prostho sheet # 12 - Ruba Jassar

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prostho sheet # 12 - Ruba Jassar

Post by Shadi Jarrar on 15/5/2011, 7:34 pm

بسم الله الرحمن الرحيم

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بسم الله الرحمن الرحيم



Metal framework try-in and the need for altered cast technique

***PART ONE: Framework Try-In
1- RPD Framework Fabrication

 Prepare Master Cast:
 Place blockout in undercuts
• Below heights of contour
• Minor connector & lingual plate embrasures
• Soft tissue undercuts (if necessary)
• Over FGM
• Under lattice and mesh
 Duplicate Master Cast using Refractory materials withstand casting temperature
 Wax-up framework Using design on secondary cast or paper equivalent. Use prefabricated patterns
 Sprue and Invest refractory Cast
 Burnout wax
 Cast in Co/Cr or other alloy
 Finish & polish and return to dentist

2- Partial Denture Framework Adjustment
This is required because 75% of RPD frameworks don’t fit perfectly.
This could be due to Active clasps that are not fully seated so they move while speaking and cause orthodontic movement. These should be deactivated, i.e: adjusted to be passive.
Incomplete seating might also cause discomfort, damage soft tissues and supporting bone.
Adjust soon after fabrication, without denture base (on the cast only)


3- Preclinical Inspection
 Check accuracy of the framework as designed >>because if there was any unclear points in the design then the technician might not understand it and produce different denture from the wanted one.
 Framework should fit master cast. If it does not, probably will not fit intraorally
 Framework should cause no abrasion on the cast >> if there was abrasions on the abutment teeth or on the soft tissues (in the cast) this means abrasion will occur intraorally. Thus, you have to adjust this area.
 Rest seats should be fully seated >> clasps should be in their appropriate place and completely fitting
 All retentive, reciprocal arms, proximal plates, superior portion of lingual plates and all maxillary major connectors should be contacting the casts (spaces will collect debris causing caries and gingival irritation).
 Major & minor connectors:
 Should have adequate distance from abutments (hygiene) >>
- In lower arch: minimally 3 mm
- In upper arch: minimally 6 mm
- Otherwise, it won’t be hygienic and collection of debris will occur
 Proper proportions (rigidity, hygiene) >> i.e: true that we should have adequate spaces as mentioned but these spaces mustn’t affect the rigidity.
 Adjust or have lab adjust or remake framework >> sometimes the denture cannot be adjusted, like if major connector is not rigid enough, so you’ll have to remake it!!
 Minor Connectors should have butt joint finish lines slightly undercut for acrylic resin and also of sufficient thickness.
 1mm relief over saddles for acrylic

 Clasps have uniform taper >> you should not change the dimensions of the clasp, if you need to adjust it then you can only do flexion using “Kammasheh or Zarradeyeh”
 Finish and Polish
- Framework should be highly polished
- No pits, nodules, scratches or sharp edges (stress concentration and might injure mucosa).











The most efficient and most used kind is diamond burs
USE CARE!!!

4- Clinical Adjustment
Now, after finishing the adjustment of the denture in the lab, you should try it in the patient’s mouth and see if any further adjustment is needed >> This is called Clinical Adjustment

 Incomplete seating of framework is a common problem (usually binding on abutments)
 For adjustment use an indicating medium:
• Aerosol Sprays (Occlude)
 Thin, accurate and not easily displaced
But
 Can dissolve in saliva
 Difficult to remove
 Can’t tell how far from seating (2D) >> because it’s very thin

• Disclosing Wax
 Sets immediately
 Inexpensive
 Shows how far from seating (3D)
But
 Can stick to teeth
 Can be distorted
>>> HOW TO USE IT? Adjust areas of significant show-through. Completely remove wax with metal particles. Repeat until full seating

• Silicone
 Three dimensional
 Minimal distortion
But
 More expensive
 Sets relatively slowly (~ 2 min)
 Can tear or pull off the framework

>>> HOW TO USE IT?
 It usually comes as base and catalyst, mix them
 Use minimal amount (expense)
 Cover all components contacting the abutments
 Mark contacts with dampened red pencil
 Remove silicone material
 Adjust marked areas

***Initial Assessment of framework fit: (u have to check the following)
- ‘How does the framework feel?’ >>> ask the patient 
- No pulling or wedging (Active engagement of abutment teeth)
- Overall comfort of the framework
- Determine if casting fits similarly on the cast and intraorally. If not, final impression is inaccurate (due to improper preparation of alginate, for example) and a new impression should be made.
*** Areas of abrasion on master cast may indicate areas of binding to abutment teeth so you have to adjust it by placing indicating medium on the framework, then align the framework, place pressure over rests (No pressure over saddles because they’re soft tissues and relief will occur). Check for ‘show-through’{i.e.: areas of preasure due to heavy contact with the denture, thus must be relieved} . Repeat this process until u determine all binding areas where abrasion of abutment teeth occurs.
*** Clinical adjustment also includes:
 Differentiate between normal & abnormal contacts
e.g: Guiding planes normally have long vertical areas of contact. Broad areas of severe show-through may indicate binding
 Avoid excessive force that might cause bending
 Avoid heat generation (could melt the acrylic)
 Retentive tip of direct retainers normally have show-through areas, but eliminate active clasp retention

*** Most common interferences that prevent complete seating:
 Rigid portions of direct retainers
 Interproximal portions of lingual plates
 Interproximal minor connectors
 Shoulder areas of embrasure clasps
*** After adjustment is completed, a thin even layer of indicating medium is applied results in greyish hue from underlying metal. Complete seating with gliding sensation and no grating or snapping.

