'Bowing' forces with IUD inserters in vitro: relevance to difficult IUD insertions.

The inserter tubes of the more modern copper bearing IUDs are flexible enough to give ('bow') when there is obstruction to the device. The MLCu 250, Copper 7 and Nova T inserters will bow considerably when forces of 1-3 N are exerted and the device obstructed up to 0.75 cm from its proximal end. This suggests that these devices are unlikely to cause cervical damage where their passage into the uterine cavity is impeded by cervical factors, provided the insertion attempt is discontinued when bowing of more than 2 cm off-centre is reached. However this does not apply to obstructions past the internal cervical os. In research circumstances great care should be used as it rapidly becomes possible to exert enough force to produce damage to the uterine muscle.