Contribution of Proteoglycans to the Fracture Mechanics of Dentin Matrices

Objectives: Small leucine rich Proteoglycans (PGs) regulate dentin formation. PGs also regulate the ultrastructure and mechanical properties of the mature dentin. The aim of the study was to determine the contribution of PGs to the apparent fracture toughness of the dentin matrix by enzymatically assisted selective removal of glycosaminoglycans chains (GAGs) and core protein. We utilized a trouser tear test loading Mode III to determine the tear energy required to tear the dentin matrix.
Methods: Sound human third molars were sectioned into standardized micron size specimens (2 x 6 x 0.45 mm) from coronal (n = 15) and radicular (n= 15) dentin. Beams were demineralized in 0.5M EDTA (14 days) and a central notch (4 mm) was prepared. Depletion of GAGs and PGs (core protein + GAGs) followed enzymatic digestion protocols using chondroitinase ABC (c-ABC) and MMP-3, respectively. The incubation solutions were 0.2U/ml of c-ABC in 0.1M Tris Acetate (pH=7.2) and 1µg/ml of MMP-3 in 100Mm Tris (pH=7.2), 0.15M NaCl, 5mM CaCl₂, 0.05% Brij 35 and 0.02%NaN₃ for 48 h at 37°C. Control groups were incubated in Tris Acetate buffer (pH=7.2). Specimens were attached to a fixture and tested in a universal testing machine (cross-speed 1mm/min). The energy to tear (kJ/m²) was calculated using the critical load from the load displacement curve and the thickness of the specimen. Data were statistically analyzed using two-way ANOVA and post-hoc Games-Howell test (α=0.05).
Results: A statistically significant interaction was observed between the study factors (dentin and digestion, p= 0.016). Regardless of the type of dentin, depletion of PGs with MMP-3 resulted in a significantly lower tear energy of the dentin matrix (p=0.007).
Conclusions: Findings indicate that PGs bound to the dentin extracellular contribute to the resistance to tearing of the dentin matrix, possibly influencing crack growth propagation.