Endodontics
Course Review
Enoch Ng, DDS 2014
Rotary Instruments
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Ideal preparation – continuously tapered funnel maintaining canal anatomy and apical constriction
Shaping Goals
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Biologic – reduce number of microbes, remove canal contents
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Mechanical – increase space for irrigants/medicaments, facilitate root canal filling
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Long term success – prevent vertical fractures, avoid procedural errors
Terminology
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Glide Path – smooth preparation from chamber orifice to root canals terminal constriction
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After straight line access and working length are determined, hand files create a glide path with
minimum size of a 20 hand file
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Master Apical File – largest file used to working length, at least 3 sizes larger than first file to bind
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Large enough for cleaning of apical portion of canal
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Maintains original canal anatomy – no strips, zips, perfs, or elbows
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Apical preparation retains obturation material
Diameter 1mm from Apex:
Small canals – 200-400microns, file size 20-40
Large canals – 400-700microns, file size 40-70
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Step Back technique – series of progressively larger files that fit successively farther from termination of canal
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Gives tapered preparation in apical to coronal direction
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Crown down technique – instruments used from larger to smaller
First instruments do coronal flaring and mid root shaping
Smaller instruments progressively taken to working length
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Decreases bio-burden carried into canal space, gives continuous coronal flare
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Decreased contact area of the file – decreased tortional force on NiTi file
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Enhances tactile awareness, minimizes changes to working length
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Rotary motion pulls debris out of the canal, instead of pushing it into canal and out apex (extrusion)
Properties and Design
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NiTi properties – austentic phase, transformation phase, martensitic phase
Transformation phase is where there is relatively little stress change with increased strain
This is the phase where NiTi can return to austentic phase (original shape)
Stainless steel (K-files) – much greater stress increase with relatively little strain (2.8%)
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Loading plateau – additional stress does not proportionally increase strain
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Shape memory – deformed files return to original shape because of crystalline form
NiTi files WILL break
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Land area – flat area between the cutting edges
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Keeps file centered in canals, adds bulk to resist file fracture
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Separates “flute” areas
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Positive (obtuse) angle – less aggressive cutting
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Negative (acute) angle – more aggressive cutting