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

The purpose of this study was to quantify the cuspal deflection produced by polymerization shrinkage, comparing 3 different techniques in the placement and polymerization of Class II posterior composite resin restorations in vivo. Thirty primary second molars in need of a Class II restoration were identified. An index, with a size 1⁄4 carbide round bur, was placed on the buccal and lingual cusp tips of the tooth that was to receive the restoration. A standardized conventional Class II preparation was completed, then each tooth was restored with posterior composite resin, using 3 different application techniques (Technique I--placement and polymerization as one complete unit; Technique II--placement and polymerization in gingivo-occlusal increments; Technique III-placement and polymerization in buccolingual increments). Preoperative and postoperative impressions were made, then poured with an epoxy resin die material. Photomicrographs were projected on a digitizing pad and measured by 3 independent investigators. Results demonstrated the mean cuspal deflection to be 19.7 ~m for Technique I, 14.7 #m for Technique II, and 5.2 #m for Technique IlL Advances in dental materials have progressively led to many changes in operative dental techniques. These advances are demonstrated by changes in preparation designs, restoration placement techniques, and improvements in the physical properties of restorative materials. Over the past 10 years composite resin has become more useful as a posterior restorative material. It has the advantages of excellent esthetics, relatively low thermal conductivity, and preservation of tooth structure in cavity preparation. Composite resins have been shown to be successful as Class I and Class II posterior restorations. 1 When considering the placement of a Class II posterior composite restoration, the knowledge of an acceptable cavity preparation design is necessary. Pa~Nelson et al. 1980; Oldenburg et al. 1985; Paquette et al. 1983; Tonn and Ryge 1985. quette et al. (1983), conducted a study in which modified preparation was placed in primary teeth involving removal of only carious enamel and dentin. Although tooth structure was conserved, this technique was successful only with Class I restorations. Class II restorations, having a failure rate of approximately 25% when using this modified preparation technique, demonstrate that a conventional type of preparation is desired. A study of composite resin restorations by Oldenberg et al. (1985), demonstrated that conventional preparation design with a bevel had the greatest success, failure rate being 2.5% (3/ 119) compared to 4.5% (5/110) failure for conventional preparations and 11.7% (15/128) for modifed preparations. A conventional preparation, with an enamel bevel, appears to be the most appropriate design for Class II composite resin restorations at this time. Increased filler content, used in current posterior composite resins for wear resistance, reduces the amount of polymerization shrinkage compared to a conventional composite with a relatively large amount of unfilled resin matrix. Although polymerization shrinkage is decreased, a closer insight into the phenomena should be viewed. Goldman (1983) analyzed the polymerization shrinkage of various chemical and photopolymerized composite resins using a volumetric shrinkage measuring method. Shrinkage ranging from 1.67 to 5.68% was observed with light-activated, highly filled materials showing the least amount of shrinkage. Significant tensile stresses have been shown to develop during polymerization of composite resins, producing a force powerful enough to create separation at the enamel-composite junction. 2 Davidson and deGee (1984) suggest that the. flow in composites compensates for the contraction 2 Bowen and Cobb 1983; Jorgensen et al. 1975; Asmussen 1975; Hansen 1981; Ehrnford and Derand 1984. 22 POLYMERIZATION SHRINKAGE--IN VIVO COMPARISON: Donly et al. stresses created by polymerization shrinkage. Bowen et al. (1982), observed that placement and polymerization of composite resin in numerous increments could create less hardening shrinkage; whereas placement in one complete unit demonstrated more shrinkage and less hygroscopic expansion. Very infrequently was hygroscopic expansion sufficient to compensate completely for the polymerization shrinkage. Polymerization shrinkage stresses were found to be less when the composite resin was placed and polymerized in buccolingual increments rather than one complete unit (Donly and Jensen 1986). A scanning electron microscope study of composite restorations showed large voids to be present in specimens restored using the bulk-pack method (Eick and Welch 1986). Gingivo-occlusal incremental polymerization showed cracks in the composite resin along the resin-adhesive-tooth interface on both the buccal and lingual surfaces of the proximal box. Buccolingual incremental polymerization showed the composite resin to produce a dense, tight bond to tooth