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.
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.