Download Lecture 16 - Pelvis and Perineum: Introduction to Pelvis

Lecture 16 - Pelvis and Perineum:
Introduction to Pelvis:
 Definitions
 The term pelvis may denote a variety of structures
 The pelvic region
 The pelvic girdle
 The anatomical pelvis
 The Pelvic Region
 Background
 In common/loose usage, the term pelvis refers to a region—the pelvic region
 A part of the trunk inferior to the abdomen and superior to the lower limbs
 An area of transition where the trunk and the lower limbs meet
 Note that this definition of pelvis carries no real anatomical meaning, as it includes parts of the lower limbs,
back, and abdomen
 The Pelvic Girdle
 The term pelvis may refer to the bony pelvic girdle, the basin-shaped ring of bones
 Note the term pelvis is Latin for basin
 The Anatomical Pelvis
 In strict anatomical terms, pelvis refers to the part of the body surrounded by the pelvic girdle
 This is sometimes referred to as the anatomical pelvis
 Note that whether the term pelvis refers to the pelvic girdle, or to the anatomical pelvis, the pelvis has
no external surface area
 Subdivisions of the Anatomical Pelvis
 The anatomical pelvis is subdivided into two parts
1. Greater pelvis
 The greater pelvis is also referred to as the false pelvis
 It is generally considered part of the abdomen
 The greater pelvis is of less clinical importance, but does provide protection to inferior abdominal
viscera
 Supports gravid uterus after 1st trimester and directs fetus into the lesser pelvis
2. Lesser Pelvis
 The lesser pelvis is also referred to as the true pelvis
 The lesser pelvis has an inlet and an outlet
 Inlet
 Completely ringed by bone
 Outlet
 Formed by bone and ligament
 Pelvic Girdle
 Orientation
 In the anatomical position the front edge of pubic symphysis and anterior superior iliac spines are in the
same vertical plane
 The pelvic inlet is tilted anteriorly
 The pubic arch is nearly horizontal
 The anterior surface of the sacrum is directed forward and downward
 Gender Differences
 Male and female pelvic girdles differ in several respects, but the differences arise for two main reasons
 Generally heavier build and larger muscles of men
 Thicker and heavier pelvis

Adaptation of female pelvis (particularly lesser pelvis) to childbirth
 Key Identifiers
 The pelvic girdle is narrow, deep, and tapering in males; wide, shallow, and cylindrical in females
 The subpubic angle is narrow in males (<70°), and wide in females (>80°)
 As well, the obturator foramina and greater sciatic notches exhibit gender-based differences
 Pelvic Diameters
 Background
 The size of the lesser pelvis is of particular importance in obstetrics
 Diameters (conjugates) of the lesser pelvis may be noted radiographically, or manually (during pelvic exam)
 In general, a conjugate (from L., joined together) is the distance between any two specified points on
the pelvic canal
 Obstetric Conjugate
 The obstetric (true) conjugate is the narrowest fixed distance through which a baby’s head must pass
 It is represented by the distance from the sacral promontory to the closest point of the pubic symphysis
(must be > 11 cm)
 Interspinous distance is less, but it can change under the influence of gestational hormones
 Diagonal Conjugate
 In a manual examination, it is the diagonal conjugate that must be measured, as the examining hand cannot
be placed in such a manner as to properly gauge the obstetric conjugate
 From the diagonal conjugate, the obstetric conjugate is estimated
 Slide 35 Image notes:

