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

Arrangement of Teeth in Complete Dentures

The arrangement of teeth in complete dentures is a critical aspect of prosthodontics that affects both the function and aesthetics of the prosthesis. The following five principal factors must be considered when arranging teeth for complete dentures:

1. Position of the Arch

• Definition: The position of the arch refers to the spatial relationship of the maxillary and mandibular dental arches.
• Considerations:
  • The relationship between the arches should be established based on the patient's occlusal plane and the anatomical landmarks of the residual ridges.
  • Proper positioning ensures that the dentures fit well and function effectively during mastication and speech.
  • The arch position also influences the overall balance and stability of the denture.

2. Contour of the Arch

• Definition: The contour of the arch refers to the shape and curvature of the dental arch.
• Considerations:
  • The contour should mimic the natural curvature of the dental arch to provide a comfortable fit and proper occlusion.
  • The arch contour affects the positioning of the teeth, ensuring that they align properly with the opposing arch.
  • A well-contoured arch enhances the esthetics and function of the denture, allowing for effective chewing and speaking.

3. Orientation of the Plane

• Definition: The orientation of the plane refers to the angulation of the occlusal plane in relation to the horizontal and vertical planes.
• Considerations:
  • The occlusal plane should be oriented to facilitate proper occlusion and function, taking into account the patient's facial features and anatomical landmarks.
  • The orientation affects the alignment of the teeth and their relationship to the surrounding soft tissues.
  • Proper orientation helps in achieving balanced occlusion and minimizes the risk of denture displacement during function.

4. Inclination of Occlusion

• Definition: The inclination of occlusion refers to the angulation of the occlusal surfaces of the teeth in relation to the vertical axis.
• Considerations:
  • The inclination should be designed to allow for proper interdigitation of the teeth during occlusion.
  • It influences the distribution of occlusal forces and the overall stability of the denture.
  • The inclination of occlusion should be adjusted based on the patient's functional needs and the type of occlusion being utilized (e.g., balanced, monoplane, or lingualized).

5. Positioning for Esthetics

• Definition: Positioning for esthetics involves arranging the teeth in a way that enhances the patient's facial appearance and smile.
• Considerations:
  • The arrangement should consider the patient's age, gender, and facial features to create a natural and pleasing appearance.
  • The size, shape, and color of the teeth should be selected to match the patient's natural dentition and facial characteristics.
  • Proper positioning for esthetics not only improves the appearance of the dentures but also boosts the patient's confidence and satisfaction with their prosthesis.

Complete Denture Occlusion

Complete denture occlusion is a critical aspect of prosthodontics, as it affects the function, stability, and comfort of the dentures. There are three primary types of occlusion used in complete dentures: Balanced Occlusion, Monoplane Occlusion, and Lingualized Occlusion. Each type has its own characteristics and applications.

Types of Complete Denture Occlusion

1. Balanced Occlusion

• Definition: Balanced occlusion is characterized by simultaneous contact of all opposing teeth in centric occlusion, providing stability and even distribution of occlusal forces.
• Key Features:
  • Three-Point Contact: While a three-point contact (one anterior and two posterior) is a starting point, it is not sufficient for true balanced occlusion. Instead, there should be simultaneous contact of all teeth.
  • Minimal Occlusal Balance: For minimal occlusal balance, there should be at least three points of contact on the occlusal plane. The more points of contact, the better the balance.
  • Absence in Natural Dentition: Balanced occlusion is not typically found in natural dentition; it is a concept specifically applied to complete dentures to enhance stability during function.
• Importance: This type of occlusion is particularly important for patients with complete dentures, as it helps to minimize tipping and movement of the dentures during chewing and speaking.

