Table 5.4 Branches of the sacral and coccygeal plexuses (spinal segments in parentheses do not consistently participate)
Table 5.5 Muscles of the anal triangle
Table 5.6 Muscles of the superficial perineal pouch
In the clinic
In certain diseases (e.g., leukemia), a sample of bone marrow must be obtained to assess the stage and severity of the problem. The iliac crest is often used for such bone marrow biopsies. The iliac crest lies close to the surface and is easily palpated.
A bone marrow biopsy is performed by injecting anesthetic in the skin and passing a cutting needle through the cortical bone of the iliac crest. The bone marrow is aspirated and viewed under a microscope. Samples of cortical bone can also be obtained in this way to provide information about bone metabolism.
In the clinic
The pelvis can be viewed as a series of anatomical rings. There are three bony rings and four fibro-osseous rings. The major bony pelvic ring consists of parts of the sacrum, ilium, and pubis, which forms the pelvic inlet. Two smaller subsidiary rings are the obturator foramina. The greater and lesser sciatic foramina formed by the greater and lesser sciatic notches and the sacrospinous and sacrotuberous ligaments form the four fibro-osseous rings. The rings, which are predominantly bony (i.e., the pelvic inlet and the obturator foramina), are brittle rings. It is not possible to break one side of the ring without breaking the other side of the ring, which in clinical terms means that if a fracture is demonstrated on one side, a second fracture should always be suspected.
Fractures of the pelvis may occur in isolation; however, they usually occur in trauma patients and warrant special mention.
Owing to the large bony surfaces of the pelvis, a fracture produces an area of bone that can bleed significantly. A large hematoma may be produced, which can compress organs such as the bladder and the ureters. This blood loss may occur rapidly, reducing the circulating blood volume and, unless this is replaced, the patient will become hypovolemic and shock will develop.
Pelvic fractures may also disrupt the contents of the pelvis, leading to urethral disruption, potential bowel rupture, and nerve damage.
In the clinic
Common problems with the sacro-iliac joints
As with many weight-bearing joints, degenerative changes may occur with the sacro-iliac joints and cause pain and discomfort in the region. In addition, disorders associated with the major histocompatibility complex antigen HLA-B27, such as ankylosing spondylitis, psoriatic arthritis, inflammatory arthritis associated with inflammatory bowel disease, and reactive arthritis (the group referred to as seronegative spondyloarthropathies), can produce specific inflammatory changes within these joints.
In the clinic
Transverse and sagittal measurements of a woman’s pelvic inlet and outlet can help in predicting the likelihood of a successful vaginal delivery. These measurements include: the sagittal inlet (between the promontory and the top of the pubic symphysis), the maximum transverse diameter of the inlet, the bispinous outlet (the distance between ischial spines), and the sagittal outlet (the distance between the tip of the coccyx and the inferior margin of the pubic symphysis).
These measurements can be obtained using magnetic resonance imaging, which carries no radiation risk for the fetus or mother (Fig. 5.33).
In the clinic