FULLY GEARED TO TAKE ON THE ENEMY - MISSION ACCOMPLISHED
FIGHTING ENEMY WITH THE WRONG WEAPON - MISSION ABORTED
WHAT DOES ROTARY MEAN?
Instruments which turn on an axis to perform work. Work may be :
Cutting Abrading Burnishing Finishing and polishing.
Historical Development: initially thick bulky chisels & excavators were used for cutting enamel & dentin.
first rotary instruments – twist drills or bur heads. refinement of twist drills – Scranton’s drill. next improvement – Drill Ring
other drill stocks, bur chucks or bit holders – forerunner of the present dental handpiece. eg: -Cheavlier drill stock -Merry’s drill stock
ďƒ˜ Dental Handpieces: - Device for holding rotating instruments, transmitting power to them & for positioning them intra-orally. - Types: a) Straight b) Angled
Journey of the Dental Handpiece:
1874 - introduction of Electric motor as power source. 1914 - incorporated in the dental unit.
- increased speed of 5000 rpm remained unchanged till 1946. - steel burs used could not cut enamel effectively & produced heat 1935 - diamond instruments & tungsten carbide burs developed which could cut enamel effectively.
- these worked on high speeds & prompted development of higher speeds handpieces. - speeds increased to 15,000 rpm by enlarging drive pulleys on existing dental engines. 1950 – speeds increased to 60,000 rpm by using speed multiplying internal belts.
Major Breakthrough: introduction of contra-angled handpieces with internal turbine drives.
- earlier units were water driven and later came the air driven units. - have free running speed of 300,00 rpm but lateral loads during cutting can reduce it to 200,00 rpm. ( excellent safety feature ) - ADV: simple, ease of control, patient acceptance, versatility. - DISADV: low torque & power output makes them unsuitable for finishing & polishing purposes. - SOLUTION: Straight handpieces which provided high torque & low speed operation.
Allow repeated sterilization Smaller head size More Torque Lower noise Better chucking mechanism Fiber-optic lighting of the cutting site.
(contemporary air turbine handpiece) ROTARY SPEEDS RANGES:
a) Acc. to CHARBENAEU: • Low speed - <10,000 rpm • High speed - 10,000 – 150,000 rpm • Ultra speed - > 150,000 rpm b) Acc. to STURDEVANT: • Low/ slow - <12,000 rpm • Medium 12,000 – 200,000rpm • High/ Ultrahigh- > 200,000 rpm c) Acc. to MARZOUK: • • • • •
Ultra/ low – 300-3000rpm Low – 3000-6000rpm Medium high – 20,000-45,000rpm High – 45,000- 100,000rpm Ultra high - >100,000rpm
SO MANY SPEEDS – WHICH IS THE RIGHT ONE TO USE ???
Low speeds: cleaning teeth caries excavation finishing & polishing ADV: better tactile sensation less chances of overheating cut surfaces. DISADV: * more heat production * vibration * time consuming * inefficient for cutting * burs have tendency to roll out & mar proximal margin or tooth surface. * carbide burs do not last long. High speeds: tooth preparation removing old restorations
ADV: faster tooth structure removal less heat production less vibration better control & ease of operation time saving diamond & carbide instruments stay longer COMMON DESIGN CHARACTERISTICS OF ROTARY CUTTING INSTRUMENTS 3 parts: • Shank • Neck • Head
Shank Design: is part that fits into the handpiece, accepts rotary motion & provides bearing surface to
control the alignment & concentricity of the instrument. Types: ďƒ˜ straight handpiece shank
ďƒ˜ latch type angle handpiece
ďƒ˜ friction grip angle handpiece
Neck Design: Neck is intermediate portion of the instrument which connects the head to the shank.
Function: to transmit rotational & translational forces to the head. Head Design: Head is the working part of the instrument, the cutting edges or points that perform the desired shaping of the tooth. Classified acc. to: • abrasive or bladed instruments • acc. to material of construction • head size & shape. Dental burs: All rotary cutting instruments that have bladed cutting heads. Purpose: finishing metal restorations surgical removal of bone tooth preparations.
