Prosthetics Skills and Knowledge Flashcards
Recall the key oral anatomical landmarks for prosthodontics
Recall the primary and secondary denture bearing areas of the maxilla and manidible
Define retention
Resistance to vertical displacement of the denture away from the denture bearing surface during function
Define stability
Resistance to lateral displacement of the denture during function
Define support
Resistance to vertical forces of occlusion towards the denture bearing area during function
Describe the post dam
The post dam aims to provide posterior retention of the maxillary denture by creating a valve-like seal. A groove is cut into the palate of the master cast:
- Locate vibrating line
- Mark 1–2 mm distal
- Inscribe post dam with sharp wax knife 0.5–1.0 mm deep
Note: angulation is chamfered into the palate with the dam at the distal
Explain the importance of the position of the post dam
If the post dam has been over-extended, the patient will get good suction and retention but may gag.
If the dam is placed anteriorly, the denture will terminate on the relatively incompressible hard palate, and so will not estabilish an adequate seal.
An inadequate post dam will not allow the retention to be held (aim for 0.5–1.0 mm deep with chamfered angulation into the palate).
It should be the dentist’s responsibility to make the post dam – not the technician’s!
Describe speech problems with new dentures with reference to phonetics
A slight change is normal with a new denture
Phonation of S, Z, T, D, N:
- Requires contact between tongue tip and palate
- Thickness of denture will alter contact
- Palatal position of incisors will alter contact
- Too thick ‘s’ becomes ‘th’ → lisp
- Too thick in canine area ‘s’ becomes ‘sh’ or whistle
Phonation of F and V:
- Requires contact of lower lip to labial surface of upper teeth
- If teeth too far back or forward difficult to make
Phonation of TH, T, D, N, S, Z, SH, ZH, CH, J, R:
- All require contact between tongue and posterior teeth
- If molars are too far buccal this may not be possible
- Also if occlusal plane too high
- If patient is overopen teeth may clatter when talking
Describe the disadvantages of an immediate denture
Immediate denture
Try in is difficult
More visits post extraction
Dentures will need replacing or relining within 3–9 months
Flanged, or open-faced (to reduce bulbous appearance), dentures are not just for immediates and are more susceptible to repeated centre line fractures. Metal base plates (e.g. chrome) may help stop the fractures.
Prevention of reline fractures → reline (new impression wash/full, take old acrylic away, replace). Tell patient to eat on one side and not on centre (avoid full arch bites). Can’t fracture chrome/high impact acrylic.
Discuss the design faults of this manidbular removable partial denture
Occlusally approaching clasp (engages undercut from the occlusal surface) too small.
This clasp should only be used on molars, and should not be used on premolars as needs to be ≥ 14 mm long (will permanently deform on repeated removal if too short).
Rest seat should be placed on the mesial area to direct masticatory forces vertically along the long axis of the tooth, and to prevent distal tilt.
Disadvantage of lingual plate is that it crosses the gingival mucosa and may be plaque retentive (however, benefit is that there is more support for denture).
Describe how to take a maxillary impression of an edentulous patient who has a flabby ridge form
Use a mucostatic impression technique:
- Perforating the special tray or cutting a window out of the anterior section where it covers the flabby areas will reduce the pressure caused by the impression material and will reduce the distortion.
- Using a fluid impression material and increasing the spacing of the tray will also help (e.g. softer mix of alginate with 2–3 mm spacing)
A patient wishes to alter the shade of their teeth on the final denture. What is the issue with this?
If incorrect tooth shade is finished on a denture, there is nothing you can do about that.
The wrong shade means a total remake of the denture.
To avoid this, always check the shade setting of the teeth at try-in before final finsih!
