Chapter 3: Pelvis and Perineum

Intro

Introduction to Pelvis and Perineum

In common usage, the pelvis (L. basin) is the part of the trunk that is inferoposterior to the abdomen, and is the area of transition between the trunk and the lower limbs. The pelvic cavity is the inferiormost part of the abdominopelvic cavity. Anatomically, the pelvis is the part of the body surrounded by the pelvic girdle (bony pelvis), part of the appendicular skeleton of the lower limb (Fig. 3.1).

Figure 3.1. Pelvis and perineum.

Pelvis and perineum

A and B.The pelvis (green) is the space within the pelvic girdle, overlapped externally by the abdominal and gluteal regions, perineum, and lower back. Thus the pelvis has no external surface area. The greater pelvis (light green) is pelvic by virtue of its bony boundaries, but is abdominal in terms of its contents. The lesser pelvis (dark green) provides the bony framework (skeleton) for the pelvic cavity and deep perineum.

The pelvis is subdivided into greater and lesser pelves. The greater pelvis is surrounded by the superior pelvic girdle. The greater pelvis is occupied by inferior abdominal viscera, affording them protection similar to the way the superior abdominal viscera are protected by the inferior thoracic cage. The lesser pelvis is surrounded by the inferior pelvic girdle, which provides the skeletal framework for both the pelvic cavity and the perineum—compartments of the trunk separated by the musculofascial pelvic diaphragm. Externally, the pelvis is covered or overlapped by the inferior anterolateral abdominal wall anteriorly, the gluteal region of the lower limb posterolaterally, and the perineum inferiorly.

The term perineum1 refers both to the area of the surface of the trunk between the thighs and the buttocks, extending from the coccyx to the pubis, and to the shallow compartment lying deep (superior) to this area but inferior to the pelvic diaphragm. The perineum includes the anus and external genitalia: the penis and scrotum of the male and the vulva of the female.

Pelvic Girdle

The pelvic girdle is a basin-shaped ring of bones that connects the vertebral column to the two femurs. The primary functions of the pelvic girdle are to:

  • Bear the weight of the upper body when sitting and standing
  • Transfer that weight from the axial to the lower appendicular skeleton for standing and walking.
  • Provide attachment for the powerful muscles of locomotion and posture and those of the abdominal wall, withstanding the forces generated by their actions.

Consequently, the pelvic girdle is strong and rigid, especially compared to the pectoral (shoulder) girdle. Other functions of the pelvic girdle are to:

  • Contain and protect the pelvic viscera (inferior parts of the urinary tracts and the internal reproductive organs) and the inferior abdominal viscera (intestines), while permitting passage of their terminal parts (and, in females, a full-term fetus) via the perineum.
  • Provide support for the abdominopelvic viscera and gravid (pregnant) uterus.
  • Provide attachment for the erectile bodies of the external genitalia.
  • Provide attachment for the muscles and membranes that assist the functions listed above by forming the pelvic floor and filling gaps that exist in or around it.

Bones and Features of Pelvic Girdle

In the mature individual, the pelvic girdle is formed by three bones (Fig. 3.2A):

  • Right and left hip bones (coxal or pelvic bones): large, irregularly shaped bones, each of which develops from the fusion of three bones, the ilium, ischium, and pubis.
  • Sacrum: formed by the fusion of five, originally separate, sacral vertebrae.

Figure 3.2. Pelvic girdle.

Pelvic girdle

 

A and B.Features of the pelvic girdle demonstrated anatomically (A) and radiographically (B). The pelvic girdle is formed by the two hip bones (of the inferior axial skeleton) anteriorly and laterally and the sacrum (of the axial skeleton) posteriorly. C.The hip bone is in the anatomical position when the anterior superior iliac spine (ASIS) and the anterior aspect of the pubis lie in the same vertical plane. The preadolescent hip bone is composed of three bones—ilium, ischium, and pubis—that meet in the cup-shaped acetabulum. Prior to their fusion, the bones are united by a triradiate cartilage along a Y-shaped line (blue). D.An adult’s right hip bone in the anatomical position shows the bones when fused.

(B courtesy of Dr. E. L. Lansdown, Professor of Medical Imaging, University of Toronto, Toronto, ON, Canada.)

The internal (medial or pelvic) aspects of the hip bones bound the pelvis, forming its lateral walls; these aspects of the bones are emphasized here. Their external aspects, primarily involved in providing attachment for the lower limb muscles, are discussed in Chapter 5. As they are part of the vertebral column, the sacrum and coccyx are discussed in detail in Chapter 4.

In infants and children, the hip bones consist of three separate bones that are united by a triradiate cartilage at the acetabulum (Fig. 3.2B), the cup-like depression in the lateral surface of the hip bone, which articulates with the head of the femur. After puberty, the ilium, ischium, and pubis fuse to form the hip bone. The two hip bones are joined anteriorly at the pubic symphysis, a secondary cartilaginous joint. The hip bones articulate posteriorly with the sacrum at the sacro-iliac joints to form the pelvic girdle.

The ilium is the superior, fan-shaped part of the hip bone (Fig. 3.2B & C). The ala (wing) of the ilium represents the spread of the fan, and the body of the ilium, the handle of the fan. On its external aspect, the body participates in formation of the acetabulum. The iliac crest, the rim of the fan, has a curve that follows the contour of the ala between the anterior and posterior superior iliac spines. The anteromedial concave surface of the ala forms the iliac fossa. Posteriorly, the sacropelvic surface of the ilium features an auricular surface and an iliac tuberosity, for synovial and syndesmotic articulation with the sacrum, respectively.

