The sacrum is a triangular bone at the base of the spine, positioned between the two hip bones and just above the tailbone. It connects the spine to the pelvis, helps transfer the body’s weight to the lower limbs, and provides stability during standing, walking, and other everyday movements. Although it is a single bone in most adults, the sacrum develops from five separate vertebrae that gradually fuse during growth.
Understanding sacrum anatomy helps explain how the lower spine and pelvis work together, why this region is important for posture and movement, and how problems affecting the sacrum can contribute to lower back or pelvic pain.
Where is the sacrum located?
The sacrum sits at the bottom of the vertebral column, below the lumbar spine and above the coccyx, commonly called the tailbone. It forms the back wall of the pelvic cavity, the space that contains organs such as the bladder and parts of the reproductive and digestive systems.
Its upper portion connects with the fifth lumbar vertebra, the lowest vertebra in the lower back. This junction is called the lumbosacral joint. The sacrum’s lower tip connects with the coccyx through the sacrococcygeal joint.
On either side, the sacrum meets the ilium, the large, wing-shaped part of each hip bone. These connections form the right and left sacroiliac joints. Together, the sacrum, hip bones, and associated joints create a strong framework that links the spine to the lower body.
The sacrum is positioned centrally, but it is not perfectly vertical. It curves forward into the pelvic cavity, contributing to the natural shape of the lower spine and pelvis. Its orientation varies among individuals, influenced by anatomy, posture, and the shape of the surrounding bones.
Although the sacrum is deep within the body, its location can be approximated from the surface. The two small dimples sometimes visible on the lower back often lie near the posterior superior iliac spines, bony landmarks on the back of the pelvis. These landmarks are close to the sacroiliac joints, but the dimples do not precisely identify the boundaries of the sacrum.
What is the sacrum made of?
The sacrum develops from five sacral vertebrae, designated S1 through S5. These bones begin as separate structures and gradually fuse during adolescence and early adulthood. The timing and completeness of fusion vary, but in most adults the five vertebrae form one solid, wedge-shaped bone.
The uppermost segment, S1, is the largest and bears much of the load transferred from the lumbar spine. Each successive segment generally becomes smaller toward the bottom, giving the sacrum its characteristic triangular shape.
The sacrum has a broad upper base, narrow lower apex, and two lateral portions that connect with the hip bones. Its front surface is relatively smooth and concave, curving toward the pelvic cavity. Its back surface is irregular, with ridges and openings associated with the fused vertebrae and their processes.
Several structural features make the sacrum well suited to its supporting role:
- Sacral base: The broad upper surface that articulates with the fifth lumbar vertebra.
- Sacral promontory: A forward-projecting edge of the upper sacrum that contributes to the boundary of the pelvic inlet.
- Sacral wings, or alae: Broad areas on either side of the upper sacrum that help distribute forces toward the pelvis.
- Auricular surfaces: Ear-shaped areas on the sides of the sacrum that meet corresponding surfaces on the ilia to form the sacroiliac joints.
- Sacral canal: A passage running through the bone that continues the vertebral canal and contains the lower end of the spinal canal’s nerve-containing space.
- Sacral foramina: Openings on the front and back surfaces through which sacral spinal nerve branches pass.
- Sacral hiatus: An opening near the lower end of the sacrum, created by incomplete formation of the back wall of the lowest sacral vertebrae.
The sacrum’s compact, fused structure provides strength while preserving openings for nerves and pathways for the transmission of forces through the pelvis.
The sacrum’s front, back and internal anatomy
The front surface of the sacrum, known as the pelvic surface, faces the pelvic cavity. It is concave and relatively smooth. Four transverse ridges usually mark the former boundaries between the five sacral vertebrae. On either side of these ridges are four pairs of anterior sacral foramina, which allow the anterior branches of the sacral spinal nerves to emerge.
The back surface, called the dorsal surface, faces outward toward the skin and back muscles. It has several prominent ridges formed by the fused vertebral processes. These include the median sacral crest, created by the fused spinous processes of the upper sacral vertebrae, and additional ridges formed by other vertebral processes. Four pairs of posterior sacral foramina allow the posterior branches of the sacral spinal nerves to pass through.
Inside the bone, the sacral canal continues downward from the vertebral canal of the lumbar spine. It contains nerve roots and associated structures, although the spinal cord itself usually ends higher up, around the upper lumbar region in adults. Below that point, the canal contains the cauda equina, a bundle of nerve roots extending downward from the end of the spinal cord.
The sacral canal narrows toward the bottom and opens through the sacral hiatus. This opening is clinically important because it can provide access to the epidural space for certain anesthetic procedures, including a caudal epidural injection.
These front, back, and internal features reflect the sacrum’s dual role: it must withstand substantial mechanical forces while allowing nerves to travel between the spine, pelvis, and lower limbs.