Describe the properties of alginate when used as an impression material (6 points)
Alginate impression material:
- Used for all types of impressions giving good surface detail
- Needs to be cast within 24 hr (deforms on drying; imibition during submersion)
- Needs to be supported fully within a tray
- Alginate is elastic when set and is therefore indicated where bony undercuts are present (to allow for safe removal of impression)
- High viscosity alginates ⇒ preliminary impressions
- Alginate preferred over ZnO:Eu for impressions with undercuts
Describe the properties of impression compound
Impression compound:
- Used for full edentulous preliminary impressions
- Not suitable for undercuts
- For tray modification (e.g. extensions)
- Poor surface detail, so not to be used for master impression
Describe the properties of zinc oxide-eugenol impression paste (7 points)
Zinc oxide-eugenol impression paste
- Rigid when set and is dimensionally stable, so it is preferred to alginate in all cases where there are no bony undercuts
- Used in close-fitting rather than spaced tray
- No need for adhesive on special tray as already intrinsically adhesive (so take care in patients with xerostomia as risk of intraoral adhesion)
- Some patients may have allergies to ZnO
- Overall bulk of the impression is kept to a minimum and so is better tolerated by the patient
- Where the sulcus is narrow, it is easier to avoid displacement of the buccal mucosa
- It is easier to obtain an impression of a resorbed lower ridge without the mucosa of the floor of the mouth becoming trapped within the border of the tray
Outline an advantage of using medium-bodied vinyl siloxane (silicone) to take an impression
Silicone is a better option for taking an impression on a Friday evening where you can’t get it to the lab until Monday morning; don’t use alginate as will shrink.
Describe the problems that may occur at the try-in stage
Incisor setup correct, but occlusion wrong:
- Strip off lower molars (or all lower teeth for a major inaccuracy)
- Build up lower rim with wax
- Make new occlusal record (re-registration)
- Trim wax to correct level
- Lightly occlude patient into relaxed or retruded position then re-seal with occlusal reg paste
- Send back to technician
- Re-try at next visit
It could be at this stage that the patient decides on changing the shade of teeth or not.
Discuss the clinical considerations for this patient case with respect to taking a dental impression
Patient presents with alveolar bone resorption, with clinically evident drift and mobility:
- Mobility indicates that load on tooth is excessive for periodontal support
- Drifting usually a result of reduced periodontal support and increased occlusal load e.g. missing posterior teeth plus periodontally involved anterior teeth
- Could splint teeth with composite material
- Or block undercuts out (e.g. fill gaps with vaseline or carding wax) to take impressions
- Or you could digitally scan the arch
- Use a nicely spaced tray, about 3 mm, with a soft mix of alginate (don’t let it set for too long)
Describe the key aims of cast surverying in the design and construction of partial dentures
Surveying is an essential step in the design and construction of patient dentures.
Surveying the cast demonstrates undercut areas.
Undercut areas need to be eliminated to insert/remove the denture (or may actually be used to aid retention).
Key aspects of cast surveying:
- Analyse cast
- Mark on model:
- Path of insertion
- Path of displacement
- Decide on final paths
- Will it work?
- If it doesn’t work, consider alternative path of insertion
Name the 6 key surveyor tools
Surveyor tools:
- Analysing rod
- Surveying graphite marker (some have red marker)
- Wax chisel
- 0.25 mm measuring guage (chrome)
- 0.5 mm measuring guage (cast gold)
- 0.75 mm measuring guage (wrought material or stainless steel)
Design a special tray (outline 5 requirements with considerations)
Special tray requirements and considerations
- Spacing:
- Spaced ⇒ 2–3 mm for alginate or silicone putty/wash
- Close-fitting ⇒ 0.5–1.0 mm for ZnO:Eu
- Peripheral extension:
- 2 mm short of sulcus
- Can be extended with tracing stick or polyvinyl siloxane (silicone) putty for accurate fit
- May be perforated (works in 2 key ways):
- Makes it a little more retentive for alginate material
- ZnO does not need adhesive so can be used in unperforated tray
- Relieves pressure to aid in mucostatic compression technique
- Makes it a little more retentive for alginate material
- Type of handle:
- Ensure appropiate shape (step vs stub)
- Should not displace lips
- Can incorporate tray stops (even amount of material):
- Made to height (e.g. 2–3 mm stop limit)
- When you take the impression, stops from pushing all the way through to the tray
Explain the use of the Willis bite gauge
Compare occlusal vertical dimension (OVD) with resting vertical dimension (RVD) to calculate freeway space (FWS):
RVS - OVD = FWS
Describe the use of the fox plane guide
Helps with assessing incisal level to establish a balanced occlusion
Line up with:
- Eyes:
- Inter-pupillary line
- Alar-tragal line
List 4 types of dental articulator
Types of articulator:
-
Non-adjustable condylar path:
- Simple hinge
- Average value*
-
Adjustable condylar path:
- Semi-ajustable
- Fully-adjustable
*commonly used as a minimum standard during prosthodontic classes
What is the Kennedy classification of this dentition?
