11.03 Bones and Joints of the Pelvis Flashcards
• Bones forming the pelvis • Pelvic types and their clinical significance • Apertures of pelvis & structures transmitted • Mechanism of the pelvis • Structure & function of sacroiliac joint • Structure & function of pubic symphysis • Applied anatomy
What bones form the pelvis?
What does pelvis roughly translate into?
The hip bone and the sacrum
Pelvis roughly translates into basin
What are the two divisions of the pelvis?
What delinetaes them?
The upper part of the basin is called the false (greater pelvis) which holds some of the abdominal viscera.
The area below that is the true (lesser) pelvis which has walls anteriorly, laterally and posteriorly.
The Iliopectineal line (pelvic brim) is the line between them
What makes up the walls that surround the lesser (false) pelvis
Anterolateral walls are formed by the hip bone and posterior wall by the sacrum.
What are important structures arising from the walls of the lesser (false) pelvis?
Muscles arise from the sacrum and get out of the pelvis and muscles from the lateral wall of the pelvis which have an important role in relation to the pelvic floor.
What is the ileopectal line?
(also called the pecten pubus)
A line/ridge from the ileum posteriorly to the pubis anteriorly. It demarcates false and true pelvis
The pectineal line forms part of the pelvic brim. Lying across it are fibers of the pectineal ligament and the proximal origin of the pectineus muscle.
What is the general difference between the male and female pelvis?
Female pelvis is broad, wide and is shallower from above downwards (sacrum is broad and width equivalent to the length).
Male pelvis is narrower but is longer.
The two hips joints home together at the midline in a typical midline joint arrangement. Describe this
Anteriorly the two pubic bones come together in a midline joint
(All midline joints in the body are symphiseal joints with a fibrocartilaginous discs).
Describe the surfaces that make up the sacroileac joint
The second joint of the pelvis.
The ileum has an ear shaped surface covered by haline cartilage called the auricular surface.
The sacrum also has an auricular surface on it lined by hyaline
Describe the sacrum
- It has a coccyx (variable structure)
- Anterior foramine which has nerves (ventral rami and sacral plexus) going through them.
Structures other than the nerves pass through the ventral foraminae of the sacrum. What are these structures and what is the significance of this?
These foraminae are also roots for veins draining pelvic viscera (plexuses of veins).
Normally this drainage is back into the inferior vena cava; but if there is interference of flow then there is passage through anterior sacral veins into spinal canal itself (also common route of cancer spread)
Describe the bones of the pelvis in terms of epiphyseal sites.
What is the importance of this?
There are many epiphyseal sites in the pelvis
Children: multiple sites of empiphysise: acetabulum, ishium, crest of the ileum
What are the two parts of the main pelvic hole?
Pelvic inlet (doing down from above)
Pelvic outlet (coming out from above)
What are the four main types of pelvic inlet shapes? What proportion of women have that particular shape?
- Gynaecoid - 50%
- Android - 30%
- Anthropoid - 18%
- Platepelloid - 2%
Describe the gynaecoid pelvic inlet shape
The pelvic inlet is broad to accommodate the head of the baby as it passes through birth canal. The true pelvis the dimension of width is retained down the pelvis and the pelvic outlet is also broad (subpubic angle)
Describe the android pelvic inlet shape \
Narrow inlet and narrow true pelvis and narrow outlet (not designed for transmission of the foetus).
The male pelvic inlet is heart shaped (is narrow).
The outlet is diamond shaped in both males and females but is more broader in females.
Describe the anthropoid and platapelloid pelvis shapes
Anthropoid (narrow from side to side but enlongated in the AP direction).
The remainder are playpelloid which is wide from side to side and narrow in the AP direction.
Describe the line of gravity through the pelvis
What is the net effect of this?
Lumbar lordosis provides flexibility in stance and locomotion.
The line of gravity passes through the vertebrae and passes anteriorly to the sacrum.
The net effect of gravity is to force the sacrum to tilt forwards and inferior in the inverse direction of the lumbar spine.
