Porcelain Therapy: Veneers vs Jacket Crowns, the ALARA Principle for Prosthodontics

ABSTRACT
This article attempts to elucidate, using two case examples, the key factors that should be considered when deciding between restoring a maxillary incisor with a veneer or with a crown. Relevant literature, providing the scientific basis of the opinions presented, is reviewed to help guide clinical judgement. Long-term predictable bonding can only be achieved to enamel, as dentine bonding degrades over time. As such, veneers should be favoured over crowns in situations where the cingulum is intact, the prep is mostly in enamel, and the margins are in enamel.

INTRODUCTION:
Bonded ceramic veneers were introduced to the profession in the early 1980s.1 At that time it was very unclear as to whether or not this treatment modality would be clinically predictable with a good long-term prognosis. Today we know that when bonded to enamel, feldspathic porcelain has a very high 10-year survival rate approaching 95 percent.2 The evidence supporting non-feldspathic by comparison remains lacking. While outcomes may prove to be clinically acceptable over time, a systematic review shows the five-year cumulative estimated survival rate for etchable non-feldspathic porcelain veneers to be over 90 percent.3

With this level of predictability in mind, it is no wonder that the ceramic veneers have become a highly recommended treatment option for aesthetically oriented dental practices. Essentially, the veneer technique involves bonding a thin porcelain laminate to the facial surface of a tooth using adhesive technologies and a luting composite cement. Veneers can be used to change the shape, shade, size and orientation of a tooth to give it a more ideal appearance. It is typically used to treat teeth that exhibit tetracycline staining, fluorosis, and wear caused by attrition. It is also used to correct a minimally malpositioned tooth, or even to repair a fractured or chipped tooth. The success of this technique is highly dependent on excellent adhesion between three components; porcelain, luting cement and the underlying enamel.

The main advantage of porcelain veneer technique when executed properly is that it can be considered extremely conservative, as they can be as thin as 0.3 to 0.5 mm while producing exquisite aesthetic results, with minimal plaque retention. The disadvantages of veneers are that they can be fragile and technically challenging to fabricate. This necessitates a heavy reliance on a highly skilled laboratory technician who even then has a limited capacity to block out severe discoloration.4

While veneer preparations can vary from no prep, minimally prepared, to essentially ¾ crown veneer preparations, there is a consensus that the preparation should stay within enamel in order to achieve the reported levels of survival.2 It has been shown that the elimination of a palatal chamfer and the creation of an incisal butt margin resulted in a stronger restoration, a more simplified tooth preparation, a more accurate impression and easier cementation procedures.5

There is less consensus and science, however, to guide us as to when a veneer preparation should be converted to a crown preparation. By comparison, a crown is significantly less conservative to tooth structure, and has a higher incidence of endodontic therapy associated with it.6 The evidence available as to when a crown is more appropriate than a veneer is anecdotal. This author’s opinion is simple. If the prepared tooth is less than 50 percent enamel, and the margins terminate on dentine or cementum then a crown should be considered. Veneers use adhesive technologies for retention as opposed to prep design. These require enamel bonding for long termlong-term predictability. The bond to dentine is known to diminish and degrade over time7 leaving our bonded veneer restorations at significant risk especially at the margins. Crowns on the other hand use cohesive technologies based on preparation design principles (retention/resistance form). The other factor to consider when deciding on a veneer or a crown for maxillary anterior teeth is the state of their cingulum. If the cingulum is intact, if 50 percent of the preparation is in enamel, and the preparation margins are in enamel, a veneer preparation should be favoured even if it is resembling a ¾ crown preparation. The reason for this is that extensive proximal cutting and restorations seemed to minimally affect crown flexure. By contrast, the lack of an intact cingulum, as noted in many traditional endodontic access preparations for anterior teeth, was observed to be responsible for most of the loss in crown stiffness.8 Regardless of which restorative solution is employed, the most important aspect of treatment is ending with a result that is satisfactory to that individual patient. Many times this has as much to do with the patient’s experience of treatment as the end result itself.9

The purpose of this article is to highlight two methods of achieving an aesthetic enhancement using porcelain restorations while using clinical judgement to determine the appropriate choice of restoration – porcelain veneer or porcelain crown.

