The finished crown will be fabricated according to the prescription that was provided to the laboratory together with the impression and all other necessary records, and returned ready to insert into the mouth.
The insertion process for the finished crown includes:
- Trial fit of the crown extra and intra-orally, including any necessary pre-cementation adjustments,
- The permanent cementation procedure and
- The final post-cementation assessment and adjustment.
If an optimal impression, appropriate opposing model and all other necessary information have been correctly provided to the laboratory, and if the tooth has been effectively temporized to maintain its position within the arch, adjustments should be minimal and the cementation appointment relatively straightforward. Care at all stages of the clinical and technical processes will minimise problems and adjustments.
The objectives of the final cementation process are to:
- Permanently secure the crown against dislodgement (i.e. to secure the inherent retention provided in the preparation design)
- Permanently seal the prepared tooth against marginal leakage (i.e. to seal the preparation margin and internal dentine from ingress of oral fluids and bacteria) and, for vital teeth,
- Ensure continued pulp vitality.
The final crown should provide optimal anatomical relationships with the remaining tooth structure, the gingiva, the adjacent teeth and the opposing dentition. For esthetic situations, the final crown should provide appropriate shape and anatomical relationships as well as optimal colour matching of shade (hue), brightness (value) and translucency (chroma) to the adjacent teeth.
Background on Major Causes of Failure of Crowns
Clinical studies have shown that a) recurrent caries and b) lack of retention are the major causes of failure of traditional crowns and bridges.1,2,3 Recurrent caries is the foremost cause of failure but displacement of crowns can occur earlier. Optimal marginal accuracy and preparation retention form are therefore two of the most important pre-requisites for crown longevity that are under the control of the dentist.
A smooth, well-marginated crown, with confluent tooth to crown continuity (no open margin and no marginal discrepancies) will provide maximum resistance to plaque accumulation around the crown margin. Caries prevention over the long-term also requires good patient oral hygiene for regular removal of plaque from crown margins. Other patient caries risk factors will also affect the potential for new decay at crown margins. It is well recognized, moreover, that even with the most optimal clinical and technical procedures, a microscopic cement line will be exposed to the oral environment at the crown margin. The smaller this cement line is, and the more resistant the enclosed luting cement to dissolution, the better - in order to prevent development of an open margin due to cement loss, with consequent plaque retention between crown and tooth. Laboratory technique that utilizes the use of die spacer to provide cement space can substantially facilitate complete seating of crowns during cementation.4
With regard to crown retention, it is well accepted that most conventional luting cements only secure the retention provided by optimal crown preparation, designed to ensure maximal preparation length, minimal occlusal taper and the presence of an optimal circumferential “cylinder of parallelism” on sound tooth structure. No cement can compensate over the long-term for inadequate retention form in the crown preparation.
Background on Dental Luting Cements
Traditionally zinc phosphate cements were used for cementing but, although used quite successfully for decades, these cements show some shortcomings in strength, solubility and lack of adhesion.5, 6 They also are extremely acidic when first mixed and require the use of varnish for vital pulp protection.
In an effort to provide improved clinical success, different classes of luting agents with improved physical properties, adhesion possibilities and potential therapeutic capabilities have been developed. Currently, four types of luting cement are commonly utilised. Each has different advantages and disadvantages.
- Glass Ionomer Luting Cements. Conventional glass ionomer cements provide fluoride release and show low, but stable, adhesion to dentine. They have been implicated in untoward pulpal sensitivity due to the low initial acidity when mixed, which is compounded if too low powder-liquid ratio is used during preparation.7 However, clinical studies have shown the material to be extremely effective when used appropriately.8 They have largely been superseded by the resin-modified glass ionomer cements.
