Bones and joints of the pelvis
The Pelvis is composed of 2 hip bones and sacrum . The hip bones form the lateral and anterior boundary of the pelvis while the sacrum forms its posterior boundary . the hip bone is an irregular bone that formed of three separate bones ileum ischeum and pubis . these bones are separate in the fetus and connected together by the triradiate cartilage in young and this cartilage ossifies by age . the sacrum is formed of five fused vertebrae at nearly the end of the vertebral column . the joints of the pelvis are : 1. the symphysis pubis ( between the two hip bones anteriorly ) 2. the iliosacral joint ( between the hip bone and the sacram posteriorly ) the first one is a fibrous joint doesn't allow movement . but the second one is a synovial joint allowing gliding movement ( plane synovial joint )
The pelvic diaphragm is the muscular sheet covering the pelvic outlet. It consists of the multi-functional muscle levator ani and the less important coccygeal muscle. Acting together, the muscles of the pelvic diaphragm raise the pelvic floor and contributes in generating intra-abdominal pressure. This causes pressure on abdominal viscera, and is thus important for vomiting, urination and defecation. Also, increased abdominal pressure is used in coughing and forced expiration. Directly, levator ani is cruicial to the voluntary suppression of defecation and urination, as both the rectum and the urethra pass through the muscle. Additionally, it provides support for the uterus in women. Weakness of the levator ani can cause problems such as incontinence and prolapse of the rectum or uterus of women, and exercises of the pelvic diaphragm is a recognized treatment option in managing urinary incontinence (particulary in stress incontinence) in women.
Levator ani (S4) The muscle is symmetrical with two attachments anteriomedially and posteriorly, and has three parts:
- M. pubococcygeus : Stretches between the symphysis pubis and the coccygeus, touching the borders of uterus and rectum. Pubococcygeus constitutes the main body of the levator ani.
- M. puborectalis: An U-shaped muscular sling that passes posteriorly to the anorectal junction, causing an anterior pull on the rectum. This creates a sharp anorectal angle (perineal flexure), which is important for maintaining fecal continence. Thus, during defecation, straightening of the flexure by relaxation of the puborectal muscle must occur.
- M. iliococcygeus is the lowest lying part of the levator ani, and it covers the posterior part of the pelvic outlet, stretching from the obturator fascia to the mid-portion of the coccygeus. This part is often small and not very important.
- M. coccyceus (S4/S5)
Forms a small part of the posterior pelvic diaphragm, and stretches for the ischial spine to the inferior medial sacrum.