Development of sealing methods and applied materials

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Available methods for caries prophylaxis by means of fluorine compounds, limited consumption of carbohydrates and systematic removal of dental plaque - which effectively inhibit caries on smooth surfaces of teeth (from 35% to over 90%) - do not significantly impact the occurrence of caries on masticatory surfaces of teeth. Since 1965 the search for effective methods of prophylaxis against caries on occlusal surfaces of teeth has been considering individual anatomical characteristics of this surface and has been leading towards isolating of fissures in premolars and molars from the impact of oral environment (quot. after 37). The method which evoked special interest among clinicians consists in fissure and pit sealing on masticatory surfaces, i.e. in mechanical isolation of clinically healthy tissues from oral environment.

Attempts to control caries of masticatory surfaces of lateral teeth have a history of over one hundred years. The oldest concept was a removal of not lesioned fissures during preparation of carious cavities on masticatory surfaces. It was a so-called prophylactic dilatation, invented by Webb and strongly propagated by Black (quot. after 72). Nevertheless, as early as 116 years ago it was observed that prophylactic activities at the time of occurrence of carious events are frequently belated or insufficient. In 1895 Wilson performed prophylactic procedure of filling fissures of permanent molars with cement. Thus he realized Hunter's idea from 1778, who observed that fissure blocking may inhibit development of caries. In 1917 Hove introduced silver impregnation applying ammonia solution of silver nitrate, and in 1950 Ast applied zinc chloride (quot. after 78). Removal of healthy enamel (odontotomy, eradication, prophylactic dilatation) was especially controversial because most applied methods yield unsatisfactory caries reduction and masticatory surfaces turned brown-black.

Willingness to use less invasive methods led to search for new materials. The development of organic chemistry, especially polymers, greatly contributed to progress in prophylaxis of occlusal surfaces of teeth. In the course of performed studies, several chemical compounds were singled out which proved useful for fissure sealing. Generally they belonged to four groups (cyanoacrylates, polyurethanes, epoxy resins and bis-GMA resins).

Cyanoacrylates, which were first adhesive materials used for this procedure, did not gain clinical approval, because they clung to tooth surfaces for a very short time and frequently caused dentine complications. Polyurethanes, which demonstrated poor adhesion to enamel, were applied in caries prophylaxis mainly thanks to content of fluorine compounds. However the two groups of chemical compounds did not act up to expectations.

Only the introduction of epoxy resins by Schröder in 1953 and later of bis-GMA resins by Bowen in 1963 raised new hopes for clinicians. The new materials allow fissure isolation without disturbing healthy enamel. Apart from many advantages, such as: mechanical endurance, chemical resistance, relatively good adhesion, low contractility during polymerization, had one major disadvantage - lack of solid bonding to tooth's hard tissue, resulting in short-term retention of material on sealed teeth.

Only Buonocore in 1965, followed by Gwinett, Sharp and Silverstone demonstrated that permanent bonding of materials with enamel can be obtained after its etching. When etched enamel became a permanent element of clinical procedures with adhesive materials, adhesion and stability of bonding of compound materials with hard tissues of teeth increased on the principle of micromechanical retention. Sealing based on application of bis-GMA resins was introduced less than 50 years ago. Due to matrix type, content of filler particles, presence (or absence) of fluorine ions and efficacy of prophylactic sealant, they were divided into four generations.

Studies on first-generation sealants (sealants polymerized with UV light, started in late 60s) demonstrated diversified prophylactic efficacy of lateral tooth sealing. Their effectiveness depended on the number and type of sealed teeth (first molars, second molars and premolars), applied methodology of procedure, methodology of assessment and the fact that procedures were generally performed by doctors. Representatives of this generation were: Nuva-Seal, Alpha-Seal, Espe 717, Saga-Sealant, Lee-System. After 10-year-long observation, retention of material was 68.0% - 180 sealed teeth in children aged 7-8 years (31). Other authors after five years of observation obtained retention from 19.3% to 63.0% (70,72) and corresponding caries reduction from 57.9% to 43.0% (64).

