Pelvic bone

The pelvic bone is a large, complex structure that forms the lower part of the trunk and connects the vertebral column to the lower limbs. It provides support for the weight of the upper body, protects pelvic and lower abdominal organs, and serves as an attachment point for numerous muscles involved in movement and posture. Understanding its anatomy is essential for clinical practice in orthopedics, obstetrics, and trauma management.

Anatomy of the Pelvic Bone

Structure

The pelvic bone is formed by the fusion of three bones on each side of the body, creating a robust and stable structure:

• Ilium: The broad, fan-shaped upper portion that provides attachment for abdominal and gluteal muscles.
• Ischium: The posterior-inferior part that forms the lower and back portion of the pelvis, contributing to weight bearing in sitting.
• Pubis: The anterior portion that meets at the midline to form the pubic symphysis, contributing to the pelvic ring.

Pelvic Girdle

The pelvic girdle consists of two hip bones joined posteriorly with the sacrum and anteriorly at the pubic symphysis. It serves as a structural ring that transmits weight from the upper body to the lower limbs while allowing flexibility during locomotion and childbirth.

Pelvic Joints

The pelvic bones articulate at several key joints that provide stability and limited mobility:

• Sacroiliac Joint: Connects the ilium to the sacrum and supports axial load transmission.
• Pubic Symphysis: A cartilaginous joint at the midline providing slight movement and shock absorption.
• Hip Joint: Formed by the acetabulum of the pelvis articulating with the femoral head, allowing a wide range of lower limb movements.

Development and Ossification

Embryological Development

The pelvic bones develop from mesenchymal tissue during embryogenesis. Initially, the ilium, ischium, and pubis arise as separate cartilaginous elements on each side. These elements gradually grow and form the acetabulum where the femoral head will articulate.

Ossification Centers

Ossification of the pelvic bones occurs in stages through primary and secondary centers:

• Primary Ossification Centers: Present in the ilium, ischium, and pubis during fetal development.
• Secondary Ossification Centers: Appear during adolescence in regions such as the iliac crest, ischial tuberosity, and pubic symphysis.
• Fusion Timeline: The three bones typically fuse to form a single hip bone by the late teenage years to early adulthood.

Blood Supply and Innervation

Arterial Supply

The pelvic bones receive a rich arterial supply primarily from branches of the internal iliac artery, ensuring adequate perfusion for bone metabolism and repair:

• Superior and inferior gluteal arteries
• Obturator artery
• Internal pudendal artery
• Contributions from external iliac and femoral arteries through anastomoses

Venous Drainage

Venous drainage of the pelvis mirrors the arterial supply and connects to the systemic circulation:

• Pelvic venous plexuses surrounding the bones and pelvic organs
• Connections to the internal iliac veins and ultimately to the inferior vena cava
• Communication with vertebral venous plexus providing collateral pathways

Nerve Supply

Innervation of the pelvic region is provided mainly by the lumbosacral plexus, contributing to both motor and sensory functions:

• Branches supplying muscles attached to the pelvis
• Sensory nerves supplying periosteum and surrounding soft tissues
• Integration with autonomic fibers for vascular regulation

Functions

Supportive Role

The pelvic bone serves as a crucial support structure for the human body:

• Transmits the weight of the upper body to the lower limbs during standing and locomotion
• Maintains balance and posture
• Serves as a stable base for muscle attachment

Protection

The pelvis provides a protective bony enclosure for several vital organs:

• Urinary bladder
• Rectum
• Internal reproductive organs
• Major blood vessels and nerves passing through the pelvic cavity

Attachment for Muscles

Numerous muscles of the lower limb, abdomen, and perineum attach to the pelvic bones, facilitating movement and function:

• Gluteal muscles for hip extension and abduction
• Adductors and flexors of the thigh
• Abdominal muscles contributing to trunk stability
• Pelvic floor muscles supporting pelvic organs

Clinical Relevance

Pelvic Fractures

Pelvic fractures are significant injuries that can result from high-energy trauma or low-impact injuries in osteoporotic individuals. They are classified based on stability and mechanism:

• Stable fractures: Involve a single break in the pelvic ring with minimal displacement.
• Unstable fractures: Involve multiple breaks or disruption of the pelvic ring, often associated with vascular injury.
• Mechanisms of injury: Includes motor vehicle accidents, falls, and crush injuries.

Developmental Disorders

Abnormal development of the pelvic bones can result in congenital or acquired conditions affecting hip and pelvic function:

• Developmental dysplasia of the hip, leading to improper acetabular formation
• Perturbations in ossification causing pelvic asymmetry or deformity
• Impact on gait, posture, and risk of early osteoarthritis

Other Conditions

The pelvic bone can also be affected by systemic and localized pathological conditions:

• Osteoporosis, leading to fragility fractures
• Paget’s disease of bone, causing enlargement and deformity
• Primary or metastatic tumors involving the pelvic bones

Imaging and Diagnosis

X-ray

Plain radiography is the first-line imaging modality for pelvic assessment:

• Anterior-posterior view for general evaluation of pelvic ring and hip joints
• Oblique or lateral views to detect subtle fractures and displacement
• Useful for follow-up of healing fractures

CT and MRI

Advanced imaging provides detailed visualization of bone and soft tissue structures:

• CT scan for precise assessment of fracture patterns and complex injuries
• MRI for evaluation of soft tissue, bone marrow pathology, and joint cartilage
• Helpful in preoperative planning for surgical interventions

Bone Scans

Nuclear medicine imaging can identify metabolic bone disorders and occult lesions:

• Evaluation of osteomyelitis, tumors, and stress fractures
• Assessment of bone turnover in metabolic diseases
• Detection of multiple lesions in systemic conditions

References

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