Cavity Design, Outline Form
Modern concepts of the extension of outline forms for direct and indirect restorations rely on more scientifically defined criteria. In the past, observation and empirical study modified by a less sophisticated understanding of the sciences helped the pioneers of operative dentistry to formulate restoration concepts. Though modified by degree, G.V. Black’s original teaching of the seven steps of cavity preparation still holds.6 Those steps are:
1. Outline form
2. Resistance form
3. Retention form
4. Convenience form
5. Removal of remaining caries
6. Finish of the enamel wall
7. Cleansing of the cavity
Modern science has altered tooth restoration outline form in significant ways. There is a call in the literature for minimally invasive preparations, though that phase has numerous and sometimes erroneous meanings, depending on author.
Decalcification/Recalcification of Enamel
Dental caries is a diet-supported disease. The Vipeholm study10 in the 1950s dramatically demonstrated the role diet plays in the etiology of rampant caries. Children who consumed sweets frequently during the day developed aggressive dental caries patterns, whereas children who consumed the same amount of sweets but confined them to the three meals did not develop such decay. Similarly, sticky sweets like toffees and caramels were more cariogenic than less retentive forms.
Teeth demineralize and remineralize based on the pH and length of exposure to a pH of 4.5-5.5. This was first proposed in 1949.11 Below that pH, enamel is demineralized; above that pH hydroxyapatite is strong enough to prevent demineralization. Remineralization is improved in the presence of fluoride and sufficient calcium phosphates normally found in saliva.12,13,14 Because of these concepts, extension of a tooth preparation need not include decalcified areas of enamel if they are confined to superficial enamel. Fluoride found in many water supplies and in many types of toothpaste is sufficient to remineralize after acid attack.15 Many other therapeutic approaches can be used to prevent caries formation even in dry mouth conditions.
Remineralization therapies utilizing fluorides and exogenous calcium phosphates are effective at hardening the enamel of previously demineralized surfaces.17,18,19,20 This should be the initial therapy, rather than surgical intervention, for incipient lesions on smooth surfaces, together with oral hygiene instruction and dietary modification. Other therapies intending to alter the microflora of an infected mouth can be used — e.g., xylitol gum and chlorhexidine.16 Because of these remineralization opportunities available daily, outline forms can be significantly smaller.
Tooth Bonding and Sealants
Bonding to enamel and dentin provides some significant benefits to restorations of all types, generally improving durability. Specifically, tooth bonding decreases microleakage,21,22 diminishes postoperative sensitivity, improves resistance to cuspal fracture and flexure,24,25,26,27 improves retention strength,28,29,30 and improves marginal stability of amalgam.31
Secondary caries is most often a reason for restoration replacement. Eighty to ninety percent of margins with secondary caries are found at the gingival margin.32 Marginal gaps most often are the cause of secondary caries.33,34,35,36,37,38 Marginal preparation is probably a key to prevention of secondary caries, as well as the use of bonding, regardless of material used for interproximal restoration.39 Use of hand instruments to plane the gingival wall and remove loose enamel rods improves marginal adaptation, especially in smaller cavity forms.
Pits and fissures are best managed in tooth preparation using pit and fissure sealants when caries may be suspected or for prevention.40 Evidence-based recommendations40 are to use composite resin restoration in combination with sealants for initial lesions to minimize tooth reduction and outline form. This combination is termed preventive resin restoration (PPR).41,42 Composite resin is most adaptable in these circumstances because it does not require placement into dentin as amalgam restorations do (resistance form). The cavity form is lesion-specific, with size and extent dictated by the presence of caries and sufficient convenience form to insert and cure the material. There is no “ideal” outline form for either composite or amalgam, as each preparation on the occlusal surfaces is lesion-specific.
Pit and fissure sealants may also be used with occlusal amalgam restorations when the lesion extends more deeply into dentin, to limit outline form. When used in combination with sealants, this is the most durable restoration,31 requiring the least removal of tooth structure, maintaining the strongest tooth. In the ten-year clinical study cited,31 the bonded amalgam with sealants showed superior clinical performance and longevity compared to unbonded amalgam. In a similar nine-year study, the bonded and sealed lesion-specific occlusal amalgam outperformed the unbonded conventional occlusal outline form amalgam.43 An additional five-year clinical study shows the bonded and unbonded amalgam had no difference in marginal integrity with conventional tooth preparation, but the bonded amalgam had excellent retention and marginal integrity in unretentive cavity forms.44 These different studies show that after a five-year period, marginal breakdown for unbonded amalgams increases.
