Nonlinear mechanical behavior of cervical tissue with increasing pre-compression

Abstract

Tissue mechanical properties may determine tissue function. This is particularly notable in the cervix, where its mechanical properties influence the outcome of pregnancy. The cervix is easily imaged with ultrasound, and its cost and portability make elastography a good candidate to use as a clinical diagnostic tool to evaluate cervical function. However, before elastography can be used clinically, the mechanical properties of cervical tissue must be characterized. In this study, we performed dynamic testing on normal human cervix tissue at several different levels of pre-compression and different testing frequencies in order to characterize its nonlinear mechanical behavior. Cervical specimens were obtained from the UW Hospital Pathology Lab (n = 12) from the uteri of patients 40 to 76 years old who underwent a hysterectomy. Patient consent was obtained for the mechanical testing of the excised tissue samples. Samples were approximately one cubic centimeter and stored in saline solution. Samples were dynamically tested using an EnduraTEC ELF 3220 for six different pre-compression ranges (1-6%) at three different compression amplitudes (2-4%) and four testing frequencies (1, 10, 20 and 30 Hz). Our results show an increase in the measured storage modulus (E') with increasing pre-compression. When the results were normalized to E' at 1% pre-compression, E' <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6%</sub> was approximately 1.4 times E' <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1%</sub> . Dynamic amplitude had little effect on E' measured at each level of pre-compression. We saw no changes in tan ? with increasing pre-compression. E' and tan ? increased significantly with frequency. E' monotonically increased from 4.6-7.8 kPa for the 1-3% compression range (lowest precompression for 2% amplitude) and from 6.3-10.3 kPa for the 6-8% range (highest precompression for 2% amplitude) when increasing frequency from 1-30 Hz. Tan ? increased montonically from 0.35-0.45 for 2% amplitude compressions from 1-30 Hz regardless of initial precompression. These results show the nonlinear behavior of cervix tissue with increasing pre-compression and testing frequency and are consistent with previous studies. In order for ultrasound elastography to be an effective tool to determine cervical function, the initial pre-compression must be considered when measuring the elastic modulus of the cervix.