Pelvis and Perineum: Bones and Ligaments

Pelvis and Perineum > Bones and Ligaments > Study Aims
STUDY AIMS
At the end of your study, you should be able to:
Identify the components of the bony pelvis
Define the boundaries of the pelvic cavity
Describe the joints of the pelvis
Describe the ligaments that strengthen the pelvis
Outline the key differences between male and female pelves
List the structures that pass through the greater and lesser sciatic foramina

Pelvis and Perineum > Bones and Ligaments > Guides
GUIDE

5-2: Pelvis and Perineum: Bones and Ligaments
Bones and Boundaries of the Pelvis
    Bony Pelvis (Plate 248)
  • Is a strong ring
  • Supports the weight of the body
  • Provides attachment for powerful muscles that move the lower limb
  • Composed of four bones
    • Two hip bones or innominate bones
    • Sacrum—five fused sacral vertebrae
    • Coccyx—four (+ 1) fused coccygeal vertebrae
  • Hip or innominate bones each formed from
    • Ilium
    • Ischium
    • Pubis
    • Fuse at puberty
    • Are united by cartilage at the acetabulum
  • Pelvic girdle
    • Is formed of hip bones and sacrum
    • Transmits weight from upper body to lower limbs
    Pelvic Walls
  • Formed by bones of bony pelvis, ligaments, muscle, and fascia
  • Surround pelvic cavity
    Pelvic Cavity (Fig. 5-2-1)
  • Basin shaped
  • Surrounded by bony pelvis
  • Boundaries:
    • Superiorly—pelvic inlet and inferior abdominal cavity
    • Inferiorly—pelvic diaphragm
    • Anterior wall—bodies and rami of pubic bone and pubic symphysis
    • Posterior wall—sacrum and coccyx, adjacent ilia and overlying piriformis muscle
    • Lateral walls—hip bones, obturator foramen and membrane, and overlying obturator internus muscle
    Pelvic Inlet, Outlet, and Brim (Fig. 5-2-1)
  • Inlet defined by an oblique plane
    • Extends from promontory to the superior aspect of the pubic symphysis
    • Lies at an angle approximately 55 degrees from horizontal
  • Rim of pelvic inlet = pelvic brim, composed of a bony line running through
    • Sacral promontory
    • Arcuate line of the ilium
    • Pectineal line of the pubis (pecten pubis)
    • Pubic crest
    • Superior edge of pubic symphysis
  • Pelvic outlet (Plate 354) is bounded by
    • Tip of coccyx
    • Sacrotuberous ligaments
    • Inferior ischiopubic rami and ischial tuberosities
    • Inferior edge of public symphysis
  • Pelvic inlet divides pelvis into two parts
    • True pelvis or lesser pelvis or pelvis minor, which
      • lies between pelvic inlet and outlet
      • contains the pelvic viscera
    • False pelvis or greater pelvis or pelvis major, which
      • lies above pelvic brim
      • between the iliac fossae
      • contains part of the ileum and sigmoid colon
  • The birth canal includes the pelvic inlet, true pelvis, cervix, vagina, and pelvic outlet.
    Joints of the Pelvis: (Plate 353)
  • Plate 353 illustrates the attachments of the ligaments stabilizing the sacroiliac, sacrococcygeal, and pubic symphysis joints.
    Lumbosacral Joints
  • Composed of
    • Intervertebral joint via intervertebral disc between L4 and S1
    • Two posterior zygapophysial joints
  • Reinforced by iliolumbar ligaments
    Sacroiliac Joint
  • Articulation between ear-shaped surfaces of the sacrum and ilium
  • Atypical synovial joint formed with fibrocartilage rather than hyaline cartilage
  • Movement is very limited
  • Stabilized by interosseous and anterior and posterior sacroiliac ligaments
    Pubic Symphysis
  • Union of bodies of right and left pubic bones
  • Secondary cartilaginous joint
  • Fibrocartilaginous interpubic disc in the joint
  • Stabilized by superior and inferior pubic ligaments
  • Affected by the hormone relaxin during pregnancy to permit freer movement between vertebral column and to increase pelvic diameter during childbirth
    Sacrococcygeal Joint
  • Articulation between sacrum and coccyx
  • Secondary cartilaginous joint
  • Stabilized by anterior and posterior sacrococcygeal ligaments
    Ligaments of the Pelvis (Plate 352)
  • The weight of the body acting through the spine will tend to rotate the sacrum, tipping the lower part backwards.
  • This movement is prevented by the sacrospinous and sacrotuberous ligaments.
    • Sacrospinous ligament: extends from lateral border sacrum to ischial spine
    • Sacrotuberous ligament: larger and extends from dorsum and lateral border sacrum and posterior surface ilium to ischial tuberosity
  • Attachments of sacrospinous and sacrotuberous ligaments enclose the lesser and greater sciatic notches, respectively, forming the greater and lesser foramina
Structures Passing Through the Greater and Lesser Sciatic Foramina ( Table 5-2-1)
    Sex Differences of Pelvis ( Plate 354)
  • Differences linked to function
    • Pregnancy and childbirth in females
    • Heavier build and larger muscles of men
  • Main differences
    • Pelvis is heavier and has more pronounced muscle attachment sites in men than in women
    • Pubic arch is narrower and the subpubic angle more acute in men than women
    • Ischial tuberosities are closer in men than in women, and the pelvis outlet is thus comparatively smaller.
    • All of the ilia are less flared in men than in women, so the greater pelvis is deeper.
    • Pelvic inlet is heart-shaped in men and more transversely oval in women
    • Obturator foremen is round in men and oval in women
    • Female pelvis is broader than in men, to allow the passage of the fetal head
Pelvis and Perineum > Bones and Ligaments > Figures
FIGURES
Plate 353: Pelvis and Perineum—Bones and Ligaments—Bones and Ligaments of Pelvis (continued)
Fig. 5-2-1: Pelvis and lower abdomen after removal of all the organs and most of the vessels to demonstrate the pelvis inlet and cavity. (From Gosling JA, Harris, PF, Whitmore I, et al. Human Anatomy, 4th Edition Mosby, 2005, p. 194, Fig. 5.4)
Fig. 5-2-2: Prosection of right hemi-pelvis. The sites of attachment of the sacrospinous and sacrotuberous ligaments are demonstrated and the lesser and greater sciatic foramina. (From Gosling JA, Harris, PF, Whitmore I, et al. Human Anatomy, 4th Edition Mosby, 2005, p. 208, Fig. 5.26)
Fig. 5-2-3: Radiograph demonstrating the bones forming the pelvis. Note the apparent gap seen at the pubic symphysis which is occupied by radiolucent fibrocartilage. The bones of the pelvic girdle can be seen to form a ‘doughnut shaped’ ring. (From Moses KP, Banks JC, Nava PB, et al. Atlas of Clinical Gross Anatomy. Mosby, 2005, p. 448, Fig. 36.12)
Pelvis and Perineum > Bones and Ligaments > Tables
TABLES

