Comparison of Bond Strength of Porcelain Laminate Veneers Bonded to Laser Treated Dentin and Acid Treated Dentin : An In Vitro Study

AND ACID TREATED DENTIN : AN IN VITRO STUDY

A Diss ertation sub mitted

in parti al fulfil men t of th e requirements

for the degree of

MASTE R O F DENT AL SURGE RY

BRANCH – I

PROSTHO DO NTICS AND CROWN & BRIDGE

THE TAMIL NADU DR.M.G.R. ME DICAL UNI VERS ITY

CHENNAI- 600032

DE PART MENT O F PROSTHO DO NTICS AND CROWN &

BRIDGE

CERTI FICATE

This i s to cert if y t hat Dr.J.RAGHUNATHAN, P ost Graduat e student (2014 -2017) in the Departm ent of Prosthodonti cs and crown & bridge , Adhiparas akt hi Dental College and Hos pit al, M elm aruvathur – 603319, has done t his diss ertation t itled “CO MPARIS ON O F BO ND STRENGTH O F PO RCEL AIN LAMINATE VENEE RS BONDED TO

LASER T REATE D DENTI N AND ACI D TRE ATED DENTIN : AN

IN VITRO STUDY.” under our di rect guidance and supervision in parti al ful film ent of t he regul ations l aid down b y the Tami lnadu Dr.M.G.R Medical Univers it y, Chennai – 600032 for M DS., (Branch - I) (Prosthodonti cs and Crown & Bri dge ) degree ex amination.

Dr.S.THILL AINAYAGAM MDS.,

Principal

Guide

Dr.A.S.RAMESH MDS.,

Professor & H ead d

Co-Guide

ACKNOWLE DGE MENT

I am extremel y grat eful to Dr.A.S.Ramesh MDS ., Guide, Professor and Head, Departm ent of Prosthodontics, Adhiparas akthi Dental College and Hospital, Mel maruvathur. Words cannot expres s m y gratitude for his quiet confi dence in m y abilit y to do the stud y, hi s willingness to help t o clear the stum bling bl ocks along the wa y and hi s tremendous pat ience till the end of t he st ud y.

It is m y dut y t o express m y t hanks to m y Co -Guide Dr.K.Prabhu MDS ., R eader for his expert gui dance and moral support duri ng t he

com plet ion of t his st ud y. I consider m ys e lf privil eg ed, to have studi ed, worked and com plet ed m y diss ert ation under them in the departm ent.

M y sincere thanks t o Dr.S.Th il lain ayagam MD S ., our beloved Principal, Adhiparas akt hi Dent al Col lege and Hos pit al, M elm aruvathur for providing m e wi th the opport unit y t o utiliz e t he faciliti es of the coll ege.

I thank our Corres pondent Dr.T .Ramesh M D., for his vital encouragem ent and s upport.

I am t hankful and express m y gratitude to m y previous t eachers Dr.K.Rijesh MD S., Professor, Dr.Lak sh mi Devi MD S., R eader, for thei r i mmens e hel p and s upport for the i nitiati on of this st ud y.

I am extremel y gra teful t o Dr.Premil a Suganth an BD S ., for granti ng me permiss ion t o conduct the stud y in her clini c and helpi n g me t o bring out m y s tud y.

I t hank Dr.Sh yam MD S ., Sri Venkates hwara Dent al Coll ege and Hospital, C h ennai, for hel ping m e with t he statisti cs in t he st ud y.

I thank AL MI GHTY GO D for ans weri ng m y pra yers and m aking me what I am toda y.

I owe m y gratitude to m y m other Mrs .J.Mannammal and m y wife Mrs .D.Ani tha who stood beside m e as a source of inner st rengt h and s acrifi ced so m uch t o make m e what I am toda y. I also thank m y loving daught ers R.Jayavarshini & R.Mith ra for t he unbound l ove and jo y the y brought to m y li fe.

Dr.J.RAGHUNATH AN

TIT LE OF THE DIS SERTATION Compari son of bond st rengt h of porcel ain l ami nate veneers bonded to laser treat ed dentin and acid treat ed denti n : An i n vit ro stud y P LACE OF THE S TUDY Adhi paras akt hi Dent al Coll ege and

Hospital, M elm aruvathur – 603319 DUR AT ION OF THE COURSE 3 years

NAM E OF THE GUIDE Dr.A.S .Ramesh MDS., NAM E OF C O-GU ID E Dr.K.P rabhu MDS .,

BACKGRO UND:

Ideal l aminat e veneer preparati ons are suppos ed to be i n enamel whi ch provides excellent bond st rength and ensures the l ongevit y of the restorations. But cert ain cli nical situations like dent al cari es extending to dent in, old restorations require the preparati on to be extended into the dentin; wherein the bond strength gets com prom is ed. Hard tiss ue lasers li ke Er,Cr:YS GG laser has the potenti al t o prepare dentinal s urface for adhesi on.

AI M:

The purpos e of this stud y is to co mpare the bond strength of the conventional acid et ch t echni que and the las er t reat ed dentinal surface bonded to porcel ai n l aminat e veneer restorat ions.

MATE RI ALS AND METHO DS:

8.5180 and .22829 with a m aximum value of 8.9 and a mi ni mum value of 8.1. The m ean and standard devi ati on val ue of Group B (Las er) is 8.4980 and .11156 respective l y wi th a maximum value of 8.7 and a minimum value of 8.3 .

CONCLUS ION:

Within the limit ati ons of t his st ud y, 37% ortho -phosphori c acid (8.5 Mpa) and l as er (8.49 Mpa) treat ed denti n s urfaces showed simil ar bond st rengt h values . The di fferences are not s t atist icall y s i gnificant . But the result s of the Las er etched bond strength were more consist ent than Aci d etched bond st rengt h.

S.NO : TITLE PAGE NO :

1. INTRODUC TION 1

2. AIM AND OBJ EC TIVES 6

3. GENER AL R EV IEW ON LASER S 7

4. REV IEW OF LITER ATURE 18

5. MATER IA LS AND METHODS 39

6. RESULTS 50

7. DISC USS ION 56

8. CONC LUS ION 70

9. REFERENCES 71

FI GURE NO : TITLE PAGE NO :

1. Dental l as er wavel engt hs on the el ect romagneti c spectrum

43

2. Collecti on Of S ampl es 43

3. Preparing crown port ion 44

4. Embedded crown in cl ear acr yl i c resin 44 5. Prepared fl at l abial s urface 44

6. Aci d et ched surface 45

7. Las er s etti ngs 45

8. Las er et ched surface 45

9. Prepared veneer 46

10. Bonded veneer t o t ooth s urface 46 11. Univers al t esti ng machi ne 46 12. Chisel pl aced at dent in -veneer int erface 47

13. Components of laser unit 47

14. Las er ti ssue i nt eracti on 47

15. Absorption curves of the prime oral chromophores

48

16. Aci d et ched Enam el 48

17. Aci d et ched Denti n 48

18. Las er et ched Enam el 49

TABLE NO: TITLE PAGE NO :

1. Shear Bond St rengt h Values 52

2. Descript ive St atist ics 53

3. Compari son between two groups 53

LIST OF GRAPHS

GRAPH NO: TITLE PAGE NO :

1. Bond St rength Values of Ei ther Group wit h R especti ve Trend Line

54

INTRODUCTION

The popul ari t y of porcelai n l aminat e veneers has increased due to its cons ervative tooth preparation and hi gh end aesthet ics .1 The long-t erm clini cal s ucces s of porcel ain veneers depends on careful cas e sel ection, t reat ment planning and tooth preparation .2

Lami nates m ade of porcel ain do offer soluti ons that are both conservati ve i n nat ure and est heti call y pleasi ng for the foll owing clini cal situations .3

