amalgamul

Amalgamul este un material utilizat de mai bine de un secol. Amalgamul reprezinta o combinatie intre mercur cu o pulbere ce contine procente variate de argint, staniu, cupru si zinc. Este durabil, usor de utilizat, foarte rezistent la uzura mecanica si relativ ieftin. Avand in vedere culoarea sa metalica se foloseste numai la dintii din spate, din considerente estetice. Desi amalgamul dentar continua sa fie sigur si un material folosit pe scara larga, exista numeroase avertizari legate de toxicitatea mercurului, dar acestea se elimina din obturatie sub forma de vapori in cantitati minime.

Dezavantajele amalgamului: sensibilitate pe termen scurt la rece sau cald dupa aplicarea obturatiei din amalgam. Aspectul metalic impiedica utilizarea sa in zona vizibila a arcadelor dentare. Nu in ultimul rand, pentru plasarea unei obturatii din amalgam, medicul dentist este nevoit sa indeparteze mai multa substanta dentara pentru a crea retentivitati in scopul ancorarii obturatiei. In timp, amalgamul poate colora tesutul dentar din jurul obturatiei.

Compozitele

Obturatiile din compozit sunt un amestec de umplutura de sticla sau cuart cu un mediu rasinos. Aceste materiale asigura o buna durabilitate si rezistenta la fractura in cazul utilizarii la obturatii cu dimensiuni mici sau medii, expuse unor forte masticatorii moderate. Medicul stomatolog nu este nevoit sa creeze retentivitati suplimentare prin indepartarea unor cantitati suplimentare din tesuturile dentare sanatoase pentru ancorarea obturatiei, ci se poate limita la curatarea structurilor cariate. Sunt disponibile sub forma de seturi, ce cuprind mai multe nuante, pentru a putea adapta obturatia la culoarea dintilor naturali ai pacientilor.

Costurile obturatiilor compozite sunt mai ridicate decat in cazul amalgamului, dar depinde si de dimensiunea cavitatii preparate si de tehnica si materialele folosite. Aplicarea obturatiilor din amalgam necesita si mai mult timp si mai multe etape de lucru (demineralizarea peretilor cavitatii preparate cu acid, spalare, uscare, aplicarea adezivului si doar apoi se introduce materialul de obturatie propriu-zis). In timp compozitul isi poate modifica nuanta initiala.

Ionomerii de sticla

Ionomerii de sticla sunt materiale translucente, de culoarea dintelui, formate dintr-un amestec de acizi acrilici si pulbere fina de sticla. Sunt destinati utilizarii indeosebi la obturatiile pe radacina si zona gingivala a coroanei dintelui. Ionomerii de sticla pot elibera cantitati reduse de fluoruri, cu efect benefic asupra structurilor dentare adiacente. Nici in acest caz nu este nevoie de indepartarea unei cantitati suplimentare de substanta dentara, intrucat materialul adera chimic de dinte.

Ionomerii de sticla sunt folositi in primul rnad in regiunile neexpuse fortelor masticatorii puternice, intrucat preiznta o rezistenta mai mica la uzura si fractura.

Rasinile ionomericeContin de asemenea umplutura de sticla, alaturi de acizi acrilici si rasini acrilice. Se folosesc la obturatii mici, in zone lipsite de forte masticatorii semnificative (ex. interdentar), pe suporafetele radacinilor si prezinta o rezistenta moderata la fractura.

Intrucat niciun material de obturatie nu este perfect si in timp obturatiile de pot fisura, fractura, colora sau disloca, este recomandata verificarea integritatii acestora cu ocazia fiecarui control stomatologic periodic (la intervale de cate 6 luni). In cazul in care o obturatie se fractureaza sau se elimina complet din dinte, este indicat sa va contactati imediat medicul stomatolog pentru a cere o programare in vederea tratarii dintelui afectat.

TIPURI DE GLASS-IONOMERI UTILIZATI IN SIGILARI

ISTORIC

Glass-ionomerii au fost introdusi pentru prima data de Wilson si Kent in 1972, au aparut in Europa in 1975,devenind disponibili in Statele Unite in 1977.Primul glass-ionomer comercial a fost facut de De Trey Company ,distribuit de Amalgamated Dental Co in Anglia si de Caulk in Statele Unite.

Cunoscuti ca ASPA( Alumino-Silicate-Poly-Acrylate), ei constand dintr-o pulbere de fluoroaluminosilicat de calciu cu particule de sticla si o solutie apoasa a unui copolymer al acidului acrylic. ASPA erau recomandati atunci pentru restaurarea cavitatilor de clasa a V-a dar le lipseau aspectul fizionomic placut si transluciditatea.

Glass-ionomerii sunt hibrizi rezultati din cimenturi silicate si cimenturi policarb.Intentia era sa se produca un ciment avand atat caracteristicile cimenturilor silicate( transluciditate si eliberare de fluor) cat si cele ale cimenturilor policarbabilitatea de a se lega chimic la structura dintelui neiritant pentru pulpa).

COMPOZITIE

PULBEREA DE STICLA - compozitia apra sticlei fluoroaluminosilicat de calciu , care reprezinta componenta de baza a pulberii cimentului glasionomer.

Aceste pulberi erau amestecate si topite (la temperaturi mai mari de 1300sC timp de 2 ore) cu un flux de fluoruri care serveau la reducerea temperaturii de topire.Sticla topita era apoi turnata intr-o tava de otel.Pentru a o fragmenta ,masa era pusa in apa si fragmentele rezultate erau zdrobite,cernute si transformate in pulbere. Particulele erau apoi trecute printr-o sita pentru a le separa dupa marime.

Marimea particulelor varia dupa producator ,aceasta fiind de la 20 microni pentru anumiti lineri pana la 50 microni pentru materiale restaurative.Pentru cimentare particulele de sticla au intre 13 si 19 microni. Pulberea contine fluoruri in concentratie de 10% pana la 23% rezultate din fluoruri de calciu, fluoruri de sodium si de aluminiu.Fluxul de fluoruri contribuie de asemenea la concentratia finala de fluor.

LICHIDUL – contine o solutie 40-cu 2:1 copolimer acid acrilic- acid itaconic in apa sau copolimer acid maleic-acid acrilic.Folosirea copolimerilor imbunatateste pastrarea compusilor,in comparatie cu solutia apoasa de acid poliacrilic folosita initial,solutie ce avea tendinta sa devina vascoasa relativ repede.

Acidul tartric - se gaseste de asemenea in lichid si este un component foarte important al GICs controland reactia de priza prin izomerii optici active.Aceasta va stimula extractia de ionilor din pulberea de sticla , va mentine neschimbat timpul de lucru si va scurta timpul de priza. Permite de asemenea utilizarea unei sticle cu continut scazut de fluor, care este mai translucida( imbunatatind astfel proprietatile fizionomice ale cimentului).

Cimentul anhidru – "anhidru" este un termen impropriu, intrucat glass-ionomerii sunt cimenturi a caror componenta de baza este apa. Totusi acidul poliacrilic poate fi uscat prin vidare si incorporate in pulberea de sticla , lichidul fiind apa sau solutie apoasa diluata de acid tartric. Prin amestecarea celor doua componente rezulta un ciment cu o vascozitate relative scazuta , indicat mai ales pentru cimentare sau ca obturatie de baza (lineri).

Clasificarea GICs dupa modul de utilizare:

Tipul I – Glass-ionomeri de cimentare

indicatii –cimentarea coroanelor, puntilor, incrustatiilor, dispozitivelor ortodontice

viteza de priza- priza rapida

raport pulbere/lichid- 1,5:1

grosimea filmului – mai mica sau egala cu 20µ

Tipul II - Glass-ionomeri de restaurare

Tip II.1.- Restaurari fizionomice

indicatii - restaurari fizionomice

viteza de priza – autopolimerizabil –rezistenta scazuta la absorbtia si pierderea apei

cu adaos de rasini- priza rapida, rezistenta imediata la absorbtia apei

raport pulbere/lichid – 3:1 sau mai mare

radioopacitate- majoritatea materialelor

Tip II.2.- Restaurari armate

indicatii – unde sunt necesare proprietati fizice crescute iar fizionomia nu este importanta

viteza de priza – priza rapida

raport pulbere/lichid – 3:1 sau mai mare

radioopacitate – intotdeauna

Tipul III- Lineri sau obturatie de baza

- Lineri

indicatii – in sectiune subtire, ca izolare termica sub restaurarile metalice

viteza de priza – priza rapida

raport pulbere/lichid- 1,5:1

- Obturatie de baza

indicatii – in combinatie cu rasini composite, in tehnica laminarii

viteza de priza – priza rapida

raport pulbere/lichid – 3:1 sau mai mare

radioopacitate – intotdeauna

Forme hibride recent introduse:

cimenturi glass-ionomere(GICs) – materiale care contin particule de sticla ce se descompun sub actiune acida si acid solubil in apa care face priza printr-o reactie de neutralizare care poate aparea si la intuneric( Ketac-Fil, Fuji Ionomer Type II)

rasini glass-ionomer modificate(RMGICs) – materiale care au componentele de mai sus, dar modificate prin adaosul unei mici cantitati de rasina – HEMA(hidrde priza este partial o reactie acid-baza si partial o polimerizare fotochimica. La unele materiale polimerizarea rasinii poate necesita o initiere chimica.

