Mechanical strength of tooth fragment reattachment.

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Mechanical strength of tooth fragment reattachment

Roberto De Santis1,*_{, Davide Prisco}2_{, Showan N. Nazhat}3_{, Francesco Riccitiello}2_{, Luigi Ambrosio}1_{, Sandro Rengo}2_, Luigi Nicolais1

Article first published online: 27 MAR 2001

DOI: 10.1002/1097-4636(20010615)55:4<629::AID-JBM1057>3.0.CO;2-2 Keywords: fragment reattachment; mechanical strength; fatigue Abstract

The aim of this study was static and fatigue test investigation of the strength of a tooth fragment reattached with adhesives to the tooth body. Central bovine incisor teeth were used, and standardized fragments were obtained by cutting the incisal edge of the selected teeth. All the fragments were reattached using a multistep dentine adhesive system, and the specimens were randomly divided into two groups (A and B). Group B specimens underwent a further dental treatment: a circumferential double chamfer prepared around the external cut interface was filled with light cured composite restorative resin. Static and fatigue bending tests were performed and linear elastic equations were used to analyze and compare the strength of the treated teeth. The results indicated that the static and fatigue bending properties were improved by using reinforcement with composite restorative resin. © 2001 John Wiley & Sons, Inc. J Biomed Mater Res 55: 629–636, 2001

INTRODUCTION

Injury to anterior teeth is a common event among children, and coronal fracture is the most frequent form of acute dental injury that represents about 8% of dental traumas.1, 2 Their restoration is achieved with different materials and techniques. Use of post and core3, 4 or laminate veneers5 results in a static strength comparable to that of an intact incisor. However, failure of teeth restored with cast posts is characterized by root fracture.6–8 Composite restorations are characterized by a lower hardness when compared to the enamel9, 10; moreover, water absorption and wear properties are further drawbacks of composite restorations.11–13 The reattachment of the fractured fragment can offer several advantages over other techniques14 (i.e., improved aesthetics15, 16 and function) that were gross, and surface anatomy is restored with increased resistance.1, 2, 17 However, the static bending strength of the restored teeth is lower when compared to the intact tooth1 and the reduction of the mechanical strength varies from 20 to 60% according to the type of bonding agents and the restoration technique employed.2, 17, 18 Moreover, the fracture strength of a rebonded fragment drops drastically with an increase in loading speed.18 About 50% of rebonded fragments failed after 2–3 years because of a new trauma or use of the restored teeth2 in an unphysiological way. Even if the reattachment technique for fractured anterior teeth is regarded as a semipermanent solution, it should be preferred, especially for children, because it helps to preserve the dental tissues during tooth development.19, 20

 Although the relationship between the mechanical properties of dental tissues and bonding to dentin is not completely clear,21, 22 enhancement in bonding to dentin has been achieved by using new adhesive systems, suggesting improvements in the fragment reattaching techniques.23

 In the present study central bovine incisor teeth were used to compare the static and dynamic strengths of restored teeth according to different restorative techniques. Bovine incisor teeth can be obtained in sufficiently large number with limited variation in size; these specimens are suitable for adhesion and fracture tests because they show mechanical properties that are similar to human dental tissues.24, 25

 The aim of this investigation was to analyze the strength of fragment reattachment with and without the use of a circumferential double chamfer2, 26 around the external fracture line reinforced with composite resin.

MATERIALS AND METHODS

Central incisors were extracted from intact mandibles of bovine animals (average age of 2 ± 0.2 years). Extractions were carried out 30 min after sacrifice. All samples were gently polished after extraction and stored in physiological saline at 37°C. Magnifying loupes (4× magnification) were used to check if the tooth

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surface had any damage. The pulp was left in the chamber in order to preserve tooth hydration, and the root apex was sealed with poly(methyl methacrylate) (PMMA) bone cement.

Specimens were selected according to the geometrical dimensions. The dimensions of the selected teeth at a cross section positioned at 7 mm from the incisal edge were 14 ± 1 mm width (b) and 6 ± 0.5 mm thickness (2a; Fig. 1). One hundred twenty selected central incisor bovine teeth were used.

Figure 1. The geometric and mechanical testing conditions: (a) the tooth. (b) The edge of the incisor tooth is modeled as a cantilever system; the grey section is the interface between the tooth fragment and the tooth body and c is the distance between this section and the applied load F. (c) A lateral view of the tooth preparation.

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Specimen preparation

All specimens were cemented in stainless steel hollow cylinders [20-mm height (h), 14-mm internal diameter (ϕi), 16-mm external diameter (ϕc)] using self-curing resin (PMMA bone cement). Custom-made equipment

was used to position each specimen with the coronal axis parallel to the cylinder axis.