The ischium has a body and ramus (L. branch). The body of the ischium helps form the acetabulum and the ramus of the ischium forms part of the obturator foramen. The large postero-inferior protuberance of the ischium is the ischial tuberosity. The small pointed posteromedial projection near the junction of the ramus and body is the ischial spine. The concavity between the ischial spine and the ischial tuberosity is the lesser sciatic notch. The larger concavity, the greater sciatic notch, is superior to the ischial spine and is formed in part by the ilium.

The pubis is an angulated bone with a superior ramus, which helps form the acetabulum, and an inferior ramus, which helps form the obturator foramen. A thickening on the anterior part of the body of the pubis is the pubic crest, which ends laterally as a prominent swelling, the pubic tubercle. The lateral part of the superior pubic ramus has an oblique ridge, the pecten pubis (pectineal line of pubis).

The pelvis is divided into greater (false) and lesser (true) pelves by the oblique plane of the pelvic inlet (superior pelvic aperture) (Figs. 3.1A and 3.2A). The bony edge (rim) surrounding and defining the pelvic inlet is the pelvic brim, formed by the:

  • Promontory and ala of the sacrum (superior surface of its lateral part, adjacent to the body of the sacrum).
  • A 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, forming the superior border of the superior ramus and body of the pubis.

The pubic arch is formed by the ischiopubic rami (conjoined inferior rami of the pubis and ischium) of the two sides (Fig. 3.2A & C). These rami meet at the pubic symphysis, their inferior borders defining the subpubic angle (Fig. 3.3). The width of the subpubic angle is determined by the distance between the right and the left ischial tuberosities, which can be measured with the gloved fingers in the vagina during a pelvic examination.

Figure 3.3. Pelvic girdles of male and female.

Pelvic girdles of male and female

Pubic arches, or subpubic angles typical for each gender (male = red; female = green) can be approximated by spreading index and middle finger (demonstrating narrow subpubic angle of male pelvis) or thumb and index finger (demonstrating wider subpubic angle of female pelvis).

The pelvic outlet (inferior pelvic aperture) is bounded by the (Figs. 3.1A and 3.2A):

  • Pubic arch anteriorly.
  • Ischial tuberosities laterally.
  • Inferior margin of the sacrotuberous ligament (running between the coccyx and the ischial tuberosity) posterolaterally.
  • Tip of the coccyx posteriorly.

The greater pelvis (false pelvis) is the part of the pelvis (Fig. 3.1):

  • Superior to the pelvic inlet.
  • Bounded by the iliac alae posterolaterally and the anterosuperior aspect of the S1 vertebra posteriorly.
  • Occupied by abdominal viscera (e.g., the ileum and sigmoid colon).

The lesser pelvis (true pelvis) is the part of the pelvis:

  • Between the pelvic inlet and the pelvic outlet.
  • Bounded by the pelvic surfaces of the hip bones, sacrum, and coccyx.
  • That includes the true pelvic cavity and the deep parts of the perineum (perineal compartment), specifically the ischio-anal fossae (Fig. 3.1B).
  • That is of major obstetrical and gynecological significance.

The concave superior surface of the musculofascial pelvic diaphragm forms the floor of the true pelvic cavity, which is thus deepest centrally. The convex inferior surface of the pelvic diaphragm forms the roof of the perineum, which is therefore shallow centrally and deep peripherally. Its lateral parts (ischio-anal fossae) extending well up into the lesser pelvis. The terms pelvis, lesser pelvis, and pelvic cavity are commonly used incorrectly, as if they were synonymous terms.

Orientation of Pelvic Girdle

When a person is in the anatomical position, the right and left anterior superior iliac spines (ASISs) and the anterior aspect of the pubic symphysis lie in the same vertical plane (Fig. 3.2B & C). When a pelvic girdle in this position is viewed anteriorly (Fig. 3.2A), the tip of the coccyx appears close to the center of the pelvic inlet, and the pubic bones and pubic symphysis constitute more of a weight-bearing floor than an anterior wall. In the median view (Fig. 3.1A), the sacral promontory is located directly superior to the center of the pelvic outlet (site of the perineal body). Consequently, the curved axis of the pelvis intersects the axis of the abdominal cavity at an oblique angle.

The pelvic girdles of males and females differ in several respects (Fig. 3.3; Table 3.1). These sexual differences are related mainly to the heavier build and larger muscles of most men and to the adaptation of the pelvis (particularly the lesser pelvis) in women for parturition (childbearing). See the blue box “Variations in Male and Female Pelves,”.

Table 3.1. Comparison of Male and Female Bony Pelves

Bony Pelvis Male (?) Female (?)
General structure Thick and heavy Thin and light
Greater pelvis (false pelvis) Deep Shallow
Lesser pelvis (true pelvis) Narrow and deep, tapering Wide and shallow, cylindrical
Pelvic inlet (superior pelvic aperture) Heart-shaped, narrow Oval and rounded; wide
Pelvic outlet (inferior pelvic aperture) Comparatively small Comparatively large
Pubic arch and subpubic angle Narrow (<70°) Wide (>80°)
Obturator foramen Round Oval
Acetabulum Large Small
Greater sciatic notch Narrow (~70°); inverted V Almost 90°