It is a structured resilient yellow-grayish substance that adheres tenaciously to the tooth surfaces including removable and fixed restorations. It is an organised bio-film that contains primarily bacteria in a matrix of glycoproteins and extracellular polysaccharides. This matrix makes it impossible to remove the plaque by rinsing or using sprays. Materia alba is similar to plaque but it lacks the organized structure of plaque and hence easily displaced with rinses and sprays. When this plaque gets mineralized it forms into calculus. Calculus is thus the hard deposit formed by mineralization usually covered by a layer of unmineralized plaque.
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