Principles of tooth preparation Flashcards
what is a indirect restoration
amalgam or composite restoration is placed directly into the mouth.
-Example-crown-few more steps outside pt mouth to produce it.
-Impression, scan – dental laboratory- cast a model from impression and construct crown-photo.
-Then cemented-photo
- VENEER
- ONLAY
- INLAY
Why use indirect restorations
Why do we need Indirect restorations
Occlusal stabiltiy
Benefits: indirect restorations can improve asesthetics: photo. Alter shape / inclination of teeth for cosmetic or functional purposes.
Heavily restored tooth-fracture risk LOSS of function-Chewing, speaking or sharp trauma – restore function, appearance and comfort via cuspal coverage.
Fractured tooth –opposing tooth over eruption or drift or challenging contact point to recreate-drifting- indirect restoration helps to maintain occlusal stability and comfort to pt to avoid food packing which can occur with open contact points.
Dental implant-require indirect restoration to construct the crown which cements onto the abutment of the implant and restore aesthetic, stability, function and comfort.
Why do we prepare for indirect restorations
For most indirect restorations to be successful some tooth preparation would be required. A small amount of tooth tissue removal required to create sufficient space or clearance for the indirect restoration.
Photo –die spacer-no preparation no space for cement and materials which make up the crown
Photo-material of MCC
Photo-space created not affect occlusal scheme
Why is preparation essential for a crown
If crown cemented then occlusion would be changed.
Patient complain-high in bite, pain, opposing tooth risk fracture due to excessive occlusal forces being concentrated on the tooth.
Tooth tissue removal to create appropriate space required for the material used to construct the crown, then the patient’s occlusion is unaltered.
This is known as conforming to the patient’s occlusion
why is shape important in preparing crowns
Shape allow the crown to come on and off with close proximity between the tooth- tissue and the crown.
This means it will have a slight taper, widest portion being at the bottom and narrowest on top-left image.
Wider at top than bottom0-undercut-overhanging of tissue-causes preparation to be over prepared and weakening the tooth as less tooth tissue-risk pulpal exposure or tooth fracture.
Undercut-affect impression-locking impression into undercut space.
Undercut not recommended-indirect restoration not conform to tooth shape.
Additional reduction to resolve-over preparing.
Aim gentle taper.
What is the path of insertion
Preparation need to consider path of insertion and adjacent teeth’s proximal contacts.
Ideal parallel allow path of insertion-image left-taper also allows crown to seat on LR6 with ease and maintain good contact points.
Bulbous tooth-LR5-path of insertion is impeded and crown locked out.
describe the 4 main points of preparing a tooth
So we prepare a tooth to ensure sufficient space, correct path of insertion for the indirect restoration and there is an appropriate shape with no undercuts present.
Underlying this preparation are seven principles to consider when preparing teeth which we will now consider.
What is the precisely of preparation 1
Balancing act-space for material / retain as much tooth tissue as possible to protect pulpal vitality and periodontium and reduce the risk of tooth fracture.
what happens if the tooth is under prepared
If too little tooth tissue remove-under-prepared-what could happen
Describe images-know tooth morphology prep to any preparation-UR1 2 planes each plane need to be reduced maintaining the existing plane angles to create a space for the indirect crown restoration / silicone putty index /coronal labial plane under prepared, only one plane reduced-coronal reduction insufficient space for material- lead to poor aesthetics due to bulbous crown or porcelain too thin and fracture or greying affect through porcelain.
Second image further reduction coronal plane to create space
Under preparation: further example in molar tooth.
Inadequate space for the indirection restoration due to under preparation-lead to thin indirect restoration-risk fracture indirect restoration or perforate exposed underlying tooth tissue.
Correct thickness of material-crown bulky-no conform to patient’s occlusion.
Under preparation of the preparation margin width-thin material thickness at the indirect restoration finishing line- which risks facture exposing underlying tooth.
Or tooth bulbous cause plaque retentive factor
Describe creating room when preserving the tooth tissue
Overall message prepare the tooth to give you necessary space for the material of choice, the design of the indirect restoration and consideration to the functional cusp which provides structural support and influence the structural integrity of the indirect restoration.
Example_if require porcelain layer on the crown to be more aesthetics more space would be required and therefore more tooth tissue removal needed.
A thin metal crown will require less tooth tissue removal because it can be stronger than porcelain in thin sections.
What is over preparation of tooth when preserving tooth tissue
Opposite of under preparing is over preparing, taking too much tooth tissue away.
Close proximity to pulpal tissues, see pink hue shining through dentine-pulp very close –RISK LOSS OF VITALITY.
IMPORTANT must follow planes of reduction/natural contours of the tooth to reduce this risk.