2. Monoplane Occlusion

• Definition: Monoplane occlusion involves a flat occlusal plane where the occlusal surfaces of the teeth are arranged in a single plane.
• Key Features:
  • Flat Occlusal Plane: The occlusal surfaces are designed to be flat, which simplifies the occlusion and reduces the complexity of the denture design.
  • Limited Interference: This type of occlusion minimizes interferences during lateral and protrusive movements, making it easier for patients to adapt to their dentures.
• Applications: Monoplane occlusion is often used in cases where the residual ridge is severely resorbed or in patients with limited jaw movements.

3. Lingualized Occlusion

• Definition: Lingualized occlusion is characterized by the positioning of the maxillary posterior teeth in a way that they occlude with the mandibular posterior teeth, with the buccal cusps of the mandibular teeth being positioned more towards the buccal side.
• Key Features:
  • Maxillary Teeth Positioning: The maxillary posterior teeth are positioned more towards the center of the arch, while the mandibular posterior teeth are positioned buccally.
  • Functional Balance: This arrangement allows for better functional balance and stability during chewing, as the maxillary teeth provide support to the mandibular teeth.
• Advantages: Lingualized occlusion can enhance the esthetics and function of complete dentures, particularly in patients with a well-defined ridge.

Understanding the anatomical considerations for upper (maxillary) and lower (mandibular) dentures is crucial for successful denture fabrication and fitting. Proper knowledge of stress-bearing areas, retentive areas, and relief areas helps in achieving optimal retention, stability, and comfort for the patient.

Maxilla

Stress Bearing Areas

• Primary Stress Bearing Area:

  • Residual Alveolar Ridge: The primary area where the forces of mastication are transmitted.

• Secondary Stress Bearing Areas:

  • Rugae: The folds in the anterior hard palate that provide additional support.
  • Anterior Hard Palate: The bony part of the roof of the mouth.
  • Maxillary Tuberosity: The rounded area at the back of the maxilla that aids in support.

• Tertiary Stress Bearing Area and Secondary Retentive Area:

  • Posteriolateral Part of Hard Palate: Provides additional support and retention.

Relieving Areas

• Incisive Papilla: A small elevation located behind the maxillary central incisors; important to relieve pressure.
• Mid Palatine Raphe: The midline ridge of the hard palate; should be relieved to avoid discomfort.
• Cuspid Eminence: The bony prominence associated with the canine teeth; requires relief.
• Fovea Palatine: Small depressions located posterior to the hard palate; should be considered for relief.

Primary Retentive Area

• Posterior Palatal Seal Area: The area at the posterior border of the maxillary denture that aids in retention by creating a seal.

Mandible

Stress Bearing Areas

• Primary Stress Bearing Area:

  • Buccal Shelf Area: The area between the residual ridge and the buccal vestibule; provides significant support.

• Secondary Stress Bearing Area:

  • Slopes of Edentulous Ridge: The inclined surfaces of the residual ridge that can bear some stress.

Retentive Areas

• Primary Retentive and Primary Peripheral Seal Area:

  • Retromolar Pad: The area behind the last molar that provides retention and support.

• Secondary Peripheral Seal Area:

  • Anterior Lingual Border: The area along the anterior border of the lingual vestibule that aids in retention.

Relief Areas

• Crest of Residual Ridge: The top of the ridge should be relieved to prevent pressure sores.
• Mental Foramen: The opening for the mental nerve; should be avoided to prevent discomfort.
• Mylohyoid Ridge: The bony ridge along the mandible that may require relief.

Posterior Palatal Seal (PPS)

The posterior palatal seal is critical for ensuring a complete seal, which enhances the retention of the maxillary denture.

Functions of the Posterior Palatal Seal

• Displacement of Soft Tissues: Slightly displaces the soft tissues at the distal end of the denture to ensure a complete seal.
• Prevention of Food Ingress: Prevents food and saliva from entering beneath the denture base.
• Control of Impression Material: Prevents excess impression material from running down the patient's throat.

Vibrating Lines

• Vibrating Line: An imaginary line that passes from one pterygomaxillary notch to the other, located 2 mm in front of the fovea palatine, always on the soft palate. The distal end of the denture should be positioned 1-2 mm posterior to this line.