Describe the alignment of the major anterior structures of the hip bone (ASIS and pubic tubercle) in normal stance
In normal orientation the ASIS is in line with the pubic tubercle (vertical alignment).
Describe the horizontal alignment betwen the sacrum and the hip bone (pelvis)
The top of the pubic symphisis is in the same horizontal plane as the tip of the coccyx
Compare the forward/inferior tilt in females with that in males
The tilt is eccentuated in females due to increased lumbar lordosis
What does the pelvic outlet transmit?
Pelvic outlet is diamond shaped and it transmits tubular viscera to the exterior (as well as important muscles and ligaments)
How is the pelvic outlet divided?
Divided into 2 triangles
- Urogenital triangle (uretha and vagina in females and beginning of membranous urethra in males).
- Boundaries are boney structures
- Anal triangle which contains the opening for the anal canal.
- Boundaries are mainly ligamentous: sacrotuberous ligament .
Describe the anterior and posterior definitions of:
- Pelvic Inlet
- Pelvic Outlet
Pelvic Inlet:
- Top of pubic symphisis to the first sacral vertebrae.
Pelvic outlet:
- From the inferior pubic symphisis to tip of the cocyx
Where and what is the narrow pelvic plane?
The narrow pelvic plane runs from inferior part of pubic symphis through the ischeal spine to S4.
It is the narrowest part of the pelvis that has to be negotiated by foetal head in childbirth.
The narrow pelvic plane also corresponds to another plane of the pelvis.
The pelvic pain line (site from which pain is referred back to the CNS).
What is the pelvic pain line?
- Any viscera located above the line (eg. body of bladder) refers pain to the lower thoracic/upper lumbar spinal cord with sympathetics.
- Below the pain line, pain is through parasympathetic pain line S2,3,4 (to the skin)
The pelvis forms an important interface to the surrounding structures. Elaborate on this
Pelvis provides interface between abdomen above, back behind and lower limb below.
Draw and describe the apertures of the pelvis
- Pelvic inlet – from abdomen
- Anterior sacral foraminae – to back
- Obturator canal – to lower limb
- Greater sciatic foramen – to lower limb
- Lesser sciatic foramen – to perineum & gluteal region
Describe the forensic significance of the pelvis
pelvis is useful to identify sex of individual and age by looking at the shape of the body of the pubis.
Male: body is narrow from front to back and the subpubic angulation is acute.
Symphiseal surface may give a rough indication of age. Bottom picture has a smooth (eroded) surface from elderly individual. Younger have ridges and grooves.
The bones of the pelvis forms 2 arches. What are these arches and what is their significance?
- Passes from acetabulum to top of sacrum and then to acetablum to other side (primary of the two arches)
- The other arch is under the two pubic bones at the pubic symphisis.
Line from inferior arch to greater arch shows buttressing effect of bone for load bearing in sitting and standing (superior ramus of the pubis transmitting considerable force)
What is the major weight function of the pelvis?
Pubic bones transfer load & absorb force between trunk & limbs
Where do the forces run down in the pelvis?
Forces depends on what we are doing:
Standing: force is directed from junction where hip bone articulates to sacrum (from trunk along lines of boney trabeculae from pelvis to lower limb and reverse)
Sitting: Bone is reinforced in the area of ischeal tuberosity and inferior ramus of the pubis.
Together they transmit forces inferiorly and superiorly (in locomotion)
What are the two joints of the pelvis?
- Pubic symphysis
- Ileosacral joint
Describe the parts of the sacroileal joint
The anterior part of the joint tends to lie more lateral than the posterior (a little more medial).
It is a mixed joint:
- Part synovial (anterior)/ part fibrous (posterior)
- There Fibrous part of the joint: bones held together by very powerful ligaments.
What are the four ligaments critical for stabilising the sacroilieac joint?
- Interosseoussacroiliac
- Iliolumbar
- Sacrotuberous
- Sacrospinous
Describe the sacral and ileal surfaces of the sacroileal joint
Has surfaces on the ileum and a mirror image of it on the sacrum. They are both called auricular surfaces.
What lies within the space of the sacroileal joint?