CASE 1:
Susan presented displeased with the appearance of her smile (Fig. 1a). Her main goal was to correct the overlap of her maxillary anterior teeth as well as to correct the discolouration caused by the breakdown of her existing restorations. After consultation with an orthodontist she decided to correct the crowding without orthodontic intervention. She was not looking for a “media driven bleached white tooth shade”, but instead was pleased with the overall age-appropriate natural shade of her dentition. As always the first step was to determine if her occlusion is acceptable and if her TMJ is in a physiologic position. Her TMJ could accept load, her range of motion was not compromised, her dentition was unchanged in the past five years as per patients account and she had a stable occlusal scheme. She did not have any parafunctional habits and did not clench her teeth during the day. As her occlusion and VDO was acceptable, there was no plan to alter either. Preliminary impressions were made and poured up to create study casts. A functional analysis was performed and it was determined with the patient that we could satisfy her immediate aesthetic goals by limiting treatment to her maxillary incisors. The patient was advised that once all restorative material and recurrent decay was removed we would determine the best preparation design to allow for predictable long term function (veneer vs crown), while at the same time satisfying her aesthetic needs. A functional aesthetic wax up was made to allow patient to preview the proposed final result (Fig. 1b). Once approved, two matrices were created. The first is a prep guide with “barn doors” allowing the prepared tooth to be viewed in all dimensions to ensure minimally adequate reduction (Fig. 1c). The second is matrix made out of putty and relined with light body VPS in order to fabricate provisionals that require minimal trimming (Fig. 1d).
Patient was then profoundly anaesthetized with local infiltration utilizing articaine 1:200 000 epi. Existing restorations and decay is removed (Figs. 1e, 1f, 1g). Depth cuts are used to guide tooth reduction (Fig. 1h) while the above mentioned prep guide matrix (Figs. 1i, 1j) is used to ensure minimally adequate reduction to achieve the required alteration. Once the veneer preparation was complete, the tooth was critically observed answering the following questions:. Is the majority of the remaining prepared tooth structure enamel? Is the cingulum intact? Are the margins and finish lines in enamel? If the answers to the above questions are yes, then we can could comfortably choose an adhesively retained porcelain veneer as a long termlong-term functional restorative solution. If the answers are no, then long-term adhesive bonding should be questioned and perhaps a conservative cohesively retained crown should be considered. This usually requires further preparation to allow for an axial path of insertion. For this particular patient, it was determined that the central incisors could be restored with veneers and the lateral incisors with porcelain jacket crowns. Adequate retraction of soft tissue was achieved (Fig. 1k) and two consecutive final impressions are were made using vinylpolysiloxane. The “best” of the two impressions was used to fabricate the final restorations; the other was poured up immediately using quick set stone and used to fabricate indirect provisionals (Fig. 1l). All four restorations were fabricated using the same material (pressed lithium disilicate–Emax) in order to ensure a predictable shade match. The veneers were adhered to the tooth using traditional bonding protocols and a light activated cement. Excess material was removed, margins are were polished and the occlusion was verified. The final result was pleasing to the patient (Figs. 1m, 1n). The patient was advised to start with a softer diet and to gradually introduce a more normal diet as she was adapting to her new teeth. A two-year follow up is shown as the patient has returned to continue on her journey to dental rejuvenation (Fig. 1o, 1p). Although this is only a two-year follow up, the restorations have satisfied the patient both aesthetically and functionally.