• Anterior Vibrating Line:

  • Located at the junction between the immovable tissues of the hard palate and the slightly movable tissues of the soft palate.
  • Identified by asking the patient to say "ah" in short vigorous bursts or performing the Valsalva maneuver.
  • The line has a cupid bow shape.

• Posterior Vibrating Line:

  • Located at the junction of the soft palate that shows limited movement and the soft palate that shows marked movement.

→ Following rules should be considered to classify partially edentulous arches, based on Kennedy's classification.

Rule 1:

→ Classification should follow, rather than precede extraction, that might alter the original classification.

Rule 2:

→ If 3^rd molar is missing and not to be replaced, it is not considered in classification.

Rule 3:

→ If the 3^rd molar is present and is to be used as an abutment, it is considered in classification.

Rule 4:

→ If second molar is missing and is not to be replaced, it is not considered in classification.

Rule 5:

→ The most posterior edentulous area or areas always determine the classification.

Rule 6:

→ Edentulous areas other than those, which determine the classification are referred as modification spaces and are designated by their number.

Rule 7:

→ The extent of modification is not considered, only the number of additional edentulous areas are taken into consideration (i.e. no. of teeth missing in modification spaces are not considered, only no. of additional edentulous spaces are considered).

Rule 8:

→ There can be no modification areas in class IV.

Articulators in Prosthodontics

An articulator is a mechanical device that simulates the temporomandibular joint (TMJ) and jaw movements, allowing for the attachment of maxillary and mandibular casts. This simulation is essential for diagnosing, planning, and fabricating dental prostheses, as it helps in understanding the relationship between the upper and lower jaws during functional movements.

Classification of Articulators

Class I: Simple Articulators

• Description: These are simple holding instruments that can accept a static registration of the dental casts.
• Characteristics:
  • Limited to hinge movements.
  • Do not allow for any dynamic or eccentric movements.
• Examples:
  • Slab Articulator: A basic device that holds casts in a fixed position.
  • Hinge Joint: Mimics the hinge action of the jaw.
  • Barndor: A simple articulator with limited functionality.
  • Gysi Semplex: A basic articulator for static registrations.

Class II: Semi-Adjustable Articulators

• Description: These instruments permit horizontal and vertical motion but do not orient the motion of the TMJ via face bow transfer.
• Subcategories:
  • IIA: Eccentric motion is permitted based on average or arbitrary values.
    • Examples: Mean Value Articulator, Simplex.
  • IIB: Limited eccentric motion is possible based on theories of arbitrary motion.
    • Examples: Monson's Articulator, Hall's Articulator.
  • IIC: Limited eccentric motion is possible based on engraved records obtained from the patient.
    • Example: House Articulator.

Class III: Fully Adjustable Articulators

• Description: These articulators permit horizontal and vertical positions and accept face bow transfer and protrusive registrations.
• Subcategories:
  • IIIA: Accept a static protrusive registration and use equivalents for other types of motion.
    • Examples: Hanau Mate, Dentatus, Arcon.
  • IIIB: Accept static lateral registration in addition to protrusive and face bow transfer.
    • Examples: Ney, Teledyne, Hanau Universit series, Trubyte, Kinescope.

Class IV: Fully Adjustable Articulators with Dynamic Registration

• Description: These articulators accept 3D dynamic registrations and utilize a face bow transfer.
• Subcategories:
  • IVA: The condylar path registered cannot be modified.
    • Examples: TMJ Articulator, Stereograph.
  • IVB: They allow customization of the condylar path.
    • Examples: Stuart Instrument, Gnathoscope, Pantograph, Pantronic.

Key Points

• Face Bow Transfer: Class I and Class II articulators do not accept face bow transfers, which are essential for accurately positioning the maxillary cast relative to the TMJ.
• Dynamic vs. Static Registrations: Class III and IV articulators allow for more complex movements and registrations, which are crucial for creating functional and esthetic dental prostheses.