OVERVIEW OF THE PELVIS 

The pelvic girdle, or pelvis, is a complete bony girdle made up of the two os coxae bones laterally and in front, and the sacrum and coccyx behind. The two os coxae bones are joined anteriorly at the symphysis pubis. Posteriorly, the iliac portions of the os coxae are joined to the sacrum at the sacroiliac joints. The pelvis is divided by an oblique plane, which passes through the prominence of the sacrum (sacral promontory), the arcuate lines, and the superior plane of the pubic or pubes bones. Above this plane is the greater (false) pelvis, and below is the lesser (true) pelvis. The circumference of this plane is termed the pelvic brim or ring. The interior diameter of the female true pelvis is important in prenatal pelvimetry.

The Os Coxa

The os coxa consists of three parts: the ilium, ischium, and pubic. These are three separate, distinct bones in the young subject, but are fused and consolidated in the adult. The bodies of these three portions meet and unite in and around a large cup-shaped socket, the acetabulum, which is situated near the middle of the lateral surface of the bone. The ilium, or flank bone, is the upper expanded portion of the bone and its body forms the upper two-fifths of the acetabulum. The ischium forms the lower and back part of the bone; its body also contributes about two-fifths to the acetabulum. The pubis forms the anterior and inferior portion of the bone and its body contributes one-fifth to the acetabulum.

The ilium is divided into a body and a flared portion called the ala, or wing. The upper border of the ala is called the iliac crest. This crest terminates anteriorly as the anterior superior iliac spine (ASIS), below which is situated the anterior inferior iliac spine (AIIS). The crest of the ilium terminates posteriorly as the posterior superior iliac spine (PSIS), below which is situated the posterior inferior iliac spine (PIIS). The greater sciatic notch is situated below the posterior inferior iliac spine.

The internal or medial surface of the ilium presents a large, smooth, concave depression called the iliac fossa. Behind the iliac fossa is a rough surface divided into two portions. The superior portion is the iliac tuberosity and the inferior portion is the auricular surface. A curved line, the arcuate line, that marks the inferior boundary of the major or false pelvis indicates the inferior boundary of the iliac fossa. The obturator foramen is a large aperture situated between the ischium and the pubis and inferior to the acetabulum.

The ischium is composed of a body, a superior ramus, and an inferior ramus. The ischial spine or sciatic spine projects posteriorly from the body. Situated below the spine is the lesser sciatic notch. The superior ramus branches downward from the body. The ischial tuberosity is situated on the posterior aspect of the superior ramus. The inferior ramus extends from the lower part of the superior ramus to join the inferior ramus of the pubis.

The pubis is divided into a body (which forms part of the acetabulum), a superior ramus, and an inferior ramus. The superior ramus projects anteromedially from the pubic body. The lateral portion of the superior ramus is marked by a rough iliopectineal eminence that indicates the fusion of the ilium and the pubis. The medial portion presents a pubic tubercle, which projects ventrally. The symphysial surface is the articulating surface by means of which the left pubis and right pubis are joined.

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