Dr. Waseem Bahjat Mushtaha Specialized in prosthodontics DENTURE BASE Dr. Waseem Bahjat Mushtaha Specialized in prosthodontics
Definition: The denture base is that part of the RPD, which contacts the oral mucosa and in which the artificial teeth are attached. Requirements for an ideal denture base 1-Accuracy of adaptation to the tissues, with minimal dimensional changes. 2- Thermal conductivity. 3-Non irritant and can maintain good finish and polish and easily kept clean. 4-Low specific gravity (light in weight) 5-Sufficiently strong to resist fracture or distortion. 6-Esthetically acceptable. 7-Can be relined. 8-Low coast.
Functions: 1 -Support artificial teeth and transfer stresses to the supporting structures. 2-Add to esthetics by reproducing natural tissue contour. 3-Stimulation of underlying tissues by the denture movement. 4-Prevent vertical and horizontal migration of the remaining natural teeth. 5-In addition, the distal extension base has the following
functions: a-Support; the broad coverage provides the best support with minimal load per unit area. b-Retention; is provided by intimate contact of the base and palatal plates with the underlying tissues and by proper molding of the polished surface.
Types of denture bases I-Acrylic denture base: Advantages: 1- Low specific gravity. 2 -Ease of relining. 3- Good esthetics. 4-Low coast. 5-Ease of fabrication. Disadvantages: 1-Less accurate fitness and high dimensional changes. 2-No thermal conductivity. 3-Low strength, so it is made more thick and bulky. 4-Law abrasion resistance, so it becomes rough and tend to accumulate food particles and calcareous deposits.
1I- Metallic denture bases: Indications: 1-For short span tooth borne denture. 2- When there is insufficient vertical space for use of resin. 3- When there is a deep vertical overlap of anterior teeth .
Advantages: 1-Accuracy and dimensional stability. 2-High abrasion resistance. 3-Contributes to the health of the underlying tissues by its cleanliness. 4-Thermal conductivity may help in patient's acceptance to the denture. 5-High strength; can be made in thin sections especially cobaltchromium and titanium alloys. Disadvantages: 1-It cannot be relined. 2- Difficult in fabrication. 3-Poor esthetic. 4-Expensive.
III- Denture base resin on metal framework Indication: l-For extension bases. 2-When denture base resin is needed to restore anatomic contour and esthetics. 3- When there is a need for relining. 4-For long span tooth supported saddle.
Design considerations: 1-The open lattice design with large openings provides the greatest retention to the resin. The retentive mesh makes the resin packing more difficult and the bond between the resin and the metal is weak. 2- A relief should be provided to the metal framework. at the ridge area to provide a space to allow resin to completely surround it. 3-The internal and external finish lines should have less than 90" angle to provide mechanical retention for the resin. 4-A metal stop should be used on the extension base framework for stabilization of the framework during packing of resin
STRESS BREAKERS (EQUILIZERS) Definition: It is any device that allows movement between the saddle unit and the retainer unit. It is used to reduce the lateral and tipping forces on the abutment teeth in the tooth- tissue borne partial dentures
Types of stress breakers: 1-Any flexible clasp design that permits tissueward movement of the extension base with minimal leverage on the abutment tooth. These clasps include the wrought wire clasp, the RPI systems, the roach clasp and the C clasp. These types of clasps are commonly used to equitably distribute the load between the abutment and the ridge. 2- A more complicated designs include: a- movable joints: between the direct retainer and the denture base: This type permits both vertical movement and hinge movement of the distal extension base. These movements prevent direct transmission of tipping forces to the abutment teeth during function. These devices include hinges, sleeves and cylinders, and ball and socket . b- A flexible connection: between the direct retainer and the denture base for example split major connector
Advantages of stress breakers: 1-Preservation of the abutment tooth alveolar support by minimizing the horizontal force falling on it. 2-Distribution of the masticatory forces between the abutment teeth and the residual ridge. 3-Physiologic stimulation of the mucosa by the intermittent movement of the denture base. 4-If relining was needed but not done; the abutment teeth are not damaged.
Disadvantages: 1-Difficult to construct and expensive. 2-More stresses are falling on the residual ridge resulting in increase bone resorption. 3- The effect of indirect retainer is reduced. 4-lt may allow food and tissue trap. 5-lt is liable for distortion and fracture. 6-It is difficult to repair. 7-If relining was not done when needed, excessive resorption of the residual ridge will occur.