K. Al-Satf, BDS, MSD*
Department of Restorative Dental Sciences, King Saud University, College of Dentistry,
P.O. Box 60169, Riyadh 11545, Saudi Arabia.

Recurrent caries,around dental restorations,has been a major problem facing dental practitioners. In 1949, Healey and Phillips1 found that 53% of amalgam failure was directly due to recurrent dental caries. Although Richardson et al2 reported a smaller percentage in 1973, 23% of all amalgam restorations were replaced because of secondary caries. This percentage increased to 54% when new caries were included. In a recent survey in 1987, Klausner and associates3 found that 54% of 5511 restorations placed by 191 dentists were replacement restorations. Of these replacements, 53% were replaced because of recurrent caries.

 
Despite the fact that operative procedures have been improved measurably, particularly in regard to the type and design of cavity preparation and in the quality of restorative materials, there is still a need to render the cavity preparation itself less sus­ ceptible to recurrent caries. This procedure is espe­ cially important because most dental materials are incapable of preventing marginal leakage and do not provide anticariogenic properties. The purpose of this review is to provide evidence showing that fluorides are effective and safe in reducing recur­ rent caries.
Fluorides in Cavity Preparation and Caries Reduction
The use of fluorides has been proven to be effec­ tive in the field of preventive dentistry. Systemic and topical applications of fluorides results in caries reduction. This fact has lead dental researchers to investigate the possibility of reducing recurrent caries by topical fluoride application inside cavity preparations. Gross et al4 reported that a 50% reduction in secondary caries was achieved by treating the cavity preparations with sodium fluoride. In a laboratory study, a 30-second appli­ cation of a 10% stannous fluoride solution to pre­ pared cavities was investigated. The result showed a 53 to 75% reduction in enamel solubility in acid.5 These authors concluded that a topical stannous fluoride treatment should be included as a routine step in cavity preparation. In a controlled two-year clinical trial, Alexander, McDonald, and Stookey6 used a 30% stannous fluoride solution to treat Class II and Class V cavity preparations in children. Cav­ ity preparations were moistened with stannous fluoride for 15 seconds, air dried, and then restored with amalgam. The control group had cavity prep­ aration with placebo. Children receiving SnF2 treat­ ment showed a 60.7% and 46.7% reduction in recurrent caries in permanent and primary teeth respectively. Shannon7 investigated the degree of effectiveness of various SnF2 concentrations on the acid solubility of walls of freshly prepared cavities. Results indicated that solubility reduction provided by the 30% or 10% SnF2 is needlessly concentrated and that a diluted solution of SnF2 is a more biolog­ ically sound approach.
Several studies investigated the effect of topical fluoride application on carious dentin.8-9 Radiog­ raphic and microradiographic examinations of SnF2-treated carious dentin showed a great increase in radiopacity compared to that before treatment. This finding suggests that a 10% SnF2 solution may cause remineralization.
The above studies clearly indicate the impor­ tance of topical fluoride treatment for reducing the incidence of secondary caries and also the possibil­ ity of promoting remineralization of small carious tissue accidently left in the cavity preparations. Freshly cut dentin and enamel should be consi­ dered similar to freshly erupted enamel in being highly susceptible to caries because it lacks the fluoride-rich protective layer that forms following exposure to oral fluids. Therefore, any safe means of reducing the dissolution of the susceptible sur­ faces would be beneficial.10
The use of topical fluoride on freshly-cut dentin is not a new concept. Hoyt et al11 suggested the use of sodium fluoride-kaolin paste as a potential means of reducing hypersensitivity of the dentin.
Another rationale for placing topical fluoride inside cavity preparation is its effectiveness as an indirect pulp-capping material. Nordstrom and co­ workers12 compared a 10% SnF2 solution to cal­ cium hydroxide for indirect pulp capping in human primary and permanent teeth. The SnF2 solution was applied for five minutes. Results showed no significant difference between the failure rates of teeth treated by both means. However, teeth treated with SnF2 showed harder dentin and greater radiodensity than teeth treated with cal­ cium hydroxide.
It is well known that fluoride in small quantities is an enzyme inhibitor whereas in large concentra­ tion it actually kills microorganism. Therefore, application of topical fluoride inside cavity prepa­ rations may have an antibacterial effect. Further­ more, fluoride decreases the free energy of tooth surfaces. This may render cavity margins less sus­ ceptible to plaque accumulation. A long term in vivo study has indicated that plaque formed on sili­  cate restorations differs in composition from plaque formed on resin or amalgam restorations. Car­ bohydrate/nitrogen and nitrogen/calcium ratios were generally lower in plaque associated with sili- cate cement. This suggests that either the car­ bohydrates are metabolized less or there is less bacteria in the plaque.13
The effect of fluorides on dental pulp
The biocompatibiiity of topical fluoride, when applied to freshly-cut dentin, has been addressed in numerous studies. Andres et al14 studied the effect of 30% SnF2 solution applied to deep cavities prepared in dog teeth. They found no adverse pulp response. Weiss et al15 also reported a very slight, if any, inflammation in pulpal tissue in human teeth when a 10% SnF2 solution was applied to freshly cut cavities. In another study by Weiss and associates,16 Class V cavities were prepared in young premolar teeth. The prepared cavities were then treated with 10% SnF2, 2% NaF, or 1.23% AFP. Histologic examinations showed no significant effects of any of these fluoride solutions on the underlying pulp. However, Branstrom et al17stated that in deep cavities, where pulp could be exposed, 8% SnF2 should not be applied more than 30 seconds. These studies indicate that application of topical fluoride to cavity preparations in moderate concentration and for a short period of time is a safe procedure.
Fluoride application procedures in cavity preparation
Fresh stannous fluoride solution is the topical fluoride of choice. Many studies have shown that SnF2 offered a higher degree of protection and resistance to acid dissolution.7,18,19 Although some studies have shown that aged SnF2 solution can be more effective than fresh solution, the aged solu­ tion has a lowered pH value which may cause an adverse effect on the pulp.10

Numerous research projects have shown fluoride to be useful as anticariogenic agent, anti­ bacterial agent, pulp capping material and desen­ sitizing solution. Accordingly, the application of fluoride solutions to cavity preparations has been suggested as beneficial. The use of topical fluorides is not required on all cavity preparations. If one elects to use a topical fluoride, the following are recommended: