L6.2 Bones & joints of pelvis

Question 1

True and false pelvis

Answer 1

• Pelvis divides into true (lesser - which contains the viscera) & false (greater - above the true pelvis) pelvis
  
  • Iliopectineal line → subdivides false pelvis from true pelvis

Question 2

Difference between female and male pelvis shape

Answer 2

• Wider and shorter pelvis and have wider pubic arches
• 50% of females have gynaecoid shaped pelvis
• 30% of females have android pelvis
  
  • Narrower, longer & funnel shaped → complications of birth
• 20% of females have Anthropoid shaped pelvis (ape-like)
  
  • Or 2% have paltypelloid
• Most males have android shaped pelvis

Question 3

Difference between male and female pelvic inlet

Answer 3

• Female pelvis:
  
  • Wide inlet
  • Ischial spine does not project MED as much
• Male pelvis
  
  • Heart shaped inlet
  • Ischial spine projects into MED plane

Question 4

Line of gravity and pelvic tilt

Answer 4

• Tilt: Pubis becomes INF (in line with lumbosacral joint)
  
  • Accommodated by changes in the lumbar spine & aligns with center of gravity
• Females have a greater pelvic tilt (due to lumbar lordosis)

Question 5

Key features of the Hip bone

Answer 5

• Epiphyseal plate b/w 3 bones at the acetabulum
• Ilium: contains the obturator foramen (covered by membrane → gives rise to muscles)
• Ischium: Ischial spine → projected MED may obstruct birth of fetus
• Pubis: G/L Sciatic notch

Question 6

Plane: Tip of coccyx to pubis

Answer 6

Ovaries located at this plane

Question 7

Pelvic inlet/outlet shape

Answer 7

• Outlet: Diamond shape (narrower in males)
  
  • Divides into urogenital triangle and anal triangle

Question 8

3 different pelvic planes

Answer 8

• Narrow pelvic plane (plane of least dimensions): S4 → INF pubis (where head has to pass)
• Tip of coccyx to pubis
• Line of gravity plane: Lumbosacral joint to INF pubis

Question 9

Dimensions of the pelvic cavity size and fetus size

Answer 9

• Avg pelvic cavity = 11cm diameter
• Vortex presentation of fetus (Chin twd chest) = 9.5cm diameter
• Brow presentation of fetus (Chin up) = 13.5cm → problematic for child birth

Question 10

Apertures of the pelvis

Answer 10

• Pelvic inlet
• ANT sacral foramen
• Obturator canal
• GSF → to LL
• LSF → perineum & gluteal regions

Question 11

How is the pelvis the most important skeleton for forensic identification

Answer 11

• Size and shape → male/female
• Transverse ridges and groove present in symphysis of younger adults (teen to late 20s)

Question 12

Mechanical structure of the pelvis

Answer 12

• 2 bony arches:
  
  • POS arch → sacrum → acetabulum
  • ANT arch → Pubic arch → bear weight in sitting
• Bones organised in trabaculae → resist forces
• Transfer of forces → pelvis acts as struts → allows movement and load transfer

Question 13

Movements affecting the pelvic bone

Answer 13

• Stance → weight from trunk to pelvis
  
  • Pelvis wants to tilt further fwd (nutation) ~S2 vertebrae
  • But Little movement takes place due to ligaments in the joint of pelvis
• Movement ~ 2-8o
  
  • Occurs from a lying down position to standing position
  • Movements increase during late pregnancy (due to relaxin hormone → relax ligaments)
    
    • Complications with POST pregnancy → SI joint pain → ligaments retighten in wrong way
    • Refer pain to MED/LOWER buttocks

Question 14

Sacroiliac joint

Answer 14

• Synovial ANT
• Fibrous POS
• Held together by strong lig → prevents movement/nutation

Question 15

Ligaments of the sacroiliac joint

Answer 15

• Sacroiliac (interosseous)
• Sacrospinous (GSF)
• Sacrotuberous (LSF)

Question 16

Reverse keystone effect

Answer 16

• Sacrum pushed downwards from standing
• Forces sacrum to pull ligaments tighter → joints stronger by drawing iliac bones together

Question 17

How can there be a force closure at the sacroiliac joint

Answer 17

• Gluteals + lats → muscular sling from on side to the other of the body
  
  • Important to maintain integrity of sacroiliac joint
  • Weak gluteal and lats → may lead to instability of sacroiliac joint

Question 18

Features of the pubic symphysis

Answer 18

• Second cartilaginous disc
  
  • May split during pregnancy
• Reinforced by lig SUP & INF

Question 19

Pubic symphysitis

Answer 19

• Due to pull from ANT ab wall muscles and ADD longus

Question 20

Pelvic fractures

Answer 20

• Due to trauma
• Common in children - ligaments attachment is stronger than bone

Question 21

What is the ring phenomenon

Answer 21

• Fracture on one side → fracture diametrically opposite on the other side