Table 5-2-1: Structures Passing through Greater and Lesser Sciatic Foramina (Fig. 5-2-2)
Greater Sciatic Foramen Lesser Sciatic Foramen
Piriformis muscle
Sciatic nerve
Inferior gluteal nerve and artery
Internal pudendal nerve, artery, and vein
Nerve to obturator internus muscle
Nerve to quadratus femoris
Posterior cutaneous nerve of the thigh
Tendon of obturator internus
Nerve to obturator internus
Pudendal nerve
Internal pudendal artery

Pelvis and Perineum > Bones and Ligaments > Facts and Hints
FACTS & HINTS

PART A: HIGH-YIELD FACTS
Box 5-2-1: Anatomic Points
Measurements of the Pelvic Inlet (Plate 354)
  • Used to determine capacity of female pelvis for childbearing
  • True conjugate diameter
    • measured radiologically on a lateral x-ray
    • distance from superior border of pubic symphysis to sacral promontory
  • Transverse diameter: widest distance of pelvic inlet
  • Oblique diameter: distance from sacroiliac joint to contralateral iliopectineal line
  • Box 5-2-2: Clinical Points
    Fractures of the Pelvis
  • A large force is required to fracture the pelvis and fractures usually result from direct trauma, such as occurs in automobile accidents
  • The bony pelvis may fracture at any point and there may be associated damage to pelvic viscera, for example, bladder and urethral rupture can occur with fractures involving the pubis
  • Pelvic fractures are classified as stable or unstable.
    • In a stable fracture, the pelvis remains stable and there is only one break-point in the pelvic ring with minimal hemorrhage.
    • In an unstable fracture, the pelvis is unstable with two or more break-points in the pelvic ring with moderate to severe hemorrhage.
  • Signs of a fractured pelvis include: pain in the groin, hip or lower back; difficulty walking; urethral, vaginal or rectal bleeding; scrotal hematoma; and shock as a result of concealed hemorrhage (contained bleeding into the pelvic cavity)
  • A fracture can be confirmed on x-ray and is seen as a break in continuity of the pelvic ring. (Fig. 5-2-3)
  • Box 5-2-3: Clinical Points
    Decubitus Ulcers
  • Also called pressure sores
  • Can be a partial- or full-thickness loss of skin, underlying connective tissue and can extend into muscle, bone, tendons, and joint capsules.
  • Two thirds of pressure sores occur in patients older than 70 years
  • Results from prolonged pressure on an area of skin, connective tissue and muscle from a mattress, wheelchair seat, or bed rail.
  • Commonly occur in those with poor mobility, bed-bound, poor nutrition, and incontinence.
  • Can become infected with bacteria from poor skin care, or fecal or urinary incontinence
  • The hip and buttock regions account for 67% of all pressure sores, with ischial tuberosity, trochanteric, and sacral locations being most common.
  • In the sitting position, the ischial tuberosity bears the weight of the whole body and thus is a prime site of ulceration

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