Crown restoration considerations

• Study notes on indirect restorations, crowns, bridges and pontics

Indirect Restorations: Crowns

• Crowns are extra-coronal restorations that are fabricated outside of the mouth
• Crowns cover all or most surfaces of a tooth to return it to its external form.
• A ferrule is a band of material that totally encircles the tooth
• The ferrule extends 1-2mm onto sound tooth tissue to guard against longitudinal fracture

Crown functions

• Restore form and function
• Protect cusps
• Prevent flexing
• Provide structural integrity
• Crowns utilize porcelain/composite anterior crowns for aesthetic purposes
• Crown preparations weaken the tooth
• There is potential for pulp damage if the tooth is still vital
• Crowns often fail, and are rarely permanent and often expensive
• When treatment planning the patient's dental state in 10 years should be considered
• Widespread caries may be more cost effective to manage conservatively as they move from dentate to edentulous
• The treatment should meet patient expectations and the patient should be able to tolerate treatment and maintain restoration
• Limited mouth opening may contraindicate crowns
• Assess whether or not the damage to be caused by the procedure can be justified

Crown pre-assessment

• Identifying occlusal discrepancies is a must
• A further management plan may be required before tooth prepping
• The mouth should be periodontally stable
• Crown margins should be placed supragingivally (or be aware of biologic width when placing subgingivally)
• The biologic width is approximately 2mm on average and is naturally maintained between the base of the gingival sulcus and the alveolar bone
• Pre-crown retreatments may be necessary if there are signs of endo failure or poor condensation, as this can lead to better foundations
• The tooth should have a stable core, adequate height for crown retention and enough space for the restoration
• Crowns can only restore what’s visible and are compromised when aesthetics are a concern (when being placed subgingivally)

Biological Width

• If a restorative margin is placed too deep below tissue it invades the biologic width
• Bone resorption or gingival inflammation can occur if the restorative margin invades the biologic width
• When possible, crown margins should be placed supragingivally
• A minimum distance of 2-3mm should be maintained between the crown margin and alveolar crest

Crowns and Endodontics

• Crown preparations place pulp at increased risk
• High-speed stripping overheats tooth and disturbs micro-circulation
• High-speed stripping opens dentinal tubules that directly communicate with the pulp
• The deeper a cut is, the more permeable the tooth is, making the pulp vulnerable to irritants
• Dangers of root treating crowned teeth includes direct pulp exposure
• Failure to accurately replicate the tooth anatomy beneath can lead to loss of orientation and increase risk of perforation, decreasing the structure strength
• When applying crowns it is pertinent to minimize damage to healthy and vital pulps
• High-speed drills require water irrigation and avoid desiccating dentine

Crown Cementation

• Ensure the patient has well fitting temporary crown
• Cementation includes the use of a post, posts should be of equal length to anticipated crown height
• A minimum of 5mm GP should be left apically
• Long posts are more retentive, but parallel-sided posts are more retentive than tapered posts and cause internal stresses, increasing risk of fracture
• Fiber posts are MORE FLEXIBLE than cast posts, making them less likely to fracture
• Remove all remnants of GP on the walls with alcohol, 2-3mm ferrule is required
• Apply posts into the straightest and bulkiest canal in multi-rooted teeth (distal for lower molars, palatal for upper molars)
• Post channel preparation should be undertaken with RCT fill

Considerations before Crowns:

• Premolars or molars with only minimal access cavities and no other coronal tissue loss can be restored with amalgam or composite
• Physical cuspal coverage must be provided by the core material on posterior teeth that have lost one or both marginal ridges
• Cusps adjacent to lost marginal ridges should be reduced in height by 3mm
• Surgical crown lengthening or forced eruption may be required if there is little/no coronal tissue remaining
• Consider whether tooth is restorable or whether best to extract the tooth and place a bridge/denture/ implant
• Ferrule refers to presence of at least 1.5-2mm of circumferential tooth structure above the crown margin
• Heavily restored or crowned guidance teeth may be at risk of fracture or de-cementation due to repetitive non-axial loading during excursions
• If a tooth feel is insufficiently robust to carry guidance, guidance can be moved onto other teeth
• If RCP involves a tooth the deflective contact can be removed before prep

