ICSI is a very effective method to fertilise mature eggs when certain types of male factor infertility has been diagnosed as long as motile sperm are present. ICSI involves the injection of a single sperm into a single egg rather than just mixing sperm and eggs together.
Not all eggs are mature and can be injected for ICSI. There is also a small risk that not every egg that is injected survives this technique as egg quality can vary e.g. if 9 eggs are injected 8 might survive.
ICSI is an invasive technique and may also use sperm that would not otherwise be able to fertilise an egg. There have now been many thousands of babies born after IVF with ICSI with follow-up after birth. Thus far, there has not been demonstrated to be any clear increased risk for birth defects as compared to IVF with conventional insemination.
In some cases ICSI may lead to transmission of genetic problems that might cause infertility in male offspring, or could possibly be associated with other (at this time unknown) medical problems in the child.