Caries Lesion Classification and Histopathology

Caries lesions are classified by where they occur, how far they have advanced, and whether they are actively progressing, and they are read histologically through characteristic zones of mineral change. This topic links the clinical appearance of a lesion to the microscopic structure that underlies it, from the earliest subsurface enamel lesion to dentine caries with its layered zones.

Definition

Caries lesion classification is the systematic description of a lesion by site, depth, and activity, while caries histopathology is the microscopic pattern of demineralization and tissue change—the subsurface enamel lesion and the zoned structure of carious dentine—that corresponds to that progression.

Scope

The topic covers the principal axes used to describe lesions—surface site, depth, and activity—and the histopathological features of carious enamel and dentine, including the subsurface enamel lesion and the recognized zones within carious dentine. It frames classification as a way of reading the disease's progress and is a reference description of structure, not a clinical detection protocol or treatment guide.

Core questions

• What dimensions—site, depth, activity—are used to classify a carious lesion?
• Why does early enamel caries form a subsurface lesion beneath a relatively intact surface?
• What are the histological zones of carious dentine and what do they represent?
• How does the microscopic structure of a lesion relate to whether it is active or arrested?
• How does histopathology connect the visible lesion to the underlying disease process?

Key concepts

• Lesion site (pit-and-fissure, smooth-surface, root)
• Non-cavitated versus cavitated lesion
• Active versus arrested lesion
• Subsurface enamel lesion with surface zone
• Body of the lesion and translucent/dark zones in enamel
• Zones of carious dentine (infected and affected dentine)
• Histopathology as a record of the demineralization-remineralization balance

Mechanisms

Histologically, early enamel caries appears as a subsurface lesion: acid diffuses through the porosities of the surface enamel and demineralizes the underlying tissue, producing a body of the lesion beneath a comparatively well-mineralized surface zone, with intermediate dark and translucent zones reflecting partial demineralization and remineralization. This structure is itself evidence that the process is dynamic, alternating between mineral loss and partial recovery (Featherstone 2004). As the lesion enters dentine, classic descriptions distinguish an outer, heavily demineralized and bacterially invaded layer from a deeper, partially demineralized but less bacterially penetrated layer, often termed infected and affected dentine. Because caries is fundamentally the histological consequence of biofilm activity at the surface, the microscopic appearance of enamel and dentine caries is read as the imprint of that biofilm-driven demineralization (Kidd & Fejerskov 2004). Classification by activity—whether a lesion is progressing or arrested—maps onto these histological features and onto the dentine-pulp complex's responses to the advancing front (Mjör 1995; Pitts 2017).

Clinical relevance

Reading lesions by site, depth, and activity, and understanding their histology, clarifies why a non-cavitated lesion differs fundamentally from a cavity and why activity, not just depth, describes the disease. This topic is a structural and descriptive reference; it does not prescribe how individual lesions should be detected, staged, or treated in practice.

History

Histological study of the subsurface enamel lesion and of zoned carious dentine, together with later reframing of caries as a dynamic process, shifted classification away from describing only the size of a cavity toward describing lesion activity and the biofilm-driven structure beneath the surface (Kidd & Fejerskov 2004; Featherstone 2004).

Debates

How sharply can infected and affected dentine be distinguished?

The classic two-layer description of carious dentine—an outer infected layer and an inner affected layer—has guided thinking about lesion structure, but the boundary is a continuum rather than a sharp line, and how cleanly the layers can be separated histologically remains a matter of interpretation.

Key figures

• Edwina A. M. Kidd
• Ole Fejerskov
• Ivar A. Mjör

Seminal works

• kidd-fejerskov-2004
• featherstone-2004

Frequently asked questions

Why does early enamel decay sit below the surface instead of on it?

Acid diffuses into the porous enamel and dissolves mineral beneath a relatively intact surface layer, producing a subsurface lesion; this is why early caries can be present histologically before any surface breakdown is visible.

What is the difference between infected and affected dentine?

Classic descriptions distinguish an outer, heavily demineralized and bacterially invaded layer of carious dentine from a deeper, partially demineralized layer with fewer bacteria; the two grade into one another rather than being separated by a sharp boundary.