Role of uterine forces in intrauterine device embedment, perforation, and expulsion

Norman D Goldstuck, Dirk Wildemeersch, Norman D Goldstuck, Dirk Wildemeersch

Abstract

Background: The purpose of this study was to examine factors that could help reduce primary perforation during insertion of a framed intrauterine device (IUD) and to determine factors that contribute in generating enough uterine muscle force to cause embedment and secondary perforation of an IUD. The objective was also to evaluate the main underlying mechanism of IUD expulsion.

Methods: We compared known IUD insertion forces for "framed" devices with known perforation forces in vitro (hysterectomy specimens) and known IUD removal forces and calculated a range of possible intrauterine forces using pressure and surface area. These were compared with known perforation forces.

Results: IUD insertion forces range from 1.5 N to 6.5 N. Removal forces range from 1 N to 5.8 N and fracture forces from 8.7 N to 30 N depending upon device. Measured perforation forces are from 20 N to 54 N, and calculations show the uterus is capable of generating up to 50 N of myometrial force depending on internal pressure and surface area.

Conclusion: Primary perforation with conventional framed IUDs may occur if the insertion pressure exceeds the perforation resistance of the uterine fundus. This is more likely to occur if the front end of the inserter/IUD is narrow, the passage through the cervix is difficult, and the procedure is complex. IUD embedment and secondary perforation and IUD expulsion may be due to imbalance between the size of the IUD and that of the uterine cavity, causing production of asymmetrical uterine forces. The uterine muscle seems capable of generating enough force to cause an IUD to perforate the myometrium provided it is applied asymmetrically. A physical theory for IUD expulsion and secondary IUD perforation is given.

Keywords: IUD; fracture forces; insertion forces; intrauterine pressure; intrauterine surface area; removal forces.

Figures

Figure 1
Figure 1
Myometrial force (N) produced by a given intrauterine pressure (mmHg) for three values of endometrial cavity surface area.
Figure 2
Figure 2
Direction of uterine forces in the normal intrauterine device containing uterus are given at (A–D) in the top diagram. Notes: Depending on the time of the cycle (and at basal pressures), any direction may be dominant. At higher pressures, the fundus to cervix force at (C) is usually dominant and the counterforces at (A) hold the IUD in place. High pressure asymmetrical forces at position (A) in the bottom diagram push the intrauterine device from left to right and towards the fundus so that it perforates at the uterotubal junction. Abbreviation: IUD, intrauterine device.
Figure 3
Figure 3
The figure shows an oval shaped internal cervical os. Note: The inserter tube and folded arms will automatically rotate (arrow) and adapt to the shape of the os to find the entry of least resistance, usually in the latero-lateral direction.
Figure 4
Figure 4
Extreme forces can act on the intrauterine device (eg, Multiload Cu-IUD [MLCu®; Multilan AG, Dublin, Ireland]) causing somersaulting and predisposing to expulsion or perforation. Note: This is the end result of progressive clockwise or anticlockwise asymmetric uterine rotation forces. Abbreviation: IUD, intrauterine device.
Figure 5
Figure 5
Secondary perforation of one extremity of the transverse arm of an intrauterine device close to the uterotubal junction; the other extremity is about to perforate (arrow). Note: This type of secondary perforation is due to asymmetrical uterine forces and cannot be produced on insertion.
Figure 6
Figure 6
3D ultrasonography of an abnormally located ParaGard® intrauterine device (left) and Mirena® levonorgestrel intrauterine system (right) causing bleeding and pain. Notes: The fundal transverse dimension in these cases is only approximately 2 cm. The fundal width shown in the picture on the right is 19.56 mm. Severe disproportion causes expulsion, embedment and sometimes secondary perforation as a consequence of severe uterine forces. Markers indicate maximum uterine cavity width. ParaGard (Teva Pharmaceutical Industries Ltd, Petach Tikva, Israel). Mirena (Bayer, Wuppertal, Germany).
Figure 7
Figure 7
T-shaped Femilis® levonorgestrel intrauterine system (Contrel Europe NV, Ghent, Belgium) with transverse arm of either 24 mm or 28 mm, showing perfect fit in the fundus of the uterus. Note: Expulsion have been less than one per 100 per year in multicenter studies.
Figure 8
Figure 8
Frameless copper and drug delivery systems occupy limited space in the uterine cavity and have no transverse arms, eliminating embedment and secondary perforation of the uterine wall. Notes: Arrows indicate uterine cavity width. Framed intrauterine devices that do not adapt to the width of the uterine cavity, or with too long transverse arms, are likely to become embedded, if not expelled, and could perforate the uterine wall in the presence of asymmetrical forces. The anchor is highly visible in the fundal muscle in both pictures.

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