Sacrum bone

The sacrum is a large, triangular bone situated at the base of the vertebral column and forms the posterior portion of the pelvic cavity. It serves as a critical structural component that connects the spine to the pelvis, transmitting the weight of the upper body to the lower limbs. The sacrum also provides protection for the spinal nerves and forms part of the bony framework of the pelvic girdle.

Introduction

Overview of the Sacrum

The sacrum is a wedge-shaped bone composed of five fused sacral vertebrae (S1–S5). It lies between the two hip bones, articulating laterally with the ilia at the sacroiliac joints and inferiorly with the coccyx. Functionally, the sacrum plays a pivotal role in supporting the vertebral column, stabilizing the pelvis, and serving as a conduit for the sacral nerves that form part of the lumbosacral plexus.

Definition and General Description

Anatomically, the sacrum forms the posterior wall of the pelvis and contributes to the pelvic ring’s stability. It tapers inferiorly to articulate with the coccyx and is curved to conform to the concavity of the pelvic cavity. Its anterior surface is smooth and concave, while the posterior surface is rough and convex. Numerous foramina perforate the sacrum to transmit spinal nerves and blood vessels, reflecting its dual role as both a structural and neuroprotective bone.

Historical Background and Nomenclature

The term “sacrum” originates from the Latin word os sacrum, meaning “sacred bone,” reflecting its significance in ancient anatomical and spiritual traditions. Early anatomists believed it to be the last bone to decay after death and regarded it as sacred. Modern anatomy recognizes it as a fusion of the five sacral vertebrae, a process that begins during adolescence and completes in early adulthood, forming a single solid structure that contributes to pelvic stability.

Functional Importance in the Skeletal System

The sacrum is a keystone in the human skeletal framework. It transmits the load from the vertebral column to the pelvic girdle and lower limbs, acts as an anchor for major ligaments and muscles, and forms the posterior boundary of the pelvic cavity. Its curvature and shape also influence posture and spinal alignment, while its foramina provide passage for the sacral spinal nerves that innervate the lower body and pelvic organs.

Gross Anatomy of the Sacrum

Location and Orientation

The sacrum occupies a central position in the posterior part of the pelvic cavity. It is wedged between the two iliac bones, with its broad base articulating superiorly with the fifth lumbar vertebra (L5) and its narrow apex connecting inferiorly with the coccyx. The anterior surface faces forward and downward, forming the posterior wall of the true pelvis, while the posterior surface faces backward and forms part of the sacral region of the back.

• Position within the Vertebral Column: The sacrum is the fourth major division of the vertebral column, situated between the lumbar vertebrae above and the coccyx below.
• Relation to the Pelvis and Coccyx: It articulates with the ilia of the hip bones at the sacroiliac joints and with the coccyx at the sacrococcygeal joint.
• Curvature and Alignment: The sacrum is concave anteriorly and convex posteriorly, contributing to the pelvic curvature that supports the abdominal and pelvic viscera.

General Shape and Structure

The sacrum is roughly triangular, with a broad base superiorly and a narrow apex inferiorly. It exhibits a gentle curvature that is more pronounced in females to accommodate the wider pelvic cavity necessary for childbirth. Its structure reflects the fusion of five sacral vertebrae, with visible transverse lines marking the points of fusion on the anterior surface.

• Base: The upper end of the sacrum articulates with the body of the fifth lumbar vertebra at the lumbosacral angle. The base contains the sacral promontory, a prominent ridge that projects forward into the pelvic cavity.
• Apex: The tapered inferior end of the sacrum articulates with the coccyx through the sacrococcygeal joint, which allows slight movement.
• Anterior (Pelvic) Surface: Smooth and concave, it bears four transverse lines indicating the fusion of the sacral vertebrae and anterior sacral foramina for the passage of ventral rami of the sacral nerves.
• Posterior (Dorsal) Surface: Rough and convex, it displays bony ridges and crests that mark the fusion of vertebral processes and contains posterior sacral foramina for the dorsal rami of the sacral nerves.
• Lateral and Superior Views: The lateral sides bear auricular surfaces that articulate with the ilium, while the superior surface includes the sacral promontory and superior articular processes for articulation with L5.

Surfaces and Landmarks

Anterior (Pelvic) Surface

The anterior surface of the sacrum is smooth and concave, facing the pelvic cavity. It serves as the posterior boundary for pelvic organs and provides attachment points for the pelvic fascia and ligaments.

