1
Q
When is a crown required?
A
- When tooth is heavily restored & existing tooth tissue is undermined
- Root-canal treated molars
- Abutments for fixed/removable partial dentures
- Cuspal protection on tooth under extreme occlusal forces
- Failing existing extra coronal restorations
- Extremely discoloured teeth
- Part of extensive restorative treatment
- Aesthetic concerns
2
Q
What is the first step of assessing a patient for a crown?
A
Ensure a crown is the best treatment option & explore what type of crown
3
Q
What preparation should be carried out prior to crown prep?
A
- Up to date PAs to rule out apical pathology/unsatisfactory root fillings
- Sensibility testing
- Periodontal tissue assessment (periodontal health required)
- Core assessment (replace if indicated)
- Occlusal assessment (especially when tooth to be prepared carried deflective contact)
- Diagnostic wax up if indicated (especially when multiple replacement planned)
4
Q
Different types of crowns?
A
Full gold crown
Ceramo-metal crowns
Composite crowns
Partial gold crowns
All ceramic crowns
5
Q
Stages of fitting crowns?
A
- Tooth preparation
- Impression of prepared tooth
- Construction of working die
- Wax pattern
- Investment of wax pattern
- Casting of restoration
- Porcelain application
- Polishing & finishing of restoration
6
Q
What are the principles of tooth preparation?
A
Preservation of tooth structures
Retention & resistance
Resistance
Retention
Structural durability
Marginal integrity
Preservation of the periodontium
7
Q
What is meant by resistance in regard to crown prep?
A
The ability to withstand compressive & oblique displacing forces
8
Q
What is meant by retention in regard to crown prep?
A
The ability to withstand occlusally directed displacing forces; theoretically, maximum retention is obtained if a tooth preparation has parallel walls.
However, it is impossible to prepare a tooth this way. Slight undercuts are created that prevent the restorations from seating.
9
Q
What is taper & what is ideal?
A
- Angle from a perpendicular dropped through the centre of the tooth to the wall of preparation
- Ideally this should be 2-3˚
10
Q
What is the convergence angle & what should it be?
A
- The angle between 2 opposing walls
- Ideally 6˚
11
Q
What are undercuts?
A
A divergence between opposing axial walls in a cervico-occlusal direction
12
Q
3 types of finishing lines/margins?
A
- Feather-edge & chisel finishing lines
- Chamfer finishing line
- Shoulder finishing line
13
Q
What is a feather edge & chisel finishing line?
A
- More conservative to tooth structure
- Not recommended because they do not provide sufficient bulk & the location of the margin if difficult to locate
14
Q
When would you use a chamfer finishing line?
A
Distinct margin, adequate bulk
- Used in full metal crowns, lingual margin (if unveneered) of ceramo-metal crowns
15
Q
When would you use a shoulder finishing line?
A
- Provides bulk of restorative material
- Used in facial margin (veneered) of ceramo-metal crowns & all-ceramic crowns
16
Q
What is meant by structural durability?
A
- Casting must be rigid enough not to flex & break
- Sufficient tooth structure must be removed to create space for an adequate bulk of restorative material to accomplish this
- Adequate occlusal reduction to allow bulk of metal
- Functional cusp bevel to allow for adequate thickness of metal
- Sufficient axial reduction
- Rounded edges
17
Q
How do we ensure the periodontium is preserved?
A
- Clearly defined margins supra or equi-gingival margins
- The margin of a restoration should not be placed within 2mm of the alveolar crest
18
Q
What can invasion of the biological width result in?
A
Gingival inflammation
Loss of alveolar bone
Pocket formation
19
Q
What does bur selection and usage depend on?