The interosseus sacroiliac ligament (between bones).
It is one of the most powerful ligaments of the body - intrinsic ligaments.
It serves as a very powerful stabiliser of the sacroileac joint.
Describe the sacrotuberous and sacrospinous ligaments
- Sacrotuberous ligament runs from the sacrum to the tuberosity of the ischium with fibres running vertical
- Sacrospinous ligament with horizontal fibres that run from the ischial spine to lateral margins of the sacrum. They resist inferior part of sacrum from tilting backwards.
Gluteal side (sacrospinous ligament) but pelvic side it is covered by muscle fibres that form part of the pelvic floor (coccygeous ligament).
Describe the iliolumbar ligament in terms of location and function
Iliolumbar ligament from the tip of the transverse processes of the lumbar spine (L5 mainly and maybe L4) to the posterior part of the ileac crest. It helps hold the ileum in position.
How does the sacrum tilt as a result of the line of gravity passing in front of it?
The sacrum wants to rotate with upper part tilting forward and lower part tilting backwards.
Where is the axis of rotation for the sacrum tilt?
The second sacral vertebra
The sacral tilt as a result of the line of gravity has a special type of movement. What is this movement called?
A “nodding” movement.
This is called nutation.
Describe the reverse keystone effect of the sacrum, the hipbones and their related ligaments
When standing, the sacrum sinks forwards & downwards into the pelvis between the ileac bones.
The posterior ligaments tighten & draw iliac bones together. Stability is due to ligaments (particularly the interosseous sacroiliac ligaments)
Bones tend to ‘open’ the joint up while the ligamentous fibres resulting in increased stability and tightness of the structures at the joint.
Describe nutation and counternutation in terms of the range of movements and what happens to the hip bones in this movement
In nutation, the sacrum tilts forwards (about the axis of S2) and the hip bones tilt inwards (superior part tilts inwards).
In counternutation, the sacrum tilts backwards and the hip bones tilt outwards.
These movements primarily occur from the lying position to standing.
The range of movement is very small (only about 2-8 degrees)
Describe the clinical significance of the nutation/counternutation movement
These movements may increase during mid-late stages of pregnancy as the pelvis widens causing a laxity of the ligaments responsible for stability particularly of the last trimester under the influence of the hormone relaxin.
Post-partum, the ligaments may tighten but not in the appropriate manner which may result in pain after.
What is the nerve supply of the sacroileac joint and where does pain refer to?
Nerve supply is from the dorsal rami of L5-S3. Pain referred to medial & lower buttock
There is an additional structure that helps to stablise the sacroileac joint.
What is this structure?
A sling of muscle that criss-crosses the sacroileac joint. These massive muscles use the lumbar fascia as an interface causing a forced closure at the SIJ
- Lower sling = gluteus maximus
- Upper sling = Latissimus dorsi.
The fibres of these large powerful muscles connect across the posterior aspect of the SIJ on the fascia.
[Exercises that recruit these muscles can be sufficient to increase stability of the joint and overcome problems with laxity]
Describe the pubic symphysis
A cartilaginous joint (hyaline cartilage on bones external to symphysis)
Located anterior and inferiorly. And is a classical symphisis joint with a broad interarticular fibrocartilaginous disc
Describe the stabilising ligaments of the pubic symphysis
The joint is reinforced by
- arching fibres above = Superior pubic ligament and
- arcuate bundle below = inferior arcuate ligament
Together they maintain the proximity of bones holding them together resisting forces in all directions.
Muscle fibres also add to the stability of the joint. Describe this
The Adductor longus muscle merges with the aponeurosis of internal oblique and transversus abdominus providing an anterior sling from trunk to opposite limb.
[Clinically: Pain may not be associated with inflammation or degeneration at the joint itself and may be due to pull of attachments of these muscles]
What makes the pelvis a common site for fractures?
Common site of avulsion fractures due to the multiple ephyseal sites (especially in children)
The ring structure of bones means fracture at one site is associated with a fracture in a diagonal direction (the ring principle).
(multiple fractures to the pelvic are often not compatible with life)
