24 April 2017
This article has been read 17,488 times.

Crowns Revision Part 3 – Imps, Temps and Cementation

Crowns Revision Part 3 – Imps, Temps and Cementation

 

Chirag Patel is a Final Year student at Liverpool School of Dentistry

This article, as part of a 3-part series, is designed to give dental students an overview into the World of Crowns.  Useful as a revision and reference tool, this article is intended to be easy to read for anyone unfamiliar with Crown provision, with tips and guidance from an existing dental student.

A PDF download version is available of the entire series of Crowns Revision Articles.

Check out Part 1 or Part 2 the Crowns Revision series if you have not already.

With Special thanks to:

Professor Callum Youngson (Head of Liverpool University Dental School)

Dr Sophie Desmons (Clinical tutor at Liverpool University Dental School)

Leona Yip (Editor)

Impressions

- The impression stage of the appointment is just as crucial as the tooth preparation stage, and any flaws in the impression sent to the lab, has a potential to produce an ill-fitting Crown.  Therefore it is important to be able to produce well defined impressions, and identify where impressions need to be retaken.  Retaking impressions, is both time consuming, costly, and can be unpleasant for the patient, and therefore the ability to consistently produce good impressions is important.

-At Liverpool University Dental Hospital, the One Stage Heavy and light impression technique is utilized, using addition silicones.  These silicones have excellent dimensional stability and are thought to have relatively good handling characteristics.

Technique guidelines:

1)      Choose an appropriately sized stock tray by trying in the patient’s mouth

-If the tray is under-extended, it can be adapted by the addition of a stiff material (i.e impression compound)

-If the stock tray cannot be adapted, a special tray would be indicated

-Metal trays are preferable, because they are rigid and reduce the risk of distortions.  Recoil is a problem that occurs more commonly in plastic stock trays, whereby the walls of the tray flex outward during occlusal pressure, followed by an inward flexion, producing impressions that are undersized bucco-lingually

2)      Retract the gingivae around the Crown preparation, so that the finish line can be recorded accurately. The most common method is the “two cord technique”, where a thin cord is wrapped around the tooth and placed into the sulcus followed by a thick cord, which is removed just before the impression is taken.  Sometimes, the cords are impregnated with solutions to prevent haemorrhage i.e. adrenaline and ferric sulphate.

Other methods include:

-Electrosurgery- controlled tissue removal using an electric current through a tip

-Rotary curettage- involves removal of epithelial tissue within the sulcus using a diamond chamfer bur

3)      Block out large embrasures using ribbon wax, to prevent the impression locking into the patients mouth

4)      Dry the tray and apply adhesive evenly.  Blow dry the adhesive lightly to encourage evaporation of the adhesive’s solvent.

-This stage can be done before tooth preparation to allow solvent evaporation and production of a good bond strength

5)      Syringe the wash material (i.e. light bodied silicone) into the sulcus, and over the tooth preparation

-Ensure there are no air bubbles or voids by ensuring the nozzle is not removed from a continuous stream of material-Remember to remove the thick retraction cord beforehand

-Work from the most difficult aspect of the tooth to access and around to the easiest

6)      Load the tray with a heavy bodied addition silicone material (i.e. Express 2 Penta) and take the impression

-This should be done before the light bodied silicone sets in the One stage technique

7)      Use excess of material to monitor setting, and remove once set

-Bear in mind the warmth of the mouth will encourage setting faster than externally

 

Temporary restorations

-Temporary Crowns are made for the period of time between tooth preparation and fitting the final restoration. The only scenario in which a temporary restoration would not be essential, is when making a Resin bonded Crown where space maintenance and aesthetics are important, because there is no or minimal dentinal exposure.

-The functions of temporary restorations are summarized below:

-Tooth vitality protection: Traditional Crown preparations expose thousands of dentinal tubules, which can lead to sensitivity and pulp death.  Therefore it is important to provide a protective covering over these tubules in the interim.

