How do you clean out cavities that are subgingival?

 The problem with cleaning out cavities that are at least partially below the gingiva is that it is easy to cause bleeding when prepping the cavity for a filling . In my practice I employ ultradent cord to retract the gums away from the margin of my preparation. Often I need to use two cords... a smaller cord first and and slightly larger cord second. These cords are saturated with either hemodent or viscostat. Hemodent is often better because it doesn't get "brown" ferric sulfate infused into the gingiva and preparation. Hemodent (aluminum chloride) is a clear solution. 

Ocasionally I will remove some of the gingiva with a rotary diamond or a periodontal currette (a gingivictomy) because there is too much gingiva to be retracted by retraction cord alone. I find that ferric sulfate (Ultradent's Viscostat) will arrest the resulting bleeding that occurs due to my gingivectomy. 

Once the gingiva is pushed away from the margin , then I am usually able to clean out the decay without injuring the gingiva because I use smaller round burs (#1,2 or 4) to gently clean out the cavity. If there is any bleeding usually rubbing the area with a pledget of cotton dabbed in Hemodent will stop any small amount of further bleeding.

Another method of getting the gingiva away from the margin is using either a soft tissue laser or electro surgery.  Both will work well and many dentists will use them instead of retraction cord, but both require the dentist to own one of these devices and may require additional time to set them up for the procedure. All dental offices have retraction cord and it is a universal and inexpensive method that can be  used to "retract gingiva" away from the margins of a filling.