Surgery
A surgical modification in this context tends to mean a procedure that is far more invasive than piercing, tattooing or scarification. Examples include; implants, tongue splitting, subincision, castration and amputations.
In an ideal world, these procedures would be offered by doctors, but unfortunately there is a great deal of pressure on doctors stopping them from offering unconventional procedures. As such, most of the time surgical modifications are left in the hands of cutters, or are self-done.
Surgical procedures are rarely legal for the untrained and unlicensed public to perform "commercially", although some procedures such as implants currently fall in a grey area. When performed non-commercially, further procedures may also be grey. Self-done procedures are almost always legal, although they can cause other problems like (unjustified) psychiatric commitment.
Non-doctors performing these modifications face the possibility of being charged with practicing medicine without a license.