Specimens were then sectioned perpendicularly to the mesial or distal surface by using a low speed micrometer (Buehler Isomet) with a diamond saw (0.3-mm width). The distance between the incisal edge and the section was 7 mm. The geometrical dimensions of the sectioned surface were measured (width b and thickness 2a, Fig. 1) and the moment of inertia about the neutral axis was computed according to the following equation: ((1))

 ((1))

The dentin and enamel were etched for 15 and 30 s, respectively, with 37% phosphoric acid gel (Kerr Corp., Orange, CA). The fragments were then reattached using the multistep adhesive system Optibond FL (Kerr Corp.). This bonding agent was used following the manufacturer's instructions. The realignment of the fragments was carried out using magnifying loupes and the light curing process was for 1 min in the vestibular, palatal, mesial, and distal side of each specimen at a distance of about 2 mm.

Specimens were randomly divided in two groups (A and B). Group A specimens were static and fatigue tested; group B specimens underwent a further dental treatment: a circumferential bevel was made along the sectioning line. A round shaped diamond bur (Intensive 201S) cooled by chilled water was used, and the long axis of the bur was kept perpendicular to the specimen surface. The depth of the circumferential bevel was half of the diamond bur diameter (ϕ = 1.5 mm).

The bevel was then filled with Prodigy resin composite (Kerr Corp.) using an incremental technique. Eight increments of resin composite were used, and polymerization of the composite resin was completed by using an Optilux 500 lamp (Kerr Corp.). The composite surfaces were refined with diamond disks (Sof-lex Pop-on, 3M). Then the specimens were stored in physiological saline at 37°C for 24 h prior to testing.

A palatal–labial bending load (F) was applied perpendicular to the main axis of the tooth (z) as shown in Figure 1.

Static tests

Forty-five specimens from each group were used. The load distance from the cutting line (distance c in Fig.

1) was 1, 3, or 30 mm. The latter condition was achieved by using a rigid arm constrained to the tooth fragment.

The load was applied using a Bionix MTS858 testing system (MTS, Minneapolis, MN) working in a displacement control mode. The displacement was increased at a speed of 0.5 mm/min and load-displacement data were acquired at a sampling rate of 10 points/s. The loading cells used were in the 2.5-kN and 250-N range.

Dynamic tests

Dynamic tests where performed using the Bionix MTS testing system in the force controlled mode. The load was applied at a distance of 3 mm from the line of fracture. The fatigue behavior was investigated at a frequency (f) of 2 Hz. The minimum load (Lmin) was 10 N, and three values were used for the maximum load

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(Lmax, i.e., 66, 50, and 33% of the ultimate static load). Tests were performed in a saline solution at 37°C. Fifteen specimens from each group were used.

Mechanical analysis

The cantilever system was analyzed using the equations derived from the linear elasticity model27_:₍₍₂₎₎

 ((2))

((3))

 ((3))

where σz and τzy are the normal stress and shear stress, respectively; Mz is the bending moment, Mz = Fz;F is

the applied load; Ix is the moment of inertia of the z cross section about the neutral axis; y is the distance

along the cross section width (Fig. 1); and a is half of the cross section thickness. The mechanical data were statistically analyzed using the Student t test.

By applying Equation (2) in the section of interest (z = c) and with the hypothesis of a homogeneous and isotropic system, the edges’ compressive and tensile stress are given by ((4))

 ((4))

where Mc = Fc.

The maximum shear value was reached when y = 0 (the central plane): ((5))

 ((5))

The equivalent stress distribution27 related to the maximum load (Fu) in the section of interest (z = c) is

given by ((6))

 ((6))

Scanning electron microscopy (SEM)

Bovine incisor teeth were used for SEM investigations. Specimens were ground flat, and the dentine surface was bonded using the Optibond FL adhesive system according to the manufacturer's instructions. Specimens were fractured, and the resin dentine interface was examined using a Leica S440 (Leica, Cambridge, UK). RESULTS

Table I compares the ultimate load according to the different distances between the applied load and the line of fracture. All specimens fractured at the interface between the reattached fragment and the tooth body. Table I. Mechanical Properties of Tooth Fragment Reattachment of Unreinforced (Group A) and Reinforced (Group B) Specimens

Group c = 1 mm c = 3 mm c = 30 mm

1. a

Each value is the mean ± standard deviation of the ultimate static load (N) of 15 specimens.

A 412 ± 51 220 ± 65 26 ± 4.2

B 399 ± 56 405 ± 88 41 ± 4.5

Figures 2 and 3 compare the mechanical behavior of each group according to Equations (4) and (5), respectively, with a distance (c) of 3 mm. The normal stress in the cross section edges and the shear stress in the middle plane are plotted against the deflection of the section z = c (the testing machine actuator displacement).

