The clinical implications of an edentulous stomatognathic system are considered under the following factors:

(1) modi?cations in areas of support .
(2) functional and parafunctional considerations.
(3) changes in morphologic face height, and temporomandibular joint (TMJ).
(4) cosmetic changes and adaptive responses

Support mechanism for complete dentures

Mucosal support and masticatory loads

- The area of mucosa available to receive the load from complete dentures is limited when compared with the corresponding areas of support available for natural dentitions.

- The mean denture bearing area to be 22.96 cm2 in the edentulous maxillae and approximately 12.25 cm2 in an edentulous mandible

- In fact, any disturbance of the normal metabolic processes may lower the upper limit of mucosal tolerance and initiate in?ammation

Residual ridge

The residual ridge consists of denture-bearing mucosa, the submucosa and periosteum, and the underlying residual alveolar bone.

The alveolar bone supporting natural teeth receives tensile loads through a large area of periodontal ligament, whereas the edentulous residual ridge receives vertical, diagonal, and horizontal loads applied by a denture with a surface area much smaller than the total area of the periodontal ligaments of all the natural teeth that had been present.

There are two physical factors involved in denture retention that are under the control of the dentist

- The maximal extension of the denture base
- maximal intimate contact of the denture base and its basal seat

 - The buccinator, the orbicularis oris, and the intrinsic and extrinsic muscles of the tongue are the key muscles that the dentist harnesses to achieve this objective by means of impression techniques.
 - The design of the labial buccal and lingual polished surface of the denture and the form of the dental arch are considered in balancing the forces generated by the tongue and perioral musculature.

Function: mastication and other mandibular movements

Mastication consists of a rhythmic separation and apposition of the jaws and involves biophysical and biochemical processes, including the use of the lips, teeth, cheeks, tongue, palate, and all the oral structures to prepare food for swallowing.

- The maximal bite force in denture wearers is ?ve to six times less than that in dentulous individuals.

- The pronounced differences between persons with natural teeth and patients with complete dentures are conspicuous in this functional context:

(1) the mucosal mechanism of support as opposed to support by the periodontium ;

(2) the movements of the dentures during mastication;

(3) the progressive changes in maxillomandibular relations and the eventual migration of dentures

(4) the different physical stimuli to the sensor motor systems.

Parafunctional considerations

- Parafunctional habits involving repeated or sustained occlusion of the teeth can be harmful to the teeth or other components of the masticatory system.

- Teeth clenching is common and is a frequent cause of the complaint of soreness of the denture-bearing mucosa.

- In the denture wearer, parafunctional habits can cause additional loading on the denture-bearing tissues

Force generated during mastication and parafunction

Functional (Mastication)

Direction -> Mainly vertical

Duration and magnitude -> Intermittent and light diurnal only

Parafunction

Direction -> Frequently horizontalas well as vertical

Duration and magnitude -> Prolonged, possibly excessive Both diurnal and nocturnal

Changes in morphology (face height), occlusion, and the TMJs

The reduction of the residual ridges under complete dentures and the accompanying reduction in vertical dimension of occlusion tend to cause a reduction in the total face height and a resultant mandibular prognathism.

In complete denture wearers, the mean reduction in height of the mandibular residual alveolar ridge measured in the anterior region may be approximately four times greater than the mean reduction occurring in the maxillary residual alveolar process

Occlusion

- In complete denture prosthodontics, the position of planned maximum intercuspation of teeth is established to coincide with the patient’s centric relation.

-The coincidence of centric relation and centric occlusion is consequently referred to as centric relation occlusion (CRG).

- Centric relation at the established vertical dimension has potential for change. This change is brought about by alterations indenture-supporting tissues and facial height, as well as by morphological changes in the TMJs.

TMJ changes

impaired dental ef?ciency resulting from partial tooth loss and absence of or incorrect prosthodontic treatment can in?uence the outcome of temporomandibular disorders.

Aesthetic, behavioral, and adaptive response

Aesthetic changes associated with the edentulous state.

- Deepening of nasolabial groove

- Loss of labiodentals angle

- Narrowing of lips

- Increase in columellae philtral angle

-  Prognathic appearance

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.

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.

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

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.

Applegate's Classification is a system used to categorize edentulous (toothless) arches in preparation for denture construction. The classification is based on the amount and quality of the remaining alveolar ridge, the relationship of the ridge to the residual ridges, and the presence of undercuts. The system is primarily used in the context of complete denture prosthodontics to determine the best approach for achieving retention, stability, and support for the dentures.

Applegate's Classification for edentulous arches:

1. Class I: The alveolar ridge has a favorable arch form and sufficient height and width to provide adequate support for a complete denture without the need for extensive modifications. This is the ideal scenario for denture construction.

2. Class II: The alveolar ridge has a favorable arch form but lacks the necessary height or width to provide adequate support. This may require the use of denture modifications such as flanges to enhance retention and support.

3. Class III: The ridge lacks both height and width, and there may be undercuts or excessive resorption. In this case, additional procedures such as ridge augmentation or the use of implants might be necessary to improve the foundation for the denture.

4. Class IV: The ridge has an unfavorable arch form, often with significant resorption, and may require extensive surgical procedures or adjuncts like implants to achieve a functional and stable denture.

5. Class V: This is the most severe classification where the patient has no residual alveolar ridge, possibly due to severe resorption, trauma, or surgical removal. In such cases, the creation of a functional and stable denture may be highly challenging and might necessitate advanced surgical procedures and/or the use of alternative prosthetic options like over-dentures with implant support.

It's important to note that this classification is a guide, and individual patient cases may present with a combination of features from different classes or may require customized treatment plans based on unique anatomical and functional requirements.