RPD Impression

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<p>Impression procedures for removable partial dentures</p> <p>IMPRESSIONIs a negative reproduction of dental</p> <p>structures from which a positive cast can be made. It is one of the most important steps in denture construction as all steps depend on it.</p> <p>TYPES OF IMPRESSIONTHERE ARE TWO TYPES OF IMPRESSION</p> <p>1.Primary impression</p> <p>Used to make a reproduction of the teeth and surrounding tissues.which a custom tray is constructed.</p> <p> It is made in a stock tray for making a study cast on</p> <p>TYPES OF IMPRESSION2.Final impression It is an impression made in custom tray Used for making the master cast on which the</p> <p>denture is constructed.</p> <p>Used to make the most accurate reproduction of the teeth and surrounding tissues.</p> <p>Impression materialsRigid materialsThermoplastics materials</p> <p>Elastic materials</p> <p>Rigid materials It record tooth and tissue details</p> <p>accurately but it cannot be removed from the mouth without fracture.</p> <p>Types of Rigid materialsPlaster of paris It have been used in dentistry for over 200 years but now elastic materials completely replaced it.</p> <p>Metallic oxide past</p> <p> Not used as primary impression materials Used for extension base edentulous ridge areas for RPD</p> <p>Thermoplastic materialsCannot record minute details</p> <p>accurately because they under go permanent distortion during removal from the tooth and tissue undercuts.</p> <p>Types of thermoplastic materialsModeling plastic Used mostly for border molding of custom impression tray for class IandII RPD.</p> <p>Impression waxes and Natural Resins</p> <p> They have the ability to record border details accurately.</p> <p>Elastic material Remain in an elastic state after they set and</p> <p>removed from the mouth. Used for making impression for RPD,</p> <p>immediate dentures, crowns, fixed partial dentures when tissue undercuts and surface detail must be record with accuracy.</p> <p>Types of Elastic materialReversible hydrocolloid (agar-agar) It is accurate for making master cast for RPD , It is mainly used for duplication of cast</p> <p>Irreversible hydrocolloid Are used for Making study cast and Master cast (alginate)</p> <p>Mercaptan rubber base impression materials Polyether impression materials Silicone impression material</p> <p> Should not be used when several undercuts are present.</p> <p> Provide good surface details and make them useful as border molding materials More accurate and easier to use than other elastic impression material</p> <p>Goals of Impression Techniques for RPD Record hard unyielding tissues (teeth)</p> <p>as well as the soft yielding tissues (mucosa) and Surfaces that will contact the RPD framework Delineate accurately Critical landmarks: preipheral extention retromolar pads, hamular notch, vestibular depths and edentulous regions.</p> <p>Impression Techniques1- Anatomic ridge form:</p> <p> for tooth suppoted R.P.D. (Kenedys</p> <p>class III, short span class IV) so the edentulous ridges dont contribute to the support of the R.P.D. Single, pressure-free imp. records the teeth and soft tissues in their anatomic form .</p> <p>Impression Techniques2-Physiologic or functional ridge form: for tooth- tissue supported R.P.D. (Kenedys class I,II,long span class IV)</p> <p>When the occlusal forces fall on toothtissue supported R.P.D., the ridge contribute to support as well as teeth This imp. recordteeth in their anatomic form and the ridge in its functional form under pressure.</p> <p>The objective of any functionalimpression technique is : to provide maximum support for the removable partial denture bases. This allows for: 1. maintenance of occlusal contact between both natural and artificial dentition 2. minimum movement of the base, which would create leverage on the abutment teeth.</p> <p>Impression Techniques2-Physiologic or functional ridge form: for tooth tissue supported R.P.D. (Kenedys class I,II,long span class IV)</p> <p>The imp. must: 1. Record and relate the tissues under uniform loading. 2. Distribute the load over as large an area as possible 3. Accurately delineate the peripheral extent of the denture base.