Dis colorati on: Teet h dis colored b y tet rac ycline st aining, devitaliz ation and fluorosis , and even t eeth darkened with age can be given younger, bri ght er -looking s mi les .3

Enamel Defects : Different t ypes of enamel h ypopl asi a and mal formations can be m asked / correct ed .3

Dias temata: Gaps and other m ultipl e unsi ghtl y spaces in t he ant eri or

region can be cl osed .3

Malposition ed Teeth: Indi cated in cases where orthodonti cs is not the treatm ent of choice or if t he pati ent has unav ai labilit y of tot al treatm ent time. An i llusion is creat ed b y changi ng the shape, position, size and surface appearance of a malpositioned / rot at ed tooth and maki ng it appear st rai ght . Ex: increasi ng the l ength or wi dth of the ant erior t eeth .3

Mal occlusi on: The confi gurati on of lingual surface of ant erior teeth

Poor Restoration: Teet h with multipl e s hall ow, unaestheti c restorations on l abi al surfaces can be rest ored t o nat uralit y .3

Aging: The on goi ng process of agi ng can result in col or changes and wear in teet h. Thes e teeth ma y be i deal candi dat es for improvem ent b y bleaching or, in cert ain situati ons , bleachi ng with subs equent veneering.3

Wear Pattern : Porcel ain l aminat es are also useful in thos e cases that exhibit slowl y progressi ve wear patterns . If s uffi ci ent enam el rem ains and the des cribed increas e in lengt h is not exces si ve, porcel ain veneer can be bonded to the rem aini ng tooth st ruct ure to change shape, col or, or funct ion .3

Porcel ai n l aminat e veneers are a powerful pros theti c tool to improve est heti cs wi th minim al los s of t ooth st ructure .4 The techni que requi res a sh allow reducti on of the enamel on the l abial surface. Som e aut hors beli eve toot h preparati on is unneces sar y for porcel ain veneer restoration. Tooth preparation is necessar y for the following reas ons , the y provi de for, t he st rength, restorative area and colo r of the restoration .2 The prepared area for porcel ain veneer should be in the enam el t o m aximize the resin bond strength and decrease t he tensil e stress es in t he porcel ain .2

simpl e and m inim al reducti on of sound toot h st ructure .6 There are different approaches in tooth preparation for porcelai n laminat e veneers. Tooth preparation for porcel ain l aminat e should be int ra -enam el t o m aximize the resi n bond s t rength.7

The resin bonding i s a quite rel iable and p redi ctabl e m ethod in the enam el . For bonding of ceramic l aminat e veneer, presence of enam el thi ckness i s important. P retreat ment of the tooth s urface is essent ial for es tablis hing a strong bond bet ween the cerami c and both the enam el and denti n .8

The acid -et ching t echnique with phosphoric acid, introduced b y Buonocore' in 1955, is still us ed to creat e an irregular s urface of bevel ed enam el .9 The ret enti on to the dentinal surface i s mai nl y due t o the format ion of hybri d la yer and the micro -m echani cal ret enti on offered b y t he resin t ags em bedded in the dentin .1 0

In recent years , there has been a growing debat e about t he us e of las ers for vari ous appli cat ions in denti str y, i ncludi ng cari ous denti n rem oval or cavit y preparati on .1 0 Las er irradi ation of de nt al hard tis sues modi fies cal ci um to phosphorus rat io, reduces carbonat e t o phos phat e rat io, and l eads to the form ati on of more stabl e and l ess aci d -s olubl e com pounds, thus reduci ng s us ceptibilit y t o aci d att ack and caries .1 2

Advancem ents i n l aser techn ol ogy have led t o multi pl e dental applicati ons such as soft tissue surger y, compos ite phot o -pol ym erizati on, toot h whi tening, endodonti c procedures, and cari es rem oval and cavit y preparati on wit h mi nimal pai n and dis comfort .1 3 Kastler developed the “optical pumping effect” mechanism. LASER is

Li ght Am pli ficati on b y Stim ulated Em ission of R adiat ion des cri bed b y Gordon Gould in 1957. The first las er used was Rub y laser by M aim an. Then various res earches were done regarding thi s and Maim an found the therapeuti c effect of the laser devi ce as a “bloodless ” surgi cal machine and used as a “dentist’s drill”. In the year 1990, introduction

of fi rst laser for general dentis tr y was done b y M yers and M yers .1 4

The pum ping source m a y be elect ri cal or opti cal. The gai n medi um us ed between pai r of mi rrors are pl aced in s uch a wa y that li ght os cill ating bet ween the m irrors pas ses ever y tim e t hroug h t he gain medi um pl aced i n between mi rrors, thereaft er att aini ng considerabl e ampli fi cation which emits through t he transmitti ng mi rror .1 4 (fi gure:1).

Las er et ching i s a painl es s procedure making the cli nical procedure hi ghl y attract ive for routi ne cli ni cal us e. The surface produced b y laser et ching is i deal for adhesion and i s also acid resist ant .1 The erbi um las ers with very short puls e durati ons easil y abl ade l a yers of calci fied tiss ue wit h minim al thermal effects. For bett er adhesion to dentin, som e authors have s ugges ted t he use of las ers for dentin conditi oni ng, as las er irradiation can provide an apparentl y mi cro retenti ve s urface, free of a smear la yer and open dentinal t ubul es .1 5

AIM AND OBJECTIVES

Ideal l aminat e veneer preparati ons are suppos ed to be i n enamel whi ch provides excellent bond st rength and ensures the l ongevit y of the restorations. But cert ain cli nical situations like dent al cari es extending to dent in, old restorations require the preparati on to be extended into the dentin; wherein the bond strength gets com prom is ed. Hard ti ssue l as ers li ke Er, C r: YSGG l aser has the potenti al to prepare dentinal s urface for adhesi on.

GENERAL REVIEW ON LASERS

HISTORY O F LAS ERS:

It all started with MAX PLANCK’s work in 1900 that provided

the underst andi ng t hat li ght is a form of elect rom agnetic radiation. Without this understanding, LASER would not have evol ved. The princi ple of the l as er was fi rst known in 1917, when ph ys ici st Al bert Einst ein des cri bed t he t heor y of stim ul at ed emiss ion. The first l as er was devel oped b y Theodore H. Maim an . Using a theor y ori gi nal l y postulat ed b y Ei nst ein, Mai man creat ed a devi ce where a cr ys tal medi um was stim ul ated b y energy, and radi ant , laser li ght was emitt ed from the cr yst al .1 4 Approximat el y, the hist or y of l as ers begi ns similarl y to much of modern p h ys ics, wit h Ei nst ein. In 1917, his paper in Ph ysi ki alis che Zeil, "ZurQuant ern Theori e der St rahlung", was the first dis cussi on of sti mulat ed emmission .1 6

At thi s st age, t he engi neers were working towards the creation of what was term ed a MASER (Mi crowa ve Ampl ifi cation b y t he Stimulated Emis sion of Radi ation), a device that am pli fied mi crowaves as opposed to l i ght and soon found us e in mi crowave comm uni cati on s yst ems. Townes and the ot her engi neers beli eved it t o be possibl e to creat e an optical m aser, a devi ce for creating powerful beam s of l i ght using hi gher frequenc y energy to stimulate what was to become t ermed the lasing m edium .1 7

Despit e t he pi oneeri ng work of Townes and P rokhorov, i t was left to Theodore M aim an in 1960 to i nvent the first Las er usi ng a lasi ng m edi um of rub y that was sti mul at ed using hi gh energy fl ashes of intens e li ght. Townes and Prokhorov were l ater awarded the Nobel Sci ence Priz e i n 1964 for thei r endeavours .1 6

