Pulp Protection by HC111129104257

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									PULP PROTECTION

RITTER & SWIFT, 2003 vol 5 Endodontic Topics, Current restorative concepts of pulp protection

The restoration of compromised dental structure requires attention to function, esthetics, and biology. Function and esthetics often can
be restored to satisfaction with current restorative materials and techniques. However, the biological requirements of dental restorations
are poorly understood. Since stimulated dental regeneration is still not a reality, dental amalgam, resin-based composites, ceramics,
and metals are used to restore missing parts of teeth. Biologically, these materials are not expected to behave entirely like dentin or
enamel, but dental restorations should restore and protect the integrity of the dentin–pulp complex.

For many years, it was believed that restorative materials themselves were toxic to the pulp. Based on that assumption, the use of
bases and liners covering the vital dentin for pulp protection was considered essential to the success of the restoration. This
recommendation was based on studies that linked pulp reactions to the low pH of dental materials (1–5).

However, it is now believed that the main reason for the biological failure of restorations is not related to pH or other attributes of the
restorative material (6–8), but rather to the poor ability of restorations to seal the tooth–restoration interface leading to marginal leakage
of bacteria and toxins (9–11).

The role of bacterial infection on pulp pathologies was proposed in the mid-1960s, and has been revisited in later years (12–14).

During the last 20 years, a new approach to pulp protection has been proposed in many countries, with more emphasis being placed on
the ability of restorative materials to prevent or neutralize bacterial penetration along the tooth–restoration interface (15–18). The
purpose of this paper is to review current restorative concepts of pulp protection of a vital non-exposed pulp.

Common causes of pulp injury

In restorative dentistry, the following can be considered as the most common causes of pulp injury, before, during, and after a
restoration is placed:

1. Presence of bacteria in the dentin–pulp complex. Cavitated carious lesions provide a niche for bacteria to aggregate and
   proliferate. Once the lesion reaches the dentinoenamel junction (DEJ), bacteria and their toxins can travel through the
   dentinal tubules and reach the pulp. Residual bacteria left after caries excavation, as well as bacteria reaching the dentin–
   pulp complex through microleakage, can also cause pulp disease.
2. Exposure of patent dentinal tubules. Patent dentinal tubules communicate directly to pulp cells. These can be present in
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   cervical areas of the tooth unprotected by enamel or cementum, and in dentin/pulp exposures after trauma. Patent dentinal
   tubules might also be present in poorly sealed walls beneath restorations, and contribute greatly to postoperative sensitivity.
3. Depth of tooth preparation. The deeper the preparation, the greater is the chance for direct or indirect pulp exposure. Deep
   preparations expose wider and more dentinal tubules per square millimeter than shallow preparations, which can lead to pulp
   injury when dentin is not properly sealed. However, preparation depth per se seems to be irrelevant for the pulp as long as the
   preparation is not contaminated and the surface seal is maintained. Torstenson and Brännström (19) histologically evaluated
   pulp responses to amalgam restorations placed in teeth that would be extracted for orthodontic reasons. When contamination
   of the preparation was avoided, no inflammatory cells were found in the pulp of most specimens, even when the remaining
   dentin thickness (RDT) in the preparations was as little as 0.15 mm, and regardless of the use or not of a pulp protection
   material.
4. Dentin dehydration. When working with vital dentin, overdrying should be avoided. Dehydration of the dentin surface by
   overdrying results in outward fluid flow, which in turn can result in aspiration of odontoblast cells (20).
5. Heat generation. Dentin has good insulative potential, but heat generation during tooth preparation, light-curing, and
   finishing/polishing of a restoration can injure the pulp. It has been shown that a 5.5°C increase in pulpal temperature result in
   a 15% chance of necrosis, and an 11°C increase result in 60% chance of necrosis (21).

The dentin–pulp complex is capable of counteracting many of these insults. However, the cumulative incidence of these and other
injuries can reduce its defense and repair potential.

The age of the tooth also influences its response to injuries. In general, a younger pulp is more resistant and can more readily offset
irritants than an older pulp. On the other hand, a young tooth has a larger pulp chamber and more permeable dentin structure than an
old tooth, due to the deposition of secondary and intratubular dentin that occurs with time. These biological factors must always be
taken into consideration when selecting a pulp protection technique or material.

