Anatomy of Uterus and Pelvic Support

Uterus and Pelvic Support

The uterus lies between the rectum and the bladder (Fig. 4.11). Various pelvic ligaments help support the uterus and other pelvic organs. The broad ligament overlies the structures and connective tissue immediately adjacent to the uterus. Because it contains the uterine arteries and veins and the ureters, it is important to identify the broad ligament during surgery. The infundibulopelvic lig-ament connects the ovary to the posterior abdominal wall and is composed mainly of the ovarian vessels. The uterosacral ligament connects the uterus at the level of thecervix to the sacrum and is therefore its primary support. The cardinal ligament is attached to the side of the uterus immediately inferior to the uterine artery. The sacro-spinous ligament connects the sacrum to the iliac spine andis not attached to the uterus. This ligament is frequently used surgically to support the pelvic viscera.

The two major portions of the uterus are the cervix and the body (corpus), which are separated by a narrower isthmus. The length of the cervix is established at puberty. Before puberty, the relative lengths of the body of the uterus and cervix are approximately equal; after puberty, under the influence of increased estrogen levels, the ratio of the body to the cervix changes to between 2:1 and 3:1. The part of the body where the two uterine tubes enter it is called the cornu. The part of the corpus above the cornu is referred to as the fundus. In a woman who has had no children, the uterus is approximately 7 to 8 cm long and 4 to 5 cm wide at the widest part. The cervix is relatively cylindrical in shape and is 2 to 3 cm long. The body is gen-erally pear-shaped, with the anterior surface flat and the posterior surface convex. In cross section, the lumen of the uterine body is triangular.

The wall of the uterus consists of three layers:

1.    The inner mucosa, or endometrium, consists of sim-ple columnar epithelium with underlying connective tissue, which changes in structure during the men-strual cycle.

2.           The middle layer, or myometrium, consists of smooth muscle. This layer becomes greatly distensible during pregnancy; during labor, the smooth muscle in this layer contracts in response to hormonal stimulation

3.   The outermost layer, or perimetrium, consists of a thin layer of connective tissue. It is distinct from the para-metrium, a subserous extension of the uterus betweenthe layers of the broad ligament.

The position of the uterus can vary depending on the rela-tionship of a straight axis that extends from the cervix to the uterine fundus to the horizontal. When a woman is in the dorsal lithotomy position, the uterus may be bent forward (anteversion, AV), slightly forward but functionally straight (mid-position, MP), or bent backward (retroversion, RV). The top of the uterus can also fold forward (anteflexion,AF) or backward (retroflexion, RF). Five combinationsof these configurations are possible (Fig. 4.12). The posi-tion of the uterus is clinically important. For example, estimation of gestational age in the late part of the first trimester may be difficult when the uterus is in the RVRF or RV positions. Risk of uterine perforation during pro-cedures such as dilatation & curettage or insertion of an intrauterine device is increased in a woman with a retro-flexed or anteflexed uterus. Applying traction on the cervix to pull the uterine canal into a straight line can greatly reduce this risk.

The blood supply to the uterus comes primarily from the uterine arteries, with a contribution from the ovarian arteries, whereas the venous plexus drains through the uterine vein.

Of particular importance in pelvic surgery is the relative position of the uterine artery to the ureter.

The arteries travel in a lateral to medial direction at the level of the internal os of the cervix. At the point where they meet the uterus, they overlie the ureter. This proximity can cause inadvertent injury during pelvic surgery. The ureters lie between 1.5 cm and 3 cm from the uterine side-wall at this point (Fig. 4.13).