TYPES O F L ASERS1 4:

According t o their s ources:

 Gas Las ers  Cr yst al Las er

 Semi conductors Las ers  Li qui d Las ers

According to the nature of emission :

 Continuous Wave  Pulsed Las er

According to their wavel ength:

 Visible Region  Infrared R egi on

 Far infrared

 Near infrared  Ultraviol et R egi on

According to their s ite of action :

GAS L ASERS :

The HeNe gas las er was first gas l as er introduced, man y other gas dis charges have been found to amplify li ght coherentl y. Gas l as ers using m an y di fferent gas es have been built and us ed for man y purpos es . The helium-neon l as er (HeNe) is abl e t o operate at a number of different wavel engt hs, however majorit y are l as ed at 633 nm; thes e rel ati vel y l ow cost but are hi ghl y coherent lasers. Comm ercial carbon dioxide (CO2) lasers can emit m an y hundreds of watts i n a single

spat ial m ode which can be concent rat ed into a tin y spot . Thi s emis sion is in the thermal infrared at 10.6 µm Argon -ion l as ers can operate at a number of l asi ng transi tions bet ween 351 and 528.7 nm. A nitrogen t rans vers e el ectri cal discharge in gas at atmospheri c press ure (TEA) laser is an inexpensi ve gas l aser, oft en home -built. M et al i on las ers are gas lasers that generat e deep like t he ultravi ol et wavel engt hs. Heli umsilver (HeAg) 224 nm and neon -copper (NeC u) 248 nm are t wo examples .1 4

CONT INUOUS WAVE:

PULSE D WAVE :

Pulsed operat ion of las ers refers to any l as er not cl assi fi ed as continuous wave, so that t he opti cal power appears in pul ses of s om e durati on at s ome repetit ion rat e. This encom pass es a wi de range of technologi es address ing a number of di fferent moti vatio ns. S ome l as ers are puls ed simpl y because the y cannot be run in continuous mode. In other cas es the appli cation requires the product ion of puls es having as large an energy as possi ble. Si nce the puls e energy is equal t o t he average power di vided b y the rep etiti on rat e, this goal can s ometi mes be s ati sfi ed b y lowering the rat e of puls es s o that m ore energy can be built up in bet ween pulses .1 6

SEMI CONDUCTO R DI ODE LASE RS:

The chromophores are pi gm ents such as hemoglobi n and m el anin, similar t o the Nd: YAG absorpt ion spectrum. Phot othermal int eractions predom inat e whereby diode t issue cutt ing is vi a t herm al energy. The y are quite effective for a host of intraoral soft tis sue procedures such as gi ngi vectom y, bi ops y, im pression t roughing, and frenectom y. Diode las ers al so exhibit bact eri ci da l capabilities and can be used for adj unctive peri odont al procedures. The y are also us ed for las er assi sted tooth whit eni ng. Di ode l as ers have photobiom odulati on properti es as wel l.1 4

EXCIME R L ASERS :

The ex cim er laser is a speci al gas laser bas ed on uns t able molecul es called excim ers. The y exist onl y i n the excit ed st at e in for nanos econds, just long enough for puls ed l as er acti on. Exci mer l as er can abl at e tis sue very effectivel y vi a photochemi cal i nteraction wit hout depositi ng heat therein .1 4

ARGO N L ASER:

The prim ar y advant age of the argon l as er is that t he l aser operat es at a wavelength t hat is abs orbed b y haemoglobi n, whi ch provides excell ent haemost asi s. Denti sts should be aware that, when used for resin curi ng, argon l as ers do not necess aril y produce a resin with ph ys ical propert ies superi or to thos e of resins cured with traditi onal hal ogen curing li ghts. In addition, som e resi ns cont ain multipl e initiators that acti vate at different wavel engt hs. This suggest s that the relat ivel y narrow s pect rum of a las er mi ght not be the best approach to activat e the initi at ors .1 4

HELIUM-NEO N L ASER:

These t ypes of l as ers are one of the forerunners of all l asers whi ch were at fi rst t heoreti call y propos ed and then demonst rat ed in t he year 1961. Generall y onl y the red 632nm emis sion was widel y appli ed as a pointing beam, however becom ing replaced b y diode las ers of similar wavel engt h .1 4

GALLI UM -ARSE NI DE / DIO DE L ASER:

the pres ence of som e d ye phot osensitiz ers. Finall y, within certain low -energy ranges, t he diode l as er can stimul ate t he proli ferat ion of fibroblasts .1 4

NEODYMIUM:YTT RIUM -AL UMINIUM-GARNET (Nd :YAG) L ASERS :

Nd: YAG lasers were the fi rst t ypes of true pul sed l as ers t o be used for dent al procedures in 1990. The y are a near infrared wavel ength of 1064 nm. The Nd: YAG l as er wi ll penet rat e into wat er to a dept h of 60 mm before it is att enuated t o 10% of it s ori gi nal strength. Therefore, the energy is s catt ered in soft ti ssue rather t han bei ng abs orbed on t he t i ssue surface as occurs with C O2 las er energy.

However, s ince this wavel ength is attracted to col o u rs, i n heavil y pi gm ent ed soft tis sue such as ski n, scatt eri ng i s about twice as great as abs orption. This heating effect with the Nd: YAG l as er i s ideal for abl ation of pot entiall y hemorrhagi c abnormal t issue, and for hemos tasis of sm all capill ari es and ver y sm a ll venous vess els . Nd:YAG also have excellent bi ostimulative properti es and unique capacit y to stimul at e fibrin form ation. This effect i s m aximized when t he pul se duration is set at 650 mi cros econds .1 4

A Nd: YAG l as er device, des i gned and prom ot ed for oral and dental appli cations, can deliver up to 3 watts of power i n ei ther a pulsed (20 pulses per s econd) or non -pul sed mode, utilizing a speci all y desi gned hand -pi ece with cont act or non -contact probes. This las er also offers good haem ost asis duri ng soft -ti ssue procedures , whi ch facili tat es a cl ear operat ing field. In addition, t he Nd: YAG l as er offers a fl exible fibre deliver y s yst em .1 4

The Nd:YAG l as er has a number of di s advantages; however, it has the great est dept h of penet rat ion of all the avail abl e dent al surgi cal las er s ys t ems, which means that tissues under the surface are expos ed to las er energy. This is cause for concern becaus e of the ris k of unwant ed coll at eral dam age, especial l y i n the underl yi ng bone as well as t he associ at ed pos toperative morbidit y .1 4

HOLMI UM: YTT RI UM -ALUMI NIUM -GARNET ( Ho:YAG) L ASER:

CARBON DIOXI DE LASERS :

CO2 Las ers have been avail able in m edi cine since the earl y

1970’s and have been used in dentistry for more than 25 years. They

are a 10,600 nm infrared wavel ength, whi ch i s hi ghl y abs orbed b y wat er. Arti culat ed arms or hollow wave guides are us ed to transmit CO2 las er beams and quartz opti cal fibres cannot be us ed .1 4

The CO2 gas is in a chamber with nit rogen and hel ium and the

active m edium is pumped with an el ectri cal current. C O2 las ers are

ver y efficient and exhibit excell ent haem ostasis . The y are currentl y for soft tiss ue us es onl y. The y are continuous wave l as ers that can be operat ed in gat ed wave modes, includ ing what are t ermed as “super -pulsed” modes. It is important to note even the super -pulsed mode is

not a free running pulsed mode .1 4

These s uper -puls ed gat ed modes offer i mproved surgi cal control with l ess charring of tissue. CO2 lasers are excell ent t ools for incising

tissue for m ultiple purposes. Incis ional and excisi onal biops ies , frenectom y, gi ngi vectom y, pre -prostheti c procedures are all achi eved with excell ent haemost asis , de-epithel ialis ati on of gi ngival ti ssue duri ng peri odontal regenerati ve pro cedures. Sutures are rarel y needed and the cont roll ed therm al effects and s ealing of nerve endi ngs often makes for a ver y com fort abl e pos t -operati ve experi ence for the pati ent .1 4

