Retention and Stability of Complete Dentures Flashcards
Define retention
Resistance of a denture to VERTICAL movement AWAY from the ridge (at rest).
Define stability
Resistance of a denture to DISPLACEMENT BY FUNCTIONAL FORCES.
4 displacive forces
- Gravity
- Muscle activity
- Sticky foods
- Function
Is the U or L and CoCr or acrylic most affected by gravity?
- The upper denture.
- CoCr affected more as it is heavier than acrylic.
What is the Interfacial surface tension?
Tension created when a thin layer of fluid is present between two parallel planes of rigid material (ex. between two glass slides with water).
What does the amount of interfacial surface tension created depend on?
The ability of fluid to wet the rigid material.
- better with serous saliva, worse with mucinic saliva.
Define Interfacial viscous tension
The force holding two parallel planes together that is due to the viscosity of the interposed liquid (ex. saliva).
What does the interfacial viscous tension mean regarding saliva?
Although highly viscous saliva is less able to flow and wet a denture (less interfacial surface tension) a degree of viscosity to saliva is required to create interfacial viscous tension
What causes interfacial viscous tension to fall? How can this be prevented?
- Viscous force falls rapidly as the thickness of the liquid increases.
- Optimum adaptation between denture and mucosa with thin salivary film.
Define adhesion
The physical attraction of unlike molecules for each other
(ex. saliva to mucous membrane, saliva to denture base).
How can denture adhesion be maximized?
By extending over potential load bearing area as possible
Does adhesion play a bigger role in U or L dentures? Why?
Upper because we can cover more area.
Define cohesion?
The physical attraction between similar molecules (ex. salivary film).
What is the importance of atmospheric pressure/ border seal in dentures (explain the analogy).
- Suction cup and glass analogy – after pressing denture (glass) into mucosa (suction cup) the mucosa wants to return to its original shape. Hence a NEGATIVE PRESSURE is generated underneath the denture.
In which denture is border seal mostly achieved? Why?
- The MAXILLARY denture.
- Because the pressure increases the greater the area that is covered.
Which two forces increase when more area is covered?
- Adhesion
- Border seal
Another term for fitting surface
Basal surface
Another term for polished surfaces
Flanges
3 things to consider to achieve optimum retention and stability in complete dentures
- Fitting surface.
- Polished surface
- Occlusal surface
What must be considered in terms of the fitting surface?
- Base shape - must we WELL ADAPTED TO THE MUCOSA
Does greater alveolor bone height happen more in the maxilla or mandible?
The mandible.
What is the importance of the basal bone? Is it greater in the U or L.
- Helps with SUPPORTING the denture.
- Always more in the MAXILLA.
Where does the support for maxillary dentures come from?
- Primary: Basal bone.
- Secondary supporting structures: Soft tissue, rugae, the tuberosities.
2 requirements for the fitting surface.
- Should sit as close to the mucosa as possible.
- Should create a negative pressure under it (maintain the tension formed between saliva, mucosa and denture).
What is the post dam? What does it do? What thickness must it be?
- Ridge of acrylic
- 0.5mm deep
- Post dam gives the seal required at the posterior border - extra pressure at the posterior edge.
Two reasons why we want the border seal/ post dam to sit before the soft palate?
- The mucosa covering the hard palate does not compress as much as the soft palate
- Soft palate has muscles which move.
Where do we want the posterior border of the denture to sit?
Behind the posterior border of the hard palate but anterior to the vibrating line.
Where are palatine fovea located? Where do the palatine fovea sit relative to the vibrating line?
- Soft palate.
- Just anterior to the vibrating line.
Where do we want the post dam depression to sit relative the palatine fovea?
Just anterior to the palatine fovea.
What is an area that can be utilized to increase the bracing + greater area to extend the denture over in a lower edentulous patient?
Retromylohyoid fossa/ lingual pouch?
Where is the retromylohyoid fossa/ lingual pouch located?
Above and anterior to where the muscles move (lingual aspect of the lower jaw).
How can you clinically investigate the retromylohyoid fossa/ lingual pouch?
- Place a mirror along the lateral border of the tongue and ask the patient to move their tongue.
- Can then see how DEEP the lingual pouch is.
What should the shape of the distolingual border of the lower denture be?
