Pelvis and Perineum

• There are 2 hip bones connected to each other anteriorly by the pubic symphysis (a secondary cartilaginous joint). Posteriorly they are connected to the sacrum at the sacroiliac joints, which are joints that are part fibrous and part synovial.
• Together, the two hips bones form the pelvic girdle. These hip bones, plus the sacrum, form a bony ring containing a space known as the pelvis.

  The purpose of the pelvic girdle is mainly for the attachment of muscles. Internally, the muscles help to support the pelvic viscera. Externally, the muscles are the abdominal muscles and back muscles, and the muscles of the lower limb.
• The pelvic girdle is tilted forward by approximately 60 degrees, so that the anterior superior iliac spines and pubic tubercles (the most anterior bony points of the girdle) lie in a vertical plane.

  The pelvic inlet is formed by a line which runs from the top of S1, along the wings or ala, of the sacrum, and then over the sacroiliac joints to the top of the pubic symphysis anteriorly.
• The pelvis is divided into a false/greater pelvis which is the part above the pelvic inlet, and a true/lesser pelvis which lies below the pelvic inlet.
• The pelvic outlet is bounded by the pubic symphysis anteriorly, the ischial tuberosities laterally, ad the tip of the coccyx posteriorly.
• The hip bone is formed by the fusion of 3 separate bones: the ilium, the ischium, and the pubis.

  These 3 bones fuse together and collectively form the socket of the hip joint. At the centre of the socket, there is a Y-shaped line of fusion of the 3 bones.

  Once the ilium, ischium, and pubis have fused together, a single bone is formed which is known as the innominate bone (meaning 'nameless' bone).

  The socket of the hip (formed by the fusion of the ilium, ischium, and the pubis) is known as the acetabulum.
• Posteriorly, there are two notches on the hip bone: the greater sciatic notch and the lesser sciatic notch. These notches allow for the passage of muscle, blood vessels, and nerves.
• Between the pubis and ischium, there is a hole known as the obturator foramen. The obturator foramen is blocked by a membrane called by the obturator membrane. However, there is a small gap that remains, which is known as the obturator canal, and this canal allows for small blood vessels and a nerve to pass through from the pelvis into the lower limb.
• The ilium has an internal surface, which is called the iliac fossa, and an external surface called the external lamina/gluteal surface. Both the iliac fossa and external lamina/gluteal surface are sites for muscle attachment (gluteal muscles attach to the external lamina of the ilium).
• Posterior of the iliac fossa is the iliac tuberosity and the auricular surface. These surfaces are the site of attachment of the sacroiliac joint. The sacroiliac join is complex, as it's partly fibrous and partly synovial. The fibrous part of the sacroiliac joint attaches to the iliac tuberosity, and the synovial part of the sacroiliac joint attaches to the auricular surface.
• At the superior margin of the ilium, there is the iliac crest; on the iliac crest is the iliac tubercle. At the inferior margin of the ilium, there is the arcuate line of the pelvic inlet.

  The ilium also has 4 spines, two anterior and two posterior. The two anterior spines are the anterior superior iliac spine (ASIS), and anterior inferior iliac spine (AIIS).
• The pubis has a body and two rami: a superior ramus and an inferior ramus. On the superior ramus, from lateral to medial, there is a ridge called the pecten, a pubic tubercle, and a pubic crest. The two pubis bones forms the pubic symphysis.
• The ischium has a spine and a tuberosity, known as the ischial spine and ischial tuberosity respectively. It also has a ramus which fuses with the inferior ramus of the pubis, which becomes the ischiopubic ramus.
• The sacrum and pelvic girdle are both tilted forwards by 60 degrees. The weight of the vertebral column on the sacrum puts a great amount of force on the sacroiliac joints and tends to cause the sacrum to become even more tilted. To prevent the sacrum from tilting too much, 2 ligaments anchor the sacrum in place: the sacrospinous ligament, and the sacrotuberous ligament.
• The sacrospinous and sacrotuberous ligaments arise from the ischial spine and ischial tuberosity respectively, and together, they not only provide stability for the sacrum, but they also create two openings: the greater sciatic foramen, and the lesser sciatic foramen. These foraminae give access to and from the lower limb.
• The pelvic floor is composed to two muscles on either side, and the bulk of this muscle meets in a raphe in the midline. This sheet of muscle arises laterally on the pelvic wall and cups downwards and forwards.
• There are a couple of holes in the pelvic floor, and these holes are known as the urogenital hiatus and the anorectal hiatus.

  In the female pelvis, the urethra and vagina posteriorly pass through the urogenital hiatus. The rectum passes through the anorectal hiatus.

  In the male pelvis, only the urethra passes through the urogenital hiatus, and the rectum passes through the anorectal hiatus.
• The pelvic muscles arise from the lateral pelvic walls, and there is a region below this, known as the perineum. Hence, the pelvic outlet isn't necessarily in the pelvis, but in the perineum.
• The perineum is divided into two pouches: a superficial perineal pouch, and a deep perineal pouch. The superficial perineal pouch contains the structures associated with the external genitalia and the deep perineal pouch contains a muscle associated with the urethra and also the vagina in females.
• The external genitalia in both sexes arise in part from the thickened inferior membrane of the deep perineal pouch. This membrane is known as the perineal membrane. It is attached posteriorly to a fibrous body known as the perineal body, and the main part of the perineal membrane lies inferior to the urogenital hiatus. There is a gap anterior to the urogenital hiatus to allow the dorsal vein of the penis or clitoris to reach the pelvis.
• The perineum is divided at the pelvic outlet into two triangles: the urogenital triangle, and the anal triangle. The bases of the triangles meet to form an imaginary line between the two ischial tuberosities. The urogenital triangle is anterior to this line, and the anal triangle is posterior to this line.