The C O2 l as er is s afe around the impl ant s because the energy is

wat er of bacteri a, t he CO2 wavelengt h can s afel y and effecti vel y treat

peri-impl anti tis and mucositis, because t he energy is not abs orbed int o the implant’s surface.1 4

With the C O2 las er, the rapi d ris e in

intracel lul ar t emperature and press ure leads t o cellul ar rupt ure, as well as rel ease of vapo ur and cell ular debris, term ed the laser pl ume. The debris arisi ng from t he s ite of imp act, t he char i s carbonized tissue b y the laser beam .1 8

ERBIUM AND ERB IUM -CHROMIUM LASERS :

The i nt erest in t he Er 3+ l as er is based on the wavel ength it can emit i .e. 1.54µm and 2.7 -2.9µm. The former coi nci des ni cel y with the abs orption minimum of opt ic al sil ica fibres , al lowing l ong -range opti cal com muni cati ons. The lat er wavel ength coincides nicel y with the peak of water absorption. As wat er is contai ned in ever y biological tissue, efficient i nt eraction and dense opti cal energy depositi on is guarant eed. In m edi cal appli cations and especiall y in denti str y, the Erbium l as ers repres ent hi ghl y devel oped commerci al l as ers with hi gh yi el d and effi ci enc y in tissue removal . For dent al hard tissue abl ation, this i s current l y t he t ype of l as er most oft en us ed .1 9

ERBIUM :YTT RI UM – AL UMINIUM-GARNET (Er: YAG) LASER:

The laser operates at a wavelength of 2.94 μm and in a pulsed

advantages as it produces clean, sharp margins in enam el and dent in, and i n addition, pul pal s afet y is not a s igni ficant concern because t h e depth of energy penetration is negli gibl e. Pulp m a y respond even bett er to preparati ons , done with the Er: YAG l aser than thos e done wit h t he bur. When the Er: YAG l aser is used for cari es rem oval, it us uall y does not require l ocal anaesthes ia. The l as er i s antimi crobi al when us ed withi n root canals and on root s urfaces, and it rem oves endotoxins from root s urfaces . Finall y, vi brati on from the Er: YAG l as er i s less severe t han that from the conventional hi gh -speed drill , and it is les s likel y t o provoke dis c om fort or pain .1 4

ERBIUM,CH RO MI UM:YTTRIUM -S CANDIUM

-GALLIUM-GARNET (E r,Cr: YSGG) L ASER:

The Er,C r:YS GG operat es at a wavel ength of 2.78 μm, with an extinction length in water of 1.0 μm. The waveform for the

Er,C r: YSGG l as er is puls ed. The Er,C r: YSGG l as er has s everal hard -tissue appli cations enam el et ching, cari es rem oval, cavit y preparati on, In-Vit ro bone cutt ing wit h no burning, melti ng or alt erati on of the calci um: phosphorus rat io, root canal preparati on .1 4

REVIEW OF LITERATURE

Marco Franchi et al (1995)9 st at ed that the best removal of dentinal smear l a yer res ult ed from the 37% phos phoric aci d t reatment; al l the dentinal tubul es ori fices appeared to be compl et el y opened. In this stud y t hree standard occlus al caviti es with bevel led enam el margins were prepared on each t oot h and et ched with the et ching s olutions of three denti nal adhesi ve s ys t ems; ( 1 ) 37% phos phori c acid s ol ution. (2) 4.3% ox alic acid and 2.6% aluminium salts solution, and (3) 10% mal ei c acid soluti on. Scanni ng elect ron micros copic anal ys is reveal ed that al l the et chi ng soluti ons affect ed the enam el s urface m orphology. The solut ion of oxal ic aci d and al umini um salts removed pri maril y the prism core m at eri al and parti all y the peripher y of the pr isms , but did not affect t he non bevell ed enamel surface. Phos phori c acid and m al ei c acid removed both prism core m at eri al s and prism peripher y; thes e specim ens al so showed areas in whi ch no pri sm morphology coul d be det ected. Thes e t wo acids also rem oved apatit e cr yst als from t he pri sm core of t he int act enamel surface.

with 37% ortho -phos phoric acid (15 seconds for enam el, 5 s econds for dentin). Las er etchi ng was wit h Er: YAG las er (four 200mJ pulses per second for enam el; four 160 mJ pulses per s econd for denti n). Brackets were bonded wit h auto -curing resi n past e, having first appli ed a prim er (denti n onl y) and then li ght cured bondi ng resin. Tensil e strength was det ermined wit h a univers al testing m achine. R es ult s hows t hat bond fai lure aft er laser et chi ng was due t o mi cro -cohesive fracture of tooth tissue.

M Peu mans et al (2000)7 st at ed that there is a general ag reement among the practiti oners that porcel ain veneers will pl a y a vit al role i n el ect ive dental aestheti cs. This pl aces hi gh demands on predi ct abilit y, especi all y wit h col our mat ching and masking met hods. In additi on, pati ent accept ance of porcelain vene ers in thes e clini cal s tudies was hi gh. P orcelain veneers are st eadil y increasi ng in popul ari t y among today‘s dental practitioners for conservative restoration of unaesthetic

insuffi ci ent wear resistance of the luti ng com posit e. Al though t hes e shortcomings had no direct impact on the cli nical s uccess of porcel ai n veneers in the m edi um term, the ir infl uence on the overal l clini cal perform ance in t he long term is still unknown and therefore needs further stud y.

Mitsuharu Okamoto et al (2003)4 stated t hat pl asm a curing for 3 second is suffici ent to obt ain simi lar bon d st rengt hs and st abili t y to specim ens cured wi th halogen l i ght for 40 s econd for 1 -mm-thi ck porcel ain and t hat plasm a curing for 5 second is sufficient even for porcel ain of 2 -mm thicknes s. Plasma and hal ogen l am p units were used to photo -cure a composit e resi n for porcelai n bonding. Mean shear bond st rengths to dentin after li ght curi ng through porcel ain of 1 and 2mm thi ckness ranged from 20 to 27 MPa before and aft er therm o -c y-cling, indi-cati ng no si gni fi -cant effe-ct s due to por-cel ain t hi-ckness, curing met hod, or thermo -c ycling, except for pl asm a curi ng for 3 second t hrough porcelai n of 2 mm thi cknes s that overcam e t hermo -c y-cling.

GC Lopes et al (2003)1 1 stat ed that for norm al occl us al dentin, no difference exist s in bond st rength when 35% phosphoric acid et chant is applied following the manufacturer‘s suggested time (15 seconds), or

that the hi gher mineral amount in sclerot ic dentin m akes it di ffi cult to bond t o thi s subst rat e, resulti ng i n a l ower µ -TBS . However, doubl ing the et ching tim e resulted in µ -TBS simi lar to norm al dentin.

cross head speed of 1 mm/min, and the maxi mum load at fracture (kg) was recorded. No stati sti cal l y si gni fi cant di fferences were found among the bond st rengths of veneers bonded to toot h surfaces et ched with Er,Cr: YSGG l as er (12.1±4.4 MP a), 37% ort ho -phosphori c acid (13±6.5 MPa), and 10% m aleic aci d (10.6±5.6 MPa). The control group dem onst rated the lowest bond strengt h values in all test groups . Statist icall y si gni fi cant differences were found bet ween the bond strengths of cervical and incisal sections (p˂.001). Invitro microtensile

bond st rengths of porcel ain laminat e veneers bonded t o tooth surfaces that were laser et ched showed results si milar t o ortho -phosphori c acid or m al ei c acid et ched tooth surfaces.

and (6) proper planning for the cont inuing m aint enance of thes e restorations. This art icl e dis cusses failures t hat could occur if meti cul ous at tention is not gi ven to such details. Failures that did occur st ructurall y and aestheti call y warned individuals who were learning t he procedure what to wat ch for. Some concerns as to newer products and m ethods and t hei r effect on the conti nued s uccess of t hi s modalit y of t reatm ent are also addressed.

minimize effects and poss ibl e probl ems, we should be precis e and careful about cas e s election and toot h preparation.

