Fracture or breakage that occurs within a single material (restorative material, luting cement, or native tooth hard tissue) rather than at the interface between two bonded materials; the failure surface traverses the bulk of the material.

Principle

Cohesive failure indicates that the internal tensile or shear strength, toughness, or fatigue resistance of the material or tissue is lower than applied or accumulated stresses, regardless of adhesive quality at interfaces.

Demonstration

Illustrative scenario: a ceramic inlay develops an internal chip under occlusal load while the cement layer and tooth margin remain adherent. Recognition: fractured pieces show material-internal fracture patterns and adhesive residues remain on both bonded surfaces. Action: evaluate material selection, occlusal scheme and parafunction; repair or replace the restoration and address contributing stresses. Consequence: loss of structural integrity of the restoration or tooth, potential propagation to larger fractures and need for restorative replacement or more extensive coverage.

Misapplication

Labeling any detachment or marginal gap as cohesive failure when the fracture actually originated at the interface (adhesive failure). The error reverses locus of weakness; correction requires microscopic or visual analysis of the fracture face to identify whether material or interface failed.

Consequence

Causal chain: intrinsic material weakness or overload → bulk material fracture → compromised occlusal anatomy and function → increased plaque retention and risk of secondary complications; clinically often necessitates repair or full replacement rather than simple re-bonding.

Reversal

In layered restorations, an apparent cohesive fracture of a superficial layer may be driven by subsurface adhesive defects or residual stresses; conversely, some cohesive microcracking without function loss may be monitored rather than immediately treated.

Boundary

Clearly within: a restoration with fracture lines entirely through the restoration material leaving adhesive layers attached to both adherends. Boundary case: failure beginning inside the cement and propagating to the interface producing both cohesive and adhesive features. Clearly outside: pure adhesive detachment at the interface with intact bulk materials.

Semantic Tension

Choosing materials with higher intrinsic strength (which may be more brittle or require more aggressive preparation) versus materials or techniques that conserve tooth structure and tolerate lower strength through reinforcement or coverage strategies.

Synthesis

Cohesive failure pinpoints which object lacks sufficient intrinsic resistance (restorative material or tooth); treatment focuses on replacing or reinforcing that material and mitigating the stress regime rather than changing interfacial bonding protocols.