Identify The Bony Posterior Wall Of The Pelvis.

The bony posterior wall of the pelvis, a critical component of the human skeletal system, provides structural support, protects vital organs, and serves as an attachment site for numerous muscles. Understanding its intricate anatomy is essential for medical professionals, physical therapists, and anyone interested in human anatomy and biomechanics. This article delves into the detailed anatomy of the bony posterior wall of the pelvis, exploring its constituent bones, key landmarks, associated ligaments, and clinical significance.

Anatomy of the Pelvis

The pelvis, a basin-shaped structure located at the base of the spine, connects the trunk to the lower limbs. It is composed of several bones:

• Two Hip Bones (Os Coxae): These form the lateral and anterior aspects of the pelvis. Each hip bone is further divided into three parts: the ilium, ischium, and pubis.
• Sacrum: A triangular bone formed by the fusion of five sacral vertebrae, located at the posterior aspect of the pelvis.
• Coccyx: Commonly known as the tailbone, it is a small, triangular bone that articulates with the inferior aspect of the sacrum.

The bony posterior wall of the pelvis primarily consists of the sacrum and parts of the ilium.

Detailed Anatomy of the Sacrum

The sacrum is a large, triangular bone that forms the posterior part of the pelvic girdle. It is formed by the fusion of five sacral vertebrae (S1-S5) during adolescence and early adulthood. The sacrum articulates with the lumbar spine superiorly at the lumbosacral joint and with the coccyx inferiorly at the sacrococcygeal joint. Laterally, the sacrum articulates with the ilium at the sacroiliac joints, which are crucial for weight transfer and stability.

Key Features of the Sacrum:

• Base: The superior aspect of the sacrum, which articulates with the fifth lumbar vertebra (L5). The superior surface of the first sacral vertebra (S1) is known as the sacral promontory, an important obstetrical landmark.
• Apex: The inferior tip of the sacrum, which articulates with the coccyx.
• Alae (Wings): The lateral masses of the sacrum, which articulate with the ilium to form the sacroiliac joints. The alae provide a large surface area for ligamentous attachments.
• Sacral Canal: A continuation of the vertebral canal, which runs through the center of the sacrum and contains the spinal nerve roots of the cauda equina.
• Sacral Foramina: Four pairs of openings on the anterior and posterior surfaces of the sacrum, which transmit the anterior and posterior rami of the sacral spinal nerves. The anterior sacral foramina are larger and more prominent than the posterior sacral foramina.
• Median Sacral Crest: A vertical ridge on the posterior surface of the sacrum, formed by the fused spinous processes of the sacral vertebrae.
• Intermediate Sacral Crests: Located lateral to the median sacral crest, formed by the fused articular processes of the sacral vertebrae.
• Lateral Sacral Crests: The most lateral ridges on the posterior surface of the sacrum, formed by the fused transverse processes of the sacral vertebrae.
• Sacral Hiatus: An opening at the inferior end of the sacral canal, located inferior to the fourth or fifth sacral vertebra. It is formed by the absence of the laminae and spinous process of these vertebrae. The sacral hiatus is an important landmark for caudal epidural anesthesia.
• Sacral Cornua: Two bony projections that flank the sacral hiatus, representing the inferior articular processes of the fifth sacral vertebra. The sacral cornua articulate with the coccygeal cornua.

Detailed Anatomy of the Ilium

The ilium is the largest and most superior part of the hip bone. It forms the upper portion of the acetabulum (the hip socket) and articulates with the sacrum at the sacroiliac joint.