Exista diferente si intre RMGICs si conventionalul GICs. Modul de eliberare a fluorului din materiale este cam acelasi, cea mai mare parte din fluor fiind eliberata in primele zile sau chiar saptamani,apoi nivelul scazand pentru un timp indelungat. RMGICs elibereaza cam aceeasi cantitate de fluor ca si GICs,dar ele sunt utilizate in leziunile carioase recurente.Rezistenta RMGICs este mai mica comparativ cu GICs,probabil datorita diferentelor de structura.Ambele tipuri de materiale prezinta o imbunatatire a rezistentei in timp,bazata pe reactiile acid/baza ale glassinomerilor.Datorita proprietatilor fizice Photac.Fil este mai asemanator cu cimenturile glassionomere decat Fuji II LC si Vitremer.

rasini composite cu modificari ale poliacidului(PAMCR) - pot contine una sau mai multe componente glassionomere dar nu pot prezenta o reactie acid / baza. Exemple de astfel de rasini sunt : Dyract AP, Compoglass F , Hytac Aplitip, F2000,Elan.

GICs si RMGICs se aseamana prin capacitatea lor de a capta si a elibera fluorul mai tarziu. Numai Dyract are o capacitate mai scazuta de capturare si eliberare a fluorului.Un studiu recent arata ca Dyract si Compoglass asigura o protectie anticarie mai mica decat GICs.

diferente : captare si eliberare fluor

GICs > RMGICs > PAMCRs

rezistenta la uzura

PAMCRs > GICs> RMGICs

rezistenta

PAMCs > RMGICs> GICs

usurinta in utilizare

PAMCRs > RMGICs > GICs

finisare si estetica

PAMCs > RMGICs > GICs

Proprietatile chimice si fizice ale GICs

Constituienti : compozitia prafului si lichidului variaza de la producator la producator. Se recomanda ca pulberile si lichidul sa nu se foloseasca in alte combinatii.

Reactii chimice : reactia cadru este initiata in momentul amestecarii pulberii cu lichidul ,avand trei etape care se suprapun intre ele :

faza I – cand pulberea si lichidul sunt amestecate,se elibereaza ioni de hidrogen prin ionizarea acidului poliacrilic in apa. Acesti ioni ataca marginile particulelor de sticla care stimuleaza eliberarea ionilor de Ca , aluminiu si fluor cu formarea unei baze hidrogel-silicat in jurul particulelor de sticla.

faza II – in faza a doua ionii de calciu si aluminiu migreaza de la hidrogel silicat in cimentul moale, pH-ul creste, si se precipita inafara ca policarbiar cimentul se intareste. Calciul policarbse formeaza primul din mai multe motive: sunt eliberati intr-o cantitate mai mare sub actiunea ionilor de hidrogen,deoarece atacul asupra particulelor de sticla se produce la nivelul situsurilor pentru calciu. Calciul este bivalent si astfel poate migra mai usor in faza de ciment moale. Calciul nu formeaza complexe stabile cu ionii de fluor asa cum face cu cei de aluminium, ceea ce inseamna ca se leaga imediat la polianioni. Policarbde Ca se formeaza in primele 5 min. in timp ce policarbde Al se formeaza in 24 de ore deci, rezulta ca cimentul are la inceput calitati fizice reduse. Aceste calitati se imbunatatesc pe masura ce se formeaza aluminium policarbIonii de fluor eliberati de particulele de sticla odata cu ionii de calciu si aluminiu nu iau parte la formarea matricii dar raman in ea.

faza III- se produce o hidratare lenta a bazei hidrogel-silicat si policarbcare are ca rezultat imbunatatirea calitatilor fizice.Aceasta faza poate dura cateva luni.

Doua rezultate clinice rezultate din acest lant de reactii sunt importante :ca proprietatile fizice ale cimenturilor GI se formeaza intr-o perioada lunga de timp datorita timpului de intarire si datorita faptului ca aceste cimunturi sunt sensibile la contaminarea cu saliva si la desicare pentru particulele glass sunt acoperite cu hidrogel-silicat.

Proprietati fizice :

GICs pot fi considerate ca fiind materiale cu duritate medie,fragile,cu o putere de comprezie relativ mare,putin reziste la facturi,putin flexibile si rezistente la uzura,de aceea nu sunt recomandate la restaurarea dintilor din zonele cu forte mari de masticatie.

Proprietatile fizice se dezvolta incet: de exemplu forta de comprezie a GIC II creste intr-o perioada de un an,se dilata in mediu umed si se contracta in mediu uscat,si au o buna stabilitate coloristica.

Coeficientul termic de dilatare este de 0,8 iar cel de difuzie termica este apracelasi cu cel al dentinei.GICs prezinta o rezistenta la uzura de 10 ori mai mare decat rasinile compozite.Fortele de tensiune reprezinta 10% din fortele de compresie,dar sunt mai mari in comparatie cu fortele cimentului cu fosfat de zinc.Elasticitatea reprezinta ½ din cea a cimentului cu fosfat de zinc.

Eliberarea de fluor

GICs contine fluor in proportie de 10 – 23%,fluorul localizandu-se in principal in particulele " glass" dar poate fi gasit si in matrice .

Fluorul este eliberat sub forma de fluorura de Na care nu participa la formarea matricii,dar eliberarea nu duce la reducerea calitatilor fizice.Eliberarea este mare imediat dupa preparatie si scade dupa o perioada de timp.Eliberarea este masiva in primele 24-48 de ore dupa care urmeaza o scadere rapida.Initial este eliberat fluorul de la suprafata,apoi cel din straturile profunde ale materialului.

Fluorul a fost gasit la 7,5 mm de marginea restauratiei cu GIC tip II.Dupa 3 saptamani s-a observat ca GICs elibereaza de 2,5 ori mai mult fluor in comparatie cu cimentul silicat.

Cantitatea de fluor eliberata scade odata cu scaderea pH-ului.

GICs in saliva artificiala elibereaza mai putin fluor decat in apa neionizata.

Studiile au evidentiat ca GICs functioneaza ca un sistem eliberare-captare a fluorului.Introdus " in vitro " intr-un gel cu fluor,GICs se incarca cu o mare cantitate de fluor pe care o elibereaza apoi fragmentat. Eliberarea este mai mare la GICs de tip II decat la cel de tip I datorita faptului ca pulberea din amestec este mai mare cantitativ la tip II ,deci contine o cantitate mai mare de particule de sticla care vor elibera o cantitate mai mare de fluor. Amestecul realizat manual elibereaza mai putin fluor decat cel realizat mecanic.Aplicarea unui sigilant reduce cantitatea de fluor eliberata.

GICs reduce solubilitatea smaltului cu 52%.

Fluorul eliberat din GIC reduce incidenta cariei,dar exista dovezi ca ar actiona si in cariile secundare.

Glasionomerii s-au dovedit a fi sigilanti foarte eficienti pentru fisurile deschise, desi nu s-au publicat multe studii pe termen lung referitoare la ei. Valoarea lor consta in adeziunea la smalt prin schimb ionic si, in plus, in eliberarea continua de fluor. Au aceleasi limite ca si rasinile, intrucat nu curg dincolo de punctual unde fisura este mai ingusta de 200µm.

Pentru o inserare corecta, suprafata dintelui trebuie conditionata cu acid poliacrilic 10% timp de 10 secunde, apoi spalata bine si uscata cu grija. Astfel se indeparteaza placa bacteriana si se scade tensiunea superficiala a smaltului, permitandu-se o buna adeziune si o adaptare corecta a materialului. Un ciment cu priza rapida de tipul II restaurativ, autopolimerizabil sau cu adios de rasini, trebuie realizat cu un raport crescut de pulbere / lichid si lasat sa curga in fisura. Se poate aplica degetul (in manusa si usor lubrifiat) peste cimentul autopolimerizabil pentru a asigura o adaptare completa in profunzimea fisurii. Se tine degetul pana la priza cimentului si apoi se modeleaza cu atentie pentru a nu deshidrata cimentul. Un glasionomer cu adaosde rasini este mai fluid si va curge mai usor in fisura, fara presiune, analog rasinilor.

In timp, rasinile composite sau glasionomerii ar putea fi indepartati ca rezultat al stresului ocluzal dar, datorita calitatilor cementului , va ramane un reziduu care va elibera fluoruri, sigiland fisura inca multi ani. Chiar daca pare pierduta, rezistenta la carie va ramane inca semnificativa.

Tipuri de rasini utilizate in sigilari

 

Sigilarea reprezinta o metoda de imunizare a suprefetelor dure dentare impotriva cariei. Materialele utilizate in sigilarea santurilor si fosetelor sunt:

a. Glasionomeri conventionali (GIC)b. Glasionomeri modificati cu rasini (RMGI)

c. Rasini compozite cu sau fara umplere (RC)

d. Compomeri (PMCR,s)

 

A. Ionomeri de sticla.

B. Ionomeri de sticla reincarcabili.

C. Rasini modificate.

D. Compomeri (ionomeri de sticla hibrizi).

E. Suspensii de rasini-ionomer.

F. Rasini compozite.

A. Glassionomerii

Cimenturile conventionale din (GI) glasionomei sunt alcatuite dintr-o sticla esentiala si un polimer acid hidrosolubil, care fac priza printr-o reactie acido-bazica in prezenta apei. Elesunt folosite ca agenti de adeziune, garnituri, baze sau materiale restaurative. Sunt originare din Europa si nu au fost niciodata acceptate in U.S. probabil din cauza ca necesita noi tehnici medicale pentru a putea fi folosite la nivel optim.Sunt mai putin rezistenti si mai putini estetici decat cele mai noi rasini compozite cu macroumplutura si microumplutura introduse in acelasi timp.

Vechile materiale conventionale din glassionomeri erau sensibile din punct de vedere tehnic, priza era intarziata, erau considerabil opace dupa realizarea prizei si sensibile atat la uscare cat si la umezeala (hidratare) in timpul prizei. Aceasta ducea la pierderea precoce de material de pe suprafata.

