Secondary impression and master cast for RPD (0).pptx

Removable partial denture

1. Secondary impression and master cast for RPD Prepared by: Dr. Mohammed Bahompos

2. INTRODUCTION An impression is defined as a negative likeness of the teeth and/or edentulous areas. Primary impression is made by using irreversible hydrocolloid (alginate) impression material due to it is economical, elastic and easy to manipulate.

3. Secondary impression Classification of : 1- Anatomical impression : impression of surface contour of ridge when it is not under any occlusal load by soft impression materials like zinc oxide eugenol or plaster of paris . This impression indication for tooth supported partial denture and contraindication for tooth-tissue supported partial denture .

4. 2- functional impression : are recorded under functional load (pressure), so the tissue surface is recorded in compressed form so the denture not exert additional stress on abutment teeth during functional loading and the load distributed between soft tissue and abutment teeth. This indicated for tooth-tissue supported partial denture .

5. Factors influence the support of distal extension base 1- Quality of soft tissue covering the edentulous ridge : a firm, thick, tightly attached mucosa best support than flabby , mobile tissue. 2- Alveolar ridge of denture bearing area : should be cortical plate with adequate thickness to resist the occlusal load not cancellous bone which undergo to resorption. 3- Design of partial denture : should designed such forces acting on edentulous ridge can be minimized .

6. 4- Amount of tissue coverage of denture base : wider tissue coverage distributes more occlusal load so protecting soft tissues and teeth from damage during function. 5- Amount of occlusal force : to avoid this the residual ridge should covered to its maximum extent. 6- Nature of denture bearing area : in maxilla , buccal slops of ridge, hard palate & crest of ridge. In mandible , buccal shelf , crest and slopes of ridge. 7- Fit of denture . 8- Type of impression registration .

7. Dual impression This means when record the ridge by two impression, one portion in functional form ( record edentulous ridge ) and the other in anatomical form ( record the teeth ).it indicates for all tooth tissue supported partial denture. It classify to: 1- physiological or functional dual impression. 2- selective pressure functional dual impression .

8. 1- physiological or functional dual impression. One anatomical impression made of entire ridge and one functional impression only made on edentulous portion, by many techniques: McLean's technique. Handle's modification of McLean's technique. Functional relining method. Fluid wax .

9. McLean's technique . Two impressions are made in this procedure First one functional impression on edentulous ridge and second anatomical impression made over the functional one recorded all structures. 1- special tray made on edentulous area on primary cast without space.

10. 2- occlusal rim made over the special tray. 3- ask the patient to close on special tray with occlusal rim to apply pressure while making impression

11. 4- alginate with stock tray over impression made by special tray with functional impression. 5- pressure applied over stock tray by finger to push the special tray towards the tissue .

12. 6- A cast is poured into the impression. This cast will reproduce the teeth in the anatomical form and the tissues in the functional form. Disadvantage : the supporting tissues may not be as compressed as they were while making the functional impression. - There will be a small quantity of alginate between the occlusal rim of the special tray and the over-impression stock tray.

13. Modification by Hindle it is similar to mclean’s technique. hindle modified mclean’s technique to overcome the disadvantages . 1- made special tray with occlusal rim with stoppers to avoid excessive pressure compression on tissue. 2- special tray is used to record supported tissue under anatomical form. 3- stock tray with large holes used to make over impression. The finger place into holes to apply constant pressure on occlusal rim.

14. Effects produced by Mclean and Handle's methods The denture compress the tissue even when there is no occlusal load, this will lead to excessive bone resorption duo to: Constant pressure stimulate osteoclasts . Constant pressure decrease blood supply to bone so by chemical mediators stimulate osteoclasts .

15. Functional relining method. 1- anatomical impression made of all oral structures and make master cast which duplicated and refractory cast made. 2- partial denture framework is constructed on refractory cast with spacer. 3-modelling compound is melt and adapt over framework to act as special tray. 4- pressure applied during impression to ensure compression of tissue. 5- border molding is done by manipulating cheeks and tongue. 6- impression made by zinc oxide eugenol past .

16. Fluid wax functional impression. The framework with 1-2mm space is modified into special tray that hold a fluid wax impression. Uses : * reline the tissue surface of existing RPD. Objective : * obtain maximum peripheral border extension. * to record stress bearing area in functional form. * record non- stress bearing area in anatomical form.

17. 1- marking the extent of special tray on the cast. 2- cast is coated with separating medium and spacer adapted by single layer base plate wax, the framework placed over spacer & auto- polymerising resin adapt over it. The excess material trimmed away. 3- relief holes prepared along the crest and retromolar pad area to escape material. 4- fluid wax is painted in layers over special tray till sufficient thickness .

18. Pouring the master cast - Poured within 12min, the first pour by stone & thickness atleast 6mm. After 10-12 min, slurry water applied over first pour. The impression inverted over mix of stone (second pour) and the base shaped.& the separation after 45-60min

19. Altering the master cast

20. thanks