Key Features of the Ilium Relevant to the Posterior Pelvic Wall:

• Iliac Crest: The superior border of the ilium, which extends from the anterior superior iliac spine (ASIS) to the posterior superior iliac spine (PSIS). The iliac crest serves as an attachment site for abdominal muscles, back muscles, and the fascia lata.
• Posterior Superior Iliac Spine (PSIS): A bony prominence located at the posterior end of the iliac crest. It is palpable beneath the skin and serves as a landmark for the sacroiliac joint.
• Posterior Inferior Iliac Spine (PIIS): Located inferior to the PSIS, it marks the superior border of the greater sciatic notch.
• Iliac Tuberosity: A roughened area on the posterior aspect of the ilium, located medial to the PSIS. It provides attachment for strong ligaments that stabilize the sacroiliac joint.
• Auricular Surface: A rough, ear-shaped surface on the medial aspect of the ilium, which articulates with the sacrum to form the sacroiliac joint. The articular cartilage on the auricular surface is hyaline cartilage.

Sacroiliac Joint (SIJ)

The sacroiliac joint (SIJ) is a complex articulation between the sacrum and the ilium. It is a strong, weight-bearing joint that transmits forces between the spine and the lower extremities. The SIJ is primarily a synovial joint, with some syndesmotic components due to the presence of strong interosseous ligaments.

Key Features of the SIJ:

• Articular Surfaces: The auricular surfaces of the sacrum and ilium are covered with hyaline cartilage, which allows for smooth movement. The joint surfaces are irregular and interlock, providing stability.
• Joint Capsule: A fibrous capsule that surrounds the joint and helps to maintain its integrity. The capsule is reinforced by ligaments.
• Ligaments: The SIJ is supported by a complex network of ligaments, which provide stability and limit excessive movement. These ligaments include:
  • Anterior Sacroiliac Ligament: A thin ligament located on the anterior aspect of the joint capsule.
  • Posterior Sacroiliac Ligament: A strong ligament located on the posterior aspect of the joint capsule, divided into short and long posterior sacroiliac ligaments.
  • Interosseous Sacroiliac Ligament: The strongest ligament of the SIJ, located deep within the joint between the sacrum and ilium. It provides significant stability.
  • Sacrotuberous Ligament: A large, strong ligament that extends from the sacrum and ischial tuberosity. It helps to resist upward tilting of the sacrum.
  • Sacrospinous Ligament: A triangular ligament that extends from the sacrum and ischial spine. It converts the greater sciatic notch into the greater sciatic foramen.
  • Iliolumbar Ligament: Extends from the transverse process of L5 to the iliac crest. Although not directly part of the SI joint, it provides stability to the lumbosacral junction and influences SIJ mechanics.

Muscles Associated with the Posterior Pelvic Wall

Several muscles attach to the sacrum and ilium, contributing to the stability and movement of the pelvis and lower extremities.

• Erector Spinae Muscles: These muscles, including the spinalis, longissimus, and iliocostalis, run along the vertebral column and attach to the sacrum and iliac crest. They are responsible for extension and lateral flexion of the spine.
• Multifidus: A deep back muscle that spans the length of the vertebral column, attaching to the sacrum and lumbar vertebrae. It contributes to spinal stability and proprioception.
• Gluteus Maximus: The largest muscle in the body, which originates from the posterior ilium, sacrum, coccyx, and sacrotuberous ligament. It is a powerful hip extensor and external rotator.
• Piriformis: A small muscle that originates from the anterior sacrum and passes through the greater sciatic foramen to insert on the greater trochanter of the femur. It is a hip external rotator.
• Iliacus: Originates from the iliac fossa and joins with the psoas major to form the iliopsoas muscle, which is a powerful hip flexor.
• Latissimus Dorsi: Although primarily a back muscle, it attaches to the iliac crest via the thoracolumbar fascia and contributes to the stability of the posterior pelvic wall.

Clinical Significance

Understanding the anatomy of the bony posterior wall of the pelvis is crucial for diagnosing and treating various clinical conditions.

