Using surgical tools like a scalpel is a grey area for piercers. Operating with these instruments, or any kind of anestheia, could be classified as practicing medicine. Without a medical license, a piercer who does this is technically committing assault on the person getting the implant.
So, an action by itself is not assault, but if you do the same action but make sure it doesn't hurt the patient, it is assault?
http://www.theverge.com/2012/8/8/3177438/cyborg-america-biohackers-grinders-body-hackers