0099-2399/91/1706-0257/$03.00/0 JOURNAL OF ENDODONTICS Copyright 9 1991 by The American Association of Endodontists
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VOL. 1"7, NO. 6, JUNE 1991
SCIENTIFIC ARTICLES Resistance to Fracture of Endodontically Treated Premolars Restored with Glass Ionomer Cement or Acid Etch Composite Resin M. Trope, DMD, and L. Tronstad, DMD, PhD
ally after acid etching. This strengthening effect occurred equally well when 2 mm of the resin was removed and replaced with amalgam as when the entire cavities were filled with the resin material. Recently these findings were confirmed by Reeh et al. (6) who found that endodontically treated teeth restored with composite resin after enamel and dentin etching were significantly stronger than those that were left unrestored and almost as strong as intact teeth. With acid etching of the dentin the superficial smear layer is removed as is the peritubular dentin near the surface, resulting in open and widened dentinal tubules (7). The resin forced into these tubules holds the tooth together, increasing its resistance to fracture (5). Glass ionomer cements have also been suggested as an intracoronal restoration for endodontically treated teeth. The characteristics which make this type of material desirable are chemical bonding to enamel and dentin (8), a coefficient of thermal expansion which is low and essentially the same as that of tooth structure (9), and an ability to be acid etched and used in conjunction with a composite resin restoration (10). In addition glass ionomers release fluoride ions to adjacent tooth structure (11), are easy to place, and can be permanently finished as a core build up at the same appointment (12). The purpose of the present investigation was to determine whether the resistance to fracture of endodontically treated premolars was increased by means of intracoronal restorations with glass ionomer cement used as the sole restoration or as a reinforcing base over which amalgam on composite resin was placed. The purpose was also to determine whether these types of restorations compared equally with the acid etch resin technique (5).
MOD cavity preparations in 64 endodontically treated premolars were restored using four different methods. Copper rings were filled with commercial hard-setting cement and the teeth were placed into the cement up to the level of the cementoenamel junction. The teeth were grouped according to restorative method, mounted in an Instrom T.T. machine, and the buccal walls subjected to a slowly increasing compressive force until fracture occurred. The force of fracture of the walls of each tooth was recorded and the results in the various groups compared. All teeth fractured in a similar manner irrespective of the restorative method used. The resistance to fracture of the teeth was the same when they were restored with glass ionomer cement as a base over which an amalgam or composite resin was placed or with acid-etched resin. When the entire cavities were filled with glass ionomer cement the resistance to fracture of the teeth decreased significantly compared with the acid etch resin technique.
Endodontically treated teeth contain less moisture, are structurally weaker, and thus fracture more easily than vital teeth (1, 2). Particularly in endodontically treated posterior teeth, the stresses of occlusion can lead to the fracture of unprotected cusps (3). A study comprising over 1000 endodontically treated teeth showed that teeth which were restored with intracoronal restorations fractured more often than those restored extracoronally (4). However, an acid etch composite resin technique for intracoronal restoration of endodontically treated teeth has reversed this tendency to fracture (5, 6). Trope et al. (5), showed that the resistance to fracture of endodontically treated premolars was significantly increased when the teeth were restored with composite resin when placed intracoron-
MATERIALS AND METHODS The material consisted of 64 freshly extracted human premolars without caries, abrasion, injury from forceps, or fractures. The premolars were randomly selected and divided into four groups of 16, each with an equal number of maxillary and mandibular first and second premolars. Access cavities were prepared and the root canals instrumented to size #60
Trope and Tronstad
Journal of Endodontics
and filled with gutta percha and Grossman's cement. MOD cavities were prepared in each tooth down to the canal orifices so that the thickness of the buccal wall measured 2 m m at the occlusal surface and 3 m m at the cementoenamel junction (Fig. 1). The teeth were then restored as described below.
amalgam ~! or resin glass ~ionomer
Group 1 The cavities were treated with a 10% polyacrylic acid solution (G. C. Dental Industrial Co., Tokyo, Japan) for 10 s, washed thoroughly, and air dried. Glass ionomer cement (Fuji, Type II; G. C. Industrial Co.) was then forcefully condensed into the cavities with a cotton pellet.
Group 2 The cavities were treated as in group 1. A 2-mm deep MOD cavity was then prepared into the glass ionomer cement and filled with amalgam (Cupralloy ESP; Syntex, Valley Forge, PA) (Fig. 2).
FIG 1. Diagram Of MOD cavity in premolar tooth. The buccal wall measures 2 mm occlusally and 3 mm at the cementoenamel junction.
Group 3 The cavities were treated as described in group 1. A 2-mm deep MOD cavity was then prepared into the glass ionomer cement and filled with a composite resin without acid etching the surface of the glass ionomer (Fig. 2).