A
- Depends on shape of preparation
- Tip of bur depends on margin
- Thickness
- How coarse they are
- Technique used
- Pressure applied
- Use of water
20
Q
Advs of FMC
A
- Best retention & resistance form of all indirect restorations
- Reduces less tooth tissue than porcelain fused to metal or all porcelain restorations
- Best control of occlusion
- Best marginal fit
- Kindest to opposing tooth tissue
21
Q
Disadvs of FMC
A
- Not the most aesthetic material
- Can be too soft in some situations
- Some non-precious metals can cause allergy or can corrode (e.g. if it has nickel in it)
22
Q
Advs of all ceramic crowns
A
- Excellent aesthetics
- Preservation of tooth structure in some areas
- Good material selection
23
Q
Disadvs of all ceramic crowns
A
- Increased destruction of tooth structure in some areas
- Longevity
- Moderate strength
24
Q
Advs of ceramo-metal crowns
A
- Good aesthetics
- Longevity
- Preservation of tooth structure in some areas
25
Disadvs of ceramo-metal crowns
Increased destruction of tooth structure in some areas
26
- What are the steps during the crown preparation appointment?
- Shade selection
- LA
- Putty indices
- Crown preparation
- Temporary crown ready
- Tray selection & adhesive
- Retraction cord
- Impression & disinfection of impression
- Cementation of temporary crown & OHI instructions
- Laboratory card prescription
27
- Steps involved in the fit & cementation
- Check your lab work
- Remove temporary restoration
- Clean the underlying prepared tooth
- Try in the restoration
- Check marginal fit, contact points & occlusion
- Check pt if happy with fit & appearance
- Cement with appropriate cement
- Remove excess cement
- Recheck occlusion
28
Function of temporary crowns?
- To protect the open dentine tubules from micro-leakage, avoiding hypersensitivity & protecting root canal treated teeth from bacterial invasion
- To maintain the occlusal relationship, preventing over-eruption of opposing teeth
- To maintain the interdental space & contacts, preventing tilting on neighbouring teeth
- To prevent gingival hyperplasia at the margins & maintain gingival health. In some cases, improve gingival health when placed over previously overhanging restorations
- To maintain appearance
29
Why do we need provisionals?
- For all the reasons we need temporaries
- To check changes in occlusion are acceptable
- To check phonetics
- To check aesthetics
- To check mastication
30
- What are the diagnostic functions of a temporary restoration?
- To confirm there is enough occlusal reduction - use a Iwanson gauge (also buccal, lingual etc.)
- To confirm there is enough retention (if temporary restoration keeps falling off - could be lack of retentive features in prep or an occlusal interference)
31
- Why do we need lab made provisionals?
- In aesthetically demanding cases to visualise as closely as possible the proposed final aesthetic result
- Where long term temporisation is required such in tissue healing implants
- For the creation of optimum tissue health around multiple preparations before taking the definitive impression
- To create & ensure occlusal stability in full arch cases before final jaw registration
32
- The procedure for lab made provisionals
1. Completion of preparations
2. Chair-side temporaries as interim crowns
3. Impression of preparations using silicone
4. Facebow & jaw registration
5. Cementation of temporaries
6. Impressions & jaw records send to lab with instructions & shade
7. Labmade provisionals returned for fit (minor adjustments required)
33
- Materials used for chair-side provisionals
- Poly vinyl ethyl methacrylate (Trim)
- Poly ethyl methacrylate (Snap)
- Bis acryl composite (Protemp II, Quicktemp)
- Poly methyl methacrylates (Duralay)
34
- How to create temporaries & provisionals
- Overimpressions - Using putty or alginate indices
- Vacuum formed matrix - Created on a cast prior to the prep appointment
- Polycarbonate crowns - Usually only for anteriors & premolars
- Aluminium crowns - Usually only for molars
- Celluloid crown formers - Usually only for anteriors
35
- 2 techniques for over impression?
