Label The Structures Of The Sacrum And Coccyx

The sacrum and coccyx, often referred to as the tailbone, are integral parts of the human vertebral column, situated at its base. These bones play crucial roles in supporting the upper body, providing attachment points for muscles and ligaments, and contributing to overall pelvic stability. Understanding the anatomy of the sacrum and coccyx, including their individual structures and relationships, is essential for healthcare professionals, students, and anyone interested in human anatomy.

Sacrum: Anatomy and Structure

The sacrum is a large, triangular bone formed by the fusion of five sacral vertebrae (S1-S5). Located at the base of the spine, it forms the posterior part of the pelvic girdle, articulating with the hip bones (ilia) at the sacroiliac joints.

Key Structures of the Sacrum

Base: The base is the superior portion of the sacrum, which articulates with the fifth lumbar vertebra (L5) to form the lumbosacral joint. This joint is a critical area for weight distribution and spinal stability.
Apex: The apex is the inferior, narrow end of the sacrum that articulates with the coccyx. This articulation is known as the sacrococcygeal joint.
Sacral Promontory: The sacral promontory is the prominent, anterior projecting edge of the base of the sacrum. It is an important obstetrical landmark used to measure the pelvic inlet.
Alae (Wings): The alae are the lateral masses of the sacrum that articulate with the iliac bones to form the sacroiliac joints. These large, wing-like structures provide stability and support to the pelvis.
Sacroiliac Joints: These joints are formed by the articulation of the alae of the sacrum with the iliac bones. They are strong, weight-bearing joints that transmit forces between the spine and the lower limbs.
Anterior (Pelvic) Surface: The anterior surface of the sacrum is concave and relatively smooth. It features four transverse lines, which represent the fused intervertebral discs between the sacral vertebrae.
Posterior Surface: The posterior surface of the sacrum is rough and irregular. It features the median sacral crest, intermediate sacral crests, and lateral sacral crests.
Median Sacral Crest: The median sacral crest is a ridge formed by the fused spinous processes of the sacral vertebrae. It provides an attachment site for ligaments and muscles.
Intermediate Sacral Crests: The intermediate sacral crests are formed by the fused articular processes of the sacral vertebrae.
Lateral Sacral Crests: The lateral sacral crests are formed by the transverse processes of the sacral vertebrae.
Sacral Canal: The sacral canal is a continuation of the vertebral canal that runs through the sacrum. It contains the sacral nerve roots and the filum terminale.
Sacral Hiatus: The sacral hiatus is an opening at the inferior end of the sacral canal, located posterior to the apex of the sacrum. It is formed by the absence of the laminae and spinous process of the fifth sacral vertebra.
Sacral Cornua: The sacral cornua are bony projections located on either side of the sacral hiatus. They represent the inferior articular processes of the fifth sacral vertebra and articulate with the coccygeal cornua.
Anterior Sacral Foramina: These are four pairs of openings on the anterior surface of the sacrum that transmit the anterior rami of the sacral spinal nerves.
Posterior Sacral Foramina: These are four pairs of openings on the posterior surface of the sacrum that transmit the posterior rami of the sacral spinal nerves.

Development and Ossification of the Sacrum

The sacrum develops from five separate vertebral segments that begin to fuse in early adulthood. The ossification process typically starts around age 18 and is completed by the mid-30s. The fusion occurs via endochondral ossification, where cartilage is gradually replaced by bone.

Coccyx: Anatomy and Structure

The coccyx, commonly known as the tailbone, is a small, triangular bone located at the inferior end of the vertebral column. It typically consists of three to five fused coccygeal vertebrae (Co1-Co5), although the number can vary.

Key Structures of the Coccyx

Base: The base is the superior part of the coccyx that articulates with the apex of the sacrum at the sacrococcygeal joint.
Apex: The apex is the inferior tip of the coccyx.
Coccygeal Cornua: The coccygeal cornua are small, superior projections that articulate with the sacral cornua. They represent the superior articular processes of the first coccygeal vertebra.
Transverse Processes: The transverse processes are small, lateral projections present on the first coccygeal vertebra and sometimes on the second.
Coccygeal Vertebrae (Co1-Co5): These are the individual vertebrae that fuse to form the coccyx. The first coccygeal vertebra is the largest and most recognizable, while the subsequent vertebrae are smaller and more rudimentary.
Sacrococcygeal Joint: This is the joint between the apex of the sacrum and the base of the coccyx. It is a symphyseal joint, meaning it is connected by fibrocartilage.

Development and Ossification of the Coccyx

The coccyx develops from separate coccygeal segments that fuse over time. The ossification process begins later than that of the sacrum, typically starting in the late teens or early twenties and continuing into adulthood. The number of fused segments can vary, with some individuals having a completely fused coccyx and others retaining some degree of separation between segments.

Ligaments of the Sacrum and Coccyx

Several ligaments support the sacrum and coccyx, providing stability and limiting excessive movement.

