What's new in the functional anatomy of pelvic organ prolapse?

Abstract

Purpose of review: Provide an evidence-based review of pelvic floor functional anatomy related to pelvic organ prolapse.

Recent findings: Pelvic organ support depends on interactions between the levator ani muscle and pelvic connective tissues. Muscle failure exposes the vaginal wall to a pressure differential producing abnormal tension on the attachments of the pelvic organs to the pelvic sidewall. Birth-induced injury to the pubococcygeal portion of the levator ani muscle is seen in 55% of women with prolapse and 16% of women with normal support. Failure of the lateral connective tissue attachments between the uterus and vagina to the pelvic wall (cardinal, uterosacral, and paravaginal) are strongly related with prolapse (effect sizes ∼2.5) and are also highly correlated with one another (r ∼ 0.85). Small differences exist with prolapse in factors involving the vaginal wall length and width (effect sizes ∼1). The primary difference in ligament properties between women with and without prolapse is found in ligament length. Only minor differences in ligament stiffness are seen.

Summary: Pelvic organ prolapse occurs because of injury to the levator ani muscles and failure of the lateral connections between the pelvic organs to the pelvic sidewall. Abnormalities of the vaginal wall fascial tissues may play a minor role.

Figure 1

Diagrammatic representation of interactions between levator ani muscle, anterior vaginal wall prolapse, and cardinal/uterosacral ligament 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 cardinal ligament (CL) and uterosacral ligament (USL). Modified from DeLancey 2012. © DeLancey

Figure 2

2D biomechanical model created from MRI scan. Left panels show model development; (a) mid-sagittal MR image and modeled element traced or projected on mid-sagittal MR image and (b) the lumped parameter biomechanical model. Pubovisceral muscle (PVM) is modeled as a spring in parallel with an active force generator. PS, pubic symphysis; SAC, sacrum; PM, perineal membrane; LA plate, levator plate; R, rectum; V, vagina; CL, cardinal ligament spring; US, uterosacral ligament spring; B, bladder; UT, uterus. Right Panels show simulated deformation of the model anterior vaginal wall, and its support system, under maximal Valsalva with various degrees of pubovisceral muscle (PVM) and cardinal and uterosacral ligament impairment (indicated in percent). The value for D, which presents the size of prolapse measured as the decent of the most dependent point of vaginal wall from end of perineal membrane is provided. Modified from Chen 2006. © Biomechanics Research Lab, University of Michigan, 2006

Figure 3

Panel a shows 3D reconstruction of the pelvic organs and bones (semitransparent) from MRI at rest with the line of the arcus tendineus fascia pelvis (ATFP) indicated. Anterior vaginal wall, AVW; bladder, B; external anal sphincter, EAS; levator plate, LP; rectum, R; urethra, U; uterus, Ut). Panel b shows the organs at rest with the measuring lines shown. Vaginal wall parameters are shown with white lines; vaginal width is measured at 5 locations and length in the midline. Green lines indicate apical support as the distance from the uterus to the top of the greater sciatic foramen where the cardinal ligament is fixed to the pelvic wall. Paravaginal distance indicated by yellow line (shown for one of five sampling locations). Panel c shows the same distances at maximal Valsalva. © DeLancey

Figure 4

Data from the comparison of women with anterior vaginal prolapse and controls with normal support. Top panel compares apical and paravaginal distances from normal and bottom panel, vaginal width. Note that “1” represents the apex at the cervico-vaginal junction (CVJ) and position “5,” the urethrovaginal junction (UVJ) at the distal end. Asterisks mark statistically significant differences (p < 0.05). Note that right and left paravaginal distance means have been combined to one overall mean. Standard deviation shown. Larson 2012. © DeLancey

Figure 5

Levator Ani Lines of Action. Panel A: The thick arrow displays the mean direction to the horizontal line in a two-dimensional graphic. The dashed line is the horizontal line from which the angles are measured. Angles above the horizontal line have a “+” sign and those below the horizontal line a “−” sign. On MRI, the PVM was found medial to the PRM; for graphical reasons their lines of actions are depicted in the same plane. Panel B: Horizontal and vertical components of the PVM and PRM in the standing position. The thick arrows show the average direction of the lines of action of the PVM and PRM muscles relative to the horizontal with a theoretical 1 N force. Thin lines indicate the portion of each force related to a closing and lifting function. (Note: vectors are shown larger than the background anatomy to avoid an overlap in the display). From Betschart 2014. © DeLancey

Figure 6

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. Right panel shows different levels of support in post-hysterectomy cadaver.

Figure 7

Left side panels show 3D models in the standing posture with (upper panel) and without (lower panel) the display of the pelvis showing. Pubic bone, PB; Sacrum, Sac; Uterus, Ut; Vaginal, Vag; Cardinal ligaments, CL; and deep uterosacral ligament, US. The middle panels show normal and prolapse ligament comparison under resting and maximal Valsalva. Shown are 3D models of one healthy individual at rest (a) and at maximal Valsalva (b); with their relationship to the normalized arcus tendineus fascia pelvis, ATFP) extending from the public symphysis, P) to the ischial spines, yellow squares). Below are shown a subject with prolapse at rest (c) and at maximal Valsalva (d). “Uterus, Ut; vagina, V; cardinal ligament, CL; deep uterosacral ligament, USLd. Ut uterus, V vagina, CL cardinal ligament, USLd deep uterosacral ligament. Right panel: Average straight length and angle of cardinal ligament (CL) and deep uterosacral ligament (USLd) are shown based on one average, mid-sagittal image from a healthy woman, with outline for uterus, vagina, pubic symphysis, and sacrum. From Luo 2013. © DeLancey