How the sacrum connects the spine to the pelvis
The sacrum serves as the structural bridge between the vertebral column and the pelvic ring. Three major articulations contribute to this connection: the lumbosacral joint above, the two sacroiliac joints on either side, and the sacrococcygeal joint below.
The lumbosacral joint connects the fifth lumbar vertebra to the upper surface of the sacrum. It includes a disc between the vertebral bodies and paired facet joints at the back. The disc cushions compressive forces, while the facet joints help guide movement and limit excessive displacement. Because the lower back transitions into the relatively rigid pelvis at this point, the junction experiences substantial mechanical stress.
The sacroiliac joints connect the sacrum to the ilia. Their surfaces are irregular and reinforced by strong ligaments, which help resist slipping and rotation. These joints permit small movements, but they are much less mobile than the joints of the hip or shoulder.
This limited movement is important. The pelvis needs enough flexibility to accommodate changes in posture and movement, but it must remain stable enough to transmit body weight efficiently. The sacroiliac joints balance these requirements through their joint surfaces, ligaments, and surrounding muscles.
At the bottom, the sacrococcygeal joint connects the sacrum to the coccyx. It generally allows only limited movement, although the degree of mobility varies. The coccyx serves as an attachment site for several muscles and ligaments involved in pelvic floor support.
The main functions of the sacrum
The sacrum is more than a connection between bones. Its shape, joints, and nerve pathways enable several essential functions.
Supporting and transferring body weight
One of the sacrum’s most important jobs is to transmit the weight of the upper body from the spine to the pelvis. When a person stands, sits, or walks, forces travel from the lumbar vertebrae into the sacrum and then through the sacroiliac joints to the hip bones.
From the pelvis, these forces are distributed toward the hip joints and lower limbs. The sacrum therefore plays a central role in supporting body weight and maintaining balance.
Its broad upper surface and wedge-like shape help it fit securely between the hip bones. Strong ligaments resist forces that might otherwise push the sacrum downward or cause it to rotate excessively. The combined effect of bone geometry and soft-tissue support allows the pelvis to remain stable under load.
Providing stability during movement
Walking, running, climbing stairs, and standing on one leg all require coordinated movement of the spine and pelvis. The sacrum provides a stable foundation for these activities by linking the lumbar spine to both sides of the pelvic ring.
The sacroiliac joints move slightly as forces change, but their primary function is stability rather than a large range of motion. Ligaments and surrounding muscles help control these small movements and distribute mechanical stress.
This stability also supports efficient movement. If the sacrum and pelvis could shift freely under load, more muscular effort would be required to maintain balance and control the legs.
Protecting nerves and supporting nerve function
The sacrum surrounds the lower portion of the spinal canal and provides protected passageways for sacral nerve roots and their branches. These nerves contribute to sensation and movement in parts of the lower limbs and carry signals important for pelvic organ function.
The sacral spinal nerves are especially important for muscles involved in bowel and bladder control, sexual function, and aspects of pelvic floor activity. Nerves from the sacral region also contribute to the sciatic nerve, the large nerve that supplies much of the lower limb.
Damage to the sacrum or the nerves passing through it can therefore affect more than local comfort. Depending on the location and severity of the injury, symptoms may include pain, numbness, weakness, or changes in bowel, bladder, or sexual function.
Forming the back of the pelvic cavity
The sacrum forms much of the posterior wall of the pelvis. Its concave front surface helps define the space occupied by pelvic organs, while its connections with the hip bones contribute to the overall shape of the pelvic ring.
The sacrum also provides attachment points for ligaments and muscles that help stabilize the spine, pelvis, and hip region. Together, these structures support posture and contribute to coordinated movement of the trunk and lower body.
How the sacrum develops and changes with age
Early in development, the five sacral vertebrae are separate structures, much like the vertebrae in other parts of the spine. As a person grows, their bones and cartilage mature, and the sacral segments progressively fuse. This process generally becomes more advanced during adolescence and early adulthood.
Fusion makes the sacrum more rigid than the individual vertebrae of the lumbar spine. That rigidity helps the bone transfer weight and resist forces generated during movement.
The degree of fusion and the precise shape of the sacrum vary from person to person. Some individuals have developmental variations in which part of the lowest lumbar vertebra is incorporated into the sacrum, a condition known as sacralization. Others have a transitional anatomy in which the uppermost sacral vertebra more closely resembles a lumbar vertebra. Such variations are not necessarily associated with symptoms.
The sacrum’s position and orientation also differ among individuals. Pelvic shape, spinal curvature, age, and other anatomical factors influence how the sacrum aligns with the lumbar spine and hip bones.
During pregnancy, hormonal changes increase the flexibility of some pelvic ligaments, and the pelvic joints may accommodate small changes in movement. The sacrum remains an important weight-bearing structure throughout pregnancy and childbirth, although the extent of joint motion varies.
