Prosthetics Skills and Knowledge

Question 1

Recall the key oral anatomical landmarks for prosthodontics

Answer 1

Question 2

Recall the primary and secondary denture bearing areas of the maxilla and manidible

Answer 2

Question 3

Define retention

Answer 3

Resistance to vertical displacement of the denture away from the denture bearing surface during function

Question 4

Define stability

Answer 4

Resistance to lateral displacement of the denture during function

Question 5

Define support

Answer 5

Resistance to vertical forces of occlusion towards the denture bearing area during function

Question 6

Describe the post dam

Answer 6

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

Question 7

Explain the importance of the position of the post dam

Answer 7

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!

Question 8

Describe speech problems with new dentures with reference to phonetics

Answer 8

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

Question 9

Describe the disadvantages of an immediate denture

Answer 9

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.

Question 10

Discuss the design faults of this manidbular removable partial denture

Answer 10

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).

Question 11

Describe how to take a maxillary impression of an edentulous patient who has a flabby ridge form

Answer 11

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)

Question 12

A patient wishes to alter the shade of their teeth on the final denture. What is the issue with this?

Answer 12

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!

Question 13

Describe the properties of alginate when used as an impression material (6 points)

Answer 13

Alginate impression material:

1. Used for all types of impressions giving good surface detail
2. Needs to be cast within 24 hr (deforms on drying; imibition during submersion)
3. Needs to be supported fully within a tray
4. Alginate is elastic when set and is therefore indicated where bony undercuts are present (to allow for safe removal of impression)
5. High viscosity alginates ⇒ preliminary impressions
6. Alginate preferred over ZnO:Eu for impressions with undercuts

Question 14

Describe the properties of impression compound

Answer 14

Impression compound:

1. Used for full edentulous preliminary impressions
2. Not suitable for undercuts
3. For tray modification (e.g. extensions)
4. Poor surface detail, so not to be used for master impression

Question 15

Describe the properties of zinc oxide-eugenol impression paste (7 points)

Answer 15

Zinc oxide-eugenol impression paste

1. Rigid when set and is dimensionally stable, so it is preferred to alginate in all cases where there are no bony undercuts
2. Used in close-fitting rather than spaced tray
3. No need for adhesive on special tray as already intrinsically adhesive (so take care in patients with xerostomia as risk of intraoral adhesion)
4. Some patients may have allergies to ZnO
5. Overall bulk of the impression is kept to a minimum and so is better tolerated by the patient
6. Where the sulcus is narrow, it is easier to avoid displacement of the buccal mucosa
7. 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

Question 16

Outline an advantage of using medium-bodied vinyl siloxane (silicone) to take an impression

Answer 16

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.

Question 17

Describe the properties of polyether impression material

Answer 17

Used in a variety of impression types

Comes in light-, medium-, and heavy-bodied sets

Used for crown and bridge impressions, or chrome impressions (good surface detail of chromes)

Question 18

Describe the problems that may occur at the try-in stage

Answer 18

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.

Question 19

Discuss the clinical considerations for this patient case with respect to taking a dental impression

Answer 19

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)

Question 20

Describe the key aims of cast surverying in the design and construction of partial dentures

Answer 20

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:

1. Analyse cast
2. Mark on model:
   
   • Path of insertion
   • Path of displacement
3. Decide on final paths
4. Will it work?
5. If it doesn’t work, consider alternative path of insertion

Question 21

Name the 6 key surveyor tools

Answer 21

Surveyor tools:

1. Analysing rod
2. Surveying graphite marker (some have red marker)
3. Wax chisel
4. 0.25 mm measuring guage (chrome)
5. 0.5 mm measuring guage (cast gold)
6. 0.75 mm measuring guage (wrought material or stainless steel)

Question 22

Design a special tray (outline 5 requirements with considerations)

Answer 22

Special tray requirements and considerations

1. Spacing:
   
   • Spaced ⇒ 2–3 mm for alginate or silicone putty/wash
   • Close-fitting ⇒ 0.5–1.0 mm for ZnO:Eu
2. Peripheral extension:
   
   • 2 mm short of sulcus
   • Can be extended with tracing stick or polyvinyl siloxane (silicone) putty for accurate fit
3. 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
4. Type of handle:
   
   • Ensure appropiate shape (step vs stub)
   • Should not displace lips
5. 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

Question 23

Describe three different connectors in partial denture design

Answer 23

Deciding on connector:

1. Bar (metal):
   
   • Kind to mucosa
   • Can feel bulky
2. Plate (metal):
   
   • Covers more mucosa than bar
   • Less noticeable to patients than a bar
   • Can make plaque control more difficult
3. Plate (acrylic):
   
   • Poorly tolerated by mucosa
   • Needs bulk to give adequate strength

Wherever possible connectors should be at least 3 mm from gingival margin

Question 24

Summarize the Dundee Replica Record Block Technique (i.e. copy denture technique)

Answer 24

Copy dentures → lab putty (condensation cured silicone).

