A randomized study of two cups for vacuum extraction.

O'Neil designed a set of suction cups which, because of more efficient lateral pull and greater maneuverability, were claimed to be more efficient and perinatally safer than the Malmström model. These claims were checked by comparing the two instruments in a randomized controlled trial comprising 410 attempted elective extractions. The two instrument groups were comparable at entry and the operators had a similar degree of experience, the sole inter-group difference being the 0.5 cm larger diameter of the O'Neil cups. No significant differences were found between the two types of instrument as to failure rate, incidence of correct cup positioning, and capacity of eliciting internal rotation nor was there a significant divergence in neonatal safety. In a few cases the perineum was lacerated by the traction cord affixed to the O'Neil instrument. The results of this study indicate that the relative advantages and disadvantages of the two cup models are unremarkable.