The bony pelvis, muscles and ligaments

Chapter 6

The pelvis

C H A P T ER C O N T E N TS

The bony pelvis, muscles and ligaments 215 The pelvic floor 217 The sigmoid colon, rectum and anal canal 221 Blood vessels, lymphatics and nerves of the pelvis 224 The lower urinary tract 227 The male urethra 228 The male reproductive organs 229 The female reproductive tract 236 Cross-sectional anatomy 242

THE BONY PELVIS, MUSCLES AND LIGAMENTS

(Figs 6.1-6.6) The pelvis (Fig. 6.1) is a bony ring comprised of paired innominate bones, the sacrum and coccyx. The innominate bones articulate w i th each other anteriorly and w i th the sacrum posteriorly. Each innominate bone is composed of three parts, which fuse at the acetabulum.

The ilium is a flat curved bone and bears the iliac crest superiorly. The anterior and posterior superior iliac spines are on either end of the iliac crest, w i th the anterior and posterior inferior iliac spines below them. The inner surface of the bone is smooth and has the iliopectineal line running from front to back, demarcating the true from the false pelvis.

The pubic bone consists of a body, and inferior and superior rami. The body of the pubic bone articulates w i th its fellow at the symphysis pubis. It bears a tubercle on its superomedial aspect. The articular surfaces of the symphysis pubis are covered in hyaline cartilage w i th a fibrocartilaginous disc between them. The pubic bone is strengthened on all sides by dense ligaments.

The ischium is composed of a body and an inferior ramus, which joins the inferior pubic ramus. The body bears a tuberosity inferiorly and a spine posteriorly. The ischial spine defines the greater and lesser sciatic notches above and below.

The obturator foramen is bounded by the body and rami of the pubic bone and the body and ramus of the ischial bone.

The sacrum

Five fused vertebrae comprise this triangular bone, which is curved posteriorly. The anterior part of its upper end is termed the sacral promontory. It articulates w i th the lumbar spine superiorly and w i th the coccyx inferiorly. Anteriorly the sacrum has four pairs of sacral foramina, which transmit nerves from the sacral canal. Lateral to these are the lateral masses or alae of the sacrum. The sacrum also bears four pairs of posterior sacral foramina

and the canal ends posteriorly in the sacral hiatus - a m i dline opening that transmits the fifth sacral nerves.

The coccyx This is composed of three to five fused vertebrae. The first segment is often separate. It articulates at an acute angle w i th the sacrum.

The sacroiliac joints The sacroiliac joints are covered w i th cartilage and lined w i th synovium. The joint surface is flat and uneven, and this irregularity helps lock the sacrum into the iliac bones. Ligaments support the front and back of the joint. The dense interosseous sacroiliac ligaments unite the bones above and behind the joint.

The sacrotuberous ligament runs from the ischial tuberosity to the sides of the sacrum and coccyx. It defines the posterior limit of the lesser sciatic foramen.

The sacrospinous ligament runs from the ischial spine to the sides of the sacrum and coccyx. It defines the posterior limit of the greater sciatic foramen.

The iliolumbar ligament runs from the transverse process of L5 to the posterior part of the iliac crest, further stabilizing the joint.

The pelvic muscles are shown in Figure 6.2. At the level of the iliac crest the paired psoas muscles lie on either