Impression Introduction and Rigid Impression Materials (Summer 2023-2024) PDF

Faculty of Dentistry Dental Biomaterials II (PDD 132) Prof. Essam Al-Wakeel  Dental Base Impression polymers Materials Dental...

Faculty of Dentistry Dental Biomaterials II (PDD 132) Prof. Essam Al-Wakeel  Dental Base Impression polymers Materials Dental Biomaterials II Model and Die Casting Materials Alloys Dental Waxes Casting Procedures and Defects Casting Investment Faculty of Dentistry Faculty of Dentistry Impression Materials Faculty of Dentistry Dental models Definitions Faculty of Dentistry l Animpression: is a negative replica of some structure. l A dental impression: is a negative replica of tooth, teeth and /oral soft tissues. l A dental impression material : is a material used to accurately record the dimensions of oral tissues (hard and soft tissues).  Impressions Faculty of Dentistry Definitions Faculty of Dentistry l Primary impression: an impression taken for patients for the first time and dose not register the fine details. l Secondary impression: an impression taken for the second time and record the fine details. Definitions Faculty of Dentistry l Tray: is the container that carries the impression materials into the patient’s mouth. - Stock tray (universal tray) with standardized sizes (1, 2, 3) used for taking primary impressions. Made of metallic or plastic. - Special Tray is a customized tray constructed on a primary model used for taking secondary impression materials. Made of acrylic or shellac. Impression trays Faculty of Dentistry  Impression and Models Faculty of Dentistry Properties of ideal impression materials (requirements) l Ease of manipulation and reasonable cast. l Adequate flow properties. l Appropriate setting time. l Good dimensional accuracy. l Safety (non toxic and non irritant). l Compatibility with all die and cast materials. l Should have satisfactory odor and taste. l Inexpensive. l Good keeping qualities (good shelf life). Faculty of Dentistry Indications for taking impression l Diagnosis and treatment planning. l Construction of indirect restorations. l Fabrication of special (custom) trays. Faculty of Dentistry Faculty of Dentistry Dental models Classifications of impression materials l According to type of setting reaction: A. Materials set by chemical reaction. B. Materials set by temperature changes (physical). l According to elasticity (mechanical behavior after setting): A. Inelastic (rigid). B. Elastic (flexible). Faculty of Dentistry Rigid (inelastic) impression materials They are restricted to applications in areas where no undercuts exist and include: 1. Impression compound (physical). 2. Zinc oxide eugenol impression paste (chemical). 3. Impression plaster (chemical). 4. Impression waxes (physical). Faculty of Dentistry Elastic impression materials Faculty of Dentistry They are used in both cases where undercuts exist or not and include: l Hydrocolloids a. Alginate b. Agar l Elastomers (rubber impression materials) a. Polysulfide rubber b. Silicone rubber c. Polyether rubber Faculty of Dentistry Rigid Impression Materials Faculty of Dentistry 1. Dental impression compound Definition Faculty of Dentistry l Impression compound is a thermoplastic impression material that softens by heat and hardens by cooling. l It is non elastic impression material. Types Faculty of Dentistry l Type I (impression compound): it is a low fusing type soften at 45-55o C. -Used to record primary impression and tracing of special tray. l Type II (tray compound): It is a high fusing type soften at 70o C. - Used as tray material to carry other impression material (wash impression). Uses Faculty of Dentistry l Primary impression for edentulous patients. l Modeling or tracing of acrylic special tray with the stick form. l Support other impression material (used as tray for wash impression).  Compound impression Faculty of Dentistry  Tracing of tray with green compound Faculty of Dentistry Packaging (form) Faculty of Dentistry l Cakes l Sticks l It may have brown, gray or green color. l It may presented as large pieces (tray compound) or sticks, cakes and cones (impression compound). Impression compound Faculty of Dentistry  Green stick compound Faculty of Dentistry  Impression Compound Faculty of Dentistry Composition Faculty of Dentistry l Thermoplastic materials - Give thermoplasticiy and flow. - eg. 40% resin and 7% waxes. l Filler - Give body and suitable working consistency. - eg. talc and iron oxide. l Coloring agents (pigments). l Plasticizer - Act as lubricant and control consistency. - eg. Stearic acid or stearin. Reaction Faculty of Dentistry l Physical (thermoplastic). l Reversible reaction. l Softenupon heating (45-55o C) and harden upon cooling (37o C). Manipulation Faculty of Dentistry l Sheets or cakes (used for complete denture primary impression) - Softening and wet kneading. - Loading softened compound in the stock tray. - Insertion in the patient’s mouth. - Cooling and impression removal. - Cast should be prepared promptly (using plaster). - Separate the cast using warm water. Manipulation Faculty of Dentistry l Sticks used for tracing the borders of acrylic special tray - Soften by holding the stick above the flame. - The softened compound should be tempered in air or in warm water. - Tracing the borders of the tray.  