Alginate - Accurate, fast-setting, cheap but dimensionally unstable, only used for same day appointment
Putty - Accurate & can be used for multiple appointments, but expensive & slow setting
36
Cementation process of temporary crown
- Once adjusted & checked
- Show patient, especially if anterior
- Ensure moisture control
- Dry temporary crown
- Use gauze to avoid pt swallowing/inhaling
- Mix small amount of temporary cement
- Apply sparingly into the fitting surface of crown
- Seat crown & apply gentle pressure
- Use cotton wool roll & ask the pt to bite gently together
- Wait for temporary cement to set & clean
- Check occlusion again
- Post op instructions
37
Instructions to pt regarding temp
- Temporary crowns are usually placed while permanent restorations are being constructed
- They are fragile & easily broken or lost since they are cemented with temporary cement
- Avoid eating until the numbness is gone as it could potentially injure lip, tongue, cheek
- Avoid chewing on hard or crunchy foods as they break the temporary crown
- Avoid extreme hot & cold foods especially on the first few days to avoid irritating the nerve
- Avoid chewing gum or sticky candy as these can pull the crown (plastic shell) loose or completely come off
38
In case of de-cementation or fracture
- Pt needs to contact clinician as it can cause issues with permanent crown or damage tooth beneath
- Discomfort may be experienced
- Might have hot or cold sensitivity for ~1 week
- Bite should feel balanced, if not then they should contact clinic for review appointment
- a high spot can lead to breaking of the temporary crown
- To continue brushing the area as normal & when flossing to pull the floss through rather than upwards
- Better use Te-Pe brushes
39
Stages of getting a crown
1. Consent
2. LA (if required)
3. Pre op shade taking
4. Pre op putty indices (x2) for your prep gauge guide & for the provisional/temporary crown
5. Crown preparation
6. Construction of provisional/temporary restoration
7. Block out undercuts (if required)
8. Tray selection
9. Soft tissue management -e.g. retraction cord & astringent if sub-gingival margins
10. Moisture control - this is very important, silicones are hydrophobic
11. Impression taking (and bite registration if required); the impression of the crown prep is taken in silicone the opposing arch in hydrocolloid or neocolloid
12. Temporary cementation of provisional/temporary crown
13. Post op instructions to the patient
14. Completion of lab card
15. 2-3 weeks later - try in & cementation/fit of indirect restoration
40
3 types of elastomers for crowns?
Addition cured silicones
Polyethers
Hydrocolloids
41
Why do you need moisture control?
- Patient comfort
- Improved operator vision
- To help obtain an accurate impression
42
How to obtain good moisture control?
- Wide bore suction
- Salivary ejector
- Cotton wool rolls in buccal/lingual sulcus
- Cellulose pads
- Rubber dam (not for impressions)
- 3 in 1 air
43
Aims of an impression material?
- To create an exact duplication of the prepared (and adjacent) teeth from the patients mouth to the dental lab
- Must be free of air bubbles - especially on the finishing line
- There must be no drags or distortion in the impression material
- Include accurate occlusal surfaces of all teeth in arch
44
Properties & uses of hydrocolloids
- Poor tear resistance
- Dimensionally stable - if they are stored dry they shrink, if they are stored in water they swell
- Inferior impression detail to addition silicones
Used for opposing model & study casts
45
Features & uses of polyethers
- Takes up water so needs to be stored dry
- Only come in one viscosity & therefore only used in a monophase single stage impression technique (impression syringed around tooth & loaded into stock tray)
- Have good elastic properties
- Particularly suited for implant prosthodontics
46
Different types of addition cured silicones for crowns
- Low viscosity (light bodied) - fluid material for syringing around the tooth preparation to obtain great surface detail
- Medium viscosity (medium bodied) - consistency is suitable for syringing & mass filling of the tray
- High viscosity (heavy bodied) - Only suitable as a tray borne material
- Putty - Not as accurate as heavy bodied as it’s less flowable
- Depend on amount of filler content
Provide accurate fine detail of tooth preparations & are dimensionally stable with time - this is because there is a delay in taking the impression, sending to the lab and the lab then casting it up
47
What is the 2 phase, single stage method of taking impressions?
- 2 materials of different viscosity are used to take a single impression with light bodied wash being syringed around the tooth preparation to record fine detail & heavy bodied loaded into the tray to take up the bulk of space between prep & the tray
- Take impression in supine position
48
What is the 2 phase, 2 stage technique?