Historical development: Earliest burs were handmade, expensive, varied in dimension and performance. Steel burs: Cut dentin well @ low speeds Dull rapidly @ high speeds while cutting enamel. Now mostly used for finishing procedures. Carbide burs: Introduced in 1947. Perform better than steel burs @ all speeds with max. superiority @ high speeds. Structure:
• Heads : of cemented carbide with particles of tungsten carbide held in a matrix of cobalt/ nickel. • Carbide head attached to steel shank by welding / brazing. Bur Classification: Facilitates description, selection & manufacture of dental burs. In U.S – burs described by an arbitrary numerical code. o Eg: 2 = 1mm diameter round bur 57 = 1mm dia straight fissure bur. FDI & ISO system – separate nos. for shape and size ( head dia in tenths of mm).
o Eg: round 010 Inverted cone 008 Shapes: ďƒ˜ Refers to the contour of the silhouette of the head. ďƒ˜Basic shapes are : round, inverted cone, straight fissure, tapered fissure.
Sizes: ďƒ˜ Given by S.S. White dental manufacturing company in 1891.
originally had 9 shapes & 11 sizes, designations like ½, ¼, 500, 900 added later.
Additional features in Head design:
Head length – long enough to reach the full depth of the preparation. Taper angle – gives desired occlusal divergence to the lateral walls. Neck diameter Too large: interferes with visibility & access
Too small: weakens instrument & unable to resist lateral forces. Spiral angle – reduced for burs used @ higher speeds to produce efficient cutting & smooth operation. Crosscutting – have notches in the blade edges to increase cutting effectiveness @ low & medium speeds. Flutes – the depressed areas b/w the bur blades.
always even in number. Greater the no. of blades – smoother is the cutting action • Excavating burs – 6-10 blades
• Finishing burs – 12-40 blades. Concentricity : Direct measurement of the symmetry of the bur head. Measures how closely a single circle can be passed by the tips of the blades. Not related to function. Runout:
Maximum displacement of the bur head from the long axis during rotation.
Avg. acceptable value- 0.023mm
Bur Blade Design: Each bur tooth terminates in to a Blade. Each blade has: 2 sides o Rake face ( towards direction of cutting) o Clearance face
3 Angles o Rake angle o Edge angle o Clearance angle
Rake Angle: Angle w/c the face of the bur tooth makes with the radial line from the centre of the bur to the blade. Can be – Radial ( “0” ) Positive Negative
For cutting hard, brittle material (enamel) negative rake angle is required as it increases the life of the tool by preventing fracture of the blade. (Sturdevant) Should be slightly positive (Harty) Edge Angle: ďƒ˜ Increasing the edge angle reinforces the cutting edge & reduces the chances of the blade to fracture.
Clearance Angle: ďƒ˜ The angle b/w the back of the bur tooth and the work. Land: ďƒ˜ Is the plane surface immediately following the cutting edge.
DIAMOND ABRASIVE INSTRUMENTS: Second major category of rotary cutting instruments & involve abrasive rather than bladed cutting. Small, angular particles of hard substance are held in the matrix of a softer substance. Cutting occurs at points where hard particles protrude from the matrix.
ADV: Effective in cutting enamel & dentin Long life of instruments. Shapes, Sizes & Designs: Come in various forms.
OTHER ABRASIVE INSTRUMENTS: 2 types: Molded abrasive instruments Coated abrasive instruments
Material used: Matrix – phenolic resins / rubbers Abrasives – vary o Silicon carbide o Aluminium oxide o Garnet o Pumice o Cuttlebone. EVALUATION OF CUTTING: Evaluated in terms of effectiveness & efficiency. Effectiveness: rate @ which tooth structure is removed. (mm/min) o Does not consider side effects s.a. heat or noise. Efficiency: % of energy actually producing cutting.
o Reduced by heat &noise. BLADED CUTTING: Tooth structure undergoes 2 kinds of fractures: Brittle fracture – involves crack production by tensile loading Ductile fracture – plastic deformation proceeded by shear. Low speed cutting – proceed by plastic deformation. High speed cutting (esp. enamel) – by brittle fracture. Requirements for a blade to initiate cutting action: Sharp Harder than the substance being cut. Pressed against the surface with sufficient force.
Mechanism of bladed cutting:
ABRASIVE CUTTING: Burs – preferred for cutting ductile materials s.a. Dentin. Diamonds: brittle materials s.a. Enamel.
