Regional anatomy 13 -- abdomen & pelvis 3

Bones of the pelvis

• Consist of the right & left pelvic (hip) bones, sacrum, & coccyx

• Sacrum articulates with vertebra L5 at the lumbosacral joint

• Pelvic bones articulate posteriorly with sacrum at the sacro-iliac joints & with each other at the pubic symphysis

Pelvic bones

• 1 right, 1 left — irregularly shaped bones that form part of the pelvic girdle (bony structure attaching axial skeleton to lower limbs)

• They have 3 main articulations —

  • Sacroiliac joint — articulation with the sacrum

  • Pubic symphysis — articulation between left & right hip bones

  • Hip joint — articulation with head of femur (between acetabulum & femur)

• 2 major parts separated by an oblique line on the medial surface of the bone — separating true & false pelvis (bony rim) —

  • Above the line — lateral wall of false pelvis, part of abdominal cavity

  • Below the line — lateral wall of true pelvis

• Linea terminalis — lower 2/3 of this oblique line, contributing to margin of pelvic inlet

• Inferior to the acetabulum is the large obturator foramen, mostly closed by a flat connective tissue obturator membrane.

  • Small obturator canal remains open superiorly between the membrane & adjacent bone, providing route of communication between lower limb & pelvic cavity

• Posterior margin is marked by 2 notches separated by the ischial spine — Greater & lesser ischial notches — with the posterior margin terminating inferiorly as the large ischial tuberosity

• The irregular anterior margin of the pelvic bone is marked by the anterior superior iliac spine, the anterior inferior iliac spine, & the pubic tubercle

Composition of the hip bone (3 parts)

• 3 main parts —

  • Ilium

  • Pubis

  • Ischium

• Prior to puberty, the triradiate cartilage separates these parts, with fusion beginning at ages 16-18

• Together, the ileum, pubis, & ischium form a cup-shaped socket known as the acetabulum (latin = vinegar cup)

  • The head of the femur articulates with the acetabulum to form the hip joint

The ilium

• The most superior, widest & largest of the 3 parts of the hip bone

• Forms the superior part of the acetabulum (acetabular roof), & immediately above expands to form the wing (ala)

• The upper part of the ilium expands to form a flat, fan-shaped ‘wing’ that provides bony support for the lower abdomen/false pelvis — also provides attachment for muscles functionally associated with the lower limb — 2 surfaces —

  • Inner surface —

    • Concave, producing iliac fossa (site of origin of iliacus muscle)

  • External (gluteal) surface —

    • Convex shape & provides attachment to gluteal muscle

• The superior margin of the wing is thickened & forms the iliac crest, extending from the anterior superior iliac spine (ASIS) to the posterior superior iliac spine (PSIS)

  • The tuberculum of the iliac crest — a prominent tubercle projecting laterally near the anterior end of the crest

  • The iliac tuberosity — thickening of posterior end of the crest

• The greater sciatic notch — indentation formed by the posterior inferior iliac spine

• Inferior to anterior superior iliac spine of the crest, on the anterior margin of the ilium, is a rounded protuberance called the anterior inferior iliac spine — serves as point of attachment for rectus femoris muscle of anterior thigh compartment & iliofemoral ligament associated with the hip joint

• The ilium is separated into upper & lower parts by a ridge on the medial surface —

  • Posteriorly, ridge is sharp & lies immediately superior to the surface of the bone articulating with the sacrum — the sacral surface has a large L-shaped facet for articulating with the sacrum & an expanded, posterior roughened area for the attachment of the strong ligaments supporting the sacro-iliac joint

  • Anteriorly, the reidge separating upper & lower parts of the ilium is rounded — called the arcuate line

Clinical drop — Bone marrow biopsy

• In certain diseases (ex. leukemia), a sample of bone marrow must be obtained to assess the stage & severity of the problem — often the iliac crest is used as it lies close to the surface & is easily palpated

• The procedure is performed by injecting anesthetic in the skin & passing a cutting needle through the cortical bone of the iliac crest, where the bone marrow is then aspirated & viewed under a microscope