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Figure 2. The maximum normal stress (σc,max) versus the deflection for a distance (c) of 3 mm (see Fig. 1

legend). Group A is the unreinforced specimens and group B is the reinforced specimens. The bar lengths are the standard deviations according to 15 specimens.

Figure 3. The maximum shear stress (τc,0) versus the deflection for a distance (c) of 3 mm (see Fig. 1

legend). Group A is the unreinforced specimens and group B is the reinforced specimens. The bar lengths are the standard deviations according to 15 specimens.

The plane stress state follows from the perpendicular incidence between the applied load and the x axis (neutral axis): σx = σy = τxy = τxz = 0. Hence, at any given point of the loaded structure, the stress tensor is

identified by only two stress components: σz (the normal stress) and τzy (the shear stress).

The σz is an increasing function of the cross section distance (z) reaching its maximum values at the edge of

each cross section [Equation (2)] while the shear stress distribution is independent of the cross section position. It is parabolic through the depth of each cross section [Equation (3)].

Fatigue tests performed with the maximum load (Lmax = 0.33 Fu) were stopped at 107 cycles without specimen failure. The number of cycles to failure related to each dynamic condition and group are shown in Table II. Table II. Number of Cycles to Failure of Unreinforced (Group A) and Reinforced (Group B) Specimens Lmax = 0.66Fu 5 × 102 ÷ 5 × 104 Lmax = 0.50Fu 5 × 103 ÷ 5 × 105 Lmax = 0.33Fu 5 × 106 ÷ Indefinite 1. a

The specimens were stressed at a frequency of 2 Hz between the minimum load (Lmin = 10 N) and the maximum load (Lmax) applied at 3 mm from the interface between the tooth fragment and the tooth body. The number of specimens used for each group was 15.

Group A range Minimum 120 6,300 Indefinite Maximum 13,000 1.7 × 105 Mean 4,900 57,000 Standard error 2,200 30,500 Group B range Minimum 550 2,700 Indefinite Maximum 25,000 2.4 × 105 Mean 6,800 66,000 Standard error 4,500 44,000

Figure 4 compares the maximum normal and shear stresses of each group computed using Equations (4) and (5) with F = Fu and c = 1, 3, and 30 mm.

Figure 4. The maximum normal stress and shear stress of the attached tooth fragment at different distances (c) of the applied load (F) from the adhesive interface of group A (unreinforced) and group B (reinforced)

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specimens. Each column height is the mean stress value of 15 specimens, and the bar length is the standard deviation.

The SEM of the fracture surfaces showed a well-defined hybrid layer between the resin and dentine. Figure 5

shows the opposing fracture surfaces of the dentine and resin. The resin tags in the tubules, which run perpendicular to the fractured surface, suggest a high quality bonding between the resin and dentine substrate.

Figure 5. An SEM micrograph (original magnification ×1500) of the interface between bovine dentine and Optibond FL. Resin tags in the tubules, run perpendicular to the fractured surface, suggest high quality bonding between the resin and dentine substrate.

DISCUSSION

A central incisor tooth must withstand a combination of bending and compressive loads. The tooth and surrounding bone may be regarded as a cantilever system. The fracture mechanics of hard tissues suggest that these materials behave as semibrittle solids.28 When the tooth is fatigue loaded under a maximum load lower than 50% of the static strength, the system is stable, even according to an open-loop loading configuration, because yielding does not occur at this low stress level and the material is characterized by elastic behavior.

Bending (or flexural) testing methods are used to evaluate the strength of brittle materials because they show a linear load-displacement behavior up to the point of fracture.27 Brittle materials are usually stronger in compression than in tension, so that the maximum tensile stress is the cause of the failure.

The normal stress distribution, Equation (2), is linear through the depth (y direction) of each cross section. The maximum tensile and compressive stresses are reached on the opposite edges. This stress behavior suggests that the central plane of the teeth is a neutral area with regard to stress concentration related to flexural loadings.29

Similar to bone, tooth structures are connective hard tissues with an inorganic mineral part of hydroxyapatite (crystals) and an organic matrix that is mainly collagen.30 The resulting composite structure is characterized by different arrangements at different points of the tooth.31, 32 Thus, a biomechanical model should take into account the information related to structural nonhomogeneity and anisotropy. Although there have been advancements in the techniques for structural investigations, the mechanical behavior of teeth is still misunderstood. In this work linear elastic models and the hypothesis of a homogeneous isotropic structure were used. This investigation was successful in comparing the mechanical behavior of teeth treated according to the two different techniques.

