a Spanish population based on sacrum The discrepancy probably reflects differences in which measurements were used, which populations were studied, and how much individual variation exists within any group. The practical takeaway is that the sacrum can contribute to sex determination, especially when combined with other skeletal features, but it is not as reliable on its own as the full pelvis or skull.
Rare Congenital Sacral Conditions
At the extreme end of sacral developmental problems is caudal regression syndrome, a rare congenital malformation in which the lower spine and spinal cord fail to develop fully. Estimates put its incidence at roughly 1 to 2 per 100,000 live births. The spectrum is wide: some affected individuals are missing only the coccyx, while others have absence of multiple sacral and lumbar segments, with corresponding deficits in lower limb function, bladder control, and bowel function. MRI findings can include coccygeal agenesis, sacralization of remaining vertebrae, and spinal cord defects.35PubMed Central. Caudal Regression Syndrome First Diagnosed in Adulthood: A Case Report and a Review of the Literature Mild forms occasionally go undiagnosed until adulthood, discovered incidentally on imaging done for other reasons.