In addition to pulp risk there is an increased risk of the tooth fracturing.
So it is a balance.
What is the Principles of preparation 2
Retention and resistance
retention - prevents removal along the path of insertion
Resistance - prevents dislodgement by later or apical forces under occlusal load
What is Taper?
The properties of retention and resistance are influenced by taper as opposing external surfaces provide retention.
Greater the taper the less resistance and retention experienced by the indirect restoration.
2 parallel sided tubes which slide into one another. They are very close fitting and therefore difficult to move up and down-more retention and resistance is experienced.
If a taper is added they move on and off one another far easier. Like plant pots stacked together-slight taper easier to lift off one another compared if parallel.
Internal surfaces can also offer retention and in a tooth where there is a cavity this can be used. Again, opposing surfaces contribute and the more parallel sided they are, the greater the retention achieved. Don’t forget though, you need to be able to get the restoration in and out of the cavity if is an indirect restoration.
This picture shows varying degrees of tapers on both the equivalent of an extra-coronal restoration and the preparation. You can visibly see the difference in taper from 10 degrees to 20 degrees.
Describe the relationship between retention and resistance
Graph-with increasing taper there is a decrease in retention and resistance and consequently an increase in the stresses concentrated within a luting cement which is helping to retain the extra-coronal restoration.
The appropriate taper required to retain an extra-coronal restoration is often called ‘Total Occlusal Convergence’/ ‘TOC’.
which is the angle subtended between the two opposing prepared surfaces and a range between 6 to 12 degrees is acceptable. This means there is a reduction of between 3 and 6 degrees on the circumference of the tooth as shown in the picture on the right to achieve this.
What should the optimal taper show
The finish line should be visible and this will help determine the presence of any undercut
the ideal Total occlusal convergence is between 6 and 12 degrees
Which burs do we use to prepare a tooth for extra coronal restoration
Remember for the future when you are preparing a tooth for an extra-coronal restoration we often use long diamond burs which already possess a taper between 5-6 degrees. Therefore the ideal TOC can be achieved when the high speed hand piece and bur are aligned parallel to the long access of the tooth.
Describe the relationship between surface area and retention and resistance
The next factor that affects retention and resistance form is the surface area of the preparation. The greater the surface area the greater the retention and resistance. A larger tooth such as a molar will give a greater surface area than a premolar when prepared as shown in the picture
Describe the relationship between height and retention and resistance
The height of a tooth will aid retention and resistance as well. The greater the height, the greater the retention and resistance and so the more tooth-tissue remaining the better. Be aware that luting cements used to retain the extra-coronal restoration perform better when loaded in compression. A preparation possessing a good height will result in the extra-coronal restoration becoming lodged against the opposing wall when it is experiencing rotational forces and therefore the cement will encounter more compression in this area rather than tension and sheering stresses.
Animation
Therefore prior to tooth preparation you should perform a coronal tooth tissue assessment to ensure a minimum height of 3 mm to have sufficient retention and resistance. Review the tooth in occlusion, there may be sufficient clearance and therefore limited occlusal preparation would be required, preserving crown height and tooth tissue.
Describe the preparation design for retention and resistance
Additional preparation features may be included to improve the retention and resistance such as grooves and boxes.
A groove can help to resist torquing effects. It needs to be placed along the path of insertion and should be no less than half a burs diameter in depth to avoid the die spacer filling in the groove. This procedure can be technique-sensitive with risks of casting errors.
Aims to change the arc of displacement as shown in the pictures from a very long arc with a short clinical crown height allowing easy displacement, to a short arc from the groove or box giving increased resistance to lateral forces.
What are cements used in retention and resistance
Today we also have cements which offer adhesion rather than just being a passive luting cement and these can overcome some, not all, deficiencies in preparation design and will help retain the crown
what is the principles of preparation 3
The structure of the restoration needs to be fit for purpose, i.e., durable. It needs to withstand the occlusal forces and to survive in the oral environment where there are radical changes in temperature, pH, as well the daily challenges of masticating a variety of foods. Maintenance needs to be minimal over its expected lifetime. This can be a challenge in the human mouth and the materials need to be used correctly. If inadequate space is provided then the required thickness of material may not be given and this can reduce its durability. Thin porcelain is prone to cracks and fractures and thin metal may perforate.
