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The concept of minimally invasive dentistry (MID) refers to a conservative, tooth-structure-saving approach when treatment-planning restorative dental procedures. Minimally invasive dentistry as it relates to the treatment of caries is typically described as a change in the principles of the traditional cavity preparation design that followed the fundamentals of "extension for prevention" described by G.V. Black to a more carious lesion-centered approach.1-3 This lesion-centered approach is possible through advancements in adhesive restorative materials, the introduction of computer-assisted methods of caries detection, a better understanding of the role of magnification, digital radiography, and caries risk assessment of the patient, all of which enable improved conservative caries management.4 For any restoration, consideration must be given to maintaining as much tooth structure as possible because in the restorative cycle of placement and replacement more tooth structure will be lost.
While the focus of minimally invasive dentistry has mainly been on a lesion-centered approach for the conservation of tooth structure, this author believes the concepts of MID should also include the word "invasive." That is, MID can be expanded to include treatment decisions when replacing existing defective restorations as well as missing teeth. In the case of replacing existing nonadhesive restorations, eg, defective amalgam restorations, with conservative adhesive composite resins, tooth structure is maintained without overextending the preparation design. In the case of large cusp replacement restorations, consideration is given to composite or ceramic inlay-onlays and not the placement of a core foundation followed by a crown.5 Minimally invasive restorative dentistry also includes elective esthetic procedures, such as the use of microabrasion and bleaching for the treatment of superficial enamel discoloration and minimally invasive, minimal-preparation porcelain veneers.6-9 In circumstances where the patient is missing a single tooth, the use of an implant is more minimally invasive than preparing the adjacent teeth as abutments for a three-unit fixed partial denture.10 This article will focus on minimally invasive dentistry as it relates to a lesion-centered approach to diagnosis, tooth preparation, and restoration.
Diagnosis for Early Caries Intervention
In order to accomplish minimally invasive dentistry, carious lesions must be detected as early as possible. If detected in early stages the progress of caries can be arrested using preventive remineralization therapy, thus avoiding a more invasive operative intervention.11 Accurate diagnosis of a noncavitated carious lesion is important because it may be an indicator of high caries activity and caries risk, and could be a condition that requires a more aggressive preventive program.12
The prevalence of dental caries in children and adults in the United States has declined in the past 40 years primarily because of increased use of fluoride, improved oral hygiene and better oral hygiene devices, a greater emphasis on disease prevention and control, and better access to dental care made available by the dental profession.13 In spite of this, dental caries continues to be a prominent oral disease and major public health problem, particularly among certain segments of the US population.14,15 Approximately 60% of caries occurs in 20% of the population, and fewer than 5% of adults are caries-free.3 While caries on interproximal surfaces is decreasing, occlusal pit-and-fissure caries has continued to increase. In general, caries on occlusal and buccal/lingual surfaces account for almost 90% of caries experiences in children and adolescents.16
With the continuing increase in occlusal pit-and-fissure caries and a concomitant change in its pattern and progression, correctly diagnosing occlusal lesions has become more difficult. This difficulty has led to an increase in research and development for better diagnostic tools for pit-and-fissure caries detection.17-19 While correctly differentiating between sound and cavitated surfaces is typically easily accomplished, noncavitated lesions create a significant diagnostic challenge.
Clinical research and observations in the past quarter century have led to changes in the way pit-and-fissure caries are detected, diagnosed, and treated. Also, early intervention for proximal carious lesions using a preventive approach before they need preparation has been advocated.20 Traditional concepts of classical cavity preparations that followed G.V. Black's principles of "extension for prevention" were criticized as early as the 1950s by Miles Markley and others.1 Dr. Markley presented a more modern clinical approach in cavity preparation design that valued sound tooth structure. His philosophy was based upon the principles that the removal of unnecessary amounts of tooth structure through cavity preparation should be regarded as a serious injury to the tooth and the tooth structure must be preserved as best as possible with conservative instrumentation during preparation.21 Dr. Markley was responsible for the design of the 330 bur. His earliest concepts were hindered, however, by the direct placement restorative materials available at the time (dental amalgam, direct gold, unfilled acrylic resins, silicate cements). With the introduction of adhesive restorative materials, the concepts of minimally invasive restorative dentistry could be followed.1,3,4,22 The earliest uses of adhesive restorative materials were for the prevention of pit-and-fissure caries with sealants. Sealants placed using good isolation are very successful at preventing pit-and-fissure caries for children and adolescents. The rates of sealant retention and caries prevention on occlusal surfaces are relatively high at more than 7 to 10 years.23-25
In recent years there has been evidence of what appears to be intact pits and fissures that are, in fact, carious lesions that are difficult to diagnose. This difficulty in establishing a diagnosis of pit-and-fissure caries is a result of the increased use of fluoride, which has changed the traditional characteristics of demineralization and the visual appearance of enamel opacities in the initiating lesion.26-28 In the enamel surface, these lesions tend to be minimal in size, but within the dentin they spread along the dentinoenamel junction (DEJ) as larger, more invasive lesions. These carious lesions that are not very readily apparent in the pit or fissure and extend more extensively into the dentin have been referred to as hidden caries.
