Model helps map prolapsed anterior vaginal wall


Researchers are working to develop a computational model that simulates the in vivo mechanical properties of the human prolapsed anterior vaginal wall to test mesh materials.

Toronto-Mesh prolapse repair has accumulated a poor track record. Some investigators who spoke at the 2010 joint meeting of the International Continence Society and International Urogynecological Association in Toronto pointed out that it's not known how mesh will behave until it's used in patients.

"We need to have some modeling so we can create meshes that have the right tensile strength, durability, and so on," said Philippe Zimmern, MD, professor of urology at the University of Texas Southwestern Medical Center in Dallas. "We have absolutely no tools to do that. I hope we can get to that without having to use our patients as human experimentation."

The data they are pouring into the model include in vivo pressure uplift data on the prolapsed anterior vaginal wall and ex vivo data on tissue mechanical properties.

To gather in vivo data, they used a device called the BTC-2000 (SRLI Technologies, Nashville, TN), which plastic surgeons use to assess elasticity and properties of skin. A laser beam captures properties of suctioned tissue, measuring tissue deflection within a 10-mm aperture when maximum suction reaches 150 mm Hg.

Similar to a previous study, Dr. Zimmern and colleagues used the BTC-2000 to measure anterior vaginal wall properties in patients with prolapse (Neurourol Urodyn 2009; 28:325-9). Dr. Zimmern's six patients, aged 35 to 85 years, all had symptomatic stage 2-3 anterior vaginal wall prolapse requiring surgical repair. But in the previous study, multiple measurements were made by the same tester, so there were no inter-rater reliability studies.

Strong inter-rater reliability

Dr. Zimmern tested ratings made by himself and a non-trained investigator not familiar with the device; eg, a medical student or a nurse present in the operating room. They made sure all the readings were taken when the bladder was empty and then filled to 300 mL, without the catheter on traction, at the same very specific point; namely, the level of the bladder neck as identified on the vaginal wall by palpating the balloon of the urethral Foley catheter. Measursements over the most protruding area of the vaginal bulge would have resulted in too much variability from patient to patient, Dr. Zimmern explained.

The BTC-2000's tube was placed against the tissue for 8 seconds while pressure was decreased from 0 to –147 mm Hg. Then suction was released, and a laser light detected the degree of tissue uplift. Their inter-rater reliability was strong.

Testing the biomechanical properties of a strip of tissue using the Instron model 5565 (Norwood, MA) gave the researchers their ex vivo data. Their tensile testing process put a cyclic load on the fresh tissue harvested from the vaginal wall. The tissue was stretched three times to 25% strain, then stretched at 0.5 mm/s to failure.

The stress-strain data were fitted to a strain energy function to describe the material. Then, the finite element model predictors compared these data to the in vivo tissue uplift measurements at peak suction pressure.

The prediction was fairly good, although with some variation between what was actually measured and what the model predicted, so the model needs to be refined, said Dr. Zimmern. He said he is refining the model by including more patients in the series and improving their measurement methods.

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