Toate aceste probleme au fost evitate la noile materiale. Materialele moderne realizeaza o priza mult mai rapida, sunt mai estetice si problemele de hidratare si sensitivitate au fost mult reduse. Oricum spre deosebire de compozite, ele nu er trebui folosite in restaurarile aszpra carora se exercita un stres crescut.

Caracteristici ale glassionomerilor conventionali:

Formeaza o substanta dura pe priza Reactie scazuta exotermicade autovindecare

Se resorb mai putin decat rasinile polimerizate

Coeficient de dilatare termica similar cu structura dentara.

Absenta monomerilor liberi

Stabilitate dimensionala la umiditate crescuta

Legaturi chimice de umplutura

Rezistente la microscurgei

Biocompatibilitate

Integritate marginala

Adera chimic la smalt si dentina in prezenta umezelii

Eliberarea de fluor inhiba infiltrarea microbiana

Rezistenta crescuta la slefuire

Estetica obisnuita scazuta.

Indicatii de utilizare:

Fixarea protezelor unidentare si a puntilor; Obturatii de baza (in general);

Obturatii de baza sub materiale compozite (sandwich);

Obturarea dintilor temporari;

Refacerea (reetansarea) inchiderilor marginale a obturatiilor vechi;

Reconstituirea leziunilor odontale coronare de clasa a II, III, V (eroziuni cervicale, abrazii);

Sigilarea santurilor si fosetelor;

Sigilari largite;

In diferite tehnici de colaj care apeleaza la componente metalice;

Obturarea canalelor radiculare;

Obturatii retrograde in chirurgia parodontiului apical;

Imobilizari adezive;

Chirurgia O.R.L. (implante de cohlee);

Chirurgia ortopedica (fixarea jonturilor articulatiilor).

 

C.Glassionomeri modificati cu rasini (RMGI)

RMGI’s se mai numesc rasini glassionomere, dar aceasta denumire este tehnic incorecta deoarece ei au fost la origine glassionomeri si apoi au fost modificati.

Glassionomeri modificati cu rasini sunt materiale in care o rasina polimerizabila este adaugata la matricea glassionomerului. Introdusi in 1991, ei reprezinta o incercare de a rezolva o parte din problemele pe care le provoacau glassionemerii. Aceste materiale au imbunatatit estetica, proprietatile fizice, au imbunatatit priza si au mai putine probleme de uscare si hidratare. Cum materialele glassionomere nu realizeaza adeziunea RMGI au o mai mare putere de adeziune la structura dintelui cand aplicarea este precedata de gravarea acida. Pe langa asta rasina poate forma o legatura chimica cu structura dintelui. Nu este cunoscut daca acest lucru are vreo importanta clinica.

Aceste materiale hibride fac priza in parte printr-o rectie acido-bazica a GIC si o polimerizare a componentei rasinoase a matricei.

Componentele rasinoase pot fi supuse tratamentului cu lumina sau tratamentului chimic. Prizarea se realizeaza prin adaugarea unei rasini-monomer hidrosolubile, cum ar fi HEMA, in lichidului hidrosolubil al acidului poliacrilic. Un initiator chimic si sau fotoinitiator determina reactia prin care se realizeaza priza.

Termenul light-cured sau dual-cured nu inseamna ca intreaga reactie de prizare este fotoinitiala. O portiune a procesului de prizare poate implica de asemenea procesul tipic acido-bazic intre materialul de umplutura si matricea poliacida.

 

C. Compomeri

Compomerii sunt mai corect numiti rasini compozite acide poliacrilice modificate sau PMCR’s. aceste materiale sunt moi, nelipicioase, nu necesita amestecare si sunt usor de folosit. Sunt o incercare dea combina cele mai bune proprietati ale glassionomerilor si a rasinilor compozite. Introduse in 1993, ele reprezinta acum 15% din totalitatea materialelor dentare vandute in intreaga lume. Folosirea lor in U.S. este mai scazuta.Sunt usor de introdus intr-o cavitate, usor de modelat, rapid de tratat, pot fi gravate si lustruite. Ele inlocuiesc rasinile compozite in restaurarile anterioare proximalesi glassionomerii in restaurarile cervicale. In aproape toate celelalte arii, compozitele si glassionomeriisunt preferati.

Compomerii au adeziune buna la structura dintelui. Sunt facuti dintr-o rasina dimetacrilata acida care poate intra intr-o reactie acido-bazica cu o pudra de glassionomer care poate fi amestecata cu un compozit conventional. Functioneaza prin absorbtia apei care mareste in timp restaurarea. Aceasta apa absorbita poate cauza apoi o reactie acido-bazica intre lanturile care reprezinta aproximativ 20% dun cantitatea eliberata de un glassionomer conventional.

Spre deosebire de glassionomeri proprietatile fizice ale compomerilor scad pe masura ce apa se absoarbe. La unele materiale acesta scadere poate fi de 50% fata de un material inferior in ceea ce priveste puterea.

Cu cat compomerul contine mai multe grupari acide carboxil, cu atat matricea devine mai hidrofila si ionica, ceea ce duce la cresterea absorbtiei apei. Spre deosebire de RMGI, cand un compomer absoarbe apa, proprietatile lui fizice scad. In unele cazuri scad chiar cu 30-50%.

 

Compomeri. Avantaje.

Biocompatibilitate; Efect cariostatic, reduce aparitia cariilor sacundare;

Aplicare economica si rapida;

Etape de aplicare reduse prin indepartarea gravajului (optional);

Prezinta proprietati hidrofilice;

Nu necesita izolare perfecta prin aplicarea de diga;

Nu adera de instrumente. Se manipuleaza si se modeleaza usor;

Adeziune performanta la S si D- asigura integritatea marginala a obturatiei; (mecanism de adeziune mecanica si chimica)

Prin prelucrare- suprafata neteda- reduce retentia de PB;

Rezistenta la abrazie buna (prezinta particule mici);

Contractii de priza mai mici decat la compozite.

Indicatii de utilizare:

Lineri ai cavitatilor; Obturatii de baza;

Restaurarea leziunilor odontale coronare din zona anterioara si posterioara a arcadelor dentare clasa I, II, III, V, eroziuni radiculare, defecte cuneiforme;

Sigilarea fisurilor molarilot temporari;

Sigilarea santurilor si fosetelor zonelor care nu sunt supuse stresului ocluzal.

 

F. Rasini compozite.

Aceste materiale se gasesc in medicina dentara de mult timp si proprietatile si caracteristicile lor sunt bine cunoscute si apreciat. In ultimii ani aceste materiale au fost imbogatite prin utilizarea unor particule de umplere mai mici. Aceste materialesunt durabile, foarte estetice, pot fi foarte mult lustruite, slefuite, culoarea lor este stabila, sunt usor de modelat, rezistente la uzura, la rupere si dusponibile in anumite nuante. Proprietatile lor de manipulare sunt cele mai elastice dintre toate sistemele restaurative.

Au si dezavantaje care constau in faptul ca se micsoreaza ca volum atunci cand se stabilizeaza si trebuie asezate in straturi atunci candsunt folosite ca materiale restaurative. Din acest motiv plasarea lor este sensibila din punct de vedere tehnic. In orice caz, majoritatea practicienilor au invatat sa lucreze cu aceste materiale si acum pot fi utilizate cu usurinta.

Performanta clinica a rasinilor compozite a fost excelenta, mai ales daca o comparam cu sistemele restaurative precedente. In ceea ce priveste adeziunea la smalt, este ceea ce poate medicina dentara sa ofere cel mai bun. Adeziunea la dentina este reprezentata de conceptul “totul sau nimic”. Daca cavitatea nu este bine inchisa sau este sparta poate aparea rapid caria recurenta din moment ce aceste materiale nu au proprietati de inhibare a aparitiei cariilor.

Chiar daca inchiderea dentinei poate sa se realizeze cu succes in laborator, in utilizarea clinica multi factori necontrolabili pot duce la apritia unor rezultate mai putin optime.

Din cauza acestor limitari ale compozitelor s-au facut multe eforturi pentru a dezvolta sisteme alternative care au efecte benefice bioterapeutice si anticarie.

Folosirea combinata a GIC, drept captuseala ti baza sub rasinile compozite a fost indelung utilizata pentru a rezolva probleme clinice. Desi combinatia a fost mult timp utila, acum este depasita iar cerintele tehnice sunt mai mari.

 

Indicatii de utilizare:

Reconstituiri coronare ale grupului frontal si lateral; Sigilarea santurilor si fosetelor;

Confectionarea fatetelor vestibulare si incrustatiilor;

Refacerea componentei fizionomice a protezelor;

Fixarea puntilor adezive, inlay-uri si fatete vestibulare din ceramica si compozit, brackets-urilor;

Proteze fixe peovizorii;

Adezivi amelo-dentinari;

Reconstituirea bonturilor;

Obturatii provizorii;

Imobilizari adezive.

Concluzii: nu este posibil sa faci recomandari generale pentru folosirea uneia dintre clasele de materiale in defavoarea alteia in toate cazurile. Fiecare are utilitatea ei.

Coafajul direct pulpar o procedurå recomandabilå

Coafajul direct se practicå in scopul conservårii vitalitåtii pulpare si constå in aplicarea pe pulpa expuså a unui pansament reprezentat de hidroxidul de calciu, material dovedit a fi eficient in timp. Tehnica ce foloseste demineralizarea totalå (total etch) este mult mai recentå dar, si aceasta, cu rezultate satisfåcåtoare pe termen lung.Pentru succesul coafajului pulpar este necesar ca dintele implicat så fie asimptomatic sau cu simptomatologie minimå. De asemenea, hemostaza este absolut necesarå.