Sacroiliac Ligaments: These strong ligaments connect the sacrum to the iliac bones, reinforcing the sacroiliac joints. They include the anterior sacroiliac ligament, posterior sacroiliac ligament, and interosseous sacroiliac ligament.
Sacrotuberous Ligament: This large ligament extends from the sacrum and coccyx to the ischial tuberosity of the hip bone. It helps to stabilize the pelvis and prevent upward rotation of the sacrum.
Sacrospinous Ligament: This ligament runs from the sacrum and coccyx to the ischial spine. It divides the greater and lesser sciatic notches, forming the greater and lesser sciatic foramina.
Anterior Sacrococcygeal Ligament: This ligament connects the anterior surface of the sacrum to the anterior surface of the coccyx, reinforcing the sacrococcygeal joint.
Posterior Sacrococcygeal Ligaments: These ligaments connect the posterior surface of the sacrum to the posterior surface of the coccyx, reinforcing the sacrococcygeal joint. They include superficial and deep posterior sacrococcygeal ligaments.
Lateral Sacrococcygeal Ligaments: These ligaments connect the lateral aspects of the sacrum to the coccyx, providing additional stability to the sacrococcygeal joint.
Intercornual Ligaments: These ligaments connect the sacral cornua to the coccygeal cornua, further stabilizing the articulation between the sacrum and coccyx.

Muscles Associated with the Sacrum and Coccyx

Several muscles attach to the sacrum and coccyx, contributing to pelvic floor support, hip movement, and spinal stability.

Gluteus Maximus: The gluteus maximus, the largest muscle in the body, originates from the posterior sacrum, coccyx, and iliac crest. It is a powerful hip extensor and lateral rotator.
Piriformis: The piriformis muscle originates from the anterior surface of the sacrum and passes through the greater sciatic foramen to insert on the greater trochanter of the femur. It is a hip external rotator and abductor.
Coccygeus: The coccygeus muscle originates from the ischial spine and inserts on the coccyx and lower sacrum. It supports the pelvic floor and helps to flex the coccyx.
Levator Ani: The levator ani is a group of muscles that form the pelvic floor. It includes the pubococcygeus, iliococcygeus, and puborectalis muscles. These muscles attach to the coccyx and surrounding structures, supporting the pelvic organs and controlling bowel and bladder function.
Sphincter Ani Externus: The external anal sphincter surrounds the anus and attaches to the coccyx via the anococcygeal ligament. It controls voluntary defecation.

Nerve Supply to the Sacrum and Coccyx

The sacrum and coccyx receive nerve supply from the sacral and coccygeal spinal nerves.

Sacral Spinal Nerves (S1-S5): These nerves exit the spinal canal through the sacral foramina and innervate the lower limbs, pelvic organs, and perineum.
Coccygeal Nerve (Co1): This nerve is the terminal branch of the spinal cord and provides sensory innervation to the skin over the coccyx.
Autonomic Nerves: The sacrum and coccyx also receive autonomic nerve fibers from the sympathetic and parasympathetic nervous systems, which regulate pelvic organ function.

Clinical Significance

Understanding the anatomy of the sacrum and coccyx is essential for diagnosing and treating various clinical conditions.

Sacroiliac Joint Dysfunction: Sacroiliac joint dysfunction can cause lower back pain, hip pain, and referred pain to the groin and legs. It may result from trauma, arthritis, or pregnancy.
Sacral Fractures: Sacral fractures can occur due to high-energy trauma, such as motor vehicle accidents or falls. They can be associated with neurological deficits and pelvic instability.
Coccygodynia (Tailbone Pain): Coccygodynia is pain in the coccyx, often caused by trauma, repetitive strain, or childbirth. It can be chronic and debilitating.
Piriformis Syndrome: Piriformis syndrome occurs when the piriformis muscle compresses the sciatic nerve, causing buttock pain, hip pain, and sciatica.
Spinal Stenosis: Spinal stenosis is the narrowing of the spinal canal, which can compress the spinal cord and nerve roots. It can affect the sacral region, causing lower back pain, leg pain, and bowel or bladder dysfunction.
Epidural Anesthesia: Epidural anesthesia is a common technique used during labor and delivery to relieve pain. It involves injecting local anesthetic into the epidural space in the sacral region to block nerve transmission.
Cauda Equina Syndrome: Cauda equina syndrome is a rare but serious condition that occurs when the nerve roots in the lumbar and sacral region are compressed. It can cause bowel and bladder dysfunction, saddle anesthesia, and lower extremity weakness.

Imaging of the Sacrum and Coccyx

Various imaging techniques can be used to visualize the sacrum and coccyx, aiding in diagnosis and treatment planning.

X-rays: X-rays are useful for evaluating fractures, dislocations, and bony abnormalities of the sacrum and coccyx.
CT Scans: CT scans provide detailed cross-sectional images of the sacrum and coccyx, allowing for better visualization of fractures, tumors, and other bony lesions.
MRI Scans: MRI scans provide excellent soft tissue contrast and are useful for evaluating nerve compression, ligament injuries, and tumors of the sacrum and coccyx.
Bone Scans: Bone scans can detect areas of increased bone turnover, which may indicate fractures, infections, or tumors.

Conclusion

The sacrum and coccyx are essential components of the human vertebral column, providing support, stability, and attachment points for muscles and ligaments. Understanding their anatomy, including the individual structures and relationships, is crucial for healthcare professionals, students, and anyone interested in human anatomy. The sacrum, formed by the fusion of five sacral vertebrae, articulates with the lumbar spine and hip bones, transmitting forces between the upper body and lower limbs. The coccyx, or tailbone, is a small, triangular bone that articulates with the sacrum and provides attachment for pelvic floor muscles and ligaments. Clinical conditions affecting the sacrum and coccyx can cause significant pain and disability, highlighting the importance of accurate diagnosis and treatment. Imaging techniques, such as X-rays, CT scans, and MRI scans, play a vital role in evaluating these bones and guiding clinical decision-making. By appreciating the complex anatomy and function of the sacrum and coccyx, we can better understand and address the conditions that affect these important structures.