Second-generation sealants (chemically polymerized, late 60s and early 70s) indicated a higher prophylactic efficacy compared to first-generation sealants (22). Their better effectiveness was due to introduced modifications, which resulted in improved physical-chemical properties (added filler particles) and improved retention to hard tissues of teeth, for example: Concise BWSS from 3M Kerr (PFS) Delton, Concise EBS (Enamel Bond System). Light-cured sealants, which also belong to the second generation, demonstrated lower microleakage (19-20%) compared to chemically cured sealants (50-67%). After 10 years 50% total retention of second-generation sealants was obtained, also a higher percentage of partial retention and of caries reduction (from 40 to 63%) compared to first-generation sealants (70,72). They were represented by light-polymerized Concise (BWSS), Prismashield resins.

Studies on third-generation sealants (polymerized with visible light - the first half of the 80s) demonstrated efficacy which was comparable with second-generation sealants as far as material retention and caries reduction are concerned. After five year of observation it was about 77% (65,66,67). This generation is represented by Concise LVC, Helioseal.

The fourth generation includes sealants with fluorine (late 80s). They are based on bis-GMA resin, urethane dimethacrylate, aliphatic methacrylates. Fluorine is released from fluosilicate glass (Helioseal F) or added in the form of sodium fluoride (Fluoroshield, Fissurit F, Ultraseal XT). The presence of fluorine in sealing material decreases caries risk on sealed surface, even when microcracks occur on sealant's surface. Lasting low concentration of this element in the oral cavity is especially significant. Fluorine level in dental plaque is from 6 ppm to 300 ppm, whereas in saliva it is from 0.001 ppm to 9.4 ppm. Sealants with fluorine such as: Helioseal F and Ultraseal XT were characterized by low viscosity, which enabled easy penetration of material into deep, narrow fissures. Moreover, they do not feature phase separation (i.e. sedimentation of ingredients), which makes them different from other fluorine sealants. Another advantage of Helioseal F and Ultraseal XT sealants is lack of air bubbles in material, which constitutes a common problem with sealing materials. Laboratory tests demonstrated that sealants with fluorine release twice as much fluorine ions within nine days compared to the number of fluorine ions released from glass-ionomer cements (68). In own experimental studies, the number of fluoride anions released from five sealants was determined by direct potentiometry method with the use of fluoride ion selective electrode. In order to obtain constant pH of contact solution and standard solutions, TISAB buffer was used, which eliminated any influence of alien ions. The potentiometer showed the potential of a given solution, while the amount of fluorine was calculated on the basis of calibration curve obtained for standard solutions. The study demonstrated that the value of fluorine ion emission level varied depending on the type of sealing material, type of fluorine compound in sealant, time of testing (short-term from 0.5 to 7 days and longitudinal during 15 study periods from 7 to 371 days). During the whole study period, the highest level of released fluorine ions was observed in Ultraseal XT (22.83 mgF/mm2 after 12 months and 28.10mgF/mm2 after 13 months of observation), and a much lower level in Fissurit F (from 13.04mgF/mm2 after 12 months of observation and 9.30 mgF/mm2 after 13 months of observation). The lowest level of released fluorine ions was observed for the three remaining sealants: Pentraseal, Ionoseal, Helioseal (4.40, 3.90, 3.40mgF/mm2 respectively) and after13 months of observation for Helioseal, Pentraseal i Ionoseal (5.8, 4.0, 3.5mgF/mm2 respectively). It was also observed that release of fluoride ions from sealants containing various fluorine compounds is a long-term process, though amounts of released fluoride ions are small (33).

Clinical studies comparing retention of sealants with fluorine to retention of sealants without fluorine, after one-year observation indicated a higher retention of sealants with fluorine. After three years of observation retention of materials with fluorine was similar. The obtained effect of reduced caries by application of sealants with fluorine was 83.9%, whereas with sealants without fluorine the result was 70% (30). The net gain index (which indicates the actual number of teeth saved from caries thanks to sealing) for 100 teeth sealed by application of sealants with fluorine was 30 teeth saved from caries, while only 15 teeth were saved by application of sealants without fluorine.

Another group of sealants which became a permanent part of caries prophylaxis are glassionomer cements (GICs). Thanks to their hydrolytic properties they are capable of forming solid and long-lasting bonds with hard tissues of teeth, non-etched enamel and dentine, and are characterized by similar release of active fluoride ions to surrounding fissures. However, it should not be forgotten that glass-ionomer cements used for sealing of lateral teeth are susceptible to impact of big masticatory forces, thus their application should be limited because of their low resistance to wear. Clinical studies showed low retention index for GICs used as sealants in observation periods from six months to seven years (73). Although a high percentage of sealant loss was observed , it was not directly related to development of caries (25,57,74,76). Examples of such sealants were: ASP A, Fuji III, Ketac Cem, Ketac Bond, Chemfil Superior.