1. Before tooth preparation, take a heavy bodied imoression of the arch
2. After tooth prep, syringe light-bodied material around the prepared tooth & inside the tooth concerned in your origincal impression. Then re-seat the impression & wait for the light-bodied silicone to set
49
What is the 2 stage injection moulding technique?
1. Before tooth prep, take a heavy bodied impression of the arch
2. Once set, remove & drill a hole/channel through the impression material, in the area of the tooth which is to be prepared
3. After prep, seat the adjusted heavy bodied impression tray & then inject light bodied through the channel & wait for it to set
**NB:**
- This can be useful in lower posterior impressions where moisture control is very difficult to achieve or in patients with a very active tongue)
50
Checking the impression of the prepared tooth?
- All prep margins should be clearly visible
- No drags or air bubbles on the prep margins, axial walls, in slots, grooves or boxes etc
51
Problems with the impression to look for?
- Marginal tearing
- Air bubbles - small ones acceptable
- Drags
52
Checking the impression of the other teeth?
- Adequate impression of entire arch to ensure occlusion & shape of other teeth are recorded
- Check for air bubbles on occlusal surfaces of other teeth because if these are not removed it will make the cast rock & will inevitable lead to a crown being made which does not fit into the patient’s occlusal scheme -e.g. it will be proud
53
Which soft tissues need to be managed?
- Gingivae
- Mucosa
- Mucosa on the edentulous ridge
54
Why and how do we manage the periodontal status before the crown prep?
- Untreated gingivitis creates swollen, inflamed, loose gingival tissues
- Leads to a suboptimal marginal crown fit, increasing the risk of further periodontal deterioration & caries
- May create difficulties when assessing & preparing the finish line you require, obtaining moisture control, reproducing the finish lines in the impression
- Give OHI & complete any perio treatment required before or after surgery
55
When do soft tissues need to be managed during the crown prep stage?
- When appearance is paramount e.g. in the aesthetic zone in patients with high lip line, sub-gingival margins may have to be accepted but ensure these do not exceed deeper than 0.5mm from the free gingival margin
- If deep extensive restorations or short clinical crown height dictates that margins should be more apically placed, then surgical crown lengthening should be considered
56
What are the effects on soft tissues when crown preps are subgingival?
Risk of encroaching on biological width → Persistent gingival inflammation, alveolar bone loss & gingival recession
Difficult to prepare, record cement & clean → May affect long term perio health & also risk marginal leakage
57
What is the ideal soft tissue management when constructing the provisional restoration?
- Good adaptation of the provisional restoration to the tooth prep margins (no overhangs, no open margins & smooth)
- Provide further OHI & ensure ability for good interdental cleaning to maintain a healthy periodontium
58
When to manage soft tissues in the impression stage?
- Assess the tooth prep margins - are they supra-gingival, in the ginigval crevice or sub-gingival?
- Soft tissue management is required at this stage if some or all of the prep finish line are at or sub gingival
- Subgingival preparations need gingival retraction to prevent bleeding, to act as a physical barrier & retract the gingival tissues & to allow an accurate impression of the prep margins
59
What is the aim of soft tissue management?
To allow reproduction of the entire preparation
60
Criteria for soft tissue retraction?
- Effective gingival displacement
- Haemostasis
- No irreversible damage
- No systemic effects
61
Mechanical methods of soft tissue retraction?
- Plain retraction cord
- Copper ring
62
Chemomechanical methods of soft tissue retraction?
- Impregnated retraction cord
- +/- Duel cord technique
- Retraction paste
63
Surgical methods of soft tissue retraction?
- Rotary curettage
- Electrosurgery
- Crown lengthening
64
Aim of plain retraction cord?
Sulcus enlargement (physically displace the gingivae away from the finish line)
65
Disadvantage of a plain retraction cord
Sulcular heamorrhage → moisture control difficulty & poor impression accuracy (impression materials are hydrophobic)
66
Aim of copper band?
