Effect of storage environment on the bond strength of reattachment of crown fragments to fractured teeth

Abstract

Aim:

The aim of this study was to examine various storage environments for storing fragments before being bonded to the remaining teeth and also estimate the required force to fracture the restored teeth.

Materials and Methods:

Sixty mandibular incisor teeth were fractured on the incisal one-third and were divided into five groups of 12 each to be stored in normal saline, water, milk, saliva and dry environments for 24 hours. All the fractured parts in each group were bonded to their relevant apical parts by an etch and rinse bonding system and a flowable composite resin. The fracture resistance was measured by a universal testing machine, and the results were analyzed using one-way ANOVA and Tukey statistical tests.

Results:

The results revealed that the difference among the five groups was statistically significant (P<0.001). Tukey tests showed that the force required for fracturing fragments kept in the milk and saliva environments were significantly higher than those for the normal saline, water and dry environments (P<0.05 ).

Conclusions:

It was concluded that keeping the fractured parts in milk and saliva environments can increase the required force for fracturing teeth more than the other environments.

INTRODUCTION

Permanent tooth crown fracture is one of the common traumatic problems among different age groups and both genders.[1,2] Several techniques have been proposed for restoring fractured crowns, including stainless steel crowns, orthodontic bonds, pin retained resin restorations, basket crowns, composite resins with acid etch adhesives techniques, porcelain veneers and jacket crowns, each of which show diverse degrees of success.[3] However, the development of composite resins has entailed the use of adhesive materials and techniques, which may also be applied for restoration and reattachment of fractured teeth.[3,4] Moreover, esthetic dimensions of restored teeth, which may be achieved by means of a crown, decrease the traumatic impact on patients with injured teeth.[2,5]

Fragment bonding has several advantages over other techniques which include: a) superior natural appearance as no composite resin appears as natural and translucent as the patient's own incisor enamel; b) harmonious wear, as most composite restorations wear out faster than the enamel while the patient's own incisor edge wears harmoniously; c) preservation of the pulp vitality; and finally, d) economical and less time consuming reconstruction of the contour and morphology of the crown.[3,6–8]

Some researchers explain how to technically reattach the fractured part or how to improve the bond strength.[2,3,9–14] Keeping the fractured part in a wet environment before reattaching has been studied by others, supporting the effect of a moist environment.[15,16] Since successful reattachment of a fractured fragment depends on the time of restoring the fractured part after trauma (which may vary from a few hours to a few days) and the patient's awareness, the fractured part may variably lose its moisture. The restoration time can affect bond strength of these restorations because dentin moisture is essential for achieving high bond strength of composite resins with dentin.[17] No studies have yet been reported on the kinds of environments, for example, saliva, water, milk or normal saline, in which parents may store fractured parts of teeth, like what should be done in case of dealing with avulsed teeth, before referring to the dentist. We may also come across avulsed teeth with fractured parts that are carried to the clinic in different storage environments. The focus of this study was to examine the effect of different storage environments on the quality of fragment restoration and the bond strength of restored teeth through reattachment methods.

MATERIALS AND METHODS

In this experimental laboratory study, 60 human mandibular incisors, which were extracted because of periodontal diseases, without any defects such as fractures, decalcification, or caries were collected. They were kept in 0.2% thymol solution. The teeth were randomly and equally divided into five groups of 12 each. After scaling the teeth crowns, they were prepared for fracturing. A line was traced on each tooth, 3 mm from the incisal edge and parallel to it. Then, using a diamond disk (Reference no. 918F, D + Z, Diamant, Lemgo, Germany), an enamel-deep fracture line was made on the lingual side of each tooth (situated on the traced line). On the labial side of the line, a small notch was made in the middle of the surface to prevent the blade used later on from slipping. They were then fractured by a perpendicular force applied labio-lingually. The force was applied on the enamel cut by a blade on the lingual surface of the tooth and by another blade positioned in the opposite direction on the labial surface. The surgical blades (No. 21, Aesculap AG, Tuttlingen, Germany) were replaced for every three samples. The teeth displaying a fracture pattern different from the premeditated line or an unclear fracture line were discarded at this stage. Then, the apical parts were kept in distilled water and at an ambient temperature.

