Paper 4: Do Pubococcygeus Muscle and the Cardinal Uterosacral Ligament Complex Impairments Interact to Affect Anterior Vaginal Wall Descent?

Abstract

OBJECTIVE: To investigate how impairment of the pubococcygeus muscle (which closes the urogenital hiatus) and of cardinal uterosacral ligament complex (which provides apical suspension) interact to affect the severity of anterior vaginal wall descent during a 200 cm H20 Valsalva. METHODS: The apical suspension of the anterior vaginal wall (AVW) was represented in a 2-D, mid-sagittal plane, biomechanical model in Matlab (Fig. 1). The levator plate (LP) was modeled as a “trapdoor” hinged on the sacrum. The pubococcygeus muscle (PCM) was considered to control levator plate inclination with both active contractile forces and passive spring-like forces. In order to focus on apex suspension mechanics, paravaginal attachments were not analyzed at this time. The suspension provided by the cardinal uterosacral ligament complex (CUS) was represented by a passive elastic spring (Fig. 1). Material properties for the PCM and CUS were based on published data and clinical measurements. An inextensible membrane was used to represent the vaginal wall. For this first simulation, elongation of the vaginal wall as a mechanism of prolapse was not considered. The impairment of the PCM was represented by a loss of active contractile force and decreased passive spring stiffness, while the impairment of the CUS was modeled using decreased spring stiffness.FIGURE 1: Biomechanical model (PS, pubic symphysis; Sac, sacrum).RESULTS: Fig. 2 shows how AVW descent changes with different degrees of PCM and CUS impairment: Panels A and D both have normal PCM and no AVW descent occurs despite a 75% loss of the CUS stiffness in D. Panels B, C, E, F show that when the PCM impairment exceeds a threshold at 40%, the AVW becomes exposed to intra-abdominal pressure unsupported by the LP and the CUS begins to stretch under load and AVW descent increases. The LP inclination depends on the degree of PCM impairment (cf, B vs C, E vs F). The AVW descent then depends on the severity of the CUS impairment (cf, B vs E, C vs F).FIGURE 2: Effects PCM and CUS impairment on AVW descent under 200 cm H20 intra-abdominal pressure. Black curve, AVW; Gray, the levator plate.CONCLUSION: The interaction between PCM and CUS impairments affect AVW support more than impairment in either of these elements alone. Funded by NIH R01 HD 038665 & P50 HD 044406.