13 cm represents the distance from the tip of the middle finger to the mark on the gloved examining hand
(the diagonal conjugate). By convention, the obstetric conjugate is then taken to be the distance from the
index finger to the mark on the gloved hand, which is ~1.5 cm less than the diagonal conjugate.
 Pelvic Cavity
 Background
 The funnel-shaped pelvic cavity is a space—the inferoposterior part of the abdominopelvic cavity
 As a space, the pelvic cavity has both walls and most importantly, a floor—the pelvic diaphragm
 Pelvic Diaphragm
 Background
 As stated previously, the pelvic diaphragm is the muscular floor of the pelvic cavity
 Note that this floor covers the pelvic outlet, and forms the lower limit of the pelvic cavity, separating
pelvic cavity from perineum
 The pelvic diaphragm is bowl-shaped, essentially suspended “hammock-like” from the pelvic girdle
 Muscles
 Two muscles (and their covering fasciae) form the pelvic diaphragm
1. Levator ani
 Puborectalis
 Pubococcygeus
 Iliococcygeus
2. Coccygeus
 Function
 Background
 The muscles of the diaphragm function in two general scenarios
 They are tonically contracted most of the time, suporting the pelvic viscera and maintaining
continence
 They must relax in order for urination and defecation to occur
 They actively contract during certain activities
 Forced expiration, coughing, sneezing
 Puborectalis
 Puborectalis forms a U-shaped sling with continuous fibers passing posterior to the anorectal
junction
 This muscle is key in maintaining continence
 Injury to Pelvic Floor
 Levator ani—the medially placed pubococcygeus and puborectalis in particular—and structures of the
perineum, may be injured during childbirth
 Injury to the above-mentioned muscles may lead to urinary incontinence, or prolapse of pelvic
viscera
 Pelvic Neurovasculature
 Background
 The major neurovascular structures of the pelvis lie extraperitoneally against the posterolateral walls
 In general, veins lie between arteries (more internal) and nerves (more external)
 Pelvic Arteries
 Background
 The major arteries of the pelvis (and perineum) are the internal iliacs
 Detail
 It is helpful to think of paired and unpaired arteries entering the lesser pelvis
 Paired
 Unpaired
 Internal iliac arteries
 Median sacral artery
 Ovarian arteries
 Superior rectal artery
 The testicular arteries do not
enter the lesser pelvis
 Collateral Circulation
 Occasionally the internal iliac artery becomes stenotic due to atherosclerosis, or it may be ligated to control
hemorrhage
 In both cases, blood flow is maintained through collateral circulation
 Notes: In the case of ligation, overall blood flow is reduced allowing more time for hemostasis.
 Internal Iliac Trunks
 At approximately the superior border of the greater sciatic foramen, the internal iliac artery divides into two
trunks
 Anterior trunk
 Posterior trunk
 Supplies mainly viscera
 Supplies posterior pelvic wall,
gluteal region
 Pelvic Veins
 The pelvic plexus of veins is mainly drained by tributaries of the internal iliac arteries
 Portions of the plexus drain into the hepatic portal system via the superior rectal vein
 Portacaval anastomoses in this area can lead to rectal varices
 Lateral sacral veins anastamose with the internal vertebral plexus
 Pathway for metastasis of prostatic or ovarian cancers
 Pelvic Nerves
 Background
 Pelvic innervation derives from three main sources
 Sacral spinal nerves
 Coccygeal spinal nerves
 Pelvic part of autonomic nervous system
 Somatic
 Background
 Somatic nerves of the pelvis and perineum originate from two plexuses—sacral and coccygeal—situated
on the posterolateral wall of the pelvic cavity
 Note bed formed by piriformis and coccygeus
 Sacral Plexus
 The bilateral sacral plexuses are formed by the anterior rami of S1 to S4 and the lumbosacral trunk
(L4,L5)
 Nerves emerging from this plexus innervate two main areas
 Lower limb
 Sciatic nerve and nerves to gluteal region
 Pelvis and perineum
 Nerves to muscles of lesser pelvis, pudendal nerve to perineum
 Coccygeal Plexus
 The coccygeal plexus is a small network of nerves formed by the anterior rami of S4, S5, and the
coccygeal nerve
 The principal nerve emerging from this plexus is the nerve to levator ani and coccygeus
 Autonomic
 Background
 Autonomic innervation within the pelvis may generally be thought of in two ways
 Fibers innervating the lower limbs
 Fibers innervating pelvic viscera


