Mechanism of perforation
Esposito et al postulated that two mechanisms of uterine perforation exist.
Immediate traumatic perforation
Later “secondary” perforation caused by gradual erosion through the myometrium
Confirmation of the latter mechanism has been achieved by serial computed tomography (CT) scans performed for nongynecologic purposes.
In the early days of IUDs, insertion instruments were rigid and so perforation was more likely as a consequence.16 An early version of the Birnberg bow had a perforation rate of one in 200 because of the rigidity of its inserter.
With the Progestasert device, there was documented evidence of a reduction in perforation rate when the inserter was made more flexible and a uterine sound included in the package.19 Nowadays, insertion tubes are made so that they are more flexible, but not so soft that they buckle easily. Other risk factors are use of a push-out insertion technique, or lack of an insertion tube, as was the case with the Dalkon Shield (Dalkon Corporation, Defiance, OH, USA).
Perforation can also happen with a uterine sound. Traditionally, metal sounds were used to measure the uterine cavity length. Plastic sounds are almost certainly safer as they are less rigid. Perforation of the uterus may occur when the uterus is being sounded, or a false track may be created which is then followed by the IUD.
The forces required to insert an IUD increase linearly with increasing inserter tube diameter.20 The force required to insert an IUD is generally 1.5–6.5 N. The forces generated physiologically within the uterus have been estimated to be theoretically as high as 50 N, sufficient to produce uterine perforation. This would explain the so-called “secondary” perforation as a consequence of embedment of a device. Except in cases of lactation or uterine atrophy (eg, due to long-term use of injectable progestins), it is thought that the forces required to cause perforation are higher than the forces needed to carry out insertion of an IUD