Hi there is no problem using flash at all. I have done that while doing an annual report for a hospital. So go ahead. Please ask the Surgeons, GP or the nurse first before proceeding.
You may one to include a another zoom lens of you have. A 70-200 maybe needed as you do not want to go to close for certain type of surgery
Last, I can tell you it is very GROSS if you see some certain surgery being perform. While I was shooting for a one of the surgery, my assistant vomited! Aiya very pai seh. So be prepared! And I mean REALLY prepared for blood gore!
Your flash is sufficient but I would highly recommend a stronger flash unit, maybe a hammer head flash. You should have another flash unit from another position as your secondary light to filling in. If you are in a theater, you may not be able to use bounch flash.
I would suggest you bring an assistant along to help out.
Unless this is a minor surgery, then your flash unit should be sufficient but please bring a small white board so you can bounch your flash. Curious, what type of surgical procedure?