Mathew C A et al (2010)3 stat ed t hat the cerami c l aminate veneer rem ai ns the prost het ic rest orati on that best compi les the pri nci ples of pres ent - da y aesthet ic dent ist r y. This ―s ubstit ute e nam el ‖ now brings us closer to achievi ng the goals of P rosthodonti cs; to repl ace hum an enamel to its proper structure, shape and colour with this ―bonded artificial enamel‖. The history of aesthetic or cosmetic dentistry can be

dat ed back to t he J apanes e customs of decorati ve tooth st ai ning called ―Ohoguro‖ which was documented 4000 years ago. Dr. Charles L.

Pincus i ntroduced the concept of veneering anteri or teeth wit h laminat es when approached b y Holl ywood di rect ors in 1928. It was Buonocore's res earch a bout the acid etching technique in 1955, whi ch provided a s impl e m ethod of i ncreasing adhesi on t o enam el s urface for acr yl ic m at eri al s. But onl y aft er the introduction of li ght cured com posit es in 1970 did t he denti st have the neces sar y worki ng tim e to pro perl y shape di rect lam inate veneers. In t he mid 70's and 80's the com posit e resi n lam i nat e veneers , (with or wit hout faci ng evolved ). At first the composit es were directl y bonded t o teet h and call ed ―bonding‖.

faci ng up. The l abi al surfaces were prepared wit h 0.5 -mm reducti on to receive composit e veneers . T hi rt y specim ens were et ched wit h Er,C r: YSGG l as er. This group was als o divided int o three subgroups , and the following three bonding s ys tem s were then appli ed on the laser groups and the ot her three unlased groups: (1) 37% phos phori c acid et ch + Bond 1 pri mer/adhesive (P ent ron); (2) Nano bond sel f -et ch prim er (P ent ron) + Nano -bond adhesive (Pent ron); and (3) all -in -one adhesi ve—si ngl e dos e (Futurabond NR, Voco). All of the groups were restored with a nanoh ybrid composit e resin (Smil e, P ent ron). Shear bond st rength was measured. There were no si gnifi cant differences in shear bond st rengt h bet ween s el f -et ch pri mer + adhesi ve and all -in-one adhesi ve s yst ems for non -et ched and las er -et ched enamel groups (P > .05).

Guilh erme Carpen a Lopes et al (2011)2 4 stated t hat bondi ng to ol d dentin with 30 s econds of et chi ng time result ed i n hi gher bond st rengt h and m ore homogen eous h ybrid l a yer formation than dent in acid et ched for 15 s econds. Accordi ng to this stud y the modificati on of dentin b y ph ysi ological s clerosis due to aging should be considered duri ng adhesi ve procedures. The clini cal appli cation of Adper Single Bond i n old denti n aft er 15 seconds of acid et ching m a y produce l ower bond strengths. B y si mpl y doubl ing etching time from 15 to 30 seconds, bondi ng to old dentin result ed in hi gher bond st rengths t hat were similar to thos e found in young denti n. Further investi g ation must be conduct ed with other et ch -and-rinse si mplifi ed adhesi ves. Whet her increasing et chi ng ti me res ults in i ncreas ed longevit y of restorat ions in elderl y pati ents rem ains to be det erm ined. This is sue requi res furt her clini cal i nvesti gation.

when denti n is cont aminat ed with various provi sional cem ents, com pared to freshl y cut dentin. Si nce some period of tim e is necess ar y to make the fi nal restoration, a provisional restoration is used for approxim at el y 10 days aft er the fix ation. This m akes mechani cal and/or chemi cal cl eaning procedures necessar y before the defi nitive restoration is cem ent ed. In this current st ud y, aft er a period of 1 week, provisional cem ent s were found to affect the final bond st rength to dentin, alt hough the bond s trength t est achi e ved acceptable values for all cas es.

Zah ra Jab eri Ansari et al (2012)8 stat ed that the mi cro -shear bond strength of groups prepared b y bur cutting and aci d -et chi ng were hi gher than t hat of groups prepared and etched b y an Er,C r:YSGG l as er withi n the condi ti ons of this st ud y. Therefore, re -et ching with phos phori c aci d would be recomm ended i f an Er,C r: YSGG las er is us ed for tooth preparation or s urface t reatment. Enamel preparati on b y the Er,C r: YSGG l as er results in a charact eri sti call y chalk y surface. Scanni ng el ect ron micros copic im ages showed that las er irradiat ion produces a surface that increas es t he restorative m at erial ret ention, whi ch i n turn makes t he surface suit abl e for the appli cation of com posit e and com poser filli ng mat eri als .

las er. Two etch and rins e adhesives (Si ngl e Bond and XP Bond) and two s elf -et ch adhesi ves (Prompt L -Pop and Xeno III) were emplo yed to bond the compos ite. Shear bond st rengt h (S BS ) was determ ined aft er storage i n wat er for 24 hours using a uni vers al t esti ng m achi ne with a cross head speed of 0.5 mm/m in. Fail ure patt erns and m odes were anal yz ed and evaluat ed usi ng a stereomi cros cope. In addition, sampl es were process ed for S canni ng Elect ron Mi cros cop y (S EM ) eval uati on. A linear mixed m odel was us ed, and pai r wis e comparisons were m ade using the Bonferroni t est. R esult s s howed si gnifi cant di fferences bet ween the l evels of denti n treatment (p=.01) in carbide bur -cut dentin and l as ed dentin, as well as si gni fi cant interact ion effects due to t he depth of denti n and t he bonding s yst em us ed. The etch and rinse adhesi ves bonded l ess effecti vel y wit h l ased dentin than wit h carbide bur-cut dentin, whi l e s elf -et ch adhesi ves bonded equall y well with las ed and bur-cut s uperfi cial dent in but much less effecti vel y with las ed deep dentin t han wi th bur -cut deep dentin. SEM revealed a predom inantl y adhesive failure mode in las er -abl at ed fractured specim ens, whil e a mixed fai lure mode was apparent i n t he bur -cut fractured s pecim ens.

buccal surfaces were ground usi ng a di am ond bur and pol ished until the dentin was exposed; the sampl es were randoml y divided into fi ve groups (n=15) according to the surface t reatm ent: (1) aci d et chi ng; (2) las er et chi ng; (3) l as er et ching followed b y acid et ching; (4) acid et ching followed b y las er et chi ng and (5) no acid et ching and no las er et ching (control group). Composit e resin rods (Point 4, Kerr Co) were bonded to treat ed dentin surfaces with an et ch -and -ris e adhesi ve s yst em (Optibond F L, Kerr Co) and li ght -cured. Aft er st orage for t wo weeks at 37°C and 100% hum idit y and then therm o -c ycling, bond strength was measured. There were no si gni fi cant di fferences betw een acid et ching and aci d+l as er groups, and between laser+aci d and l as er groups.

