NEET MDS Lessons
Prosthodontics
LIMITING STRUCTURES
A) Labial, lingual & buccal frenum
- It is fibrous band extending from the labial aspect of the residual alveolar ridge to the lip containing a band of the fibrous connective tissue the that helps in attachment of the orbicularis oris muscle.
- It is quite sensitive hence the denture should have an appropriate labial notch.
- The fibers of buccinator are attached to the buccal frenum.
- Should be relieved to prevent displacement of the denture during function.
- The lingual frenum relief should be provided in the anterior portion of the lingual flange.
- This anterior portion of the lingual flange called sub-lingual crescent area.
- The lingual notch of the denture should be well adapted otherwise it will affect the denture stability.
B) Labial & buccal vestibule
- The labial sulcus runs from the labial frenum to the buccal frenum on each side.
- Mentalis muscle is quite active in this region.
- The buccal sulcus extends posteriorly from the buccal frenum to outside back corner of the retromolar region.
- Area maximization can be safely done here as because the fibers of the buccinator runs parallel to the border and hence displacing action due to buccinator during its contraction is slight.
- The impression is the widest in this region.
C) Alveololingual sulcus
- Between lingual frenum to retromylohyoid curtain.
- Overextension causes soreness and instability.
It can be divided into three parts:
i) Anterior part :
- From lingual frenum to mylohyoid ridge
- The shallowest portion(least height) of the lingual flange
ii) Middle region :
- From the premylohyoid fossa to the the distal end of the mylohyoid region
iii) Posterior portion :
- From the end of the mylohyoid ridge end to the retromylohyoid curtain
- Provides for a valuable undercut area so important retention
- Overextension causes soreness and instability
- Proper recording gives typical S –form of the lingual flange
D) Retromolar pad
- Pear-shaped triangular soft pad of tissue at the distal end of the lower ridge is referred to as the retromolar pad.
- It is an important structure, which forms the posterior seal of the mandibular denture.
- The denture base should extend up to 2/3rd of the retromolar pad triangle.
E) Pterygomandibular raphe
SUPPORTING STRUCTURES
A) Primary stress bearing area / Supporting area
1. Buccal shelf area
- Extends from buccal frenum to retromolar pad.
- Between external oblique ridge and crest of alveolar ridge.
Its boundaries are:
1. Medially the crest of the ridge
2. Laterally the external oblique ridge
3. Distally the retromolar pad
4. Mesially the buccal frenum
The width of this area increases as the alveolar resorption continues.
B) Secondary stress bearing area / Supporting area
1. Residual alveolar ridge
- Buccal and lingual slopes are secondary stress bearing areas.
RELIEF AREAS
A) Mylohyoid ridge
- Attachment for the mylohyoid muscle.
- Running along the lingual surface of the mandible.
- Anteriorly: the ridge lies close to the inferior border of the mandible.
- Posteriorly it lies close to the residual ridge.
- Covered by the thin mucosa which may be traumatized by denture base hence it should be relieved.
- The extension of the lingual flange is to be beyond the palpable position of the mylohyoid ridge but not in the undercut.
B) Mental foramen
- Lies on the external surface of the mandible in between the 1st and the 2nd premolar region.
- It should be relieved specially in case it lies close to the residual alveolar ridge due to ridge resorption to prevent parasthesia.
C) Genial tubercle
- Area of muscle attachment (Genioglossus and Geniohyoid).
- Lies away from the crest of the ridge.
- Prominent in resorbed ridges therefore adequate relief to be provided.
D) Torus mandibularis
- Abnormal bony prominence.
- Bilaterally on the lingual side near the premolar area.
- Covered by thin mucosa so it should be relieved
Biomechanics & Support
-
Clasp Arm Flexibility
-
Inversely proportional to diameter: thinner clasp arms are more flexible.
-
-
Posterior Tooth Support
-
Teeth with divergent roots offer better periodontal support and stability.
-
Abutment Selection
-
Lateral Incisor
-
Not ideal as abutment due to low pericemental area and poor root morphology.
-
Phonetics & Tooth Placement
-
Labiodental Sounds (‘f’, ‘v’)
-
Help determine vertical positioning of maxillary anterior teeth during try-in.
-
Diagnostic Tools
-
Gnathodynamometer
-
Instrument used to measure biting force—valuable in occlusal analysis and prosthesis design.
-
Muscles & Movement
-
Orbitoaxial Opening
-
Controlled by lateral pterygoid muscle—key in mandibular translation.
-
-
Tongue Movement
-
Influences the palatoglossal arch—important in denture border molding and speech.
-
| Muscle | Function | Clinical Relevance |
|---|---|---|
| Masseter | Elevates mandible (closes jaw) | Key in mastication; affects occlusal forces |
| Temporalis | Elevates and retracts mandible | Guides centric relation; helps in CR recording |
| Medial Pterygoid | Elevates and protrudes mandible | Assists in lateral movements |
| Lateral Pterygoid | Protrudes and depresses mandible | Controls orbitoaxial opening; guides condyle |
| Digastric | Depresses mandible; elevates hyoid | Important in opening jaw and swallowing |
| Mylohyoid & Geniohyoid | Floor of mouth support | Influence denture stability and border molding |
Finish lines are the marginal configurations at the
interface between a restoration and the tooth structure that are intended to be
refined and polished to a smooth contour. In prosthodontics, they are crucial
for the proper adaptation and seating of restorations, as well as for
maintaining the health of the surrounding soft and hard tissues. Finish lines
can be classified in several ways, such as by their location, purpose, and the
burs used to create them. Here's an overview:
1. Classification by Width:
a. Narrow Finish Lines: These are typically 0.5mm wide or less
and are often used in areas where the restoration margin is tight against the
tooth structure, such as with metal-ceramic restorations or in cases with
minimal tooth preparation.
b. Moderate Finish Lines: These are 0.5-1.5mm wide and are
commonly used for most types of restorations, providing adequate space for a
good margin and seal.
c. Wide Finish Lines: These are 1.5mm wide or more and are
often used in areas with less than ideal tooth preparation or when a wider
margin is necessary for material manipulation or when there is a concern about
the stability of the restoration.
