RP 2 - Retention, support and stability of complete dentures Flashcards
What is retention?
resistance of a denture to VERTICAL movement away from the tissues
What is stability?
the resistance of a denture to displacement by functional forces
What are some of the displacive forces?
- gravity (for upper denture)
- muscle activity
- sticky foods
- function
What is interfacial surface tension?
the tension that is generated when a thin layer of fluid is present between 2 rigid materials
air seal around the edge of it which allows the tension to be retained
What is the amount of interfacial surface tension which can be generated linked to?
linked to the ability of the fluid to wet the rigid material
- a runny liquid is able to flow and wet the rigid material, and the material needs to be able to be wetted rather than repelling weather
When does fluid have the ability to wet acrylic?
- pure water on acrylic would bead, but once a pellicle layer from saliva has coated the acrylic then it is able to be wetted
- serous (watery) saliva has a greater ability to do this compared with mucinic saliva
What is interfacial viscous tension?
the force holding two parallel plates together that is due to the viscosity of the interposed liquid (e.g. saliva)
What is the relationship between interfacial viscous tension and liquid viscosity?
- viscous tension increases as the viscosity of the liquid increases
i.e. thick mucinic saliva is less able to flow and wet a denture base and generate surface tension, however saliva does need to have a degree of viscosity in order to generate some viscous tension - viscous force falls as the thickness of the liquid increases
i.e. liquid needs to be in a thin layer
In terms of retention, why is adaptation of the denture base to the underlying mucosa so important?
a close fitting denture will mean a thinner layer of fluid or saliva will be present between the denture and the mucosa, which then increases the tension and therefore retention of the denture
What is adhesion?
the physical attraction of unlike molecules for eachother
In terms of dentures, what adhesion is present?
saliva - mucous membrane
saliva - denture base
How is adhesion maximised?
the bigger the area covered, the bigger the adhesive force can be
- extend the denture base over as much of the potential load bearing area as possible (greater effect in maxilla than mandibe)
What is cohesion?
the physical attraction between similar molecules e.g. salivary film
What features of a complete denture must be considered to achieve optimum retention and stability?
- fitting (basal) surface
- polished surfaces
- occlusal surfaces
In what jaw is loss of alveolar bone height usually greater?
in the mandible
What is the primary supporting tissue of a denture?
basal bone
What are secondary supporting structures of a denture?
soft tissues, rugae, tuberosities
Why does a maxillary complete denture need to have a border seal?
to create a negative pressure under it (to do with atmospheric pressure)
In terms of retention, why is border moulding important during impression taking?
tells us the space that the muscles will need to function, allows for correct extensions in the buccal sulci
What provides the posterior border seal?
the post-dam
What is a post dam?
a ridge of acrylic that’s about 0.5mm deep which will apply extra pressure at the posterior edge of the denture
Anatomically, where is the post dam placed?
between the hard palate and the Ahh line/vibrating line of the soft palate - to compress soft but relatively immobile tissue
What are the palatine fovea?
2 small dots which are either side of the midline of the soft palate, sit just anterior to the vibrating line
What is an area that can be utilised to gain retention in a lower complete denture?
the retromylohyoid fossa (lingual pouch)
Where is the retromylohyoid fossa?
sits above and anterior to where the muscles move
Why is adequate extension into the retromylohyoid fossa important?
extension of the denture around the corner of the curve of the mandible will offer some bracing from lateral movement, and offer a greater area to extend the denture over
What undercuts may be used to increase retention of a complete denture?
- labial undercut
- bilateral soft tissue undercuts
- bilateral honey undercuts
Why is retention more difficult when there is an atrophic ridge?
there is less surface area to cover to gain retentive factors, and also less scope to engage useful undercuts
What is a ‘flabby ridge’?
a soft, mobile, fibrous band of tissue overlying where the alveolar ridge once sat but has now resorbed
Where is a ‘flabby ridge’ most commonly seen?
in the anterior part of the maxilla
Why does a ‘flabby ridge’ make retention and stability more difficult?
when the denture is loaded the underlying tissues will compress and move which will make the denture feel unstable, and possibly break the retentive forces keeping the denture in place
Why can tori make retention and stability difficult?
if tori are present in the mandible, due to paths of insertion the acrylic can not engage the bony growths, so the extension of the denture would only be to the top of the tori, this may reduce potential stability of the denture as the lingual extension is reduced
Why may a strong gag reflex make retention and stability more difficult?
if a patient has a strong gag reflex they may not be able to tolerate a fully extended upper denture, in these cases you may have to reduce the palatal coverage which will compromise the border seal and reduce the retentive forces
When making an upper complete denture, if you think the patient’s gag reflex may be an issue what might you do?
ask the technician to place a double post dam into the finished denture
- one at the ideal location and a second about 1mm inside of this, so if the pt can’t tolerate the full extension you can trim it to the second
What factors can make retention and stability of complete denture difficult?
- atrophic ridge
- damaged alveolar ridge - trauma or surgery
- ‘flabby ridge’
- tori
- strong gag reflex
- insufficient saliva/quality of saliva (disease or medication)
What occlusal surface factors are important to consider for retention and stability of a complete denture?
- arch form
- occlusal plane level
- tooth position
As a very general guide, where should the teeth on a lower denture be positioned?
over the centre of the ridge
As a very general guide, where should the teeth on an upper denture be positioned?
just to the buccal aspect of the ridge
What is the ‘neutral zone’ in terms of tooth position?
- the ideal area to place the artificial teeth
- area where the outward forces from the tongue are balanced by the inwards forces from the cheeks
- placing teeth in this zone increases denture retention, stability and comfort
In what way does a lingual overhang of teeth on a lower denture impact stability?
will make a lower denture very unstable as it encroached on the tongue space, as the tongue lifts in function it would pick the denture up with it
Why is creating a balanced occlusion important for retention?
the border seal can be broken by uneven loading
What role do the polished surfaces of a denture have in retention?
need to give support to the soft tissues, needs to be shaped to work in harmony with the muscles so as not to result in displacive forces during function
What are denture valves?
valves inserted into the base of a denture which act similarly to a suction cup, helping to generate a negative pressure under the denture
(the use of these typically restricted to private practice)
What are denture anchors?
- implants which pop into place in the denture
- need 2 at minimum to prevent rotation of the denture
- requires excellent hygiene and is very expensive