Common conditions that affect the sacrum
Pain in the lower back or buttock region does not automatically indicate a problem with the sacrum. Muscles, spinal discs, facet joints, hip joints, and other structures can produce similar symptoms. Identifying the cause depends on the location of the pain, associated symptoms, physical examination, and, when appropriate, imaging.
Several conditions can directly or indirectly affect the sacrum.
Sacroiliac joint dysfunction
The sacroiliac joints can become a source of pain when their surrounding structures are irritated or when joint mechanics are altered. Symptoms often occur near one or both buttocks and may spread into the lower back, groin, or upper thigh.
The pain can resemble discomfort caused by lumbar spine problems, making diagnosis difficult. No single symptom reliably identifies the sacroiliac joint as the source. Clinicians may use a combination of examination maneuvers and other findings to determine whether the joint is likely contributing to the pain.
Sacral fractures
A sacral fracture is a break in the bone. It can occur after substantial trauma, such as a vehicle collision or a fall, but it can also develop when weakened bone cannot withstand ordinary loading.
In older adults and people with osteoporosis, a sacral insufficiency fracture may occur without a major injury. Osteoporosis reduces bone strength, allowing normal stresses from standing or walking to produce a fracture.
Symptoms commonly include pain in the lower back, buttocks, or pelvis that worsens with standing or walking. Some fractures are difficult to identify on ordinary X-rays, so additional imaging may be necessary when clinical suspicion remains high.
Fractures that affect the openings or canal through which nerves travel can cause neurological symptoms. Severe or worsening weakness, numbness around the groin or saddle region, or new loss of bowel or bladder control requires urgent medical assessment.
Sacral nerve compression or injury
The sacral nerve roots can be affected by fractures, tumors, inflammatory conditions, or other disorders involving the spine and pelvis. Depending on which nerves are involved, symptoms may include radiating pain, tingling, reduced sensation, weakness, or impaired pelvic organ function.
The term sciatica describes pain that follows the course of the sciatic nerve, often extending from the buttock down the leg. Although sacral nerve roots contribute to the sciatic nerve, sciatica is frequently associated with conditions in the lumbar spine rather than a problem within the sacrum itself.
New urinary retention, loss of bladder or bowel control, numbness around the saddle region, or rapidly progressing leg weakness may indicate serious compression of the lower spinal nerve roots. These symptoms require emergency evaluation.
Sacral tumors and other bone disorders
Tumors can develop in the sacrum, either originating in the bone or spreading there from another part of the body. One uncommon primary bone tumor associated with this region is chordoma, which arises from remnants of embryonic notochord tissue.
Persistent or progressive pain, particularly when accompanied by other concerning symptoms, may require investigation. However, most lower back and buttock pain is not caused by a sacral tumor.
Other conditions, including infections and inflammatory disorders, can also affect the sacrum or the sacroiliac joints. The appropriate evaluation depends on the symptoms, medical history, and clinical findings.
How clinicians evaluate the sacrum
Evaluation begins with a medical history and physical examination. A clinician may ask where the pain occurs, when it began, whether it followed an injury, and whether it changes with movement or weight-bearing. The examination may include assessment of spinal and hip movement, tenderness, strength, sensation, reflexes, and other signs of nerve involvement.
Imaging is used when the history and examination suggest a fracture, significant structural problem, infection, tumor, or another condition that requires further investigation.
X-rays can show the general shape and alignment of the sacrum and pelvis, although overlapping structures can make some fractures difficult to see. Computed tomography, or CT, provides detailed images of bone and can help identify fractures and complex bony changes. Magnetic resonance imaging, or MRI, is particularly useful for evaluating bone marrow, soft tissues, and nerve-related abnormalities, as well as certain fractures that are not apparent on initial X-rays.
Not every episode of lower back pain requires imaging. Many cases improve with time and appropriate conservative care, and imaging findings do not always explain a person’s symptoms. Clinicians consider the overall pattern of symptoms and examination findings rather than relying on a scan alone.
Treatment depends on the underlying cause. It may include activity modification, pain management, physical therapy, treatment of an underlying bone disorder, or specialized procedures when indicated. Significant fractures, neurological complications, infections, and tumors may require more targeted care.
Why the sacrum matters to everyday movement
The sacrum works largely out of sight, but it is central to the mechanics of the lower body. By joining the spine to the pelvis, it transfers weight, supports balance, limits unwanted movement, and protects important nerve pathways.
Its strength comes from the fusion of five vertebrae, while its joints and ligaments allow the small amount of movement needed for the pelvis to adapt to changing loads. This combination of rigidity and controlled flexibility is essential for activities ranging from standing still to walking and lifting.
Understanding sacrum anatomy also helps explain why pain in this region can have several different causes. The sacrum, sacroiliac joints, lumbar spine, muscles, and nerves work closely together, so symptoms in one area may reflect a problem elsewhere. Recognizing the sacrum’s position and function provides a useful foundation for understanding both normal movement and disorders affecting the lower back and pelvis.