2. Classification by Location and Application:
a. Shoulder Finish Line: This finish line is at a 90-degree
angle to the tooth structure and is often used for metal-ceramic and all-ceramic
restorations. It provides good support and can be easily visualized and
finished.
b. Knife-Edge Finish Line: This is a very thin finish line that
is beveled at an approximately 45-degree angle to the tooth structure. It is
typically used for all-ceramic restorations and is designed to mimic the natural
tooth contour, providing excellent esthetics.
c. Feather Edge Finish Line: Also known as a chamfer, this
finish line is beveled at approximately 90-degrees to the tooth structure. It is
used in situations where the tooth structure is not ideal for a shoulder margin,
and it helps to distribute the forces evenly and reduce the risk of tooth
fracture.
d. Butt-Joint Finish Line: This is when the restoration margin
is placed directly against the tooth structure without any bevel. It is often
used in the lingual areas of anterior teeth and in situations where there is
minimal space for a margin.
3. Classification by Function:
a. Functional Finish Lines: These are placed where the restoration will be
subject to significant occlusal or functional stresses. They are designed to
enhance the durability of the restoration and are usually placed at or slightly
below the height of the free gingival margin.
b. Esthetic Finish Lines: These are placed to achieve a high level of cosmetic
appeal and are often located in the facial or incisal areas of anterior teeth.
They are typically knife-edge margins that are highly polished.
Advantages and Disadvantages:
- Narrow finish lines can be more challenging to clean and may be less visible,
potentially leading to better esthetics and less irritation of the surrounding
tissues. However, they may also increase the risk of recurrent decay and are
more difficult to achieve a good margin seal with.
- Moderate finish lines are easier to clean and provide a better margin seal,
but may be more visible and can potentially lead to increased tooth sensitivity.
- Wide finish lines are more forgiving for marginal adaptation and are easier to
clean, but they can be less esthetic and may require more tooth reduction.
Burs Used:
- The choice of bur for creating finish lines depends on the restoration
material and the desired margin design. For example:
a. Diamond Burs: Typically used for creating finish lines on natural tooth
structures, especially for knife-edge margins on ceramic restorations, due to
their ability to produce a smooth and precise finish.
b. Carbide Burs: Often used for metal-ceramic restorations, as they are less
likely to chip the ceramic material.
c. Zirconia-Specific Burs: Used for zirconia restorations to prevent chipping or
fracture of the zirconia material.
When creating finish lines, the dentist must consider the patient's oral health,
the type of restoration, the location in the mouth, and the desired functional
and esthetic outcomes. The correct selection and preparation of the finish line
are essential for the longevity and success of the restoration.
Most Tested Values
- 6 gauge lingual bar - standard specification
- <90° rest seat angle - prevents food impaction
- 0.010 inch undercut - optimal retention
- ⅔ guide plane length - adequate guidance
- 1.5 mm acrylic thickness - tissue adaptation
- ≥8 mm palatal strap - adequate strength
Procedural Sequences
- Surveying: Guiding plane → path of insertion
- Construction: Stress-bearing design → framework → tissue contact
- Retention: Direct (clasps) + Indirect (rugae area)
Design Principles
- Far from fulcrum = better indirect retention (rugae area)
- Gingival third = optimal retention zone
- Broad buccolingually = minor connector strength
- Complete palate = maximum rigidity
- Stress-bearing first = foundation principle
Clinical Correlations
- Rugae = indirect retention - anatomical advantage
- Guiding planes = predictable insertion path
- Bennett shift = lateral mandibular movement
- Translation motion = superior TMJ compartment
- Facebow = axis-orbital plane transfer
Fluid Wax Method
- Proper technique indicator: Glossy appearance
- Significance: Indicates intimate tissue contact
- Defective flow sign: PPS butt joint formation
- Butt joint cause: Inadequate wax flow
Special Conditions
- Submucous fibrosis: Addition silicone (preferred material)
- Reason: Better flow properties and biocompatibility
Surveying
Surveying is the process of analyzing a dental cast to determine the optimal path of insertion and identify undercuts, guiding planes, and suitable abutments for RPD design.
Components of a Dental Surveyor
| Part | Function |
|---|---|
| Vertical Column | Holds the surveying arm |
| Surveying Arm | Moves horizontally to analyze contours |
| Tabletop | Rotates and tilts the cast |
| Analyzing Rod | Identifies height of contour |
| Undercut Gauge | Measures depth of undercuts for clasp design |
| Carbon Marker | Marks survey lines and guiding planes |
Key Surveying Objectives
-
Determine Path of Insertion
- Establish a direction that allows smooth placement and removal of the prosthesis.
-
Identify Height of Contour
- Line encircling the greatest bulge of a tooth; helps in clasp placement.
-
Locate Undercuts
- Areas below the height of contour used for retention via clasps.
-
Mark Guiding Planes
- Flat surfaces prepared on abutment teeth to guide insertion and enhance stability.
-
Evaluate Soft Tissue Undercuts
- Helps avoid interference and discomfort during prosthesis placement.