Introduction
In dentistry, traumatic tooth damage is a difficult emergency to treat and can result in many complications [1]. Traumatic dental injuries occur more commonly in children and adolescents. Uncomplicated crown fractures are the most com-mon traumatic dental injuries, representing 17 - 48% of dental injuries worldwide [2,3]. Crown fractures occur more commonly in permanent teeth than in deciduous teeth and more often in maxillary central incisors because they protrude forward [4,5].
The maxillary central incisors are aesthetically important. Hence, restoring teeth with crown fractures is essential for pulp protection and functions such as mastication, as well as for restoring aesthetics [6]. With the development of adhesive systems, composite resin restorations are increasingly being used for treating coronal fractures; however, discoloration and poor margin compliance are often observed during composite resin restorations [6]. According to the International Association of Dental Traumatology (IADT) guidelines, if the tooth fragment is available and intact, it can be bonded back to the tooth, and the fragment should be rehydrated before bonding [7]; however, rehydration recommendations and protocols are not specified in recent guidelines [8].
Given this lack of guidance, studies have been published on the bonding strength and degree of color recovery based on the rehydration time. Garcia et al. [9] recommended immersion in distilled water for 30 minutes, whereas Shirani et al. recommended 24 hours. Poubel et al. [10] found no difference in bond strength between immersion-rehydration protocols lasting longer than 15 minutes. Toshihiro and Rintaro [11] observed that reattaching the fractured part resulted in discoloration due to moisture loss; however, the fragment regained some of its original color and translucency after 1 month and had satisfactory aesthetics after 1 year.
Before arriving at the dentist, parents may store fractured parts of teeth, as suggested, when dealing with avulsed teeth [12]. However, research on the ideal storage medium for improving bonding strength and color recovery after tooth fracture is lacking. Hence, this study aimed to investigate the effects of various common storage media on color recovery and bonding strength after tooth fragment reattachment.
Materials and Methods
The protocol was approved by the Institutional Review Board (IRB) of Kyungpook National University Dental Hospital (IRB No: KNUDH-2024-09-04-00).
1. Preparation of tooth specimens
This study selected 60 central incisors extracted due to periodontal reasons. The study included permanent incisors with no developmental defects in the coronal region, traumatic injuries, and caries. A simple crown fracture of the anterior tooth was reproduced by cutting diagonally at 1/3 of the way from the incisal edge using a rotary disc (Fig. 1A, 1B). This formed an oblique fracture, typical of a crown fracture, in which the fracture line ran downward from the lingual to the labial side. For this purpose, the fracture line was angled at 60° relative to the long axis of the tooth. Coronal fracture fragments were separated to avoid invading the pulp space.
According to the storage medium used for the fractured crowns, the dehydrated teeth were classified into six groups: control (dry), water, 0.9% isotonic saline solution, milk (pasteurized milk), artificial saliva, and casein phosphopeptide-amorphous calcium phosphate (CPPACP, GC Tooth Mousse, GC Corporation, Tokyo, Japan). The remaining root portion was soaked in artificial saliva, and the coronal portion was dehydrated for 24 hours to replicate most situations in which patients could not visit the hospital immediately after trauma. The International Association of Dental Traumatology (IADT) recommends rehydrating the fractured coronal portion for 20 minutes. Accordingly, in this study, the fractured fragments were reattached after 20 minutes of rehydration in each storage medium. The experiment was conducted at room temperature.
The fractured portion was rinsed, dried, and bonded to the remaining portion using a bonding agent (Single Bond Universal, 3M ESPE, St. Paul, MN, USA). The fracture surfaces were etched with phosphoric acid 32% (Uni-Etch, Bisco, Schaumburg, IL, USA) for 15 seconds (Fig. 1C), rinsed with water for 15 seconds, and dried using a paper towel, leaving a small amount of moisture on the attachment surfaces. Two layers of single-bond adhesive were applied to the etched surfaces (Fig. 1D). The first layer was thinned gently using blown air for 3 seconds, and the second layer was immediately applied and thinned again for 3 seconds to obtain a shiny surface. The bonding agent was cured using a light-curing unit (Ivoclar Vivadents, Schaan, Liechtenstein). For reattachment, a flowable composite (Filtek Flow; 3M ESPE) was used on both fracture surfaces, and the two parts were pressed together and cured for 40 seconds on the buccal and palatal sides (Fig. 1E, 1F). Excess composite resin was removed using a sharp scalpel blade, and the reattached samples were subsequently stored in artificial saliva. In this study, a bevel was not applied to evaluate the fracture resistance of reattached fragments across different storage media.