• Samples of cortical bone can also be obtained in this way to provide information about bone metabolism

Pubis

• Makes up anterior & inferior part of the pelvic bone — has a body & 2 arms/rami —

  • Body —

    • Flattened dorsoventrally & articulates with body of pubic bone on other side at the pubic symphysis

    • Has a rounded pubic crest on its superior surface that ends laterally as the prominent pubic tubercle

  • Superior ramus —

    • Projects posterolaterally from the body, joins with ilium & ischium at its base

    • Sharp superior margin of the surface — known as the pectineal line — forms part of linea terminalis of the pelvic bone & inlet

      • Anteriorly, this line is continuous with the pubic crest, & is also marked on its interior surface by the obturator groove (forms upper margin of obturator canal)

  • Inferior ramus —

    • Projects laterally & inferiorly to join with the ramus of the ischium

Ischium —

• Posterior & inferior part of pelvic bone, with a —

  • Large body — projects superiorly to join with ilium & superior ramus of pubis

  • Ramus — projects anteriorly to join with inferior ramus of pubis

• Posterior margin of the bone is marked by a prominent ischial spine that separates the lower lesser sciatic notch from the upper greater sciatic notch

• Most prominent feature — ischial tuberosity — large tuberosity on the posteroinferior aspect of the bone, an important site for attachment of lower limb muscles & supporting body when sitting

Sacrum

• Appearance of an inverted triangle formed by the fusion of 5 sacral vertebrae

• Base of sacrum articulates with L5, apex articulates with coccyx, & each lateral surface bears a large L-shaped facet for articulation with the ilium of the pelvic bone

  • Posterior to the facet is a large roughened area — attaches ligaments supporting sacroiliac joint

• Superior surface — characterized by superior aspect of body of S1 & flanked on each side by an expanded wing-like transverse process — ala

• Anterior edge of the vertebral body projects forward as the promontory — anterior surface of sacrum is concave, posterior is convex

• Because the transverse processes of adjacent sacral vertebrae fuse lateral to the position of the intervertebral foramina & lateral to the bifurcation of spinal nerves into rami, the posterior & anterior rami of spinal nerves S1-S4 leave the sacrum via separate foramina —

  • 4 pairs of anterior sacral foramina, 4 pairs of posterior

• The sacral canal is a continuation of the vertebral canal that terminates as the sacral hiatus

Coccyx

• Small terminal part of the vertebral column — consists of 4 fused coccygeal vertebrae shaped like an inverted triangle

• Base of the coccyx — directed superiorly, bearing a facet with the sacrum & 2 horns/cornue that project upward to articulate/fuse with similar downward projecting cornua from the sacrum

• These processes are modified superior & inferior articular processes present on other vertebrae — each lateral surface of the coccyx has a rudimentary transverse process from the first coccygeal vertebra

• Vertebral arches are absent from the coccygeal vertebrae — thus no bony vertebral canal present

Joints of the pelvis

• Lumbosacral joint —

  • Symphysis (secondary cartilaginous) joint between L5 & sacral base, allowing flexion, extension, lateral flexion, & minimal rotation of torso with respect to pelvis & lower limbs

  • Consist of 2 zygapophyseal joints (between inferior & superior articulatory processes) & an intervertebral disc between L5 & S1

  • Reinforced by strong iliolumbar ligaments (L5-ilium) and lumbosacral ligaments (L5-sacrum), which specifically originate from the expanded transverse processes of this vertebrae

• Sacrococcygeal joint —

• Sacroiliac joint —

  • L-shaped synovial joint between ala of sacrum & auricular surface of ilium

  • Allows very little mobility through slight gliding & rotation movments

  • In women, ligaments of the joint soften during pregnancy to enable increase of pelvic diameter during childbirth

  • Each joint stabilized by 3 ligaments —

    • Anterior sacro-iliac ligament — thickening of fibrous membrane of joint capsule, running anteroinferiorly to the joint

