Sacrum

The sacrum is a triangular bone located at the base of the spine, forming the posterior part of the pelvis. It serves as a critical structure for weight transmission from the upper body to the lower limbs and provides protection for the sacral nerve roots. Its anatomy and function are essential for stability, locomotion, and pelvic integrity.

Anatomy of the Sacrum

Gross Anatomy

• Shape, Size, and Curvature: The sacrum is a wedge-shaped, curved bone that typically consists of five fused vertebrae. It forms a concave anterior surface and a convex posterior surface.
• Relation to the Lumbar Spine and Coccyx: Superiorly, it articulates with the fifth lumbar vertebra, and inferiorly, it connects with the coccyx.
• Anterior and Posterior Surfaces: The anterior surface is smooth and concave, providing space for pelvic organs, while the posterior surface is rough and convex with prominences for ligament and muscle attachment.

Vertebral Components

• Sacral Vertebrae (S1–S5): The sacrum consists of five vertebrae that fuse during adolescence to form a single bone.
• Sacral Foramina: Anterior and posterior foramina allow passage of sacral spinal nerves and blood vessels.
• Median, Intermediate, and Lateral Sacral Crests: These crests represent fused spinous and transverse processes and serve as attachment points for ligaments and muscles.

Articulations

• Lumbosacral Joint: Connects the sacrum to L5 and provides weight transfer from the spine to the pelvis.
• Sacroiliac Joints: Formed between the sacrum and iliac bones, these joints contribute to pelvic stability.
• Sacrococcygeal Joint: Connects the sacrum to the coccyx, allowing limited movement.

Ligaments and Muscles

• Anterior and Posterior Sacroiliac Ligaments: Stabilize the sacroiliac joints and prevent excessive movement.
• Sacrospinous and Sacrotuberous Ligaments: Provide additional pelvic support and limit sacral rotation.
• Muscular Attachments: Includes gluteal muscles, piriformis, and other pelvic floor muscles that attach to the sacrum for movement and stability.

Vascular and Neural Supply

• Arterial Supply: Branches from the lateral sacral arteries supply the sacrum.
• Venous Drainage: Drains into the internal vertebral venous plexus and pelvic veins.
• Nerve Roots and Sacral Plexus Contributions: The anterior and posterior sacral foramina transmit sacral nerves, which form the sacral plexus for lower limb and pelvic innervation.

Development and Embryology

• Ossification of Sacral Vertebrae: The sacral vertebrae develop from separate ossification centers that appear during fetal life and early childhood.
• Fusion Process and Timeline: The five sacral vertebrae gradually fuse between adolescence and adulthood, completing by the third decade of life.
• Congenital Variations: Variations include sacralization of L5, lumbarization of S1, and spina bifida occulta, which may impact spinal stability or predispose to pathology.

Physiology and Function

• Load-Bearing and Weight Transmission: The sacrum transfers the weight of the upper body to the pelvis and lower limbs, playing a critical role in maintaining upright posture.
• Role in Pelvic Stability: By forming the posterior wall of the pelvis and articulating with the iliac bones, the sacrum provides structural support and stability for the pelvic girdle.
• Contribution to Posture and Locomotion: The sacrum supports spinal alignment and facilitates movements such as walking, running, and bending by serving as a keystone for musculoskeletal attachments.
• Protection of Sacral Nerve Roots: The sacral canal and foramina protect the sacral spinal nerves that contribute to lower limb and pelvic organ innervation.

Clinical Significance

Common Disorders

• Fractures and Trauma: Sacral fractures can result from high-energy trauma, falls, or pelvic injuries and may compromise stability or nerve function.
• Sacroiliitis and Sacral Osteoarthritis: Inflammatory and degenerative conditions of the sacroiliac joints can cause chronic pain and reduced mobility.
• Congenital Anomalies: Variations such as sacralization or spina bifida occulta may be asymptomatic or contribute to lower back pain and functional impairment.
• Tumors of the Sacrum: Primary or metastatic lesions can compress nerves, cause pain, and destabilize the pelvis.

Symptoms of Sacral Pathology

• Low Back and Pelvic Pain: Pain may be localized to the sacral region or radiate to the lower limbs.
• Neurological Deficits: Compression of sacral nerves can result in sciatica, numbness, or in severe cases, cauda equina syndrome.
• Gait Disturbances: Sacral instability or nerve involvement may alter walking patterns and reduce mobility.

Diagnostic Evaluation

• Radiography (X-ray): Initial imaging modality to assess sacral alignment, fractures, and congenital anomalies.
• Computed Tomography (CT): Provides detailed visualization of bone structures, fractures, and tumors for precise evaluation.
• Magnetic Resonance Imaging (MRI): Useful for assessing soft tissue involvement, nerve compression, and sacral tumors.
• Electrophysiological Studies: Nerve conduction studies and electromyography may be employed to evaluate sacral nerve function in cases of neurological deficits.

Treatment and Management

Conservative Management

• Physical Therapy and Exercises: Focus on strengthening core and pelvic muscles to support sacral stability and improve posture.
• Analgesics and Anti-Inflammatory Medications: Used to manage pain and inflammation associated with sacral disorders.
• Bracing or Support Devices: Sacral belts or orthoses may be applied to reduce stress on the sacroiliac joints and enhance stability during healing.

Surgical Management

• Fracture Fixation and Stabilization: Indicated for unstable sacral fractures or severe trauma to restore structural integrity.
• Resection of Sacral Tumors: Surgical removal may be required for symptomatic or malignant lesions affecting sacral bone or nerves.
• Sacroiliac Joint Fusion Procedures: Considered in chronic sacroiliitis or degenerative joint disease refractory to conservative therapy to relieve pain and improve function.

Prevention and Rehabilitation

• Posture and Ergonomic Strategies: Maintaining proper posture during sitting, standing, and lifting helps reduce stress on the sacrum and sacroiliac joints.
• Strengthening and Flexibility Exercises: Targeted exercises for core, pelvic, and lower back muscles enhance sacral stability and prevent injury.
• Fall Prevention and Safety Measures: Implementing environmental modifications and safety protocols reduces the risk of traumatic sacral fractures, especially in elderly or at-risk populations.

References

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