Hemostaza

Precede coafajul pulpar, fiind obligatorie. Dupå irigarea pulpei cu ser fiziologic steril, se usucå cu bulete de vatå. Sunt recunoscute trei metode de hemostazå: aplicarea de bulete de vatå imbibate in apå oxigenatå 3% sau hipoclorit de sodiu 2,5% sau folosirea de agenti hemostatici ca trombina.In cazul in care sangerarea continuå se va considera tratamentul endodontic al dintelui.

Hidroxidul de calciu

Se aplicå pe podeaua uscatå a camerei pulpare, uniform si in contact direct cu tesutul pulpar expus (printre alte produse, se pot folosi: Dycal, Life sau Ultradent Calcium Hydroxide).Peste stratul de hidroxid de calciu se aplicå obturatia de bazå cu glass-ionomer ce contine o råsinå iar obturatia provizorie se poate realiza fie cu eugenat de zinc (oxid de zinc-eugenol), fie cu ajutorul unui adeziv dentinar (dentin bonding).

Dacå dintele råmane asimptomatic si isi mentine vitalitatea, peste trei luni se poate aplica obturatia definitivå.

 

RESTAURAREA PREVENTIVA CU RASINI

Restaurarea preventiva cu rasini – RPR- se adreseaza leziunilor carioase incipiente,dar si situatiilor de diagnostic incert de carie.Dezvoltarea continua a materialelor bioadezive faciliteaza indeplinirea obiectivelor acestui tip de tratament in caria dentara:

1. realizarea unor preparatii minime ,cu sacrificiu dentar redus – RESTAURARE =R

2. Inlocuirea extensiei preventive mecanice cu cea chimica,neinvaziva – PREVENTIVA = P

3. Imunizarea suprafetelor ocluzale cu ajutorul rasinilor sigilante – RASINI =R

Abordarea leziunilor carioase in maniera preventiva( sigilari ) sau conservativ –curativa (RPR I,II,III) va fi aplicata dupa stabilirea certitudinii diagnosticului decarie.Coroborarea datelor de diagnostic : clinic ( examinare, inspectie, palpare), paraclinic (radiologic: bite-wing, raper, transluminare, masurarea rezistentei electrice,cariodetectia cu laser) si speciale,va trebui sa reduca sau cel putin sa minimalizeze erorile dediagnostic.

Stomatologii ,cercetatorii din domeniul biomaterialelor si toxicologii au considerat ca materialele de restaurare in leziunile incipiente trebuie sa prezinte urmatoarele calitati:

adezivitate pentru o preparatie minima,conservativa; rezistenta la abrazie asemanatoare smaltului;

blocarea demineralizarii si favorizarea remineralizarii ( eliberarea de fluor);

stabilitatea in timp ( fara contractii de priza si absorbtie de apa) si sub stresul ocluzal( rezistenta la presiune);

biocompatibilitate

radioopacitate mai mare ca smaltul

usurinta in manipulare,inserare;

finisare precisa,fara alterarea structurii dentare restante;

culoare asemanatoare dintelui

Materialele existente,privite individual nu au putut raspunde exigentelor impuse de clinica leziunilor incipiente de aceea modificarea continua a rasinilor compozite,incearca sa depaseasca esecurile legate de compozitia acestora.Astfel se explica necesitatea corelarii stricte intre: diagnosticul clinic,compozitia materialului si modul de utilizare,dupa indicatiile producatorului,cu toate componentele sistemului: acizi,adezivi,primeri,lampa de fotopolimerizare,pene,instrumentar de finisare.

Datorita duritatii crescute,radioopacitatii,compozitele hibride ( umplutura anorganica ultrafina cu diametrul de 0,7mm) sunt preferate pentru restaurarile posterioare( RPR I,II) in locul celor conventionale.Particulele ultrafine maresc gradul de finisare si prelucrare,de maniera similara compozitelor microfilled.Cercetarile actuale se axeaza asupra compozitelor cu nanoparticole ( nanocompozite ) si particule noi de umplutura,pentru micsorarea contractiei de polimerizare.

O descoperire revolutionara ar fi introducerea monomerilor expandabili,fara contractie de priza.Compozitele cu fluor reduc semnificativ demineralizarea smaltului fata de cele nefluorurate.

Glassionomerii sunt o categorie relativ noua de materiale,cu aplicabilitate si in stomatologia preventiva.Proprietatile cimenturilor G.I difera in functie de utilizare ( cimentare,restaurare,baza) si depind de raportul pulbere-lichid si timpul scurs de la prepararea materialului.Daca G.I conventionali prezentau inchidere marginala buna,dar rezistenta mecanica scazuta( cu utilitate limitata in zonele ferite de stres ocluzal),modificarea compozitiei a determinat implicit si modificarea mecanismelor de priza si a calitatilor.Proprietatile mecanice ale materialelor hibride( PAMR sau RMGI) sunt imbunatatite considerabil fata de G.I conventionali,in dauna eliberarii de fluor , care este mult mai redusa.

Pentru a nu limita utilizarea G.I in RPR II,III se aplica peste G.I un sigilant,necesitatea aplicarii sigilantului fiind justificata de porozitatea crescuta a materialului( asemanatoare cu a compozitelor conventionale) si microfisurile care apar datorita tensiunilor interne in momentul prizei.Raportul pulbere lichid conditioneaza aspectul suprafetei.

Cementurile G.I armate ( G.I CERMET:Ketac-silver,Chelon-Silver) prin proprietatile mecanice,superioare fata de celelate G.I.,rezista la abraziune,duritate,pot fi utilizate in cazurile in care G.I. conventionale nu au reusit sa se impuna sau au dat rezultate mai slabe (RPR III- leziuni multiple,separate prin punti de smalt sanatos).Dupa intarire si prelucarae suprafetele cermet au aspect metalic,fara a atinge luciul acestora,insa mai apropiat de aspectul smaltului fata de amalgam.

RMGI ( galssionomeri –rasini modificate) sunt cementuri G.I. carora li s-a adaugat o rasina care permite usurarea aplicarii;utilizand lumina sau un catalizator chimic,se initiaza reactia acid-baza a glassionomerului.Astfel,aceste materiale se vor aplica cu usurinta si fara a modifica proprietatile esentiale ale glassionomerului.

PAMR(rasini compozite poliacid modificate) au un continut mai mare de rasina si reactia acid-baza a glassionomerului nu mai are loc.Desi aceste materiale sunt usor de utilizat( sunt predozate in capsule),exista nelamuriri in ceea ce prezinta beneficiile in timp,comparativ cu rasinile compozite conventionale.

Ormocerii resprezinta o clasa noua de materiale,care vor trebui sa anuleze dezavantajele celor existente,apropiindu-se de dezideratele materialului ideal.In. laborator ormocerii se pot compara cu silicatii si polimerii organici.Experimentele arata ca prezinta o uzura mai mica ca la compozite si lipsa toxicitatii.Sub forma de pasta,usor de manipulat,polimerizarea va fi indusa termic( auto) sau fotochimic.Rezistenta la uzura,lipsa toxicitatii si proprietatile reologice( de curgere) vor justifica indicatiile acestor materiale in sigilari largite( fisuri in palnie) si RPR II si II.

Proprietatile fizice si mecanice impun stabilirea unor noi principii de tratament:

forma de acces: trebuie sa fie mult mai conservativa,se abordeaza fiecare leziune in parte si se pastreaza puntile de smalt integre.

managementul leziunii: se indeparteaza numai dentina ramolita

forma de adeziune: caracteristici care asigura adeziunea,etanseitatea,rezistenta la dislocare

curatirea cavitatii ( mecanica si chimica): mecanic, cu instrumentar rotativ si chimic,prin sisteme primer si conditioner specifice.;acestea indeparteaza debriurile rezultate in urna preparatiei,impreuna cu smear-layerul si biofilmele incluzand microorganismele reziduale.

Atitudinea corecta in ceea ce priveste restaurarile preventive este data de corelatia dintre situatia clinica si dotarea materiala de care dispunem.Compromisule sau extinderea nejustificata in utilizarea acestor tehnici vor afecta principiul preventiv in sine.

 

 

Tehnica de demineralizare totalå (total etch)

Dupå realizarea hemostazei, se aplicå pe podeaua camerei pulpare o pastå pe bazå de hidroxid de calciu care nu se intåreste (de ex. Pulpdent) si se continuå prepararea cavitåtii.Dupå dezinfectarea acesteia se procedeazå la demineralizarea smaltului si a dentinei (total etch) cu ajutorul acidului fosforic 32% care se mentine timp de 15 secunde. Apoi, cavitatea se spalå si se usucå cu aer.Se aplicå adezivul dentinar: cateva straturi succesive de primer hidrofil urmate de fotopolimerizarea in douå straturi a adezivului aplicat pe tesutul pulpar expus (pentru a preveni

intruzia materialului de obturatie), dentinå si smalt. In continuare, se aplicå obturatia de bazå cu glass-ionomer iar obturatia provizorie se realizeazå prin metodele clasice acceptate.

In cazurile in care metoda de conservare a vitalitåtii pulpare prin coafaj esueazå, råspunderea apartine medicului stomatolog: contaminare microbianå, resturi de dentinå pe suprafata expuså a pulpei, sigilarea neadecvatå a dentinei, etc.

Ambele tehnici folosite pentru coafajul direct al pulpei sunt eficiente, desi experienta cu hidroxid de calciu este mai indelungatå.Coafajul pulpar va fi mai eficient in cazurile de expunere mecanicå a pulpei (deschidere accidentalå) decat in cazurile de expunere prin evolutia proceselor carioase.

Semnificatia clinicå

Procentul de succes prin metoda de coafaj direct pulpar este foarte ridicat, ambele tehnici (hidroxid de calciu sau total etch) avand acelasi prognostic favorabil.

