The bonded porcelain veneer restoration (BPVR) has undergone a dramatic transition over the past two decades. It was initially introduced as a conservative alternative to a complete coverage porcelain crown. This is because instead of achieving retention from a circumferential tooth preparation, it comes from micro-mechanical resin tags. The lifelike esthetics of modern ceramic materials, coupled with the conservation of tooth structure and the convenience of a laboratory-fabricated restoration, made the BPVR an extremely popular procedure. Unfortunately, today’s approach to the BPVR by some clinicians, bears little resemblance to the original conservative concept. Some of these changes may have been influenced by improved dentin adhesive technology and others likely came from dental laboratory technicians. This disturbing transition of the BPVR should concern the dental profession because it may adversely impact the prognosis of the restoration as well as the long-term oral health of the patient.
A HISTORICAL REVIEW OF ACID ETCHING
The concept of acid etching enamel, initially described by Buonocore, coupled with Bowen’s formulation of the backbone resin for composite restorative materials, has changed the practice of dentistry forever.1-3 By the late 1970s, the conservation of healthy tooth structure became a mainstream philosophy in the dental profession. Specific cavity preparation designs for posterior teeth were being replaced with pit and fissure sealants and preventive resin restorations (PRRs) in an effort to preserve and protect as much natural tooth structure as possible.4-6
Enamel became the revered substrate for adhesive dentistry procedures and this trend was particularly beneficial for young patients.7 These conservative, interceptive procedures reduced a young patient’s risk of requiring large occlusal restorations and it is reasonable to assume that the need for more extensive restorations, gold castings, ceramic crowns and root canal therapy as an adult, will be reduced as well. The positive impact of acid etching enamel also changed the restorative philosophy for anterior teeth. Adhesive dentistry not only provided techniques for greater conservation of tooth structure, but also ushered in an era of direct filling materials that looked like natural tooth structure. When visible light activated composites were introduced, the field of elective, esthetic restorative dentistry grew at a rapid pace.8-9
The increased interest in esthetic restorative dentistry made direct composite bonding a highly sought-after procedure.10-12 Because it could be performed with little if any rotary instrument preparation, it was promoted as a reversible procedure. Direct composite bonding appealed to the artistic dentist and it removed the variability factor of a laboratory- fabricated restoration. Moreover, the dental practitioner now had the materials and methods to conservatively alter a patient’s smile with little risk to long-term dental health. It is reasonable to speculate that the reversibility aspect of direct composite bonding may be in part, why it was such a popular procedure with dentists and patients alike.
Irrespective of a practitioner’s artistic skill and clinical expertise with direct composite bonding, resin materials eventually exhibit changes. Cohesive fractures, wear and discoloration are evident over a period of time.13-14 By mid-1980, countless numbers of patients had undergone direct composite bonding and many of these restorations were no longer esthetically pleasing. In certain instances, the underlying teeth were more esthetically pleasing than the overlying composite bonding and correction involved simply removing the bonding. In other instances replacement restorations were required because the underlying conditions were not esthetically acceptable. The practitioner is faced with a decision when a patient presents with deteriorated direct composite bonding. Either replace the composite bonding or select another restorative option that would provide better longevity and possibly require less artistic skill and chair time to perform (Figs. 1-3). In the early 1980s, Calamia and Simonsen published their classic paper on bonded porcelain veneer restorations.15-17 They could never have anticipated the “tsunami” of esthetic, restorative dentistry that would result from their contribution to the literature.
THE EARLY PORCELAIN VENEER RESTORATION
The bonded porcelain veneer restoration (BPVR) is an innovative concept. It is an ultra-thin, porcelain shell retained to the tooth surface through micro-mechanical retention via resin tags. The early porcelain veneers were fabricated from stacked, feldspathic porcelain on refractory die material or platinum foil.18 The composite resin that represented the “weak-link” in direct composite bonding in terms of fracture, abrasion and stain resistance, is protected beneath the more impervious porcelain veneer surface. This limits the risk of composite degradation to the small seam of material that bridges the marginal interface between the porcelain and the tooth surface. Like direct composite bonding, a BPVR can be performed with little if any tooth preparation and that is how it was initially promoted.19-20 However, many clinical situations dictate the need for some tooth preparation to prevent unacceptable over contouring.
