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Suverying and Eliminatino of Unwanted Undercuts (Steps for Surverying…
Suverying and Eliminatino of Unwanted Undercuts
Principles and Objectives
Principles
Easily inserted and removed
by patient
Retained
against reasonable
dislodging forces
Best possible
appearance
Objective of Surveying
Determined the most
advantageous path of insertion/ withdrawal
bearing above principles in mind
What is Surveying?
The diagnostic procedure which has to be done on the study cast prior to designing the denture
Using a diagnostic tool (dental surveyor) to analyse the cast of a patient's mouth and draw lines on them
lines indicate undercut areas in relation to the paths of insertion/ removal of the dentures
(def.) Procedure of locating, delineating and appraising contour and position of the teeth and alveolar bone prior to designing a removable prosthesis
Important because it enables a denture to be designed with the
easiest path of insertion
, coupled with
greatest retention and resistance to displacement
Terminology
Guide planes/ surfaces
(
def.) Two or more vertically parallel surfaces of abutment teeth so oriented as to direct the path of placement and removal of RPDs
May occur naturally or more commonly need to be prepared
Path of insertion/ withdrawl
(def.) Refers to the path along which a prosthesis is placed/ removed intraorally
An RPD is usually fabricated to have a single path of insertion
Path of displacement
Direction in which denture tends to be displaced in function
Assumed to be at
right angles
to the occlusal plane
The disadvantage of having a vertical path of insertion/ removal is that it coincides with the path of displacement of the denture when the wearer eats sticky food
Retention depends entirely on clasps when there is vertical path of insertion
Elimination of undercut
Failure to eliminate undercut results in acrylic resin being processed into the area --> gets stuck/ cannot be inserted into the mouth
Choosing the Path of Insertion
Determined by
Potential guide planes
Undercuts for direct retention
Hard and soft tissue interferences
Aesthetic considerations
Considerations
Angulation of posterior abutment teeth
Two situations must be considered
Proximal surfaces
of bound saddle are
both angled in the same plane
, the path of insertion should be averaged along the plane
Advantages
Efficient guide planes utilized
Path of withdrawal not at right angles
to occlusal plane so displacement by sticky food is less likely
If the
distal surface
of an
abutment tooth
on a free-end saddle is
angulated
, it may be better for the
path of insertion to follow that angulation
--> makes the saddle more resistant to rotational displacement (but be careful about using a full guide plane)
Path of insertion will sometimes need to follow the angulation of anterior teeth to avoid otherwise unsightly spaces
Angulation of anterior abutment teeth
Path of insertion will sometimes need to follow angulation of anterior teeth to
avoid otherwise unsightly spaces
Position of undercuts
Undercuts may be created by tilting model on surveyor table but engagement of
undercuts is only effective
if
guide planes
ensure a
positive path of withdrawal
of the denture
Dexterity of the patient
Elderly and physically handicapped patients can find
inserting dentures with sophisticated guide planes difficult
or impossible
Purpose of Surveying
Identifying suitable
path of insertion
or withdrawal, keeping in mind aesthetics
Locating
maximum contour
of teeth, alveolar ridges, and residual ridges (soft and bony tissue undercuts)
Identifying which
undercuts need to be blocked out
Identifying which
teeth undercuts can be used to aid retention
(i.e placing clasps etc.)
Locating existing or probable
guide planes
on abutment teeth
Identifying
restorative purposes and mouth preparation
Record the cast position in relation to the selected path of insertion (
tripoding
)
Can be done by
Lock vertical arm of surveyor at a height that allows the tip of the marker to contact palatal surface of ridge in molar and incisal regions; mark 3 points
Place analyzing rod/chisel against one side of base of cast and draw line on cast parallel to rod; repeat on other side and at the back of the cast- get 3 widely spaced lines parallel to path of insertion
The Dental Surveryor
Parts of a Surveyor
Vertical Arm
Horizontal arm
Mandrel
Surveying platform
Surveying table
Tools
Lead protector
Carbon marker identifies and
marks position of maximum convexity
(survey line) separating non-undercut from undercut areas
When surveying, the tip of the marker should be level with the gingival margin, allowing the side of the marker to produce the lines
Analysing rod
Metal rod placed against teeth and alveolar ridge to
identify undercut areas
and
determine parallelism
without marking the cast
Chisel
Undercut guages (0.25mm, 0.50mm, 0.75mm)
To measure the
extent of horizontal undercut
Gauge adjusted till shank and head contact cast simultaneously -- point at which specific undercut occurs can be identified and marked
Diagnostic tool used to survey a cast prior to the construction of an RPD
Used to determine the
relative parallelism
of two or more surfaces of the teeth or other parts of the cast
When is the surveryor used?
To survey the diagnostic cast
bearing in mind the purposes of surveying (select path of insertion, look for undercuts, guide planes etc.)
To survey the master cast
prior to framework fabrication
To
eliminate unwanted undercuts
on the cast
Steps for Surverying
Orientate cast on table such that the occlusal plane is parallel to the base of the surveyor stand (final orientation should not vary too much from this)
Use analyzing rod to evaluate positions of proximal surfaces of abutments for parallelism
Adjust anteroposterior tilt of cast until maximal parallelism is obtained. By aligning majority of proximal walls, the need for alteration of tooth contours is reduced
Without adjusting anteroposterior tilt, vary the lateral tilt until undercuts for clasp tips are approximately equal (use undercut guages)
Reassess the guide planes. Remember mechanical retainers will only function if path of insertion/ removal is dictated by guide planes
Evaluate hard and soft tissue prominences and undercuts which may interfere with insertion of framework/ acrylic resin
Examine height of contour of abutments where clasps originate-- the rigid originating portion of a circumferential clasp cannot be placed in an undercut and must not be too near the occlusal surface or it will interfere with occlusion
Decide whether to eliminate interferences by recontouring. sugery or alter tilt of cast to avoid interferences
Consider asethstic requirement especially if anterior teeth need to be replaced; whether gum fit or short flange can be used to avoid alteration in path of insertion
The labial extent of guide planes for the anterior teeth must be limited to the linguo-proximal surfaces so that the metal of the guide planes will not be visible between the natural and artificial teeth
Final Path of Placcement
Should be that which is most compatible with
Guide planes location
Hard and soft tissue interferences
Required retention
Aesthetics
Tilting of cast during surveying changes
Path of insertion
Position of survey lines
Location of undercut and non-undercut areas