Szockton LW: Vital pulp capping: a worthwhile procedure.J Can Dent Assoc 65: 328, 1999

Dental Composites (an overview)

A composite is any material that is composed of hard, pebble-like filler particles similar to sand or pebbles, surrounded by a hard matrix of a second material which binds the filler particles together.  The filler particles can be any coarseness varying from large rocks to microscopically fine powder or virtually any shape varying from spherical through fibers to flakes.  The matrix material generally starts out as a paste or liquid and begins to harden when it is activated, either by adding a catalyst (which may be mixed with the filler particles), or by adding water or another solvent to allow chemical reactions to take place.

Before it hardens, it can be pressed into a mold, or stuffed into a hole.  The most commonly understood composite material is concrete, or "Portland cement".  It is composed of sand, sometimes mixed with pebbles, bound together by a matrix of lime, alumina and Iron.  This material can be formed into bricks, poured into molds, or used to "cement" iron rods into the ground. Composites are an increasingly important part of everyday life, from wooden particle board to Corian® countertops.

The image on the right shows the microscopic structure of a typical composite material.  The filler particles are the darker, irregular granules.  The matrix is the lighter

material that surrounds them.  This particular composite is not highly "filled", which means that there is

a low density of filler particles compared to the amount of matrix material.  Compare that with the micrograph on the left.  This shows another composite material with differently shaped filler particles which are much more closely packed together.  This is a " highly filled" composite.  Because the characteristics and relative volumes of both the matrix materials and the various filler particles can be manipulated by the manufacturer of the composite, it is obvious that these materials show an almost infinite range of physical properties.   

In dentistry, The material commonly called "composite" is made of an acrylic matrix called BIS-GMA mixed with a finely ground glass particle filler.  The acrylic will harden with the addition of a catalyst, similar to the way fiber-glass hardens.  In the case of light cured composites, the catalyst is already mixed into the paste, but does not become active until illuminated with a strong light.  To ensure bonding between the filler and the matrix, the filler particles are coated with a silane-coupling agent that contain a methacrylic group able to co-polymerize with the matrix-forming dimethacrylate monomers and functional groups able to interact with the filler.

Dental amalgam is also a composite, although it is not customary to refer to it as such.  It is made up of finely ground silver/tin metal powder mixed with mercury.  The mercury dissolves the outside layers of the metal powder particles to form a matrix of silver-tin-mercury which hardens around the unreacted metal powder particles to form the finished amalgam composite.  For much more on dental amalgam, please click here.

Dental cements are all composite materials made from different  powders mixed with different liquids.  The liquid partially dissolves the powder particles and forms a matrix which becomes hard enough to act as a "glue" and is used to cement Crowns and Posts.  All non

metallic composite filling materials are really just more highly filled versions of their respective cements.

Porcelain is not generally thought of as a composite material, but it is in fact composed of a glass matrix filled with crystalline particles.  While ceramics are an extremely important part of dentistry, very few dental professionals really understand them.  For this reason, I have written a Beginners course in dental ceramics to help fill this void.

What is Bonding, and how is it done?

Prior to the age of bonding, dental restorations (fillings, crowns, onlays etc.) had to be attached to teeth mechanically.  This is still done in the case of most fillings by the use of undercuts placed inside the cavity preparation (the "hole" in the tooth). The filling material is condensed into the cavity preparation so that it flows into the undercuts.  When hardened, the filling will not be able to dislodge because it is larger at the bottom of

the hole than it is at the top.  When placing a cast restoration such as a crown or an inlay, there can be no undercuts.  Otherwise, the casting would not be able to seat.  The vertical walls of the preparation are made nearly parallel, usually slightly tapered.  The space between the restoration and the tooth is filled with a waterproof cement such as zinc phosphate which hardens and "locks" the restoration onto or into the tooth.   The cement flows into the tiny imperfections in the sides of both the preparation and the restoration and acts as a "lock and key" to keep the restoration from sliding out or

Bonding is a different process entirely.  Restorations that are bonded "stick" to the tooth without the aid of undercuts or "lock and key" cementation.  There are four types of bonding used in dentistry today.

Acid etch enamel conditioning 

In this technique, a 10% solution of phosphoric acid is placed on the enamel portions of the tooth and left in place for fifteen seconds.  When it is washed off, the formerly shiny enamel surface now looks like it is chalky, or frosted.  Under a microscope, the surface looks like a ragged landscape of jagged mountains and valleys (see micrograph to the right).  These microscopic

irregularities are then filled with a liquid acrylic plastic which hardens in place.  Since the filling material is composed of the same sort of plastic, mixed with glass particles (see filled resins below) it will bond onto the plastic which becomes mechanically adhered to the conditioned enamel.  Click the image to learn more about the structure of enamel

Dentinal bonding

The micrograph on the left shows what dentin looks like when it is sliced perpendicularly to the dentinal tubules.  The tubule openings are clearly visible, but the hard material between them is still fairly smooth and will not bond to a layer of liquid plastic in the same way as it does to etched enamel.  Etching the dentin dissolves a small amount of the

hard dentin material around the tubules allowing the strands of collagen that permeate the dentin to project beyond the cut surface, and partially opening up the the tubules (image to the right).  An aqueous solution of 2-hydroxyethyl methacrylate (HEMA)--a hydophylic (water soluble) polymer (plastic)--is applied to the conditioned dentin.  This material flows into the tubules and between the exposed collagen fibers.  This acts as a bridge between the otherwise hydophylic collagen fibers and a subsequent layer of hydrophobic (water insoluble) resin, allowing the resin to thoroughly infiltrate between the collagen fibers.  Once the resin hardens, it serves as the basis of dentinal bonding.   Click either image to learn more about the structure of dentin.

 

Chemical adhesion

Certain materials such as Glass Ionomer, and polycarboxylate cements may be applied directly to unconditioned enamel and dentin.  They are applied in a liquid form, and this liquid is fairly acidic.  Metallic polyalkenoate salts combine with the hydroxyapatite by replacing phosphate ions.  The carboxylic groups of the polyalkenoic chains can chelate (chemically combine with) the calcium of the hydroxyapatite to bond the cement to both dentin and enamel. This cross linking of restorative material and tooth structure gives excellent chemical bonding strength.

  

Amalgam bonding

The bonding of a dental amalgam to a tooth involves any or all three of the above mechanisms to bond a filled resin cement to the tooth structure and a mechanical mechanism to bind the amalgam to the resin.  The enamel and dentin are conditioned with 10% phosphoric acid, HEMA is applied to the dentin for dentinal bonding, and a layer of very loose filled resin is applied over the tooth structure.   Dental amalgam is condensed into the tooth while the resin is still unset.  This causes tags of amalgam and filled resin to intermingle at the interface, and when both materials set, they are securely mechanically locked together.  Thus the amalgam is locked to the resin, and the resin is bonded to the tooth. 

Dental Cements and the composite restorations derived from them

Interestingly, all dental cements, and all tooth colored filling materials are made of combinations of only two different powders ( top row), and four different liquids (left column) .  In most cases, the chemical combination of the various powders with the various liquids creates a  material which begins as a paste and "sets" as a hard cement.  Most of these materials are water soluble during the setting phase, but become waterproof after they become hard.  

Liquid  \/          Powder-->

Zinc Oxide powder Glass powder

Phosphoric AcidZinc Phosphate cement

Silicate Cement and filling material

Polyacrylic acidPolycarboxylate Cement

Glass Ionomer Cement and filling material

BIS-GMA Acrylic  Resin Composite Cement and filling material

Eugenol (oil of clove)ZOE (Zinc oxide and Eugenol cement and filling material)

 

Types of Non metal Composite material

Zinc phosphate cement

Zinc phosphate cement is one of the oldest and most reliable dental materials.  It has been used for at least two hundred years.  It is still used for cementing cast metal crowns and onlays. It is made by mixing a strong solution (37%) of phosphoric acid with zinc oxide powder.  The zinc oxide powder partially dissolves in the acid creating zinc phosphate which when dry is a very hard, waterproof matrix which bonds unreacted zinc oxide particles together.   Mixing and cementing with this material is something of an art since it must be mixed slowly or else it will harden too quickly, and the work must be kept dry until the cement is set or else it will dissolve in saliva or water.  Once set, it is still one of the most reliable and most durable cements for luting (cementing) cast metal crowns and onlays on teeth.  It is also used to cement posts in teeth and was used until quite recently as a base under amalgam fillings.  (A base is a layer of material placed under a filling to protect the nerve from hot and cold while the overlying filling is in service.  Some bases can also be useful as a method of desensitizing the nerve.)  

Zinc oxide has an added benefit since the acidity of the phosphoric acid etches the enamel on the tooth creating the irregular surface seen in the micrograph above.  The cement flows into these irregularities to create a tight mechanical seal with the tooth itself.  It also flows into irregularities in the structure of the casting to form a "lock and key" type of bond between the tooth and casting thus locking it in place.  With the advent of newer cements with a quicker working time and less demanding technique, zinc phosphate is used less and less today.  Note that zinc oxide is an opaque white powder.  While it can be manufactured to be any color, the set material remains perfectly opaque.  For this reason, and the fact that it lacks wear resistance, zinc oxide is not esthetic or tough enough to be used as a "tooth colored" filling restorative.

Polycarboxylate cement

Polycarboxylate cement is a newer innovation than zinc phosphate cement.  In this case, zinc oxide powder is mixed with polyacrylic acid. Sometimes the polyacrylic acid is freeze dried into a powder and mixed with the zinc oxide powder, in which case the powder is mixed with distilled water.  As with zinc phosphate, the zinc oxide dissolves and creates a matrix which eventually becomes quite waterproof, and though not nearly as strong a cement as zinc phosphate, it is much easier to work with, sets much more quickly and is less irritating to the nerve of the tooth.  As with zinc phosphate, the zinc oxide remains opaque and the color of this material is not easily controlled.  It is rarely used as a restorative filling material.  Like zinc phosphate, this cement is somewhat technique sensitive in that it too must be kept dry until it is completely set.