• Transverse Lines: Four horizontal ridges represent the fused intervertebral discs of the sacral vertebrae, providing a landmark for vertebral segmentation.
• Anterior Sacral Foramina: These are four pairs of openings on each side of the anterior surface through which the ventral rami of the sacral spinal nerves exit to supply the pelvis and lower limbs.
• Promontory: The anterior projecting edge of the base of the sacrum forms the sacral promontory, a key landmark in obstetrics for measuring the pelvic inlet.
• Ala (Wings) of Sacrum: Lateral expansions at the base of the sacrum articulate with the ilium to form the sacroiliac joints.

Posterior (Dorsal) Surface

The posterior surface of the sacrum is convex and irregular, providing attachment sites for muscles and ligaments that stabilize the pelvis and vertebral column. It features several ridges and foramina for neurovascular passage.

• Median Sacral Crest: Formed by the fused spinous processes of S1–S5, this ridge serves as an attachment for ligaments of the vertebral column.
• Intermediate Sacral Crest: Corresponds to the fused articular processes and provides sites for ligament and muscle attachment.
• Lateral Sacral Crest: Formed by fused transverse processes and serves as an attachment for the sacroiliac ligaments.
• Posterior Sacral Foramina: Four pairs of openings allow the dorsal rami of the sacral nerves to exit and supply the muscles and skin of the back.
• Sacral Hiatus: An inferior opening resulting from non-fusion of the S5 vertebral laminae, it is a clinically significant site for caudal epidural anesthesia.
• Sacral Cornua: Bony projections flanking the sacral hiatus that articulate with the coccygeal cornua.

Lateral Surfaces

• Auricular Surface: Smooth, ear-shaped area that articulates with the ilium at the sacroiliac joint.
• Sacral Tuberosity: Roughened area posterior to the auricular surface for ligament attachment.

Superior and Inferior Features

• Superior Articular Processes: Project upward to articulate with the inferior articular processes of L5 vertebra.
• Sacral Canal: Continuation of the vertebral canal within the sacrum, housing the sacral nerves and filum terminale.
• Apex: Narrow inferior end that articulates with the base of the coccyx via the sacrococcygeal joint.

Osteological Features and Landmarks

The sacrum possesses several distinctive osteological features that serve as important landmarks for anatomical identification, surgical intervention, and neurovascular passage. These features are crucial for understanding sacral function and relationships within the pelvis.

Sacral Canal and Its Openings

The sacral canal is a continuation of the vertebral canal and extends from the base to the sacral hiatus. It contains the sacral portion of the spinal cord, nerve roots, and meninges. The canal opens anteriorly through the anterior sacral foramina and posteriorly through the posterior sacral foramina, providing exit points for sacral nerves.

Sacral Hiatus and Cornua

The sacral hiatus is an inferior opening of the sacral canal, formed due to incomplete fusion of the S5 laminae. Flanking the hiatus are the sacral cornua, which articulate with the coccygeal cornua. These structures serve as important landmarks for administering caudal epidural anesthesia and for surgical orientation.

Promontory of the Sacrum

The sacral promontory is the anterior projecting edge of the S1 vertebra at the base of the sacrum. It forms the posterior boundary of the pelvic inlet and serves as a reference point in obstetric and orthopedic measurements. Its prominence and location are significant for assessing pelvic dimensions and planning surgical procedures.

Intervertebral Fusion Lines

Four transverse ridges on the anterior surface of the sacrum mark the lines of fusion between the five sacral vertebrae. These ridges are palpable landmarks and provide information about the degree of sacral ossification, which can be relevant in radiographic evaluation and forensic identification.

Articulations

The sacrum articulates with multiple skeletal elements to form the posterior aspect of the pelvic girdle and connect the vertebral column to the lower limbs. These articulations are essential for weight transmission, pelvic stability, and movement.

Lumbosacral Articulation (L5-S1 Joint)

The superior articular processes of the sacrum articulate with the inferior articular processes of the fifth lumbar vertebra, forming the lumbosacral joint. This joint allows limited flexion, extension, and rotation while supporting the weight of the upper body. The orientation of the articular surfaces contributes to spinal stability and the distribution of mechanical forces.

Sacroiliac Joint (Articulation with Ilium)

The lateral surfaces of the sacrum contain auricular surfaces that articulate with the ilium to form the sacroiliac joints. These joints are strong, weight-bearing synovial and syndesmotic joints reinforced by multiple ligaments. They transmit forces from the spine to the pelvis and lower limbs and provide limited mobility necessary for walking, standing, and childbirth.

