AIM: To investigate the relationship between radiographically and macroscopically well-defined carious lesions and the dentine-pulp complex with regard to: a) level of bacterial penetration, b) inflammatory status including the presence of hyperplastic pulp stroma; and c) formation of hard and/or ectopic connective tissue.

METHODOLOGY: The material comprised 68 untreated cavitated permanent teeth divided into well-defined radiographic categories based on the lesion-penetration depth: a) Deep lesions ( ≥¾ of the dentine thickness with a radio-dense zone separating the lesion from the pulp) and b) extremely deep lesions (the carious lesion penetrated the entire thickness of the dentine, without a radio-dense zone). After extraction, the teeth were processed for histology. The material was scored with regard to coronal break-down, macroscopic variables describing caries activity, and histological variables describing the dentine-pulp complex (bacteria, inflammatory infiltrate, partial pulp necrosis, hyperplastic changes, hard tissue/ectopic presence of connective tissue). Interrater agreement was assessed using Cohen´s kappa. Associations between variables were assessed using Pearson's χ2 or Fisher's Exact test. The effect size was reported by odds ratio (OR) and associated 95% confidence interval (CI). Level of significance was set to 5%.

RESULTS: There were significant associations between a closed environment (1-2 surfaces involved) and the presence of biofilm, retrograde demineralization and light-coloured demineralized dentine. Whereas radiographically defined deep lesions tended to have bacteria only in the primary dentine (P = 0.000, OR = 20.55, 95% CI [4.44, 107.89 ]), extremely deep carious lesions tended to have bacteria in contact with the pulpal tissue (P = 0.007, OR =6.84, 95% CI [2.00, 62.83]), presence of an inflammatory infiltrate (Fisher's Exact; P = 0.000); and partial pulp necrosis. Hyperplastic pulps were seen only in extremely deep lesions.

CONCLUSIONS: Unlike deep lesions, extremely deep carious lesions were often associated with severe pulp inflammation and infection. A radiographic threshold between deep and extremely deep lesions is suggested as indicator of the bacterial penetration level and the severity of the pulpal response prior to intervention.