Pulp capping

Zinc Oxide Eugenol (ZOE) is a commonly used material in dentistry. The use of ZOE as a pulp capping material remains controversial. This is due to Eugenol, being cytotoxic to the pulp are present in large quantity in this formulation. Also due to its nature of non-adhesive, it leads to poor coronal seal hence increases micro-leakage. Studies have demonstrated unfavourable results for ZOE when compared to calcium hydroxide as a direct pulp capping material as it causes pulpal necrosis.[9]

Both Glass Ionomer (GI) and Resin Modified Glass Ionomer (RMGIC) has been widely used as a lining or base material for deep cavities where pulp is in close proximity. This is due to its superior properties of good biocompatibility and adhesive nature, providing coronal seal to prevent bacteria infiltration. However, they are not a material of choice for direct pulp capping. When the use of RMGIC and calcium hydroxide has been studied as direct pulp capping agents, RMGIC has demonstrated increase in chronic inflammation in pulpal tissues and lack of reparative dentine bridge formation.[9]

Materials that fall under this category include 4-META-MMA-TBB adhesives and hybridizing dentine bonding agents. The idea of using adhesive materials for direct pulp capping has been explored two decades ago. Studies have demonstrated that it encourages bleeding due to its vasodilating properties hence impairing polymerisation of the material, affecting its ability to provide a coronal seal when used as a pulp capping agent. In addition, the material triggers chronic inflammation even without the presence of bacteria makes it an unfavourable condition for pulp healing to take place. Most importantly, its toxicity to human pulp cells once again makes it an unacceptable material of choice.[9]

Calcium hydroxide (CaOH) is an organo-metallic cement that was introduced into dentistry in the early twentieth century[10] and there have since been many advantages to this material described in much of the available literature. CaOH has a high antimicrobial activity which has been shown to be outstanding.[11][12] In one experiment conducted by Stuart et al. (1991), bacteria-inoculated root canals of extracted human teeth were treated with CaOH for 1 hour against a control group with no treatment and the results yielded 64-100% reductions in all viable bacteria.[11] CaOH also has a high pH and high solubility, thus it readily leaches into the surrounding tissues.[13] This alkaline environment created around the cement has been suggested to give beneficial irritancy to pulpal tissues and stimulates dentine regeneration. One study further demonstrated that CaOH causes release of growth factors TGF-B1 and bioactive molecules from the dentine matrix which induces the formation of dentine bridges.[14]

CaOH does however have significant disadvantages. The set cement has low compressive strength and cannot withstand or support condensation of a restoration.[13][15] It is thus good practice to place a stronger separate lining material (e.g. glass ionomer or resin-modified glass ionomer) over CaOH before packing the final restorative material.[9] CaOH cement is not adhesive to tooth tissues and thus does not provide a coronal seal.[9] In pulp perfusion studies, CaOH has shown to insufficiently seal all dentinal tubules, and presence of tunnel defects (patent communications within reparative dentine connecting pulp and exposure sites) indicate a potential for microleakage when CaOH is used.[13][16] It is suggested that an adhesive coronal restoration be used above the CaOH lining to provide adequate coronal seal. Because of its many advantageous properties and long-standing success in clinical use, it has been used as a control material in multiple experiments with pulp capping agents over the years[17][18] and is considered the gold standard dental material for direct pulp capping to date.[19]

Mineral trioxide aggregate (MTA) is a recent development of the 1990s[20] initially as a root canal sealer but has seen increased interest in its use as a direct pulp capping material.[9] The material comprises a blend of tricalcium silicate, dicalcium silicate and tricalcium aluminate; bismuth oxide is added to give the cement radiopaque properties to aid radiological investigation.[20] MTA has been shown to produce CaOH as a hydration product[21] and maintains an extended duration of high pH in lab conditions.[22] Similar to CaOH, this alkalinity potentially provides beneficial irritancy and stimulates dentine repair and regeneration.[23] MTA has also demonstrated reliable and favourable healing outcomes on human teeth when used as a pulp cap on teeth diagnosed as nothing more severe than reversible pulpitis.[24] There is also less coronal microleakage of MTA in one experiment comparing it to amalgam[25] thus suggesting some tooth adhesion properties. MTA also comes in white and grey preparations[26] which may aid visual identification clinically. Disadvantages have also been described for MTA. Grey MTA preparations can potentially cause tooth discolouration.[9] MTA also takes a long time (up to 2 hours 45 minutes) to set completely[27] thus preventing immediate restoration placement without mechanical disruption of the underlying MTA. It has been suggested that a pulp capped with MTA should be temporised to allow for the complete setting of MTA,[9] and the patient to present at a second visit for placement of the permanent restoration.[24] MTA also has for difficult handling properties and is a very expensive material, thus is less cost effective as compared to CaOH.[9]

Although MTA shows great promise which is possibly attributed to its adhesive properties and ability to act as a source of CaOH release,[9] the available literature and experimental studies of MTA is limited due to its recency. Studies that compare pulp capping abilities of MTA to CaOH in human teeth yielded generally equal and similarly successful healing outcomes at a histological level from both materials.[28][29]