CCA Prosthetic Fabrication & Fitting 3 — Questions and Answers
Question 1: Which platinum catalyst system is most commonly used to vulcanize addition-cured (condensation-free) silicone for facial prostheses?
- Peroxide-based catalyst at 150°C
- Tin-based condensation catalyst at room temperature
- Platinum-based addition catalyst at body or room temperature (Correct answer)
- UV-activated photoinitiator under 365 nm light
Correct answer: Platinum-based addition catalyst at body or room temperature
Platinum-catalyzed addition-cure silicones produce no by-products, offer excellent biocompatibility, and cure at room or mild elevated temperatures used in anaplastology.
Question 2: In a magnetic retention system for an auricular prosthesis, the magnets are usually embedded in the:
- Silicone flange and the titanium implant abutment simultaneously
- Prosthesis base and the corresponding bar or cap on the abutment (Correct answer)
- Patient's skin tissue directly opposite the prosthesis
- Acrylic substructure under the pinna only
Correct answer: Prosthesis base and the corresponding bar or cap on the abutment
One magnet set is encapsulated within the silicone prosthesis base and the opposing magnets are housed in keeper attachments on the implant bar or individual abutments.
Question 3: During impression-taking for a mid-facial defect, a dentist's tray adhesive is applied to the impression tray to:
- Reduce patient gag reflex during the procedure
- Chemically bond the impression material to the tray, preventing distortion on removal (Correct answer)
- Disinfect the tray surface before contact with tissue
- Speed the setting time of the elastomeric material
Correct answer: Chemically bond the impression material to the tray, preventing distortion on removal
Tray adhesive creates a chemical bond between the elastomeric impression material and the tray, preventing the material from separating or distorting when removed from the defect.
Question 4: Extrinsic coloring of a cured silicone prosthesis is applied using silicone-compatible paints primarily because:
- They dry faster than oil-based paints under UV light
- Silicone-based surface paints flex with the prosthesis and resist peeling better than non-compatible paints (Correct answer)
- They are invisible under tungsten lighting conditions
- They permanently fuse into the silicone substrate upon contact
Correct answer: Silicone-based surface paints flex with the prosthesis and resist peeling better than non-compatible paints
Silicone-compatible extrinsic paints have the same elastic modulus as the prosthesis, so they flex without cracking and adhere longer than incompatible media.
Question 5: A stock ocular prosthesis differs from a custom-fabricated ocular prosthesis in that the stock version:
- Is made of silicone rather than PMMA acrylic
- Has a pre-formed, standardized shape not individually contoured to the patient's socket (Correct answer)
- Incorporates real blood vessel simulation using silk threads
- Requires osseointegrated implants for retention
Correct answer: Has a pre-formed, standardized shape not individually contoured to the patient's socket
Stock prostheses come in pre-made sizes and iris colors and are not custom-fitted to the individual socket anatomy, often requiring more frequent adjustment and less optimal comfort.
Question 6: When mixing pigments into platinum silicone, the anaplastologist should avoid iron oxide pigments that are contaminated with sulfur compounds because sulfur:
- Causes silicone to become permanently transparent
- Acts as a platinum catalyst inhibitor, preventing proper cure (Correct answer)
- Increases the Shore A hardness excessively
- Reacts with silicone to produce toxic hydrogen gas
Correct answer: Acts as a platinum catalyst inhibitor, preventing proper cure
Sulfur-containing compounds and certain amines inhibit platinum catalysts, resulting in a tacky or uncured silicone surface even after extended vulcanization time.
Question 7: The primary advantage of using a bar-retained overdenture design for a maxillofacial prosthesis is:
- It requires fewer implants than individual abutment attachments
- It splints multiple implants, distributing occlusal and retention forces across all fixtures (Correct answer)
- It eliminates the need for a wax try-in stage
- It allows the patient to bond the prosthesis with standard skin adhesive
Correct answer: It splints multiple implants, distributing occlusal and retention forces across all fixtures
A connecting bar rigidly splints implants, spreading functional loads and improving prosthesis stability compared to independent abutment attachments.
CCA Prosthetic Fabrication & Fitting 3
Which platinum catalyst system is most commonly used to vulcanize addition-cured (condensation-free) silicone for facial prostheses?