Mineral trioxide aggregate (MTA) has been a revolutionary material in end- odontics. Since its introduction in the 1990s several studies have demonstrated its use in various clinical applications. MTA has been extensively studied and is currently used for perforation repairs, apexifications, regenerative procedures, apexogenesis, pulpotomies, and pulp capping. This article will review the history, composition, research findings, and clinical applications of this versatile endodontic material.
Over the past two decades, min- eral trioxide aggregate (MTA) has become one of the most widely studied endodontic ma- terials. The trioxide aggregate in MTA consists of calcium, aluminum, and selenium. MTA has several desirable properties in terms of its biocompatibility, bioactivity, hydrophilicity, radiopacity, sealing ability, and low solubility. The most important of these properties in dentistry are its biocom- patibility and sealing ability. High biocompat- ibility encourages optimal healing responses. This has been observed histologically with the formation of new cementum in periradicular tissue areas and a low inflammatory response with bridge formation in the pulp space. The sealing that is achieved is due to the mate- rial’s expansion and contraction properties being very similar to dentin, which results in high resistance to both marginal leakage and to bacterial migration into the root canal system. A stable barrier to bacterial and fluid leakage is one of the key factors that facilitates clinical success.
A very practical advantage of MTA is that, unlike many other dental materials, it sets in the moist environment that is omnipresent in dentistry. When in contact with moisture, the material’s main component, which is cal- cium oxide, converts into calcium hydroxide, which many clinicians are familiar with.6 This conversion results in a high pH microenvi- ronment, which has beneficial antibacterial effects. Unlike calcium hydroxide, however, MTA has very low solubility and maintains its physical integrity after placement.
MTA materials are derived from a Portland cement parent compound. Although these compounds are similar in some respects, Portland cement and MTA are not identical. MTA products undergo additional processing and purification. When compared to Portland cements MTA products have a smaller mean particle size and contain fewer toxic heavy metals.
When first introduced, clinicians had diffi- culty handling MTA due to its wet sandlike consistency, which was unlike most other con- ventional dental materials. Following the introduction of several customized application devices, the handling and application of this material has become more predictable.
Clinical Applications
More than 24 million endodontic procedures are performed in the United States on an an- nual basis, with 5.5% of these procedures being advanced treatments such as periapical mi- crosurgeries, perforation repairs, and apexi- fication treatments. All of these endodontic procedures and some operative procedures have greatly benefitted from the availability of MTA, and they are discussed here in turn.
Pulp Capping
Pulpal exposures are sometimes inevitable when addressing large carious lesions. While some clinicians are hesitant to perform direct pulp capping procedures due to documented unpredictability as a definitive treatment op- tion, MTA may help to improve the outcome of this treatment in the near future. MTA has the advantage of being less soluble than cal- cium hydroxide and offers an enhanced seal due to its setting expansion, which hermeti- cally seals the pulp space, preventing bacterial contamination from the outside. Studies have shown that in asymptomatic cases or in cases with reversible pulpitis (where the infection has not spread into the pulp chamber proper), MTA pulp capping can serve as a viable treat- ment option.19,20 Histological studies have also shown less inflammation and more dentinal bridging when MTA is placed compared to conventional pulp capping with calcium hy- droxide. In pulp capping, the rapid 15-minute set of MTA Angelus allows the final restora- tion to be placed without delay and in direct contact with the set MTA.
Vital Pulp Therapy (Pulpotomy and Apexogenesis)
In cases of irreversible pulpitis where bacteria have invaded the pulp chamber, a pulpotomy procedure can sometimes be considered. This procedure is also termed apexogenesis be- cause its ultimate goal is to facilitate the com- plete formation of the apex and root. This pro- cedure is carried out in immature teeth with incomplete root formation that contain vital pulp tissue. The radicular pulp, which is con- sidered to be relatively free of inflammation, is retained. When this is done, at a histological level odontoblasts will differentiate, dentin will continue to be laid down, and root devel- opment should continue. This will result in thickening of the root walls, which reduces the risk of root fracture, and apical closure should occur (apexogenesis), forming a natural apical constriction that would facilitate any future root canal obturation procedures.
Clinical procedure: Once in the pulp chamber, clinicians should use a diamond bur, because it will cauterize the tissue and minimize the bleeding. After this is done, the area should be disinfected with an antimicrobial agent (so- dium hypochlorite or chlorhexidine), followed by a saline rinse. Hemostasis is obtained with light pressure from a damp cotton pellet. The pellet is removed after 2 or 3 minutes, and the area is then ready to be filled with MTA.
