Margins, Spruing & Investing. C & B Stage 3

Transcription

1 Margins, Spruing & Investing C & B Stage 3

2 The Margin Topics 1. The Preparation 2. Types of Margins 3. Contouring the margin 4. Sealing the margin

3 The Preparation 2mm reduction on Incisal and Occlussal 1.5mm reduction on, facial, lingual, mesial and distal

4 The Preparation Temporary Filling Root Canal

5 The Preparation Amalgam Filling Preparation for Crown

6 The Preparation Impression of the Preparation

7 Types of Margins The margin is the outline of the preparation where the prepared tooth surface and unprepared tooth surface meet. It is also the junction between the tooth preparation and the artificial restoration.

8 Types of Margins Bevel Feather Chamfer Shoulder Chamfer shoulder

9 Types of Margins Bevel Uses Mandibular anterior teeth a restorations with metal margins Advantages Easily defined because of ½ shoulder Disadvantages Thin feather edge - possible distortion

10 Types of Margins Feather Uses Full gold crown posterior teeth VMK with 360 fine metal margin Advantages Good marginal seal Good escape of cement Disadvantages Hard to define margin/finish line May distort after casting Over contouring problems

11 Types of Margins Chamfer Uses Full gold crowns Popular design for posterior teeth Advantages Defined margins Strength due to bulk of gold at margin area Disadvantages None

12 Types of Margins Shoulder Uses Maxillary anterior teeth. Mainly used for all ceramic crowns Resin / acrylic crowns Advantages Easily defined Allows correct contouring Supports porcelain Disadvantages During cementation the crown may not seat properly

13 Types of Margins Shoulder Chamfer Uses All ceramic restorations Advantages Easily defined Allows correct contouring Supports porcelain Disadvantages None

14 Contouring the Margin Wax margin finishing Check carefully for these conditions Over-waxed margins Short margins Rippled margins Open margins Thick margins

15 Contouring the Margin Over-waxed Margins Wax has been carried past the finish line and may break off when the pattern is removed from the die resulting in a short margin. If the over-waxed area does not break during withdrawal of the pattern, it may spring back. When cast in metal this area will no longer bend as it once did and the casting maybe prevented from seating all the way down.

16 Contouring the Margin Short Margins A wax margin that is not waxed all the way to the tooth preparation margin will not provide an adequate seal for the finished restoration.

17 Contouring the Margin Ripples Any roughness in the wax near the margin will be duplicated in the casting. Do not count on being able to remove these irregularities once the casting has been made, since their elimination usually entails an unwanted change in contour.

18 Contouring the Margin Open Margins It can be the result of any of the above-mentioned problems or the contraction of wax during cooling. A gap will be seen. A poor seal for the finished restoration will occur. Remove the wax pattern from the die

19 Contouring the Margin Thick Margins A thick, rounded margin will result in poor sealing of the restoration and poor contours. The margin of the wax must come to a fine edge.

20 Contouring the Margin Correct This is how the margin should look in the finished state. That is your polished crown, ready to send to the client.

21 Spruing & Investing

22 Spruing and Investing Topics 1. Preparing to Sprue Resealing the wax margin Adding wax to contact areas 2. Spruing Technique Items required The correct technique and dimension 3. Investing Ring liners Steps when Investing (demo in prac lesson)

23 Steps Preparing to Sprue Resealing The Wax Margin 1. Freshly lubricate the die with separator 2. Place the wax pattern back on the die 3. Make certain you can see the margin on the die 4. Margin is remelted with hot instrument 5. A depression remains near the margin (see arrows) 6. Wax is added to the margin with hot instrument 7. Excess wax is lightly removed and margin re-contoured

24 Preparing to Sprue Adding wax to contact areas Extra wax is added to the contact area to allow for polishing. Only for full crowns

25 Spruing Technique Items required for spruing and investing Ring liner Sprue Former Sprue wax Casting ring Investment

26 Spruing Technique The correct technique and dimension A Successful casting depends on the correct spruing technique The sprue creates a passage way for molten alloy to pass through into the heated mold Sprues are made from wax or plastic that burn completely out of the mold

27 Spruing Technique Wax Pattern should be a minimum thickness of 0.5mm Sprue thickness (diameter) should be wider than the thickest cross-section of the wax pattern. Sprue Placement - on the thickest part of the wax pattern, at an angle allowing for the best flow of the molten alloy

28 Spruing Technique A sprue former is a cone-shaped base made of plastic to which the sprue is attached. It forms a crucible in the investment material, that allows the dental alloy to pass into the sprue and wax pattern. It is also called crucible former.

29 Spruing Technique When placing the wax pattern and sprue on to the sprue former, consider the flow of the molten alloy into the hot mold and the angles the molten alloy must pass.

30 Spruing Technique Distance between top of ring & wax pattern 5mm Ring Wax pattern Ring liner Ring liner short of ring 3mm Sprue Length 5mm Contact area extra wax Sprue Former

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32 Investing Ring liners Ring liner is placed inside the casting ring and should be 3mm short of the metal ring at both ends

33 Investing Steps when investing Investing will be demonstrated in class today

34 A finished Gold Crown