Behaviour of primary incisor caries: a micromechanical study

OBJECTIVES: To date, there has been no attempt to assess the mechanical properties of the entirety of a smooth-surface carious lesion in primary teeth, despite the fact that these lesions are not only common, but clinically challenging. Therefore, the aim of this study was to describe the hardness and modulus of elasticity across smooth surface lesions of primary incisors. STUDY DESIGN: An in vitro study of the micromechanical properties of primary incisors. MATERIALS AND METHODS: Carious primary incisor teeth were set in resin, sectioned and polished. A series of indentations using the ultra-micro-indentation system were conducted in fully hydrated carious and sound dentine from the minimally affected pulpal region towards the tooth surface. A single set of indentations were duplicated for sound dentine. RESULTS: Although the mechanical properties of the carious dentine varied between the test teeth, the median hardness of the surface, middle and inner (pulpal) region of the carious dentine was 0.01, 0.10 and 0.28 GPa, respectively. The modulus of elasticity of the surface, middle and inner (pulpal) carious dentine was 0.12, 2.16 and 5.98 GPa, respectively. The mechanical properties of the sound dentine varied less, and were consistent between the pulpal and surface regions. Examination of the individual series of indentations indicated that, although the majority of the test teeth showed a decrease in the mechanical properties from the 'unaffected dentine' to the surface of the lesion, in the last 300-500 microm, both the hardness and modulus of elasticity showed a dramatic increase. CONCLUSIONS: This study has confirmed that the carious process has a deleterious effect on the mechanical properties of dentine in primary incisors. This, in turn, increases the likelihood of restorative failure. However, the slight increase in mechanical properties seen at the surface of the carious lesion suggests an increase in mineral content.