- Resin-Modified Glass Ionomer Luting Cements. More recently developed, the resin-modified glass ionomer luting materials also provide fluoride release but show increased resistance to dissolution and improved physical and biological attributes over the conventional glass ionomers. They are easy to use and are widely used in general practice for routine cementation of crowns and bridges. They do not require any tooth pre-conditioning, provide stable bonding to tooth structure and have intermediate physical and mechanical properties. They have not been associated with increased levels of post-operative sensitivity. A resin-modified luting cement is currently used in the clinic for routine crown cementation.
- Resin Luting Cements. The success of current-day dentine bonding materials and resin composite materials has led to the development of resin luting cements with the potential to provide greatly increased bonding capability, retention potential and resistance to dissolution.9 However, resin luting cements involve multiple steps and are much more technique sensitive than conventional water-based cements. They are essential for cementation of many all-ceramic crowns to prevent crown fracture,10 but are also useful in low retention conventional crown or post situations.11,12 Critical usage factors and idiosyncrasies make the use of resin cements for routine procedures more difficult and these cements are not a guarantee against postoperative sensitivity.13 They are not essential for cementation of routine metallic-based crowns and bridges with adequate retention form. A resin luting cement is available in the clinic for low retention crown and/or post clinical situations.
- Self-Etch Resin Luting Cements. In order to overcome the technique-sensitivity of most resin-based luting cements, which require additional tooth conditioning, priming and bonding procedures, a self-etching resin cement was first introduced in 2002. Such cements do not require any additional bonding procedures prior to use and offer a simpler, less technique-sensitive, one-step approach to resin luting and bonding. Although described as a single “class” of materials, substantial differences in properties and performance exist between the different proprietary products, some of which have been very recently introduced to the market-place.14 Further investigation of properties and performance will be required for these and other new proprietary materials of this type as they are introduced. Only use of products with substantive evidence of appropriate properties is recommended. Good examples of such cements offer excellent potential for improved cement performance.
Background on Sealing and Desensitisation of Dentine
The use of dentinal resin primers alone can provide extremely effective sealing and desensitization for freshly cut vital dentine after crown preparation. Tooth reduction and exposure of large areas of freshly cut dentine during crown preparation can lead to increased permeability and risk of pulpal inflammation.15,16 Dentinal primer/sealers have been recommended to decrease sensitivity caused by increased permeability and/or microleakage. At the faculty the resin primer/sealer “Hurriseal” is available for this purpose. Such procedures are best carried out at the tooth preparation appointment, after fabrication of the temporary crown and prior to temporary cementation. They also may be performed immediately prior to permanent cementation, especially when using acidic conventional luting cements - to prevent postoperative sensitivity. Current proprietary haemostatic agents, often used prior to the impression, have been shown to have an extremely low pH, can remove the dentinal smear layer opening up dentinal tubules on the root surface which can potentiate the effect of any subsequent cement acidity.17 If acidic haemostatic agents have been used, a dentine primer-sealer is recommended in all cases to seal the dentinal tubules of vital teeth. Resin sealers can be used successfully with glass ionomer and resin-modified glass ionomer cements without reducing crown retention.18
Permanent Cementation Procedure
1) Trial Fit of Crown (“Try-In”)
a) Extra-oral assessment. The delivered crown will first be assessed on the working die and model that was poured by the laboratory from the final impression. While seated on the die the appropriateness of the overall anatomic form in relation to the adjacent teeth and arch, the adequacy of the proximal contacts, the general surface smoothness and the quality of the crown margins should be assessed. Ideally the restoration should be checked on the model before the patient appointment.
The working die will have been “trimmed” by the technician to clarify the exact margins of the crown preparation. It is considered advisable (and is expected at the faculty) to request an “untrimmed sectional pour” from the same impression that will be available to assess the accuracy of the trimming process and to confirm the marginal accuracy of the finished crown on the prepared tooth. The internal surface of the crown, sometimes called the intaglio surface, should also be examined. There should be no evidence of pitting, voids or discrepancies that are not present on the crown preparation. Evidence of internal adjustments should be minor.
