RP 2 - Retention, support and stability of complete dentures

Question 1

What is retention?

Answer 1

resistance of a denture to VERTICAL movement away from the tissues

Question 2

What is stability?

Answer 2

the resistance of a denture to displacement by functional forces

Question 3

What are some of the displacive forces?

Answer 3

• gravity (for upper denture)
• muscle activity
• sticky foods
• function

Question 4

What is interfacial surface tension?

Answer 4

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

Question 5

What is the amount of interfacial surface tension which can be generated linked to?

Answer 5

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

Question 6

When does fluid have the ability to wet acrylic?

Answer 6

• 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

Question 7

What is interfacial viscous tension?

Answer 7

the force holding two parallel plates together that is due to the viscosity of the interposed liquid (e.g. saliva)

Question 8

What is the relationship between interfacial viscous tension and liquid viscosity?

Answer 8

• 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

Question 9

In terms of retention, why is adaptation of the denture base to the underlying mucosa so important?

Answer 9

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

Question 10

What is adhesion?

Answer 10

the physical attraction of unlike molecules for eachother

Question 11

In terms of dentures, what adhesion is present?

Answer 11

saliva - mucous membrane

saliva - denture base

Question 12

How is adhesion maximised?

Answer 12

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)

Question 13

What is cohesion?

Answer 13

the physical attraction between similar molecules e.g. salivary film

Question 14

What features of a complete denture must be considered to achieve optimum retention and stability?

Answer 14

• fitting (basal) surface
• polished surfaces
• occlusal surfaces

Question 15

In what jaw is loss of alveolar bone height usually greater?

Answer 15

in the mandible

Question 16

What is the primary supporting tissue of a denture?

Answer 16

basal bone

Question 17

What are secondary supporting structures of a denture?

Answer 17

soft tissues, rugae, tuberosities

Question 18

Why does a maxillary complete denture need to have a border seal?

Answer 18

to create a negative pressure under it (to do with atmospheric pressure)

Question 19

In terms of retention, why is border moulding important during impression taking?

Answer 19

tells us the space that the muscles will need to function, allows for correct extensions in the buccal sulci

Question 20

What provides the posterior border seal?

Answer 20

the post-dam

Question 21

What is a post dam?

Answer 21

a ridge of acrylic that’s about 0.5mm deep which will apply extra pressure at the posterior edge of the denture

Question 22

Anatomically, where is the post dam placed?

Answer 22

between the hard palate and the Ahh line/vibrating line of the soft palate - to compress soft but relatively immobile tissue

Question 23

What are the palatine fovea?

Answer 23

2 small dots which are either side of the midline of the soft palate, sit just anterior to the vibrating line

Question 24

What is an area that can be utilised to gain retention in a lower complete denture?

Answer 24

the retromylohyoid fossa (lingual pouch)

Question 25

Where is the retromylohyoid fossa?

Answer 25

sits above and anterior to where the muscles move

Question 26

Why is adequate extension into the retromylohyoid fossa important?

Answer 26

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

Question 27

What undercuts may be used to increase retention of a complete denture?

Answer 27

• labial undercut
• bilateral soft tissue undercuts
• bilateral honey undercuts

Question 28

Why is retention more difficult when there is an atrophic ridge?

Answer 28

there is less surface area to cover to gain retentive factors, and also less scope to engage useful undercuts

Question 29

What is a ‘flabby ridge’?

Answer 29

a soft, mobile, fibrous band of tissue overlying where the alveolar ridge once sat but has now resorbed

Question 30

Where is a ‘flabby ridge’ most commonly seen?

Answer 30

in the anterior part of the maxilla

Question 31

Why does a ‘flabby ridge’ make retention and stability more difficult?

Answer 31

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

Question 32

Why can tori make retention and stability difficult?

Answer 32

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

Question 33

Why may a strong gag reflex make retention and stability more difficult?

Answer 33

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

Question 34

When making an upper complete denture, if you think the patient’s gag reflex may be an issue what might you do?

Answer 34

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

Question 35

What factors can make retention and stability of complete denture difficult?

Answer 35

• atrophic ridge
• damaged alveolar ridge - trauma or surgery
• ‘flabby ridge’
• tori
• strong gag reflex
• insufficient saliva/quality of saliva (disease or medication)

Question 36

What occlusal surface factors are important to consider for retention and stability of a complete denture?

Answer 36

• arch form
• occlusal plane level
• tooth position

Question 37

As a very general guide, where should the teeth on a lower denture be positioned?

Answer 37

over the centre of the ridge

Question 38

As a very general guide, where should the teeth on an upper denture be positioned?

Answer 38

just to the buccal aspect of the ridge

Question 39

What is the ‘neutral zone’ in terms of tooth position?

Answer 39

• 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

Question 40

In what way does a lingual overhang of teeth on a lower denture impact stability?

Answer 40

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

Question 41

Why is creating a balanced occlusion important for retention?

Answer 41

the border seal can be broken by uneven loading

Question 42

What role do the polished surfaces of a denture have in retention?

Answer 42

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

Question 43

What are denture valves?

Answer 43

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)

Question 44

What are denture anchors?

Answer 44

• 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