Annotated Bibliography

Vacuum extraction as an effective alternative to forceps delivery began in the late 1950s when  T Malmström of Gothenburg introduced his vacuum delivery device1. The main feature of the Malmström device was the unique design of the cup which has formed the basis for all subsequent metal and rigid plastic cup modifications (the so-called ‘mushroom’ cups). However, the initial enthusiasm that greeted Malmström’s extractor was tempered by reports of a tendency for the cup to detach from the fetal scalp when traction was directed obliquely.

GC Bird, working initially in Kenya and then Australia, recognised that a weakness of the Malmström design was the common suction and traction port situated at the centre of the dome of the cup. He identified two major problems. Firstly, because the suction tube was attached to the cup by a metal nozzle, oblique traction exerted a leverage effect at the end of the nozzle that tended to lift the edge of the cup resulting in a detachment. Secondly, the central attachment of the tube to the cup limited the ability to place the cup correctly on the fetal scalp in many cases particularly in deflexed occipito-posterior and asynclitic occipito-transverse positions of the fetal head.

Bird addressed the problem of cup detachment by separating the suction from the traction system to avoid the leverage factor and thereby reduce the tendency for the cup to detach2. This modification was called the ‘anterior cup’. Bird realised however that the manoeuvrability of this cup design was still limited by the dome-attached suction tube. He therefore shifted the attachment of the suction pipe to the cup’s lateral wall so that the tubing, being in the same plane as the body of the cup, would not impede the cup’s movement in the confined space of the lower birth canal3. This design is known as the ‘posterior cup’ and is recommended for use in posterior (OP) and transverse (OT) positions of the occiput.