Lower Limb Innervation
 The sacral sympathetic trunks provide sympathetic innervation (vasomotor, sudomotor, pilomotor)
to the lower limbs
 Presynaptic fibers (originating in IMLs of T11-L2/3) synapse in ganglia of the trunks, with
postsynaptic fibers entering the sacral plexus for distribution to lower limbs via spinal nerves
Pelvic Viscera
 Autonomic nerve fibers are distributed to viscera and blood vessels of the pelvic cavity in two basic
ways
 Via periarterial plexuses
 Via inferior hypogastric plexuses
1. Periarterial Plexuses
 Periarterial plexuses accompany most blood vessels entering the pelvic cavity
 The plexusese distribute postsynaptic sympathetic fibers mainly to blood vessels (vasomotion),
and some to viscera
 These postsynaptic fibers originate in the ganglia and plexuses of the aorta
 The periarterial plexus route represents a relatively minor contribution to sympathetic supply
2. Inferior Hypogastric Plexuses
 Background
 Most autonomic nerve fibers to the pelvic viscera are distributed by the inferior hypogastric
plexuses (also known as pelvic plexuses)
 These are mixed (sympathetic/ parasympathetic) plexuses, directly or indirectly
receiving fibers from splanchnic nerves (lumbar, sacral, and pelvic)
 From the inferior hypogastric plexuses, nerve fibers are distributed to sub-plexuses
associated with specific organs
 Sympathetic
 In general, the inferior hypogastric plexus receives sympathetic fibers both directly and
indirectly
 Lumbar splanchnic nerves carry fibers (L1-L2/3 cord levels) to the superior hypogastric
plexus
 The fibers then travel via bilateral hypogastric nerves to the inferior hypogastric
plexuses
 Sacral splanchnic nerves carry fibers (L1-L2/3) directly to the inferior hypogastric plexus
 Parasympathetic
 Pelvic splanchnic nerves carry presynaptic parasympathetic fibers (S2-S4 cord levels) to the
inferior hypogastric plexuses
 The presynaptic fibers then travel to subplexuses to synapse on the organ
 General Autonomic Function
 Sympathetic fibers
 Vasomotion, as elsewhere in the body
 Inhibition of peristaltic contraction of rectum
 Ejaculation in male
 Parasympathetic fibers
 Contraction of rectum and bladder
 Engorgement of erectile tissues
 Visceral Afferents
 Background
 All fibers carrying reflex information travel with parasympathetic fibers
 Pelvic splanchnic nerves
 Pain fibers travel with both sympathetic and parasympathetic fibers
 The course of visceral pain afferents differs based on their relationship to the pelvic pain line

Pelvic Pain Line
 The pelvic pain line is the inferior limit of the peritoneum
 Exception is alimentary canal where pain line is in the middle of sigmoid colon
 Structures (or portions thereof) in contact with the peritoneum are above the pain line
 Fibers travel with sympathetics
 Structures (or portions thereof) deep to the peritoneum are below the pain line
 Fibers travel with parasympathetics
Introduction to Perineum:
 Background
 In common parlance the perineum is the region of the body between the thighs
 Crotch?
 Other Meanings
 In a restricted sense, the perineum is a small area between the vulva or scrotum, and the anus
 Clinical term
 Anatomically, the perineum has both depth, and superficial boundaries
 Perineal region is a term sometimes used
 Superficial Boundaries
 With legs abducted (lithotomy position) the superficial boundaries of the perineum are the scrotum/mons pubis
anteriorly, the medial thighs laterally, and the superior end of the intergluteal cleft posteriorly
 Bony Boundaries
 The previous boundaries can also be related to osseofibrous structures
 Pubic symphysis, anteriorly
 Ischiopubic rami, anterolaterally
 Ischial tuberosities, laterally
 Sacrotuberous ligaments, posteriorly
 Coccyx, posteriorly
 Depth
 The perineum, defined anatomically, has a certain depth
 Superior border
 Inferior fascia of the pelvic diaphragm

Inferior border
 The skin
 Triangles of the Perineum
 An arbitrary line between the ischial tuberosities divides the diamond-shaped perineum into two triangles
1. Anterior urogenital triangle
2. Posterior anal triangle
 Anal Triangle
 Background
 The main contents of the anal triangle are the anal canal and its orifice, the anus
 These are surrounded by ischioanal fat
 The anal triangle, compared to the UG triangle, is “open”
 No perineal membrane
 Ischioanal fossae
 Background
 Because the levator ani muscles essentially form a bowl, wedge-shaped spaces are created on either
side of the anal canal
 Ischioanal fossae
 The fossae are filled with fat and loose connective tissue


 Permits movement of pelvic diaphragm and expansion of anal canal during defecation
External Anal Sphincter
 The large, voluntary external anal sphincter forms a broad band on either side of the inferior two-thirds
of the anal canal
 The sphincter is attached to the perineal body and the anococcygeal ligament
 The sphincter is supplied by the inferior rectal nerve (mainly S4)
Pudendal Canal
 The pudendal canal is a nearly horizontal passageway within the obturator fascia
 Its main contents are the internal pudendal artery and vein, and the pudendal nerve
 These vessels supply most of the perineum
 Prior to entering the canal, the internal pudendal artery and the pudendal nerve give rise to the inferior
rectal artery and nerve