Posterior composite polymerization shrinkage in primary teeth: an in vitro comparison of three techniques.

This in vitro study investigated strain produced in the placement and polymerization of Class II posterior composite resin restorations in primary teeth. Mesio-occlusodistal preparations were placed in primary teeth, followed by posterior composite resin restoration placement, using 3 different application techniques (technique I--placement and polymerization in 1 complete unit; technique II--placement and polymerization in gingivo-occlusal increments; and technique III--placement and polymerization in buccolingual increments). A precision strain gage was attached to the buccal surface of each tooth, balanced at O, and after each increment was polymerized, the strain appearing on the strain gage indicator was recorded. Each tooth was restored using all 3 techniques. Results demonstrated the mean microstrain units to be 60.3 for technique I, 46.5 for technique II, and 38.5 for technique III. Scheffe’s test indicated that the buccolingual incremental polymerization produced a statistically significant lower amount of strain on the tooth than polymerizing the restoration as 1 complete unit (P < 0.05). The utilization of composite resin is becoming more popular for the restoration of posterior primary teeth. Use of composite resin has many advantages: esthetics, relatively low thermal conductivity, and most importantly, the preservation of tooth structure in cavity preparation. Researchers have demonstrated the volumetric polymerization shrinkage of composite resin and discussed techniques to measure accurately this shrinkage quantitatively. 1-3 A study by Goldman analyzed the polymerization shrinkage of various chemical and photopolymerized composite resins using a volumetric shrinkage measuring method. The values of the polymerization shrinkage ranged from 1.67 to 5.68%, light-activated materials showing the least. 4 Bowen et alo reported measurements showing significant tensile stresses developed during the polymerization of composite resins. 5,6 The bond of the cornposite resin to the enamel and dentin wails must be stronger than the polymerization shrinkage tensile strength, therefore preventing the shrinkage contraction from breaking the composite-tooth interface bond. 7 Although studies have shown that polymerization shrinkage can produce a force powerful enough to create separation at the enamel-composite junction, thereby allowing for marginal leakage, 8-12 the significance of this occurrence remains a controversial issue. Davidson and deGee suggest that the flow in the composite can compensate for the contraction stresses created by polymerization. 13 A study by Hegdahl and Gjerdet indicated that the stresses produced on the enamel by polymerization shrinkage were low compared to the tensile strength of the enamel, providing minimal force on the enameU 4 Bowen et al. observed that placement of composite resin in numerous increments could create less polymerization shrinkage, whereas placement by the bulk method demonstrated more shrinkage and less hygroscopic expansion2s This study also demonstrated that hygroscopic expansion infrequently is sufficient to compensate completely for the polymerization shrinkage. Composite restorations are becoming more widely accepted as a posterior restoration in primary teeth. Several studies report that composite is a reasonable restoration for Class II preparations in primary teeth. 16-~9 The purpose of this study is to quantify the stresses created by 3 different techniques in the placement and polymerization of Class II posterior composite resin restorations. Methods and Materials Ten primary second molars were obtained from patients treated in the University of Iowa Pediatric Dentistry Clinic. Five teeth were maxillary second primary molars and 5 teeth were mandibular second primary molars. None of these teeth were affected by PEDIATRIC DENTISTRY: September 1986/Vol. 8 No. 3 209 caries--each was extracted for orthodontic treatment. The teeth were placed in a 0.1% Thymol ®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 dried. A precision strain gageb was attached to the buccal surface of each tooth with a cyanoacrylate ester bonding cement, c The tooth was situated in a I-in. retention tube, the tooth roots being retained within the tube by acrylic, d leaving the crown and strain gage exposed. The mounted tooth had 2 stabilized leads from the strain gage connected to the digital strain gage indicator, e The baseline strain indicator was balanced at 0 and recorded. A mesio-occlusodistal preparation was placed in the tooth, the isthmus being approximately one-half of the intercuspal width and a 45°, 0.5 mm b, evel placed on all enamel margins. The standardized preparation was essentially the same as that used for conventional amalgam preparations in primary molars. 2° The tooth was acid etched with 37% phosphoric acidf for 60 sec, then thoroughly rinsed for 30 sec and air dried. Each tooth then was restored, using 3 different techniques described as follows: Technique I. Scotchbond ®g unfilled resin was applied to the etched surface, followed by P-30 ®h being condensed into the preparation and polymerizedi (2 rain) as 1 complete unit. Technique II. Scotchbond ® unfilled resin was applied to the etched surface, followed by a gingivo-occlusal incremental placement of P-30. ® The first increment was condensed to the gingival half of the cavity preparation and polymerized (1 rain). The second increment, filling the remainder of the preparation, was polymerized (1 rain). Technique IIL Scotchbond ® unfilled resin was applied to the etched surface, followed by a buccolingual incremental placement of P-30. ® The first increment was placed against the buccal wall and extended lingually to an imaginary plane approximately 1.5 mm from the lingual wall. This increment was polymerized (1 rain), followed by the Thymol®--Mallinckrodt Inc: St Louis, MO. CEA-09-032UW-/20 Precision Strain Gages--Measurement Group Inc: Raleigh, NC. Permabond, 910 Adhesive--Permabond International Corp: Englewood, NJ. Fastray--Harry J. Bosworth Co: Skokie, IL. V/E-20A Strain Gage Indicator--Measurements Group Inc: Raleigh, NC. Etching Liquid--3M Dental Products: St Paul, MN. Scotchbond®--3M Dental Products: St Paul, MN. P_30®_3M Dental Products: St Paul, MN. Visilux® Visible Light Curing Unit--3M Dental Products: St Paul, MN. placement and polymerization (1 min) of P-30® in the remainder of the preparation. After each increment was polymerized, the strain appearing on the strain gage indicator was recorded. Each tooth was restored using all 3 techniques. After 1 technique was completed and findings recorded, the restoration was cut from the preparation and the next technique initiated. Three teeth were started with technique I, 4 teeth were started with technique II, and 3 teeth were started with technique III. After each tooth had been restored using the 3 techniques, the strain gage was removed and the tooth was disengaged from the acrylic. The roots were cut from the tooth at the cementoenamel junction leaving the entire crown intact. This portion of each tooth was weighed.i The composite resin then was cut from the teeth and the teeth were weighed again. The mass of the composite restoration was calculated by subtracting the weight of the crown, in which the restoration had been removed, from the weight of the crown with the restoration intact. By multiplying this mass by the density of the composite, the actual volume of composite resin was determined. The percentage of the total crown weight that was composite resin then was calculated (Table 1).