To displace the gingivae & help to carry the impression material to ensure the finish line is captured in the impression
67
Diasadvantages of of copper band?
- Traumatic
- Not effective
- Not accurate
68
What is impregnated retraction cord?
Retraction cord soaked in:
- Ferric sulphate 15%
- Alum AlK(SO4)3
- Aluminium sulphate
Chemomechanical process combining gentle packing of retraction cord to enlarge the sulcus, with chemical action to control sulcular haemorrhage creating a more accurate impression of the prep margins
69
Disadvantages of impregnated retraction cord?
- Systemic side effects
- Inflammation & tissue necrosis
- Staining
70
Factors that influence the perception of colour?
- Light source
- Object
- Observer
71
What is the spatial classification of colour?
- Hue
- Value
- Chroma
- Translucency
72
What is hue?
- The quality by which it is possible to distingush one colour family from another
- It corresponds to the wavelength of the light reflected by the teeth
73
What is value?
- An achromatic measure of the lightness or darkness of a particular colour, ranging from pure black to pure white
- High value → Light shade
- Low value → Dark shade
- Most important factor in colour determination
74
What is chroma?
- The degree of strength or saturation of a colour or particular hue
- It describes the intensity or vividness of a colour
75
What is translucency?
- Difficult parameter to quantify
- Light is permitted to pass through the object but is dispersed rather than absorbed or reflected
- Highest translucency = transparency
- Lowest translucency = opacity
- Varies between individuals & teeth
- Depends on thickness of enamel & dentine
- Susceptible to changes with age
76
What are the elements affecting colour?
- Lighting conditions
- Value contrasts effect (relative lightness of an object is affected by the lightness of the contrasting background or surrounding)
- Hue contrast effect (when viewed against different background colours, the teeth appear to take on the hue of the background’s complementary colour)
- Metamerism → phenomenon when 2 objects appear to have same colour under certain lighting conditions & different under changed conditions, due to non-matching spectral analysis curves
- Opalescence → Light scattering phenomenon caused by the presence of fine particles (opalisers)
- Fluorescence → Emission of visible light by an object when exposed to ultraviolet light
- Light reflection is affected by surface appearance of object (smooth/flat vs. rough)
77
Physical factors affecting colour?
Natural tooth colour
Surface texture
Special characteristics
78
How can we measure tooth colour?
Subjective comparison - e.g. coloured ceramic shade guides/coloured acrylic rein shade guides
Instrumental objective measurements - e.g. spectrophotometers, colourimeters, computer imaging systems
79
Short comings of shade guides?
- Generally not designed for systematic assessment of colour
- Restricted & inadequate range of shades
- Made from thick layers of high fusing porcelain - do not allow for variations in thickness
- No surface texture or other characteristics
- Ceramic material used, different from that of the restoration - exhibit different optical properties
- None of the commercial shade guides are identical
80
What is the shade matching technique?
1. Select shade prior to tooth prep & early on the appointment - reduces eye fatigue, tooth dehydration under rubber dam & change of tooth colour during prep
2. Work under controlled lighting conditions: natual light (not direct sunlight at 10am or 2pm on a clear, bright day) or colour corrected artificial light source (5500K)
3. Remove brightly coloured makeup
4. Set Vita shade tags in order of decreasing value & not of hue
5. By squinting eyes, make rapid comparisons with shade tags (no more than 5 secs) & gaze at blue colour between attempts
6. First choose the value & then the dominant chroma & hue
7. Try different lip positions, viewing angles & distance
8. Always accept your first decision, if in doubt better select shade with higher value (lighter)
9. Look for special characteristics - white spots, enamel crack lines, staining - preferably under magnification & draw them on a diagram
10. Determine degree & extent of translucency & surface texture
11. Have the shade selected checked by someone else (dental assisstant or technician) & compare choices
12. Give clear & precise written prescription to the technician, including a shade diagram & a photograph preferably for anterior teeth
81
Advantages of technology based shade matching?