Sacrococcygeal Joint

The apex of the sacrum articulates with the base of the coccyx, forming the sacrococcygeal joint. This joint is a cartilaginous symphysis reinforced by ligaments and allows slight movement of the coccyx, which is important for sitting and absorbing mechanical stresses transmitted through the pelvis.

Muscle Attachments

The sacrum serves as an important site for the origin and insertion of several muscles and ligaments that contribute to spinal stability, pelvic movement, and postural support. These attachments are critical for locomotion, maintaining upright posture, and supporting pelvic organs.

Muscles Originating from the Sacrum

• Erector Spinae: Portions of the erector spinae muscles, including the sacral fibers, originate from the dorsal surface of the sacrum, aiding in extension and lateral flexion of the vertebral column.
• Multifidus: Arises from the posterior sacral surface and lateral crests, contributing to spinal stabilization and posture control.
• Gluteus Maximus: Some fibers originate from the posterior aspect of the sacrum and sacrotuberous ligament, assisting in hip extension and lateral rotation.
• Piriformis: Originates from the anterior surface of the sacrum, exiting the pelvis via the greater sciatic foramen, and functions in lateral rotation and abduction of the thigh.

Muscles Inserting onto the Sacrum

• Pelvic Floor Muscles: Portions of the levator ani and coccygeus muscles attach to the lateral aspects of the sacrum, providing support to pelvic organs and maintaining continence.

Ligamentous Attachments

• Sacrospinous Ligament: Connects the sacrum to the ischial spine, stabilizing the sacroiliac joint and forming the greater and lesser sciatic foramina.
• Sacrotuberous Ligament: Runs from the sacral apex and lateral sacral crest to the ischial tuberosity, resisting sacral rotation and providing stability to the pelvis.
• Iliolumbar Ligaments: Attach superiorly from the sacrum to the transverse processes of L5, stabilizing the lumbosacral junction.

Relations and Boundaries

The sacrum occupies a central position within the pelvic cavity and is related to numerous bony, muscular, vascular, and visceral structures. Understanding these relations is essential for surgical approaches, anesthetic procedures, and evaluation of pelvic pathologies.

Relations to Pelvic Organs

• Rectum: Lies directly anterior to the sacrum, separated by the presacral fascia, with a potential space for surgical access or abscess formation.
• Uterus (in females): Positioned anterior to the sacral promontory, contributing to the curvature of the sacral concavity in the pelvic cavity.
• Bladder: Located anteroinferiorly, in close proximity to the lower sacral segments and the sacrococcygeal junction.

Relations to Major Blood Vessels and Nerves

• The sacrum is closely associated with the median sacral artery, lateral sacral arteries, and their venous counterparts, which run along its anterior surface.
• The sacral plexus lies anterior to the sacrum, with sacral nerve roots exiting through the anterior sacral foramina to supply the pelvis and lower limbs.

Relations to the Coccyx and Pelvic Floor Musculature

• The apex of the sacrum articulates with the coccyx, forming the sacrococcygeal joint, and serves as an anchor point for pelvic floor muscles including the levator ani and coccygeus.
• The lateral surfaces provide attachment for ligaments and fascia that integrate the sacrum into the pelvic floor, supporting pelvic organ function and contributing to continence mechanisms.

Vasculature and Nerve Supply

The sacrum is richly supplied by arterial, venous, and neural structures that ensure its functional integration within the pelvis and vertebral column. These neurovascular pathways also have clinical significance in surgery, anesthesia, and trauma management.

Arterial Supply

• Lateral Sacral Arteries: Branches of the internal iliac artery that run along the anterior surface of the sacrum, supplying the sacral vertebrae, muscles, and overlying soft tissues.
• Median Sacral Artery: A small unpaired vessel arising from the posterior aspect of the abdominal aorta, descending over the midline of the sacrum to supply its vertebral bodies and coccyx.
• Contribution from Iliolumbar Arteries: These vessels provide minor branches to the sacral base and lateral aspects, reinforcing the sacral blood supply.

Venous Drainage

• The sacral venous plexus collects blood from the sacral vertebrae and surrounding tissues and communicates with the internal vertebral venous plexus and the pelvic venous network, providing a pathway for venous return and potential spread of infections.

