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Research Papers

System-Level Biomechanical Approach for the Evaluation of Term and Preterm Pregnancy Maintenance

[+] Author and Article Information
Hussam Mahmoud

Assistant Professor
Department of Civil and
Environmental Engineering,
College of Engineering,
Colorado State University,
Fort Collins, CO 80523

Amy Wagoner Johnson

Associate Professor
Department of Mechanical
Science and Engineering,
College of Engineering,
University of Illinois at Urbana-Champaign, Urbana, IL 61801

Edward K. Chien

Associate Professor
Department of Obstetrics and Gynecology,
Division of Maternal Fetal Medicine,
Alpert Medical School of Brown University,
Women and Infants Hospital of Rhode Island,
Providence RI 02905

Michael J. Poellmann

Ph.D. Candidate
Department of Bioengineering,
College of Engineering,
University of Illinois at Urbana-Champaign,
Urbana, IL 61801

Barbara McFarlin

Associate Professor
Department of Women, Children and
Family Health Science,
University of Illinois at Chicago,
Chicago, IL 60612

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical Engineering. Manuscript received October 1, 2012; final manuscript received January 20, 2013; accepted manuscript posted January 28, 2013; published online February 8, 2013. Editor: Victor H. Barocas.

J Biomech Eng 135(2), 021009 (Feb 08, 2013) (11 pages) Paper No: BIO-12-1456; doi: 10.1115/1.4023486 History: Received October 01, 2012; Revised January 20, 2013; Accepted November 28, 2013

Preterm birth is the primary contributor to perinatal morbidity and mortality, with those born prior to 32 weeks disproportionately contributing compared to those born at 32–37 weeks. Outcomes for babies born prematurely can be devastating. Parturition is recognized as a mechanical process that involves the two processes that are required to initiate labor: rhythmic myometrial contractions and cervical remodeling with subsequent dilation. Studies of parturition tend to separate these two processes rather than evaluate them as a unified system. The mechanical property characterization of the cervix has been primarily performed on isolated cervical tissue, with an implied understanding of the contribution from the uterine corpus. Few studies have evaluated the function of the uterine corpus in the absence of myometrial contractions or in relationship to retaining the fetus. Therefore, the cervical-uterine interaction has largely been neglected in the literature. We suggest that a system-level biomechanical approach is needed to understand pregnancy maintenance. To that end, this paper has two main goals. One goal is to highlight the gaps in current knowledge that need to be addressed in order to develop any comprehensive and clinically relevant models of the system. The second goal is to illustrate the utility of finite element models in understanding pregnancy maintenance of the cervical-uterine system. The paper targets an audience that includes the reproductive biologist/clinician and the engineer/physical scientist interested in biomechanics and the system level behavior of tissues.

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Figures

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Fig. 1

Schematics showing geometric changes in the uterine and cervical anatomy. The mass and volume of the human uterus and cervix increase with gestational age. The uterus typically increases in length from 6.5 cm to 32 cm and from 60 g to 1000 g in mass. The cervix increases in overall size, but gets progressively shorter after 24 weeks, from 4–6 cm. As the cervix nears term, the internal os increases in diameter and first opens with progressive cervical shortening. The anatomy of the cervical canal is typically described in terms of its shape, beginning as a ‘T,’ opening to a ‘V,’ and finally progressing to a ‘U’ as labor nears [20].

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Fig. 2

Schematic of the forces that act on the uterus and cervix before (left) and during dilation (right) include myometrial contraction, gravitational force, and boundary conditions from round, broad, and uterosacral ligaments. Cross-sections of the cervix illustrate the changes in dilation, collagen fiber arrangement, and lateral boundary conditions from the ligaments.

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Fig. 3

Cervical remodeling in normal pregnancy starts with cervical softening very early in pregnancy. Later in pregnancy, the collagen disorganizes and collagen fibers disorganize [48].

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Fig. 4

Representative data from the literature showing the complex mechanical behavior of uterine tissue. (a) The nonlinear elastic behavior in the stress-elongation data and differences for pregnant versus non pregnant tissue [69]. Nonpregnant uterine tissue is stiffer than pregnant tissue for the entire range of stress and elongation shown. (b) Representative examples of anisotropic and viscoelastic behavior in uterine tissue. The complex modulus depends, to a great extent, on the collagen fiber orientation, loading frequency, and amount of precompression. In addition to the complex modulus being greater for loading parallel to fibers, it also increases with increasing loading frequency [71].

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Fig. 5

The 2D finite element models of the ‘T’ and ‘U’ geometries

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Fig. 6

Deformation (widening) of the seven springs representing the cervical canal. An increase in the pressure results in increased widening ((a) versus (b), (c) versus (d)), as does softening of the tissue ((a) versus (c), (b) versus (d)). The greatest deformation of ‘T’ cervices is located at the internal os, while the greatest deformation of ‘U’ cervices is shifted towards the external os.

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