Group 4 The cavities were etched with 37% phosphoric acid for 60 s, washed with copious amounts of water, and air dried. A composite resin (Concise; 3M, St. Paul, MN) was then forcefully condensed into the cavities with a cotton pellet. The restored teeth were allowed to air dry for 1 h and then placed in 100% humidity for 48 h. Copper rings of 25 m m in length and 10 m m in diameter were filled with a commercial hard-setting cement (Rockite; Hartline, Cleveland, OH) and the teeth were placed into the cement to the level of the cementoenamel junction. The cement was allowed to harden for 1 h and the copper rings with the teeth were placed into an Instron T.T. machine (Instron, Canton, MA). The buccal walls of the premolars were then subjected to a slowly increasing force at the junction of the buccal cusp and the filling material. The force was applied at a 150-degree angle to the long axis of the teeth (5). The force necessary to fracture each tooth was recorded and the results between groups compared. The data were subjected to a one-way analysis of variance for the four experimental conditions. The three glass ionomer treatment methods were further compared with the acid etch resin method using a Dunnett's t statistical test (13). RESULTS One tooth was lost from groups 1 and 2. All teeth fractured in a similar manner irrespective of the method used. The mean force of fracture value and the standard deviation for each of the four experimental conditions are presented in Table 1. With the one-way analysis of variance, overall differ-
FIG 2. Diagram of filled MOD cavity. The glass ionomer cement was condensed into the cavities. A 2-ram cavity was prepared into the glass ionomer cement and filled with amalgam or resin. TABLE 1. Resistance to fracture (Ibs.) of endodontically treated premolars following different methods of restoration
Group 1: Group 2: Group 3: Group 4: Glass Ionomer Glass Ionomer Glass Ionomer Acid Etch Restoration + Amalgam + Resin Resin Mean SD
ence in statistical significance between the groups was found at the 0.05 level (F = 3.48, df = 3.34, p < 0.05). In addition, statistically significant differences were found between the teeth restored solely with glass ionomer cement and those restored with acid-etched composite resin (group 1 versus group 4, 89.3 versus 146.4, t = 3.16). Statistically significant differences were not found between the teeth in which amalgam or resin was placed in the preparations made in glass
Vol. 17, No. 6, June 1991
ionomer cement and the teeth restored with acid-etched composite resin (group 2 versus group 4; 125.2 versus 146.4; t = 1.17 or group 3 versus group 4; 127.0 versus 146.4 t = 1.17).
Fracture of Restored Premolars
when used as a base the glass ionomer increased the resistance of the cusps to fracture. Therefore, if the clinician feels that its other characteristics warrant its choice, its use as a base in these teeth may be justified.
DISCUSSION CONCLUSIONS Each group comprised different types and thus different sizes of premolars. To compensate for these differences, the buccal walls of the teeth were prepared in a uniform manner and each type of premolar was equally distributed to the four groups. Therefore, the large standard deviation found for the force of fracture of the cusps was in all likelihood due to variation within the groups which comprised both maxillary and mandibular first and second premolars. Previous studies have shown conclusively that the acid etch composite resin technique significantly increases the strength of the cusps of endodontically treated teeth compared with unrestored teeth or teeth restored with a nonbonding material (5, 6). This increase in strength occurs equally well if the composite resin is placed as a reinforcing base or as the sole restoration (5). Glass ionomer cement increased the resistance of the cusps to fracture when used as a base but surprisingly not when used as the sole restoration. Glass ionomer is an extremely technique sensitive material (14, 15). According to the manufacturer's instructions, it is essential that glass ionomer cement be kept free from moisture for 15 to 30 rain to allow maximal strength and adhesion of the material to dentin. Even though we allowed the material to sit for 1 h before placing the teeth in 100% humidity, the bond between the glass ionomer and dentin was still broken. Possibly, the material became too dry in the first hour resulting in a loss of the bond between it and the tooth. However, a more likely reason is that the material had not completely set at 1 h and the subsequent moisture caused the loss of the chemical bond to the dentin and enamel (16). When used as a base, the resin or amalgam placed over the glass ionomer cement insulated it from the external environment, thus maintaining its bond to the tooth. New glass ionomer cements which are claimed to be less sensitive to moisture are now available. Also, manufacturers now recommend covering glass ionomer restorations with a layer of varnish or light-cured unfilled resin in order to protect them from moisture until they have completely set, thus conceivably maintaining the bond to the dental tissues. However, the compressive strength of glass ionomer cements is not sufficiently high for use in large posterior restorations and its use as the sole restoration in these situations is contraindicated (17). Amalgam placed over an appropriate strengthening base is still the intracoronal restoration of choice in these teeth (17). The acid-etched composite resin restorations gave the best numerical results and remain an excellent method for internally strengthening endodontically treated teeth. However,
1. The resistance to fracture of endodontically treated premolars was highest when the intracoronal cavities prepared in the teeth were acid etched and filled with a composite resin. 2. Glass ionomer cement used as a base under either a resin or an amalgam restoration increased the resistance to fracture of endodontically treated premolars, but to a lesser degree than the use of the acid etch resin technique. 3. Under the conditions of this study, glass ionomer cement did not increase the resistance to fracture of endodontically treated premolars when it was used as the sole restorative material. Dr. Trope is chairman, Department of Endodontology, Temple University School of Dentistry, Philadelphia, PA. Dr. Tronstad is chairman, Department of Endodontics, University of Pennsylvania, Philadelphia, PA. Address requests for reprints to Dr. Martin Trope, Department of Endodontology, Temple University School of Dentistry, 3223 North Bond Street, Philadelphia, PA 19140.
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