Silicate  and Glass Ionomer Cements

Silicate cement was probably the very first tooth colored filling material (if you discount whalebone). Glass Ionomer restoratives came later.  However, in order to understand silicate cement, and, indeed, in order to understand the characteristics of most modern composites, it is very important to understand the composition and chemistry of the glass powder that gives them their special characteristics.  

Glass is composed of silica (silicon dioxide) which is essentially quartz.  Silica is the chief component in ordinary sand.  The melting temperature of quartz is very high, but it was discovered early in human civilization that the addition of certain metallic oxides could serve to lower the melting point of the glass quite a bit.  These additional components, when added to sand in order to lower the melting temperature are called "fluxes".  When the glass mixture melts, it becomes a liquid with the consistency of syrup on a very cold day.  Glass does not have a specific melting temperature, and when it cools, it remains a "supercooled" liquid (think of a hard candy, like a lollipop), however contrary to mythology, it does not continue to flow at normal temperatures.  A third component of glass is a stabilizer.  Stabilizers make the glass strong and water resistant. Calcium carbonate, (limestone) is a stabilizer. Without a stabilizer, water and humidity attack and dissolve glass. Glass lacking a stabilizer is often called "waterglass" since it can dissolve in water.

When lead is used as the stabilizer, the resulting glass has superior clarity and durability, and will ring like a bell when tapped. It is also fairly insoluble, even in acidic solutions.  Lead is NOT used in dental glass.  The FDA (US food and drug administration) has recommended that lead modified glass not be used to store liquids as small amounts of lead have been known to leach out of the glass and into the liquid.  Historically, lead "crystal" has been used for years in the manufacture of fine tableware including drinking glasses and wine canisters (Reference Waterford crystal).  Lead is not used to flux or stabilize any dental glass manufactured in North America or Europe.

Boron oxide is, like silica, a glass former.  When added to silicon based glass at a minimum of 5% by weight, the glass becomes a borosilicate.  Glass fortified in this way is resistant to mechanical and thermal shock and is used to make baking pans (Pyrex), laboratory ware and sealed beam headlights.

Alumina (aluminum oxide) is found combined with silicon in naturally occurring glasses called feldspars.  It is used in molecular form to toughen the glass and and is also used as a crystalline structure dispersed throughout the  glass that acts as a sort of framework or skeleton.  This "framework" stiffens the glass during firing and makes it less likely to slump.  The inclusion of crystalline structures transforms the glass into porcelain which is much tougher and less

prone to fracture than the same glass without such a matrix.  Alumina is a major component in ordinary clay and is present in nearly all the ceramic products you buy such as the plates and cups in your dinnerware and your mother's bone china.  It is generally added to dental porcelain in the form of aluminum oxide.

The addition of trace metals can give color to the glass. Cobalt imparts a blue color, while gold imparts red and copper a green color.  (These metals are added as oxides, and they generally have fluxing qualities, but they are added in such small amounts that they are not considered fluxes for purposes of calculating glass formulas.)

The addition of zirconium and titanium oxides add opacity to the glass.  These oxides form a crystalline structure within the otherwise translucent glass, and this diffuses light as it penetrates, creating a milky or pure white appearance depending on the amount of zirconium or titanium oxides used.

Fluxes are oxides of alkaline metals such as sodium, potassium, lithium, boron and lead.  They serve to dissolve the silica, a bit like water dissolves sugar.  This is important, since glass is composed of silicon dioxide which has a very high melting temperature. ( Pure quartz melts at 1713 degreed centigrade.  The addition of 25 % sodium oxide can lower the melting temperature to 793 degrees centigrade.)  The most common fluxes used in ceramics are sodium and potassium oxides, but there is a long list of fluxes, each one with its own set of characteristics and uses.

Alumino-Fluoro-Silicate glass

The glass powder that is used in the production of both Silicate cement and Glass Ionomer cement is made from a glass made with Sodium Fluoride and stabilized with minimal alumina.  It is technically known as Alumino-Fluoro-Silicate glass.  This glass is ground into a very fine powder.  While this glass is stabilized to make it insoluble in water, it is formulated to remain partially soluble in very highly acidic solutions.  (It is not soluble in saliva or in any food or liquid that can be consumed by mouth.)  By the use of various trace metals, zirconium, and other components, the glass can be fabricated to match the various colors and opacities of tooth structure.  The major characteristic of this type of glass, however is its ability to partially dissolve and form a hard, waterproof matrix when mixed with either of the two types of acids shown in the table above.  When the powder to liquid ratio is varied correctly, a stiff paste results.  This paste can then be used to fill cavities, and the paste will set in time to form a very hard and insoluble solid.  The hardness, durability and appearance of the resulting restoration is largely dependent on the nature of the chemistry of the matrix formed when the glass particles begin to dissolve in the acidic solution. 

Restorations and cements made with alumino-fluoro-silicate glass have a number of advantages and disadvantages:

Alumino-fluoro-silicate glass cements and restorations bond chemically with both enamel and dentin (and also metalic structures).

o This means that they can be applied directly to clean tooth structure without etching or bonding or even cutting retentive undercuts.

o These materials will also chemically bond to metallic substructures such as gold and base metal crowns and bridges, so they can be used to anchor esthetic facings made of resin composite to these structures.

Alumino-fluoro-silicate glass cements will slowly release fluoride into the adjacent tooth structure.  This converts hydroxyapatite into fluoroapetite, thus strengthening the tooth structure and making it more resistant to decay.

The major disadvantages of restorations  and cements made from unmodified alumino-fluoro-silicate glass are:

o The materials are very water soluble during the setting phases, and if they are allowed to get wet during placement, they can leach out allowing the final restoration to leak.

o They are also not especially resistant to abrasion, and are not suitable as restorations on occlusal or stress bearing areas.

Silicate Cement

Silicate cement is made by mixing a powder made of Alumino-Fluoro-Silicate glass  with a 37% solution of phosphoric acid.  The acid partially dissolves the glass, chemically combining with it, thus creating a very hard and brittle matrix.  A fluid mixture of this cement can serve the same purpose as the zinc phosphate cement described above, however, its main use in dentistry has been as a tooth colored filling material.  While the matrix is very hard, its brittleness and lack of wear resistance limits its use as a restorative in stress bearing areas.  Until the advent of resin composites, silicates were the only tooth colored filling material available, and the only alternative to silver amalgam as a simple (non gold) permanent filling material.  Its use was limited to front teeth, or areas of decay on non stress bearing surfaces of  back teeth.  

Its largest single advantage, other than its color, is that the fluoride from the glass, (which is also a component of the matrix material due to the chemical reactions involved in mixing the powder with the liquid), tended to prevent further decay around the margins of the filling. (In fact, it is a characteristic of all the formulations using an Al-Fl-Si glass/acid combination that the finished restoration continues to leach small amounts of fluoride into the surrounding tooth structure throughout its life.  This is true of glass ionomer restorations as well.)   Its major disadvantage is its appearance.  Real teeth are somewhat translucent.  Silicate cements tend to be lacking in this characteristic.  In addition, the glass particles are prone to dislodging from the surface of the filling leaving a rough surface which is prone to staining.  The brittleness of the matrix is another esthetic difficulty since it causes surface crazing and marginal chipping as the restoration ages and creating more potential places for stains to lodge.  

Glass Ionomer  (polyalkenoate cement)

Glass Ionomer cements and restoratives (filling materials) are a fairly recent advent in dentistry.  While Silicate cements have been around for years, Glass Ionomer had to await the invention of poly-acrylic acid.  The mixture of poly-acrylic acid with Alumino-Fluoro-Silicate glass causes a partial dissolving of the glass particles.  The poly-acrylic acid chemically combines with the dissolved glass components and produces a hard matrix material similar to that in silicate cement.  (This is essentially an acid-base reaction resulting in the formation of a "metallic polyalkenoate salt" which precipitates and begins to gel until the cement sets hard.)  The characteristics of this matrix material, however, are strikingly different than

the characteristics of the matrix found in silicate cements. Unlike silicates, the matrix is reasonably translucent allowing the color of the glass particles to dominate the esthetics.  It is also much less brittle than the matrix of Silicate cement making it a bit less prone to fracturing over time.  Since the filler is a glass, its esthetics can be precisely controlled.  The less brittle matrix means that the margins and surface of the restoration are less prone to chipping and crazing so there is much less staining with Glass Ionomer restorations than there is with silicates.   As a restorative, glass ionomers can be used in all esthetically sensitive areas with no reservations.  Of all the composite restoratives, glass ionomers are some of the prettiest restorations available.  

On the plus side, these restorations not only look good, but they bond to tooth structure quite well.   Bonding between the cement and dental hard tissues is achieved through an ionic exchange at the interface.  Polyalkenoate chains enter the molecular surface of enamel and dentin, replacing phosphate ions. Calcium ions are displaced equally with the phosphate ions so as to maintain electrical equilibrium.  This leads to the development of an ion-enriched layer of cement that is firmly attached to the tooth.  Glass ionomer restorations, like silicates also leach fluoride into the tooth structure throughout the life of the restoration and thus tend to reduce the likelihood of recurrent decay around the margins.  For an excellent detailed technical explanation of the chemistry of glass ionomer, click on this link to the Canadian Dental Association review of glass ionomers.