At the beginning of the 90s the quality of glass-ionomer cements was improved by addition of bis-GMA and HEMA (2-hydroxyethyl methacrylate) resins. HEMA monomer was combined with a copolymer based on chains of itaconic acid, having - through amide groups - metacrylan groups polymerizing when exposed to light, which led to development of resin-modified glass-ionomer cements (RMGICs) and polyacid-modified resins which were tested as sealants. In in vitro studies, resin-modified glass-ionomer cements demonstrated the same or better adhesive properties and release of fluorine ions. Moreover, the addition of resins made glass-ionomer cements stronger and less brittle. As far as protection against secondary caries is concerned, no significant differences were observed between materials. A low retention index was obtained for Ketac GIC and Vitremer RMGIC after five years (57). The total retention index was less than 2% (1.6%) for both materials. The authors explained low retention with long-term use and insufficient resistance to abrasion of both materials. In another research Forss and Halme noticed 10% total retention after seven-year-long observation of conventional glass-ionomer cement (26). There have been numerous published reports comparing the efficacy of GIC sealants with efficacy of materials based on bis-GMA resins. They covered observation periods from one month to four years. However, due to various research methods, diversification of age groups, anatomical characteristics of observed teeth and number of repeated sealant applications, the results can not be compared, therefore their value is limited. Nevertheless the studies demonstrated decidedly worse retention among glass-ionomer cements compared to retention of sealants based on bis-GMA resins (25,62). Low retention of GICs was due to the fact that before application of sealant, tooth enamel was not subjected to any preparatory procedures aimed at improving retention, but only - according to instruction - teeth were cleaned of dental plaque, rinsed and dried. Probably the main cause of GIC loss may be its inadequate adhesion to enamel. Such explanation seems to confirm results of in vivo and in vitro studies on the impact of various preparatory procedures on GIC's adhesion to tissues of teeth. They indicate that adhesion can be improved by proper conditioning of enamel before application of cement. 30-second etching with 50% citric acid before application of cement or 10-second etching with 10% polyacrylic acid resulted in 26% retention after two years and 10% after seven years. The reasons for such unsatisfactory results are poor adhesion of cement to enamel, lack of procedures improving adhesion and brittleness of GICs. An improvement of cement's physical properties suggested by the authors, e.g. liquidity of cement which was created as a result of perfecting Fuji III, was not confirmed in two- and four-year studies reporting 4% retention, whereas with application of a comparable sealant based on bis-GMA resins - Delton - the values obtained after two and four years were respectively 79% and 61%. The discussed issue is GIS's ability to release fluorine, which enables anticarietic activity on neighboring tissues and facilitates remineralization of existing early carious centers in enamel. Observation of carious process after six and twelve months of observation in several cases in both study groups prevents formulation of an opinion on anticarietic activity of glass-ionomer cements. Mejer and Mjor after six and twelve months of study observed considerable loss of cement on 66% of masticatory surfaces sealed with Fuji III, however they did not observe carious lesions, whereas in case of sealing with bis-GMA resins (Delton and Concise), whose total retention after five years was as high as 90%, caries was present on 5% of masticatory surfaces (48). This suggests that even a small amount of cement which remains in fissures is enough for long-lasting caries prevention. In vitro studies demonstrated that fissures sealed with GIC are more resistant to demineralization than unsealed fissures, even if presence of sealant is not confirmed clinically, authors associate the anticarietic effect it with possible residue of cement remains at the fissures' bottom and continual release of fluorine by glass-ionomer cements. Some attention should also be paid to antibacterial activity of glass-ionomer cements against S. mutans and S. sorbinus. Suppression of colony growth is related to the ability to release fluorine ions. Therefore the choice of GIC as fissure sealant must result from other reasons than good retention or cariostatic property. Certainly it can be an alternative e.g. in the absence of proper humidity control, with partial tooth eruption or with mentally or physically handicapped patients. Apart from that, glass-ionomer cements have another advantage of simple application technique. Increased efficacy of sealing can be obtained by reapplication of sealant during routine visits, however this will increase cost of treatment. That is why only certain clinical situations can constitute an indication for such treatment, for example difficult children or those with a high caries risk (23).