Clinical experience has demonstrated that as little as 0.5 mm of axial and incisal tooth reduction allows adequate space to fabricate a stacked porcelain veneer and still maintain a mostly enamel substrate.21-24 Although challenging for the dental technician, a veneer fabricated at this minimal thickness can also provide reasonable esthetics, even when masking a dark undesirable discoloration such as that found in patients who have tetracycline-like staining25 (Figs. 4-5). However, it is also axiomatic that the greater the porcelain thickness, the more room there is for maximizing the inherent, esthetic characteristics of the material.
Unfortunately, many dentists today are not electing to use a conservative approach for BPVRs. Many well written clinical articles promote an aggressive tooth preparation for BPVRs.26 This is a significant reversal in philosophy and the trend is especially disturbing because many patients seeking these procedures are young individuals who may not fully comprehend the long-term ramifications of invasive, esthetic restorative dentistry. Recent television programs promoting a “fix-all” approach using BPVRs, have further added to the confusion and misrepresentation of this procedure.
LABORATORY PRESSURE FOR GREATER VENEER THICKNESS
As more clinicians add BPVRs to their “menu” of available services, greater numbers of laboratory technicians are needed to meet the increased demand. Some of these technicians may not be highly experienced in fabricating porcelain at ultra-thin dimensions. Traditional porcelain techniques dictate fabrication thickness greater than 0.5mm.27 In addition, many laboratories are highly invested in pressed ceramic equipment and workflow.28 This procedure is calibrated to produce restorations at 0.75mm – 1.5mm in thickness or more. If a tooth is in ideal arch position and the BPVR is fabricated to a thickness of 1.5mm, the restoration will be over-contoured unless the dentist removes that amount of tooth structure.
The trend for greater axial tooth reduction for porcelain, pressed ceramic or CAD-CAM veneers may stem directly from influence by the laboratory technician. Most dentists probably rely on their dental technician for guidance about new ceramic materials and techniques, so it is reasonable to speculate that the practitioner will follow the technician’s recommendations relative to the design of a tooth preparation. A central question to consider is what price is paid for a change in philosophy regarding tooth preparation? Some tooth preparation designs for A BPVR look identical to a reverse three quarter crown (Fig. 6).
When the tooth preparation
is entirely into dentin, the reliance on dentin bonding becomes paramount since very little frictional retention from reciprocating walls exists with a veneer. The net result is an increased potential for short-term failure of the adhesive bond and the subsequent need for re-treatment.
BENEFITS, RISKS AND ALTERNATIVES
The lay public may consider that the average porcelain veneer restoration is as innocuous as direct composite bonding was 20 years ago, but that is certainly incorrect. Every dental practitioner has the ethical responsibility to develop and present treatment plans that are based on the clinical conditions presented, patient concerns/desires and the most current scientific evidence that substantiates a particular approach.29
Interdisciplinary options should also be presented whenever they are appropriate.30 In other words, patients must be fully informed about the benefits, the risks and the alternatives before any treatment is initiated. This author has observed scores of failed restorations in recent years where these guidelines are being completely ignored. The concept of “instant orthodontics” is another disturbing concept that emerged from the BPVR era.31-33 Fortunately, some dental professionals are voicing objectives to these negative trends in the profession, but more dialogue is needed on this subject.34-38
Beyond the issue of misrepresentation to a patient, is the projected longevity and follow-up treatment requirements when restorative dentistry is initiated in lieu of orthodontics, for example. If a BPVR does fail due to micro-leakage or de-bonding, does a replacement restoration have a greater or lesser chance of success (Fig. 7). The answer is not a simple one and will be based on a number of factors such as; the condition of the substrate, the amount of enamel that still exists, the occlusal scheme and the experience of the practitioner. However, if the patient with failing BPVRs expresses concern regarding the maximum reliability against de-bonding and fracture of the replacement restorations, the most reliable alternative is traditional crowns.39-40
The complete coverage crown imparts frictional retention via a ferrule design of the tooth preparation and does not rely completely on adhesives for retention. The complete coverage crown restoration is the antithesis of a conservative restoration and yet many are being fabricated simply to rectify a failed porcelain veneer restoration. The possibility of crown replacement, root canal therapy and even tooth loss are increased once the first crown is placed on a tooth.
