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Effect of thickness of cavity wall on fracture strength of pulpotomized primary molar teeth with class II amalgam restorations. F. Mazhari*, M. Gharaghahi**. Depts.

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RESEARCH Effect of thickness of cavity wall on fracture strength of pulpotomized primary molar teeth with class II amalgam restorations F. Mazhari*, M. Gharaghahi**. Depts. *Paediatric Dentistry, Faculty of Dentistry, ** Endodontics; Mashhad University of Medical Sciences, Mashhad, Iran. Abstract Aim: This was to evaluate the effect of different thicknesses of cavity walls on fracture strength of pulpotomized primary molar teeth with class II amalgam restorations. Methods: 80 carious extracted human primary molar teeth were selected for pulpotomy preparation. The teeth were divided into four groups. Mesio- or disto-occlusal (20 teeth) and mesio–occluso-distal (20 teeth); cavities were prepared in both first and second primary molar teeth. Each group was divided into two subgroups of ten teeth according to the thickness of their walls (1.5 or 2.5 mm). After restoring teeth with amalgam, all groups were stored in distilled water at 37°C for seven days. They were then thermo cycled 1,000 times between 5° to 55°C. The specimens were then subjected to a compressive axial load in a universal testing machine at a crosshead speed of 0.5 mm min-1. The t-test was used for statistical analysis. Results: Mean fracture resistances of the first and second molar teeth were 975.5 ± 368.8 and 1049.2 ± 540.1, respectively. In the first molars, fracture resistance of 2-surface cavities was significantly more than 3-surface cavities (p=0.001), but this difference was not statistically significant in second molars. In second molars, fracture strength in 2- and 3-surface cavities with a 2.5 mm thickness in the walls was more than those with 1.5 mm thickness. However, in first molars this difference was only statistically significant in 3-surface cavities (p=0.045). Conclusions: The fracture strength in pulpotomized primary molar teeth with amalgam restorations was high, more than maximum bite forces in primary teeth, even in extensive 3surface cavities.

Introduction Tooth substance loss can reduce fracture resistance due to dental caries and cavity preparation [Caron et al, 1996], especially in endodontically treated teeth with extensive (MOD) restorations [Linn and Messer, 1994; Panitvisai and Messer, 1995; Ortega et al, 2004]. The loss of marginal ridge has been shown to weaken teeth and to increase susceptibility to fracture [Linn and Messer, 1994; Ortega et al, 2004]. In preschool children with large proximal carious lesions,

pre-formed metal crowns (PMC) are preferred to amalgam because of their durability. Similar-sized lesions in teeth that are within two to three years of exfoliation may be restored with amalgam because the anticipated lifespan is fairly short [Waggoner, 2005]. However, as amalgam lacks the ability to adhere to tooth structures there is no significant change in the fracture resistance of the cusps when compared to prepared, non-restored teeth [Pilo et al, 1998; Gorucu and Ozgunaltay, 2003]. Therefore, sufficient thickness of the tooth structure in the cavity walls must be left in prepared teeth (resistance form in GV Black principles) to resist occlusal forces without fracture. Currently, there is no report in the literature concerning minimal thickness of cavity walls with enough fracture resistance in primary teeth restored with amalgam. The purpose of this study, therefore, was to compare the fracture strength of minimal thickness of cavity walls (1.5 mm) with a greater thickness (2.5 mm) in 2 and 3-surface cavity preparations in primary molar teeth after pulpotomy.

Methods and Materials Eighty extracted primary molar teeth (40 first and 40 second primary molar teeth) were collected and stored in physiologic solution for less than 3 months. Each tooth was cleaned and examined using a fiber optic light and teeth with cracks or other visible defects were excluded. The teeth were selected based on the size of the carious lesions, so as to allow preparation of standardized cavities (described later). The teeth with more extensive lesions were excluded. At no stage in the study were the teeth allowed to dehydrate. The teeth were mounted vertically in acrylic resin to 2 mm below the CEJ, approximately the level of the alveolar bone in a healthy tooth, such that the cusp tips aligned in the same plane to ensure a more equal distribution of the load during testing. After a pulpotomy had been completed the teeth were divided into two groups of first molar teeth (FMT) or second molar teeth (SMT). Each group was then subdivided into two sub-groups according to cavity preparation

Key words: cavity wall thickness, fracture strength, p ulpotomized primary molar teeth, amalgam restoration Postal address: Dr. F. Mazhari, Paediatric Department, Dental School, Park Square, Mashhad, Khorasan, IRAN, 91735. Email: mazharif @mums.ac.ir

