Increased ligamentous laxity is associated with pelvic floor distension in pregnant women. This considered, it may also be related to the risk of obstetric anal sphincter injury (OASI). Our objective was to assess the association among increased ligamentous laxity, perineal tear severity, and OASI occurrence.
This is a prospective study. We assessed ligamentous laxity between the 36th week of pregnancy and the onset of labor, by measuring the passive extension of the nondominant index finger for a 0.26 N.m torque applied to the second metacarpophalangeal joint (MCP laxity). We collected perineal tear occurrence and classification (Royal College of Obstetricians and Gynecologists). We investigated the MCP laxity distribution according to the stage of perineal tears, and then we looked for a predictive level of MCP laxity for OASI.
A total of 272 of the 300 pregnant women included had a vaginal delivery and were considered for the analysis. Mean age was 29 years, mean body mass index was 24.5 kg/m ² and 39.2% of women were nulliparous. We reported 12 cases of OASI. MCP laxity significantly increased with the stage of perineal tears from 58° in stage 0 to 69° and 66° for stages 3a and 3b respectively. An MCP laxity higher than 64° was associated with OASI with sensitivity of 75%, specificity of 56%, and an area under the curve of 0.65.
Increased ligamentous laxity seems associated with OASI occurrence, which is the opposite of the initial hypothesis. This suggests that the stiffest tissues might be at a lower risk of injury.