Role of retention in avoiding expulsion of IUDs--measuring devices for basic research

320 medicated IUDs were removed from 292 women by means of the dynamometer. This instrument indicates the highest retention force (HRF) in newtons (N). Ages of the women ranged from 17-50 years. The IUDs were fitted in nulligravidae (56%), nulliparous with 1 or more abortions (13%), primiparous (18%), and multiparous (13%) women. The IUDs used were the ML Cu 250-standard, 7 Cu 200, T Cu 200-adapted, with a shortened transversal arm of 15-30 mm and T Cu 200 standard. With the low expulsion rate, the ML Cu 250-standard IUD needed 5.8-5.4N and the 7 Cu 200 3.8-2.6N. The adapted T Cu 200 IUD needed 2.6-1.6N, i.e., about 1N more thant the T Cu 200-standard with an HRF of 1.7-1.0N. As the dynamomoeter only indicated the highest retention force, a retentiograph was devised to demonstrate the different retention forces and phases of the various IUDs by characteristic curves. The possible connections between retention and expulsion of IUDs are discussed. The retention forces of the different IUDs compared are congruent with the published expulsion rates. The retentiograph showed new patterns to test IUDs before they demonstrate side effects in vitro.