- No influence of surroundings
- No influence of lighting
- Reproducible results
- Improved communication between dentist & lab
- Integration with available hardware (digital camera, printer) & image enhancing software
82
Definition of centric relation/RCP
- In a dentate patient, the RCP is an unstrained position of the mandible relative to the maxilla occurring at initial tooth contact(s)
- This contact follows closure about the terminal hinge axis where the condylar heads are in their most anterior & superior position in the glenoid fossae
83
How to guide a pt into CR?
- Chin-point guidance
- Bimanual manipulation
84
Definition of ICP?
- The position where there is most tooth-tooth contact for that individual’s occlusion
- Position varies as we change the occlusal scheme from tooth movement as in orthodontics to morphological changed in even simple restorations
- Most people can adapt to small changes in the ICP but large changes need to be tested in some way before proceeding with tooth preparation
85
When do we record RCP?
Only reproducible position of occlusion when we want to make large changes to the occlusion in fully dentate individual or if we have no guide as to what the occlusion is
Edentulous → no guide what occlusion is, can also be seen with very few contacting teeth remaining
To alter occlusion like this = re-organised approach - only done by specialists
86
When do we use ICP?
When we conform to patient’s occlusion
= CONFORMATIVE APPROACH
This is done in practice
87
How & why do we maintain occlusion?
- Must create enough space so that we can ensure we have enough structural durability in the crown that we provide
- Must ensure that we maintain the occlusion, not doing so will lead to teeth over-erupting, tilting or drifting → could lead to development of working & non-working side interferences
88
What materials do we use for bite registration?
- Beauty wax
- Pink wax (should not be used)
- Polyvinylsiloxane bite registration paste
- Duralay - quite exothermic & must set whilst in the mouth
89
When do you not need a bite registration?
If you have a bounded prep with a stable occlusion you shouldn’t need a bite registration
90
When do you need a bite registration?
- When preparing the last tooth in the arch
- Multiple preparations
- Multiple anterior preparations
91
What are the 2 types of luting agents?
Non-adhesive
Adhesive
92
When would you use a non-adhesive luting agent?
When you are reliant on retentive preparation
e.g.
- Crowns, retentive onlays, cast custom made posts, some prefabriacted posts
- Whether it would need retrieving - consider whether the crown would need taking off
93
Examples of non-adhesive luting agents
- Zinc phosphate
- Zinc polycarboxylate
- Glass ionomer
94
When would you use adhesive luting agents?
- Reliant on micro-mechanical retention/bond
- Unretentive preparations
e.g.
- Crowns
- Resin bonded bridge
- Inlay/onlay
- Veneers
- Prefabricated posts - non metal posts
- Irretrievable - consider prognosis
95
Examples of adhesive luting agents?
- Resin-based cement
- Glass ionomer - compomer based
96
Steps in try-in cementation of crowns appt?
- Decide material of choice for cementation
- Examine the crown on the die
- Remove provisional restoration
- Seat the crown on the tooth
- Examine - margins, contacts, occlusion
- Adjust occlusion
- Polish, clean restoration, prepare fit surface for cementation
- Rubber dam
- Clean tooth - prepare for cementation
- Cement the crown
97
What is the try in procedure (3 steps)?
- Check crown on die
- Seat crown
- Assess seated crown
98
Common errors affecting marginal fit & failing to seat the crown?
- Tight proximal contacts
- Casting blebs on fit surface
- Over/under-extended crown margins
- No die spacer
- Impression distortion (would need to remade from a new impression)
99
What to visually check for when trying in the crown?
- Margins
- Direct/mirror
- Magnification
100
How do you evaluate complete seating of crown?
- Explorer - sub-gingival margins
- Use the correct size tip
- Angle of approach is such that it gets to the margin
101
Types of defective margins?
Over-extended (beyond finish line)
Under-extended margin (ledge)
Over contoured (thick)
Open margin
Fixed Pros
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