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anatomy of the pelvis
The bony pelvis is formed by the sacrum, coccyx, and
two hip bones or innominate bones .
The sacrum is a triangular-shaped bone formed by the
fusion of five vertebral segments. The first sacral segment
has a prominent ridge located on the anterior surface of
the body termed the sacral promontory, which acts as a
bony landmark separating the abdominal cavity from
the pelvic cavity . The transverse pro-
cesses of the five sacral segments combine to form the
lateral mass (ala), which articulates with the hip bones at
the sacroiliac joints . The lateral mass
contains sacral foramina that allow for the passage of
sacral nerves . Articulating with
the fifth sacral segment is the coccyx, which consists of
three to five small fused bony segments.
The hip bones are made up of three bones: the ilium,
pubis, and ischium .
Ilium. The ilium, the largest and most superior portion,
consists of a body and a large winglike projection called
the ala . The concave, anterior surface
of the ala is termed the iliac fossa, which is separated
from the body by the arcuate line. This arch-shaped line,
which is located on the anterior surface of the ilium,
forms part of the pelvic brim . The
superior ridge of the ala is termed the iliac crest; it slopes
down to give rise to the superior and inferior iliac spines.
on both the anterior and posterior surfaces . The body of the ilium creates the upper portion of
the acetabulum, which is a deep fossa that articulates
with the head of the femur .
Pubis. The pubis, or pubic bone, forms the lower anterior portion of the acetabulum and consists of a body
and superior and inferior pubic rami. The bodies of the two pubic bones meet at the midline to form
the pubic symphysis . The superior pubic ramus projects inferiorly and medially from
the acetabulum to the midline of the body . Located on the upper surface of the superior
pubic ramus is a ridge termed the pectineal line, which is
continuous with the arcuate line of the ilium, forming
the pelvic brim . Also on the superior surface
is the pubic tubercle, which is the attachment site for the
inguinal ligament . The inferior pubic
ramus projects inferiorly and laterally from the body to
join the ischium at an indistinct point; therefore the two
together are often referred to as the ischiopubic ramus.
Ischium. The ischium is the inferior portion of the hip
bones and, like the pubis, is composed of a body and two
rami. The body of the ischium forms the lower posterior
portion of the acetabulum . The superior ischial ramus extends posteriorly
and inferiorly to a roughened, enlarged area termed
the ischial tuberosity. From
the ischial tuberosity, the inferior ischial ramus extends
anteriorly and medially to join the inferior pubic ramus.
The ischial spine projects from the superior ischial ramus
between two prominent notches on the posterior surface
of the hip bones . The greater
sciatic notch extends from the posterior inferior iliac
spine to the ischial spine, and the lesser sciatic notch
extends from the ischial spine to the ischial tuberosity
. The two notches are spanned by
ligaments that create foramina for the passage of nerves
and vessels. The union of the pubic rami and ischium
creates a large aperture termed the obturator foramen,
which is enclosed by the obturator muscles