Contents

SacrumEdit

The sacrum is a large, triangular bone at the base of the spine that forms a central part of the pelvic girdle. It results from the fusion of five sacral vertebrae and serves as a keystone that distributes the weight of the upper body into the pelvis and lower limbs. The bone anchors a network of ligaments and muscles that stabilize the spine and pelvis, and it provides channels for nerves and vessels that traverse the pelvic region. As the lower portion of the axial skeleton, the sacrum interacts closely with the Vertebral column above and the Pelvis below, and its integrity is central to posture, locomotion, and overall biomechanical efficiency.

The sacrum’s position and shape reflect its role in transferring forces from the upper body to the hips. Its curved base links with the lumbar spine at the lumbosacral junction, while its apex articulates with the coccyx. The wide upper end (the base) faces anteriorly and superiorly, and the narrower lower end (the apex) points downward and anteriorly. This configuration helps convert bending and compression loads into stable pelvic support. The sacrum also contributes to the formation of the pelvic cavity, the posterior boundary of which is shaped in part by the dorsal surface of the bone.

Anatomy

Structure

  • The sacrum is formed by fusion of five sacral vertebrae (S1–S5). In early adulthood these elements typically unite to create a single bone, though small variations can persist. The base of the sacrum interfaces with the L5 vertebra at the lumbosacral joint, while the apex rests near the coccyx at the sacrococcygeal joint.
  • The median sacral crest on the posterior surface represents the fused spinous processes of S1–S4, and the transverse ridges indicate the fused transverse processes. Lateral features form the alae, or winglike expansions, which give the sacrum its broad, geographically evident lateral extent.
  • The anterior (pelvic) surface is smoother and presents four anterior sacral foramina on each side, through which ventral rami of sacral nerves exit. The posterior surface has four corresponding posterior sacral foramina for dorsal rami.

Articulations

  • Superiorly, the sacrum meets the L5 vertebra at the lumbosacral junction, forming a pivotal load-bearing joint with limited motion that enhances stability.
  • Laterally, the sacrum articulates with the ilia of the pelvis at the Sacroiliac joints, forming a strong, weight-transferring connection between the spine and the pelvis.
  • Inferiorly, the sacrum connects with the coccyx at the sacrococcygeal joint, completing the posterior part of the pelvic ring.
  • The sacral foramina transmit sacral nerves, while the sacral canal encloses the distal portion of the spinal cord or cauda equina in adults.

Surfaces and features

  • The base bears the sacral promontory, a prominent anterior projection that has clinical and anatomical significance in obstetrics and pelvic surgery.
  • The anterior surface is marked by the ventral sacral foramina, which convey the anterior rami of the sacral nerves.
  • The posterior surface features the median crest, lateral crests, and the posterior sacral foramina, guiding the origins and paths of the sacral nerves.

Nerves and vessels

  • The sacrum is intimately related to the sacral plexus, through which several significant nerves pass, including branches that contribute to the sciatic and pudendal nerves.
  • Blood supply is primarily from branches of the internal iliac arteries, with contributions from surrounding pelvic vessels. The vascular network supports the bony matrix and the adjacent neural structures.

Development and variation

  • The sacrum develops from the fusion of five originally separate vertebrae (S1–S5). The fusion typically completes in late adolescence or early adulthood, producing a single bone that can vary slightly in its degree of fusion and shape.
  • Variants include sacralization, in which the fifth lumbar vertebra partially fuses with the sacrum, and lumbarization, where S1 shows segments that resemble a lumbar vertebra. These variations can influence spinal biomechanics and susceptibility to certain conditions at the lumbosacral junction.

Clinical significance

  • Sacral fractures, though less common than injuries higher in the spine, can accompany high-energy trauma and can destabilize the pelvic ring or injure sacral nerves. Recognition is important in trauma assessment and pelvic radiography or CT imaging.
  • Sacroiliac joint disorders, including sacroiliitis, involve pain and dysfunction at the joint between the sacrum and the ilium. These conditions can arise from mechanical overload, inflammatory disease, or trauma and are a frequent source of low back and buttock pain.
  • Pathologies affecting the sacral nerves, such as compression or irritation at the sacral foramina or within the sacral canal, can lead to radicular symptoms in the buttocks and legs or to deficits in pelvic floor function.
  • The sacrum’s role in weight transfer and pelvic stability makes it a focal point in orthopedic and rehabilitative care, particularly in procedures involving the pelvis, lower spine, or during childbirth considerations where pelvic alignment and sacral positioning are relevant.

See also