FACTS & HINTS

High-Yield Facts

Clinical Points

Pregnancy:In pregnancythe placenta can be palpated above the pubic symphysis at 12 weeks, at the umbilicus at 12 weeks, and at the xiphisternum at 40 weeks.

Acute UrinaryRetention:The bladder, if distended, maybe palpated and percussed up to the umbilicus. On examination, the bladder is dull to percussion and in acute urinaryretention, the patient mayalso complain of tenderness on palpation in the suprapubic region. Supracristal Line:Auseful landmark when performing a lumbar puncture since it corresponds to the 4th lumbar vertebral body. Lumbar puncture in adults is performed in the lateral decubitus position in the L4-L5 interspace.

Pilonidal Sinus:Ablind ending hair-filled tract most commonlyfound in the midline of the natal cleft overlying the lower sacrum and coccyx and occurs in 26/1000 persons in the United States. The sinus can become infected, creating a so-called pilonidal abscess that usually requires drainage and/or excision of the sinus.

257 / 425

33 Bones and Ligaments

STUDYAIMS

At the end of your study, you should be able to:

Identifythe components of the bonypelvis

Define the boundaries of the pelvic cavity

Describe the joints of the pelvis

Describe the ligaments that strengthen the pelvis

Outline the keydifferences between the male and female pelvis

List the structures that pass through the greater and lesser sciatic foramina

258 / 425

GUIDE

Pelvis and Perineum: Bones and Ligaments

[Plate 333, Radiographs of Male and Female Pelvis]

259 / 425

[Plate 335, Bones and Ligaments of Pelvis]

Bones and Boundaries of the Pelvis

Bony pelvis

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 bycartilage at the acetabulum

Pelvic girdle

Is formed of hip bones and sacrum

Transmits weight from upper bodyto lower limbs

Pelvic walls

Formed bybones of bonypelvis, ligaments, muscle, and fascia

Surround pelvic cavity

Pelvic cavity

page 168

page 169

Basin shaped

Surrounded bybonypelvis

260 / 425

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

Inlet defined byan oblique plane

Extends from promontoryto the superior aspect of the pubic symphysis

Lies at an angle approximately55 degrees from horizontal

Rim of pelvic inlet = pelvic brim, composed of a bonyline 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 is bounded by

Tip of coccyx

Sacrotuberous ligaments

Inferior ischiopubic rami and ischial tuberosities

Inferior edge of pubic 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

Lumbosacral joints

Composed of

Intervertebral joint via intervertebral disc between L4 and S1

Two posterior zygapophysial joints

Reinforced byiliolumbar 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 verylimited

Stabilized byinterosseous and anterior and posterior sacroiliac ligaments

Pubic symphysis

page 169

page 170

Union of bodies of right and left pubic bones Secondarycartilaginous joint Fibrocartilaginous interpubic disc in the joint

Stabilized bysuperior and inferior pubic ligaments

Affected bythe hormone relaxin during pregnancyto permit freer movement between vertebral column and to increase pelvic diameter during childbirth

Sacrococcygeal joint

Articulation between sacrum and coccyx

Secondarycartilaginous joint

Stabilized byanterior and posterior sacrococcygeal ligaments

Ligaments of the Pelvis

The weight of the bodyacting through the spine will tend to rotate the sacrum, tipping the lower part backwards. This movement is prevented bythe 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

261 / 425

Structures Passing Through the Greater and Lesser Sciatic Foramina

Differences linked to function

Pregnancyand 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 comparativelysmaller.

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 transverselyoval in women

Obturator foramen is round in men and oval in women

Female pelvis is broader than in men, to allow the passage of the fetal head

262 / 425