The aim of this study was to analyse influence of the fetal head position and the type of instrument used (forceps, vacuum, OdonAssist™) on perineal deformation, during simulated vaginal deliveries monitored by stereophotogrammetry. An exploratory study was conducted using mannequins simulating vaginal births. Fifty simulated deliveries were performed with different fetal head positions and instruments: Pajot's forceps, Kiwi-vacuum, and OdonAssist™. Stereophotogrammetry measured perineal deformations called equivalent strains Eeq. Eeq during spontaneous deliveries were minimal, average, and maximal at 0.8%, 5.8%, and 11.6%, respectively. Assisted vaginal births (AVB) showed slightly higher strains compared to spontaneous deliveries. In the occipito-posterior position (OP), strains were significantly higher (p<0.05). Equivalent strain Eeq trend towards higher mean (6.7±0.6%) and maximum (12.3±0.8%) with forceps compared to other methods in the occiput anterior position (OA), especially against spontaneous (mean: 5.3±0.5% and max: 9.9±0.6; p=0.06), vacuum AVB (mean: 4.6±1.0% and max: 9.4±0.1%, p=0.06) and OdonAssist™ AVB (mean: 3.8±0.9% and max: 8.8±1.0%, p=0.06). Forceps induced greater strains compared to vacuum and OdonAssits™. In OP position, OdonAssist™ tend to lower mean equivalent strain Eeq compared to spontaneous vaginal deliveries (4.9±0.6% vs 6.4±0.5%, p=0.06). Results confirmed increased mechanical perineal stress for OP fetal position and for forceps assisted deliveries compared to others devices. The OdonAssist™ may offer a less invasive alternative, reducing perineal strains. Stereophotogrammetry provides valuable data on the mechanical effects of childbirth.
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