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What is Gingival recession and explain in detail?
Gingival recession, also known as receding gums, is the exposure in the roots of the teeth caused by a loss of gum tissue and/or retraction of the gingival margin from the crown of the teeth. Gum recession is a common problem in adults over the age of 40, but it may also occur starting in adolescence, or around the age of 10. It may exist with or without concomitant decrease in crown-to-root ratio (recession of alveolar bone).

Classification
Various classifications have been proposed to classify gingival recession, Miller’s classification system being the one that is most widely followed. Many cases which are encountered in daily clinical practice cannot be classified according to the criteria of the present classification systems. Kumar & Masamattis classification system gives a comprehensive depiction of recession defect that can be used to include cases that cannot be classified according to present classifications. A separate classification system for palatal recessions (PR) has been given. A new comprehensive classification system classifies recession on the basis of the position of interdental papilla and buccal/lingual/palatal recessions. Kumar & Masamattis classification system tries to overcome the limitations of Millers classification.

Causes
There are many possible causes for gingival recession:

By far the most common cause is gum disease (periodontal disease).
Overaggressive brushing also causes gum recession. One systemic review of the literature concluded that "The data to support or refute the association between tooth brushing and gingival recession are inconclusive," although aggressive or forceful brushing was not specifically addressed. A subsequent study found horizontal tooth brushing technique (versus Bass technique or circular methods), medium-hardness toothbrush use and brushing only once daily were associated with gingival recession.
Improper flossing (i.e., flossing too roughly or aggressively) which may cut into the gums.
Hereditary thin, fragile or insufficient gingival tissue predisposes to gingival recession.
Dipping tobacco, which affects the mucous membrane lining in the mouth and will cause receding gums over time
Self-inflicted trauma, such as habits like digging a fingernail or pencil into the gum. This type of recession more commonly associated with children and persons with psychiatric disorders.
Scurvy (lack of dietary vitamin C)
Acute necrotizing ulcerative gingivitis
Abnormal tooth position, such as tooth crowding, giving inadequate cover of one or more teeth by the jaw bone.
Piercings in the lip or tongue that wear away the gum by rubbing against it.
Intentional gingival retraction. For example, the adult tooth may not grow out of the gum, and to remedy this, a procedure called an exposure is done. It involves the gum tissue being cut open to allow the adult tooth to grow out. This is a less common cause of gum recession.

Symptoms
Gum recession is generally not an acute condition. In most cases, receding of gums is a progressive condition that occurs gradually over the years. This is one reason that it is common over the age of 40