Ayoub Pahl evan et al (2014)2 st at ed that the enam el t hi cknes s in different part s of the labi al surface is very import ant . The thi cknes s of enam el in t he gingival area does not permit a cha m fer preparation. The knife edge preparat ion is preferabl e i n the gingival area. But is beli eved to end wit h the over contouri ng. In this st ud y, the thi cknes s of enam el i n di fferent pl aces of l abi al surface was meas ured. Thi s measurement i s an i mportant gu ide for t he preparation of the t ooth in laminat e veneer. The most criti cal area is the labi al gi ngival t hird. This stud y showed t he m ean t hickness of enam el at the gingi val thi rd is 410 μ on the maxillary central incisor and 367 μ on the maxillary lateral

inci sor. There are two approaches i n the preparat ion of tooth for porcel ain l aminat e in the cervi cal area, cham fer and knife -edge preparati ons . The i nterest i n cham fer preparation is becaus e of cosm eti cs and avoidance of over contouring. The knife -edge preparati on is believed to end up wit h the over cont ouring. The result of this stud y showed that i n t he knife -edge preparation there is no risk of denti n expos ure, whereas in chamfer preparati on the ri sk of denti nal exposure is si gni fi cantl y hi gher in t he pre paration of porcel ain laminat e.

sectioned into crowns and roots along the cem ento -enam el juncti on, and then the crowns were cut l ongi tudi nall y int o s heet s about 1.5 mm thick with a cutting machine. The denti n samples were fixed on a st age at focus pl ane. The l as er beam was irradi at ed onto the s amples through a galvanom etric s canning s ystem, so rect angula r m ovem ent could be achi eved. Aft er ablation, the sampl es were exami ned with a scanning el ect ron mi cros cope and l aser t hree -dim ensi onal profi le m easurem ent micros cope for morphology and roughnes s stud y. With increasing l as er fluence, denti n s amples exhibit ed more m elting and re -solidi fi cation of dentin as well as debris -li ke s truct ure and occluded parts of denti nal tubul es . When at t he s canni ng speed of 2400mm/ s and scanni ng distance of 24μm, the surface roughness of dentin ablated with

fem tos econd puls ed l as er decreased si gnifi cantl y and varied bet ween val ues of denti n surface roughness grinded with two kinds of diam ond burs with di fferent grits. When at the scanning s peed of 1200mm/s and scanning distance of 12μm, the surface roughness decreased slightly,

and the surface roughness of dentin abl at ed with fem tos econd puls ed las er was al most equal t o that grinded wi t h a low grit di amond bur.

―micro-explosions‖ weakened the enamel and gave rise to a more

obt ained aft er acid et ching, and can be an al ternati ve to aci d etching was accept ed.

Rafael Massunari Maenosono et al (2015)1 5 stat ed that t he associ ati on of di ode las er i rradiation wi t h sim plified adhesi ve s ys t ems alread y appli ed to denti n, but pri or to pol ym eriz ation, i s a prom ising alt ernat ive for achi eving hi gher bond st rength values . Las er i rradi ati on aft er the immediat e applicati on of Denti n Bonding S ys t ems (DBSs) and prior t o their pol ymerizati on has been proposed to increase bond strength. The obj ecti ve of this s tud y was to evaluat e the effect of diode laser irradiation (λ = 970 nm) on simplified DBSs through microtensile

bond st rength tests . Fort y health y human mol ars were random l y distribut ed among four groups (n = 10) according t o DBSs us ed [Adper™ SingleBond 2 (SB) and Adper™ EasyOne (EO)], and the

MATERIALS AND METHODS

This stud y is conducted wit h the approval of Inst ituti onal Ethics Committ ee ( IEC ) , Adhi paras akt hi Dent al College & Hospital, Melm aruvathur - 603319.

IR B/ IEC R eference No : 2014-M D-Br I-R IJ -03.

INTRODUCTI ON:

Ideal l aminat e veneer preparati ons are suppos ed to be i n enamel whi ch provides excellent bond st rength and ensures the l ongevit y of the restorations. But cert ain cli nical situations like dent al cari es extending to dentin, old res torati ons , require t he prep arati on to be extended into the dentin; wherein the bond strength gets com prom is ed. Hard tiss ue lasers l i ke Er,Cr: YS GG l as er mi ght have the potenti al to prepare dentinal surface for adhesion.

OVERVIEW O F TH E PRO CE DURE:

COLLECT ION O F SAMPLES:

Hundred extract ed hum an maxill ar y cent ral inci sors with approxim at el y 10 m m anatomi c crown l ength and 8 mm mesio -dist al width are select ed. Each t oot h is free of dental cari es, res torations and morphol ogi cal abnormaliti es like att rition, h ypoplasi a etc. Th e t eet h are cleaned and s tored in s ali ne solution at room t emperature immediat el y aft er extracti on. Periodi call y the s aline s olution is changed until t he required sampl es are col lected (fi gure : 2).

PRE PARING CRO WN PORT ION:

The t oot h are sectioned 2 mm bel ow the cem ent o -enamel junction with a slow speed di amond s aw s ection machi ne and the crowns are em bedded in the blocks of clear aut o -pol ym erisi ng acr yli c resin with the l abi al s urface faci ng upward. The l abi al surface i s prepared fl at, deep enough to expose the dentin, in the mi ddle of the tooth .1 (fi gure : 3 , 4 & 5).

ETCHI NG THE TO OTH SURFACE BY ACID:

Aft er preparing the l abi al s urface flat , the y are di vided i nto t wo groups (Group A & Group B). In group A (control), dent i n s urfaces are treat ed with 37% ort ho -phosphori c aci d et ching gel (3M ESP E) for 15 sec, t hen ri ns ed for 20 sec with dist illed wat er and air dri ed .1 (fi gure:6).

ETCHING THE TO OTH SURFACE BY LASER :

2780 nm; pul se durat ion, 140 µs; energy, 3.0 W; repetiti on rat e, 20 Hz) with water s pra y (45% wat er, 60% air), in accordance with t he manufacturer’s instructions. The laser beam is used in a noncontact

mode 7–9 mm from the target area, and the 600 µm di am et er sapphi re tip turbo hand -pi ece is held perpendicul ar to the dentin surface and moved in a sweepi ng fashi on b y hand duri ng the exposure time (15 seconds ) (fi gure : 7 & 8).

BONDING PO RCE LAIN LAMINAT E VENEE RS:

Porcel ai n veneers of siz e 5mm widt h, 5mm hei ght , 2mm thickness are m ade. The i nner surfaces of the porcel ain veneer are et ched with h ydrofl uori c acid and subsequentl y sil aning the et ched surface i s done us ing sil ani ng agent s. Then, this is bonded t o group A and group B with R el y X veneer cement (3M ESP E) li ght cure resin. Then, the embedded specim ens are st ored in di still ed wat er for 24 hours before t esti ng of bond st rength (fi gure : 9 & 10).

MEAS URING BOND STRE NGTH :

Comp on ents :

Load Frame – Usuall y cons isting of t wo strong supports for t he

machine.

Load Cell – A force transducer or ot her means of m easuri ng the

load is requi red. Episodi c calibration is usuall y needed b y governing s ys t em.

Cross h ead – A movable cros s head i s controlled to move up and down. Usuall y this i s at a const ant speed.

Means of recording extensi on or d eformation – Many tests needs a m easurem ent of response of t he t est specimen t o t he movement of the cros s head. Extensom et ers are som eti mes us ed.

Output Device – A means of delivering the tes t result that is needed. Some of the older equi pm ents have di git al dis plays and chart recorders. M any newer machi nes have a comput er i nt erface for anal ys is and print ing.

