Dentin is the hard, calcified tissue that forms the bulk of a tooth’s structure, positioned beneath the enamel in the crown and beneath the cementum along the root surface. Unlike enamel — the hardest substance in the human body — dentin is permeated by thousands of microscopic channels called dentinal tubules, giving it both a degree of sensitivity and ongoing biological activity throughout the life of the tooth.

Composition and Structure

By weight, dentin consists of approximately 70% inorganic mineral (primarily hydroxyapatite crystals), 20% organic matrix (largely type I collagen), and 10% water. This composition places it in hardness between enamel and bone. The dentinal tubules extend radially from the central pulp chamber to the outer dentin surface, housing the cytoplasmic extensions of odontoblasts — specialized cells that continue to produce dentin throughout life and serve as the tissue’s primary defense and repair mechanism.

Types of Dentin

• Primary dentin — formed during tooth development prior to eruption; constitutes the majority of the tooth’s dentin volume
• Secondary dentin — deposited slowly and continuously after root formation is complete, gradually narrowing the pulp chamber with age
• Tertiary (reparative) dentin — rapidly produced by odontoblasts in direct response to injury, caries, or restorative procedures as a protective barrier
• Sclerotic dentin — results from mineral deposition within tubules in aging teeth or under mild chronic irritation, reducing tubule permeability

Clinical Significance

When the overlying enamel or cementum is lost — through caries, erosion, abrasion, or gingival recession — exposed dentinal tubules become a primary cause of dentin hypersensitivity. According to the widely accepted hydrodynamic theory, fluid shifts within the tubules stimulate nerve fibers at the pulp-dentin junction, producing sharp, transient pain in response to cold, heat, sweet stimuli, or air exposure.

In restorative dentistry, achieving reliable adhesion to dentin is more technically demanding than bonding to enamel, requiring careful management of tubule fluid and surface moisture. In cases of deep decay, the proximity of carious dentin to the pulp directly influences clinical decisions — ranging from indirect pulp capping with biocompatible materials to endodontic intervention when the pulp is compromised.

Preserving dentin through early caries detection, targeted remineralization, and conservative restorative technique is one of the most effective strategies for maintaining long-term tooth vitality and structural integrity.