21 July 2017
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Crowns Revision Part 2 – Tooth Preparation Guidelines

Crowns Revision Part 2 – Tooth Preparation Guidelines

 

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 will be available of the entire series in Part 3 of this great revision resource.

Check out Part 1 of 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)

Tooth Preparation Guidelines

Tooth preparation considers a balance of all the factors mentioned in the pre-operative assessment, with the ultimate goal of producing a preparation that has maximal height and minimum taper for optimal resistance and retention form.  Retention form is the preparation’s ability to resist vertical displacement forces, and resistance form is the ability to resist rotation about the vertical axis.

-There are seven key principles of preparation, as outlined by Herbert Shillingberg that determine the shape and form of preparations.

Seven Key Principles:

  1. Conservation of tooth structure – To maintain pulp health and tooth strength
  2. Retention Form- To prevent the Crown dislodging
  3. Resistance Form- To prevent rotational displacement along any of the Crown’s path of insertion, including long axis
  4. Structural Durability-  To provide sufficient space for the material of Crown, in order to prevent fracture, distortion or perforation
  5. Marginal Integrity- To implement a finish line that accommodates a robust margin, and allowing close adaptation to prevent microleakage
  6. Periodontium Preservation- To place the margin so that it is accessible for optimal oral hygiene, and avoid recession
  7. Aesthetics- To create sufficient space for an aesthetically pleasing restoration, in particular where Veneers are indicated

-These principles should be adhered to, however, quite often; compromises need to be made between them, because of a patient’s individual requirements.  For example, conservation of tooth structure; this is sacrificed where a Metal-Ceramic Crown is used, because an adequate thickness of material needs to be made.  In such cases, if preparations are too minimal, Crown’s may end up being excessively bulky and therefore unaesthetic.

Tooth preparations vary, with regards to-

-Amount of tooth removed- Most for Metal-Ceramic Crowns, Least in Resin Bonded Porcelain Crowns

-Margin position- Supragingival is ideal; however aesthetic importance may depict the position of the margin. Subgingival preparations are indicated where metal margins are visible such as: Anterior teeth and MCC Crown preparations with a bevelled shoulder

-Accessory retention components- Grooves, Boxes

-Degree of Taper- 6 degrees of taper ideally, apart from in Resin Bonded Porcelain Crown preparations, where around 20 degrees is preferable, and also in High Strength Porcelain Crowns.

-Margin Finish-


TIPS to consider:

-Hold the bur parallel to the path of insertion at all times, to prevent undercut and create the correct taper

-Assess for undercuts using one eye only whilst directing the mirror adjacent to the tooth

-Slightly under-prep, then smooth and refine the preparation, inevitably you will then get the desired reduction amount

-Complete smoothing is not essential, as it poses a threat to pulp health due to overheating, especially where a water coolant is not used. However, it is important to remove any irregularities, sharp lines and corners

-It is helpful to know the exact diameters of the burs used during production of depth grooves, because they can be used as a measurement tool

Full Metal Crown Preparation

-Generally restricted to the back of the mouth due to metal appearance, however some patients may prefer a metal appearance anteriorly

-Relatively little removal of tooth structure compared to other Crown preparations

  1. Take two initial Silicone putty indexes- one for the provision of a temporary Crown and the second to use as a reference for how much tooth has been removed. If the existing tooth contour is incorrect, an index can be made from diagnostic wax up
  2. Using a Fissure Bur, carry out a 1.0mm occlusal reduction, using depth grooves as a guide to reduction. Occlusal reduction is not necessary if there is already a minimum of 1mm occlusal space in ICP
  3. Reduce the functional cusps of the tooth by an additional 0.5mm, angled at approximately 45 degrees, producing the functional cusp bevel. This is essential to prevent thin casting and fracture of the functional cusp of the Crown.
  4. Assess occlusogingival height, and decide on whether auxillary features such as grooves are needed to aid retention.
  5. At this stage you can check occlusal clearance using 2.0mm red wax strip, asking the patient to bite, and then holding up the strip to the light
  6. Reduce axial walls by up to 1.0mm using a Tapered chamfer diamond; the tip creates a chamfer for Crown seating.  Chamfer is best for strength in this preparation.
  7. Cut Interproximal areas with the Long needle diamond, which then creates enough room for preparation with the Tapered Chamfer diamond bur
  8. A 0.5mm depth Seating groove can be placed, on the functional cusp side to prevent rotation.
  9. Smooth sharp lines, corners and irregularities with a Finishing Diamond Bur