-Prevention of tooth movement: Without the provision of a temporary Crown, there is a risk of drifting and over-eruption of the teeth in the long term, therefore disturbing the existing occlusion

-Maintenance of function: Allows the patient to masticate and speak normally

-Aesthetics: Especially important in anterior teeth, where it is important for the patient to have an acceptable appearance.  However, a diagnostic wax-up or computer imaging may be sufficient in some cases to show the final appearance.

-Diagnostic purpose: For the patient to assess function and appearance of a Crown before a permanent Crown is cemented in place.  This is especially important where there are plans to change the existing aesthetics or occlusion (i.e. increasing OVD or changing guidance surfaces).  In cases where Crown lengthening has been planned, you should provide a temporary restoration for a minimum of 6 months before a definitive restoration, to allow stabilisation of the periodontium.

Types of Temporary Crown

-There are a number of different methods to temporise a Crown preparation, and a major factor influencing the choice of material is the length of time between tooth preparation and cementation of the final Crown.  Generally, a laboratory made temporary Crown will last longer than a chair side Crown, and can be tailored to the patient’s needs aesthetically, however the additional cost of manufacture needs to be balanced against the pros of doing so.

1)     Preformed Crowns: Plastic (Opaque: polycarbonate or acrylic) or Metal (aluminium, stainless steel or nickel chromium)

-Non-Custom: These come in a variety of different sizes and the dentist needs to pick according to the most appropriate marginal, proximal and occlusal fit.

-Plastic preformed Crowns are indicated for anterior teeth and metal Crowns for the posterior

-Colour matching is needed when choosing an opaque plastic Crown (i.e. Direct-a-Crown), as the outer shell is retained, differing from a Strip Crown matrix (mentioned later).

1) Once a preformed Crown of appropriate size has been selected, the margins are adjusted for a closer fit and a small hole placed incisally on the lingual/palatal surface for excess flow, using a high speed bur.

2) The tooth prepared is coated with petroleum jelly and the Crown is filled with a material such as Trim plus (polymethyl methacrylate/PMMA) or Integrity (chemical cured composite), and placed over the preparation.

3) Excess is trimmed away from the margins during the gel phase of the material using a sharp bladed instrument such as a carver

4) Remove the Crown and adjust occlusal surfaces and any excess using steel or tungsten carbide burs and soflex discs

5) Fill the preparation impression area with a small amount of temporary cement material, and position back on the preparation.  Remove excess from the margins.

6) Once set, further adjust the Crown margins using steel or tungsten carbide burs and soflex discs

7) Check occlusion using articulating paper

-As these Crowns are not custom-fitted, it is likely that considerable adjustments will need to be made.  For long term temporisation, or multiple Crown cases, lab made preformed Crowns can be requested and these are usually made in acrylic, or poly-methyl methacrylate.

Temporary cement materials:

-Temp bond – commonly used, however may soften preparations with a composite core.  If a preparation is very retentive, a small amount of modifier can be added so removal is possible.

-Temp bond NE (non-eugenol) – used where there is a composite core or a patient has a eugenol allergy

-Zinc Polycarboxylate (i.e. Poly-F) – Where preparations may be unretentive and there is a risk of the temporary Crown falling off

2) Matrices: Impression Matrix, Vacuum Formed Matrix, Odus Pella/Strip Crowns

-A matrix is a mould made in the shape of existing teeth or from a diagnostic wax up, and is used to help fabricate a temporary Crown.

-Impression matrix: a quick and easy way of producing a matrix, frequently involving the use a polyvinysiloxane to take an impression of the teeth before tooth preparation.  The matrix duplicates the existing tooth form back onto the Crown preparation, utilizing a self-curing composite such as Integrity.

-Vacuum Formed Matrix:  Made of a clear vinyl sheet that is pulled over a stone cast or diagnostic wax up, duplicating the form of the existing dentition.  Due to the flexibility of the vinyl sheet, distortions may occur on seating.  A benefit of this type of matrix is that a light cured resin may be used, and set through the clear material.

-Odus Pella/Strip Crowns: These are clear, Crown shaped matrices, made of cellulose acetate, and come in a variety of different sizes.  The procedure for creating a temporary restoration using Strip Crowns is the same as for Preformed Crowns, except the clear outer shell is removed before adjusting and light cured composite may be used.

