The pelvic floor

side of the spine. They descend anteriorly fusing w i th the iliacus muscle, which arises from the inner surface of the i l i u m. The fused iliopsoas muscle passes anteriorly under the inguinal ligament to insert into the lesser trochanter of the femur.

The piriformis muscles pass obliquely from the anterior aspect of the sacrum through the greater sciatic foramen behind the acetabulum to insert into the greater trochanter of the femur.

The obturator membrane closes the obturator foramen. The obturator internus muscle arises from the anterior and lateral walls of the pelvis covering the obturator foramen. Its fibres converge towards the lesser sciatic foramen as it hooks around the posterior part of the ischium between its tuberosity and spine. It passes through the lesser sciatic foramen to insert into the greater trochanter of the femur.

The aponeurosis of the abdominal wall muscles inserts into the superior surface of the pubic bone. A thickening of the aponeurosis is the inguinal ligament, which runs from the pubic tubercle to the anterior superior iliac spine. A ll the muscles of the anterior, lateral and posterior abdominal walls insert, to some degree, into the iliac crest, inguinal ligament and pubic bone.

The gluteal muscles arise from the external surface of the iliac bone and the iliac crest and insert into the upper femur. Gluteus maximus is the largest, the most superficial and the most posterior gluteal muscle, covering the posterior part of the i l i um and the sacroiliac joints. Gluteus medius and minimus are more anteriorly placed, gluteus minimus being the smallest and the most deeply placed.

Radiology of the pelvic ring

Plain films

The bony landmarks may be identified on the plain radiograph (Fig. 6.3). The sacral promontory and superior part of the pubic bone define the pelvic inlet and separate the true pelvis below from the false pelvis above. The sacroiliac joints are not optimally seen on the frontal view owing to their obliquity. Special views may be performed so that the X-ray beam passes through the joint to demonstrate it clearly.

Some variations of the lower lumbar spine and sacrum occur. The first sacral segment may be partially or completely separate - so-called lumbarization of the sacrum. Similarly, the lowest lumbar vertebra may be partially or completely fused to the sacrum - known as sacralization of the lumbar spine. The posterior elements of the lower lumbar vertebra or the sacral vertebrae may not be fused in some people, w i th no apparent sequelae.

The male and female pelvises have several differences: • The muscle attachments are more prominent in the male. • The pelvic inlet is heart-shaped in the male and oval in the female. • The angle between the inferior pubic rami is narrow in the male and wide in the female.

Cross-sectional imaging The muscles of the pelvis may be seen on CT and MR images (Figs 6.4-6.6). The ability of MRI to yield sagittal and coronal views of the pelvis gives a clear appreciation of the anatomy of the pelvic floor. CT is an excellent way to image the sacroiliac joints. High-resolution axial scans taking narrow slices give very good bony detail.

THE PELVIC FLOOR (Figs 6.5-6.7; also Figs 6.18 and 6.20) A sling of muscles closes the floor of the pelvis. The urethra and rectum and the vagina in the female pierce the pelvic floor. The floor is composed of two muscular layers, the levator ani / coccygeus complex and the perineum.

The levator ani muscle is the principal support of the pelvic floor. It arises from the posterior aspect of the pubis, from the fascia covering the obturator internus muscle on the inner wall of the i l i um (in an arc known as the tendinous arch or white line) and from the ischial spine. Its fibres sweep posteriorly, inserting into the perineal body (a fibromuscular node behind the urethra in males or the urethra and vagina in females), the anococcygeal body (a fibromuscular node between the anus and coccyx) and the coccyx. The midline raphe of the levator ani anterior to the coccyx is also known as the levator plate. The fibres of levator ani sling around the prostate gland or vagina and rectum, blending w i th the external anal sphincter. The components of levator ani are named according to their attachments: puborectalis, pubococcygeus and i l i ococcygeus. Levator ani provides muscular support for the pelvic organs and reinforces the urethral and rectal sphincters. The coccygeus muscle is in the same tissue plane as levator ani. It arises from the ischial spine and sacrotuberous ligament and inserts into the side of the coccyx and lower sacrum. It aids levator ani in supporting the pelvic organs.

The perineum is the diamond-shaped space between the pubis, the ischial tuberosities and the coccyx. It is divided into two compartments by the transverse perineal muscles, which arise from the ischial tuberosity and run medially to insert into the perineal body.

The anterior compartment is the anterior urogenital triangle. The anterior urogenital triangle contains a tough sheet of fascia - the perineal membrane - which is pierced by the urethra and in females by the vagina as well. The external urethral sphincter is reinforced by this layer. Its inferior surface gives attachment to the bulb and crura of the penis or clitoris (bulbocavernosus and ischiocavernosus).

