PG Cert Restorative Dentistry- Porcelain Veneers, October 2020

Porcelain Veneers The preparation technique and type of materials used will influence the chance of long-term success but depend upon an assessment of patient general factors (including patient expectations and consent), pre-operative factors (including occlusion, tooth position, the presenting substrate quality including: colour; existing tooth surface loss and presence of restorations) and a consideration of the anticipated intraoperative factors to be encountered (such as moisture control, access and the dynamic operative challenges faced). Post-operative factors might also include mitigating measures to ensure long term protection of the final restorations in the form of occlusal splints and ensuring clinical follow-up and robust OH maintenance. As a general rule conventional and/or slice preparation techniques are utilised to deliver a pre-planned and agreed end result. The importance of pre-planning is facilitated by using lab made diagnositic wax ups, preparation guides and dudicious modification of working prototypes. Pre-operative clinical photography to document the patient journey provides utility in patient education and managing expectations and offers valuable information to the lab in the delivery of a quality end result.

Workflow materials
Planning: Study models, wax-up, silicone index, reduction guides, shade guide.
Preparation: Speed increasing handpiece (control); selection of diamond burs of variable thickness (0.5mm, 0.6mm,0.7mm, 1mm) to deliver predetermined/planned reductions. Digital calipers to facilitate bur selection. Interdental finishing strips and soflex discs to provide rounded line angles and smooth preparation finish.
Hard tissue management: A suitable adhesive to perform hybridisation (4^th,5^th, universal or system of choice) after preparation (in order to seal dentine from bacterial ingress and protect the pulp). Glycerine to cure the oxygen inhibition layer. Pumice/cups/brushes (to prevent excess resin onto enamel and reduce the chance of the prototype bonding to the hybridised preparations).
Impression materials (light and heavy body e.g imprint 4), modified celluloid matrices to facilitate interproximal separation during impression (creating separate dies at lab stage).
Prototype materials – luxatemp/protemp/luxatemp glaz and flowable composite for additions if required. Alginate to copy final prototypes. Bonding system – 3m system: Rely X ultimate; Scotchbond Universal or Choice 2 System. Silination of veneer 24hrs prior to fit (lab prescription to include request for 120sec 9.5% buffered hydrofluoric acid etch step; requires phosphoric acid etch step in office to remove salt prior to silination).
Moisture control – optragate/ Rubber dam. Other lab prescription requests: only 2 die spacers (reduce risk of fracture during curing of resin cement due to excessive polymerisation shrinkage).

Conventional Preparations (CP)
CP is used where a tooth is in desired position, without interproximal restorations but with labial enamel for bonding present and where the occlusion is favourable without protrusive guidance concerns. A three plane labial reduction is performed taking into account the existing enamel thickness which varies by tooth and labial location. On average and according to Shillingburg, centrals have 0.36mm cervical enamel; laterals 0.39mm and canines 0.29mm. This determines the maximal amount enamel that should be removed to avoid exposing dentine. Bonding to dentine has been shown to be suboptimal and avoided where possible. Suboptimal bonding can lead to leakage and failure at the bond interface, reducing the chance for long term success. Measuring the thickness of our burs with digital calipers ensures suitable selection to perform reductions of 0.3mm, 0.35mm and 0.25mm respectively using 0.6mm, 0.7mm and 0.5mm thickness burs cervically. Suitable bur selection also importantly allows suitable depth cuts and preparation in the other 2 planes to achieve the desired overall reduction of 0.5mm-0.7mm mid-labially and incisally. Incisal edge reduction can be by 1.5 to 2mm but will be dependent on the requirement for space, so might be less in an additive case and may be incorporated into a window style preparation design. Checking the 3 plane reduction to ensure a convex labial shape is important and should be monitored with a silicone index as the reductions progress to avoid over preparation. Proximal preparations extend ½ way interproximally. The incisal preparation can be a butt joint; slanted butt joint (30 degrees) or incorporate a 0.5mm incisal/palatal chamfer. The variations allow for incorporation of additional resistance and retention form and may be part of the decision making process when appraising the availability of sufficient enamel for bonding, particularly at the cervical margins. Interproximal cuts used at the level of the incisal/palatal chamfer are carried out mesially and distally to create a table mountain effect to complete the preparation form and aid in seating and retention. The palatal chamfer feature can be useful in scenarios where additional retention form is required but care to ensure the path of insertion is correct is required to avoid stress concentration or seating problems during fit. Interdental finishing strips are useful to round enamel corners to eliminate sharp line angles which may otherwise fracture off during casting and reduce the accuracy of the fit of the final restoration.

Slice Preparations Where preparations require a more detailed consideration to address other intra-operative factors, a slice preparation is required. This factors may include the need for the closure of a diastema or to manage a missing interdental papilla or prepare teeth with existing interproximal restorations or where teeth are crowded and position movements are desired. The incisal reductions and three plane labial reductions are the same as CP. Slice preparations differ by the need to prepare through the interproximal contact and create 15 degree axial inclination and a 25 degree convergence angle. Preparation of a 0.5mm interproximal chamfer margin using a 1mm chamfer bur is required. All sharp line angles are rounded at the incisal edge to ensure stress reduction transference in the final restoration. A palatal mini-chamfer can be introduced similar to those in CP but depending on the occlusal scheme and where additional retention form is required. Pascal Mange (2002) informs us of the importance of considering the mid-palatal concavity in planning occlusion and finishing margins of the veneer. The palatal chamfer should be 1 mm incisal or 1mm palatal to the veneer margin and not encroach on the mid-palatal concavity. This avoids forces being directed onto the restoration where risk fracture of the restoration may ensue. If occlusal assessment suggests the finish line may otherwise conincide with this area, then a 360 degree veneer may be more suitably planned to achieve a palatlly positioned finish line. Pre-operative planning is therefore pertinent and will require consideration in the early consent phase with the patient informed of the requirement and expectations for the extent of expected tooth tissue removal.

Impression techniques Impression taking using PVS materials should consider the prior hybridisation satge of the dentine. The hybridisation stage is important to seal the dentine and protect the pulp from bacterial leakage. The hybridised tooth surface should be adequately cured under glycerine and cleaned and polished with pumice to remove the OIL. This will also reduce any impeding of the PVS setting reaction and facilitate accurate impression taking. A celluloid matrix technique should be used if a conventional preparation has been carried out. means the interproximal contacts are intact. The matrix strip is prepared with either a rubber dam punch or a fine bur to create a small hole that allows impression material to lock in allowing its withdrawal with the tray upon removal. Without this and where a contact point is maintained in a conventional preparation technique, it will be impossible for the lab to pour a model that has separation between the teeth. They will be required to cut the contacts artificially with a saw blade and this will inevitably damage the preparation margins. The matrix strip placed creates space free of plaster for the tech to successfully handle the model and create accurate restorations. An alternative to this is of course to create additional interproximal space before impression taking using interproximal strips but this may not always be desired. Protoypes placed in luxatemp may be shink fitted and modified to the required aesthetic proportions. It may be appropriate to review the prototypes at a subsequent visit in order to adjust and trial by the patient. An alginate impression and resulting model of the final prototypes can be issued to the lab along with shade and additional photos with a request to copy the final result.

References
Magne, P. Bonded Porcelain restorations in the anterior dentition – A Biomimetic approach. 2002.

Shillingburg, H. Fundementals of fixed prosthodontics. 2012, 4^th Ed.

Shillingburg, H. Fundamentals of tooth preparation: for cast metal and porcelation restorations. 1987.

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