Figure : 1 Dental laser wavelengths on the electromagnetic spectrum

Figure : 3 Preparing crown portion

Figure : 4 Embedded crown in clear acrylic resin

Figure : 6 Acid etched surface

Figure : 9 Prepared veneer

Figure : 10 Bonded veneer to tooth surface

Figure : 12 Chisel placed at dentin – veneer interface

Figure : 13 Components of a laser unit

Figure : 15 Absorption curves of the prime oral chromophores

Figure : 16 Acid etched Enamel

Figure : 18 Laser etched Enamel

RESULTS

STATISTI CAL ANALYSIS

The debonding values for the t est s s am ples were m easured and tabulat ed. M ean val ues and thei r st andard deviat ions were calculated for each group. Since both the groups are different, unpaired „t‟ test is

perform ed. Al l datas are process ed b y SPSS soft ware version 19.0 (SPSS Inc., Chi cago, IL, US A). The power of the stud y was set at 90% with β at 5%. Differences are considered significant for a value of P <

Table : 1 Shear Bond Strength Values

S.No Group “A” (Acid)Stress Value

in Mpa S.No

Group “B” (Laser)Stress Value in Mpa

1 8.3 1 8.6

2 8.4 2 8.4

3 8.5 3 8.6

4 8.9 4 8.4

5 8.7 5 8.5

6 8.1 6 8.3

7 8.6 7 8.6

8 8.2 8 8.4

9 8.7 9 8.4

10 8.8 10 8.6

11 8.6 11 8.4

12 8.8 12 8.3

13 8.1 13 8.4

14 8.5 14 8.6

15 8.4 15 8.6

16 8.8 16 8.5

17 8.2 17 8.6

18 8.2 18 8.4

19 8.7 19 8.6

20 8.6 20 8.7

21 8.3 21 8.4

22 8.4 22 8.5

23 8.6 23 8.7

24 8.5 24 8.6

25 8.3 25 8.5

26 8.9 26 8.7

27 8.4 27 8.6

28 8.5 28 8.7

29 8.6 29 8.5

30 8.3 30 8.4

31 8.6 31 8.5

32 8.2 32 8.5

33 8.4 33 8.6

34 8.6 34 8.5

35 8.5 35 8.4

36 8.5 36 8.4

37 8.4 37 8.5

38 8.2 38 8.4

39 8.5 39 8.3

40 8.8 40 8.5

41 8.1 41 8.4

42 8.8 42 8.5

43 8.7 43 8.5

44 8.9 44 8.6

45 8.7 45 8.5

46 8.7 46 8.6

47 8.3 47 8.4

48 8.7 48 8.3

49 8.6 49 8.4

Table 2 : Descriptive statistics Groups (Stress

Value in Mpa) Sample N Mean Std. Deviation

Acid (A) 50 8.518 0.22829

Laser (B) 50 8.498 0.11156

Table 3 : Comparison between two groups

Groups (Stress Value in Mpa)

Sample

N Mean

Std.

Deviation Mean difference t value p value Acid (A) 50 8.518 0.22829

0.02 0.557 0.579 Laser (B) 50 8.498 0.11156

GR

A

P

H :

GR

A

P

H :

DISCUSSION

The aim of this st ud y is to compare the bond strength of porcel ain l aminat e veneer bonded to acid t reat ed denti n and l as er treat ed denti n. M os t dentin bonding s ystems rel y on som e t ype of dentin etching and primi ng to m aximize micromechanical adhesi on. Etching enamel affects t he prism core and prism peri pher y. Et chi ng dentin affect s the i nter -tubular and peri -tubul ar dentin, res ulting i n enl arging the tubul ar openings, removi n g a l arge amount of t he surface h ydrox yapatit e cr ys t als, and l eaving an organiz ed network of coll agen fibrils .2 0 Etching of t ooth surface can be done either b y acid or l as er.

BRIE FL Y ABO UT COMPO NENTS O F LASER UNIT :

Las er unit has acti ve m edium, laser resonat or and a power source. The acti ve medi um m a y be gas , soli d, li quid, or s emi -conductor. Active medi um is cont ained wit hin an opti cal enclosure. Act ive medium needs to be charged t o rel ease phot ons . The external source of energy m ay be el ect ri cal, che mical, or fl as h l amp. The gain medi um is pum ped by an ext ernal energy source. The gai n m edium t hen emits photons, whi ch bounce back and forth between the refl ectors . Part of t he radiati on is allowed to exit through an aperture in one of t he refl ectors, res ulting i n the l as er beam1 4 (fi gure : 13).

SPO NT ANEO US E MISSIO N:

speci fi c amount of energy to reach the next shell farther from t he nucleus, which is at a higher state of energy level. A “population inversion” can occur when there is more electrons in the higher energy

level t han the ground st at e. The elect rons wh i ch are in that excit ed stat e spontaneous l y rel ease that energy in the form of a photon. This is called s pont aneous emission .1 6

EMISSIO N MO DE O F LASE RS :

Dental l as er devi ces can emit li ght energy in two m odaliti es as a functi on of tim e:

(1) Const ant / Continuous (2) Pul sed

 Gat ed -puls e m ode

 Free -runni ng pul sed mode

1.Continu ous -wave mod e:

Las ers depend on a beam whose output power is const ant over time and st ead y when averaged over an y longer tim e periods , wi th the ver y hi gh frequenc y. The power vari ation has l ittl e or no i mpact in t he intended applicati on. Such a l aser is known as cont inuous wave . Man y t ypes of l as ers can be m ade to operat e in continuous wave mode to satisf y s uch an appl ication. The beam i s emitt ed at onl y one power level for as long as t he operat or depress es the foot s wit ch .1 7

2. Gated-pul se mod e:

clos ing of a shut ter whi ch is pl aced in front of the li ght beam pathwa y of a continuous -wave emissi on. All surgi cal devi ces that operat e i n continuous wave have thi s gat ed -pulse feature. The more advanced units have computer -cont roll ed shutters that all ow for t hese ver y s hort pulses.1 7

3. Free-running pul sed mod e:

It is a t rue -pul sed mode. This em ission is unique i n that large peak energi es of l aser li ght are emitt ed for us ual l y mi croseconds , foll owed b y a rel ati vel y long tim e i n whi ch the l as er is off. Thes e devices have a rapidl y st riki ng fl as h lamp that pumps t he acti ve medi um. With every puls e, hi gh powers are generat ed. However, becaus e the puls e durat ion of this mode i s short, the average power that the t issue experi ences i s sm all. Free -running pul sed devi ces cannot have a continuous -wave or gated -puls e output .1 7

Contact mod e-

Terminal end of fibro optic is pl aced in direct co ntact of target tissue and the operat or will have a t actil e feedback .1 4

Non con tact mod e:

The hand pi ece is held awa y from the tis sue and the operator has to adjust t he focus of the beam b y var ying dist ance bet ween the hand piece and t arget to have t he d esi red effect .1 4

LASER ENE RGY AND T ISSUE TEMPERAT URE :

depends upon the temperature ri se and t he corresponding reaction of the int erstiti al and i ntracellular water .1 8

TARGET TISSUE E FFECTS I N RELAT ION TO TE MPE RATURE1 8

TISSUE TE MPE RATURE OBSERVED EFFECT

37–500C - H yperthermia

60–700C - Coagul ation, protei n denaturation 70–800C - Tissue W eldi ng

100–1500C - Vaporizati on, abl at ion >2000C - C arbonizati on.

EFFECTS O F LAS ER O N TISS UES :

Las er li ght has eit her four di fferent i nteracti ons with the t arget tissue, depending on the opti cal properti es of that ti ssue (fi gure : 14). The y are as follows

 ABSORP TION  TRANSM ISS ION  REF LEC TION  SCATTER ING

ABSORPTIO N:

wavel engths are m ore int eracti ve wit h wat er and h ydrox yapatit e2 1 (fi gure : 15).