Bonding can very significantly decrease outline form. Consider the Class II amalgam preparation (Figures 3, 4, 5). If the restoration of a virgin lesion will be bonded, the facial extension (much like that for a Class II posterior composite) need not be extended all the way through the proximal contact facially. The preparation need only remove loose enamel and the carious lesion. Gingival margins must still extend about 0.5 mm from the adjacent tooth (i.e., they must clear the contact, as caries occurs most often lingual and gingival to the proximal contact).45 Though requiring hand instruments to finish all enamel margins, this much smaller outline form results in a considerably stronger tooth, less likely to fracture. In fact, if desirable, the occlusal extension of this outline could be managed with a pit and fissure sealant with the use of a proximal box-only. The material still requires convergent walls occlusally and proximal undercuts (line angle retention grooves)46 together with the bonding to be retentive enough with adequate resistance form to support occlusion. This type of restoration requires very careful condensation of the amalgam in order to achieve an adequate marginal finish neither under-filled, nor overfilled.47 This is an outline form far different from what many Class II restorations look like today.
Effect of Tooth Preparation on the Strength of Teeth
Tooth preparation leads to a loss of tooth strength, with increases in the amount of tooth structure removal causing greater weakness.48,49 In these and further studies, increasing the isthmus width from one quarter to one third of the intercuspal distance (measured from facial to lingual cusp tip) significantly decreases tooth strength. Even preparations cut with MOD cavities have no significant strength difference between occlusal cavities cut with the same isthmus width.50 However, the fracture pattern of the larger MOD cavities invariably leads to a more catastrophic fracture compared to cavities such as box-only Class II preparations.51 More conservative amalgam restorations exhibit fewer breakdowns than larger restorations.52
Fracture toughness of teeth is related to propagation of dentinal tears caused by tooth preparation and by cyclic loading of the teeth from activities such as bruxing. The behavior of dentin in testing suggests a distribution of pre-existing defects in dentin and that normal mastication and bruxing can initiate dentinal cracks, especially when the normal anatomy of the tooth has been altered by tooth preparation.53,54 Fatigue fractures are caused by more than successive static force applications, but instead by a cyclic fatigue mechanism that involves crack-tip blunting and resharpening as a result of the cyclic application of force.55,56
Any minimization of tooth preparation size and extension of Class I or Class II preparations will increase the longevity of both the restoration and the tooth, and will limit tooth fracture because it creates fewer and smaller dentinal tears, located more centrally rather than facial or lingual.57,58
In an interesting in vitro study, molar cusps were intentionally undermined leaving no dentin support and then restored with unbonded amalgam, bonded amalgam, and bonded composite. The cusps were fractured with an Instron machine. The force required to fracture the undermined but restored cusps was intermediate between cusps with no support and cusps supported by dentin. There were significant differences between the unbonded and bonded specimens. There was a significant difference between the restored versus dentin- supported cusps and between the dentin-supported and unsupported cusps.58 While unsupported enamel is made stronger by bonding, it is still weaker than enamel supported by dentin.59
Restorative Material Improvements
The key to decreasing the development of secondary or marginal caries is to maintain marginal integrity of the materials. Secondary caries is thought to be associated with marginal gaps, voids, and openings along the margins of existing restorations.60,61 As stated previously, bonding of materials will generally improve their durability and marginal integrity.31 Studies of restoration longevity generally indicate that indirect materials outlast direct materials. Two meta reviews of clinical studies found the most common reasons for restoration replacement were secondary caries, fracture, marginal discrepancies, wear, and postoperative sensitivity.62,63
Differences in materials can be significant, as recent research has shown that composites have a higher rate of secondary caries the further posteriorly they are placed compared to amalgam.64 The rate of secondary caries of composites in clinical studies has been shown to be 3.5 times that of amalgam restorations.65 From these and other studies it is apparent that the choice of material for specific restoration locations becomes critical to their longevity.
Amalgam as material has significantly improved since the days of Dr. Black. High copper amalgams, popularized in the late 1960s/early 1970s, have become the standard amalgam materials used. High copper amalgam decreases the gamma two phase, increasing edge strength and decreasing creep. Both properties cause significant issues with marginal breakdown, leading to more secondary caries. High copper amalgams exhibit fewer breakdowns, with dispersed-phase alloys superior to spherical particle alloys.57 Zinc as an additive (approximately one percent by weight) improves the fatigue resistance and results in better marginal adaptation with superior resistance to marginal breakdown.66
Black and others believed the extension of a margin sub-gingivally imparted some protection from plaque and caries formation.2 As research on caries and periodontal disease has improved, today we know this assumption was incorrect. Periodontal research is conclusive in finding that placement of subgingival margins results in loss of attachment in a one- to three-year time frame and is detrimental to the periodontal health.67 An in vivo study over a 26-year period demonstrates subgingival margins do not protect gingival health. Supragingival compared to subgingival margins improve gingival health.68 Bacteriological studies from various patient populations show convincingly that subgingival locations impart no lack of bacteria that cause caries, both smooth surface caries and root caries.69,70,71 Extension of gingival margins subgingivally is only necessary to include caries or old restoratives, to create greater length of preparation for retention, or for esthetics.