Pulp protection materials

The selection of pulp protection material is a function of (1) the restorative material being used and (2) the RDT between the pulp and
the pulpal or axial walls of the final tooth preparation (Fig. 1). Tooth preparations are often classified according to their depth as shallow
or deep. However, the concept of preparation depth is better applied when it relates to the RDT as opposed to the distance from the
cavosurface margin to the pulpal or axial wall.

Pulp protection materials can be generically classified as bases, liners, varnishes, sealers, and dentin adhesives.
1. Bases. Bases are used in relatively thick layers (>1 mm) between the restorative material and the tooth preparation. These
   have been traditionally used to provide thermal and electrical insulation, mechanical pulp protection, and to create an ideal
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  tooth preparation form in deep preparations. Currently, bases are almost exclusively used only as internal buildups to block
  undercuts in preparations for indirect inlays and onlays. Examples of bases are zinc phosphate cement, zinc polycarboxylate
  cement, zinc oxide–eugenol cement, and glass ionomer cements and derivatives. By virtue of their adhesive and fluoride-
  releasing properties, resin-modified glass ionomer cements (e.g. Vitrebond, 3M ESPE; Fuji Lining LC, GC America, Alsip, IL,
  USA) should be favored when a base is required.
2. Liners. Liners are more fluid than bases, and used in thin layers (approximately 0.5 mm). Liners have been used traditionally
   to protect the dentin–pulp complex from the potential toxic effects of restorative materials. Currently, liners are used to seal
   the dentinal tubules reducing dentin permeability, as antibacterial agents, and as fluoride-releasing agents. Examples of liners
   are hard-set calcium hydroxide (CH) cements and glass ionomer cements. Due to their biological properties (high pH,
   antibacterial, stimulation of reparative dentin formation), CH cements (e.g. Dycal, Dentsply Caulk, Milford, DE, USA; Life,
   Sybron Kerr, Orange, CA, USA) are indicated for direct and indirect pulp caps, and when the RDT is judged to be less than
   0.5 mm. CH cements do not adhere to dentin, have poor physical and mechanical properties, and are extremely soluble.
   Therefore, they have to be covered by a layer of resin-modified glass ionomer cement before the final restoration is placed.
   When the RDT is judged to be more than 0.5 mm, sealers and adhesives should be used in lieu of liners.
3. Varnishes. Varnishes are synthetic or natural resins suspended in organic solvents. When applied, varnishes form a non-
   uniform 5 μm-thin pellicle covering the tooth preparation walls. For many years, copal resin varnishes have been used under
   amalgam restorations and crowns to seal the dentin. Varnish use has decreased substantially due to their high solubility and
   poor sealing ability (22–24), and because sealers are more advantageous than varnishes.
4. Sealers. Sealers are aqueous solutions of resins (e.g. 2-hydroxyethylmethacrylate (HEMA)), antibacterial agents (e.g.
   benzalkonium chloride, chlorhexidine), and/or desensitizing agents (e.g. glutaraldehyde). Combinations of these compounds
   are found in specific commercial products (e.g. Gluma Desensitizer, Heraeus Kulzer, Armonk, NY, USA, is an aqueous
   solution of 35% HEMA and 5% glutaraldehyde). Sealers are compatible with a variety of restorative materials. Sealers are
   used in place of varnishes under amalgam restorations and full crowns. Other examples of sealers include HurriSeal (Beutlich
   Pharmaceuticals, Waukegan, IL, USA) and Aqua-Prep F (Bisco, Inc., Schaumburg, IL, USA).
5. Dentin adhesives. The adhesion of restorations to dentin can be considered a truly new concept of pulp protection, even
   though this technique was first described many years ago (25–27). Virtually all adhesives today bond simultaneously to
   enamel and dentin; therefore, they should be more correctly referred to as dental adhesives. Dental adhesives provide pulp
   protection by making possible conservative tooth preparations and by sealing out bacteria from the tooth–restoration
   interface. Adhesion and its role in pulp protection are discussed in more detail in the next section of this article.