Extended flare.
What does a well adapted fitting surface depend on?
Master impressions with good surface detail.
What does border moulding do?/ what does it show? What advantage does this result in/what does it prevent?
- Replicates the functional movements the patient will make - shows the space the surrounding muscles need to move in.
- Prevents an over-extended flange which would dislodge the denture during function.
What do undercuts increase? What does this mean about the path of insertion?
- Undercuts increase retention.
- The path of insertion will have to be tilted slightly (not straight).
3 types of undercuts in the lower edentulous patient?
- Labial undercut.
- Bilateral soft tissue undercuts.
- Bilateral boney undercuts.
2 downsides of a resorbed ridge:
- Less surface area to cover to gain retention.
- Less scope to engage useful undercuts.
Factors that can make retention and stability difficult?
- Atrophic ridge.
- Damaged alveolar ridge (trauma or surgery ex. skin graft, flabby ridge).
- Tori
- Gagging
- Insufficient/ quality (mucinic vs serous) saliva.
What is the disadvantage of a flabby ridge?
When denture is loaded, the underlying tissues will compress/ move and make the denture feel unstable and possibly break the retentive forces keeping the denture in place.
What are tori?
- Benign bony growths that occur in the midline of the palate or the lingual aspect of the premolar region in the lower.
Where do tori cause more of a problem and why?
- More problematic in the lower.
- Cannot bend acrylic to engage them, hence the base would have to be shortened in the area to sit on top of the tori –> may reduce the potential stability of the denture as the lingual extension will be reduced.
What is a flabby ridge and where is it most common?
- Soft, mobile, fibrous band of tissue overlying where the alveolar ridge once sat.
- Most common in the anterior part of the maxilla.
What is the effect of gagging on retention?
Would have to reduce the palatal coverage, hence compromising border seal and reduce the other retentive forces.
What can be used if the patient gags from a full upper acrylic denture?
- Use a double post dam.
- One in the ideal area, one 1mm inside of that.
- Provides space to shorten the palatal extension (improve tolerance of denture) yet still provide some compression and border seal (although not at the ideal location).
What can be used to treat a patient with insufficient saliva?
- Saliva orthana (SPRAY).
- Dry mouth saliva replacement GEL (ex. oral balance).
3 considerations regarding occlusal surfaces when designing complete dentures
- Arch form.
- Occlusal plane level.
- Tooth position.
Ideal tooth position for the artificial teeth in U and L complete dentures?
- Lower: over the center of the ridge:
- Upper: Just to the buccal aspect of the ridge.
Where must all teeth (in complete dentures) be positioned?
In the NEUTRAL ZONE
What is the neutral zone?
An area where the outward forces from the tongue are balanced with the inward forces from the cheeks (buccinator and orbicularis oris)
3 advantages of placing the teeth in the neutral zone?
Increased:
- Stability.
- Retention.
- Comfort
What is lateral tongue spread and when does it occur?
- Tongue spreads laterally/ relaxes into this space.
- Occurs when a patient has not had natural teeth or worn a prosthesis for some time.
2 disadvantages of lateral tongue spread?
- Difficulty in finding sufficient space to position teeth.
- Difficult for patient to wear any prosthesis made.
What is the effect of a lingual overhang?
- Makes denture unstable as it encroaches into the tongue space.
How to check the retention of an upper denture?
- Gentle downwards pressure on the incisor teeth of an upper denture.
What does the term polished surfaces refer to?
- The contouring of the buccal, lingual and palatal surfaces.
What must the polished surfaces do?
Must work in harmony with the muscle function.
What is seabond?
Film inserted along the entire fit surface of the denture - helps with denture retention and stability.
Name two examples of denture adhesives. What is a caution with them and how should they be applied? Where do they work best?
- Fixodent, poligrip
- Can be toxic if used in high quantity.
- Ideally use 3 small dots in a triangular pattern on the fit surface of the denture.
- Works much better in the upper compared to the lower.
Two other methods (private) of achieving a good base fit?
- Valves
- Implants
What is the advantage of implant retained dentures?
- Help improve retention, especially in mandible.
How many implants are required?
- Minimum 2 implants are required to prevent denture rotation.
2 requirements for implants?
- Adequate bone levels.
- Excellent OH.
