A full understanding of pelvic anatomy is required to treat pelvic fractures, to prevent iatrogenic injuries, and to provide the best results.

Learning Pelvic Anatomy is composed of learning bones, muscles, ligaments, nerves and vascular supply.

See Also: Pelvic Fractures

Osteology

  • Pelvic anatomy is composed of two innominate (coxal) bones that articulate with the sacrum and proximal
    femora.
  • Each innominate bone is composed of three united bones: ilium, ischium, and pubis, meeting in the acetabular fossa at the triradiate fusion center.

Ilium Bone:

  • Important landmarks include iliac crest, anteriorsuperior iliac spine (ASIS), anterior-inferior iliac spine
    (AIIS), and posterior-superior iliac spine (PSIS)
  • Iliac crest: under-skin palpable rim of ilium, important site for bone graft harvest (iliac tubercle 5 cm posterior to ASIS).
  • ASIS: palpable at lateral edge of inguinal ligament; origin of sartorius muscle and transverse and internal abdominal muscles.
  • AIIS: less prominent; origin of direct head of the rectus femoris and iliofemoral ligament (Y ligament of Bigelow)
  • PSIS: 4 to 5 cm lateral to S2 spinous process; important landmark for posterior iliac crest bone graft harvest.

The bone in the ilium has two columns, which are extremely thick and strong:

  1. The first column runs from the ischial tuberosity to the SI joint and allows for force transfer during sitting.
  2. The second column runs from the dome of the acetabulum to the SI joint and allows for force transfer during standing.

Ischium Bone:

  • Ischium bone forms the posterior column of acetabulum.
  • Iliac spine separates greater and lesser sciatic notch; sacrospinous ligament (anterior sacrum to ischial spine) separates greater and lesser sciatic foramina.
  • Greater sciatic notch: posterior and superior to acetabulum.
  • Ischial tuberosity: origin of hamstrings; sacrotuberous ligament (posterolateral sacrum to ischial tuberosity) inferior border of lesser sciatic foramen.

Pubis Bone:

  • Pubis bone forms the anterior pelvic ring.
  • Bilateral pubic rami articulate at pubic symphysis anteriorly.
  • Iliopectineal eminence: anterior pelvic rim prominence at the union of the ilium and pubis.
  • Iliopsoas muscle/tendon traverses a groove between iliopectineal eminence and AIIS.

Acetabulum

  • Normal acetabulum is anteverted (15 degrees) and obliquely oriented in coronal plane (45 degrees caudally)
  • Posterosuperior articular surface (The roof) thickened to accommodate weight bearing.
  • Inferior surface contains the acetabular (cotyloid) notch, which is bound by the transverse acetabular ligament.
See Also: Sacral Fractures

Pelvic Ligaments

The three bony structures (ilium, ischium, and pubis) are held together with strong ligaments that are important in understanding pelvic anatomy and biomechanics.

  • Pelvic floor consists of two ligaments:
    1. Sacrospinous ligaments: resist external rotation forces.
    2. Sacrotuberous ligaments: resist shear and flexion forces.
  • The SI joints are held together with anterior, intra-articular, and posterior sacroiliac ligaments.
  • Posterior sacroiliac complex is the strongest ligaments in the body, and consists of 4 ligaments:
    1. Anterior sacroiliac ligaments: resist external rotation.
    2. Interosseous sacroiliac ligament: resist anterior-posterior translation of pelvis.
    3. posterior sacroiliac ligament: resist cephalad-caudad displacement of pelvis.
    4. Iliolumbar ligament: resist rotation and augment posterior SI ligaments.
  • The posterior ring structures are responsible for the majority of pelvic ring stability.
  • The symphyseal ligaments, which hold the pubis together, resist external rotation and account for only 15% of the stability to the entire ring.

Biomechanics of the Pelvis

The forces on the pelvic ring vary according to load, principally, either single-leg or bilateral-leg stance.

  • When a person stands on both legs, both the symphysis and inferior SI joints are under tension, whereas the superior aspect of the SI joint is under compression.
  • During a single-leg stance, the symphysis actually undergoes compression while the SI joint forces flip, with compression inferiorly and tension superiorly.

Neurovascular Structures

The internal iliac vessels and its major trunks pass anterior to the SI joints and then exit the pelvis through the greater and lesser sciatic notches and the obturator foramen. The superior gluteal vessels exit the greater sciatic notch.

  • The L5 nerve root lies on the anterior aspect of the sacral ala and is at risk with pelvic ring fractures or during surgical intervention. The sacral nerve roots exit from each of the five sacral foramina. These coalesce to form part of the lumbosacral and all of the sacral plexus, which innervate the perineal structures and lower extremities.
  • The primary named peripheral nerves that exit the pelvis via the greater sciatic notch are the sciatic nerve, the superior and inferior gluteal nerves, and the internal pudendal nerve. The obturator nerve exits the pelvis via the obturator foramen.

A full understanding of pelvic anatomy is required to treat pelvic fractures, to prevent iatrogenic injuries, and to provide the best results.