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Technical Brief

Biaxial Mechanical Assessment of the Murine Vaginal Wall Using Extension–Inflation Testing

[+] Author and Article Information
Kathryn M. Robison

Mem. ASME
Department of Biomedical Engineering,
Tulane University,
6823 St. Charles Avenue,
New Orleans, LA 70118
e-mail: krobison@tulane.edu

Cassandra K. Conway

Department of Biomedical Engineering,
Tulane University,
6823 St. Charles Avenue,
New Orleans, LA 70118
e-mail: cconway2@tulane.edu

Laurephile Desrosiers

Department of Female Pelvic Medicine
& Reconstructive Surgery,
Ochsner Clinical School,
1514 Jefferson Highway,
New Orleans, LA 70121
e-mail: laurephile.desrosiers@ochsner.org

Leise R. Knoepp

Department of Female Pelvic Medicine
& Reconstructive Surgery,
Ochsner Clinical School,
1514 Jefferson Highway,
New Orleans, LA 70121
e-mail: lknoepp@ochsner.org

Kristin S. Miller

Mem. ASME
Department of Biomedical Engineering,
Tulane University,
6823 St. Charles Avenue,
New Orleans, LA 70118
e-mail: kmille11@tulane.edu

1Corresponding author.

Manuscript received February 8, 2017; final manuscript received August 1, 2017; published online August 24, 2017. Assoc. Editor: Jonathan Vande Geest.

J Biomech Eng 139(10), 104504 (Aug 24, 2017) (8 pages) Paper No: BIO-17-1053; doi: 10.1115/1.4037559 History: Received February 08, 2017; Revised August 01, 2017

Progress toward understanding the underlying mechanisms of pelvic organ prolapse (POP) is limited, in part, due to a lack of information on the biomechanical properties and microstructural composition of the vaginal wall. Compromised vaginal wall integrity is thought to contribute to pelvic floor disorders; however, normal structure–function relationships within the vaginal wall are not fully understood. In addition to the information produced from uniaxial testing, biaxial extension–inflation tests performed over a range of physiological values could provide additional insights into vaginal wall mechanical behavior (i.e., axial coupling and anisotropy), while preserving in vivo tissue geometry. Thus, we present experimental methods of assessing murine vaginal wall biaxial mechanical properties using extension–inflation protocols. Geometrically intact vaginal samples taken from 16 female C57BL/6 mice underwent pressure–diameter and force–length preconditioning and testing within a pressure-myograph device. A bilinear curve fit was applied to the local stress–stretch data to quantify the transition stress and stretch as well as the toe- and linear-region moduli. The murine vaginal wall demonstrated a nonlinear response resembling that of other soft tissues, and evaluation of bilinear curve fits suggests that the vagina exhibits pseudoelasticity, axial coupling, and anisotropy. The protocols developed herein permit quantification of biaxial tissue properties. These methods can be utilized in future studies in order to assess evolving structure–function relationships with respect to aging, the onset of prolapse, and response to potential clinical interventions.

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Figures

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Fig. 1

Murine reproductive system pre- (left) and postexplant (right) with white lines denoting the border between the cervix and vagina and approximate distances from the border to the base of the vagina and split of the uterine horns

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Fig. 2

Postexplant murine reproductive system demonstrating the point at which the bifurcated external os (two rings) of the cervix merges into the vaginal canal (one ring)

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Fig. 3

Vaginal sample mounted onto cannulas in preparation for testing. The unloaded length (left) can be found by identifying the point at which the ridges on either side of where the urethra was removed from (white dotted lines added for emphasis) begin to buckle. Note that the ridges become more linear in the in vivo configuration (right).

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Fig. 4

Schematic of the biaxial testing protocol for pressure (top) and force (bottom) over time. The protocol consists of: (I) five cycles of pressure–diameter preconditioning from 0 to 25 mm Hg, (II) five cycles of force–length preconditioning with pressure held at 2 mm Hg and the stretch ranging from −4% to 4% of the unloaded length, (III) equilibration period with the pressure held at 2 mm Hg at the estimated in vivo stretch, (IV) pressure–diameter tests performed at (a) −4% in vivo stretch, (b) in vivo stretch, and (c) 4% in vivo stretch over 0–25 mm Hg, and (V) force–length tests: (a) pressure held at 2 mm Hg, (b) 8 mm Hg, (c) 12 mm Hg, and (d) 25 mm Hg over the range of in vivo stretches.

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Fig. 5

Schematic of a bilinear curve fit wherein the built-in matlab function lsqcurvefit is used to identify the moduli of the toe and linear regions as well as the transition stress and stretch at the transition point

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Fig. 6

Testing data (mean ± SEM) from pressure–diameter (a) and (b) and force–length (c) protocols conducted on n = 16 specimens. Estimation of the in vivo axial stretch ratio based on the near constancy of the transducer-measured force–pressure response during the cyclic pressure–diameter testing (b), and typical force–length responses for which the intersection in the force–axial stretch data, denoted by the black line, reveals the in vivo axial stretch (c) as described previously in Refs. [52] and [55].

Grahic Jump Location
Fig. 7

Circumferential Cauchy stress–stretch curves (mean ± SEM of n = 16 specimens) calculated from p–d testing data for loading (filled) and unloading (unfilled) cycles at the estimated in vivo axial stretch, denoting the effect of cycle on vaginal wall mechanical behavior

Grahic Jump Location
Fig. 8

Axial Cauchy stress versus circumferential stretch curves (mean ± SEM of n = 16 specimens) calculated from p–d testing data for loading (filled) and unloading (unfilled) cycles at 4% above (gray) and below (black) the in vivo axial stretch, denoting the effect of cycle and axial coupling on vaginal wall mechanical behavior

Grahic Jump Location
Fig. 9

Axial Cauchy stress–stretch curves (mean ± SEM of n = 16 specimens) calculated from f–l testing data for loading (filled) and unloading (unfilled) cycles at 2 (black) and 25 mm Hg (gray), denoting the effect of cycle and axial coupling on vaginal wall mechanical behavior

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