Diagnostic Records

• For planning crowns:
• Occlusal examination
• Study models give unimpeded view of ICP, provide information regarding inter-occlusal space, and help with articulated study models mounted on semi-adjustable articulator
• Main purpose of articulator is to construct restorations that require little, if any, chair-side occlusal adjustment
• Copying old methods helps maintain proper occlusion and function (anterior guidance, canine guidance or group function)

Crown tooth guidance is necessary in cases such as:

• Full-mouth rehabilitation
• When pt has stable, comfortable occlusion
• Restoring pathologic wear cases
• Tooth guidance can be copied from previous methods (putty matrix, custom incisal guide table)

Preparing a tooth for a crown

• Functional cusps can be remembered as PUBL (Palatal upper, buccal lower)
• Crown dimensions
• Full metal, 1mm NF cusp (1.5mm F cusp)
• Crown dimensions
• Porcelain requires more space (2mm NF cusp/ 2.5mm F cusp)
• Crown dimensions
• PFM(Porcelain Fused to metal) requires same as porcelain on occlusal and axial 1.2mm for porcelain (0.5-1mm for metal)
• It is crucial prepare the least taper compatible with the elimination of undercut
• The exception is resin-bonded porcelain crowns: they must require a taper of 20 degrees to avoid crown fractures

Luting

• Sealing generating high seating hydrostatic pressures with a luting agent
• The general rule is to reduce for porcelain and 0.5-1mm for metal on the incisal edge by 2mm and lingual aspect by 1mm
• Shoter over-tapered restorations are at high risk of decementation and retention grooves should be cut in the axial surface of a preparation to reduce the radius of rotation
• Conventional cements are strong in compression, but weak in tension, minimize stresses by ensuring prep is not over-cut or over-tapered
• Axial grooves increase retention by reducing radius ofrotation and should be kept 0.5mm away from the finish line to reduce micro-leakage and must be parallel with the POI
• Resin cements overcome this by bonding dentine

Provisional Restorations

• Function of provisional restorations: comfort by preventing sensitivity and PAP
• Prevent tooth movements by maintaining intercuspal and proximal contacts, as well as maintain esthetics and aid in diagnosis
• BisAcryl resin is commonly used to create provisional restorations; occasionally a stronger cement may be needed if retention limited
• Eugenol cements can significantly reduce the bond of resin cements to composite cores (use tempbond)

Bridges

• Bridges are prosthetic appliances definitively attached to remaining teeth and replaces a missing/teeth.
• Support comes from the abutment tooth that provides attachment and support for the bridge
• The pontic sits on the saddle in place of the missing tooth that is being suspended by the connector and retained by a retainer,
• Abutment teeth need to withstand occlusal load on the pontic
• In fixed fixed bridges retainers are ridigid with minimum tooth reduction with good support

Types of Bridges

• Fixed-fixed pontic is anchored to retainers with rigid connectors at either end of the edentulous span -Abutment prep means tooth reduction with adequate support on parallel preps
• A Moveable bridge has The major abutment that allows for retention and gives flexibility in cases such as non parallel abutments
• A direct- cantilever bridge Pontic anchored at one end of the span with reduced tooth prep
• A spring cantilever bridge consists of the upper incisors with a metal bar attached on the metal bar in place of premolars
• Resin bonded bridges are a conservative measure that requires minimal drilling with a metal piece, but are prone to de-bonding
• compound hybrid bridges have components that account for different abutment angulations and bone states, but provide flexibility and balance

Considerations when chosing a bridge:

• Caries and periodontal status
• Pulp vitality
• Existing restorations
• retention, size, and root geometry
• In assessing support, keep the crown : root ratio ideally at 2:3
• Root configuration and spread provide a strong support
• Anter's law refers to having a combined periodontal area as a backup but has been invalidated
• Cannines are v difficult to place pontics
• Pontic pressure on the gingival tissues results in inflamed gums and require extra floss
• In hygienic bridges the missing tooth is replaced with a easy to clean gap, in ovate bridges excellent OH is required

Providing a bridge requires the following stpes:

1. Impressions and facebow record Models mounted on semi-adjustable articulator
2. Diagnostic wax-up for temo bridge
3. Abutment preps
4. elastomer impression + occlusal registration
5. Test metals and ensure a test run of occlussion, fix where neccessary and apply permanent set
6. Cement and provide a review

This quiz covers key considerations for crown restorations, including ferrule importance, biologic width, occlusal discrepancies, and risks of violating biologic width. It also addresses pulpal risks during tooth preparation and the impact of endodontic access on crown integrity.