Kennedy Class I
Bilateral free-end saddles
Class I is a situation where the edentulous areas are bilateral and lie posterior to the remaining natural teeth.
There are no teeth standing distal to the abutment teeth. Therefore the free-end or distal extension saddles can only derive tooth support.
What is the Kennedy classification of this dentition?
Kennedy Class II
Unilateral free-end saddle
Class II is a situation where the edentulous area is unilateral and lies posterior to the remaining teeth. All the teeth remain on one side of the arch and a number are missing on the opposite side, with no natural teeth posterior to the edentulous area.
There are no teeth standing distal to the abutment tooth. Therefore the saddle can only derive tooth support at one end.
What is the Kennedy classification of this dentition?
Kennedy Class III
Unilateral bounded saddle
Class III is a situation where the edentulous area is unilateral and has natural teeth remaining posterior as well as anterior to it.
The bounded saddle has an abutment tooth at each end. Therefore the saddle could, if required, derive tooth support for the whole saddle.
What is the Kennedy classification of this dentition?
Kennedy Class IV
Anterior bounded saddle (crossing the mid-line)
Class IV is a situation where the only edentulous area is entirely anterior to the remaining teeth and crosses the midline.
The anterior saddle has an abutment tooth at each end, and by definition crosses the mid-line (if it did not cross the mid-line, then it would be a unilateral bounded saddle).
Describe the use of the alma gauge for occlusal registration
Only useful if a patient already has an existing set of dentures!
The Alma Gauge allows the position of the incisal edges of an existing full upper denture to be reproduced on a registration block.
This is important if the appearance of the existing denture needs to be reproduced exactly.
It can be used to compare the new denture dimensions with the old denture.
It can save chairside time if the technician has trimmed the registration block to the dimensions of the existing denture rather than using the accepted standard height of 22 mm.
Compare the results of a too great vertical demension with a too small vertical dimension
Vertical dimension
Too great:
- Feels like ‘a mouthfull’
- Muscular effort to close lips
- Teeth touching at rest
- Teeth clacking when eating and speaking
- Pain under dentures
- Ulceration
- Ridge resorption
Too small (downward smile and chin too high, reduced FWS):
- Patient looks ‘old’
- Freeway space is too small ⇒ no reprieve for patient and the ridge is under constant load → pain and discomfort
- Patients should not be complaining about pain underneath dentures (unless due to direct cause such as blebs and sharp bits)
- Lips roll inwards or outwards on closure (risk of angular cheilitis)
- Deep lines at corners of mouth (pronounced Marionette lines)
- Often very uncomfortable
Describe the compensating curves
Compensating curves
The curvature of the occlusal plane of dentures, created to permit balanced occlusion, to compensate for the paths of the mandibular condyles as the mandible moves from centric to eccentric positions
Blanced occlusion:
- Static
- Maximum cusp contact
Balanced articulation:
- Dynamic
- Movement (centric and eccentric) without cuspal interference
Eccentric movement ⇒ protrusive, retrusive, right and left lateral
Recall the curve of Monson
The curved plane on which lie the occlusal surfaces of the posterior teeth; it conforms to the segment of a sphere of 102 mm (4 inches) radius.
Recall the curve of Spee
An imaginary line joining the buccal cusps of the upper or lower posterior teeth, viewed from the side the curve runs upwards towards the posterior of the mouth.
What are the clinical stages of the wash impression?
Wash impression – used in copy tecnique and for reline where you can replicate existing denture which is currently loose and then use wash impression technique to improve the fit of the new copy.
Use a fluid material, e.g. ZnO:Eugenol if no undercuts, or polyether, or light-bodied silicone. Never use alginate for wash impression as does not work in thin section.
- Remove all the undercuts which might be a problem
- Extend using green stick compound
- Always remember to apply adhesive
- Ideal impression shows areas where there is contact with good surface detail
- Take straight to finish stage (investing and flasking)
Recall the use of mold seal during the flasking process
Flasking ⇒ processing dentures
There are various designs of processing flasks. Each flask usually consists of two halves.
Flasks are made of either brass or aluminium.
We apply cold mold seal (sodium alginate) over entire plaster to act as a separator when the two halves of plaster are pressed against each other via curing clamp (see photo).
We do not put mold seal on wax because we don’t need to separate wax (only for 2 like surfaces → 2 plasters as they will stick together).
