Pelvis

I. BONY PELVIS consists of the following bones:

A. Two coxal (hip) bones, each consisting of three parts—ischium, ilium, and pubis—that join at the acetabulum of the hip joint.

B. The sacrum is the posterior portion of the bony pelvis, formed by the fusion of vertebrae SI—5. It contains the dorsal sacral foramina, which transmit the dorsal primary rami of the sacral spinal nerves; the ventral sacral foramina, which transmit the ventral primary rami of the sacral spinal nerves; and the sacral hiatus, which is formed when the laminae of the S5 vertebra do not fuse. The pedicles form the sacral cornua, which are important landmarks in locating the sacral hiatus for administration of caudal anesthesia.

C. The coccyx (tailbone) is formed by the fusion of vertebrae Col—4.

II. GREATER AND LESSER SCIATIC FORAMINA. The sacrotuberous ligament, which runs from the sacrum to the ischial tuberosity, and the sacrospinous ligament, which runs from the sacrum to the ischial spine, help define the borders of the foramina.

A. The greater sciatic foramen is divided by the piriformis muscle into the suprapiri-formis recess and the infrapiriformis recess. It transmits the following important structures as they exit the pelvic cavity to enter the gluteal and thigh regions: superior gluteal vein, artery, and nerve; piriformis muscle; inferior gluteal vein, artery, and nerve; sciatic nerve; internal pudendal vein and artery; and pudendal nerve.

B. The lesser sciatic foramen transmits the internal pudendal vein and artery and the pudendal nerve as they reenter the pelvic cavity and proceed to the perineum. The internal pudendal vein and artery and the pudendal nerve exit the pelvic cavity through the greater sciatic foramen and then reenter the pelvic cavity through the lesser sciatic foramen and proceed to the perineum.

III. PELVIC INLET (Figure 16-1)

A. General features. The pelvic inlet is defined by the sacral promontory (vertebra SI) and the linea terminalis. The linea terminalis includes the pubic crest, the iliopectineal line, and the arcuate line. The pelvic inlet divides the pelvic cavity into two parts: the major (false) pelvis, which lies above the pelvic inlet between the iliac crests and is part of the abdominal cavity; and the minor (true) pelvis, which lies below the pelvic inlet and extends to the pelvic outlet.

Xrays Pelvic Inlet Outlet

Figure 16-1. (A) Lateral view of the pelvis. The diameter of the pelvic inlet is measured by the true conjugate (TC) and diagonal conjugate (DC) diameters. The opening of the pelvic outlet is shown (line with arrowheads) extending from the pubic symphysis to the coccyx. In the natural position of the bony pelvis, the anterior superior iliac spine and the pubic tubercle lie in the same vertical plane (X). (B) Lateral view of the pelvis. Note that during childbirth the fetal head must pass through the pelvic inlet. The TC and DC diameters measure the diameter of the pelvic inlet. (C) Superior view of the pelvis. The diameter of the pelvic outlet is measured by the transverse diameter (not shown) and the interspinous (IS) diameter. (D) Frontal view of the pelvis. Note that during childbirth the fetal head must pass through the pelvic outlet. The IS diameter measures the diameter of the pelvic outlet. The TC, DC, and IS diameters are important during childbirth, because the fetus must travel through the birth canal, which consists of the pelvic inlet - minor pelvis > cervix > vagina ► pelvic outlet. (A adapted with permission from Rohen JW, Yokochi C, Lutjen-Drecoll E: Color Atlas of Anatomy, 4th ed. Baltimore, Williams & Wilkins, 1998, p 414; B and D adapted with permission from Callahan TL, Caughey AB, Heffner LJ: Blueprints in Obstetrics and Gynecology. Maiden, MA, Blackwell Science, 1998, p 18; C adapted with permission from Mathers LH, Chase RA, Dolph J, et al: Clinical Anatomy Principles. St. Louis, Mosby-Year Book, 1995.)

B. Measurements of the pelvic inlet

1. True conjugate diameter is the distance from the sacral promontory to the superior margin of the pubic symphysis. It is measured radiographically on a lateral projection.

2. Diagonal conjugate diameter is the distance from the sacral promontory to the inferior margin of the pubic symphysis. It is measured during an obstetric examination to assess potential difficulty in childbirth.

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