Nerve Innervation

• Sacral Nerve Roots: Ventral and dorsal rami exit through the anterior and posterior sacral foramina, forming part of the sacral plexus which innervates the lower limbs, pelvis, and perineum.
• Autonomic Fibers: Sympathetic and parasympathetic fibers traverse the sacral foramina to supply pelvic organs.

Lymphatic Drainage

• Lymph from the sacrum drains primarily into the internal and external iliac lymph nodes, with connections to presacral and sacral lymphatic chains, playing a role in immune surveillance and metastatic spread.

Development and Ossification

The sacrum develops from the fusion of five sacral vertebrae during adolescence. Its ossification process and fusion patterns are critical for pelvic stability and vertebral alignment.

Embryological Origin

The sacrum originates from the sclerotome portions of the mesoderm of the somites, which form the vertebral bodies. Each sacral vertebra initially ossifies independently before undergoing progressive fusion to form a single, solid bone that integrates with the pelvis.

Primary and Secondary Ossification Centers

• Five primary centers of ossification exist, one in each sacral vertebral body, appearing in early fetal life.
• Secondary ossification centers develop in the vertebral arches and apices, facilitating growth and fusion during adolescence.

Timeline of Fusion (Sacral Vertebrae S1–S5)

• Fusion of the sacral bodies begins in late adolescence and is typically complete by the third decade of life.
• Fusion starts at S1–S2 and progresses downward to S5, resulting in a rigid, triangular structure that supports the pelvis.

Formation of Sacral Canal and Curvature

The sacral canal, a continuation of the vertebral canal, forms as the vertebral arches fuse dorsally. The anterior concavity and posterior convexity develop in response to biomechanical forces, shaping the sacral curvature that is essential for weight transmission and pelvic anatomy.

Histology

The sacrum, like other vertebral bones, consists of compact and cancellous bone tissue organized to provide strength, support, and flexibility. Its histological structure allows for weight transmission, muscle attachment, and protection of neural elements.

Composition of Compact and Cancellous Bone

The outer surface of the sacrum is composed of dense compact bone, which provides rigidity and protects the internal structures. The interior contains cancellous (spongy) bone, consisting of trabeculae oriented along lines of stress. This arrangement allows the sacrum to withstand compressive forces from the vertebral column and transmit them to the pelvis.

Microscopic Structure (Haversian Systems and Trabeculae)

Under the microscope, the compact bone of the sacrum is arranged into Haversian systems or osteons, each containing a central canal with blood vessels and nerves. Volkmann’s canals interconnect osteons, facilitating nutrient and waste exchange. The cancellous bone features trabeculae lined with endosteum, providing structural support while housing bone marrow.

Bone Marrow Distribution

The cancellous spaces within the sacral vertebrae contain red bone marrow in younger individuals, which gradually converts to yellow marrow with age. This marrow is involved in hematopoiesis and contributes to the metabolic activity of the bone.

Functions

The sacrum serves multiple critical functions in the human body, including structural support, protection, and facilitation of locomotion and pelvic organ function.

Structural Support and Weight Transmission

The sacrum acts as a keystone in the pelvic girdle, transmitting the weight of the upper body from the lumbar vertebrae to the pelvis and lower limbs. Its triangular shape and fused vertebral bodies provide stability and distribute mechanical loads efficiently during standing, walking, and other movements.

Stabilization of the Pelvis

The sacroiliac joints and ligamentous attachments to the sacrum stabilize the pelvis. These structures prevent excessive movement while allowing slight mobility for shock absorption and flexibility during activities such as walking or childbirth.

Protection of Neural Elements

The sacral canal and foramina protect the sacral spinal nerves as they exit to form the sacral plexus. These nerves provide motor and sensory innervation to the lower limbs, pelvis, and perineum. The sacrum’s rigid structure safeguards these neural pathways from compression and injury.

Attachment for Ligaments and Muscles

The sacrum provides attachment sites for numerous muscles and ligaments, including the erector spinae, gluteus maximus, piriformis, and pelvic floor muscles. Ligaments such as the sacrospinous and sacrotuberous ligaments anchor the sacrum to the pelvis, contributing to stability and facilitating coordinated movement.

Anatomical Variations

The sacrum exhibits several anatomical variations that can influence pelvic structure, spinal alignment, and clinical interventions. Awareness of these variations is important for surgical planning, radiological interpretation, and understanding biomechanical differences among individuals.