-Procedure for using Impression and Vacuum Formed Matrices:

1) The prepared tooth and adjacent teeth are coated with a thin layer of petroleum jelly and the matrix is filled with a material such as Trim plus (polymethyl methacrylate), Integrity (chemical cured composite) or Light cured composite (if clear matrix) and placed over the preparation.  Care is taken to avoid air blows, and over/under filling the impressed area

2) Allow the material to set, or command set with light as necessary, and remove the matrix

3) A mould of the temporary Crown should now be formed, and trimmed with stainless steel or tungsten carbide burs and soflex discs

4) Cement the temporary Crown in place with the temporary cement materials mentioned before

3) Direct Syringing

-This is used in situations where a pre-formed Crown or matrix cannot be made for any reason.  A polyethyl methacrylate such as Trim or Trim II is indicated in such cases because of its handling properties and low exothermic reaction.

-As the name suggests, the material is syringed all around the tooth preparation from base to tip, and later trimmed down to the correct shape in occlusion.

4) Lab made Temporary Crowns

- These are indicated if:

- The plan is to change a patient’s teeth aesthetically, and the patient needs a “trial”

- Occlusal changes are to be made i.e. increase a patient’s OVD

- Crown lengthening is to be done, and periodontal stability is needed before final cementation of a permanent Crown

- Multiple restorations are to be done, as the occlusion can be controlled on an articulator

- There is any other reason for long term temporisation

- Heat cured or Self-Cured acrylic is usually used to manufacture the temporary Crowns, and these materials are more durable than other materials

- There is an increased cost associated with lab made temporary Crowns, and an extra appointment is needed, unless there is an on-site technician.  If an extra appointment is needed, which is the case in most situations, other methods of temporisation will be needed in the interim.

- Usually lab made temporary Crowns will simply need cementing in place with little or no change to occlusion.

Temporary restorations for Resin Bonded Crowns

- If required, the following methods can be used to temporise a Resin Bonded Crown:

1) Seal and Protect or Duraphat- If aesthetics are acceptable after tooth preparation, apply a thin layer to protect exposed dentinal tubules

2) Composite resin – can be applied, with a small spot etched, either freehand or with a matrix. If the sole reason for application is to prevent over eruption,  a composite stop can be placed on the opposing tooth only, and removed when required.

3) Lab made temporary – if a long term solution is required

4) Conventional temporary restorations – as those mentioned above, except use a cement such as a Zinc Polycarboxylate (i.e. Poly-F)

5) Zinc Phosphate Cement- applied in a thin layer in areas where aesthetics is not crucial, and movement of teeth are unlikely.  There is a risk of pulpal damage due to the drop in pH after mixing.

Try in and Cementation

-This is usually the final stage in Crown provision, during which the final Crown is assessed, and material is chosen for final cementation in place.  It is important to note that once a Crown has been cemented, removal for modification is impossible without damaging the Crown and/or tooth preparation.

-A systematic approach in Crown assessment is important, so as not to miss any defects or potential future problems.  Prior to the try-in procedure, the temporary Crown (if provided) must be removed and the preparation cleaned thoroughly from any temporary cement for example, by using an ultrasonic scaler.

Try in procedure:

1)      Check the Crown on the die

-Look for obvious defects, such as casting blebs, which can be removed with a bur 

-Check occlusion

-Look for any fitting surface defects, marginal fit, aesthetics and articulation.  Keep in mind any faults that will appear in the mouth too.

-If there are obvious faults in the Crown, where it is not related to impression defect, then it may be necessary to get a lab to remake the Crown

2)      Place gauze in the back of the patients mouth, to prevent the patient swallowing the Crown

3)      Attempt to seat the Crown on the tooth preparation.  If the Crown does not seat:

-Ensure the preparation is completely clean of any temporary cement material

-Check proximal contacts: if too tight> grind and polish

-Check for over-extended margins > adjust from the axial surface and not the base, using soflex discs

-Check the internal fitting surface for burnished areas where the preparation has come in contact with the Crown.  Disclosing wax, aerosol spray or light bodied silicone placed in the fitting surface can help with identification of imperfections, which can then be ground down with a white stone