For the purposes of this study, five storage environments including normal saline, water, common cow's milk (Pegah Milk, Isfahan, Iran), saliva (Artificial saliva; Bioxtra, Biohealth care, Belgium) and a dry environment were prepared in which the fractured parts of the teeth were to be preserved for 24 hours. The apical parts of the teeth were stored in normal saline at 37°C. Table 1 summarizes the environments and their features.

Table 1.

Storage environments for the teeth fragments according to their groups

In the first group, the fractured parts were rinsed, dried and bonded to the remaining part using a bonding agent (Single Bond, 3ME SPE, St. Paul, MN, USA, Batch No. 6KR). Both the fractured surfaces were etched by phosphoric acid 35% (Ultra etch, Ultradent Products, Inc., South Jordan, UT, USA) for 15 seconds; they were rinsed with water for 15 seconds and dried with paper towel in a way that slight moisture remained on the surfaces to be attached. Two layers of Single Bond were applied on the etched surfaces in such a way that first, one layer of the bonding agent was used and after 10 seconds, when priming was done, it was thinned gently by air blow for 3 seconds. Then, the second layer was immediately placed and thinned for 3 seconds to get a shiny and glossy surface.[3,18] Following the manufacturer's instruction, the bonding agent was cured using a light curing unit (Coltolux 75, Coltene/Whaledent Inc., Mahwah, NJ , USA) at a wavelength of 480 nm. For reattaching the fractured part to the tooth, a flowable composite (Filtek Flow, 3M ESPE, St. Paul, MN, USA) was used on both the fractured surfaces, and the two parts of each tooth were pressed together and cured for 40 seconds from the labial and lingual directions and for 30 seconds from the mesial and distal sides. Any excessive composite resin was removed with a sharp scalpel blade and the reattached samples were kept in normal saline in an incubator at 37°C. Similar procedures were repeated for Groups 2, 3, 4 and 5; however, the fractured parts were stored in water, milk, saliva and dry environments, respectively. In the next step, each sample was mounted in an acrylic block (Acropars, Marlic Co., Tehran, Iran). The teeth roots were embedded in acrylic resin up to the cingulum, so that the line axis of the tooth was parallel with the line axis of the acrylic block and the incisal edge was parallel with the horizontal line.

Fracture test

The specimens were loaded on a universal testing machine (Dartec HC 10, Dartec Ltd., Stourbridge, England). The load was applied using a ball-shaped stainless steel device, measuring 3 mm in diameter. The force application tip was positioned exactly on the fractured line, at 90° on the facial surface of the crowns and the machine was activated at a speed of 0.5 mm/min until the specimens fractured. The force required to fracture each specimen was measured in Newtons. The results were compared using one-way analysis of variance (ANOVA) and Tukey's statistical tests at a 95% level of significance (α < 0.05). After the samples were fractured, they were recollected in order to study the fracture mode in each group.

RESULTS

The forces required for fracture in each group are shown in Table 2. One-way ANOVA indicated differences in the amount of force required for making fractures in different groups (P < 0.001). There was no statistically significant difference between Groups 3 and 4, but values for these groups were larger than those of the other groups [Table 3]. The required force for fracture was lesser in Groups 1 and 2, compared with Groups 3 and 4. Group 5 had the lowest required force for fracture, though not significantly different from Groups 1 and 2 (P > 0.05).

Table 2.

Required force (N) to fracture specimens according to the storage environment for each experimental group

Table 3.

Level of significance according to Tukey's test

The fracture pattern in restoration with reattachment in all groups revealed adhesive fractures in the reattachment line.