Variation in Sacral Curvature and Size

The degree of sacral curvature can vary significantly between individuals, with a more pronounced anterior concavity seen in females to accommodate the pelvic cavity for childbirth. The overall size and width of the sacrum also differ, affecting the dimensions of the pelvic inlet and outlet.

Lumbarization and Sacralization

Lumbarization refers to the condition where the first sacral vertebra (S1) remains separate from the sacrum, resembling a lumbar vertebra. Sacralization occurs when the fifth lumbar vertebra (L5) fuses partially or completely with the sacrum. Both variations alter spinal biomechanics and may predispose to back pain or altered pelvic mechanics.

Differences Between Male and Female Sacrum

Male sacra are typically longer, narrower, and more curved posteriorly, while female sacra are shorter, wider, and less curved to accommodate the dimensions of the birth canal. These sexual dimorphisms have clinical relevance in obstetrics and orthopedics.

Accessory Sacral Foramina or Fusion Anomalies

Occasionally, additional sacral foramina or incomplete fusion of sacral vertebrae may be observed. These anomalies can impact the exit paths of sacral nerves and influence the approach for caudal epidural injections or spinal surgeries.

Clinical Significance

The sacrum is involved in numerous clinical conditions and surgical procedures. Its anatomy, variations, and relationships with surrounding structures have significant implications for diagnosis, intervention, and patient outcomes.

Fractures and Dislocations of the Sacrum

Sacral fractures typically occur due to trauma, falls, or high-impact injuries. They may involve the sacral ala, body, or canal, potentially compromising nerve roots and pelvic stability. Dislocations are rare but can result from severe trauma. Management includes immobilization, surgical fixation, or conservative approaches depending on fracture type and neurological involvement.

Sacroiliitis and Degenerative Disorders

Inflammation of the sacroiliac joint, known as sacroiliitis, can cause low back pain and radiate to the lower limbs. Degenerative changes, such as osteoarthritis, may affect the sacroiliac joints, leading to stiffness and impaired mobility. Imaging studies and clinical evaluation are essential for diagnosis and treatment planning.

Spina Bifida Occulta and Sacral Hiatus Abnormalities

Incomplete closure of the sacral vertebral arches results in spina bifida occulta, often asymptomatic but occasionally associated with neurological deficits. Variations in the sacral hiatus are important in anesthetic procedures, as they influence the administration of caudal epidural anesthesia.

Epidural and Caudal Anesthesia Applications

The sacral hiatus and canal provide access points for epidural and caudal anesthesia. Knowledge of sacral anatomy ensures proper needle placement, effective analgesia, and reduced risk of nerve injury. This is particularly relevant in obstetric, pediatric, and orthopedic procedures.

Tumors and Metastatic Lesions

The sacrum can be affected by primary bone tumors such as chordomas or by metastatic lesions from the prostate, breast, or lung. These lesions may cause pain, neurological deficits, or structural instability. Imaging and biopsy are used for diagnosis, while surgical resection, radiotherapy, or chemotherapy form the mainstay of treatment.

Radiological Identification and Imaging Considerations

The sacrum is routinely assessed in X-rays, CT scans, and MRI to evaluate fractures, congenital anomalies, tumors, and degenerative changes. Key landmarks such as the sacral promontory, sacral foramina, and sacral hiatus are critical for accurate interpretation. Imaging also aids in surgical planning and guiding interventions such as spinal instrumentation or epidural injections.

Comparative Anatomy

The sacrum demonstrates notable variations across different species, reflecting adaptations to posture, locomotion, and reproductive functions. Comparative anatomy provides insights into the evolutionary development of the sacrum in humans and other vertebrates.

Sacrum in Other Mammals

In quadrupedal mammals, the sacrum is typically elongated and composed of more fused vertebrae to support weight transmission along the spine during locomotion. The sacroiliac articulation is robust, providing stability for walking and running. In bipedal species, including humans, the sacrum is shorter and broader, facilitating upright posture and efficient load transfer from the spine to the pelvis.

Evolutionary Adaptations in Bipedal vs Quadrupedal Species

In bipedal humans, the sacrum exhibits a pronounced curvature and a wider base to accommodate the vertical load of the upper body while maintaining pelvic stability. The anterior concavity of the sacrum contributes to the curvature of the pelvic cavity, which is particularly important in childbirth. In quadrupeds, the sacrum aligns more horizontally, optimizing weight distribution and locomotor efficiency.

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