On the negative side, the matrix material is much less hard than the matrix of silicate cement, so the restorations wear faster than silicates.  They also lack fracture resistance.  Glass Ionomers are excellent fillings on the front surfaces of front teeth, but should not be used to rebuild top edges of these teeth due to their inherent weakness.  They are also used extensively in dentistry as  luting agents ("dental glue" for cementing crowns).  The material is very sensitive to water contamination during placement, and poor technique on the part of the dentist (or poor cooperation on the part of the patient) can shorten the lifespan of the resulting restoration considerably.  Most dentists have switched to using a version of glass ionomer mixed with acrylic resin known as a resin modified glass ionomer for cementing cast metal restorations.  The major uses of glass ionomer cements today are as bases under resin composite restorations and as luting agents for cementing crowns and bridges which have metallic substructures.  

Resin-glass composites (filled resins)  

The most widely used tooth colored filling materials in use today are the resin (plastic)-glass reinforced composites.  These restoratives, like the composites discussed above, are composed of

A powdered filler material (in this case glass (quartz) particles) A hard plastic matrix which binds them together.   The plastic is a

form of acrylic known as bisphenol A glycidyl methacrylate, most commonly refered to as BIS-GMA.  This material is in a viscous liquid form until it is cured either by the addition of a peroxide catalyst or by applying a light source to a pre-catalyzed form of BIS-GMA.

Unlike the glass ionomer and silicate restoratives discussed above the composition of the hard, plastic matrix does not depend upon a chemical reaction between an acid and the glass particles.  This means that the glass used in resin based composites are not formulated to be soluble in acidic solutions.  Like everything else, this has some advantages, and a few disadvantages.  

The glass particles are pre-mixed with the acrylic liquid into a paste.  When the dentist is ready to place the restoration in the tooth, he or she mixes a catalyst into the paste and this causes the acrylic to harden around the glass particles.  Thus the material resembles a refined version of fiber glass or auto body putty.  As an alternative, the catalyst may already be mixed into the paste, but it is not activated until the dentist shines a very bright light on it, causing it to harden.   This procedure is known as light curing.

The acrylic resin has certain characteristics which make it unsuitable as a restorative material if used by itself without the glass filler particles.  The unfilled resin is prone to abrasive wear, but its major disadvantage is that the material tends to shrink while it is setting.  This would  create large spaces between the filling and the walls of the cavity preparation in the tooth, or in combination with the bonding process, would cause intolerable stresses on the tooth and could possibly even break the tooth. The addition of substantial amounts of rigid glass filler prevents most of the shrinkage associated with the resin.  The glass particles are also much more wear resistant than unfilled resin, and if the particles are of irregular shape, they are less likely to dislodge from the resin matrix under stress. Thus the glass filler solves the durability problem as well.

The fact that the glass particles do not have to react with the matrix allows the manufacturer a great deal of leeway in the manufacture the glass powder.  He can flux and stabilize the glass with materials that give it characteristics like better wear, workability and esthetic qualities than he could achieve if he were constrained by the need to manufacture the glass  according to solubility specifications.  The glass can be formulated with

virtually unlimited variations for esthetics.  Special formulations allow for particles of differing size for different restorative situations.  The particles may also have different shapes which allow for an attachment between adjacent particles thus strengthening the material.  Particle size and shape may be varied to allow for differing consistencies without compromising strength or wear characteristics.  He can also vary the qualities of the acrylic matrix independently of the filler particles.  

One disadvantage to standard resin systems is that unlike with Al-Fl-Si  glass/acid mixtures, there is no mechanism for fluoride fluxed into the glass to enter the resin matrix, and thus no way for fluoride to leach into the tooth structure offering a measure of decay resistance to the margins of the cavity preparation.  This problem has been overcome to a certain extent with the introduction of the compomers, and also by advances in the composition of the unfilled resin matrix itself.

A second disadvantage is that resin composites do not bond to tooth structure unless the tooth is acid-etched and a layer of thin plastic bonding resin is placed on the prepared surface first.  Al-Fl-Si  glass/acid mixtures chemically bond with tooth structure without the need for etching or special resin bonding agents. 

Even with these disadvantages, however, the advantages of resin composites are impressive.  By decoupling the chemical link between the glass filler particles and the surrounding matrix, the resulting flexibility has created huge developmental possibilities for manufacturers.  The evolution of dental composites is so advanced, that the industry is now working on a sixth generation of materials, and resin/glass composites have even begun to replace the ever popular silver amalgam as the inexpensive restoration of choice for back teeth.  

Types of resin composites

Traditional (Macrofill) Composites---This was the first type of resin composite marketed for filling front teeth.  As the name implies, the particles in a macrofill are fairly large.   Crystalline quartz was ground into a fine powder containing particles 8 to 12 microns in diameter.   As mentioned above, the acrylic matrix in a composite tends to shrink on setting.  Excessive shrinkage in a filling material is undesirable because it would either leave a gap between the tooth surface and the filling material, or, if well bonded, would cause cracks in the tooth structure as the filling contracts during setting.  The inclusion of glass particles reduces this problem because they reduce the volume of acrylic, and act as a mechanical "skeletal structure" within the composite to help maintain the original volume of the filling.  The advantage of large particle size is that more of them can be

incorporated into the mixture without making it too stiff to work with.  Macrofills are 70% to 80% glass by weight, 60% to65% by volume.  Unfortunately, macrofill composites have two undesirable qualities: 

o Due to large particle size, macrofills are not very polishable.  Furthermore, they feel rough and are prone to accumulation of plaque.  The relatively soft acrylic polymer tends to wear below the level of the glass particles, which constantly pop out of the surface leaving holes in their place.  This leads to a surface which, on a microscopic level, looks like a series of craters interspersed with boulders.

o Large particles are relatively easily dislodged from the surface of the restoration during function exposing the relatively soft acrylic polymer which wears away exposing more filler particles which again pop out ad infinitum.  This tendency to abrade away makes macrofils unsuitable for posterior restorations.

The first macrofill appeared on the market in the mid 1960's.  Most older dentists affectionately remember it by its brand name, Adaptic.  Adaptic had the additional disadvantage of containing no radiopaque materials which made it hard to distinguish from decay on x-rays.

Microfilled and Nanofilled composites---In dentistry, microfillers are particles that are smaller than 1 micron, while nanofillers are particles that are smaller than 0.1 micron.  In reality, most microfill composites use particles that vary between .04 and .2 microns, while nanofill composites are those that contain filler particles no larger than 0.1 micron.  Thus nanofill composites are technically just a category of microfill composites, although the term "nano" has come to imply the newer agglomerated microfill composites (defined below).   The smallest nano particles are in a form called a colloidal silica, which is produced by "burning" silica compounds such as SiCl4 in an oxygen  atmosphere to form macromolecular structures which fall into this size range.  Microfilled composites were originally invented to overcome the esthetic liabilities of the macrofills.  Microfilled composites polish beautifully and can be formulated to be quite translucent.  However, the tiny particle sizes affect many more of the fundamental properties of composites than just the ability to maintain a high polish.  

Older macrofill composites were formulated from a simple mixture of a light-cured acrylic matrix and standard quartz particles.  These composites contained 70% to 80% by weight of quartz particles (60%-65% by volume) to avoid the problem of curing shrinkage, and also to avoid excessive wear from the opposing dentition.  The resin matrix,

by itself, is not a suitable filling material.  To reiterate, it shrinks as much as 10% during curing and would leave large gaps between the filling and the tooth structure if no bonding techniques were used.   If bonding techniques are used, the resin, upon setting, would draw the edges of the cavity preparation together placing great stresses on the tooth structure, most likely causing fractures.  Furthermore, any filling made from resin alone would wear very rapidly in service.

Glass (quartz) particles, on the other hand, resist both wear and shrinkage.  A high density of glass particles displaces the resin and reduces its volume thus mitigating much of the shrinkage that happens as the composite cures.  These particles also form a sort of rigid skeleton which mechanically counteracts much of the remaining shrinkage.  Finally, the glass particles themselves do not wear in service.  A composite restoration wears exclusively because the glass particles are slowly dislodged from the surface.  If there were a way to keep them in place forever, the restoration would never wear down.  In theory, the less acrylic and the more glass a composite contains, the better.  An ideal composite filling would contain only glass, and no acrylic at all.  This, of course, is impossible, since the resin is the material used to glue the silica particles together.  It is also the component that gives the unpolymerized material the handling characteristics that allow the dentist to work with it in the first place.  

Large macrofill particles have the unfortunate property of popping out of the surface of the finished restoration.  This exposes the resin matrix around it to wear.  This property makes older macrofilled composites unsuitable for posterior restorations, since the occlusal (top) surfaces of the back teeth receive a lot of abrasive challenges.  Any filling that wears excessively would allow the bite to change, and the teeth will move over time.  In persons who brux (grind their teeth), this could cause a collapsed bite and contribute to Temperomandibular Joint Dysfunction (TMJ, or TMD). 

A smaller particle has a relatively greater surface area in relationship to its volume than a bigger one.  This gives micro particles a major advantage over macro particles.  The greater surface area, combined with the smaller volume of micro sized particles, makes them more difficult to dislodge from the plastic matrix.  The more microsized particles the composite contains, the more resistant the finished composite is to wear in the mouth.

Unfortunately, micro particle sizes have one major disadvantage when compared to macro particles.  Since friction is a function of involved surface area, the increased surface area of micro

particles also increases internal friction and makes the composite so stiff that it becomes very difficult for the dentist to manipulate.    According to Phillips Science of Dental Materials, "Colloidal silica particles, because of their extremely small size, have extremely large surface areas ranging from 50 to 400 square meters per gram."  Macrofilled composites are much easier for the dentist to handle than micros.