Composite restorations generally are lesion specific, regardless of their location in the mouth. That is, the outline form is created with adequate convenience form to excavate carious tissue, establish a sound perimeter, and insert and polish the material. Bonding of these restorations has lengthened their durability. As noted earlier, in Class II situations, the further posterior the composite, the more likely it is that secondary caries will form. This can be attributed to several significant factors.72 The surface free energy of composite materials attracts the Gram-negative bacteria that cause caries. The gingival margin of the preparation must be extended 0.5 mm from the adjacent tooth and must be properly finished. If the operator does not use a hand instrument to remove loose enamel rods, the loose enamel will be fractured due to the polymerization contraction of the composite resin while setting, leaving a significant marginal gap (Figure 6). Marginal gaps cause secondary caries. The gingival margin is the most difficult to isolate properly. Proper isolation is a key to good adaptation and bonding, as is adequate condensation force with no pull back of the material by the condenser.
As explained earlier, Class I outline forms are lesion-specific, as are Class II, III, IV, V, and VI, all limited to removal of caries and convenience for insertion. The preparation outline must extend to an area of the tooth where enamel is sound, not friable. Caries must be removed and sufficient convenience form allowed for insertion. Blind margins in contact with adjacent teeth are adequate as long as they can be planed to remove loose enamel and can be bonded. Leaving undermined enamel is risky, as its strength is between that of unbonded enamel and fully supported enamel.58
The extensions of tooth preparations for glass ionomer are the same as for composite materials, but are applicable to generally Class I, III, V due to the weakness of the material. The outline form is lesion-specific, made to accommodate adequate convenience form for insertion. As this material bonds to tooth structure and provides therapeutic release of fluoride, it need not be extended beyond the lesionspecific criteria. Occasionally glass ionomer materials are used for ART (Atruamatic Restorative Treatment) for the temporary restoration of carious teeth, and are used in Class II, IV, and VI as well. Glass ionomer is also used as a temporary material for use in rampant caries patients.16
The efficacy of bonding has improved our ability to fit the indirect materials with significant durability. Generally used for broken down teeth, for the replacement of large restorations, or to correct some esthetic or functional concern, indirect restorations, including cast gold, porcelain, and indirect laboratory processed composite, can utilize site-specific preparation outlines required to correct the variety of these problems.
Supra-gingival margins generally provide for better gingival health over long periods of time.67 On many occasions porcelain margins can be bonded at or within 0.5 mm of the gingival crest with very little effect on the health of the gingival tissues. Extending sub-gingivally should be done for either significant esthetic concerns or for length necessary for retention. The most favorable response from the gingiva is achieved with the margins at or above the crest of the gingival tissue.73,74
Extensions of partial-coverage indirect restorations should allow for the direct finishing of the margins to permit the best marginal fit with the smallest gap possible. As gap sizes increase, secondary caries increases.32,33,34,35,38,39
Occlusal forces can cause breakdown of margins if the margin is exposed to direct occlusal pressure.75,76,77,78 Most commonly, margins fracture, leaving gaps that promote secondary caries. With cast gold, margins that undergo direct occlusal pressure may also open due to the force of occlusion, especially from bruxing, leaving a gap that can become carious. Extension of preparation margins for indirect materials to avoid direct occlusal contact in either maximum intercuspation or lateral movement will improve durability.
Improved materials and advancing the understanding of science has led to lesion-specific and site-specific outline forms for all restorative materials. While vastly different by degree from the outline forms described by G.V. Black in the early 1900s, the modern designs are determined by scientific principles following the principles of cavity preparation outlined originally by G.V. Black.
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*Dr. Larson is Associate Professor, Department of Restorative Sciences, Division of Operative Dentistry, University of Minnesota School of Dentistry, 8-450 Moos Tower, 515 Delaware Street S.E., Minneapolis, Minnesota 55455. Email is email@example.com.
**“Why Do We Polish? Part One”, Northwest Dentistry, Volume 90, Number 3, May-June, 2011, pages 17-22; “Why Do We Polish? Part Two”, Northwest Dentistry, Volume 90, Number 4, July- August, 2011, pages 31-38; “The Clinical Signifi cance of Marginal Fit”, Northwest Dentistry, Volume 91, Number 1, January- February 2012, pages 22-29.