Tracing of tray with green compound Faculty of Dentistry Properties Faculty of Dentistry l Thermoplastic behavior l Flow - Minimum flow is required at mouth temperature, but sufficient flow at only 8o C above mouth temperature. - Flow is higher for impression compound than tray compound. l Thermal conductivity - It is bad thermal conductor. - It should be left in the hot water bath for sufficient time and left in the mouth to cool for sufficient time. Properties Faculty of Dentistry l Has high coefficient of thermal expansion - Cooling the impression gradually. - Pouring the impression immediately after removal from the mouth. l Residual stress - Causes dimensional changes and warpage. - Other causes of warpage are: - Incomplete homogenous softening. - Removal of the impression before complete hardening. - Delaying in pouring the impression. Advantages of impression compound l Easy manipulation. l Not expensive. l Has adequate shelf life. l Needs no separating medium. l Can be electroplated with copper. Faculty of Dentistry Disadvantages of impression compound l Can not reproduce the undercut. l Does not record fine details. l Non hygienic (if it is reused). l Dimensional changes due to thermal contraction. Faculty of Dentistry Faculty of Dentistry 2. Zinc oxide eugenol (ZOE) impression material ZOE impression material l It is a rigid impression material. l Used for taking secondary impression for edentulous patients. l Generally, it is manufactured as two past system in collapsible tubes and sometimes paste and liquid. Faculty of Dentistry Packaging or forms Faculty of Dentistry Two pasts with contrasting color: l Base past: white l Catalyst past (accelerator or reactor): brown or red. ZOE impression material Faculty of Dentistry Composition of ZOE Faculty of Dentistry l Base Paste - ZO (80-85%) : reactive ingredient. - Inert oil (15-20%): acts as a plasticizer and masks the action of eugenol. l Catalyst Paste - Oil of clove that contains 70-85% eugenol. - Various oils and gum rosin. - Silica filler - Zinc acetate, MgCl2 or CaCl2 catalyst (accelerator) and water. Reaction of ZOE (Chelation) l Irreversible chemical reaction called chelation occurs in two steps: 1. ZnO hydrolysis ZnO + H2O Zn (OH)2 2. Formation of chelate Zn (OH)2 + Eugenol Zn eugenolate + ZO (unreacted) l Water accelerates setting reaction and shorten working time. l Once mixed, the material is sticky until set. Faculty of Dentistry Manipulation Faculty of Dentistry l Equal lengths are squeezed from the tubes. l Mixing is done on a glass slab or waxed paper pad. l A stiff spatula is used for mixing. l Mixing can be accomplished in 30-45 sec. l Mixing is done until a homogenous mix is formed. l Loading the traced acrylic special tray. l Using a Vaseline coating or facial cream on the patient lips makes cleaning easier. Manipulation Faculty of Dentistry l After removal from the mouth, disinfection is done by immersion in 2% glutaraldehyde. l The impression is poured with dental stone. l No need for separating medium. l To separate impression from the cast, use warm water at 60o C. Mixing of ZOE Faculty of Dentistry ZOE impressions Faculty of Dentistry Properties of ZOE l Initial setting time is 3-5 min. l Final setting time is less than 10 min. l Water, high humidity, and high temperature shorten setting time. l The impression is accurate with dimensional changes during setting of 0.1%. l They have good adhesion with tray compound or acrylic tray. Faculty of Dentistry Properties of ZOE l Model materials are restricted to gypsum products. l Easily separated from impression. Does not need separating medium. l It causes irritation to the soft tissues. Faculty of Dentistry Factors affecting setting of ZOE l Adding a drop of water accelerate reaction. l Cooling the spatula or glass slab increases setting time. l The longer the mixing time, the shorter the setting time. l Increasing the base paste increases the setting time. Faculty of Dentistry Uses of ZOE impression past l It is used as final impression for edentulous patients using border molded acrylic special tray. l Wash impression on a tray compound impression. l Taking impression in old denture for relining. Faculty of Dentistry Advantages of ZOE l Fluid enough to record the fine details. l High dimensional stability. l No separating medium needed. Faculty of Dentistry Disadvantages of ZOE l Don’t record undercut. l Sticky to dry skin and lips of the patient. l Can not be electroplated. l Eugenol causes burning sensation to some patients. Faculty of Dentistry  Non eugenol paste l Used for patient with eugenol allergy. l Supplied as two tubes one containing base paste (white) and one catalyst paste (blue). l Craboxylic acid derivatives (orthoethoxy benzoic acid) are used instead of eugenol l The reaction is chemical irreversible reaction called saponification. Faculty of Dentistry 3. Impression Plaster Impression Plaster l Now is rarely used because it is rigid and fracture easily. l Now, it is used to mount casts or to record occlusal bite registrations. l Supplied as powder and mixed with water. l It has exothermic reaction (produces heat) and causes dryness of the patient’s mouth. l The model materials is restricted to gypsum products with separating medium. Composition of powder l It is mainly CaSO4.1/2H2O similar to other gypsum products. l Accelerators and anti-expansion, and hardening solutions and flavoring agents are added. Faculty of Dentistry Reaction l Hemihydrate react with water to form dihydrate. l CaSO4.1/2H2O + 1½ H2O CaSO4. 2H2O + Heat l It is crystallization exothermic reaction. l It starts by the formation of a suspension of hemihydrate, formation of saturated solution then precipitation of dihydrate. l Crystals form and grow. Faculty of Dentistry 4. Impression waxes Faculty of Dentistry Impression waxes l One of the old rigid impression materials. l They set by physical reaction. l Not used now. Faculty of Dentistry