Calculus (from Creek calcis for lime stone) aka tartar in dentistry refers to hardened plaque.  The process of calculus formation is not fully understood. However, it normally forms in the regions that are well irrigated with saliva e.g., the buccal side of upper molars and the lingual side of the bottom canines.  It is visible as a pale formation above the gum line. It is assumed that minerals contained in the gingival crevicular fluid (GCF) in the plaque and the saliva are precipitated on to the teeth, leading to the deposition of calculus. Any plaque which remains uncleaned for a long time will ultimately calcify and form a calculus. Although this precipitation kills the bacteria within the plaque, that is no relief.

The rough and hard surface of the newly built plaque becomes a good base for further plaque formation, and in due course another layer is formed above it. Calculus will form either above the gum line where it is visible and is called supra-gingival calculus, or below the gum line in the sulcus between the gum flesh and the tooth.  This calculus is called sub-gingival calculus.

Mineral (inorganic) content of calculus varies from 40 to 60% and comprises mostly of calcium phosphate in the form of octacalcium phosphate, hydroxyapatite, whitlockite, and brushite. In addition to this mineral content calculus also has inorganic content which is 85% cellular and 15% extracellular. The cellular content is primarily bacterial but may consist of some species of yeast and archaea. Non cellular organic matter in the calculus consists

Sublingual calculus build up is bad for the health of the gingiva. Irritation and inflammation of the gingiva caused by it is called gingivitis. In extreme cases, the connective tissue between the gum and the teeth gets damaged leading to gum separation and periodontitis. Calculus is considered the primary cause of gingivitis although it is not the only one.

Factors which influence the incidence of dental calculus are many and varied. They include ethnic background, gender, age, diet, host genetics, physical disabilities, systemic diseases e.g., diabetes, use of tobacco and drugs. It is logical that oral hygiene and access to dental care reduce the incidence of calculus. Calculus is best prevented by regular and thorough oral hygiene, and regular dental inspections, in addition to avoiding any factors which build up plaque.

Treatment of calculus is scaling and root planning by a dentist or an oral hygienist. Administration of anti-biotics may also be required in severe cases.