ICSI is a procedure where a single sperm is injected into an egg. It is often used when the infertility comes from the male, perhaps there is low sperm motility or a low sperm concentration. ICSI can be used in these circumstances to increase the number of eggs that are successfully fertilised.
Using ICSI with Pre-implantation genetic diagnosis (PGD) is common practice. It is done to reduce the chances of contamination (extraneous DNA, perhaps from the lab environment).
I asked my contact at CRGH (Karen Fordham) to check what I had written in this post and she told me that ICSI is used for all cases of PGD in some PGD centres but at CRGH, they sometimes use sing (without ICSI) for fertilisation for PGD for chromosome abnormalities, where such contamination would not be a concern.
Here is a video of ICSI being perfomed. It shows a sperm being injected into an egg.
What is ICSI?