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CASE 2:
Cassie presents displeased with the overall shape and colour of her maxillary incisors (Fig. 2a). She suffered a traumatic incident as a child leaving her a necrotic tooth 11 that subsequently discoloured and required endodontic therapy (Fig. 2b). She wanted “fuller looking” more symmetrical front teeth. She wanted the incisal embrasures to be less noticeable, and the central incisors to be more dominant (Fig. 2c). Her TMJ and occlusion were evaluated and determined to be physiologic. Preliminary impressions were made to pour casts for evaluation as well as for diagnostic aesthetic and functional wax up keeping her request in mind (Fig. 2d). A clear matrix was fabricated that acts both as a prep guide and a matrix for fabrication of provisionals (Fig. 2e). The patient was pleased with the proposed changes. Profound local anaesthesia was administered. Using the clear matrix, bisacryl was added to the teeth to provide us a preview of final restorative outline of the teeth (Fig. 2f). Depth cuts were placed into the teeth through the bisacryl in (Fig. 2g) order to minimize the amount of preparation required to achieve the result necessary. Again, once the veneer preparation was complete, the tooth was critically observed answering the following questions:. Is the majority of the remaining prepared tooth structure enamel? Is the cingulum intact? Are the margins and finish lines in enamel? If the answer to the above questions was yes, then we can could comfortably choose an adhesively retained porcelain veneer as a long-term functional restorative solution. If the answer was no, then long-term adhesive bonding should be questioned and perhaps a conservative cohesively retained crown should be considered. This usually requires further preparation to allow for an axial path of insertion. For this particular patient, the endodontic access in tooth 11 structurally weakened the tooth by removing a significant portion of the cingulum. For this reason, it was decided that a full coverage restoration would be a more appropriate restorative solution for this tooth. The remaining three teeth, 1.2, 2.1, and 2.2 were prepared to accept veneer restorations. Adequate retraction of soft tissue was achieved (Fig. 2h) and two consecutive final impressions are were made using vinylpolysiloxane. The “best” of the two impressions was used to fabricate the final restorations, the other was poured up immediately using quick set stone and used to fabricate indirect provisionals (Fig. 2i). All four restorations were fabricated using the same material (pressed lithium disilicate–Emax) in order to ensure a predictable shade match. The veneers were adhered to the tooth using traditional bonding protocols and a light activated cement. Excess material was removed, margins were polished and the occlusion was verified. The final result was pleasing to the patient (Figs. 2j, 2k). The patient was advised to start with a softer diet and to gradually introduce a more normal diet as she was adapting to her new teeth. At a two-week follow up, gingival health was obvious and the small “dark triangle” between the central incisors was closed (Figs. 2l and 2m). This case highlights the decisions that are required of us every day in practice and attempts to support the clinical decisions made with science. The final result was able to satisfy the patient’s aesthetic needs while not adding excessive functional risk to her dentition.

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CONCLUSION:
Patients present with aesthetic concerns every day in practice. It is often challenging to balance the aesthetic needs of the patient while minimizing the amount of tooth structure that needs to be removed. Not too long ago, veneers were considered a panacea for the aesthetically driven patient. It would be exclaimed from the podium that we should remove as much tooth structure as necessary to achieve the aesthetic result with utter disregard to the enamel required to achieve a predictable bond. Thankfully the pendulum has swung back to a more conservative approach. Just as there is the ALARA principle in radiology (As Low As Reasonably Achievable), we should apply the same principle to our treatment plans and tooth preparations ALARA (As Little As Reasonable Achievable). We should always attempt to be as conservative as possible when treating patients to minimize their risk. As my good friend and mentor Dr. John Kois says, “There is no dentistry better than no dentistry”.OH

Dr. Joseph Fava earned his DDS, MSc. and prosthodontic specialty certificate at the University of Toronto. His MSc focused on dental implants in the aesthetic zone. He currently instructs in the IPU at the UofT and is actively involved in clinical research. He maintains a specialty practice in Yorkville whose scope includes cosmetic dentistry, surgical and prosthetic components of dental implants as well as advanced restorative and reconstructive full mouth therapies. He is a distinguished Kois Mentor and Co-director of the University of Toronto’s Implant Residency Program.

Oral Health welcomes this original article.

References:
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2. Layton DM, Clarke M, Walton TR, A systemic review and meta-analysis of the survival of feldspathic porcelain veneers over 5 and 10 years, Int J Prosthodont 2012:25:590-603

3. Layton DM, Clarke M. A systematic review and meta-analysis of the survival of non-feldspathic porcelain veneers over 5 and 10 years, Int J Prosthodont 2013:26:111-124

4. Peumans M., Van Meerbeek B., Lambrechts P., Vanherle G, Porcelain Veneers: a review of the literature. Journal of Dentistry 28 (2000) 163-177

5. Castelnuovo J, Tjan A., Phillips K., Nicholis J., Kois JC., Fracture load and mode of failure of ceramic veneers with different preparations J Prosthet Dent 2000:83:171-80

6. Whitworth JM, Walls AW, Wassell RW, Crwons and extra-coronal resotrations: endodontic considerations: the pulp, the root-treated tooth and the crown. Br Dent J, 2002 Mar 23; 192(6): 315-20, 323-7

7. Shokati B, Tam LE, Snterre JP, Finer Y, Effect of salivary esteraseon the integrity and fracture toughness of the dentin-resin interface. J Biomed Mater Res B Appl Biomater 2010 Jul; 94(1) 230-7

8. Magne P, Doublas WH, Cumulative effects of successive restorative procedures on anterior crown flexure: Intact versus veneered incisors Quintessence Int. 2000;31(1):5-18
Fava J, Lin M, Zahran M, Joksatd A. Single implant supported crowns in the aesthetic zone: patient satisfaction with aesthetic appearance compared with appraisals by laypeople and dentists. Clin Oral Impl. Res, 2014, 1-8