Cover Story: The Power of the Pulp (Part 2) by Dr Kishan Sheth

The second of this two-part article presents a review of the different types of materials on the market which clinicians are using or have used as pulp-capping materials

by Dr Kishan Sheth

The ideal properties of direct pulp-capping materials
In the first part of this two-part article, I discussed the importance of the pulpal tissues in regeneration and function of the living tooth. This second part will present a review of the different types of materials on the market which clinicians are using or have used as pulp-capping materials.

The long-term status of an exposed pulp is a vital consideration when assessing methods by which to definitively restore the tooth. The pulp should be protected from bacteria, toxins and electrical/thermal stimuli. Cohen and Combe (1994) state that pulp protective materials should exhibit certain features, characteristics and properties.

Pulp-protective materials should:

• Be able to resist and withstand forces such as restoration placement.
• Be insulators to heat and chemicals.
• Adhere to the dentine and restoration.
• Be nontoxic and biocompatible.
• Be radiopaque.
• Be bactericidal or bacteriostatic.
• Act as a mild irritant, for example through pH to stimulate and upregulate odontoblasts and the repair and regenerative processes.
• Not dissolve or break down.
• Be able to insert into the dentinal tubules and reinforce the dentinal structure.
• Provide an excellent bacterial seal.

The role of odontoblasts
As the neural crest ectomesenchyme proliferates and condenses down, it forms the dental papilla, which is where mature pulp tissue is derived from. Formed, mature pulp tissues have striking resemblance with embryonic connective tissues (Walsh, 1997).

The pulp resides in a rigid chamber consisting of dentine, enamel and cementum, and responds to initial irritations such as caries, trauma, bacteria, and by acute and chronic inflammation (LeJeune and Amedee, 1994). If this initial irritation is not addressed, the pulp will undergo necrosis.

The pulp is home to specialised cells, odontoblasts, which harness the power throughout life and in response to chemical, bacterial or traumatic irritants to lay down reparative or reactionary dentine (Walsh, 1997). Odontoblasts differ from other matrix-forming cells in that their cell bodies do not lay within the matrix they are responsible for secreting, and that they have long, fine processes which radiate through the dentinal tubules and toward the enamel–dentine junction (LeJeune and Amedee, 1994).

What is also unique is that histological studies have demonstrated cellular adhesions and gap junctions between these processes allowing the transmittance of low electrical resistance, giving the indication that the cells have the capacity and capability of communicating between themselves. The pulp is equipped with cellular components required for recognition and processing of antigens (Jontell et al., 1988).

The challenge of endodontic treatment is ensuring that all canals are found and prepared, and that obturation also fills up the lateral canals and accessory foramina, through the action of the sealant.

Regarding the microvasculature, the microcirculation toward the pulp is brought about through the maxillary artery, which stems from the external carotid. The maxillary artery forms the dental artery, which is inserted into the pulpal space through the apical foramen via very fine arterioles.

The pulp has a relatively high blood flow, estimated to be 50ml/min/100g in the mature molar tooth. The sensory nerves responsible for pulpal detection of pain and thermal energy are divisions of the trigeminal nerve. These sensory fibres branch numerously in the cell-rich region of Raschkow’s plexus, near the odontoblastic cell bodies.

AÕ fibres are around 5 micrometres and Aß fibres around 1 micrometre in diameter. As the tooth ages, the pulp tissue’s innervation and vascularisation decreases and the pulpal matrix becomes less cellular and more fibrous. The pulp’s regenerative capability is considered to be reduced (Jontell et al., 1987; Okiji et al., 1997; Linden et al., 1995).

Siqueira (2002) described the various characterisations of pulpal endodontic infections.

• A primary infection is one that arises before endodontic therapy.
• A secondary infection progresses despite endodontic intervention having already been employed—for example, due to bacteria still being able to invade the root canal system despite attempts at creating coronal and apical seals.
• A persistent infection can be considered to be a primary infection which persists, despite endodontic treatment having been done.

Direct pulp-capping materials
Zinc oxide eugenol (ZOE) is now considered to be a relatively poor pulp-capping agent, owing to its high cytotoxicity factor and high levels of interfacial leakage and the fact its antibacterial effects diminish the longer it remains in the oral environment (Chang et al., 2000; Ho et al., 2006).

There is debate over whether the high level of interfacial leakage is important, because the material demonstrates excellent biological seal characteristics (Hume, 1984). While ZOE is now considered to be a relatively poor pulp-capping agent, it was widely used in the past.

Glass ionomer cements (GICs) also exhibit cytotoxicity, but not as much as ZOE. In addition, GICs have excellent interfacial adhesion/biological seal abilities, and are therapeutic, too, because they can transport fluoride ions to any remaining carious tissues (Schmalz et al., 1996; Hume, 1984; Koulaouzidou et al., 2004; Torbinejad, 1995).

