Assisted sperm fusion insemination improves fertilization rates and increases usable embryos for transfer: a clinical sibling-oocyte study.

To evaluate the impact of assisted sperm fusion insemination (ASFI), a microinsemination method using sperm bound to the zona pellucida (ZP-sperm) without penetrating the oocyte membrane, compared with conventional intracytoplasmic sperm injection (C-ICSI), on fertilization and embryonic development. Prospective clinical sibling-oocyte study. A total of 197 oocytes from 24 patients who underwent 35 oocyte retrieval cycles from January 2023 to April 2024 were analyzed. Patients who underwent retrieval of both, at least 2 metaphase II (MII) oocytes and at least 1 immature or degenerated oocyte, were recruited. Metaphase II oocytes were alternately allocated to 2 groups: ASFI and C-ICSI groups. To obtain ZP-sperm for ASFI, immature or degenerated oocytes were incubated with 10,000 motile sperm for 3 hours. After harvesting the ZP-sperm, it was pressed onto the membrane of an MII oocyte for 10 seconds in the ASFI group. Conventional intracytoplasmic sperm injection was performed conventionally in the C-ICSI group. The rates of 2 pronuclei (2PN), degeneration, blastocyst formation, and usable embryos, defined as the total number of transferred or cryopreserved embryos divided by the number of MII oocytes, were compared between the 2 groups. The 2PN rate of the ASFI group was 88.0% (73/83), which was significantly higher than that of the C-ICSI group (70.2% [80/114]). In addition, a significantly lower degeneration rate was observed in the ASFI group (0% [0/83]) than in the C-ICSI group (8.8% [10/114]). The blastocyst formation rate was equivalent in the 2 groups (ASFI group, 63.9% [39/61]; C-ICSI group, 62.0% [44/71]). However, the usable embryo rate was significantly higher in the ASFI group (45.8% [38/83]) than in the C-ICSI group (28.1% [32/114]). The ASFI group yielded significantly higher 2PN and lower degeneration rates than the C-ICSI group. Consequently, the ASFI group experienced a higher number of embryos usable for implantation, although there was no significant difference in the blastocyst formation rate between the 2 groups. Further studies with a larger number of cases will be needed for more general application of these findings.