DISCUSSION

Mandibular incisors are suitable for studying the required force for fracture, have little difference and variation in their dimensions, and are easily available. In this study, other retentive methods such as enamel bevel, chamfer preparation or retentive dental grooves were not used, in order to only investigate the effect of the storage environment on fractured strengths. Also, Single Bond was used in order to have a durable and strong enamel bond along with an acceptable dentin bond, and not the self-etching systems which have a weaker enamel bond.[19,20]

In a study, Toshihiro et al. observed that reattachment of the fractured part resulted in discoloration because of losing its moisture; but after 1 month, the fragment had regained some of the original color and translucency, and after 1 year, the reattached fragment had satisfactory esthetics and excellent function.[21] However, in the present study, focus had been on achieving better bond strengths and the fractured parts had to be kept in proper storage environments. No major changes were seen in the appearance of the teeth.[14] The teeth were fractured in order to optimize the simulation conditions.

Some studies recommend keeping fractured parts in moisture in order to prevent dehydration or discoloration. Some suggest a normal saline solution at 37°C and others recommend water or salt solution in a closed container.[5] Milk, normal saline, saliva, and water were selected for this study because they are easily available. The first choice for moist environment is milk and saliva, and the second choice is solutions which are hypertonic. Moreover, when the tooth is avulsed, parents are advised to store it in milk or saliva. Therefore, in case of other tooth injuries such as a crown fracture, some parents tend to keep the fragment in milk or saliva rather than water. As there was no study related to storage environments of fractured parts, and different storage environments are suggested, this study was conducted to examine the best storage environment before reattachment.

The required force for fracturing the healthy teeth is twice as much as that needed for the reattached samples.[8,22] Since the average force imposed on the anterior teeth is about 150 N[23] and the force needed to break the reattached parts is higher than that, the reattachment of the fractured part can be durable. The durability can be further enhanced by using other retentive methods.[2,11,14,22]

It was noted that the fractured part which was kept in a dry environment before reattachment had the lowest bond strength, supporting Farik's finding.[14] Intact sound dentin which is stored in a dry environment for 24 hours retains only about 25% of the total amount of its moisture.[24] It seems that this partial loss of dentin moisture and its shrinkage results in the reduction of the composite surface contact with dentin. Besides, over acid-etching due to loss of moisture in the fractured part's dentin may occur and result in unfavorable effects on the bonding condition.

The bond strength of dry group in our study was not significantly different from that of the water or normal saline groups. Due to the difference in the storage environments (milk or saliva in this study) and the role of rinsing and etching itself in rehydration of the samples, each group was rinsed only once. Rinsing was also done in the negative control group; this may be the reason why the control group produced results similar to those of the groups with normal saline and water storage environments. According to Farik, moisturizing the fractured part affects the bond strengths and the bond strength improves by increasing the moistening time.[15] Also, Capp et al. found that a dried fragment has a lower bond strength compared to a fractured part which is kept in a moist environment or is moisturized before reattachment.[16]

In groups whose fractured parts were kept in normal saline or water, their bond strength increased more than those of the groups whose fractured parts were kept in a dry environment. This also shows the important role of moisture in the bonding mechanism.

The best storage environments, as observed in this study, are milk and saliva. It seems that in these two environments, little osmotic and dimensional changes happen in the dentin surface and a stronger bond strength is achieved. There is no mention of this in the current literature and more investigations are required on the subject. It has been proved that milk elements such as calcium and phosphate can harden and stiffen both demineralized and healthy dentin by permeating the surface.[25] This is probably the reason why enhancement of bond strength was observed in the milk and saliva groups which were rich in calcium and phosphates. Calcium and phosphate sedimentation can also affect the surface topography[26] of dentin and the degree of its dissolution during acid etching, which may also explain the better bond strength in the milk and saliva groups. However, further SEM (Scanning Electron Microscopy) investigations of fractured surfaces before and after acid etching are suggested for evaluation of the surface roughness in different groups, and also for measuring their thickness and depth of the hybrid layer.

CONCLUSIONS

This study shows that the force required to fracture the restored teeth is affected by the environment where the fractured part is kept before bonding. The best result is obtained when the fragment is kept in milk or saliva.