Figure 2

Biophysical phenomena during cervical dilations. (A) Biophysical model of Hegar dilator (HeD) dilatation. Under external force F _e (required to dilate the cervix at each stage of dilation), the dilator of diameter D _i moves at speed v through the cervical canal having length L. F _e must overcome the resultant sum of forces that appeared in the direction of HeD movement. During dilation with the N⁰9 HeD (diameter D _i  = 9 mm) the mean recorded value of external force ${\bar{F}}_e$was 11 N (with vaginal sodium nitroprusside (SNP) gel) and 17 N (with vaginal misoprostol) [A]. The mean recorded ${\bar{F}}_e$ during dilation with the N⁰10 HeD (diameter D _i  = 10 mm)was 13 N [B]. The mean ${\bar{F}}_e$ required to complete cervical canal dilation points to the complex biophysics of processes in the contact zone of tissue and the HeD. HeD motion along the cervical canal (length L) in the direction of the internal uterine os leads to changes in geometry of the cervical canal. Tissue, in different ways, opposes this change of geometry in contact with the HeD, characterized by the pressure distribution p(x). (B) Biophysical model of balloon dilatation (BD) by continuous controllable balloon dilator (CCBD). (a) Initial BD form diameter is approximatelyD _i  ≈ 4.5 mm,enabling insertion into the cervical canal with very low resistance to penetration. (b) Pumping of incompressible fluid in the BD leads to increased pressure and outer diameter of the BD, causing dilatation of the cervical canal. The dilatation process is performed simultaneously on the entire length (L) of the cervical canal, where relative movement (sliding) between the tissue/BD contact pair is almost reduced to zero. In this case the cervix tissue opposes less the change in geometry characterized by p(x).