-If the Crown still doesn’t fit, and no obvious impression defect can be found, a remake may be needed

4)      Assess the fully seated Crown:

-Proximal contacts: check with floss, if too tight> grind and polish, if open contacts> return to lab or build up adjacent teeth

-Marginal fit (the gap between the Crown margin and tooth preparation margin): A poor marginal fit could render a tooth more susceptible to cement dissolution, plaque retention and secondary caries. Data suggests that a marginal gap of 100µm is at the borderline of acceptability for long term success.  If there is an overhanging margin > adjust from the axial surface until a probe can pass without catching.  If there is a deficient margin > the Crown may need to be remade

-Aesthetics: Check the shade and contours.  ACC shades can be altered slightly by using coloured luting cements if necessary.  MCC contours can be altered by grinding with diamond burs, and colour adjusted by staining and refiring in the lab.

-Occlusion: Assess by eye, patient feedback, articulating paper and shimstock.  Patient feedback can only be relied upon if anaesthetic has not been given.  Initially, check the Crown visually in ICP, using shimstock foils passed through the teeth in occlusion.  In ICP a firm grasp is needed between posterior teeth, and the amount of grasp with anterior teeth is dependent on existing anterior grasp, so compare with other anterior teeth.  The patient should then tap lightly on coloured articulating paper with a thickness of around 40µm, highlighting any areas for adjustment.

Occlusal adjustment can be done using high speed diamond burs, after which polishing may be necessary again. Care must be taken not to perforate the tooth, and a Svensen gauge is useful for checking Crown thickness.  Once the Crown has been checked in ICP, occlusion needs to be checked using articulating paper in lateral and protrusive excursion, with adjustment as necessary.

5)      Finish and polishing –this is particularly important with porcelain, because unpolished porcelain can rapidly wear away opposing teeth.

-Metal Surface, polishing sequence: Finishing diamonds > Rubber abrasive points > Soflex discs (interproximally) > Felt wheel or rubber cup with diamond polishing paste or zinc oxide

-Porcelain Surface, polishing sequence: Finishing diamonds > Soflex discs (interproximally) > Rubber abrasive points > Felt wheel or rubber cup with diamond polishing paste

-Composite Surface, polishing sequence: Composite finishing diamonds > White Stones > Soflex discs > Silicone enhancer > Diamond polishing paste

Cementation of the Crown

Choosing a cement

-There are a variety of different cements available for use, and they are categorized into 1.Luting Cements and 2.Adhesive cements.

-Luting Cements: Achieve retention by wetting and micro-mechanical interlocking, of the fitting surface of the Crown, which is often sandblasted for further retention.

-Adhesive Cements: Achieve retention by molecular adhesion between the tooth preparation and fitting surface of the Crown.

-Currently, the four major classes of luting cement include: Zinc Phosphate, Polycarboxylate, Conventional Glass Ionomer Cement and Resin Modified Glass Ionomer Cement.

-There is only one type of Adhesive cement and that is Composite Resin Cements.

-There are a number of clinical factors that need to be considered in the choice of cement and these include: occlusal forces, aesthetic demands, ability to achieve moisture control, preparation retention and margin location.

-The following table considers the properties and indications of various cements:

-As a summary, a strong Crown with good retention can be luted with any cement, whereas weak restorations and those with poor retention must be bonded with strong cements such as composite resins.

Cementing Procedure        

1)      Isolate the tooth, using dental dam (where possible) or cotton wool rolls.  If the gingivae inhibit seating, retract using gingival retraction cords

2)      Clean the tooth preparation, and dry, but do not desiccate

3)      Mix the cement according to manufacturer’s instructions

4)      Coat the entire fitting surface with a small layer of cement

5)      Seat the Crown quickly, applying finger pressure, and maintain

6)      Excess conventional cement should be removed after complete setting.  Composite resin based cements should be removed at the gel phase of set, as it can be difficult to remove later on. –Gold margins should be burnished before the cement sets, as doing this afterwards can crack underlying cement.

 

For all the references and the complete Crowns Revision Guide for the Undergraduate Dental student, including Parts 1, 2 and 3, click here to download the complete PDF.