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A new pulp protection concept: total adhesion

As discussed earlier, the preservation of tooth structure and the maintenance of a bacteria-free interface in the restoration contribute
equally to pulp protection. Adhesive restorative techniques contemplate these objectives. Dental adhesives enable clinicians to
generate more conservative tooth preparations because:

(1) most (if not all) of the retention is obtained through adhesion and micromechanical retention,
(2) resistance form is less critical with an adhesive restoration than with a non-adhesive restoration, and
(3) unsupported enamel frequently can be preserved, minimizing the extension of the preparation outline and improving marginal seal.

In vitro evaluations of enamel and dentin adhesives have been extensively performed, with promising results (28–38).

The clinical performance of adhesive restorations has been validated by prospective studies (39–48)

and retrospective studies (48–50),

and has been described in review articles (50–52).

Despite this evidence, unfortunately it is still not possible to categorically state that current dental adhesives are a final answer to the
adhesion challenge, for the following reasons: (1) more long-term controlled clinical trials are needed to confirm the good performance
of adhesive materials; (2) adhesive systems are sensitive to application technique, storage, manipulation, and substrate; and (3) long-
term in vitro investigations suggest deterioration of bonds over time (53, 54).

 The latter might or might not be clinically relevant, but is certainly a matter of concern. It is important to know and recognize these
limitations when discussing the role of adhesive systems in preventing microleakage.
In restorative dentistry, microleakage is defined as the leakage of ions, fluids, bacteria, and bacterial byproducts through the tooth–
restoration interface (55, 56).

Microleakage occurs when there is poor seal or even a lack of effective contact between the tooth preparation and the restorative
material. The pathological component of microleakage is bacterial infection of dentin and pulp, and its consequence is secondary caries
and pulp pathology. Microleakage also can lead to the failure of the restoration through partial or total loss of the restorative material.



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When properly applied, current adhesive systems can substantially minimize marginal microleakage in restorations made with
amalgam, composite, ceramics, or metal. This potential is even greater when all cavosurface margins of the preparation are in enamel,
since enamel bonding is still more predictable and stable than dentin bonding.

For contemporary adhesive systems, dentin bonding requires removal or modification of the smear layer, a semi-permeable 0.5–2.0μm-
thick film composed of denatured collagen and mineral crystals generated during cavity instrumentation and superficial demineralization
through the application of an acidic conditioner or primer. Although chemical reactions between adhesive resins and dentin are not
excluded (57–59),

it is generally accepted that dentin bonding relies primarily on a micromechanical interaction similar to enamel bonding, mediated by the
permeation of resin monomers into etched and primed dentin (27, 60–62)
.
The entanglement of the in situ polymerized adhesive resin with collagen fibrils and residual hydroxyapatite crystals is called the 'hybrid
layer' or 'resin–dentin interdiffusion zone' (63, 64).

The composition of the hybrid layer is reported to be approximately 70% resin and 30% collagen (65).

This layer of resin-saturated demineralized dentin can provide high bond strengths and a tight seal against microleakage.

By virtue of their resinous composition, dental adhesives are most compatible with composite resins and resin cements). However,
dental adhesives can also be used to seal the tooth preparation walls under amalgam restorations and full crowns.

Conclusions

In conclusion, the following recommendations summarize current pulp protection materials and techniques:

1. Bases are typically not needed for direct restorations (amalgam and composite). Bases can be used to block out undercuts or
   for other internal buildup or foundations for indirect restorations (inlays, onlays and crowns).
2. CH liners are used as a therapeutic agent only when the RDT is believed to be <0.5 mm. Glass ionomer liners are used under
   amalgam and composite restorations when the RDT is believed to be >0.5 and <1.5 mm. Liners are not needed when the
   RDT is >1.5 mm.
3. When used, bases and liners should always be applied prior to finishing the preparation and placing bevels, to avoid
   contamination of the cavosurface margin.
4. Bases and liners should only be applied to the deepest portion of the preparation, leaving enough enamel and dentin exposed
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  for adhesion.
5. Sealers are used routinely to reduce dentin permeability under amalgam restorations. Water-based sealers can also be used
   as re-wetting agents in the total-etch technique under composite restorations.
6. Dental adhesives are necessary when restoring with composites and ceramics, and provide excellent pulp protection as long
   as the interface remains sealed. The sealing ability of dental adhesives is better, and likely more durable, when enamel is
   present at the entire periphery of the preparation.




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