Preparation of the master cast - Minia...Treating the master cast with a surface sealer using a brush or an aerosol spray to seal the cast and protect the drawn design during the other

Preparation of the master cast A-Spraying. B-Beading of maxillary major connector. C-waxing the master cast

A-Spraying.

Treating the master cast with a surface sealer using a brush or an aerosol spray to seal the cast and protect the drawn design during the other steps of cast preparation and during duplication. The sealer is a mixture of acetone, diethyl phthalate, and cellulose acetate. The cast must be allowed to dry for 5 minutes.

B-Beading of maxillary major connector.

scraping the outline of the major connector into the master cast The bead line is approximately 0.5-1 mm & becomes less distinct as it approaches the

gingival margin or midline of hard palate

modification of master cast 1.Block out 2.Relief. 3.Tissue stops 4.Formation of internal finish lines

1.Blocking •Parallel block out:

1-Proximal surfaces cervical to guiding planes.

2-soft tissue undercuts that are crossed by rigid connectors.

3-Soft tissue undercut crossed by origin of bar clasp.

Shaped blockout

Arbitrary Block out

Undercuts not in the design but would complicate the duplication procedure (labial and buccal) or distobuccal area of lower cast

Crossed gingival margins

Gingival margins (crevices) Interproximal Spaces between teeth

2.Relief Any addition of wax intended to make the

framework stand away from the master cast is termed “Relief”.

In addition to serving as a spacer, one margin of the relief wax forms the “ Internal finish line” of the framework.

It is essential that this finish line be sharply defined.

Indications of relief

1.Acrylic retention Allows a space for acrylic to flow around the denture base retention

a.Internal finish lines a.Establishes a definite junction for metal framework and acrylic resin b.Located on the tissue side of RPD c.Used as a guide for placement of external finish line

a.Relief of lingual bar and approach arms a.Eliminates the possibility of tissue ulceration due to framework rubbing during function b.Lingual bars when used in conjunction with distal extension defects, usually require relief c.Relief is rarely, if ever, indicated under palatal connectors or tooth borne RPDs

Lingual bars

Sensitive areas Prominent tissues

Tissue Stops

Tissue

stops

Internal finish line