SPECIAL IMPRESSION PROCEDURE FOR TOOTH-TISSUE SUPPORTED REMOVABLE PARTIAL DENTURES This situation requiring special impression procedure includes not only distal extension partial edentulous situations like Class I and II but also includes long span anterior edentulous ridge (including atleast 6 anterior teeth). Where the ridge must supply some support for the partial denture (Long span Class IV). A tooth supported removable partial denture can be constructed on a master cast made from a single, pressure free impression that records the teeth and the residual ridge in their anatomic form, because here, the denture bas doesnâ€™t contribute to the support of the partial denture much significantly as most of the stress is transmitted through abutment teeth. A dual impression technique is used to equalize as much as possible the support derived from the edentulous ridge and that received from abutment teeth. Tooth is captured in its anatomic form while impression of soft tissue, must be made in such manner as to record the tissues in a functioning form. IMPRESSION TECHNIQUES Basically two dual impression techniques Physiologic / functional Impression technique 1. McLean & Hindelâ€™s techniques 2. Functional Relining Technique 3. Fluid wax technique
Selective pressure Impression technique
Altered cast impression technique Or Corrected cast impression technique
In both the fluid wax functional impression technique and selective pressure technique, an impression of the displaced edentulous ridge is made using an impression tray attached to the framework, and the master cast is altered to accommodate the new ridge impression. For this reason technique is often referred to as the altered cast impression technique. McLEANâ€™S PHYSIOLOGIC IMPRESION For this dual impression a custom impressin tray was constructed over a preliminary cast of the arch, a functional impression of distal extension ridge was made with patient applying steady biting and then a second impression with hydrocolloid material was made with the first impression held in its functional position with finger pressure. Drawback: Finger pressure could not produce the same functional displacement of the tissue that biting force produced. The apparent advantage of technique was lost with this weakness. HINDELâ€™S MODIFICATION 1. Hindel developed trays for second impression [with irreversible hydrocolloid] those were provided with holes so that finger pressure could be applied through the tray. 2. Another main change was the impression of edentulous ridge was not made under pressure but was an anatomic impression of the ridge at rest made with a free-flowing zinc oxide eugenol paste.
So here, teeth and ridge were reproduced in anatomic form only, but two were related to each other in a functional form. FUNCTIONAL RELINING METHOD In this method, functional impression for support of a distal extension denture base is obtained after the partial denture has been completed. Partial denture is constructed on a cast made from a single impression, usually of irreversible hydrocolloid. This is an anatomic impression and no attempt is made to alter it. To allow room for impression material between the denture base and the ridge, space must be provided. Uniform space can be obtained by adapting a soft metal spacer over the ridge on the cast before processing the denture base. After processing the metal is removed leaving an even space. Or Flow low fusing modeling plastic over the tissue surface of the denture base, tempering the modeling plastic in a waterbath and seating it in patients mouth. Heating, tempering and resection must be done several times. Determine border extensions correctly. To provide space for the impression material, the entire surface of modeling plastic may be scraped to a depth of approximately 1mm or remove entire thickness of modeling plastic over the crest of the ridge. Make final impression with ZOE Or If undercuts are present, one can use elastomers.
It has the advantage that the amount of soft tissue displacement can be controlled by the amount of relief given to the modeling plastic before final impression is made. Greater the relief, less will be the tissue displacement. FLUID-WAX FUNCTIONAL IMPRESSION: can be made 1. To correct the distal extension edentulous ridge portion of the original master cast or 2. To make a reline impression for an existing partial denture. Term: Fluid wax is used to denote waxes that are firm at room temperature and have the ability to flow at mouth temperature for e.g. IOWA Wax & Korrecta Wax No. 4.0. Key for functional impression with these waxes lies in the consistency of fluid wax used: basically impression wax flows sluggistly and a thin layer of wax will flow less readily than a thicker layer. Relief of 1 or 2 mm is given. Wax is fluid at 51째 to 54째 C pf water bath temperature. Another advantage is that the maximum extension of peripheral borders of denture base can be recorded with Korrecta Wax No. 1, (which is more viscous comparatively) without interfering with function. Borders must be short of all movable tissues, but not more than 2mm, as material does not have sufficient strength to support itself beyond that distance. Tray must remain in mouth for minimum 5mins after each addition of wax. Finally impression should be left in the mouth for 12 minutes. New cast should be poured as soon as possible because wax is fragile and subject to distortion.
SELECTIVE PRESSURE PLACEMENT IMPRESSION
What is different?; Previously discussed techniques, produced a generalized displacement of the mucosa to a greater or lesser degree. Selective pressure impression attempts to direct more force to those patients of the ridge able to absorb stress without adverse response. To do this, the tissue surface of the tray is selectively relieved. In some patient if very easily displaceable tissue is present, holes may be made through impression tray. More viscous the impression material greater will be the tendency for displacement. Zinc oxide eugenol paste can be used as it is an accurate impression material of intermediate viscosity, especially when undercuts are not a problem. Otherwise rubber base impression materials are useful. The artificial point for operator is to determine visually that all rests and indirect retainers are completely seated while impression making. Avoid applying any downward pressure on the impression tray. To prepare altered cast, line is drawn 1mm posterior to distal abutment tooth at right angles to long axis of ridge. Second line is drawn at right angles to first, beginning medial to lingual sulcus. Remove outlined area with handsaw. Make longitudinal retention grooves on cut surface of cast to provide mechanical retention for new portion of cast to be poured. Beading and boxing is done and minimal set, boxing is removed and cast is trimmed and corrected, cast is used to complete partial denture.
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