Up until now the traditional tool for pit-and-fissure caries detection has been a sharp explorer or occlusal probe. Many clinicians directly relate the tug-back of an explorer in a deeply invaginated pit as a "stick" and a diagnosis of caries. In fact, many times the sharp explorer is merely wedging into the deep "V"-shaped fissure, and caries is not present. Recent studies, however, have caused the explorer to lose favor since it has been determined that it may actually create an opening, a cavitation in the slightly demineralized enamel through which cariogenic microbes may penetrate, as well as carry caries-producing bacteria from an infected tooth to one that is uninfected.29-31 Further, it has been demonstrated that there is no overall improvement in the accuracy of a diagnosis with a sharp probe compared to judicious visual examination accomplished with careful drying, good light, and magnification.29,31-34 The changes in the appearance of the enamel adjacent to pit and fissures due to increased use of fluorides have led to a different evaluation of pit-and-fissure caries, based on caries risk assessment and combined with new technologies used as aids to traditional clinical methods of caries detection.4,35-38
Several new technologies seem to be more promising for detecting hidden and incipient caries.4,39,40 From non-invasive to slightly invasive, these new devices include quantified light-induced fluorescence (QLF), optical coherence tomography (OCT), laser fiber optics transillumination, the operative microscope, and operative exploration of the fissure or pit using air abrasion, water lasers, and fissurotomy burs.
Minimally Invasive Restorative Interventions
As described earlier, the use of minimally invasive cavity preparation designs are clinically successful. The clinician has many choices for instrumentation for use in minimally invasive cavity preparations. Traditional cavity preparations can be done using smaller burs and innovative bur shapes to provide for conservative hard tissue removal.34,41 Air abrasion, sono-abrasion and waterlase are other choices of fissure and pit exploration and preparation.33,34,41,42
Restorative materials for minimally invasive dentistry include pit-and-fissure sealants, flowable composites, resin-based composites, glass ionomer, and conservatively prepared and sealed amalgams.29,31,32,41,43
Historically, the earliest minimally invasive treatment of suspected pit-and-fissure caries has been referred to as a prophylactic odontotomy.44 Hyatt described this preventive and prophylactic procedure in which at-risk pits and fissures were minimally prepared with small burs and restored with amalgam before visible attack by caries.45 He referred to prophylactic odontotomy as being the preferred treatment method for children because it conserved tooth structure. More than 70 years since Hyatt's description of minimally invasive cavity preparation, it has been generally accepted that one of the primary goals for treating carious lesions is conserving as much tooth structure as possible combined with placement of durable and long-lasting restorations so the restorative cycle of placement and replacement is reduced.
In 1978 Simonsen described the treatment of small occlusal pit-and-fissure carious lesions with a minimally invasive preparation using small burs and then restored with a combined adhesive- composite resin-sealant technique that he named preventive resin restoration (PRR).46 Using a carious lesion-centered approach, the caries was removed with small burs to create these PRR preparations, removing only tooth structure that was carious. Initially, these small preparations were restored with a more highly filled hybrid composite resin with a sealant placed over the restoration and a sealant placed over the entire occlusal surface using an etch-and-rinse adhesive technique. Today, PRRs are restored with an adhesive technique using only a single restorative material—a flowable composite placed after either etching and adhesive placement or following the use of a self-etch adhesive. The "all-in-one" flowable composite resin is both the preparation restorative and sealant. Research reports on PRRs have demonstrated the clinical success of these conservative restorations.47-49
Initial proximal carious lesions to the DEJ have typically been treated with a surgical approach—preparing the lesion with a bur. For incipient proximal carious lesions still within the enamel, the use of remineralizing treatment with fluorides and calcium phosphates has been successful. Recently, an innovative treatment using an acid-etch approach with resin infiltration has also been demonstrated to be successful for these incipient carious lesions.50 In cases where there is no evidence of pit-and-fissure caries, but the approximate carious lesion has penetrated past the DEJ and/or the proximal enamel smooth surface is cavitated, a minimally invasive intervention using a minibox or slot preparation that includes the marginal ridge has been recommended.1,51,52 For these Class II preparations, the facial and lingual extensions of the proximal walls can be more conservative in design. The enamel margins of these preparations can maintain proximal contact because the preparations are lesion-centered; only the caries should be removed and the restoration done with an adhesive composite resin.3,53 As described earlier, minimally invasive dentistry also includes restoration replacement. In the case of a defective amalgam restoration or for a patient who desires the replacement of an amalgam restoration for esthetic reasons, the use of magnification with high-speed cavity preparations can minimize the removal of sound, healthy tooth structure during cavity preparation.
The many advances in instrumentation, materials, and techniques have all assisted clinicians in making the transition from traditional principles of cavity preparation and restoration to more conservative, minimally invasive dentistry. These advances combined with accurate and early diagnosis of caries and caries risk assessment and management allow the clinician to conserve tooth structure, eliminating the needless destruction of healthy enamel and dentin. This article has presented an overview of the principles of minimally invasive dentistry.
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