Cuspal reinforcement in primary teeth: an in vitro comparison of three restorative materials.

The purpose of this study was to evaluate cuspal flexure in posterior primary teeth following restoration with three different materials. Twelve primary second molars were obtained. A precision strain gage was attached to the buccal surface of each tooth and balanced at zero. The teeth were mounted and then loaded with a 10 kg force; the strain appearing on the strain gage indicator was recorded. A mesiocclusodistal preparation was placed, and then each tooth was restored using amalgam, posterior composite resin, and glass ionomer silver. Following restoration placement, the tooth again was loaded with the lO-kg force. Each tooth was restored using all three materials. Results demonstrated that composite resin restorations recovered an average stiffness of 75% of the original intact tooth, glass ionomer silver recovered 52% of the original tooth stiffness, and amalgam recovered 34% of the uncut tooth stiffness. The analysis of variance demonstrated that significant differences in external cuspal deflection stress were associated with the different restorative materials. Scheffe’s test demonstrated that loading a tooth restored with composite resin created significantly less strain than res toting the tooth with glass ionomer silver or amalgam. Loading a tooth restored with glass ionomer silver created significantly less strain than restoring the tooth with amalgam (P < 0.001). During the past 10 years, many advances have occurred in pediatric operative dentistry. More recently, glass ionomer silver has become available for use as a posterior restorative material. The advantages of glass ionomer silver include: (1) the continual release fluoride ions; (2) ability to chemically bond to tooth structure; (3) a coefficient of thermal expansion near that of tooth structure; (4) radiopacity; (5) the capacity to etched for the mechanical bond of composite resin; and (6) resistance to abrasion with the addition of silver particles to the glass ionomer. 1 1McComb et al. 1984; Smith 1985; McLean et al. 1985. The ability of glass ionomer to bond to tooth structure may add to increased tooth fracture resistance, due to increased cuspal reinforcement, and promote conservation of tooth structure in cavity preparation. Traditionally, the restoration of interproximal carious lesions, that extended even slightly beyond ideal limits of cavity preparation resulted in full coverage with stainless steel crowns. More contemporary materials, with the potential to bond tooth structure, may offer a more conservative approach during the restoration of the primary dentition (Croll and Phillips 1986). Morin et al. (1984) showed acid-etch composite resin restorative procedures to have significantly higher cuspal reinforcement than nonbonding restorative procedures. McCullock and Smith (1986b) found composite resin and glass ionomer restorative materials to increase the fracture resistance of teeth, while amalgam produced no increase in fracture resistance. These same authors found less internal cuspal deflection when restoring teeth with glass ionomer cement, compared to the deflection forces produced by polymerization contraction of composite resin (1986a). The purpose of this study was to evaluate cuspal flexure in posterior primary teeth following restoration with three different materials. Materials and Methods Twelve primary molars were obtained from patients treated in the University of Texas Dental Branch Clinics. None of these teeth were affected by caries or had previous restorations placed; each had been extracted prior to orthodontic treatment. The teeth were placed in 10% formalin a solution immediately after extraction and retained in this preservative until the study was initiated. One at a time, each tooth was taken from the preservative solution, rinsed with distilled water, and air a Formaldehyde solution -Fisher Scientific; Fair Lawn, NJ. 102 CUSPAL REINFORCEMENT 1N PRIMARY TEETH: Donly et al. dried. A precision strain gageb was attached to the buccal surface of each tooth with an epoxy adhesive, c The tooth was situated in a 1-inch retention tube, the tooth roots being retained within the tube by acrylic, d leaving the crown and strain gage exposed. The mounted tooth had two stabilized leads from the strain gage connected to the digital strain gage indicator, e A 10-kg load, generating a force comparable to the mean chewing force for children ages 6-12 years (Fields et al. 1986), was applied to each tooth. The force was exerted approximately parallel to the long axis of the tooth. The load was applied to the tooth by bringing a sphere, attached to the upper member of the testing instrument, into contact with both the buccal and lingual cusps (Figure). The points of contact placed lateral deflecting forces on the tooth, situated so the sphere did not touch the occlusal table. The strain appearing on the strain gage indicator, after loading the intact tooth, was recorded. A mesiocclusodistal preparation was placed in the tooth, the isthmus being approximately one-half of the intercuspal width. The standardized preparation was essentially the same as that used for conventional amalgam preparations in primary molars (McDonald and Avery 1983). Each tooth then was restored using three different techniques described as follows: Technique 1: An amalgamf restoration was placed. Technique 2: Polyacrylic acid was placed on all exposed dentin for 10 sec and rinsed. A glass ionomer silverg restoration was placed as recommended by the manufacturer. Technique 3: A 45°, 0.5-mm bevel was placed on all enamel margins. The enamel margins were etched with 37% phosphoric acid h for 60 sec. Unfilled resin i was applied to the etched surface, followed by a buccolingual incremental placement of posterior composite resini presented previously (Donly and Jensen 1986). Each tooth was restored using all three techniques. After one technique was completed, and the strain appearing on the strain gage indicator recorded, the restoration was cut from the preparation and the next CEA-09-032UW-/20 precision strain gages -Measurement Group Inc; Raleigh, NC. Devcon epoxy adhesive -Devcon Corp; Danvers, MA. Fastray -Harry J Bosworth Co; Skokie, IL. V/E o 20A strain gage indicator -Measurements Group Inc, Raleigh, NC. Premalloy -ESPE-Premier Sales Corp; Norristown, PA. Ketac® Silver -ESPE-Premier Sales Corp; Norristown,PA. Etching gel -3M Dental Products; St Paul, MN. Scotchbond® -3M Dental Products; St Paul, MN. P_30® __ 3M Dental Products; St Paul, MN. ~ L~PRESSURE ROD RESTORATIVE /.~ /~ MATERIAL "’~/~ EPOXY ADHESIVE