Effect of angular traction on the performance of modern vacuum extractors.

ABSTRACT

OBJECTIVE: Our objective was to describe the effect of off-axis traction on the performance of modern vacuum extractors. STUDY DESIGN: Eight vacuum extractors were examined in the laboratory with a force indicator and fetal cephalic model. Devices evaluated included the 6 cm Malmstrom, Mity-Vac, M-Type, O'Neil, Posterior, Silc, Tender-Touch, and silicone elastomer. Maximal tractive force (pop-off) was measured for each device at 10-degrees increments from the vertical. RESULTS: Multivariate regression analysis described the best model as follows Maximal tractive force = Constant + Angle + Vacuum (p < 0.05). The partial regression coefficients for angle were negative in all devices except the Posterior cup. At increasing angles of off-axis traction, maximal tractive force decreased in the following order Silc, silicone elastomer, Tender-Touch, M-Type, Mity-Vac, O'Neill, Malmstrom, and Posterior. CONCLUSIONS: Application of oblique traction resulted in a linear decline in maximal tractive force. An understanding of in vitro performance may allow tailoring of cup selection to the clinical situation.