The current scientific literature does not provide convincing evidence to support aggressive tooth preparations for bonded porcelain veneer restorations.41-43 It is clear that methods and materials for dentin bonding have been vastly improved over the past 20 years. Nonetheless, the longevity of the BPVR still favors an enamel substrate for best predictability.44 It will require five to 10 more years to scientifically validate whether or not a porcelain veneer restoration that is bonded exclusively to dentin, with materials currently available, will share the same longevity characteristics as those that are bonded to enamel. Therefore, what should be included in the informed consent discussion with patients? If a failure occurs, will the remaining tooth structure require further tooth preparation for frictional retention from a traditional crown? Will a replacement porcelain veneer have a reasonable chance of success?
The answers to these questions are certainly based on numerous factors beyond the scope of this paper. However, if more patients were informed that a porcelain veneer restoration failure might result in a traditional complete coverage crown, would that influence their decision to electively undergo the procedure? If a patient were not informed of this possibility and it became a reality, would the dentist be liable for incomplete informed consent? These are important issues that every practitioner who provides such services needs to consider and include in their dialogue with patients.
THE IDEAL BONDED PORCELAIN VENEER RESTORATION
There are some clinical conditions that favor long-term success of porcelain veneers based on current available literature research.45 A maxillary lateral incisor, for example with a tooth size discrepancy (peg lateral) on an adult patient, is the most ideal scenario for a porcelain veneer restoration (Figs. 8-9). In terms of informed consent, the option of direct composite bonding and orthodontics (if potentially beneficial) is considered and disclosed to the patient as well as the benefits and limitations. The reason that a peg lateral is ideal for a porcelain veneer on an adult patient is that the tooth preparation is minimal in depth, virtually insuring an enamel substrate will remain. Only a peripheral finish line is required at a depth of 0.5mm or less in some instances. More importantly, the restorative material will increase the tooth thickness and usually it is not masking an undesirable color. Both the added thickness and the lack of a masking requirement, makes it easier to achieve ideal esthetics from the porcelain material.
A translucent porcelain blend has a positive impact on porcelain esthetics; provided a translucent composite is used for cementation. The most ideal finish line placement for any bonded porcelain veneer restoration is coronal to the free gingival crest. This not only obviates the potential risk of a negative periodontal response from the restoration, but it also increases the probability that the finish line will end on an enamel substrate for more predictable bonding. Furthermore, it simplifies the operative procedures by removing the need for soft tissue management strategies during the tooth preparation and impression stage. In addition, it makes isolation and moisture control much easier to achieve at the cementation visit.
ENAMEL CERAMICS AND CONTACT LENS EFFECT
Adapted from the term metal ceramics, enamel ceramics describes a tooth preparation approach that preserves a substrate of enamel, especially at the peripheral finish lines.46-48 The metal coping of a metal ceramic restoration provides both a reliable bonding substrate and needed stiffness of the veneering porcelain layer.49 Therefore the term enamel ceramics refers to a bonded porcelain restoration where enamel is the primary substrate for bonding. Even if areas of dentin “exposure” occur within the confines of the peripheral finish lines of the preparation, the term enamel ceramics still applies as long as the margins of the veneer end on etched enamel.
Like the metal of a metal ceramic restoration, the enamel provides the stiffness to the ceramic material that is bonded to its surface.49 Research suggests that enamel ceramic conditions greatly reduce the risk of restoration failure from micro-leakage and de-bonding.50-51 When enamel ceramic conditions exist, the most likely failure mechanism will be cohesive fracture of the porcelain. Teeth that are axially retroclined (such as Class II, Division II malocclusion) make enamel ceramic conditions easier to achieve. In situations where the teeth are proclined, orthdodontic treatment can re-position the teeth such that enamel ceramic conditions can be developed. If a tooth is proclined or significantly rotated, it is unlikely that a porcelain veneer preparation will meet the criteria for enamel ceramic conditions.
