Anatomy: The Pelvis

• The pelvis is a ring-like bony structure located at the base of the trunk, connecting the spine to the lower limbs

Anatomy of the Pelvis

• The pelvic girdle is formed by the paired hip bones (also known as innominate bones or os coxae)
• Each hip bone is composed of three bones that are fused together in adults: the ilium, ischium, and pubis
• The posterior aspect of the pelvic ring is completed by the sacrum and coccyx
• The acetabulum is the cup-shaped socket on the lateral aspect of the hip bone that articulates with the head of the femur to form the hip joint
• The ilium is the largest and most superior part of the hip bone, forming the upper part of the acetabulum
• The iliac crest is the superior border of the ilium
• The anterior superior iliac spine (ASIS) and anterior inferior iliac spine (AIIS) are important palpable landmarks on the anterior ilium
• The posterior superior iliac spine (PSIS) and posterior inferior iliac spine (PIIS) are located on the posterior ilium
• The ischium forms the posteroinferior part of the hip bone and contributes to the posterior part of the acetabulum
• The ischial tuberosity is a large, palpable prominence that serves as an attachment site for hamstring muscles.
• The pubis forms the anterior part of the hip bone and contributes to the anterior part of the acetabulum
• The superior and inferior pubic rami extend from the pubic body to connect with the ilium and ischium, respectively
• The pubic symphysis is the cartilaginous joint where the two pubic bones meet anteriorly.
• The obturator foramen is a large opening in each hip bone formed by the ischium and pubis, it is largely covered by a membrane
• The sacrum is a triangular bone formed by the fusion of five sacral vertebrae (S1-S5)
• The sacrum articulates with the ilium at the sacroiliac joints (SI joints)
• The coccyx is a small bone located inferior to the sacrum, representing the vestigial tailbone

Radiographic Projections of the Pelvis

• Anteroposterior (AP) Projection:
• Patient position: Supine with legs internally rotated 15-20 degrees to place the femoral necks parallel to the image receptor
• Central ray: Perpendicular to the midpoint of the pelvis, approximately 2 inches inferior to the ASIS
• Structures shown: Entire bony pelvis, including the hip joints, sacrum, and coccyx
• AP Axial Projection (Outlet View):
• Patient position: Supine with knees flexed
• Central ray: Angled 20-35 degrees cephalad, entering at the pubic symphysis
• Structures shown: Pubic and ischial rami, pubic symphysis
• AP Oblique Projection (Judet View):
• Patient position: Supine, rotated 45 degrees toward the affected side
• Central ray: Perpendicular to the hip joint
• Structures shown: Acetabulum, anterior and posterior iliac columns
• Lateral Projection (Hip):
• Patient position: Lateral recumbent position with the affected hip joint centered to the image receptor
• Central ray: Perpendicular to the femoral neck
• Structures shown: Lateral view of the hip joint and proximal femur

Pelvic Trauma Assessment

• Pelvic fractures can result from high-energy trauma, such as motor vehicle accidents or falls from height
• Fractures are classified based on their mechanism of injury and stability
• Common fracture types include:
• Open book fracture: Disruption of the pubic symphysis and one or both sacroiliac joints, causing the pelvis to "open" like a book
• Lateral compression fracture: Fracture caused by lateral force, resulting in inward rotation of the hemipelvis
• Vertical shear fracture: Fracture caused by vertical force, resulting in upward displacement of the hemipelvis
• Acetabular fracture: Fracture involving the acetabulum, often extending into the hip joint
• Sacral fracture: Fracture of the sacrum, which may be associated with neurological deficits
• Radiographic assessment of pelvic trauma typically includes an AP pelvis radiograph
• Additional views, such as inlet and outlet views, may be obtained to further evaluate the extent of injury
• CT scanning is often used to provide more detailed visualization of pelvic fractures and associated soft tissue injuries.

Imaging Techniques for the Pelvis

• Radiography (X-ray)
• Uses ionizing radiation to produce images of the bony structures of the pelvis
• Commonly used as the initial imaging modality for evaluating pelvic trauma and suspected fractures
• Can identify fractures, dislocations, and some soft tissue abnormalities
• Computed Tomography (CT)
• Uses X-rays to create cross-sectional images of the pelvis
• Provides detailed visualization of bony structures, soft tissues, and vascular structures
• Useful for evaluating complex pelvic fractures, acetabular fractures, and sacroiliac joint injuries
• Can also be used to assess for pelvic hematomas and other soft tissue abnormalities
• Magnetic Resonance Imaging (MRI)
• Uses strong magnetic fields and radio waves to produce images of the pelvis
• Provides excellent soft tissue contrast and is useful for evaluating soft tissue injuries, such as ligamentous injuries, muscle strains, and nerve impingement
• Can also be used to assess for avascular necrosis of the femoral head and stress fractures of the pelvis
• Ultrasound
• Uses sound waves to create images of the pelvis
• Can be used to evaluate the uterus, ovaries, and bladder in females
• Can also be used to assess for fluid collections and abscesses in the pelvis
• Bone Scan
• Uses radioactive isotopes to detect areas of increased bone turnover
• Useful for evaluating stress fractures, infections, and tumors of the pelvis