The posterior compartment is the anal triangle. It contains the anus and its sphincters, w i th the ischiorectal fossa

Fig. 6.3 AP radiograph of male pelvis.

1. Spinous process of L5 vertebra

11. Anterior superior iliac spine 20. Pubic symphysis 2. Transverse process of L5 vertebra 12. Anterior inferior iliac spine 2 1. Superior pubic ramus 3. Spinous process of first sacral segment 13. Pelvic brim 22. Acetabulum 4. Lateral sacral mass 14. Ischial spine 23. Head of femur 5. Ilium 15. Body of ischium 24. Fovea 6. Sacroiliac joint 16. Ischial tuberosity 25. Neck of femur 7. Sacral foramen of S2 vertebra 17. Ramus of ischium 26. Greater trochanter of femur 8. Spinous process of S3 vertebra 18. Inferior pubic ramus 27. Lesser trochanter of femur 9. Coccyx: first segment of three 19. Body of pubic bone 28. Intertrochanteric femur 10. Iliac crest

on either side. The ischiorectal fossa is the space below and lateral to the posterior part of levator ani and medial to the inner wall of the pelvis (Fig. 6.2a). It is bounded posteriorly by sacrotuberous ligaments and the gluteus maximus muscle, laterally by the fascia of the obturator internus muscle and anteriorly by the perineal body. It is largely f u ll of fat. It is of importance in pathological conditions of the rectum. The anococcygeal body extends from the anus to the coccyx posteriorly. It receives fibres from the anal sphincter and levator ani muscles.

Radiology of the pelvic floor

Magnetic resonance imaging The muscles of the pelvic floor are elegantly demonstrated by MRI (Fig. 6.5). The low-signal intensity of the muscles contrasts w i th the high-signal intensity of the pelvic fat. No other imaging modality approaches MRI for demonstration of the pelvic floor anatomy. The bones have a low-intensity cortex w i th a high-intensity marrow cavity. The pubic symphysis is of intermediate signal intensity.

Fig. 6.4 CT scan of pelvis: axial section through male bladder. Rectal and intravenous contrast have been given. The full bladder displaces small bowel loops superiorly.

1. Subcutaneous fat 2. Rectus abdominis muscle 3. Iliacus muscle 4. Psoas muscle and tendon 5. External iliac vein 6. External iliac artery 7. Inferior epigastric vessels 8. Sartorius muscle 9. Tensor fascia lata muscle 10. Gluteus minimus muscle 11. Gluteus medius muscle 12. Gluteus maximus muscle 13. Piriformis muscle 14. Obturator internus muscle 15. Acetabulum 16. Lower sacral segment:

S4 vertebra 17. Bladder: full of layering urine and contrast 18. Perivesical fat 19. Seminal vesicle 20. Rectum containing air, faeces and contrast medium 2 1. Wall of rectum 22. Perirectal mesorectal fat 23. Perirectal fascia (known as

Denonvillier's fascia anteriorly) 24. Pararectal fat

Fig. 6.5 MRI pelvis - coronal section through male pelvis; (a) anterior section; (b) posterior section.

[a) (b)

1. Upper sacrum 2. Common iliac 3. Internal iliac 4. External iliac 5. Bladder 6. Urogenital diaphragm 7. Iliacus muscle

8. Gluteus minimus muscle 1. Attachment of levator ani 8. Sciatic nerve 9. Ilium 10. Femoral head to obturator internus 2. Ischiorectal fossa 9. Sigmoid colon 10. Left seminal vesicle

11. Obturator internus muscle 3. Levator ani 12. Posterior part of prostate 4. Ilium 13. Lesser trochanter of femur 5. Sacrum 11. Hamstring muscles 12. Obturator internus muscle 13. Ischium

14. Ischium 6. Sacroiliac joint 14. Rectum 7. Common iliac artery bifurcation 15. Long head of biceps

Fig. 6.6 Axial anatomy of the pelvis: (a) CT section through male perineum; (b) MR section through female perineum.

1. Pectineus muscle 2. Obturator externus muscle 3. Iliacus muscle 4. Obturator internus muscle 5. Pubic bone 6. Ischial tuberosity 7. Gluteus maximus muscle 8. Levator ani 9. Anus 10. External anal sphincter 11. Vagina 12. Urethra surrounded by external urethral sphincter 13. Sciatic nerve and inferior gluteal vessels