*** Soft Tissue Impingements >>> it’s detected using pressure-indicating paste (PIP):
 Apply a thin layer with streaks. Place with moderate pressure
 Areas of show-through should be relieved WHILE areas of remaining streaks indicate no contact
 Maxillary major connectors have broad even palatal contact

5- Occlusal Adjustments
 RPDs are fabricated on unmounted casts. So, occlusal interferences usually present
 Occlusal vertical dimension should be unchanged
 Centric and eccentric contacts should be identical with or without the framework >>> this is important!!! Again, you must check the occlusion while the denture is placed and when it’s not… in both cases it must be the same, if not then adjust it.
 With highly polished metal, articulating paper marks poorly. So, check opposing occlusal contacts or slightly roughen framework with air abrasive or rubber impregnated abrasive
 Adjust individually opposing frameworks, then adjust them together
 Eliminate interferences between the frameworks
 If occlusal rest thickness is ≤ 1.5 mm after adjustment, rests will be subject to fatigue and possible fracture
 May require additional tooth preparation and remake
 Last resort - occlusal reduction of opposing teeth (esp if over erupted)
 Adjust minor interferences caused by retentive arms. Reduce opposing cusp - last resort (esp if double acre clasps were used because they cause too much embrasions)
 With heavy contacts: Lower height of contour (HOC) and remake >>> sometimes, if HOC is high then the survey line will be high, this should have been reduced from the beginning but if not, this is the last chance to do it
 Don’t relieve claps (this will alter flexibility and fracture resistance)












***PART TWO: Altered Cast Technique OR Corrected (Modified) Cast Technique
This technique is done to overcome the difference in compressibility between the teeth and the edentulous saddle.
The difference in compressibility between the denture bearing mucosa and the periodontal ligament of the abutment teeth will cause the free-end saddle to sink under occlusal load and RPD to rotate about the support axis

 The Purposes for altered cast technique:
- Reduces the support differential between ridge and abutments by obtaining a compressive impression mimicking functional loading. >> we have to take impression of the saddle (only this particular area)
- Provides a more accurate relationship between abutments & ridge
- Improves load distribution and denture stability
- Corrects peripheral adaptation
 Indications for altered cast technique:
- Class I & II RPDs
- Framework most likely to be adjusted in the future (need for relining and rebasing)
- Extensive Class III & IV cases
- Tooth mobility + compressible mucosa
- Less necessary in maxilla





So far, we tried in the framework, made adjustment of the denture, and now Altered Cast Technique…

 Technique:
- Ensure well-fitting framework on the cast
- Place relief over ridge (1 mm wax relief)
- A custom acrylic impression tray is fabricated over the framework
- Check seating of the framework on the cast. If not seated, remove, repeat tray construction (rests fully seated, tissue stop contacts cast, metal adjacent abutment contacts cast, no resistance as framework seated).
- Check peripheries of the tray (2-3 mm short of vestibular)
- No displacement when cheeks and lips are pulled or when the patient activates tongue
- Border moulding is undertaken to simulate final denture border
- Before making the impression, ensure tray is well retained by framework
- Remove wax spacer
- Coat tray with adhesive (of very minimal thickness) and wait for 10-15 minutes
 The adhesive that can be used:
• Polyvinyl siloxane (light or medium body)
• Zinc-oxide eugenol
- Carefully load tray and make sure no material is under rests, guiding plates, max. major connector, etc. >> because if there was any material underneath them then the denture won’t be fully seated.
- Seat the framework applying pressure over rests. No pressure should be applied on saddles or unoccluding teeth (Fulcruming or tissue compression). This might cause spring back and lack of tissue contact.
- Remove the impression and inspect it:
 Absence of voids
 Minimal show-through
 The impression must cover supporting tissues
 Framework is fully seated.
- Modify small errors or retake impression
- Send to the laboratory. Residual ridge is sectioned from the original cast
- Ensure no contact between impression & cast
- Place retentive grooves in cast
- Sticky wax in place >>> to ensure that the denture is completely seated
- Box the impression ensuring water tight seal
- Seal retainer, major & minor connector borders
- Pour new ridge areas in different color stone
>>>> the result is a Hybrid Cast with an old part and a new part in edentulous areas

 Problems with the Altered Cast Technique:

- If tray is added carelessly, it can alter passive relationship between framework and teeth
- Excess impression material under framework, might cause incomplete seating
- If inadequately sealed, stone over teeth, can’t articulate model
- An alternative procedure involves rebasing the completed denture by applying zinc-oxide eugenol impression on the acrylic fitting surface of the relevant saddles and taking an impression while the denture is being seated by pressure on the rests. This might disrupt the evenness of the occlusal contacts in the saddle area by creating premature contacts posteriorly.

*** Finally, after the altered cast tech. is done, we’ll do Jaw Relation Records:
Place record base on the edentulous saddle. This record base material could be:
 Hard base plate wax
Easier to remove during processing
But….
Can melt or distort
 Acrylic resin
More rigid and stable
But….
Harder to remove
Mount Casts on the articulator

*** The following step will be setting of the teeth 

THE END
Corrections & comments are more than welcomed…
Ruba M. Jassar







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Shadi Jarrar
مشرف عام

عدد المساهمات : 997
النشاط : 12
تاريخ التسجيل : 2009-08-28
العمر : 26
الموقع : Amman-Jordan

http://jude.my-rpg.com

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