In most teaching institutions, as part of the learning process and for quality assurance reasons, the metal coping of a PFM crown will be assessed at an initial try-in appointment. Assessment at this stage will include ensuring that the metal coping provides optimal thickness for strength, appropriate marginal finish and adequate space for porcelain build-up to ensure optimal esthetics and contour of the finished crown. Intra-oral assessment will include the relevant elements of fit and form appropriate to the metal coping.
b) Intra-oral assessment. If satisfactory on the model, the crown will next be assessed intra-orally. Note that all copings and completed crowns should be placed in an appropriate disinfectant for 3 minutes prior to placement in the mouth. At the faculty the all-purpose laboratory disinfectant “Lab-X” is available for this purpose.
The prepared tooth should be asymptomatic. Do not attempt cementation if the tooth displays any untoward symptoms or the patient has discomfort. The cause should be ascertained and treated to resolve any symptoms. The temporary crown may be inadequate and should be checked to ensure that all prepared tooth structure is covered, that the contacts are satisfactory and that the occlusion is not “high” in centric occlusion or in lateral movements of the jaw. If pulpal symptoms are present that are subsiding, it is advisable to allow a prolonged period of temporization for total resolution. If persistent pulpal symptoms are present and attempts at resolution are unsuccessful then endodontic treatment may be required.
[Note: It is sometimes recommended that the permanent crown be temporarily cemented where symptoms are present or where certain aspects of the crown (esthetics or gingival response) are being monitored. This is considered ill-advised for the reason that a well prepared crown preparation with a well fitting crown is invariably impossible to remove when the decision is made to perform permanent cementation. Temporary cementation of the permanent crown is therefore not allowed at the faculty. A prolonged period of temporization should be provided with an optimal temporary crown.]
The need for local anaesthesia for the cementation process is both patient and tooth specific. The presence of anaesthesia can prevent the patient assisting in assessment of the occlusion, however many prepared teeth can be extremely sensitive. Endodontically-treated teeth will not usually require anaesthesia. Vital teeth with extensive freshly-prepared dentine will typically require anaesthesia, particularly in the young patient, teeth with large pulps and for patients who tend to have sensitive teeth. Previously restored teeth in older patients with receded pulps are typically less sensitive. The use of dentine primers for dentine sealing prior to the placement of the temporary crown can significantly reduce the need for anaesthesia at the cementation appointment. For resin luting cements that require acid conditioning of the prepared dentine, anaesthesia will definitiely be necessary.
The temporary crown should be carefully removed and all temporary cement fragments thoroughly cleaned from the tooth using hand instruments. No visible remnants at all should remain as they may interfere with accurate seating and cement attachment. The effect of eugenol containing temporary cements on the bond strength of resin luting cements has been implicated in reduced bonding with resin cements. However, microscopic remnants of temporary cements in general, rather than eugenol-containing cements in particular, have been implicated in reduction of bonding of adhesive cements to tooth structure. Eugenol-containing cements provide anti-bacterial effects and have a unique sedative effect on the prepared tooth and therefore can be extremely effective in preventing post-operative problems after crown preparation.
Prior to placing the crown in the mouth the patient should be advised of the fact that a try-in of the crown will occur and that to avoid crown dislodgement the patient should not try to talk, nor move suddenly. Further, it should be explained to the patient that he/she should remain alert and should the crown slip from the tooth he/she should immediately bring his/her head up and discharge the crown into their own hand. Safety during the try-in is extremely important to avoid the patient swallowing or inhaling the crown.
With the patient in a semi-supine position the disinfected crown is placed on the tooth and the position maintained with a finger placed on the occlusal surface at all times. The fit of the crown margin is first assessed using the tine of an explorer around the total margin periphery, ensuring that the crown remains completely seated on the tooth. There should be a relatively smooth transition from crown to tooth with no visible crown opening, overhang or other discrepancy. The crown should be “flush” with the tooth surface and adjacent contours of the crown should be continuous with the contours of the tooth. A totally imperceptible margin is not possible. It has been suggested that because the smallest detectable ledge is 100microns, this is a useful criterion for evaluating adequate marginal fit.
