Pelvis and Perineum

Question 1

Overview of the pelvis and perineum

Answer 1

-The pelvis is inferoposterior to the abdomen/ the area of transition between the trunk and the lower limbs
-Greater pelvis is occupied by some inferior abdominal viscera.
-Lesser pelvis is between the pelvic inlet and outlet
-Lesser pelvis houses the pelvic cavity and the perineum (separated by the pelvic diaphragm)
-The perineum includes the anus and external genitalia

Question 2

What is the pelvic girdle?

Answer 2

-Basin-shaped ring of bones that connects the vertebral column to the two femurs
Primary functions are:
-Bear the weight of the upper body when sitting and standing
-Transfer the weight from the axial to the lower appendicular skeleton for standing and walking
-Provide attachment for the powerful muscles of locomotion/posture and those of the abdominal wall, withstanding the forces generated by their actions
-Protect the pelvic viscera (inferior parts of the urinary tract and internal reproductive organs)

Question 3

Bones of the pelvic girdle

Answer 3

-Pelvic girdle (bony pelvis) -ring of bones formed by three bones in mature individuals:

-Right and left hip bones, each of which develops from the fusion of ilium, ischium and pubis
-Sacrum, formed by the fusion of five sacral vertebrae (which are originally separated) + the fused coccyx

The hip bones are joined:
-anteriorly at the pubic symphysis (secondary cartilaginous)
-posteriorly with the sacrum at the sacroiliac joints (anteriorly synovial; posteriorly syndesmosis)

Question 4

Hip bones

Answer 4

Question 5

Hip bones in children

Answer 5

-In infants and children, each hip bone consists of three separate bones united by a triradiate cartilage at the acetabulum
- Acetabulum - a cup-like depression on the lateral surface that articulates with the head of the femur
-After puberty, the ilium, ischium and pubis fuse to form the hip bone

Question 6

Pelvic inlet

Answer 6

-Also called superior pelvic aperture - marks the boundary between the greater (false) and lesser (true pelvis)

It is formed by:
-Promontory and ala of the sacrum
-Right and left linea terminalis (terminal line) together form a continuous oblique ridge consisting of the:

=>Arcuate line on the inner surface of the ilium
=>Pecten pubis (pectineal line) and pubic crest

Question 7

Pelvic outlet

Answer 7

-Also called the inferior pelvic aperture - marks the boundary between the pelvic cavity and the perineum

It is formed by:
-Pubic arch anteriorly
-Ischiopubic rami (combined inferior pubic rami and ischial rami) anterolaterally
-Ischial tuberosities laterally
-Sacrotuberous ligament posterolaterally
-Tip of coccyx posteriorly

Question 8

Ligaments of pelvic girdle

Answer 8

-Abundant interosseous sacroiliac ligaments (not shown in the diagram) lie deep between the tuberosities of the sacrum and ilium and facilitate transfer of weight from the trunk to the two ilia of the appendicular skeleton
-The interosseous sacroiliac ligaments and posterior sacroiliac ligament run obliquely upward and outward from the sacrum
-Hence the axial weight pushing down on the sacrum pulls the ilia inward and compress the sacrum in between, further interlocking the sacroiliac joints
-Ligaments form greater/lesser sciatic foramen

Question 9

Pelvic girdle sexual differences

Answer 9

Question 10

Pelvic walls and floor structure

Answer 10

-Pelvic floor (diaphragm) is the inferior boundary of the pelvic cavity
-It is suspended superior to (but descending centrally) the pelvic outlet forming a bowl-shaped structure
-Inferior to the pelvic floor is the perineum hence the pelvic floor is also the roof of the perineal cavity

Pelvic floor = levator ani + coccygeus
Levator ani = puborectalis + pubococcygeus + iliococcygeus

Question 11

Pelvic floor function

Answer 11

-Levator ani forms a dynamic floor for supporting the abdominopelvic viscera.
-Stays tonically contracted most of the time to support the organs and to assist in maintaining urinary and faecal continence
-Actively contracted during forced expiration, coughing, sneezing, and fixation of the trunk during strong movement of the upper limbs (e.g. when lifting heavy objects) - to increase support for organs during increased intra-abdominal pressure

Question 12

Perineum

Answer 12

-Shallow compartment of the body bound by the pelvic outlet and separated from the pelvic cavity by the pelvic floor

Question 13

Boundaries of the perineum

Answer 13

-Boundaries of the perineum are the same as the pelvic outlet
-A transverse line joining the ischial tuberosities divide the diamond-shaped perineum into two triangles
-The anal triangle lies posterior to this line
-The anal canal and anus are the major structures here and they lie centrally in this region surrounded by ischio-anal fat
-The urogenital triangle lies anterior to this line. It is lined by a thin sheet of tough, deep fascia, the perineal membrane. -It is perforated by the urethra in both sexes and by the vaginal canal of the female

Question 14

Strcutures through pelvic floor to perineum

Answer 14

Question 15

Perineal body

Answer 15

-Location of the perineal body is the midpoint of the line joining the ischial tuberosities
-It is also the central point of the perineum
-Perineal body is the site of convergence of fibres of multiple muscles including:
-levator ani
-external anal sphincter
-external urethral sphincter

Question 16

Injury to perineal body

Answer 16

-During childbirth, levator ani (particularly the puborectalis and pubococcygeus) may be injured
-These are important because they support the urethra, vagina and anal canal
-Weakening could decrease the support of these structures causing:
-urinary stress incontinence
-uterine prolapse
-faecal incontinence when intra-abdominal pressure is raised
-This applies to damage to the perineal body too as it is the site of convergence of majority of these muscles