    • Interosseous sacro-iliac ligament — largest & strongest of the 3, attaches to adjacent expansive roughened areas on the ilium & sacrum, thus filling the gap between the 2 bones

    • Posterior sacro-iliac ligament — covers the iterosseous sacro-iliac ligament

• Pubic symphysis —

  • secondary cartilaginous joint between medial surfaces of the pubic bones — the surfaces lined by a layer of hyaline cartilage & connected by the fibrous symphyseal cartilage between them by fibrocartilage

  • Usually no movements except in pregnancy when ligaments & cartilage soften to increase pelvic diameter during labor

  • The joint is surrounded by interwoven layers of collagen fibers & the 2 major ligament associated to it are —

    • Superior pubic ligament — above the joint

    • Inferior pubic ligament — below the joint

Clinical drop — pelvic fractures

• Reminder — pelvic bones, sacrum, & associated joints form a bony ring surrounding the pelvic cavity — soft tissue & organ damage is suspected when the pelvis is fractured

• Patients with multiple injuries & evidence of chest, abdominal, & lower limb trauma should alos be investigated for pelvic trauma

• Pelvic fractures can be associated with appreciable blood loss (concealed exsanguination) and blood transfusion is often required. Additionally, this bleeding tends to form a significant pelvic hematoma — can compress nerves, organs & inhibit pelvic visceral function

• Classification —

  • Type 1 — don’t disrupt bony pelvis ring (ex. iliac crest fracture) — unlikely to represent significant trauma, though if iliac crest is fractures, blood loss needs to be assessed

  • Type 2 — a single break in bony pelvic ring (ex. single fracture with diastasis (separation) of the symphysis pubis) — again relatively benign, but should assess for blood loss

  • Type 3 — double breaks in bony pelvic ring, including bilateral fractures of pubic rami, which could cause urethral damage

  • Type 4 — occur at & around the acetabulum

Pelvis orientation

• When a person is in the anatomical position, the right & left anterior superior iliac spines (ASISs) & the anterior aspect of the pubic symphysis lie on the same verticle plane

• When a pelvic girdle in this position is viewed anteriorly, the tip of the coccyx is close to the center of the pelvic inlet, with the pubic bones & pubic symphysis constituting more of a weight-bearing floor than an anterior wall

• The sacral promontory is located directly superior to the center of the pelvic outlet (site of the perineal body) — consequently, the curved axis of the pelvis intersecs the axis of the abdominal cavity at an oblique angle

Differences in the pelvis between males & females (chart)

Obturator internus

Flat, fan-shaped muscle of the gluteal region in the lower limb, forming part of the lateral wall of the pelvic cavity

• Origin — pubis & ischium at the obturator foramen

• Insertion — travels through lesser sciatic foramen to attach onto the greater trochanter of the femur

• Functions — lateral rotation & abduction of the lower limb

• Innervation — nerve to obturator internus

• (blood supply — obturator artery)

Piriformis

Triangular muscle of the gluteal region in the lower limb, also serving as an important landmark dividing the gluteal region into an inferior & superior part

• Origin — anterior surface of sacrum (bridges of bone)

• Insertion — fibers travel inferiorly & laterally through greater sciatic foramen to insert onto greater trochanter of the femur

• Functions — lateral rotation & abduction of the lower limb

• Innervation — nerve to piriformis

• (blood supply — superior & inferior gluteal artery)

Foramen & canals — Greater sciatic foramen

• Passageway for structures entering/leaving the pelvis (ex. sciatic nerve & piriformis muscle)

• Bordered by —

  • Superiorly, anterior sacroiliac ligament

  • Posteromedially, sacrotuberous ligament

  • Anterolaterally, greater sciatic notch of the ilium

  • Inferiorly, sacrospinous ligament & ischial spine

• It is then divided into 2 parts by the piriformis muscle —

  • Suprapiriform foramen

    • Superior gluteal artery, vein, & nerve

  • Infrapiriform foramen

    • Sciatic nerve

    • Pudendal nerve

    • Inferior gluteal atery, vein, & nerve

    • Posterior femoral cutaneous nerve

    • Nerve to obturator internus

    • Nerve to quadratus internus

Foramen & canals — Lesser sciatic foramen

• Passageway for structures entering/leaving the perineum (ex. pudendal nerve)

