External bending of cryodevice during vitrification leads to cryoprotectant cracks and damage to embryo blastomeres

Research question: Embryo blastomeres and the zona pellucida are occasionally damaged during vitrification; is this a result of crack -induced mechanical damage in the glass state, caused by external bending of the device? Design: A stereomicroscope was used to observe external bending -induced cracks in a cryoprotectant. Thereafter, 309 human cleavage -stage embryos derived from abnormally fertilized eggs were used to assess embryo damage under two external bending conditions: forward bending and backward bending, with three bending degrees applied. Three distinct embryo positions were used to examine the correlation between bending and embryo damage. Damage was assessed by looking at blastomere lysis rates, and overall rates of damaged and surviving embryos. Results: A series of parallel cracks were identified in the cryoprotectant used for external bending, which led to damage to the embryo blastomeres. Compared with forward bending and control, the embryos were found to be more easily damaged by backward bending, indicated by significantly higher blastomere lysis and embryo damage rates, and lower embryo survival rate of backward bending than forward bending (P < 0.001). The degree of embryo damage also increased as the degree of external forces increased. Embryo position correlated with degree of embryo damage. Conclusions: Cryoprotectant crack -induced damage was identified as the cause of embryo damage. Mechanical damage to the glass state occurs because of improper external bending of the cryodevice strip in liquid nitrogen during vitrification. To prevent damage, bending of the strip should be avoided and the embryos should be placed near the tip of the strip.