If the margin appears to be open along at least one surface the crown may not be completely seated. The commonest cause of non-seating is overly tight proximal contacts. These should be assessed using dental floss and adjusted if hindering the seating of the crown. Other causes of non-seating include remnants of temporary cement or other debris on the tooth surface, trapped gingiva and/or other discrepancies on the internal surface of the crown. The latter should be carefully assessed and, if necessary, the internal fit can be confirmed using a coating or disclosing medium to reveal areas of binding. The internal surface is coated and the crown seated and removed. Areas where the metal shows through the coating can be adjusted to allow accurate seating. “DeMark” coloured disclosing solution is available at the faculty which dries quickly on contact and is the preferred method for accurately locating proximal contact and internal interferences. The use of disclosing powder sprays (eg “Occlude”) is discouraged due to the widespread travel of the coloured spray. If contacts and internal surface are satisfactory and an open margin persists that cannot be overcome, a new impression will be necessary for fabrication of a new crown.
Further assessment of the shape and adequacy of the proximal contacts should be performed using dental floss. The proximal contact of the new crown should provide optimal tightness against the adjacent tooth as well as appropriate contact area and contour. There should be a definite “snap” as the floss traverses the contact. Care should be taken during this process to ensure that the crown does not displace during the procedure. One finger should always be kept firmly on the tooth. If difficulties are encountered, a dental assistant should be asked to provide assistance. To assess the contact area the floss should be wrapped around the contact and the ends held parallel a) towards the occlusal surface to assess the bucco-lingual width and b) towards the facial surface to assess the occluso-gingival width. Contact measurements and placement should be harmonious with other contacts in the patient’s mouth. Typically the width of the floss bucco-lingually should approximate one-third the width of the tooth and the width occluso-gingivally should approximate one-third the height of the crown from the gingival. Adequate facial, lingual, gingival and occlusal embrasures must be present to prevent food impaction and periodontal problems.
The final assessment involves occlusal assessment. With the crown accurately seated the patient is asked to close the teeth and the position of the crown during maximum cuspal position is assessed. Use of a mirror in the buccal sulcus will show if there is any significant occlusal discrepancy. Significant occlusal opening will require the use of coloured fine articulating paper to determine the areas requiring adjustment. If there are no visible signs of significant occlusal discrepancy using a mouth mirror, shimstock should be used to assess for the presence of perfect occlusal contact in the areas of normal centric stops. For a posterior crown the shimstock is placed on the occlusal surface of those teeth immediately adjacent and on either side of the crown and for anterior crowns on the nearest posterior teeth. “Tug-back” should be present. If “tug-back” is not present on appropriate adjacent teeth, the crown is judged “high” in occlusion and will require identification of high spots using fine coloured articulating paper and adjustment until “tug-back” is present. Articulating paper will mark a dry crown better than a wet crown but sometimes the highly polished surface is resistant to marking. Use of the working model and opposing model is often helpful to identify the areas most likely to be “high”. Slight adjustment in these areas can be followed by confirmation with articulating paper intra-orally which is more likely to produce a mark on a slightly roughened surface. If “tug-back” is present on appropriate adjacent teeth using shimstock in centric, the shimstock is then moved onto the crown itself, to ensure the crown is itself in occlusion. For posterior crowns and canines, “tug-back” should also be present on the crown. For incissor crowns, the shimstock should just slip through the lingual surface.
Once centric occlusion is satisfactory lateral movements – any gross discrepancies (working or non-working interferences) should be adjusted. The patient is asked to bite and then guided laterally from one side to the other keeping the teeth in contact. Final assessment of lateral movements is safer when carried out post-cementation – particularly where group function is present. Once the occlusion is considered perfect, any areas that have been adjusted will require smoothening. Final polish is best left until the crown occlusion has been finally assessed as satisfactory after cementation.