</p> <p>Factors influencing support from distal extension bases (factors influencing the amount of tissue displacement1- Quality of soft tissues covering edentulous ridge 2- Type of bone making up denture bearing area 3- Design of partial denture 4- Amount of tissue coverage of denture base: 5- Amount of occlusal forces 6- Anatomy of denture bearing area: 7- Fit of denture base: 8. Type and accuracy of the impression registration:</p> <p>Factors influencing support from distal extension bases (factors influencingthe amount of tissue displacement</p> <p>1- Quality of soft tissues covering edentulous ridge It should be firm, dense fibrous C.T. of even</p> <p>thickness slightly compressible and firmly attached to the bone</p> <p>Factors influencing support from distal extension bases (factors influencing the amount of tissue displacement</p> <p>2- Type of bone making up denture bearing area:The ideal ridge would consist of: Cortical bone that covers dense Cancellous bone with broad rounded crest and high vertical slops. Cortical bone can resist vertical forces better than cancellous bone.</p> <p>Factors influencing support from distal extension bases3- Design of partial denture: Knowledge of basic principles of designs</p> <p>guides the management of functional forces. The use of indirect retainer will control</p> <p>rotational movement of distal extension RPD.</p> <p>Factors influencing support from distal extension bases4- Amount of tissue coverage of denture base: The broader the coverage of the edentulous ridge, the greater the distribution of the load &amp; the smaller the force per unit area</p> <p>Factors influencing support from distal extension bases5- Amount of occlusal forces:1- Number of</p> <p>artificial teeth. 2-Width of the occlusal table. 3- Efficiency of occlusal table. 4- type of the opposing dentition 5-powerfull musculature of the patient</p> <p>It influences the amount of support required to stabilize the denture base..</p> <p>Factors influencing support from distal extension bases6- Anatomy of denture bearing area:</p> <p> To distribute the forces of mastication</p> <p>to the ridge most efficiently, the majority of force must be directed to the primary stress bearing areas, that are capable of withstanding that force.</p> <p>Factors influencing support from distal extension bases7- Fit of denture base: Support is enhanced by intimate contact between the mucosa and the fitting surface of the partial denture; 8. Type and accuracy of the impression registration: the majority of the force must be directed to portions of the ridge that are capable of withstanding the force</p> <p>Impression for distal extension R.P.D.</p> <p>1.Impression of the anatomic form of the ridge. Equalizing the support between ridge &amp; abutment teeth by the use of stress breaker</p> <p>2- Physiologic or functional impression technique which records the ridge portion of the</p> <p>cast in its physiologic or functioning form by placing an occlusal load on the impression tray as the impression is being made.</p> <p>3-Selective tissue placement impression technique. In cases of soft displaceable mucosa</p> <p>Impression for distal extension R.P.D.At the imp. stage: Mcleans and Hindels methods = dual imp. Technique = pseudo-functional imp. or Impressions with custom trays. At the framework stage: Altered cast method either by functional imp.method (fluid wax) or by selected pressure imp.method At the finished denture stage: Functional relining method using fluid wax or zinc oxide euginol or rubber base relining method.</p> <p>Imp. for Dis. Ex. R.P.D.</p> <p>1. At the imp. stage:</p> <p>Imp. for Dis. Ex. R.P.D.1- At the imp. stage:</p> <p>McLeans technique (closed mouth)The technique consists of making an impression of the edentulous ridge in border-moulded denture base tray which is provided with occlusion rims. Impression paste is used to record ridge areas under biting stresses After setting of ZnO eugenol it is removed, tested, reinserted; overall alginate impression is made with the ZnO imp.seated in the mouth.</p> <p>Imp. for Dis. Ex. R.P.D.1- At the imp. stage:</p> <p> McLeans technique (closed</p> <p>mouth)</p> <p>Since the tray used for the overall imp. is in contact with the occlusal rims, finger pressure is necessary to hold the original imp. in its functional position while the hydrocolloid material geles.</p> <p>Imp. for Dis. Ex. R.P.D.1- At the imp. stage:</p> <p>Hindles technique (opened mouth)</p> <p> the same idea of McLeans</p> <p>technique but instead of the occlusion rims, use finger pressure through 2 circular openings in the posterior region of the hydrocolloid imp. Tray.</p> <p>Imp. for Dis. Ex. R.P.D.1- At the imp. stage:</p> <p>Disadvantages If the clasp action is sufficient to maintain the</p> <p>denture base in its intended position, This may result in compromised blood flow with adverse soft tissue reaction and bone resorption. If clasp action is not sufficient to maintain that functional relationship of the denture base to the soft tissue, this will result in floating denture with premature contact and patient dissatisfaction.