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type (2-surface or 3-surface cavities). Again, each group was further subdivided into another two groups according to the thickness of the cavity wall (1.5 mm or 2.5 mm). The distribution of groups is shown in Fig. 1. It is important to note that teeth with similar crown sizes were selected for each group. Moreover, in each group of ten, five were maxillary and five mandibular. Overall, from the 80 selected teeth, 40 were maxillary and the other 40 mandibular. The cavity preparations were standardized as follows: 3-surface cavities – The mesiodistal extension of the cavity on mesial and distal boxes of all prepared teeth was up to 1 mm to adjacent cusp tips (Fig. 2). 2-surface cavities – The extension was done up to the point that 2/3 of the mesiodistal width of the teeth in both buccal and lingual surfaces remained, and in the occlusal cavity up to the point that the remaining width of the marginal ridge was the same as that of a bur 256 (0.8 mm), (Fig. 3). The gingival wall was finished at the CEJ in all cavities. As a whole, an attempt was made to retain the same bulk of tooth structure in each group. A new bur was employed for every five teeth. The thickness of the cavity walls was standardized with an orthometer gauge (Korkhaus Orthometer Kit,

Molar type

75228 Ispringen, Dentaurm, Germany). After placing zinc oxide eugenol and zinc phosphate pastes in access cavities, the teeth were restored with amalgam using a matrix retainer (Tofflemire matrix retainer, Teledyne Dental products, Saratoga, Calif.). The restored teeth were stored in water at 37°C for seven days prior to testing. All specimens were thermocycled (1,000 cycles, 5 to 55°C with a dwell time of 30 sec) and tested on the Instron universal testing machine for resistance to fracture with a crosshead speed of 0.5 mm min-1. A steel ball was selected to contact the inclined planes of the occlusal surfaces of the teeth and not the restorations. Type of failure and the place of fracture line in cavity walls were recorded. Data were analyzed by the student t-test. A level of significance of 0.5% was accepted.

Results Mean fracture resistances of first and second molar teeth were 975.5 ± 368.8 and 1049.4 ± 540.1, respectively (p=0.47) (not shown in the table). First molar teeth (Table 1): Fracture resistance of 2-surface cavities was more than the 3-surfaces, significantly (P=0.001). Fracture strength in 2- and 3-surface cavities with 2.5 mm thickness in walls was more than the ones with 1.5 mm thickness, but the difference was statistically different only in 3-surface cavities(P=0.045).

Cavity type

Thickness of cavity wall

DO (20)

1.5(10)** 2.5(10)

FMT (40)

MOD (20)

1.5(10) 2.5(10)

Teeth (80)* MO (20) SMT (40)

1.5(10) 2.5(10)

MOD (20)

1.5(10) 2.5(10)

Figure 1. Distribution of types of the teeth, cavities and thicknesses of cavity wall used in a study on wall thickness and resistance to fracture * Figures in parentheses are numbers of the teeth in each group. ** Each group consisted of five upper and five lower teeth.

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Effect of thickness of cavity wall

Figure 2. Illustrations of MOD cavities in second primary molar teeth. a) Carious lesion, b) MOD cavity preparation, c) MOD restoration

Second molar teeth (Table 1): Although the fracture resistance in 3-surface cavities was more than 2-surface ones, the difference was not significant (P=0.32). The fracture strength of cavities with 2.5 mm remaining walls was more than the 1.5 mm thick walls in both 2- (p=0.001) and 3(P=0.015) surface cavities. Pattern of failures: The commonest failure was the fracture of cavity walls and the most common place for the fracture line was below the CEJ (Fig 4). Different distances of fracture lines from the CEJ in cases with the fracture line being below the CEJ are shown in Fig 5.

Discussion In this study, the MO cavity preparation was prepared in second molars and the DO cavity preparation in first molars because carious lesions usually follow this pattern clinically. In addition, 1.5 mm thickness was selected as the least pos-

sible thickness in cavity walls because the thickness of enamel in primary teeth is 1 mm. Therefore, 1.5 mm means that at least 0.5 mm dentine still remains. Overall, fracture strength in second molars, irrespective of cavity design and thickness of cavity wall, was higher than first molars. However, the difference was not statistically significant. This may be due to the fact that the larger teeth may resist fracturing better than smaller teeth [Blaser et al., 1983]. First molar teeth: Fracture resistance of the 2-surface cavities, irrespective of wall thickness, was twice that of 3-surface cavities. Loss of marginal ridge integrity has been reported as the greatest contributing factor to loss of tooth strength; thus, whenever possible the ridge should be preserved in order to maintain tooth strength [Mondelli et al., 1980; Reeh et al., 1989; Hansen et al., 1990]. This is especially important in first molars which are smaller than second