Requirements for effective cutting: Contra angle handpiece, Air water spray High operating speeds (>20,000rpm) Light pressure Carbide bur / diamond point Carbide burs: End cutting procedures Punch cuts Intracoronal tooth preparation Amalgam removal Placing retentive features Diamond points: Intra / extracoronal preparations Beveling enamel margins Enameloplasty. ENGINE DRIVEN ROTARY CUTTING INSTRUMENTS IN ENDODONTICS:
Aim of rotary instruments in endodontics is primarily to provide – - gain entry to the pulp chamber - provide straight line access Instruments used for gaining entry to the pulp chamber: Friction grip burs Round burs Safe ended burs – used after gaining initial access to the pulp space & non cutting tip prevents gauging of the pulpal floor
Instruments used for straight line access are: Gates – Glidden drills Peeso reamers Engine driven reamers & files. Gates – Glidden drills: Elliptical / flame shaped burs used to : o Open the orifices of the root canals. o Provide straight line access by removing dentin shelf & flaring the coronal & middle third of the canal.
Designed to break high in the shank region – facilitating easy removal from the canal. Sizes: o 15mm –posterior teeth o 19mm – anterior teeth Peeso reamers: Used for: Preparation of post space Canal preparation
Similar to Gates – Glidden drills but have Parallel cutting sides than elliptical shape.
Less controlled than Gates – Glidden drills. Engine Driven Reamers: Used for cleaning & shaping of canals. Made up of stainless steel Stiff – not very effective in curved canals Use not recommended.
Engine driven Ni-Ti files: ď‚§ Have greater control in curved canals. ď‚§ Do not have a cutting end & have fewer tendencies to transport the apical preparation. Various rotary cutting systems available:
NEW DESIGNS IN ENDODONTIC ROTARY CUTTING INSTRUMENTS: Variable taper: Concept is to maximize the cutting efficiency by minimizing the contact area b/w surface of the instrument & the canal wall. Directly using desired taper rather than flaring the canal by using different sizes of standard 0.02 taper files. Larger the taper, more conical the instrument.
Flute Design: Shape of the flute in cross – section determines the cutting efficiency & ability to remove debris. Ex: “U” shaped cross – section Rake Angle: Most instruments have negative rake angle so cutting blade scrapes and does not cut dentin. Positive rake angle results in efficient cutting but excessive angle results in digging of the blade in the dentin. So rake angle should be “slightly positive.” Helical flute angle (HFA):
Is the angle at w/c the flutes encircle around the shaft of the instrument. Ideal HFA: allows efficient removal of the dentinal debris without clogging. Should increase along the length of the file, from the tip to the handle. Core diameter / flute depth: Governs the strength & flexibility of the instrument. Larger the core diameter, more robust is the instrument. Core diameter inversely proportional to the flute depth. Core diameter -------------------Outer diameter
- greatest @ tip.
Uniformly decreasing this proportion as flutes move up – results in more strength, flexibility & efficient removal of debris. Non – Cutting tip: (Safe ended / Batt tip) Pilots the instrument down the canal and prevents gauging or apical transportation. Radial Lands: Flat, radial surface w/c provides support to cutting blade & prevents its fracture. Also helps to centre the instrument and prevents canal transportation.
HAZARDS WITH ROTARY CUTTING INSTRUMRENTS: Can be classified into: Pulpal dangers Soft tissue dangers Eye, ear, inhalational dangers. Pulpal precautions: Steel burs produce more heat than carbide burs Burs/diamond points clogged with debris produce more heat. When used without coolant, diamond instruments are more damaging than carbide burs. Prevention: use of air-water coolant.
Use of air alone is not advised as it can cause desiccation of the dentinal tubules. Soft tissue precautions: Lip, tongue, cheeks most common areas of injury. Prevention: use rubber dam, cotton rolls, mouth mirrors, evacuator tips etc. Eye precautions: Patient, operator, dental assistant ALL should wear protective eye glasses. Ear precautions: Noise levels more than 75db may cause hearing damage. Prevention: o Use ear plugs o Sound proofing of rooms
Inhalational precautions: Aerosols & vapors produced while cutting with rotary instruments all hazardous. May cause alveolar irritation & tissue reactions. Prevention: o Use rubber dams, o Mouth masks etc.
Published on Apr 30, 2014
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