</p> <p>Imp. for Dis. Ex. R.P.D.2. At the framework stage:</p> <p>Altered cast method :Steps:1- after the RPD frame work is constucted on anatomic imp.cast.it should be evaluated for any metal projections and sharp edges.2-check the RPD metal frame work in the patients mouth</p> <p>Altered cast method3-the impression tray is made using chemically activated resin, a the frame work with the attached impression tray is placed in the patients mouth and correct peripheral extension 4-border molding the impression tray using low fusing modeling plastic &lt; green or grey sticks &gt;</p> <p>Altered cast method5-the final impression is made by using zinc-oxide euginol paste</p> <p>with the mouth opened and tripod pressure is applied on occlusal rests and indirect retainer6-after the impression material is set, the tray is removed and checked for any discrepancies</p> <p>Altered cast method7. The metal framework with the attached imp. is positioned on the master cast with all occlusal rests properly seated in their prepared recesses.</p> <p>8. The entire assembly is boxed and poured in a different colored stone.</p> <p>Imp. for Dis. Ex. R.P.D.3- At the finished denture stage: Functional relining method:</p> <p>Imp. for Dis. Ex. R.P.D.3- At the finished denture stage: Functional relining method:</p> <p>The finished denture is relined by applying for example ZnO eugenol imp. paste to the acrylic fitting surface of the distal extension saddle the impression is made with the denture being seated by pressure on the occlusal rests and indirect retainers only. No pressure is applied to the occlusal surface of the artificial teeth</p> <p>Gage reflex controlled by:1.Tell patient to relax and breathe through their nose during the procedure. 2.All the instrument must be out of the sight of the patient and he must not see the mixing of impression material as these will initiate the gage reflex 3.Avoid touching the dorsum of the tongue with the back of the tray and seat the impression as quickly as possible</p> <p>4. Use thicker mix of Alginate 5. Set the patient in upright position 6.Carry out the impression technique using as little material as possible. 7. Desensitize the surface of the mucous membrane with: phenol mouth washes Sucking a tablet making for this purpose Application of local anesthesia on the surface</p> <p>8. The posterior border of</p> <p>the tray is shortened or post-damming is made. 9. Remove the viscous present on the soft palate. 10. Seat the tray posteriorly first. 11. The patient's head should be brought forwards and downwards.</p> <p>Methods of forming casts:There are 2 methods to form a cast, either the two-step inverted method or the boxed method.</p> <p> The two step inverted method:The impression is poured with stone and left to reach its initial set with the face up. A second mix of stone is made and placed on the bench top; then the impression with the hardened stone is inverted onto it and contoured while it is still soft. This method is suitable for alginate impression.</p> <p> The boxed method:</p> <p>Boxing as we know is done using wax or plaster and pumice 2:1 complaster. Alginate impression should be boxed by complaster because wax will not stick to alginate. The complaster is mixed and placed on a clean, smooth surface, and the impression is partly embedded with its face up; form the cast shape and the tongue space with spatula. Then after setting of the complaster, it is trimmed to suitable cast outline and wrapped in boxing wax which is sealed to the gypsum with hot wax. The complaster land is painted with a separator and the cast is poured.</p> <p>Boxing the impression, separate the cast</p> <p>Possible causes of inaccurate castsDistortion of the hydrocolloid impression: a) by partial dislodgment from the tray. b) by shrinkage caused by dehydration. c) by expansion caused by imbibition . d) by pouring the cast with too resistant stone. 2. High water powder ratio, results in a weak cast. 3. Improper mixing, results in a weak cast with chalky surface.1.</p> <p>Possible causes of inaccurate casts4. 5.</p> <p>6.</p> <p>7.</p> <p>Trapping of air, either in the mix or in pouring, because of insufficient vibration. Soft or chalky cast surface resulting from the retarding action of the hydrocolloid or the absorption of necessary water for crystallization by the dehydrating hydrocolloid. Premature separation of the cast from the impression. Delayed separation of the cast.</p>