Week 3 Class II Cav for Amalg Lect Overheads PDF

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CLASS II CAVITY FOR AMALGAM TWO-SURFACE OR THREE-SURFACE CAVITY -- MO, DO, MOD Main principles to keep in mind Because access to the Class II lesion is normally achieved from the occlusal surface, the completed Class II amalgam cavity preparation consists of a proximal portion and an occlusal portion. 1.

Outline form a. Occlusal portion Similar to Class I cavity for amalgam. The exception is where, for reasons of retention, the cavity is modified specifically to provide an occlusal lock. This specific modification is frequently not required because adequate occlusal lock already exists after extending along the existing fissure pattern. In some teeth, when this specific modification is required, the occlusal lock takes the form of a "dovetail" and is referred to as an occlusal dovetail.

b.

Proximal portion In the CLINICAL situation : The outline should be minimal, consistent with, i.

ii.

iii.

Removal of all caries in enamel, at the enamel-dentine junction and approx 0.5mm into the dentine. Margin of cavity COMPLETELY free of contact with adjacent tooth; yet only sufficiently free of contact to pass the tip of a probe. For preclinical work, we define the extensions as: Minimum --- just sufficient to pass the tip of a probe Maximum (for conservative cavity) --- a 010 bur stood on the gingival wall, cannot be moved out between the B or L margin and the adjacent tooth Outline will reflect the slight divergence of the buccal and lingual walls towards the gingival. This slight divergence: 1. provides retention 2. minimizes tooth loss The divergence from the vertical is not necessarily equal for each wall. (Compare Diagrams 3 and 4, below)

iv.

Gingival margin usually (but not necessarily) horizontal.

v.

Rounded corners at junction of buccal, gingival and lingual walls.

vi.

Remove any substantially weakened enamel that has been unsupported by the presence of caries.

In addition, for cavities in plastic/extracted teeth, regardless of the gingival extension required to achieve freedom of contact, A STEP OF AT LEAST 1.5 mm (1.5-2.0 mm) MUST ALWAYS BE PLACED FROM THE PULPAL WALL DOWN TO THE GINGIVAL WALL. This ensures that students have a clinically realistic experience. Clinically, the step may be required to be much greater than 1.5 mm. Where required by the need to achieve freedom of contact, further extension (beyond 1.5 mm) may be necessary. Examples of possible occlusal and proximal outlines for Class II amalgam cavities are shown below:

2.

Resistance and Retention form (Depth, Angle of walls, Undercuts etc) a.

Occlusal portion Identical with Class I cavity for amalgam To provide retention against proximal movement of the restoration, it is necessary to ensure adequate occlusal lock. This i. May already exist (eg. Diagram 2, above); ii. May need to be specifically placed (eg. Diagram 1, above). Do not over-emphasize this.

b.

Proximal portion i.

Depth towards gingival -- only sufficient to satisfy requirements of outline form.

ii.

Depth of axial wall (towards pulp) -minimal necessary to allow preparation of proximal box without prejudicing the surface of the adjacent tooth.

iii.

Retention provided by slightly diverging buccal and lingual walls towards the gingival wall.

Resistance form (continued) iv.

Buccal and lingual walls approximately at right angles to tooth surface at the margin.

v. Walls smooth and free of irregularities. vi.

Rounded axio-pulpal line angle to reduce stress concentration.

vii. Lightly planed proximal margins to remove any enamel already fractured or unsupported.

NOTE To minimize the risk of fracture at the junction of the proximal and occlusal portions, this "isthmus" region should not be the position at which the occlusal outline is narrowest. Rounding of the outline form, as illustrated, reduces the danger of fracture.

3.

Convenience form Adequate for i) preparation of cavity ii) placement of lining material iii) condensation of amalgam into all aspects of the cavity

ALSO: Removal of deep caries Finish of cavo-surface angle Clean cavity

Preparation of the Class II cavity for amalgam . Burs

Instruments required F.G.:eg. Diamond 008 and 010 burs

GMT:-

U 1/2, U 3/4 or 28/29 (Possibly 77, 78, 79, 80) GMT = Gingival Margin Trimmer

Preparing the Cavity Step 1

Preparation of the occlusal portion. Prepare, finish and assess this portion BEFORE commencing the proximal portion. Prepare as directed for Class I amalgam cavity with two exceptions: 1.

At the end away from the proximal portion: May need to specifically create a slight occlusal lock. (See Diagram, below) Avoid undermining the marginal ridge.

2.

At the end of the proximal portion: Initially extend only as far as marginal ridge actually, only to the groove or fissure inside the ridge. Do not extend any buccal or lingual grooves or fissures in this region until after forming the proximal box (Step 4 iv.).

Finish and assess this portion of the cavity

Step 2

Positioning the proximal portion i.

Regardless of the general direction of the main M-D fissure , use F.G. bur at the established depth and extend from inside the marginal ridge directly towards the B-L (Bucco-lingual) centre of the contact region wherever this is. Leave a thin intact layer of plastic (enamel) between the bur and the adjacent tooth.

ii.

Form a slot at this depth (the depth of the occlusal portion). Dimensions of slot: a.

Bucco-lingual dimension is just short of the width of the contact region of the teeth.

b.

Mesio-distal dimension is minimal (one bur diameter).

Step 3

Form a proximal box i.

Using F.G. bur and maintaining the thin layer of plastic (or, if possible, enamel) separating the bur from adjacent tooth, increase the depth of this slot towards the gingival, maintaining its existing bucco-lingual dimension. Assess, as accurately as possible, the gingival extent of the slot. When this is considered to be adequate, slightly diverge (towards the gingival) the buccal and lingual walls of the slot to provide retention.

ii.

Use F.G. bur at this new depth to thin the plastic separating layer as much as possible. When preparing the proximal portion in a natural tooth, it is common for some of the thin enamel to be lost during deepening of the slot -- because this enamel is weak and not supported by dentine.

iii.

Use small GMT's with a pushing and/or scraping action to remove all the very weak plastic (or enamel), thereby converting the slot into a box.

Step 4

Finishing the box

NOTE:

i.

Assess the adequacy of the outline form: 1. Requirements of Outline form 2. Slight divergence of buccal and lingual walls towards the gingival.

ii.

Extend or modify as required using F.G. bur to, in sequence, 1.

extend within the tooth and

2.

thin the plastic/enamel remaining on the surface and then

3.

plane as before with GMTs to establish the final margin of correct extension and with buccal and lingual walls slightly diverging towards the gingival.

If a bur can reach the buccal or lingual cavosurface margin, it usually indicates overextension (of a minimal cavity).

iii.

With proximal outline now adequate, smooth walls and remove all irregularities. With a bur, blend the occlusal margins, of the buccal and lingual walls of the proximal portion, to those of the occlusal portion to remove any sharp corners.

D.

iv.

If any further extension of deep fissures on the occlusal surface near the proximal box is required, this may be carried out (see Step 1, part 2).

v.

Round or bevel the axio-pulpal line angle using a small (#1 or 2) round bur or GMT

vi.

Lightly plane (scrape) all margins of the box with GMT's, slightly rounding the corners. Clinically, it is safest to use the largest GMT that is able to fit in the cavity -- there is less risk of the GMT "slipping" out of the cavity.

Assessing the cavity 1.

Is the occlusal outline adequate deep fissures? occlusal lock?

2.

Is the proximal outline adequate?

3.

Is there adequate retention form occlusal retention against occlusal movement? occlusal lock? divergency of proximal walls?

4.

Are the margins smooth and free of ridges, steps, irregularities, "unsupported" areas?...