Apexification
The treatment of a necrotic pulp in an im- mature root has long presented a challenge to clinicians due to the lack of an apical stop. This has classically been addressed with long- term calcium hydroxide treatment, which may require several years of treatment time, involve multiple visits, and, theoretically at least, increase the fracture potential of the root involved.24 MTA has become an excel- lent predictable alternative to address these issues by creating a biocompatible apical plug in a single visit.
Clinical procedure: If apical bone loss is pres- ent (Figure 1) a collagen/gelatin sponge (eg, Gelfoam®, Pfizer Inc., www.pfizer.com) can be placed apically so that the MTA can be deliv- ered to the desired working length. (Any other surgical resorbable sponge would also work, such as OraPlug® [Salvin Dental Specialties, www.salvin.com], Surgifoam® [Midwest Dental, www.mwdental.com], or Surgispon® [Aegis Lifesciences, www.surgispon.com]). This is done by taking a small piece (2 mm x 2 mm) of the resorbable sponge and pushing it down to and through the root apex with an endodontic file. Once this is done, MTA is packed down the canal with a custom-fitted cone. The clinician can use a rubber stop- per on the gutta-percha cone to know the exact length of MTA placed in the apical third (Figure 2). Once the apical third is sealed with 3 mm to 5 mm of MTA, the remaining coronal canal space can be back-filled using a warm gutta-percha technique (Figure 3).
Regeneration
The treatment of a necrotic pulp in an im- mature root with very thin walls is problem- atic due to the high potential for root fracture. Regeneration of the dentin–pulp complex is a contemporary approach that involves dis- infecting the root canal system with a triple antibiotic paste followed by tissue repair and regeneration.27,28 This should allow contin- ued thickening of the lateral dentinal walls through deposition of new dentin-like hard tissue.27,28 More research is still needed to pre- cisely assess if and how this newly deposited dentin-like hard tissue strengthens the pre- existing thin root. Clinical procedure: Regeneration of the end- odontic pulp space is indicated for cases with very thin dentinal walls and an open apex that is more than 1 mm in diameter radiographi- cally (Figure 4). Disinfection of the root canal system is performed using sodium hypochlo- rite irrigation followed by a triple antibiotic paste dressing that is left in place for 1 week. At the second visit, ethylenediaminetetraacetic acid (EDTA) is used to condition the dentin walls, which results in the release of growth factors, and bleeding is stimulated in the peri- apical tissues (where stem cells are located), with the aim of filling the pulp space with a stable blood clot, which would serve as the scaffold. MTA is then placed at the canal ori- fice in contact with the clot to protect it from coronal microleakage (Figure 5 and Figure 6). In time, the clot should be replaced with a reparative tissue of variable composition, and the root walls should continue to thicken due to the deposition of a dentin-like material on the pre-existing root dentin (Figure 7 and Figure 8).
Root Perforation
Perforations are generally the result of iatro- genic conditions in which a communication between the pulp canal and the periradicular tissue occurs during either access preparation or canal shaping procedures. Perforations can also happen in cases of internal root resorp- tion, where the entire thickness of the root becomes affected by the resorptive process. Because of its excellent sealing ability and bio- compatibility, MTA has been used to repair root perforations with predictable results.29,30
Clinical procedure: Once a perforation oc- curs, the extent of the perforation must be assessed. If there is an adjacent bony defect, the bony defect should first be filled with an osteoconductive or osteoinductive material. This can be done with a bone graft, calcium sulfate, or collagen/gelatin sponge. The den- tinal portion of the tooth that has been per- forated is then restored with MTA (Figure 9 through Figure 14).
Root-End Filling A root-end filling (also known as a retrofill- ing) is performed when an extraradicular microsurgical approach is needed to address endodontic pathology. Most cases treated surgically cannot be predictably treated by orthograde conventional root canal meth- ods due to either complex canal anatomy or iatrogenic misadventures during root canal treatments.31,32 MTA exhibits excellent physi- cal sealing properties, and, furthermore, an additional biological seal is obtained by the proliferation of cells directly on the cemen- tum during the healing process
Clinical procedure: In the case of a root- end filling (Figure 15), once the apical 3 mm of the root has been resected (Figure 16) the canal system is then opened and cleaned with surgical ultrasonic tips to create the retro-preparation (Figure 17). After this is completed, the retro-preparation is dried and MTA is then placed and condensed in that space creating the retro-filling (Figure 18 through Figure 20).
Conclusion
Scientific research has demonstrated the ef- fectiveness of traditional MTA when used in a range of endodontic procedures. The history, chemical composition, and clinical applications of MTA have been discussed in this review article. When considering seal- ing effectiveness and biocompatibility, MTA is unique and has advantages compared to other dental materials. With the recent in- troduction of a fast-setting MTA, which also offers excellent handling properties, MTA- based endodontic products are likely to re- main at the heart of good dental practice for many years to come.
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