Study question

Can Cryo-loop increase the rate of intact embryos after thawing and clinical outcomes by complementing the weakness of Hemi-straw?

Summary of answer

Cryo-loop with high thermal conductivity significantly improved the rate of intact embryos after thawing and clinical outcomes compared to Hemi-straw.

What is known

To increase the success rate of vitrification technology, a carrier with high thermal conductivity that can minimize vitrification solution volumes is essential. With the Hemi-straw, it is difficult to adjust vitrification solution volumes, and thermal conductivity is affected by the external environment and the skill level of embryologists. On the other hand, Cryo-loop has ultra-rapid cooling rate (>20,000/min) even when it is directly exposed to LN2, and the volumes can be adjusted to the minimum volume (<1ul). In our study, we aimed to determine whether Cryo-loop can improve the survival rate and clinical outcomes of embryos compared to the Hemi-straw.

Study design, size, duration

The non-donor in the transfer cycles of vitrified-thawed embryos performed from January 2015 through January 2016 (n=208) was included in this study. A total of 101 patients underwent the procedure using Cryo-loop while 107 patients underwent the procedure using Hemi-straw.

Participants/materials, setting, methods

After day 3 embryos transfer, the surplus embryos were randomly vitrified with Cryo-loop or Hemi-straw. After thawing, the embryos were cultured in vitro for 24 hours to select the embryos showing subsequent development for transfer. The survival rate of the vitrified-thawed embryos was assessed with the proportion of the embryos that has intact blastomeres without any damage. After transfer, the pregnancy and implantation rates were compared between the Cryo-loop and Hemi-straw groups.

Main results and the role of chance

Patient characteristics were similar in the two groups. In each group, age (Cryo-loop: 35.7±3.9 vs. Hemi-straw: 35.5±3.4 P=0.38), endometrial thickness (9.6±2.1 vs.9.7±1.7mm p=0.40) and number of embryo transferred (1.9±0.4 vs. 1.9±0.3 p=0.17) did not show significant differences. However, the number of thawed embryos (3.0±1.3 vs. 5.1±2.1 p<0.001) revealed a significant difference. In the Cryo-loop group, the rate of intact embryos after thawing (77.9% vs. 62.4% p<0.001), the rate of subsequent development (60.5% vs. 31.6% p<0.036), pregnancy rate (56.4% vs. 42.1% p=0.038) and ongoing pregnancy rate (50.0% vs. 36.4% p=0.041) were all significantly higher than those in the Hemi-straw group.

Limitation, reasons for caution

Since our clinic mainly performs day 3 cryopreservation, they were only a small number of day 5 cryopreservation cases, and the results have not been compared.

Wider implication of the findings

The Cryo-loop group showed significantly higher rates of survival, pregnancy and subsequent development than that of Hemi-straw group. Cryo-loop can freeze a small number of embryos, and the survival rate of embryos is higher. Therefore, Cryo-loop can reduce unnecessary freezing of embryos and provide more chances of conception.