Metal Ceramic Crown aka Porcelain fused to Metal

-Metal coping with a ceramic layer fused onto it

-Combines strength of metal substructure with aesthetics of porcelain

-Greater strength than some ACC’s

-In most cases, Porcelain coverage is restricted to visible areas, and therefore facial surfaces have a heavy shoulder preparation to accommodate the thickness of metal and porcelain, and lingual/palatal surfaces have a lighter chamfer preparation for metal only

-Adequate prep needs to be done or else there will be: 1- poor contouring of tooth by lab, 2- the shade won’t match adjacent teeth due to incorrect thickness

Anterior tooth preparation

  1. Take two initial Silicone putty indexes- one for the provision of a temporary Crown and the second to use as a reference for how much tooth has been removed. If the existing tooth contour is incorrect, an index can be made from diagnostic wax up
  2. Using a Fissure Bur, carry out a 2.0mm occlusal reduction, using depth grooves as a guide to reduction. Occlusal reduction is not necessary if there is already a minimum of 2mm occlusal space in ICP
  3. Now remove 1.2mm of tooth from the facial surface, using a Shoulder bur via a 2 (or 3) planar reduction to achieve adequate reduction and avoid pulp. This involves creating two plane depth grooves- gingival and incisally, followed by smoothing. Facial reduction is carried around and stopped 1.0mm away from the proximal contacts. If bevelled shoulder is chosen (0.3mm using flame shaped diamond bur), a subgingival preparation would be needed to hide the metal lining it
  4. Remove 0.5mm from the lingual concavity using a “Rugby ball” Diamond Bur.
  5. Then remove 0.5mm from the lingual surface, creating a chamfer, using a Tapered Chamfer bur
  6. Reduce interproximal areas initially using a Long Needle Diamond bur, then finish using a Tapered Chamfer diamond bur. This leaves a preparation with a “winged appearance”.  The primary reason for wings, is to preserve tooth structure, and it’s secondary effect is that it provides resistance to rotation. A winged preparation is not essential, and can be removed by blending the chamfer margin with the shoulder margin using a Tapered Chamfer Bur.
  7. Smooth sharp lines, corners and irregularities with a Finishing Diamond Bur

Posterior Tooth Preparation

-Usually involves Maxillary Premolars and First Molars, and mandibular Second Premolars as these are often visible

-Other posterior teeth may require MCC for appearance if patient wishes so too

-Occlusal full ceramic coverage- involves extensive tooth removal and threatens opposing teeth as dental porcelain is 40x more abrasive than gold to tooth enamel 

  1. Take two initial Silicone putty indexes- for the provision of a temporary Crown after preparation, and to use as a reference for how much tooth has been removed. If the existing tooth contour is incorrect, an index can be made from diagnostic wax up
  2. Using a Fissure Bur, carry out a 1.0mm occlusal reduction(if occlusal metal) or 1.5mm occlusal reduction (if occlusal porcelain), using depth grooves as a guide to reduction. Reduce groove depth if the tooth is not in occlusion
  3. Reduce the functional cusps of the tooth by an additional 0.5mm, angled at 45 degrees, producing the functional cusp bevel. This is essential to prevent thin casting and fracture of the functional cusp of the Crown
  4. Now remove 1.2mm of tooth from the facial surface, using a Shoulder bur via a 2 planar reduction to achieve adequate reduction and avoid pulp. This involves creating two facial plane depth grooves- gingival and occlusal, followed by smoothing. Facial reduction is carried around and stopped 1.0mm away from the proximal contacts. If bevelled shoulder (0.3mm using flame shaped diamond bur) is chosen, a subgingival preparation would be needed to hide the metal lining it.
  5. Using a Tapered Chamfer bur remove 0.5mm lingually/palatally with 2 planar reduction, leaving a chamfer margin. Finishing line initially just above gingival level, and then finished off to at gingival level or slightly below
  6. Reduce interproximal areas initially using a Long Needle Diamond bur, then finish using a Tapered Chamfer diamond bur. This leaves a preparation with a “winged appearance”.  The primary reason for wings, is to preserve tooth structure, and it’s secondary effect is that it provides resistance to rotation. A winged preparation is not essential, and can be removed by blending the chamfer margin with the shoulder margin using a Tapered Chamfer Bur-Winged prep or blend in
  7. Smooth sharp lines, corners and irregularities with a Finishing Diamond Bur

All Ceramic Crown

-Capable of producing best aesthetics of all Crowns

-More susceptible to fracture due to brittleness

-Make as long preparations as possible to give maximum support for porcelain

-Overshortened preparations- stress concentrates in labiogingival areas producing “half-moon” fractures

-Best suited for incisors, due to risk of fracture posteriorly

-Avoid in edge-to-edge occlusion

-Should not be used where opposing teeth occlude in the cervical fifth of the lingual surface

