Noncollagenous structures: the ‘springs' within the human dentine

Noncollagenous proteins (NCP) have long been suggested to regulate the mechanical behavior of mineralized tissues by connecting collagen fibrils and regulating hydration, however little is known about how these structures might affect the mechanical properties and microstrucutre of dentin. Objective: This study sough to gain insights into the role of NCP on the mechanical behaviour and microstructure of dentin. Methods: Dentin specimens were cut, polished, half masked and demineralized with 10 vol% citric acid for 2 min. The normal and demineralized side of all specimens was indented in water using an UMIS system and imaged using SEM, following dehydration and coating. Indentations using 5mN loads were performed on specimen's surface to determine values of elastic modulus (E) and hardness (H). Additionally, creep and recovery behavior were determined using a holding time of 30 sec at maximum (5mN) and minimum (1 mN) loads. Subsequently, all specimens were treated with 1 mg/mL TPCK-treated Trypsin containing 0.2M NH4HCO3 (pH 7.9) for 48 h at 37°C to remove proteoglycans of the matrix. Specimens were rinsed with DI-water, indentation and SEM analyses were repeated. Data was analysed using ANOVA and Tukey with a significance level of 5%. Results: Normal dentin had significantly higher E (23.3±4.1 GPa) and H (0.7±0.09 GPa) than demineralized dentin, which yielded E=5.0±0.7 and H=0.3±0.07 GPa (p.<01). The trypsin digestion decreased hardness of normal and demineralized dentin significantly to values of 0.6±0.14 GPa and 0.1±0.06 GPa respectively (p<.01). Further, NCP removal increased the relative creep displacement and reduced backcreep recovery dramatically. SEM showed that NCP removal exposed the collagenous matrix and changed the microstructure of fibrils substantially. Conclusion: NCP contribute significantly for the architecture of the organic matrix of dentin and might act like springs, preventing dislocation of collagen fibrils under load and bringing fibrils together after loads are removed.