Dental stiches and mikveh - Igrot Moshe Yoreh Deah 2:87

With regard to whether or not stitches as a result of oral surgery present a problem of chatzitza for a woman who has to go to the mikveh, Rav Moshe rules that since the stitches cannot be seen and can only be manipulated via a fine instrument they are considered to be in a מקום בלוע and thus do not need to have the water reach them and thus are not considered to be a chatzitza.