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a)

Wall thickness = 1.5mm b)

Wall thickness = 1.5mm

c)

Thickness of marginal ridge = 0.8mm Amalgam Wall thickness = 1.5mm Buccal view Occusal view

Figure 3. Illustrations of MO cavities in second primary molar teeth. a) Carious lesion, b) MO cavity preparation, c) MO restoration

Table 1. Mean and SD of resistance to fracture force (in Newton) for 2- and 3- surface cavities in the teeth with regard to cavity wall thickness Type of tooth

Type of cavity

Thickness (mm)

Number

Mean

SD

P-value

2-surface

1.5 2.5

10 10

1148.80 1276.60

235.1 330.3

0.316

3-surface

1.5 2.5

10 10

610.4 886.0

280.1 249.2

0.045 *

2-surface

1.5 2.5

10 10

803.7 1475.4

311.4 447.2

0.001 *

FMT

0.001 *

SMT

0.32 3-surface

1.5 2.5

* P< 0.05

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10 10

665.9 1261.9

494.0 500.5

0.015 *

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Effect of thickness of cavity wall

60%

55%

50% 40% 30% 21%

20% 10% 0%

10%

13% 1%

cusp crushed

cusp fr.

wall fr. Above CEJ

wall fr. at CEJ

wall fracture under CEJ

40% 35% 30% 25% 20% 15% 11% 10% 5% 0% 0.5

36% 25% 20%

5% 1

1.5

2

2.5

3% 3

Types of fail

Distances between fracture lines and CEJ(mm)

Figure 4. Frequency of types of failure in primary molars in a study on wall thickness and resistance to fracture.

Figure 5. Frequency of various distances between fracture lines and CEJ in the teeth where fracture lines occurred under the CEJ.

molars. Panitvisai and Messer [1995] studied cuspal deflection in endodontically treated molars with 2- or 3-surface restorations. They concluded that endodontic access in the MOD group weakened the tooth significantly more than in the MO group. The remaining marginal ridge supported the cuspal stiffness of the tooth, leading to lesser cusp deflection. This occurred even though the remaining marginal ridge was not adjacent to the cusp under load Therefore, with regard to loss of marginal ridge in both the mesial and distal surfaces in 3-surface cavities, this group showed lower resistance to fracture than the 2-surface cavities.

Fracture strength in 2.5 mm thickness cavity walls was significantly higher compared with those with 1.5 mm thickness in both 2- and 3-surface cavities. This may be because there is greater sound dentine that remains in the former. Previous workers have shown that preservation of tooth structure is important for its protection against fracture under occlusal loads and for its survival. The main factor affecting the survival of pulpless teeth is loss of dentine [Johnson et al., 1976; Assif and Gorfil, 1994; Linn and Messer, 1994]. Therefore, it seems that in second molars, thickness of cavity walls is more important than type of cavity (2- or 3-surface) for resistance against occlusal bars.

In both the 2- and 3-surface cavities, resistance to fracture of teeth with 2.5 mm thickness in their cavity walls was higher than teeth with 1.5 mm thickness. However, the difference was significant only in 3-surface cavities. This result implies that in FMT, what is important is preserving the marginal ridge, such that in 2-surface cavities, in which fracture strength is significantly higher than 3-surface cavities, the thickness of cavity walls does not play a significant role in increasing fracture strength, but in 3-surface cavities with loss of both marginal ridges, preserving more thickness in cavity walls can result in a significant increase to fracture. Second molar teeth: Although resistance to fracture in 2-surface cavities was higher than 3-surface ones, the difference was not statistically significant. Comparing this result between first and second molars implies that loss of the marginal ridge in the latter is less important than the former. It has been shown that the weakening effect of loss of the marginal ridge in molars is much less than in premolars [Reeh et al., 1989]. This lesser degree of weakening effect, that was observed in the larger teeth (molars versus premolars and second versus first molars), may be due to their greater bulk of tooth structure thus making them correspondingly more resistant to fracture.