• Bordered by —

  • Superiorly, sacrospinous ligament & ischial spine

  • Anteriorly, ischial spine, lesser sciatic notch, & ischial tuberosity

  • Posteriorly, sacrotuberous ligament

• Following structures pass through —

  • Internal pudendal artery & vein

  • Pudendal nerve (*first leaves pelvis via greater sciatic foramen, then re-enters via lesser sciatic foramen)

  • Obturator internus tendon

  • Nerve to obturator internus

Obturator canal

• Passageway formed in the obturator foramen by the obturator membrane & the pelvis, connecting the pelvis to the thigh

• Originates from obturator foramen

  • An opening between ischium & pubic bones inferior to the acetabulum

  • Covered almost entirely by the obturator membrane, with a small gap left between the superior margin of the obturator membrne & the pelvic bone above — the obturator canal

• Allows 3 structures to pass out from the pelvic cavity to communicate with the lower limb —

  • Obturator artery

  • Obturator vein

  • Obturator nerve

Pelvic muscular diaphragm

• The muscular part of the pelvic floor — shaped like a bowl/funnel attached superiorly to the pelvic walls, consisting of the levator ani & coccygeus muscles

• Its circular line of attachment to the cylindrical pelvic wall passes on each side between the greater & lesser sciatic foramina, thus —

  • Greater sciatic foramen is situated above the level of the pelvic floor & is a route of communication between the pelvic cavity & gluteal region of the lower limb

  • Lesser sciatic foramen is situated below the pelvic floor & is a route of communication between the gluteal region of the lower limb & the perineum

Coccygeus

2 muscles, one on each side, triangular in shape & overlie the sacrospinous ligaments — together they complete the posterior part of the pelvic diaphragm

• Attachment — by their apices to the tips of the ischial spines, by their bases to the lateral margins of the coccyx & adjacent margins of the sacrum

• Innervation — Branches from anterior rami of S3 & S4

• Functions — participate in supporting posterior aspect of the pelvic floor, thus supporting the pelvic viscera. Also pulls coccyx forward after defecation

Levator Ani

A broad sheet of muscle, composed of three separate paired muscles —

• Puborectalis

• Pubococcygeus

• Iliococcygeus

Puborectalis

1st & most important levator ani muscle for maintaining faecal continence (not leaking shit)

• Attachments —

  • Originates from posterior surface of pubic, where it then forms a U-shaped sling around the anal canal to attach to the pubis on the contralateral side

• Functions —

  • Tonic contraction bends the anal canal anteriorly, creating the anorectal angle, thus contributing to faecal continence — voluntarily inhibited during defecation

• Innervation —

  • Nerve to levator ani and pudendal nerve

• ** some fibers form another U-shaped sling around male urethra & female urethra + vagina — preserve urinary continence, especially during abrupt increase of intra-abdominal pressure (ex. during sneezing)

Pubococcygeus

Forms the bulk of the levator ani complex, located between the puborectalis & iliococcygeus in the pelvic floor

• Origin —

  • posterior surface of the pubis

• Insertion —

  • blends with contralateral muscle in the midline of the pelvic floor

• Functions —

  • stabilize & support abdominal & pelvic viscera

• Innervation —

  • nerve to levator ani & branches of pudendal nerve

Iliococcygeus

Thin muscle forming the posterolateral part of the levator ani muscles —

• Origin —

  • the ischial spines & posterior tendinous arch of the internal obturator fascia

• Insertion —

  • the coccyx, perineal body, & anococcygeal ligament, also blending with the fibres of the contralateral muscle in the midline of the pelvic floor

• Function —

  • elevates the pelvic floor & anorectal canal

• Innervation —

  • nerve to levator ani & branches of pudendal nerve