Structural Factors that Prevent Prolapse
As is true throughout the body, both muscles and connective tissues work together and are essential for structural support. Normal pelvic organ support is provided by the interaction between the levator ani muscles (LAMs) and the connective tissues that attach the uterus and vagina to the pelvic sidewalls (cardinal and uterosacral ligaments).
There are three basic structural strategies involved in normal pelvic support (Fig. 3.1
). First, connective tissues attach the uterus and vaginal walls to the inside of the pelvis and provide stability by absorbing load and limiting movement. Second, the pelvic floor muscles provide closure of the vaginal opening, which prevents prolapse from occurring. Vaginal closure within the high-pressure zone acts in the same way that, for example, the anal sphincters work. Stool does not fall out of the rectum because of the occlusive effect of the high-pressure anal canal. And third, as seen in a lateral view, these two individual biomechanical systems interact with and complement one another, thereby maintaining support even if an imbalance in one of these systems occurs.
The way in which these factors interact to provide pelvic organ support can be seen in Figure 3.21
and a glossary of selected biomechanical terms needed to understand pelvic organ support is provided in Table 3.1
The LAMs hold the pelvic floor closed and provide closing forces to prevent pelvic floor descent by creating a high-pressure zone in the lower vagina.2
In this situation, the closing forces in the anterior and posterior compartments are equal and opposing, creating balanced pressures
(pressure is the measure of how much force is acting on an area). The LAMs maintain a balanced system—and therefore remain closed—in response to increased forces (e.g., rise in abdominal pressure during Valsalva), thereby preventing prolapse. The cardinal and uterosacral ligaments provide lifting forces that resist persistent downward descent of the pelvic organs by temporarily lengthening in response to increased force (due to their elastic properties) and absorbing any increases in force (due to their stiffness). This illustrates the principle of alignment
within the biomechanical system of the pelvis, which depends on the muscles being strong enough to keep the hiatus closed and the connective tissues strong enough to resist deformation in order to hold the organs in place in response to increased load (e.g., during a cough).
When the muscles, nerves, and fascial structures involved in creating the vaginal high-pressure zone that holds the hiatus closed are damaged or weakened, the hiatus in the LAM complex can easily be pushed open. This is the principle of deformation under load
. If the muscles, under the control of complex neural reflexes, fail to hold the hiatus closed, the vaginal walls descend so that one or both vaginal walls protrude below and through the levator hiatus. When the vaginal walls
descend below the hymen to the level where the levators no longer maintain proper alignment of the system, unbalanced pressures
occur, creating a pressure differential
between abdominal and atmospheric pressure. The force
created by this pressure differential is like the one that moves a sailboat. The difference in pressure between two sides of a sail, for example, creates a force that propels the boat in a certain direction. In an anterior vaginal wall prolapse, abdominal pressure acts on the vaginal wall to create a downward force due to its misalignment
. This force then places abnormal tension on the tissues that attach the uterus and vagina to the pelvic walls.
FIGURE 3.1 Concepts of pelvic organ support. The basic principles of support are shown. Attachment of the vagina to the pelvic walls, closure of the vagina by muscle action at the hiatus, and the role of alignment allowing interactions between vaginal walls and surrounding structures. The yellow arrow indicates increased pressure, and the red arrow, support provided by the pelvic muscles. NOTE: Right panel is a lateral view. (© John O. L. DeLancey.)
FIGURE 3.2 Diagrammatic representation of interactions between LAM, anterior vaginal wall prolapse, and cardinal ligament (CL)/uterosacral ligament (USL) suspension. With normal levator function (A), the vaginal walls are in apposition, and anterior and posterior pressures are balanced. Levator damage (B) results in hiatal opening, and the vagina becomes exposed to a pressure differential between abdominal and atmospheric pressures. This pressure differential (C) creates a traction force on the CL and USL. (© John O. L. DeLancey.)
The size of the surface that is exposed to this pressure differential of exposed vagina matters. Pressure can be expressed as force per unit area
. That means that if there is 1 sq in of vagina exposed and the pressure differential is 1 lb/sq in, then there would be 1 lb of force on the ligaments and fascia resisting this downward force. The same 1 lb/sq in of pressure applied to 2 sq in results in 2 lb of force, etc. In more familiar units, 100 cm H2
O applied during Valsalva to 1 sq in results in 1.42 lb. Now, the larger the exposed area, the greater the force. As a cystocele enlarges, the force produced by the same pressure increases in a positive feedback loop
. This explains why a woman can push hard with only a small descent of the anterior wall, but as the prolapse becomes a little more exposed, a greater change is seen.
TABLE 3.1 Glossary of Terms
The amount of force exerted over an area. This parameter has no direction.