• Sacroiliac Joint Dysfunction: SIJ dysfunction is a common cause of lower back pain. It can result from trauma, pregnancy, or repetitive stress. Symptoms may include pain in the lower back, buttocks, and legs. Diagnosis is often based on physical examination and imaging studies. Treatment may include physical therapy, pain medication, and injections.
• Sacral Fractures: Sacral fractures can occur as a result of high-energy trauma, such as motor vehicle accidents or falls from height. They can also occur in individuals with osteoporosis or other conditions that weaken the bones. Sacral fractures can be classified as transverse, longitudinal, or comminuted. Treatment depends on the severity and stability of the fracture. Surgical fixation may be necessary in some cases.
• Coccyx Injuries: The coccyx is vulnerable to injury from direct trauma, such as falls onto the buttocks. Coccyx injuries can cause pain and discomfort, particularly when sitting or standing for prolonged periods. Treatment typically involves pain medication, physical therapy, and the use of a cushion to relieve pressure on the coccyx.
• Piriformis Syndrome: Piriformis syndrome is a condition in which the piriformis muscle compresses the sciatic nerve, causing pain, numbness, and tingling in the buttocks and legs. It can be caused by muscle spasm, hypertrophy, or injury. Treatment may include stretching exercises, massage, and injections.
• Spinal Fusion: In cases of spinal instability or deformity, spinal fusion surgery may be performed to stabilize the spine. The sacrum is often included in spinal fusion constructs to provide a solid foundation for the spine. Understanding the anatomy of the sacrum is essential for surgeons performing spinal fusion procedures.
• Childbirth: During childbirth, the pelvis undergoes significant changes to accommodate the passage of the baby. The sacroiliac joints and pubic symphysis widen, and the ligaments surrounding the pelvis stretch. These changes can lead to pain and discomfort in some women. Understanding the anatomy of the pelvis is important for obstetricians and midwives to manage labor and delivery effectively.
• Pelvic Inflammatory Disease (PID): Although not directly related to the bony structures, PID can cause inflammation and pain in the pelvic region, which may be referred to the posterior pelvic wall. Understanding the anatomy helps in differential diagnosis.
• Tumors: Primary bone tumors or metastatic lesions can affect the bones of the posterior pelvic wall. Knowledge of the anatomy is crucial for diagnosis, staging, and treatment planning.

Aging and the Posterior Pelvic Wall

With age, the bones of the posterior pelvic wall, particularly the sacrum and ilium, undergo several changes:

• Bone Density Loss: Osteoporosis, a common age-related condition, leads to a decrease in bone density, making the sacrum and ilium more susceptible to fractures.
• Cartilage Degeneration: The articular cartilage of the sacroiliac joint thins and degenerates, leading to osteoarthritis. This can cause pain, stiffness, and reduced mobility.
• Ligament Laxity: The ligaments surrounding the sacroiliac joint lose their elasticity and strength with age, contributing to instability.
• Muscle Atrophy: The muscles that support the pelvis, such as the gluteus maximus and erector spinae, can atrophy with age, further compromising stability.

Diagnostic Imaging

Several imaging modalities are used to evaluate the bony posterior wall of the pelvis:

• X-rays: X-rays can be used to visualize fractures, dislocations, and other bony abnormalities.
• Computed Tomography (CT) Scans: CT scans provide detailed images of the bones and soft tissues of the pelvis. They are useful for evaluating complex fractures, tumors, and infections.
• Magnetic Resonance Imaging (MRI): MRI provides high-resolution images of the soft tissues of the pelvis, including the ligaments, muscles, and nerves. It is useful for evaluating sacroiliac joint dysfunction, piriformis syndrome, and other soft tissue conditions.
• Bone Scans: Bone scans can be used to detect areas of increased bone turnover, such as fractures, infections, and tumors.

Conclusion

The bony posterior wall of the pelvis, comprised of the sacrum and parts of the ilium, is a complex and critical anatomical region. Its intricate structure provides support, stability, and protection for vital organs. A thorough understanding of its bony landmarks, ligaments, and associated muscles is essential for medical professionals to diagnose and treat a wide range of clinical conditions. Furthermore, recognizing the impact of aging on these structures can aid in managing age-related musculoskeletal issues. Through detailed anatomical knowledge and appropriate diagnostic imaging, clinicians can effectively address pathologies affecting the posterior pelvic wall, improving patient outcomes and quality of life.