structure with little evidence of porosity. The purpose of this study was to evaluate any measurable dimensional change created by 3 techniques in the placement and polymerization of Class II posterior composite resin restorations in primary teeth. Methods and Materials Thirty primary teeth were identified in the University of Iowa College of Dentistry Pediatric Dentistry Clinic. These teeth were fully erupted, in functional occlusion, and in need of a Class II restoration. Criteria used to assess the need for acceptance in this study included: (1) radiographic evidence of interproximal decay, (2) decay not encroaching on the pulp, (3) no evidence of tooth fracture, (4) asymptomatic status, and (5) restoration requiring a preparation design that did not extend to the buccal or lingual surfaces. The patients were appointed for a restorative appointment after the parents had consented for the patients to take part in the study. The clinical treatment procedures were performed in the following manner: 1. The patient was seated and local anesthesia was administered. 2. The tooth and adjacent teeth were isolated with a rubber dam. 3. Indices, with a size 1~ carbide round bur, were placed on the buccal and lingual cusp tips of the proximal surface of the tooth that was to receive the restoration. 4. A preoperative impression was made using a polyvinylsiloxane impression material, a a Express®_3M Dental Products, St. Paul, MN. 5. A pretreatment color photograph was taken. 6. The tooth was prepared, removing all carious tooth structure and unsound enamel, following the usual preparation principles of operative dentistry (McDonald and Avery 1983). The isthmus of the preparation included approximately twothirds the buccolingual width of the tooth, using cusp tip to cusp tip as reference points. A 45°, 0.5 mm bevel was placed in enamel around the entire preparation with a flame-shaped carbide finishing bur. 7. An impression of the preparation was made. 8. The tooth was rinsed and dried, then a calcium hydroxide base b was placed over the exposed dentin. 9. The tooth was acid etched with 37% phosphoric acid gel c for 60 sec, then thoroughly rinsed for 30 sec and air dried. 10. The entire calcium hydroxide base then was removed using an enamel hatchet, thereby exposing all dentinal surfaces for application of the dentin bonding agent. 11. A T-band matrix band was fit to the tooth and a wooden wedge driven interproximally to prevent overhangs and provide tooth separation to obtain postrestorative contact. 12. The 30 teeth were restored randomly, with 10 teeth being used for each of the following techniques: Technique h Scotchbond®~ unfilled resin was applied to the etched surface, followed by P-30 ®e being placed into the preparation and polymerizedr (2 min) as 1 complete unit. Technique Ih Scotchbond ® unfilled resin was applied to the etched surface, followed by a gingivo-occlusal incremental placement of P-30. The first increment was placed into the gingival half of the cavity preparation and polymerized (1 rain). The second increment, filling the remainder of the preparation was polymerized (1 min). Technique IIh 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. The increment was polymerized (1 min), followed by the placement and polymerizaLife Base®-Kerr, Romulus, MI. Etching Gel--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. PEDIATRIC DENTISTRY: March 1987/Vol. 9 No. 1 23 tion (1 rain) of P-30 in the remainder of the preparation. 13. Excess composite was removed with carbide finishing burs followed by polishing of the restoration with composite finishing discs. 14. The rubber dam was removed and the restoration was evaluated for proper occlusion with articulating paper. 15. Any premature contact was relieved and the restoration polished. 16. A postoperative impression was made, using the same polyvinylsiloxane material as before, and a 2 x 2 color slide was taken. 17. The tooth, once again, was isolated with a rubber dam and the indices on the cusp tips were restored with composite resin following the accepted technique presented before. The impressions were taken to the laboratory and the following procedures completed. 1o All impressions were poured with an epoxy resin die materialg as suggested by the manufacturer. 2. The epoxy resin dies were placed on a photomicroscope h, where the indices on the cusp tips were brought into focus under five times magnification, and a photograph exposed with Tungstun film. 3. The 2 x 2 slides were developed, placed in a projector and magnified ten times, displaying the photograph on the digitizer pad. i 4. The slides were projected at random onto the digitizer pad; measurements were made from the buccal index to the lingual index using the most interior edge of the indices for the reference points. All measurements were recorded. The slides were arranged randomly and measured on the digitizer by 3 independent investigators. The 3 measurements of the independent investigators were averaged, and the means used for statistical analysis.