  The urogenital triangle is tilted superiorly to it's parallel with the inferior rami of the pubic bones, and the anal triangle is inclined superiorly to the coccyx.
• There are two muscles on each side of the pelvic wall: the piriformis and the obturator internus.
• The piriformis arises from the sacrum, passes out through the greater sciatic foramen, and inserts onto the trochanteric notch of the greater trochanter of the femur.

  The piriformis muscle is a lateral rotator of the hip joint.

  The piriformis muscle has a variable nerve supply, but usually, it's supplied by the superior gluteal nerve, which arises from L4-S1, and the ventral rami of S1 and S2.
• The obturator internus muscle arises from a large area around the inside of the obturator foramen. This muscle inserts onto the trochanteric fossa of the femur and is also a lateral rotator of the hip joint. It has little role in the pelvis, but it is covered on it's surface by pelvic fascia/obturator fascia).
• About two-thirds of the way down the obturator internus muscle, the obturator/pelvic fascia thickens and forms an arched tendon called the arcus tendineus (or tendinous arch). This tendon gives an origin to a large part of the pelvic floor muscles.
• The pelvic floor or pelvic diaphragm is made up of two muscles on either side: coccygeus (posterior of the two muscles), and the levator ani (anterior of the two muscles).
• The coccygeus muscle has its proximal attachment to the sacrum and coccyx, and its distal attachment to the ischial spine. This muscle is non-functional in humans, but does serve to complete the pelvic floor. (It's functional in animals to move their tail).

  The coccygeus is supplied by the anterior rami of S4 and S5, and there is a tonic contraction of this muscle to support the pelvic organs.
• Levator ani is a sheet of skeletal muscle that arises from the pubis, arcus tendineus, and ischial spine. It inserts into the sacrum and coccyx, and forms a raphe in the midline with its counterpart on the other side.

  Levator ani is also attached in the midline to a pair of fibrous masses called the perineal body and the anococcygeal body.
• The levator ani is also under tonic contraction and it is innervated mostly by S3 and S4 which enter the muscle on its superior surface. In some individuals there may also be a supply via the pudendal nerve (arises from S3 and S4) and when present, supplies the muscle from below. The levator ani is covered by fascia, which thickens in places to form ligaments. These also help to support the viscera.
• Levator ani is made of 3 main parts: pubococcygeus, iliococcygeus, and puborectalis.
• Iliococcygeus is the most lateral part of levator ani. It arises from the posterior part of the arcus tendinous and ischial spine, and inserts onto the anococcygeal body and coccyx.
• Pubococcygeus is the middle part of levator ani. It attaches anteriorly to the pubis, and more laterally to the anterior part of the tendinous arch. It inserts onto the anococcygeal ligament and coccyx.
• Puborectalis is the most medial part of levator ani. It slings around the anorectal hiatus and forms a raphe with its counterpart on the opposite side. Puborectalis pulls the anorectal junction forward, which is important in aiding rectal continence.
• The perineal body gives anchorage to some of the muscle fibres of levator ani, and the deep perineal pouch.

  The perineal body lies anterior to the anorectal hiatus, and posterior to the anorectal hiatus there is another fibrous body known as the anococcygeal body.

  The anococcygeal body can appear more like a ligament in some individuals, and in these cases, the structure is referred to as the anococcygeal ligament or anococcygeal raphe.
• Attached between the perineal body and the anococcygeal body in the external anal sphincter, which is a complex of skeletal muscle lying inferior to the pelvic floor.
• The coccygeus (or ischiococcygeus) is excluded from the levator ani group because it doesn't form a sling for the anorectal junction.
• There are muscles medial to puborectalis that sling around the urogenital hiatus. These muscles are:

  • the pubourethralis muscles. These are a sling of muscle fibres around the urethra in both males and females.
  • the puboprostaticus in males, which wrap around the prostate gland, and the pubovaginalis in females which wrap around the vagina.
• The pelvic floor is skeletal muscle, so there is some degree of control of these muscles. In females, these muscles can go into spasm when something is inserted into the vagina. This is known as vaginismus.
• Underneath the levator ani is the deep perineal pouch, covering the under-surface of the urogenital hiatus. The inferior fascia of the pouch is referred to as the perineal membrane, and this gives rise below to attachment of structures associated with the external genitalia. It is thicker than the superior fascia of the pouch.

  The deep perineal pouch is traversed by the urethra in both sexes, and by the vagina in females. The muscles within the pouch therefore act to constrict these structure
• Between the pelvic diaphragm and deep perineal pouch there is a space. which is continuous with the rest of the perineum and extends either side of the anal canal. These spaces are known as the ischioanal fossae. The space above the deep perineal pouch is known as the anterior recess, whilst below it is the superficial perineal pouch. Infections are common in the ischioanal fossa, and in turn, these recesses of the ischioanal fossae are also prone to such infection.