2. theory of retention and stability Flashcards
exam question : define stability
Quality of denture to be firm, steady, constant and not subject to
change of position when forces are applied
(subjective
assessment mostly by the patient)
exam question: define RETENTION
Resistance of denture to removal from the denture bearing tissues
in a vertical direction
(test by pulling down or lifting up anterior
teeth with finger and thumb)
exam question: define support
Resistance of a denture to occlusally-directed forces.
Determined
by the form and consistency of the denture-bearing tissues. Good if
well formed ridges with good bony support
Patient complains of loose dentures
– is it because of ?
Lack of stability
* Lack of retention
* Lack of support
* Commonly a combination of all 3
Average upper DBA
24cm2
Average lower
DBA
14cm2
Good Stability is likely if
Retentive forces > displacing forces
+ adequate support
RETENTIVE FORCES
offer resistance to dislodgement of a denture from the underlying
tissue in vertical direction:
exam question- explain how retention is maximised (retention depends on: )
- Acquired muscular control
- Physical forces of retention
+ minimal displacing forces
- Acquired muscular control
control with lips, cheeks and tongue (via the polished surfaces)
and by
the muscles of mastication (via the occlusal surfaces of the teeth)
Acquired muscular control depends on:
i) ability of patient to acquire necessary skill
ii) design of denture
ability of patient to acquire necessary skill -
reduced in elderly or
debilitating illness, esp. muscular disease i.e. stroke / Parkinson’s disease
design of denture:
- correct extension
- correctly contoured polished surfaces
- lower teeth placed on top of alveolar ridge (neutral
zone) - level of lower occlusal plane below level of
resting tongue - correct occlusal relationships
- Physical forces (mostly apply to upper denture)
Dependent on adhesion and cohesion of saliva between mucosa and
acrylic producing a negative pressure to cause retention
(via the fit
surface – think analogy of 2 glass slides stuck together) and is
maximised by a combination of good design/construction regards:
- physical forces
i) border periperal seal
ii) greater area of impression surface
iii) accuracy of fit
i) border periperal seal
- contact between denture periphery and mucosal tissue at all time: requires correctly
border-moulded impression (full functional depth and width of sulcus) and effective
post-dam
ii) greater area of impression surface
- Retentive forces directly proportional to area of impression surface: requires correctly
extended impression
iii) accuracy of fit
thinner saliva film provides greater retentive forces: requires accurate impression
DISPLACING FORCES
- Muscles surrounding oral cavity (lips and cheeks)
during normal function – are the teeth in the
‘neutral zone’ (also called ‘area of minimal
conflict’) ? - Tongue – as above and during excessive
movements - Occlusal interference +/- locking of cusps / teeth
during chewing (lack of balanced
articulation/occlusion) - Viscous and sticky foods
How to achieve stability in a complete denture?
achieve adequate support… ie
maximise muscular forces (ability of px to acquire necessary skill + denture design)
maximise physical forces (peripheral seal+ accuracy of fit + max coverage of denture bearing area)
minimise displacing forces (muscles esp. tongue+ interference and locking of cusps in function +sticky, viscous foods)
Exam question: Describe how retentive forces, displacing forces and support interact to contribute towards stability?
Retentive forces must be larger than displacing forces.
Alongside adequate support
= good stability
Retentive forces -
aqc musc control (px skill+ denture des)
+
phys forces of retention (border per seal+ greater area imp surf+ acc of fit)
Displacing forces-
muscles surr oral cav
tongue
occ interference +/ locking of cusps/ teeth in chewing
viscous and sticky foods
Adequate support-
maximise muscular forces (ability of px to acquire necessary skill + denture design)
maximise physical forces (peripheral seal+ accuracy of fit + max coverage of denture bearing area)
minimise displacing forces (muscles esp. tongue+ interference and locking of cusps in function +sticky, viscous foods)
!!Denture Design Assessment (exam)
(Burnett tip – )
always assess complete dentures for diagnosis and during
construction by examining these 3 surfaces!
fit sufrace
polish surface
teeth
how to examine fit surface?
extension- is it right or wrong
how to examine polished surface?
form- is it right or wrong ?
teeth
a) aesthetics of upper anteriors
b) BL placement of lower teeth (are they on top of the ridge (neutral zone) ?
c) lower occlusal plane below level of resting tongue?
- occlusion
does ICP=RCP at correct OVD?
(see pic in lecture) upper anatomical landmarks of alveolar ridge?
Incisive papilla
Posterior limit of denture extension (non-moving junction of hard and soft palates, just anterior to the ‘vibrating line’)
Vestige of palatal gingival margin
Tuberosity
Hamular notch
lower anatomical landmarks of alveolar ridge
retromolar pad
external oblique ridge (buccal shelf)
lingual sulcus
buccal fraenum
sub-mandibular salivary ducts
How to ensure correct extension for a complete denture?
maximum functional coverage off full upper and lower denture bearing areas ie
ensure maximum extension
ensure functional extension and
best fit
polished surfaces correctly shaped
ensure lower teeth are on the alveolar ridge and below the level of the resting tongue
the neutral zone
zone of ‘minimal conflict’ or ‘denture space’
applies to the position of the mandibular prosthetic teeth and allows for ‘muscular balance’
defined as ‘the potential space between the lips and the cheeks on one side and the tongue on the other. That area or position where the forces between the tongue and cheeks or lips are equal.’
how to maintain a neutral zone?/ zone of minimal conflict / denture space?
use special clinical impression techniques to identify the neutral zone to allow teeth to be set in muscular balance
aim to set the lower teeth on top of the lower ridge to achieve muscular balance
how to make sure teeth achieve muscular balance?
assess the denture and make sure that prosthetic teeth are not within the neutral zone
how to achieve a well designed lower denture?
occlusal plane should be below level of resting tongue ie
extended plane should merge with the retromolar pad
the land area present which is a space behind the last tooth for the tongue to spread over
what other ways could you solve stability issues?
mandibular overdentures ie minimum 2 implants, up to 4
maxillary overdentures ie minimum 4 implants, could be 2-6
