Master Impressions (Clinic) + Master Cast and working cast (Lab)

Question 1

How should the special tray be designed for alginate?

Answer 1

• Cold cure acrylic (CCA)
• Spaced Two thickness of wax
• Perforated

Question 2

How shoudl a special tray be designed for PVS or polyether?

Answer 2

• CCA
• Spaced one thickness of wax
• Not perforated

Question 3

What steps should be followed to take a master impression?

Answer 3

1. Assess tray conforms to specifications, extensions correct (if over redcue with acrylic bur, if under reject or modify with greenstick)
2. Coat tray in adhesive and load with impression material (medium body)
3. Place tray ensure adequate material flows over periphery
4. Retract lips and cheeks to release air trapped at periphery
5. Once material achieves consistency in which maintains position when muscle trimmed, Manipulate pt’s lips and cheeks to mould impression at periphery. For lowers, also get patient to perform functional movements (licking lips, placing tongue in left and right buccal sulci)
6. After moulding hold impression in place to avoid distortion

Question 4

What should be done after the impression has set?

Answer 4

• Remove impression
• Rinse with water + sprayed with dilute detergent
• Inspect for air bubbles, tearing, adequate thickness, adequate moulding of periphery
• Forward to laboratory to create a master cast with Class 3 (densite) type stone
• Also construct a working cast from greenstone (serves as copy of master cast)

Question 5

What are the steps to surveying?

Answer 5

1. Place cast on surveying table
2. Position occlusal plane of cast parallel to surveyor base initially
3. Use analysing rods to check contours of abutment teeth and if undercuts appropriate. If not (e.g. too undercut on one side and insufficient on the other) tilt the model slightly to adjust, fixing mesial/distal before buccal (this stage determines denture’s path of insertion)
4. Tri-pod the model (put three vertical lines on 3 sides so that the angulation of the model used when surveying is recorded
5. Use carbon marker to mark survey line around the tooth–> shows region of greatest convexity and where undercuts start (only survey teeth involved in clasping; area above=reciprocation, area below=clasp)
6. Use 0.25mm gauge to determine where the tip of the clasp shoudl be placed (measures the 0.25mm horizontal undercut required for most clasps to be retentive). Do this by placing the shaft against the tooth until the flange contacts the tooth. Use the flange to make a small cut to mark the area
7. Using red pencil: Draw in rests
8. Draw in clasps (these should originate from the rest and terminate below the survey line at the tip previously drawn, if possible can follow survey line initially)
9. Draw in reciprocating arm (these should lie above the survey line/NOT in the undercut area)
10. Draw in major connector
11. Indicate where the acrylic will finish

Question 6

How is a special tray created?

Answer 6

1. Place a sheet of wax over the model, mold it into place, and trim off the excess (may place an extra sheet over for 2 sheet thickness if using alginate, else one sheet for elastomeric materials)
2. Mould the cold cure acrylic over the wax and trim off the excess
3. Use half of a second sheet of cold cure acrylic to form the handle
4. Place the tray (while it is still on the model) into a eurolight light curing machine and switch it on
5. Remove the tray from the machine, remove wax sticking to it, replace tray into machine and cure
6. Smooth the edges of the tray with a grinding wheel