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Research Papers

Swine Vagina Under Planar Biaxial Loads: An Investigation of Large Deformations and Tears

[+] Author and Article Information
Jeffrey A. McGuire

STRETCH Lab,
Department of Biomedical
Engineering and Mechanics,
Virginia Tech,
Blacksburg, VA 24061
e-mail: jeffmcg8@vt.edu

Steven D. Abramowitch

Department of Bioengineering,
University of Pittsburgh,
Pittsburgh, PA 15261
e-mail: sdast9@pitt.edu

Spandan Maiti

Department of Bioengineering,
University of Pittsburgh,
Pittsburgh, PA 15261
e-mail: spm54@pitt.edu

Raffaella De Vita

STRETCH Lab,
Department of Biomedical
Engineering and Mechanics,
Virginia Tech,
Blacksburg, VA 24061
e-mail: devita@vt.edu

Manuscript received June 8, 2018; final manuscript received December 3, 2018; published online February 13, 2019. Assoc. Editor: Thao (Vicky) Nguyen.

J Biomech Eng 141(4), 041003 (Feb 13, 2019) (9 pages) Paper No: BIO-18-1270; doi: 10.1115/1.4042437 History: Received June 08, 2018; Revised December 03, 2018

Vaginal tears are very common and can lead to severe complications such as hemorrhaging, fecal incontinence, urinary incontinence, and dyspareunia. Despite the implications of vaginal tears on women's health, there are currently no experimental studies on the tear behavior of vaginal tissue. In this study, planar equi-biaxial tests on square specimens of vaginal tissue, with sides oriented along the longitudinal direction (LD) and circumferential direction (CD), were conducted using swine as animal model. Three groups of specimens were mechanically tested: the NT group (n =9), which had no pre-imposed tear, the longitudinal tear (LT) group (n =9), and the circumferential tear (CT) group (n =9), which had central pre-imposed elliptically shaped tears with major axes oriented in the LD and the CD, respectively. Through video recording during testing, axial strains were measured for the NT group using the digital image correlation (DIC) technique and axial displacements of hook clamps were measured for the NT, LT, and CT groups in the LD and CD. The swine vaginal tissue was found to be highly nonlinear and somewhat anisotropic. Up to normalized axial hook displacements of 1.15, no tears were observed to propagate, suggesting that the vagina has a high resistance to further tearing once a tear has occurred. However, in response to biaxial loading, the size of the tears for the CT group increased significantly more than the size of the tears for the LT group (p =0.003). The microstructural organization of the vagina is likely the culprit for its tear resistance and orientation-dependent tear behavior. Further knowledge on the structure–function relationship of the vagina is needed to guide the development of new methods for preventing the severe complications of tearing.

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Figures

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Fig. 1

Diagram of the vaginal tract within the swine displaying the three regions and two cross sections from which samples were taken for histological analysis. Blue squares represent specimens stained with MT stain and magenta squares represent specimens stained with VVG stain.

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Fig. 2

Schematic of the specimen in the (a) undeformed configuration and (b) deformed configuration. HLD, HCD, hLD, and hCD represent the distances between hooks in the LD and CD as indicated by the subscripts. 2A and 2a represent the lengths of the major axis of the tear, 2B and 2b represent the lengths of the minor axis of the tear, and A and a represent the areas of the elliptically shaped tear.

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Fig. 3

Histology of full thickness vaginal cross section: (a) MT-stained slide and (b) VVG-stained slide. (c) Percent content of smooth muscle, collagen, and elastin reported as mean ± standard deviation (n =12).

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Fig. 4

Stress–strain data in the LD (solid lines) and CD (dashed lines) for specimens in the NT group (n =9)

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Fig. 5

Mean (± standard deviation) stress–strain data in the LD (solid lines and circles) and CD (dashed lines and triangles) for specimens in the NT group (n =9)

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Fig. 6

Axial Lagrangian strain maps in the (a) LD and (b) CD of a single specimen at four values of NHD

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Fig. 7

Stress versus NHD data in the LD (solid lines) and CD (dashed lines) of specimens in the LT group (n =9)

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Fig. 8

Stress versus NHD data in the LD (solid lines) and CD (dashed lines) of specimens in the CT group (n =9)

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Fig. 9

Mean (± standard deviation) stress data in the LD (solid colored bars with solid lines) and CD (patterned color bars with dashed lines) of the NT (n =9), LT (n =9), and CT (n =9) groups at three levels of NHD. Significant differences in stress were found between the LD and CD for the NT group at all three levels of NHD.

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Fig. 10

(a) Normalized length, a/A, versus NHD data in the LD (solid lines and circles) and normalized length, b/B, versus NHD data in the CD (dashed lines and triangles) for the LT group (n =9). (b) Normalized length, a/A, versus NHD data in the CD (dashed lines and triangles) and normalized length, b/B, versus NHD data in the LD (solid lines and circles) for the CT group (n =9). Mean (±standard deviation) data are also reported (black symbols).

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Fig. 11

Normalized changes in the areas, a/A, versus the average of the NHD data in the LD and CD for the LT and CT groups. Mean (± standard deviation) data are also reported (black symbols).

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