These dueling facts bring us back to square one.  Macro composites are easy for the dentist to work with, have minimal shrinkage, reasonable esthetics and are fine for anterior teeth, but they do not wear or polish well.  Certainly, they are unsuitable for posterior applications.  Highly filled micro filled composites would not only look great and resist shrinkage, but they would wear very well in any area of the mouth.  Unfortunately, any composite that contains a very high percentage of micro and nano sized quartz particles would be so stiff that it would be impossible for the dentist to handle.  When first formulated in the late 1970's, microfilled composites were filled to a maximum of 38% by weight, 25% by volume.  They were used mostly to veneer over larger macrofill restorations in anterior teeth to make them more polishable.  Furthermore, even though the particles are smaller and thus retain better in the plastic matrix, the low density of glass particles in the micros made them wear almost as badly as the macros, so they were not suitable for posterior restorations. 

Manufacturers came up with a solution to this problem by pre-polymerizing the micro filled composite before putting it into the paste form distributed to dentists.  This pre-polymerized composite can be fabricated to 70-80% by weight of glass using industrial machines.  The composite is allowed to polymerize and is then milled into a fine powder with particle sizes between 10 and 20 microns. This composite powder (called agglomerated microfiller) is then mixed with resin to make the composite paste that is sold to the dentist.  The final composite contains between 50% to 60% glass particles by weight, 32% to 50% by volume. 

In other words, the dentist is supplied with a composite that handles a bit like a macrofil due to the larger size of the agglomerated particles, but has most of the properties of a microfill due to the microstructure of the agglomerated particles themselves.  Furthermore, since the

glass particles in the agglomerated microfiller are so small, they are not easily dislodged from the surface of the restoration during service.  This means that modern microfills (now generally called nanofills to differentiate them from the older, less filled microfills) wear quite well and are suitable for restorations in the occlusal (top) surfaces of the posterior teeth. 

The agglomerated microfiller particles do not give the final composite paste quite the same handling characteristics found in traditional macrofills.  Each agglomerated particle is, after all, coated with the same plastic that is found in the liquid resin.  This changes the flow characteristics of the paste making it more difficult to work with than macrofilled or microhybrid composites.   The major problem with microfilled composites is that they tend to be sticky, and to slump while the dentist places them.  Their main advantages are their superior esthetics and their ability to resist wear during service.  The viscosity of these nano composites can be adjusted by varying the size and density of the agglomerated microfiller particles.

 

Microhybrid composites--- Microhybrids contain a range of particle sizes ranging from 0.4 to 1 micron.  Developed in the late 1980's, these composites achieve between 70 to 75 percent by weight of filler particles.  The first generation hybrids achieved excellent wear characteristics which made them acceptable as posterior filling materials.  Their main advantages were good polishability and excellent handling characteristics.  The second generation of hybrids achieved greater polishability and superior color optics by using uniformly cut small filler particles between the larger particles, as well as resin hardeners which help to maintain a surface polish during prolonged function.  Microhybrids also have unique color reflecting characteristics which gives them a chameleon-like appearance.  They are now used primarily in anterior restorations.  Their larger particle sizes gives them better handling characteristics than the micros, and their superior esthetics make them especially useful for anterior restorations.  Several microhybrids are marketed as posterior filling materials, however they wear much faster than the agglomerated microfill (nano) composites.

Flowable composites---Flowable composite restorative is formulated with a range of particle sizes about the same as hybrid composites.  The amount of filler is reduced and the amount of unfilled resin matrix material is increased.  This makes for a very loose mix.  It is delivered into a cavity using a syringe.  It flows freely over the inside surface of the cavity preparation.  This material has made it possible to fill small cavities in the tops of teeth without a shot since the area of decay is often small enough to be removed with little or no sensation in the tooth, and the flowable composite will bond even if there are no undercuts in the cavity preparation.  Flowable composites are often used to seal the dentin of a tooth prior to placing the filling material.  Due to the low level of filler particles, flowable composites are more prone to shrinkage, so they are generally not used in bulk to fill large cavities. 

Resin (Composite) Cements---When formulated as loose, sticky, chemically cured substances (i.e. with a separate catalyst that is manually mixed into the base at the time of use), filled resins make remarkably strong cements for crowns, veneers, onlays, posts, Maryland bridges, orthodontic brackets and other bonded appliances. Since both porcelain and tooth structure can be etched with acids, the resin component can flow into the microscopic irregularities in the appliances to be cemented as well as the irregularities etched into the tooth structure.  This etched bond is, by itself, quite strong, however the presence of the filler particles adds a second "lock and key" type of mechanism to help cement the appliance as well.   

Resin modified glass ionomers

Resin modified glass ionomers are glass ionomer cements that contain a small quantity of a polymerizable resin component.  These  materials have most of the advantages of glass ionomer materials with the added advantage of water insolubility while setting. These materials are always dispensed in two component systems and begin hardening only when both components are mixed together.  The resins included in some systems have dual curing capability, which means that they will cure chemically once the pastes are mixed, but the curing can be accelerated by the use of high intensity light.  The ability to light cure the excess material reduces chair time. 

Resin modified glass ionomer cementso These are a real success story in dentistry.  Resin modified glass

ionomer cements have become the standard material used to cement metal and zirconia based crowns and bridges onto prepared teeth.  They reduce post operative sensitivity and

reduce the likelihood of cement washout.  They chemically bond to both the metal and the tooth structure.  They have much less shrinkage on setting than resin based composites.  They are also easy to use and simple to mix, unlike zinc phosphate cement which was the industry standard up until the introduction of these cements.

Resin modified glass ionomer restoratives

o These are used mostly as bases under composite resin restorations.  They lack the ability to resist occlusal wear, but their major virtue is that they shrink very little while setting and thus reduce post operative sensitivity while reducing compressive stresses on the tooth.  They also release fluoride into the tooth structure.  They are also useful for filling cavities around the gum line.  In this capacity they leach fluoride into the tooth throughout their service life thus reducing the likelihood of recurrent decay.

The Compomers (polyacid-modified resin composites)

A compomer is really a modified composite resin.  These materials have two main constituents: A resin modified with dimethacrylate monomer(s) with two carboxylic groups present in their structure, and a filler that is similar to the ion-leachable glass present in glass ionomer cements. The filler particles are only partially silanated to help the adhesion of the resin to the glass particles, while at the same time allowing some of the soluble fluoride in the glass to leach out into the tooth structure.  When first marketed, it was claimed that the carboxylic groups in the resin would allow adhesion to tooth structure without the acid etch bonding technique, similar to glass ionomer cements.  This turned out to be a false assertion.  Even so, compomers are still popular with dentists for filling deciduous (baby) teeth, and, due to their high degree of translucency, they are highly esthetic when used for the repair of cervical (gum line) caries.  They confer a degree of fluoride release into the tooth, although less than that found in glass ionomer cements.  Thus, at least in the short term, they prevent recurrent decay while allaying parents' concern about the presence of mercury in standard amalgam fillings.  They do not have the surface durability of standard composite resins, but will wear quite well for the life of a deciduous tooth.  Unlike glass ionomer restorations, they do NOT adhere to tooth structure without an acid etch bonding technique.   They are esthetically pleasing and seem to resist recurrent decay for several months after placement when used to fill cavities near the gum line.

Paste compomer restorative (filling) material; These materials are excellent tooth colored filling materials when used on front teeth

in non stress bearing areas, such as for filling cavities at the gum line, or in larger restorations if they are fully supported by natural tooth structure and do not involve incisal or occlusal surfaces.  They are especially good on the buccal or labial (front) surfaces of teeth where esthetics is extra important.  They are often used to cover exposed, sensitive root structure on both front and back teeth.  

In spite of the fact that they are less wear resistant than regular composites, some dentists use light activated compomers to  fill baby teeth due to their extended fluoride release, and also to allay parents' fears about the mercury in amalgam fillings.  The baby teeth generally exfoliate (fall out) before the wear becomes a problem.  Compomers are also useful in geriatric dentistry since oral hygiene is often poor in elderly patients, and they frequently suffer xerostomia (dry mouth).  The combination of poor oral hygiene and dry mouth causes rampant decay in these patients, and the constant release of fluoride at the tooth/restorative junction can be helpful to prevent recurrent decay.

Flowable compomers; These are like the paste compomer restorative, but they contain much more of the unfilled resin.  They are used in the same fashion as flowable composites, except they are rarely used in stress bearing areas such as the occlusal surfaces of adult teeth. 

A note on radiopacity of dental materials

X-rays are an essential part of dental diagnosis, and it is very important that any material that remains implanted in any part of the patient's body, including his teeth, be radiographically distinguishable from natural structures or disease processes.  In other words, any material or device implanted in teeth or in any other part of the body must be visible on an x-ray.  Materials like amalgam, gold and titanium (for implants or posts) are made of metal and are naturally radiopaque (ie. they block x-rays and cast a white shadow on s-ray film).

Materials like restorative composites, porcelain, or various dental cements are not inherently radiopaque and without modification of their composition, would not be visible on an x-ray film except as a dark spot if deposited in bone or tooth structure.  Unfortunately, decay in teeth shows up as a dark area on an x-ray film, and in the early days of composite technology, before the addition of radiopacifiers, it was often difficult to distinguish between a composite filling or an area of decay in a tooth when looking at an x-ray.   The addition of zirconium dioxide, barium oxide or  Ytterbium oxide to any radiolucent (the oposite of radiopaque) material will impart the property of radiopacity.  These three oxides are chosen for their

compatibility with the chemistry of composites.  Note that Barium Sulfate is used as a "milkshake" or enema  when taking medical x-rays for the observation of the gastro-intestinal tract. 

The addition of radiopacifiers is especially important in the production of dental cements used to lute crowns and bridges.  Even though the cement will spend its lifetime under the crown, excess cement will be forced out from between the crown and the tooth during placement, and often end up between the teeth or under the gums where it cannot be seen by direct observation.  When this happens, it can cause inflammation of the gums and even eventual loss of the tooth.  As long as the cement is visible on the x-ray, it will reveal the presence of the cement so that it can be removed.