Periodontal disease occurs when plaque (the sticky stuff on your teeth) is not consistently and adequately removed. Over time, this plaque will become embedded with minerals from your saliva along with food debris to harden and create something called Calculus (Tarter).
Calculus acts as a kind of safe-house for bacteria against your natural immunity, and allows these bacteria to proliferate.
When certain types of bacteria reach a high enough level, your body starts attacking that area aggressively. Unfortunately, due to the amount of calculus, and bacteria, your body is unable to remove enough of the bacteria to bring your gum tissue back to health. As it continue to attack over and over again, the bone and gum tissue holding the tooth in place starts to deteriorate, and break down. Eventually this leads to a loss of the structures holding the teeth in place, and the tooth is eventually lost.
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The Periodontal Pocket is a space around your teeth. Much like a moat around a castle. This is where the aggressive bacteria reside. If this pocket is not kept clean with brushing and flossing, the bacteria start to increase in numbers. As they proliferate, your body starts to actively attack the area.
Unfortunately, due to the calculus which acts as a kind of bunker for the bacteria, your immune system ends up damaging the area around the calculus. This is the active form of periodontal disease. Patient's often have no discomfort, other than when they brush or floss the area. However, if there is bleeding in the area during brushing or flossing....there is Disease.
As your body continue to attack, the Periodontal Pocket continue to deepen and destroy the bone around the tooth. Once the pockets have deepen beyond 4mm, your floss and the bristles of your brush can no longer reach bottom. Despite how often you brush and floss, you are not capable of cleaning the base of this pocket. If you hear 5's and higher when you are having your periodontal assessment done, it generally means that the gum tissue is starting to break down and periodontitis is setting in.
Check out this great video explaining periodontal disease