Lecture 15: Bones & Joints of Pelvis Flashcards
What separates the true (lesser) and false (greater) pelvis?
Iliopectineal line - top of pubis along bony ridge to lieum
How is the female pelvis different to the male pelvis?
Wider than it is longer, narrower in height, wide subpubic arch, ischial spine does not project far medially, round pelvic inlet
What are the 4 types of female pelvis and the occurrence of these?
Gynaecoid - 50%
Android - 30%
Anthropoid - 18%
Platypelloid - 2%
What is characteristic of a gynaecoid pelvis?
Designed to allow foetus to move through
What is characteristic of an android pelvis?
Not ideally suited for childbirth, narrower side to side and longer, funnel shaped in midpelvis
What is characteristic of the anthropoid pelvis?
Narrow medial to lateral, elongated anteroposterioly (related to head shape of ape)
What is characteristic of the platypelloid pelvis?
Flattened
What are the features of the male pelvis?
Tall and narrow, heart shaped inlet, narrow subpubic angle, ischial spine projects medially into mid pelvic region
How is the pelvis tilted?
Tilted forwards so pubis is more inferior than rest of pelvis.
Which gender has greater lumbar lordosis?
Females - more pelvic tilt
What is the shape of the pelvic outlet and how does it differ between genders?
Diamond shaped, narrowed in both directions in males
What is the pelvic outlet divided into?
Urogenital triangle and anal triangle
What are the boundaries of the urogenital triangle?
Bony
What are the boundaries of the anal triangle?
Ligamentous
What is the narrow pelvic plane?
Plane of least dimensions in the pelvis. Passes through ischial spine, S4 and pubic bones
What is the average diameter of foetus skull at birth?
9.5cm
What is the average diameter of the female pelvis?
10.5-11.5cm
How does the foetus move through the pelvis?
Head is transverse and fits through pelvic inlet, then pelvic floor rotates head of baby so that when it reaches the true pelvis the head is in an anteroposterior direction and the chin is on the chest, moves through pelvic outlet in same posture
What is the vertex presentation?
When the baby is moving through the true pelvis and the head is tilted down - smallest diameter 9.5cm
What happens if the baby presents with a brown presentation, how is this accommodated for?
Head is up (13.5cm diameter) and delivery is made difficult.
Baby has fontanels in skull that allow cranial bones to slide across each other
When does the frontal fontanel close?
18 months
What is characteristic of the pubic symphysis in younger individuals?
Has ridges and grooves - these are worn away with age
What are the two bony arches in the pelvis and what do they do?
- Acetabulum to acetabulum, bears weight when standing
- Between ischial bones and pubis, bears weight when sitting
How is weight transferred when standing?
Weight transmitted from trunk to pelvis and causes pelvis to rotate forwards
What does the pelvis rotate relative to?
S2
Why does little rotation actually occur at the pelvis?
Strong ligamentous support - interosseous, sacroiliac, sacrospinous, sacrotuberous
What is the reverse keystone effect?
Sacrum sinks forwards and downwards into the pelvis, posterior ligaments tighten and draw iliac bones together
What are the features of the sacroiliac joint?
Synovial anterior and fibrous posterior, held together by powerful ligaments that resist tendency of sacrum to rotate, auricular and iliac surfaces, only 2-8 degrees of movement – usually when you get up from lying down, movements may increase during mid-late stages of pregnancy
What happens when the ligaments supporting the SIJ are damaged in pregnancy?
SIJ pain L5-S2 posteriorly, L3-S1 anteriorly, pain referred to medial and lower bum
What is the function of the posterior layer of lumbar fascia?
Acts as an intermediary in transferring loads between trunk and lower limb muscles (lat dorsi and gmax)
What are the features of the pubic symphysis?
Midline secondary cartilaginous joint, superior, arcuate and anterior (resist vertical forces) ligaments, reinforced by criss crossing fibres from rectus abdominus and transversus (+ adductor longus and internal oblique) – tie beam, prevent lateral separation, site of pubic symphysitis
What is nutation and counter nutation?
Pelvis tilting forwards and backwards
What is characteristic of a pelvic fracture, and what is unique in kids?
Because it is a ring it fractures in multiple places, in kids the ligament is stronger than bone so common to get avulsion fractures
