Pelvic Wall and
Floor Anatomy
The pelvic wall is formed by the
bones of the pelvic girdle and their associated ligaments, muscles and fascia.
The bony component comprises the right and left hip bones anterolaterally and
the sacrum and coccyx posteriorly. The pelvic cavity is usually wider and
shallower in females because of the differences in the shapes of the
surrounding bones.
Only the medial or pelvic surface
of the hip bone is considered here; the external surface is described on p.
269. Each hip bone is formed by the fusion of three components: ilium, ischium
and pubis (Figs 5.26 & 5.27). The anterosuperior part of the ilium
contributes to the abdominal wall and gives attachment to iliacus. The lower
portion of the ilium extends below the pelvic inlet and contributes to the
lateral wall of the pelvis. On the posterior
part of the bone is the auricular surface, which articulates with the
corresponding surface of the sacrum at the sacroiliac joint.
The ischium has a rounded
tuberosity inferiorly, which bears body weight in the sitting position.
Posteriorly is the pointed spine, which separates the greater and lesser
sciatic notches, while anteriorly the ramus of the ischium ascends to fuse with
the inferior pubic ramus.
The pubic bone has an iliopubic
ramus that merges with the ilium near the iliopubic eminence, and an inferior
ramus, which is continuous below the obturator foramen with the ramus of the
ischium. The bodies of the right and left pubic bones articulate at the pubic
symphysis.
The obturator foramen is a large
aperture, which is almost completely occluded by the obturator membrane (Fig.
5.27). Superiorly, the membrane leaves a small gap, the obturator canal, which
provides access between the pelvis and the medial compartment of the thigh.
The pelvic floor (or diaphragm) is
a muscular partition separating the cavity of the pelvis above from the
perineum below. It slopes downwards towards the midline, forming a trough that
inclines downwards and forwards (Figs 5.28 & 5.29). In the midline
anteriorly, a narrow triangular gap, the urogenital hiatus, between the muscle
fibres transmits the urethra in both sexes and the vagina in the female (Figs
5.1 & 5.18). Posteriorly, the pelvic floor is pierced by the anal canal.
The pelvic floor is formed
principally by the right and left levator ani muscles, which are supplemented
posteriorly by the coccygeus muscles (Fig. 5.29). The coccygeus muscle is
applied to the medial surface of the sacrospinous ligament. Medially, it
attaches to the lateral border of the sacrum and coccyx, and laterally to the
ischial spine.
Each levator ani muscle has a
linear attachment to the pelvic wall. The attachment commences anteriorly on
the pelvic surface of the body of the pubis and continues backwards as the
tendinous arch along the obturator fascia as far as the ischial spine (Fig.
5.29). The levator ani muscle has two parts: the anterior part comprises
pubococcygeus and the posterior part is iliococcygeus.
Pubococcygeus runs backwards and
downwards. Its most anterior fibres lie near the midline and pass close to the
urethra. In the male, they support the prostate (Fig. 5.23); in the female,
they attach to the vagina (Fig. 5.18). The intermediate fibres of
pubococcygeus, puborectalis, reach the anal canal and either attach to its wall
or loop behind the anorectal junction. The posterior fibres attach to the
coccyx or fuse in the midline with fibres from the other side at the
anococcygeal raphe.
The fibres of iliococcygeus pass
downwards and medially below those of pubococcygeus and attach to the coccyx
and to the anococcygeal raphe.
The levator ani muscles support the
pelvic contents, actively maintaining the positions of the pelvic viscera. In
particular, the pubococcygeus muscles compress the urethra and vagina and
provide support for the bladder and uterus. The levator ani fibres that loop
behind the anal canal help to maintain the angulation of the anorectal junction
and play an important role in the continence of faeces. During defecation, the
fibres attaching to the wall of the anal canal pull the organ upwards. Levator
ani and coccygeus are innervated from above by the fourth sacral nerve and from
below by branches of the pudendal nerve. The levator ani may be weakened by
multiple vaginal deliveries, predisposing to stress incontinence (of urine) and
uterine prolapse.
The pelvic girdle forms a stable
ring because its constituent bones are bound together at the two sacroiliac
joints and the pubic symphysis.
The symphysis is a secondary
cartilaginous joint containing a pad of fibrocartilage, the interpubic disc (Fig.
5.28), that separates
the bodies of the right and left pubic bones. The joint is stabilized by
ligaments attached around the articular margins.
The sacroiliac joints allow very
little movement because the articulating surfaces of their synovial cavities
are irregular and behind each cavity is the thick posterior interosseous
ligament. Each joint is further supported by the anterior and posterior
sacroiliac ligaments and iliolumbar, sacrospinous and sacrotuberous ligaments.
Body weight acting downwards through the
lumbosacral disc tends to rotate
the sacrum, tipping its lower part backwards, a movement prevented by the sacrospinous
and sacrotuberous ligaments (Fig. 5.27).
The iliolumbar ligament attaches
medially to the transverse process of the fifth lumbar vertebra and laterally
to the iliac crest and front of the sacroiliac joint. The sacrospinous ligament
passes from the lateral margins of the sacrum and coccyx to the ischial spine.
The larger sacrotuberous ligament passes from the side and dorsum of the sacrum
and the posterior surface of the ilium to the ischial tuberosity. These two
ligaments convert the greater and lesser sciatic notches into the greater and
lesser sciatic foramina (Fig. 5.27). Pregnancy-related hormones may produce
ligamentous laxity, especially at the joints of the pelvic girdle, reducing
joint support and contributing to lumbar and pelvic pain during pregnancy.
Piriformis is a flat muscle
attached to the pelvic surfaces of the second, third and fourth pieces of the
sacrum (Fig. 5.27). Running laterally through the greater sciatic foramen, it
enters the buttock and attaches to the upper part of the greater trochanter of the femur (p. 271). Piriformis rotates
the hip joint laterally and is innervated by the first and second sacral
nerves. Numerous vessels and nerves accompany the muscle through the greater
sciatic foramen (Fig. 5.28).
Obturator internus is a fan-shaped
muscle with an extensive attachment to the margins of the obturator foramen and
the pelvic surface of the obturator membrane (Fig. 5.28). The muscle fibres
converge on the lesser sciatic foramen to form a tendon, which turns laterally
to enter the gluteal region. The tendon is attached to the medial aspect of the
greater trochanter (p. 271). The muscle laterally rotates the hip joint. The
nerve to obturator internus (L5, S1 & S2) enters the muscle within the
perineum, having traversed the greater and lesser sciatic foramina.
Pelvic fascia
This term includes the fascial
lining of the pelvic walls and the extraperitoneal con-nective tissue
surrounding the pelvic viscera (Fig. 5.7). The pelvic surfaces of obturator
internus (Fig. 5.29), piriformis and levator ani are covered by fascia that is
continuous superiorly with the transversalis and iliac fasciae. Between the
pelvic organs, the pelvic fascia mostly comprises a loose meshwork of
connective tissue. Pelvic infections can spread widely through these looser
tissues. The fascia is condensed anterior to the rectum to form the
rectovesical septum; and some of the arteries to the pelvic organs, notably the
uterine and vaginal vessels, are accompanied by thickened bands of fascia termed
ligaments. Radiating from the uterine cervix to the pelvic walls are the
transverse cervical (lateral sacral), uterosacral ligaments and pubocervical
ligaments, the latter passing below the bladder neck to reach the cervix.