The dental literature has demonstrated that GICs are biocompatible and appropriate to use as indirect pulp caps, but their use as direct pulp-capping agents is not clinically appropriate (Tewari and Tewari, 2002; Heys et al., 1991; Murray et al., 2002).

GICs exhibit excellent interfacial adhesion and biological seal abilities and are therapeutic, too.

Calcium hydroxide, introduced to dentistry in 1921 by Dr B.W. Hermann, has a long track record of being the gold standard pulp-capping agent (Barthel et al., 1997). Its high antibacterial nature ensures that there is a 100 percent reduction of microorganisms associated with pulp pathology after just one hour of clinical contact time (Kitasako et al., 2008). What has let this material down is the high solubility, poor seal and resultant effect of tunnel defects in the underlying dentine.

The primary theory thought to be the result of calcium hydroxide’s success was the theory of irritation whereby the highly alkaline material would irritate the odontoblasts and cause upregulation of underlying mesenchymal stem cells to form new immature odontoblasts, which could lay down reactionary dentinal tissue (Cox et al., 1996; Ulmansky et al., 1972). This may still be the case; however, there is growing evidence that calcium hydoxide’s success is due to the release, promotion and activation of bioactive molecules such as bone morphogenic protein (BMP) or transforming growth factor (TGF–B1) (Graham et al., 2006; Duque et al., 2006).

It seems as though BMP 2, 4 and 7 play the key roles in the differentiation of stem cells to odontoblast cells. BMPs were found to be the core responsible proteins for differentiation of preodontoblastic cells to odontoblasts when they were used as direct protectors of pulpal tissue (Lianjia et al., 1993).

Calcium hydroxide has also demonstrated a unique ability to mineralise tissues that can inherently not self-mineralise (Mitchell and Shankhwalker, 2014).

Mineral trioxide aggregate (MTA) is a silicate-based cement. It is composed of tricalcium silicate, dicalcium silicate and tricalcium aluminate with bismuth oxide added into the mix for radiopacity.

Upon mixture with water, the primary compound to form is calcium hydoxide, which explains why the properties of both these materials have similarities (Camilleri, 2008; Camilleri and Pitt, 2006; Fridland and Rosado, 2003). MTA is highly antibacterial, biocompatible and alkaline, and comes in two colours—white and grey. The grey formulation contains iron, which has been demonstrated to cause some degree of staining (Camilleri et al., 2005; Torabinejad et al., 1995; Islam et al., 2006).

MTA is also highly soluble; there was a 24 percent loss of MTA material after it was stored in distilled water for 78 days (Fridland et al., 2005). It has a setting time of 2 hours, 50 minutes (Tomson et al., 2007; Song et al.. 2006).

Calcium silicate came onto the market in 2009 in a material called Biodentine from Septodont. It was primarily marketed as being a dentine replacement, but it has also been used successfully for root perforations, retrograde fillings and direct pulp caps.

The material consists of powder and liquid that is machine-mixed chairside. The powder consists of tricalcium silicate, dicalcium silicate, calcium carbonate and oxide filler, with zirconium oxide as the radiopacifier. The liquid is calcium chloride (Kayahan et al., 2013; Camilleri, 2008).

Although this material resembles MTA, it has better sealing capability and a setting time of just 9 minutes, is less soluble, and has better handling features and a much higher compressive strength.

It also can release silica ions into the surrounding pulp, and can break down collagen fibres to form porosities whereby minerals can diffuse more rapidly in the tissue (Camilleri et al., 2012; Tay, 2007; Mitchell and Shankhwalkar, 2014). The material has demonstrated superior abilities of generating and activating TGF-B1, therefore stimulating the reparative dentine formation process (Laurent et al., 2012).

Conclusions
Minimally invasive dentistry relies on the basis that a functionally healthy pulpal–dentine interface can successfully heal any small exposure of the pulpal space.

Certain situations are more favorable when considering pulp capping. These include small exposures, young patients, traumatic and noncarious exposures, and exposures in uninflamed pulpal tissues. Carious exposures may have better prognosis if the teeth are asymptomatic.

The pulpal tissue is considered to be unique in its features and plays a major and significant role in ensuring the long-term vitality of the tooth.

Despite innovations in endodontics, dental professionals should ensure that they fully understand the physiological, pathological and regenerative features of the pulp. Where possible and appropriate, clinicians should endeavour to preserve pulpal vitality.

The success of direct pulp-capping ultimately depends on several factors. Clinicians must ensure that they select appropriate cases (e.g., size of pulpal exposure), isolate the tooth or teeth being restored to prevent saliva/bacterial contamination, and then select appropriate materials, handling and using them exactly as outlined by the manufacturer.

The ‘power of the pulp’ then has at least a fighting chance to keep the tooth vital.

References
Full bibliography for this paper (part 1 and part 2) can be found via this link.

Dr Kishan Sheth recently graduated from KCL as a runner-up for the prestigious Jose Souyave Prize and will embark on his vocational training in Central London. He has become the most recent and youngest honorary editor of DentaltownUK.