  1. Take two initial Silicone putty indexes- for the provision of a temporary Crown after preparation, and to use as a reference for how much tooth has been removed. If the existing tooth contour is incorrect, an index can be made from diagnostic wax up
  2. Using a Fissure Bur, carry out a 2.0mm occlusal reduction, using depth grooves as a guide to reduction. Occlusal reduction is not necessary if there is already a minimum of 2mm occlusal space in ICP. The reduction should be inclined linguogingivally to prevent shearing
  3. Now remove 1.2mm of tooth from the facial surface, using a Shoulder bur via a 2 (or 3) planar reduction to achieve adequate reduction and avoid pulp. This involves creating two plane depth grooves- gingival and incisally, followed by smoothing.
  4. Remove 1.0mm from the lingual concavity using a “Rugby ball” Diamond Bur.
  5. Then remove 1.0mm from the lingual surface, creating a shoulder, using a Shoulder bur
  6. Reduce interproximal areas initially using a Long Needle Diamond bur, then finish using a Shoulder bur, merging the labial and lingual finish lines
  7. Smooth sharp lines, corners and irregularities with a Finishing Diamond Bur

Resin Bonded Porcelain Crown

-Excellent aesthetics

-Relatively conservative of tooth tissue

-Strength comes largely from the resin bond rather than an underlying core of material

-Ideal for younger patients with large pulps

-May not be suitable for areas of high occlusal load and where there is parafunction due to thin porcelain, and therefore reserved for anterior teeth

 

  1. Take two initial Silicone putty indexes- for the provision of a temporary Crown after preparation, and to use as a reference for how much tooth has been removed.
  2. Using a Fissure Bur, carry out a 2.0mm occlusal reduction, using depth grooves as a guide to reduction. Occlusal reduction is not necessary if there is already a minimum of 2mm occlusal space in ICP
  3. Now remove 0.5mm of tooth from the facial surface, using a Tapered Chamfer bur via a 2 (or 3) planar reduction to achieve adequate reduction and avoid pulp. This involves creating two plane depth grooves- gingival and incisally, followed by smoothing and production of a chamfer.
  4. Remove 0.5mm from the lingual concavity using a “Rugby ball” Diamond Bur.
  5. Then remove 0.5mm from the lingual surface, creating a chamfer, using a Chamfer bur
  6. Reduce interproximal areas initially using a Long Needle Diamond bur, then finish using a Chamfer bur, merging the labial and lingual finish lines
  7. Smooth sharp lines, corners and irregularities with a Finishing Diamond Bur

High Strength Porcelain Crowns

-Have no metal substructure, however utilize high strength ceramics for robustness

-Usually used on posterior teeth

-Relatively destructive preparation

-Degree of taper is greater than for a traditionally cemented Crown

Posterior tooth preparation guideline:

  1. Take an initial Silicone putty index- for the provision of a temporary Crown after preparation, and to use as a reference for how much tooth has been removed.
  2. Using a Shoulder bur, carry out a 2.0mm occlusal reduction using depth grooves as a guide to reduction (reduce groove depth if the tooth is not in occlusion)
  3. Reduce the functional cusps of the tooth by an additional 0.5mm, angled at 45 degrees, producing the functional cusp bevel.
  4. Now remove 0.8-1.0mm of tooth from the facial surface, using a Shoulder bur via a 2 planar reduction to achieve adequate reduction and avoid pulp. This involves creating two facial plane depth grooves- gingival and occlusal, followed by smoothing. In this case, create a greater taper than traditional Crown preps
  5. Using a Shoulder bur remove 0.8-1.0mm lingually/palatally with 2 planar reduction, leaving a shoulder margin. Finishing line initially just above gingival level, and then finished off to at gingival level or slightly below. Can use a heavy chamfer, depending on company to produce HSPC
  6. Reduce interproximal areas initially using a Long Needle Diamond bur, then finish using a Shoulder bur, merging the labial and lingual finish lines
  7. Smooth sharp lines, corners and irregularities with a Finishing Diamond Bur

Composite Crowns

-Indications have not yet been fully researched

-Preparation is the same as for High Strength Porcelain Crowns

Trouble shooting

My preparation has ended up too tapered, what should I do?

-You could straighten the axial walls up by further preparing from the base, however this risks pulpal health and is destructive.  Another option is to add accessory retention features such as grooves and boxes, however once again this is further destructive.

-The best option here may be to use a resin cement to aid retention

My preparation has ended up too short, what should I do?

-3mm is the minimum preparation height, disregarding other factors such as cement used, occlusal loads, type of Crown.

-If the preparation has become too short for the clinical situation, you may consider adding retention features such as grooves and boxes; once again this is further destructive.

-Consider the use of a Composite Resin Cement

-Consider Crown lengthening surgery, to extend the margins further down the tooth and therefore increasing preparation height