Laboratory comparison of modern vacuum extractors.

The Malmström, Soft Cup, and Mity-Vac vacuum extractors were compared in the laboratory using a tensile strength tester and a fetal cephalic model. In the test system, a linear relationship emerged between applied vacuum and maximal tractive force for all of the vacuum extractors. At their recommended working pressures, they allowed a maximum of 18-20 kg of tractive force to assist in a delivery. Calculation of the distribution of this force per unit area would appear to favor the Soft Cup. At high vacuums, all showed large amounts of tractive force and high forces per unit area. The data clearly demonstrate that high vacuum would have potentially adverse effects on the fetus and thus should be avoided.