A synthetic cervix model and the impact of softness on cerclage integrity

Abstract

Cervical insufficiency is a medical condition of pregnancy which causes the cervix to soften, shorten and dilate before full term, typically between 18 and 22 weeks of gestation, such that a preterm birth occurs. It is a common cause of second trimester pregnancy loss. Before meeting the diagnostic criteria, a patient that experiences early cervical remodelling or the development of a short cervix may receive surgical intervention. Once detected, the typical treatment is a cerclage procedure (a purse string suture to close the cervix) and progesterone medication. There are conflicting studies on the efficacy of the cerclage procedure, with conclusions drawn from clinical evidence as opposed to mechanical properties. The purpose of this study is to understand the mechanical limitations of the cerclage procedure. Working with physicians at George Washington University School of Medicine and Health Sciences, we created generalized three dimensional models of the cervix from ultrasound images. To fabricate the synthetic cervices, we used a silicone rubber to mimic the qualitative feel of the cervix according to collaborating physicians. Using this qualitative information, we performed material testing for quantitative analysis. The synthetic cervices were then sutured using clinical techniques. The maximum force required for the synthetic tissue to rupture due to the cerclage stitch was recorded. The impact of material softness on the integrity of the cerclage is investigated.

Keywords: biomechanics; cervical insufficiency; human reproduction; reproductive biomechanics.

Figure 1.

This figure highlights important anatomical features of a uterus and cervix during pregnancy. (Online version in colour.)

Figure 2.

This figure diagrams the suture method for three cerclage protocols. In this study, the physician used a 4 step McDonald method (far left and boxed).

Figure 3.

This image shows an ultrasound of a patient at 20 weeks with a normal length cervix. Specific anatomical features such as locations of cervical tissue, amniotic fluid and fetus are labelled in the figure. (Online version in colour.)

Figure 4.

Panel (a) shows the outer piece of the cervical mould; (b) shows the inner section of the cervical mould used in these experiments. Panel (c) shows the outer and inner pieces assembled into one and (d) shows the resulting silicone model removed from the mould after curing. (Online version in colour.)

Figure 5.

The material testing results of the silicone samples (a) The stress–strain curves for each of the 10 types of silicone tested. (b) The modulus of elasticity of the linear region is reported as a function of silicone thinner added to the sample. (Online version in colour.)

Figure 6.

At left is a schematic of the experimental set-up and at right is a photo of the experiment running. The experimental set-up consists of a custom steel insert with conical end and a maximum diameter of 25 mm attached to a 5 kN load cell. The stitched cervix is bound inside a plastic capsule designed to restrict its lateral movement. The steel insert moves through the cervix, applying force to the stitch until rupture. (Online version in colour.)

Figure 7.

The three curves in this plot are single, non-averaged trials for a hard (yellow line), a medium (green line) and a soft (purple line) cervix. The black markers define the maximum force prior to rupture. (Online version in colour.)

Figure 8.

The three curves in this plot represent the averaged force traces for the hard (yellow line), medium (green line) and soft (purple line) cervices. The shaded regions indicate the standard deviation of these results. (Online version in colour.)

Figure 9.

The average rupture force for the tests is reported as a function of material elasticity. The vertical lines are the standard deviations of these results. (Online version in colour.)