Easiest way of copying dentures is to take impressions of dentures directly, not by taking impressions of patients.

Good thing about it is: 2 bits, mix them together, then stop tray, push denture into putty.

Important, use Vaseline as two like surfaces will stick together

Sets with strong Shore hardness

Question 25

**Explain the use of the Willis bite gauge**

Answer 25

Compare occlusal vertical dimension (OVD) with resting vertical dimension (RVD) to calculate freeway space (FWS): RVS - OVD = FWS

Question 26

**Describe the use of the fox plane guide**

Answer 26

Helps with assessing incisal level to establish a balanced occlusion Line up with: 1. Eyes: * Inter-pupillary line 2. Alar-tragal line

Question 27

**List 4 types of dental articulator**

Answer 27

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

Question 28

**What is the Kennedy classification of this dentition?**

Answer 28

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.

Question 29

**What is the Kennedy classification of this dentition?**

Answer 29

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.

Question 30

**What is the Kennedy classification of this dentition?**

Answer 30

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.

Question 31

**What is the Kennedy classification of this dentition?**

Answer 31

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).

Question 32

Recall the modifcation rules to the Kennedy classifcations

Answer 32

Modification rules: It is simple to decide into which class a dental arch falls, provided there are no extra edentulous areas. For Kennedy classes I–III, any _extra edentulous areas are called modifications_. Modification 1 of class I would be an additional single edentulous area due, for example, to the loss of a canine or first premolar tooth. This situation does not become Class III modification 2, because the edentulous areas with no natural teeth at their posterior ends take precedence over the edentulous area with teeth at both ends. So the _most posterior edentulous area determines the classification_. Modification 2 of Class I would be _two additional edentulous areas not connected_. Modification 3 of class I is _three edentulous areas_; and so on. The same principle applies to Classes II and III. Class IV _does not have any modifications_; any extra edentulous spaces cause the arch to fall into one of the other classes.

Question 33

**Describe the use of the alma gauge for occlusal registration**

Answer 33

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.

Question 34

**Compare the results of a too great vertical demension with a too small vertical dimension**

Answer 34

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

Question 35

**Describe the compensating curves**

Answer 35

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

Question 36

**Recall the curve of Monson**

Answer 36

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.

Question 37

**Recall the curve of Spee**

Answer 37

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.

Question 38

**Summarize the important patient and denture design checkpoints at the full wax try-in stage before proceeding to the finish stage**

Answer 38

Check everything! * Incisors (i.e. incisal level) * Shade (change while still in wax as cannot be altered after finishing stage) * Size * Shape * Position and setting * Soft tissue support * OVD, RVD and FWS * Labial contour * Speech and phonation – ‘th’ & ‘ss’ * How do changes affect phonation? Occlusion * Reproducible, relaxed, retruded * Even contact on closure * Balanced on excursion? * Do incisors meet? ⇒ Anterior open bite? * Fit * Periphery * Post dam * Do dentures ‘bounce’? * Do they lift when lips and cheeks are tensed? * Are they underextended?

Question 39

**What are the clinical stages of the wash impression?**

Answer 39

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. 1. Remove all the undercuts which might be a problem 2. Extend using green stick compound 3. Always remember to apply adhesive 4. Ideal impression shows areas where there is contact with good surface detail 5. Take straight to finish stage (investing and flasking)

Question 40

**List 6 key points of advice for denture care**

Answer 40

Denture care advice to give to patient: 1. Clean using soft brush and water 2. Use denture cleaning solutions with care (don’t over use) 3. Clean denture and mouth after meals 4. Remove at night while sleeping 5. Keep moist 6. Continue with regular check ups

Question 41

**Recall the use of mold seal during the flasking process**

Answer 41

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).

Question 42

**Describe 2 keys types of acrylic denture base porosities**

Answer 42

Gaseous porosities: * Incorrectly mixed * Monomer in the resin is allowed to boil * Insufficiently packed * Not clamped tightly enough during cure Granular porosities: * Loss of monomer while resin mix is left to stand until dough stage is reached * If resin mix is dry ⇒ high powder to liquid ratio (not enough liquid)