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The objective of this study was to investigate the association between the trigonometric properties of episiotomy in operative vaginal delivery (OVD) and obstetric anal sphincter injuries (OASIS).

The study included 72 primiparous women who had an OVD and episiotomy. Cases (n = 36) had sustained OASIS at birth, while controls (n = 36) had not. The groups were matched for instrumental delivery. The episiotomy scar was identified and its trigonometric characteristics were measured at 8-12 weeks postpartum. Data were analysed using conditional logistic analysis.

The angle of episiotomy behaves as a factor associated with anal sphincter injury, so women with a mediolateral episiotomy and an angle greater than 20A degrees have an 87 % less risk of having an OASIS (odds ratio 0.13, 95 % confidence interval 0.03-0.58). The study showed that scarred episiotomies at 8-12 weeks after OVD with an angle a parts per thousand currency signaEuro parts per thousand 20A degrees, depth and distance between the episiotomy and anus a parts per thousand currency signaEuro parts per thousand 15 mm, total upper triangle perimeter a parts per thousand currency signaEuro parts per thousand 75 mm, para-anal triangle perimeter a parts per thousand currency signaEuro parts per thousand 15 mm and areas between scar and midline a parts per thousand currency signaEuro parts per thousand 250 mm(2) were significantly associated with higher risk of OASIS.

When a mediolateral episiotomy is performed in OVD the technique has a strong effect on the occurrence of OASIS. Additional research is needed to determine if the optimal technique for mediolateral episiotomies produces less OASIS than deferring the performance of episiotomy.