Correct positioning of the muscles, ligaments, and tendons within a biomechanical system that allows the system to effect optimal functioning
Change in shape, length, or location in response to increased load
Deformation under load
The ability of a material, tissue, or complex structure to withstand permanent deformation in response to applied forces
Any interaction (e.g., push or pull) on an object that affects the object’s position or movement
Interaction between two or more subsystems allowing them to withstand and respond to forces on the system as a whole
The ability of a material to return to its original size and shape after deformation
Nonlinear elastic response to large deformation loads
The cardinal and uterosacral ligaments that attach the uterus and vagina to the pelvic walls are important to support of the pelvic organs by providing lifting forces. Their ability to maintain alignment in the pelvis is dictated by their material properties of stiffness, which is the ability to resist deformation (change in shape or length) in response to increased load, and elasticity, which is the ability to stretch (or lengthen) in response to increased force and then return to the original state when that force is removed. The cardinal and uterosacral ligaments have a normal range of lengthening, and in response, the pelvic organs have a normal range of movement. When you pull on the cervix as you would during a dilation and curettage (D&C), it lengthens the cardinal ligament somewhat like a spring. However, unlike an elastic spring, where doubling the force doubles the descent, the ligament gets stiffer the more it is elongated (Fig. 3.3
). This is called a hyperelastic property
. With increasing load, there is less and less elongation until an elastic limit is reached. At this point, additional load does not increase descent, as the cardinal ligament cannot stretch anymore. Any further descent would involve tearing in the short term or tissue changes in the long term.
FIGURE 3.3 Location of the cervix relative to the hymeneal ring with varying amounts of traction force in 73 patients without clinical evidence of prolapse. Decreasing location of the cervix with varying amounts of traction. Positive numbers are cephalic to the hymen and measurements are made to the lateral border of the cervix.
Because the ligaments are elastic, this lengthening phenomenon is reversible and the pull on the cervix during D&C does not cause prolapse. However, Davis law states that connective tissue responds to chronic and excessive force with tissue adaptation. A skin expander is a good example of this, where it is possible to double the amount of skin present in 6 to 8 weeks by subjecting it to constant increased force. If the cardinal ligament is subjected to excessive downward force over time because the hiatus is not closed and the vaginal wall is subjected to a pressure differential, it can remodel to become longer.
The degree to which the vaginal wall moves downward in this situation has to do with the properties of the ligaments and fascial structures connecting it to the pelvic walls. Both the stiffness and length of the ligaments are involved. If the connective tissues are too lax or too long to hold the organs in alignment, the vaginal walls and uterus descend below the normal LAMs and the same imbalance in pressure can occur. Because these tissues are “stretchy,” some movement is normal, such as that seen during D&C, where the cervix can easily be drawn down to the introitus.
FIGURE 3.4 Illustration of bilinear relationship, or threshold effect. The threshold effect is illustrated here in the relationship between bladder descent and the size of a cystocele, measured as the length of the vagina exposed at the introitus to atmospheric pressure on magnetic resonance imaging (MRI). Note that at about 4 cm, the relationship changes.
Another important concept concerns the principle of structural interactions
. All the different anatomical elements in the pelvic floor interact with one another. The muscles that close the pelvic floor may be strong enough to support the organs under light loads. Under these conditions, increases in abdominal pressure may not cause any displacement. However, once the threshold that overcomes the ability of the muscles to resist the downward force is reached, the pelvic floor opens. This is a threshold effect
In some systems, if you double the load, you double the displacement. In other systems, a certain load must be reached before any change happens. The pelvic floor remains closed until enough force develops to open it. Once the hiatus is opened, the vaginal wall descends and is exposed to a pressure differential. With less than that opening force, it does not matter how much pressure there is—no opening occurs and no exposed vagina is subjected to the pressure differential. But once this threshold is exceeded, the pelvic floor opens, the structures become misaligned, and the pressure differential comes into play (Fig. 3.4
). Failure of the hiatus to remain closed affects connective tissue loading.
FIGURE 3.5 Lateral attachments of the vagina and cervix to the lateral sidewalls. The left panel shows a view of the pelvic organs from above looking over the pubic symphysis showing structures of the pelvic sidewall in relationship to the vagina (outlined by dotted line) after removal of the bladder and uterine corpus. The right panel shows different levels of support in a posthysterectomy cadaver. The parametrium and paracolpium are two portions of the cardinal ligament. Arcus tendineus fasciae pelvis and arcus tendineus levator ani refer to the fascial and levator arches respectively. (Reprinted from DeLancey JO. Anatomic aspects of vaginal eversion after hysterectomy. Am J Obstet Gynecol 1992;166[6 Pt 1]:1717-1728. Copyright © 1992, with permission from Elsevier.)
Failure analysis is a foundational aspect of engineering. If a plane crashes or a bridge falls, the exact cause of the failure is rigorously sought and lessons learned. This type of failure analysis has now been applied to the pelvic floor to determine the causes of prolapse and, as a powerful tool, to understand why certain women have recurrence after surgery. The next section describes the evidence to support how these support systems fail. 3D stress magnetic resonance imaging (MRI) has facilitated this type of analysis, allowing the many competing hypotheses to be tested scientifically.