It is well established in the periodontal literature that soft tissues will not demonstrate a negative biological reaction if margins are placed coronal to the free gingival crest.52-53 Esthetic demands dictate that the restorative margins of metal ceramic crowns and even some all-ceramic crowns be positioned apical to the free gingival crest. One of the unique characteristics of ultra-thin porcelain veneer restorations is that they can be positioned coronal to the free gingival margin and still remain virtually invisible.
The optical phenomenon of contact lens effe
ct (CLE) observed with porcelain veneer restorations, was first defined in 1991 and it has been widely misunderstood.54-55 It describes how the porcelain veneer restoration can be intentionally designed to blend almost imperceptibly with the adjacent natural tooth structure. It is achieved by modifying the relative translucency of both the porcelain veneer restoration and the luting composite to insure that the value and chroma of the underlying tooth structure can subtly influence the overlying veneer.
A translucent porcelain blend is used at the periphery of the veneer such that the periphery virtually disappears in the same manner that a clear contact lens disappears on the moist cornea of the eye. Stacked porcelain veneers can be fabricated to titrate the CLE and the opacity at very specific locations.
For example, if the objective of a porcelain veneer restoration is to mask a dark underlying substrate, the technician will incorporate opacious (slightly opaque) blends within the body porcelain to reduce the inherent translucency of the restoration and increase the value. To insure a CLE at the margin, the opaque blend needs to be diluted in the area of the periphery. Otherwise, the margin will have a much greater value than the adjacent tooth structure and this will make it visibly distinct from the tooth structure. In contrast, if the opacity is gradually reduced at the peripheral 0.5 – 0.75mm. of the restoration, the underlying value and chroma will help make the porcelain margin blend almost imperceptibly with the tooth structure (Fig. 10).
When a highly translucent composite is used, the contact lens effect is maintained. If the luting composite contains a high concentration of metal oxide particles making it opaque, the CLE is cancelled and the periphery of the restoration will be distinct from the adjacent tooth structure (Fig. 11). Thus, the CLE promotes a conservative approach to tooth preparation as well. Because the margin can be virtually invisible, it can be intentionally positioned coronal to the soft tissue. This insures better periodontal health, simplifies the operative procedure and extends the esthetic longevity.
Even if the free gingival crest migrates apically over a period of time, the esthetics of the porcelain veneer restoration with CLE is relatively unaffected. More importantly, this approach also promotes an enamel ceramic tooth preparation because the concern for a visible line of demarcation at the veneer margin is reduced. Even in clinical situations where apical soft tissue migration has occurred, the tooth preparation for the veneer can still terminate on enamel (Fig. 12). Contact lens effect cannot be precisely titrated in veneer restorations that are fabricated with a pressed or milled technique. This is due to a greater thickness of these fabrication techniques as well as a lack of precision control of translucency at specific locations within the restoration.
CONCLUSION
The bonded porcelain veneer restoration was first described over 20 years ago as a conservative alternative to a traditional porcelain crown. Stacked, feldspathic porcelain as well as pressed and milled ceramic methods can be used to fabricate veneers. The inherent limitations of the various fabrication techniques, affects the thickness of the restorations and impacts the tooth preparation design. Presently, the most reliable long-term resin bonding is achieved on an acid etched enamel substrate. Interdisciplinary options such as orthodontics should always be considered to preserve enamel or remove the need for a restoration altogether.
Patients must be fully informed about benefits, risks and alternatives to the BPVR. This is particularly important with young individuals because the dental restorations may need multiple replacements in a lifetime. Hopefully, the vast majority of BPVRs will not leak or de-bond in an abbreviated period of time. Otherwise, countless of these restorations may be transitioned to complete coverage crowns and the inevitable cycle of replacement, possible root canal therapy and occasional tooth loss. It is a sad commentary on the dental profession if that is beginning to occur already.
Even the most skilled and meticulous restorative dentist, enlisting the most experienced and artistic dental technician, employing the finest dental materials available today, pales by comparison to the perfection of a natural tooth. Accepting that irrefutable fact should give pause to every restorative dentist before he or she removes enamel from an non-restored tooth for an elective, esthetic restorative procedure.
Mark J. Friedman, DDS: Graduate and professor of clinical dentistry, University of Southern California, School of Dentistry. Private practice at the Center for Dental Aesthetics in Encino, CA
Oral Health welcomes this original article.
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