In dental tissues, two types of pathological dentin have been identified after the adverse effects due to either slow dental caries or rapid pulp exposure. In carious-exposed teeth, odontoblasts and cells from the sub-odontoblastic Hoehl’s layer were implicated in the formation of reactionary dentin. Beneath calciotraumatic lines, indirect capping induced the formation of bone-like (osteo) dentin. Deeper lesions combined with pulp exposure contributed to the construction of reparative dentin by pulp cells. Indirect capping produced reactionary dentin, while direct pulp capping led to the formation of reparative dentin by a dentinal bridge, closing almost the whole pulp exposure.
Since the pioneering work of Hermann (Hermann 1930), calcium hydroxide [Ca(OH)2] is recognized as a beneficial pulp capping agent in dentistry. The high alkaline pH causes a controlled wound and subsequently a scar at the surface of the exposed pulp. Reparative cells are recruited in the central part of the pulp. The high pH (pH 12) constitutes an important parameter for the biological properties of Ca(OH)2. Inflammatory cells migrate towards the wounded area. At the border located between the necrotic and vital tissues, treatment with Dycal induces the necrosis of a limited area. Adjacent to the necrotic zone, a reparative dentinal bridge starts to be formed (Tronstad 1974).
The initial reaction of Ca(OH)2 on the dental pulp is vascular and associated with cell migration, proliferation and liquefaction necrosis (necrotic pulp) (Schroder 1985). These conditions are required to promote the biomaterial mineralization and cellular differentiation. Matrix vesicles initiate the formation and diffusion of mineralized granules in the newly formed collagen layer. Secondary odontoblasts differentiate, including the so-called Hoehl’s cells and mesenchymal stem cells/pulp progenitors. After capping, crystallite structures are observed at the interface between the superficial necrotic zone and the underlying pulp tissue (Yoshiba et al. 1996). They are positively immunostained for fibronectin and still remain positive a few days after capping, with corkscrew fiber-like structures visible between the cell bodies (Yoshiba et al. 1996). After Ca(OH)2 implantation for longer periods of time, 89 % of the dentin bridges display tunnel defects, failing to provide a hermetic seal against infection to the underlying pulp. Due to microleakage, after 6 months, most of the Ca(OH)2 capping material disintegrates and disappears (Cox et al. 1996).
Similar molecular events have been identified when other Ca(OH)2 capping agents were used (Simon et al. 2008). After mild necrosis, the cells proliferate. At day 5, cells implicated in an extracellular matrix formation express the cytoskeletal intermediary filament nestin. Just beneath the necrotic area, the cells are immuno-positive for osteopontin (OPN). Of note, OPN is implicated both in the control of inflammation and triggers the initiation of the pulp reparative process (Kuratate et al. 2008). Dentin matrix protein-1 (DMP-1) is one of the dentin noncollagenous phosphorylated extracellular matrix proteins (SIBLINGs) involved in the regulation of mineralization. DMP-1 induces the cytodifferentiation of dental pulp stem cells, becoming odontoblast-like cells (Lourenço Neto et al. 2015; Almushayt et al. 2006).
Capping with Ca(OH)2 is apparently the most efficient material utilized in direct reparative dentin formation (Decup et al. 2000; Andelin et al. 2003). Implantation with Ca(OH)2 revealed positive immunostaining for tenascin (TN) and fibronectin (FN). The reparative dentinal bridge was also immunostained for bone sialoprotein (BSP). After capping the pulp of the first maxillary molar, the pulp exposure is occluded by a bridge formed in 30 days (Decup et al. 2000; Andelin et al. 2003).
Hydroxide-containing pulp capping agent is widely used by dental practitioners, and Dycal appears to be the capping material most frequently employed by clinicians. In addition to the available Ca(OH)2 liners, recent studies shed lights on the possibility that self-assembling peptide hydrogels may contribute to support the differentiation of dental pulp stem cells (Li et al. 2014). As new tools for tissue engineering, hydrogels constitute injectable scaffolds, and cell and bioactive molecule carriers (Cavalcanti et al. 2013; Goldberg et al. 2015).
A novel protein hydrogel forming a biodegradable cavity liner (in this report named Hydrogel) was developed, which has the potential to be used as a pulp capping agent. The chemical constituents of Hydrogel are purified protein, the bovine serum albumin (BSA), and glutaraldehyde (GA), a cross-linking agent. When activated by mixing and dispensed to target tissue sites, GA covalently cross-links the surface-proteins of the patient’s tissue to BSA. The US Patent Application indicates the composition of BSA/GA, and it approves its sealing and regenerative features for tooth pulp. This material constitutes a resorbing and curable wound dressing and/or a biodegradable adhesive in dental restorative and endodontic procedures (Angeletakis 2014). It also contributes to the formation of a barrier against acidic monomers, namely to the etching agents used in dentistry. However, the mechanisms sustaining how hydrogels are acting and their molecular targets have not yet been elucidated.
The aim of this in vivo investigation was to compare the effects of Hydrogel, a bovine serum albumin (BSA)/glutaraldehyde, and Dycal, a calcium hydroxide cavity liner used as pulp capping materials, on reparative and reactionary dentin formation in rat dental pulps following pulp exposure and capping.