Comparison of fracture strength of teeth in the present study with maximum bite force in primary teeth: There is great variability in studies regarding occlusal forces. Facial structure, general muscular force and gender differences, location of the recorded bite force within a dental arch, mental state during experiment, state of dentition, malocclusions, temporomandibular dysfunction, and the extent of the vertical separation of the teeth and the jaws due to the bite, may influence the values found for bite force [Fields et al., 1986; Bakke et al., 1990; Kiliaridis et al., 1995]. However, the value of maximum bite force has been reported between 151.9 ± 141.8 and 374.4 N in different studies [Carlsson, 1979; Maki et al., 2001; Rentes et al., 2002; Kamegai et al., 2005]. In this present study mean fracture strengths were 975.5 ± 368.8 and 1049.2 ± 540.1 in first and second molars respectively, which were much higher than the value of maximum bite force reported in the literature. Types of failure: The most common failure was cavity wall fracture (77%). In most cases, the fracture line was below the CEJ (55%) and the distance between the fracture line and the CEJ was 1 mm in almost half of those cases. In these conditions with 2-surface cavities, if it is possible to



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prepare a retentive cavity, the tooth may be deemed restorable, but in cases with the fracture line >1 mm under the CEJ and when preparing a retentive cavity is not possible, the tooth must be considered for extraction, or crowning. In this study, from 40 teeth with 2-surface cavities only seven teeth (17.5%) were able to be restored again. Of course it is necessary to mention that the force used to fracture teeth in this study was much more than the maximum bite force in children. Thus, it appears that in the mouth there is a very little possibility for these types of restorations fracturing under normal occlusal forces. However, clinical trials are required to prove this.

Conclusion Preserving the marginal ridge in first primary molars was especially important for increasing tooth strength. On the other hand, in second primary molars, preserving more tooth structure at the cavity walls played a significant role in increasing tooth strength. As a whole, the fracture strength in pulpotomized primary molar teeth with amalgam restorations was high (more than maximum bite force in primary teeth), even in extensive 3surface ones.

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Caron GA, Murchison DF, Cohen RB, Broome JC. Resistance to fracture of teeth with various preparations for amalgam. J Dent.1996;24(6):407-10. El-Sherif MH, Halhoul MN, Kamar AA, Hourel-Din A, Fracture strength of premolars with class 2 silver amalgam restorations. Oper Dent 1988;13:50-3 Fields HW, Proffit WR, Case JC, Vig KW. Variables affecting measurements of vertical occlusal force. J Dent Res. 1986;65(2):135-8. Gorucu J, Ozgunaltay G. Fracture resistance of teeth with class II bonded amalgam and new tooth-colored restorations. Oper Dent. 2003;28(5):501-7. Hansen EK, Asmussen E, Christiansen NC. In vivo fractures of endodontically treated posterior teeth restored with amalgam. Endod Dent Traumatol. 1990;6(2):49-55. Johnson JK, Schwartz NL, Blackwell RT. Evaluation and restoration of endodontically treated posterior teeth: A Review. J Am Dent Assoc. 1976;93(3):597-605. Kamegai T, Tatsuki T, Nagano H, et al. A determination of bite force in northern Japanese children. Eur J Orthod. 2005;27(1):53-7. Kiliaridis S, Johansson A, Haraldson T, Omar R, Carlsson GE. Craniofacial morphology, occlusal traits and bite force in persons with advanced occlusal tooth wear. Am J Orthod Dentofacial Orthop. 1995;107(3):286-92. Linn J, Messer HH. Effect of restorative procedures on the strength of endodontically treated molars. J Endod. 1994;20(10):479-85. Maki K, Nishioka T, Morimoto A, Naito M, Kimura M. A study on the measurement of occlusal force and masticatory efficiency in school age Japanese Children. Int J Paediatr Dent. 2001;11(4):281-5. Mondelli J, Steagall L, Ishikiriama A, et al. Fracture strength of human teeth with cavity preparations. J Prosthet Dent. 1980;43(4):419-22. Ortega VL, Pegoraro LF, Conti PC, do Valle AL, Bonfante G. Evaluation of fracture resistance of endodontically treated maxillary premolars, restored with ceromer or heat-pressed ceramic inlays and fixed with dual-resin cements. J Oral Rahabil. 2004;31(4):393-7. Panitvisai P, Messer HH. Cuspal deflection in molars in relation to endodontic and restorative procedures. J Endod.1995;21(2):57-61. Pilo R, Brosh T, Chweidan H. Cusp reinforcement by bonding of amalgam restorations. J Dent. 1998;26(5-6):467-72. Reeh ES, Messer HH, Douglas WH. Reduction in tooth stiffness as a result of endodontic and restorative procedures. J Endod. 1989;15(11):512-6. Rentes AM, Gaviao MB, Amaral JR. Bite force determination in children with primary dentition. J Oral Rehabil. 2002;29(12):1174-80. Waggoner, W.F.: Restorative dentistry for the primary dentition; in Pinkham, JR. (ed): Pediatric Dentistry. Infancy through adolescence.4th Ed, Philadelphia, W.B. Saunders Co; 2005. pp. 348.