Early Caries in an In Vivo Model: Structural and Nanomechanical Characterization

The utilization of rat models in cariology research has made substantial contributions to decipher mechanisms of caries formation and to develop preventive treatments. The existing rat models still have potential for improvement toward establishing a more accurate standard caries protocol to utilize in testing and/or developing new dental technologies. The current caries-scoring methods rely on optical microscopy-based techniques, which necessitates formation of highly advanced lesions. Moreover, models that facilitate the implementation of cariogenic bacteria by shifting the balance of oral flora through desalivation and/or antibiotic treatment create a nonnatural environment. Furthermore, there is a paucity of detailed structural and mechanical characterization on the resulting carious lesions. The purpose of this study was to develop a rat model that induces formation of mild carious lesions and to provide comprehensive structural and mechanical characterization. With this aim in mind, an in vivo model promoting progression of mild lesions was established with specific pathogen-free Sprague-Dawley rats. Cariogenic bacteria, Streptococcus mutans, was implemented into the oral flora without the use of antibiotics or desalivation surgery. During caries formation, progression of the infection was monitored by quantifying the relative abundance of S. mutans in oral flora with quantitative real-time polymerase chain reaction. A significant increase in colonization efficacy of S. mutans was detected during cariogenic challenge ( P < 0.01). The resulting carious lesions were analyzed by conventional light optical and scanning electron microscopy. A detailed structural and morphological characterization on fissure caries with different degrees of severity was provided. The changes in the morphology and demineralization state of the sound and carious tissues were quantified by energy-dispersive X-ray spectroscopy, and local mechanical properties were acquired with nanoindentation. The principles laid out in this work can be utilized in cariology research and developed into a standard protocol for future studies.