It was observed that resin-modified cements demonstrate much stronger bonding forces with dentine (almost threefold) compared to conventional glass-ionomer cements, but considerably weaker than sealants based on bis-GMA resins. Retention of glass-ionomer cements after 6 months equaled 93%, after 24 months from 82.5% to 86%, and after four-year observation only 35% (50,51,86). Baseggio et al. assessed the efficacy of Vitremer modified glass-ionomer cement compared to Fluoishield conventional sealant based on bis-GMA resins containing fluorine. During three-year studies on sealants in second molars in 320 patients aged 2-16 years, retention of sealant modified with glass-ionomer cement was 5.10%, whereas retention of sealant based on bis-GMA resin equaled 91.08%, and total loss of retention was observed in 6.37% and 7.65% of sealed teeth respectively. Caries on masticatory surfaces was observed in 20.06% of teeth sealed with materials modified with glass-ionomer cement and in 8.91% of teeth sealed with materials based on bis-GMA resin

Compomers, also called composite materials modified with polyfunctional acids, contain bis-GMA monomers, deionized glass which constitutes mechanical filler with 42-67% of volume and particle size 0.7-5^m, and monomers containing COOH acidic functional groups. Thanks to glass content they are capable of releasing fluorine contained in glass, but in a much lesser quantity than GICs or resin-modified GICs (63). A two-year assessment of sealants based on bis-GMA resins (Fissurit F, Fissurit FX, compomer Dyract Seal and ormocer Admira Seal) demonstrated that retention of the compomer (Dyract Seal) was much lower compared to the other sealants. However, no significant differences were observed in marginal tightness of assessed sealants and in caries presence on surfaces of sealed teeth (88). In research comparing two materials - one compomer and one based on bis-GMA resin - special emphasis was placed on advantages of compomers, which are easy to use, chemically and physically durable, and they release fluorine. No difference was observed between applied sealant and its retention and between the degree of caries reduction and material retention (quot. after 24). In another study in which GIC-based material with zinc and fluorine was used (Jonosit Seal DMG), after six years of observation in 77.5% of cases retention on occlusal surfaces of teeth was confirmed. Presence of caries was closely related to retention of material. In case of total retention of sealant, caries reduction was 99.6%, whereas with total loss of material after five years caries reduction was 69% (34). Other authors, such as Aranda et al. and deLuca-Froga et al. , who assessed activity and preventive performance of resin-modified glass-ionomer cements (RMGICs) and composite materials modified with polyfunctional acids, observed that retention index changed after one-year observation, rising from 20% to 95.9% (4,18). An important factor which must be considered in case of glass-ionomer cements used as sealants is the fact that even after clinical loss of material, a small amount of sealant remains at the fissure's bottom with fluorine still being released, thus protecting masticatory surface. Higher grow of caries was observed in the control group compared to the group with applied sealant. Anticarietic protective effect of glass-ionomer cements remained even after their loss. Masticatory surface was protected in the period most susceptible to caries, i.e. during the first year after tooth eruption (82). In this period there is no contact of opposing teeth and children have difficulties with appropriate hygienic procedures. According to Forss and Halme, after this period the risk of caries is lower and consequences of sealant loss would be less significant (26). Therefore it was assumed that children from the study group would have a higher caries development than children from the control group, because the latter were older. However the research yielded opposite results. It should be stressed that glass-ionomer cements are recommended for sealing erupting teeth due to difficulties with proper isolation of teeth during application, while glass-ionomer cements are less sensitive to humidity, and because these teeth are susceptible to caries (62).

In a research where retention and efficacy of fissure sealing with glass-ionomer cements was assessed after reapplication of material, caries reduction on masticatory surfaces three years after sealing was 66.5% compared to untreated control group (42). This suggests that lost material should be supplemented in order to ensure the best protection of occlusal surface. The obtained results were better compared to sealing without supplementation of lost material (82). Repetitive application requires time an results in higher cost of treatment. Therefore such prophylactic program may be directed rather at patients from high risk groups, though it may also be applied for the whole population (55). With tooth eruption, all permanent molars should be sealed, which will prevent caries development and consequently will lead to a decrease in the value of the DMF score. However, costs of promoting such a program will be higher, because surfaces which probably would never be carious, would also be sealed (41). In case of sealing only teeth with high caries risk, occurrence of caries would decrease, as well as cost of treatment, which constitutes an important factor in public health protection (9). Thus it is necessary to establish which teeth indicate a higher caries risk and to choose appropriate material.

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