Integrated approach to the study of biodegradation of composite materials for the restoration of hard dental tissues

Objective. To study the stress-strain state of demineralized tooth enamel after its impregnation with a low-viscosity composite in a long-term period of functioning in the oral cavity in a series of equivalent cyclic tests; to study the correlation of the experimental results with clinical observations of the treatment of enamel caries using a modified infiltration technique. Materials and methods. The parameters of laser holographic interferometry and atomic force microscopy, reflecting the timing, type and severity of deformation defects in samples of demineralized tooth enamel were analyzed in a series of equivalent cyclic tests. During the experiment the samples underwent classical infiltration with composite using ICON technology (main group) and a 2-stage enamel caries treatment technique, including carrying out a time-modified conditioning of classical infiltration and the final stage (stages) of enamel coating with a bioactive hybrid glass ionomer (comparison group). Results. The first signs of the deformation defects generation in demineralized enamel filtered on the classical method (the appearance of perifocal microcracks) were recorded in 11.2 % of the tested samples after 0.9 · 106 cycles (equivalent to 9 months of functioning of a tooth with treated enamel caries in the oral cavity), during further cyclic tests the percentage of samples with local defects increased progressively. Signs of a violation of the stress-strain state in the enamel after combined treatment (samples of the comparison group) were revealed in cycles corresponding to 2–3 years of being in an artificial environment of the oral cavity. Experimental data correlated with the clinical observations both in time and frequency of complications (secondary/recurrent caries) in the long term after treatment of enamel caries by various methods. Conclusions. In the process of equivalent cyclic tests, heterogeneous elastic-plastic deformations develop in the thickness of a section of demineralized enamel impregnated with a flowing composite and along its perifocal zones, most pronounced at the interface between intact and treated enamel, which leads to the development of local defects, delaminations, cracks, and destruction of the structurally heterogeneous environment of tooth enamel. According to the results of the comparative analysis, the most favorable results in terms of the timing of development, frequency and severity of deformation defects were obtained in samples of demineralized enamel after its treatment using a modified caries infiltration technique followed by coating with a bioactive hybrid glass ionomer.