[A prospective study to compare the efficiency of oocyte vitrification using closed or open devices]

Oocyte vitrification using an open device is thought to be a source of microbiological and chemical contaminations that can be avoided using a closed device. The principal purpose of this study was to compare the two vitrification protocols: closed and open system. The secondary aim was to study the effects of the storage in the vapor phase of nitrogen (VPN) on oocytes vitrified using an open system and to compare it to those of a storage in liquid nitrogen (LN).

Forty-four patients have been included in our study between November 2014 and May 2015. Two hundred and fourteen oocytes have been vitrified at germinal vesicle (GV), metaphase I (0PB) and metaphase II (1PB) stages. We vitrified 96 oocytes (59 GV/37 0PB) using a closed vitrification device and 118 oocytes (57 GV/31 0PB/30 1PB) using an open device. The vitrified oocytes were then stored either in LN or in VPN. The main outcome measures were the survival rate after warming (SR), meiosis resumption rate (MRR) and maturation rate (MR).

The global post-thaw SR was significantly higher for oocytes vitrified using an open system (93.2%) compared to those vitrified using a closed one (64.5%; P<0.001). On the contrary, there was no significant difference in terms of global MRR and MR (82.1% vs. 87.5% and 60.7% vs. 61.2% using closed and open system respectively). The SR, MRR and the MR were not significantly different when vitrified oocytes were stored in VPN or LN (91.6, 83.8, 64.5% vs. 93.9, 89.8, 59.1% respectively).

Taking into account the limits of our protocol, the open vitrification system remains the more efficient system. The use of sterile liquid nitrogen for oocyte vitrification and the subsequent storage in vapor phase of nitrogen could minimize the hypothetical risks of biological and chemical contaminations.