Figure 4 shows that for a distance of 1 mm there is no statistical difference between the bending strength of group A and B specimens (p = 0.51). However, for a distance of 3 mm the ratio (t value) between the sample mean difference and the standard error of this difference is 13, and there is a statistically significant difference (p < 0.01) between the mean strength values of group A and group B. A statistical difference is also obtained for a distance of 30 mm (t = 11 and p < 0.01).

According to Equations (2) and (3), the bending or the shear distribution is negligible when the distance of the surface of interest in the z direction tends to zero or to infinity, respectively. Unfortunately, these simple conditions are difficult to match experimentally; thus, the distances of 1 and 30 mm were used to investigate the behavior in these extreme stress conditions.

Our results suggest that the normal and shear stress are both important in the section of interest and the resulting stress distribution is a composition of the stress fields related to bending and shear.

The strength values obtained in this study (Table I) for a distance of 1 mm were consistent with those found by Badami et al. by using the adhesive system Scotchbond 2 (Fu = 393 ± 97 N).1 Figure 6 shows the

equivalent stress distribution in half-thickness of the tooth–fragment interface at different distances of the applied load from the adhesive interface. The equivalent stress versus the position in the y direction [see Fig.

1(b)] is computed according to Equation (6) by using the ultimate load values presented in Table I. These

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values take into account the strength of the restored tooth according to the adhesive system and the restoration techniques. The values of the distance between the applied load and the adhesive interface (1, 3, and 30 mm) were used to compute the stress distribution according to the different testing conditions. Figure

6 shows that the stress distribution shape is different: if the distance is 1 or 30 mm the curve decreases or increases, respectively, in the middle plane-edge direction. This different behavior is consistent with Equations (2) and (3). For a distance of 1 mm the shear distribution σ [Equation (3)] is more significant than the normal distribution τ [Equation (2)]; thus, the maximum equivalent stress is approximately at the middle of the section of interest. For a distance of 30 mm the maximum equivalent stress is approximately at the edge of the same section; thus, notch effects at the border of the tooth fragment reattachment lead to lower Fu values. Moreover, the effect of the reinforcement (the bevel) is negligible when the distance is 1 mm and important when it is greater than 1 mm.

Figure 6. The equivalent stress distribution in half-thickness of the tooth–fragment interface at different distances (c) of the applied load (F) from the adhesive interface. The equivalent stress versus the position in the y direction [see Fig. (b)] is computed according to Equation (6) by using the ultimate loads values (Fu) presented in Table I.

The SEM observation showed a well-defined hybrid layer characterized by deep resin tags and lateral branches together with intertubular infiltration. The result was intimate contact between the adhesive resin and tooth hard tissues, which is a characteristic of a high quality hybrid layer (Fig. 5).

Studies on the reattachments of enamel–dentin crown fragments suggest that fracture strength is lower than in the intact teeth.1, 2 This decrease depends on the type of bonding agent. By using bonding agents that enhance the hybrid layer performance,33, 34 the strength decrease was estimated to be about 30–40%.17, 18

Prevention of fatigue fracture is a vital aspect of structures that are subjected to repeated loading27; thus, the fatigue behavior has to be analyzed before any prosthetic device and/or restoration technique becomes a viable solution. This study shows that in the low-cycle fatigue testing27 (i.e., fatigue life typically up to 102_÷ 104_{cycles) the fatigue strength (i.e., the stress amplitude corresponding to a fatigue life of N}

f cycles) of the

restored teeth was higher for group B specimens. However, the fatigue behavior was similar for groups A and B (Table II): there was no statistically significant difference (p = 0.69) between the mean number of cycles to failure of group A and group B specimens stressed in the same dynamic conditions (c = 3 mm and

Lmax = 33, 50, and 66% of Fu). In particular, the fatigue limit at 107 cycles was lower than 50% of the ultimate static stress (Table II).

CONCLUSIONS

Mechanical static tests showed that resin composite reinforcement of the fracture (cutting) line increased both the static and fatigue bending strength.

The fatigue tests results were consistent with the static results: resin composite reinforcement of the bevel improves the retention of the reattached fragment; however, both groups were characterized by a fatigue limit lower than 50% of the ultimate static load.

From the present results it can be concluded that crown fragment reattachment represents a realistic treatment alternative to composite resin buildup restoration. Better bonding strength may result from improved bonding resins and techniques. The mechanical static and fatigue tests suggested that the double chamfer preparation reinforced with composite resin improves the fragment retention. This reinforcing technique becomes more relevant when the distance of the applied load from the attachment interface was increased.

Acknowledgements

The authors are grateful to Dr. A. Tonelli for her assistance and the supply of the specimens References

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