When the crown has been completely assessed and no further adjustments are necessary the crown is ready for permanent cementation. The occlusal surface of the crown should be smooth and the internal surface clean and dry.
2) Cementation Process
During cementation a layer of the chosen cement is placed inside the casting and the crown is seated firmly on the tooth. The objective is to provide optimal permanent seating of the finished crown (i.e. to the same marginal seating as that which was produced at the try-in) resulting in the minimum possible cement line. The luting cement fills the microscopic gap, seals the margin and secures the crown. All cements should be mixed according to the manufacturer’s directions to provide a smooth mix and used without delay. Delayed use allows initiation of the setting process causing reduced cement flow, incomplete seating, increased thickness of the marginal cement line and crown elevation requiring occlusal adjustment.21
Isolation from saliva during the luting process is essential, using a saliva ejector on the floor of the mouth and appropriately placed cotton wool rolls. The latter are placed in the adjacent buccal sulcus for all maxillary crowns and mandibular anterior crowns. To control salivary flow adequately for mandibular posterior crowns, rolls are necessary both in the maxillary and mandibular buccal sulcae, as well as the lingual sulcus.
The crown should be coated evenly with cement over the internal surface avoiding trapping air bubbles and be no more than half filled. The crown is immediately seated firmly on the clean, isolated and dry tooth providing significant force and steady pressure to express the excess cement and ensure complete seating. The marginal adaptation should be quickly checked at this point with an explorer (to ensure the situation is exactly as prior to the introduction of cement) and firm pressure maintained until the cement has set. Many practitioners find that steady pressure can be better maintained by having the patient bite down on a cotton roll however, it is essential that the crown not be displaced by eccentric movements. Use of patient biting pressure is therefore only advised on posterior teeth in arches with well aligned occlusal planes to avoid tipping or rotational forces that might result in inaccurate seating. For the same reason the technique is contraindicated for anterior teeth. Very firm steady axial finger pressure is advised.
All cements are sensitive to early moisture contamination which results in a cement with inferior properties that is more prone to leakage and/or dissolution. A dry field is therefore essential to allow development of optimal physical and mechanical properties. If the required seating pressure is to be performed by the patient, it is necessary to maintain cotton rolls in the lingual sulcus to prevent salivary contamination and early cement dissolution.
Once the cement has set, the excess cement is fractured away from the margins with hand instruments and the interproximal areas cleared with floss. Attempting to clear excess cement prior to setting will disturb the marginal integrity. Cement removal is best achieved immediately after the cement is “chippable”. Waiting for further maturation of high strength cements will make them extremely difficult to remove. It is essential that all excess marginal and intra-sulcular cement be totally removed as the presence of cement will cause gingival irritation and potential periodontal problems.
3) Post-Cementation Assessment
A final check of the occlusion is necessary prior to dismissing the patient as it is possible that, if seating pressure is inadequate or if the cement film thickness is unsatisfactory, the crown will be elevated. Any occlusal adjustments required at this point however should be minor. The initial use of shimstock, in the same way as above on adjacent teeth, will immediately indicate whether the occlusion is satisfactory or not. If tug-back is not present then marking of the crown with articulating paper and appropriate adjustment should be carried out. Occlusion during lateral movements is now assessed carefully for the presence of working side or non-working side interferences which must be corrected. They can be the cause of porcelain fracture and/or patient pain during function and/or tooth mobility.
When the occlusion is finalized, the occlusal surface should be polished. A specific metal polishing kit (Shofu) and porcelain polishing kit (Brasseler) are available in our clinics for this purpose. After a final check that all cement remnants have been removed the patient can be reminded of the need for and care of the crown over the longterm.
At future comprehensive recall appointments crown margins should be checked carefully for signs of plaque build-up or recurrent caries. A well-placed crown can be an excellent long-term restoration if is maintained optimally by the patient.
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