TRANSMISSION:

Li ght energy pass es freel y t hrough the ti ssue, without i nteraction of an y kind and has l ittle or no effect . It i s an invers e of absorption .2 1

RE FLE CTIO N:

Li ght energy reflects off tiss ue surface with l ittl e or no abs orption and cons equentl y has no effect on tiss ue. The l as er beam generall y becomes m ore divergent as the di st ance from the head pi ece increases , whi ch become dangerous because the energy is di rect ed to an unint enti onal t arget s uch as e ye. This is maj or s afet y concern for las er operation .2 1

SCATTE RI NG:

Li ght energy is re -emitted in a random direction and ultim atel y abs orbed over a great er s urface area which produces less intens e and less preci sel y dist ributed therm al effect. When laser li ght emerges from a laser, it usuall y enters in the form of pencil thin beam of energy travelling at s peed of li ght .2 1

TISSUE CHANGES BY L ASER:

There are fi ve import ant t ypes of bi ol ogi cal effects that can occur once t he l as er photons ent er the ti ssue: fluores cence, phototherm al , photodi srupti ve, photochemical, and photobiom odulati on .1 4

Fluores cence happens when activel y carious toot h st ructure is expos ed to t he 655 nm visi ble wavel engt h and the amount of fluores cence is related to t he siz e of the l esion, and is useful i n diagnos ing incipi ent cari ous lesions. e.g., Di agnodent .1 4

Photot herm al effects occur when the chromophores absorb the las er energy and heat is generat ed. This heat is us ed to perform work such as inci sing ti ssues or t o coagul at e the blood. Phototherm al interact ions predom inat e when most soft tiss ue procedures are perform ed with dent al lasers . Heat i s generat ed duri ng thes e procedures and great care mus t be taken to avoi d thermal dam age t o the t issues .1 4

Photodisrupti ve effects: Hard ti ssues are removed t hrough a process known as photodisrupti ve ablat ion. Short -puls ed l aser li ght with trem endousl y hi gh power i nt eract with wat er in the tissue and from t he hand -piece causi ng rapi d t hermal expansion of the wat er molecul es .1 4

vaporiz ed but pul verized instead t hrough the photom echani cal abl ation process . This s hoc k wave creates the distinct poppi ng sound heard duri ng erbium laser use. Therm al dam age can be prevented when us ed properl y, especi all y when the concept of thermal relax ation is considered.1 4

Photochemi cal react ions occur when photon energy creat es a chemi cal reaction. Thes e reacti ons are utiliz ed in s ome of the benefi cial effects found in bios ti mulat ion. e.g, Photod ynamic therap y.1 4

Photobiomodul ation or Biosti mul ation refers to lasers abilit y to speed heal ing, increas e circulat ion, reduce edema, and mini mize pain. The exact m echani s m of t hese effects i s not cl ear, but i t is theoriz ed the y occur m ostl y through photochemi cal and photobiological interact ions withi n the ce l lul ar matrix and mitochondria. Biostimulat ion is us ed dent all y t o reduce post operati ve dis comfort and to treat mal adi es s uch as recurrent herpes and apht hous stom at itis. e.g., Low Level Laser therap y.1 4

It was in 1975 when Rochette descri bed the concept of et ching porcel ain wi th acid and bonding t o the t ooth, and des cribed a techni que for m aki ng cerami c restorations for fractured in cisors without operative influence, which made the pioneers in veneers to turn towards porcelai n, one of the most popular and att ract ive mat eri als i n the dent al arm am entarium and thus was born, the cerami c laminat e veneer. Aft er whi ch, Graham J. R obert s in 1983, cli ni cal l y eval uat ed the t echnique of m astique acr yl ic veneers whi ch were bonded to t eet h with composit e res ins aft er the acid et ch t echni que .3

EFFECT O F ACI D ETCHI NG O N E NAMEL AND DENT IN :

Aci d etching of the enam el appears to i mprove the ret ent ion b y sel ectivel y eroding cert ai n h ydrox yapatit e cr yst als . The sm ear l a yer is gentl y rem oved b y the appli cat ion of acid on the enam el surface. Micros copi c i rregul ariti es and acid treat ed enam el surface energy are enhanced b y removi ng prism ati c and i nter -pris mat ic mi neral cr ys t als and facil itat ing the penet ration with the development of res in tags .2 3 (fi gure : 16)

Despit es advances i n the chemist r y of adhesive s yst em s, dentin rem ai ns a chall engi ng substrat e for bonding due to it s heterogeneit y. The bonding m echanism of composit e resin to acid -etched denti n is wel l known. The formation of a h ybrid l a yer and resin tags i s ess enti al to the est abli shm ent of a st rong bond at the dentin level and ma y be achi eved b y com plet e diss olut ion of the s mear l ayer and demi neralization of inter -tubul ar and peri -tubular dentin b y means of acid etching, res ul ting in an ex pos ed coll agen mat rix whi ch is infilt rat ed b y resin t hat pol ym eriz es i n si tu .2 5 (fi gure : 17).

The clinical success of porcel ain l ami nate veneers is att ributed to the i ntim ate bond achi eved bet ween t he rest orati on and tooth struct ure t hrough t he luti ng c em ent.2 6 Adhesive dent al restorative mat eri al s have som e major advant ages over non adhes ive m at erials. For exampl e, bett er adhesion with t he toot h surfaces and preventi on of secondar y cari es thereb y preserving the nat ural toot h s tructures as limited cavit y prepara tion is needed .2 7

EFFECT O F LASE R ET CHING O N E NAMEL AND DENTIN :

These resin t ag formation shows advantage for onl y a fract ion of the bond s trength in norm al h ybri dized denti n. Som e res earchers have explored the use of l as ers to modi f y the surfaces of t eet h int enti onall y and improve bonding of rest orati ons .2 5

Gri nding with rot ary inst rum ents is the most comm onl y used clini cal m ethod for tooth preparation and cari es removal. It bri ngs effectivit y and efficienc y as well as some i nevit abl e drawbacks, such as the nois e and vi bration produced which al wa ys m ake t he pati ent s feel scar y about t he t herap y.2 9 R ecentl y, new t echniques were introduced for cari es removal . One of the m ai n advant age in thes e techni ques is provi ding mini mal dam age to tooth structure duri ng cari es removal. Among new techniques , Erbium laser showed prom ising results .3 0 Erbium l aser has a wide range of advant ages , such as, abs ence of vibrat ion effect , s ound, and stres s and reduci ng t he need of local anest hesia .3 1

Due to its benefici al effects, the y are used in the hard tis sue treatm ents. The advantage of an Erbium l as er wave is that it i s hi ghl y abs orbed b y wat er and dent al hard ti ss ue. The st rong absorpti on of wat er decreas es t he level of heat througho ut t oot h preparati on. Laser radi ation are bet ter absorbed b y wat er than dental hard tissue, it reduces the increasi ng t em perature of t he tissue during the preparation. Water reaches the boiling point and caus es mi cro -explosi on of the tooth .2 3

This m echani sm breaks up the s urrounding tiss ue into s mall fragm ents and dissi pat es t hem at the s am e ti me. This mi cro -explosion of t oot h is so -call ed a preparation tri ggered b y wat er. Although most radi ation i s hi ghl y utilized b y wat er, a cert ain amount of heat transmi ssion is unavoidabl e. Therefore, a wat er spra y is used for cooling. Lack of adequate wat er spra y causes pulpal dam age .2 3

This laser can al so be used for etching of enam el and dentin surfaces for the purpose of bonding the compos ite res in t o enam el an d dentin s urface. Er,C r:YSGG, whi ch has a hi gh absorpti on coefficient in wat er, enam el and denti n l ed researchers to explore its us e in denti n conditioning .2 3