What is the principle of preparation 4
There are many materials to choose from – porcelain fused to metal, shown on the upper left canine, precious metals such as gold, although due to cost non-precious metals are being used more. They may not be as aesthetically pleasing as the warm yellow of gold but they are functional. Today many people desire the all ceramics without any metal and colours to match their natural dentition. There are advantages and disadvantage for all these – metal is kind in preserving the tooth-tissue, not only in having less radical preparations but also being less abrasive to the opposing dentition.
what is the principle of preparation 5
Describe the impact of thickness on marginal integrity and position
There are different margin designs which depend on the material to be usedand each material has its own dimensions to provide the best outcome. If these requirements are not respected and a reduced or excessive dimension is used there can be failure in the material such as cracks or fracture. The margin types can be divided into two groups ‘horizontal’,, where a well-defined preparation finishing line is prepared or a ‘vertical margin. A shoulder is an example of a horizontal margin and is often used for metal ceramic crowns. The preparation margin width needed for a metal ceramic crown is at least 1.2mm. Less than this and there may be inadequate room for the porcelain to give the necessary aesthetic outcome or the crown could be constructed too bulbous in contour..
A knife-edge is a vertical margin and just as it sounds, a very fine thin edge which fades away and is usually made of a metal.
Described he importance of marginal integrity and position
The more accurate an indirect restoration’s margin seats within the preparation margin width and meets the preparation finishing line, if not exposed cement occurs and risks washing the cement away and it creates the development of a plaque retentive factor which increases the risk of periodontal disease or secondary caries.
How does marginal integrity and position relate to finishing on sound tooth
If feasible, finish a margin of an indirect restoration on sound tooth-tissue. You many need to consider the management of existing restorations especially if they are carious or possess defects.
A decision will need to be made whether to follow the direct restoration into the sulcus and take the margin of the extra-coronal restoration subgingival, with possible consequences on gingival health, to stay equigingival where aesthetics are of some concern, impression taking can be more difficult, may require a retraction cord and more difficult to maintain, or whether the margins can be supra-gingival if aesthetics don’t really matter-aids impression and maintenance.
Consideration may also be needed for the overall crown height and the implications on retention and resistance.
what are the principles of preparation 6
The periodontium should be preserved via
To achieve this, good oral hygiene is needed, supra-gingival margins help with cleaning making the margins accessible. Avoid any overhanging/bulbous margins due to inadequate tooth reduction which can be plaque retentive.
The photograph shows recession of the gingival tissue around metal ceramic crowns of the anterior incisors and this leads to ‘long’ teeth which are not always pleasing to the eye. A low lip line may hide this. The diagram on the right shows a bulbous restoration due to inadequate tooth reduction and this may be plaque retentive leading to gingival inflammation which may progress to periodontal disease.
describe the biological considerations of tooth prep
Subgingival margins may be desirable for aesthetics and also to maximise the height of the preparation for increase retention and resistance but be aware of the possible consequences.
The soft tissues of the periodontium need to be carefully managed.
Determine the biological width
When considering the soft tissue there is an area of specific interest whenplacing restoration margins. This is the biological width which is the distance from the depth of the sulcus to the crest of the alveolar bone and includes the junctional epithelium and connective tissue attachment.
This soft-tissue area can be difficult to manage when taking impressions and following placement of a restoration there may result in persistent gingival inflammation. There is also a risk of increased probing depths and loss of attachment when extra-coronal restoration margins are placed subgingival. Even 1mm subgingivally increases the risk of recession by 2.65 times.
Sometimes it may be deemed desirable, especially if more than 1.5mm of preparation height is required, to actively increase the crown height with surgery via crown-lengthening.
The photograph on the left shows the presenting crown height of the anterior teeth and the photograph on the right shows the same teeth following crown-lengthening. The process has also helped level the gingiva, as there had been some alveolar compensation.
Bone is removed in this process and the periodontal tissue is re-positioned in a more apical location to expose the crown.
Describe the biological considerations for preservation of the pulp
As the clinician, you will need to take care with the preparation of the teeth to retain as much of the mineralised tissue as possible and avoid excessive reduction which approaches the pulp tissue. Follow the correct planes of reduction as shown in A to protect the pulp. B and C risk poor aesthetics or proximity to the pulp which can be detrimental.
what is the 7th principles of preparation
In society today, aesthetics are key and many people will not accept metal extra-coronal restorations. Some will only consent to placement of all ceramic restorations. This is a personal choice and as a clinician you will need to be able to explain the advantages and disadvantages of the restorations available to allow your patient to give informed consent.
The type of restoration dictates the amount of tooth removal to satisfy the requirements of strength and structural durability.
For the metal ceramic crown a minimum of 1.2mm is needed to place the 6 layers of the crown. If this is not provided, aesthetics or periodontal health may be compromised.
Take a close look at the two central incisors and see which you think is the most aesthetic?
Take a moment now to consider why do you think this may have happened?
The central incisor on the right has been under prepared and there is insufficient space for placement of all the porcelain layers to give the best aesthetic outcome therefore a second plane of reduction should have been completed.
