A uniaxial force and stiffness model of the vagina during laparoscopic sacrocolpopexy

Background: Laparoscopic sacrocolpopexy is the preferred procedure for restoring vaginal vault prolapse. An assistant uses a vaginal manipulator to position and tension the vault such that the surgeon can dissect the bladder, rectum and vault to eventually suture a synthetic mesh used to suspend the vagina to the longitudinal anterior vertebral ligament. Vaginal vault manipulation requires application of high forces for long periods of time. Methods: This work quantifies the task by measuring and analyzing the interaction forces and the workspace during vaginal vault manipulation. From the measurements we developed a uniaxial model, expressing the increase in interaction force and stiffness of the vagina. By adapting the model parameters, the difference in interaction force and stiffness between moderate and severe prolapse is predicted. Findings: For moderate prolapse the average interaction force and stiffness start at 2.56 N and 0.11 N mm⁻¹ in the tensionfree state, and go up to 20.14 N and 0.53 N mm⁻¹ after complete insertion of the instrument. For severe degrees of prolapse, tissue interaction is much lower starting at 1.68 N and 0.06 N mm⁻¹ while staying limited to 12.20 N and 0.30 N mm⁻¹ at full extension. Interpretation: Population data shows that the stage of prolapse and total vaginal length increase with age and parity. The interaction force and stiffness of the vagina are correlated with this degree of prolapse. By adapting the model parameters a good estimation of the tissue interaction is found for patients with mild and severe prolapse.