2. Color change measurement
The color change was measured using a spectrophotometer (CM-2600d; Konica Minolta, Tokyo, Japan; Fig. 2). Color measurements were taken on the fractured segments immediately before reproducing the tooth fracture (T0), after a 24 hours drying period following tooth fracture (T1), and immediately after rehydrating the fractured crown in the storage medium (T2) for 20 minutes. After reattachment, measurements were taken at 12 hours (T3), 24 hours (T4), 1 week (T5), and 3 weeks (T6). The timeline for color change measurement is illustrated in the diagram (Fig. 3). All teeth were kept in saliva to replicate post-reattachment conditions in the oral cavity, and their color was measured and recorded seven times.
The colors were measured based on the CIE L*a*b* color space presented by the International Commission on Illumination. Before each measurement, the device was zero-adjusted using the manufacturer’s standard. The lens was placed at the center of the reattached fragment, and the color measurement was taken under a D65 light source, the standard main light for color measurement. Measurements were repeated three times for each specimen at each time point, and the average value was recorded.
The change in tooth color was calculated as the color difference (ΔE*ab), using the individual component differences (ΔL*, Δa*, Δb*), between the CIE L*a*b* value at the relevant time point and the CIE L*a*b* value at the reference time point (T0): ΔE*ab = [(ΔL*)2 + (Δa*)2 + (Δb*)2]1/2. For this analysis, a ΔE*ab > 3.3 was set as the threshold standard for a color change that can be confirmed visually [13].
3. Fracture resistance test
Fracture resistance tests were performed 3 weeks after reattachment. Before measurement, all specimens were thermocycled 500 times, and the fracture resistance strength was measured using an Instron 3366 Universal Testing Machine (Instron®, Norwood, MA, USA). The specimen was fixed such that the long axis of the tooth was at an angle of 125° relative to the crosshead, reflecting the interincisal angle of a normal child (Fig. 4). A compressive load was applied to the lingual surface of the tooth at a crosshead speed of 1.0 mm/min. The maxi-mum load was recorded as the pressure at which the adhesive surface between the tooth and the fractured fragment failed and the fractured portion fell off. The fracture resistance strength (MPa) per unit area was measured by dividing the maximum load by the fracture surface area. The fracture surfaces of all sectioned teeth were photographed at 1 : 1 magnification using a digital camera, and the fracture surface area of each specimen was calculated using ImageJ FIJI 1.46 software (National Institutes of Health, Bethesda, MD, USA) to measure the fracture resistance per unit area.
4. Statistical analysis
The results were analyzed and statistically processed using SPSS version 25.0 (SPSS Inc., Chicago, IL, USA). Based on a 95% confidence level, color differences over time in each storage medium and the interactions between these factors were analyzed using a two-way repeated analysis of variance. Statistical significance and correlations among the individual experimental groups were analyzed using post-hoc pairwise between-group and within-group comparisons with Bonferroni correction. Statistical differences in fracture resistance were evaluated using the Kruskal-Wallis test.
Results
1. Color change
At T0 and T1, no significant difference was observed in the L* values between the six groups (Fig. 5). However, at T2, immediately after 20 minutes of rehydration, significant differences in the L* values were observed between the control group and experimental groups (p < 0.05). The CPP-ACP group induced a significantly greater recovery of L* values than the other experimental groups (p < 0.05). No significant differences from the initial T0 values were detected in any group 3 weeks after reattachment at T6, indicating the recovery of L* values (Table 1).
At T0 and T1, no significant difference was observed in the ΔE*ab values between the six groups (Fig. 6). However, at T2, immediately after 20 minutes of rehydration, significant differences in ΔE*ab values were observed between the control group and experimental groups (p < 0.05). The CPP-ACP group produced the most rapid recovery of the tooth’s original color, significantly reducing ΔE*ab values compared to the other experimental groups (p < 0.05).
At T6, 3 weeks post-rehydration, no significant differences in ΔE*ab values were observed between the six groups (Table 2).
2. Fracture resistance
Table 3 shows the results of the fracture resistance per unit area tests of tooth fragments rehydrated in various storage media. The CPP-ACP group had the highest fracture resistance per unit area (29.4 ± 17.0 MPa), followed by those stored in saline (28.4 ± 12.7 MPa). The control group showed the lowest fracture resistance (19.9 ± 7.1 MPa). No statistically significant differences were observed between the six groups (p > 0.05).
Discussion
Tooth fragment reattachment is a tooth restoration method that attaches a fractured tooth fragment to the secondary tooth using a resin material [14-16]. This treatment can be considered when the fracture line minimally invades the gingiva, or when an intact fragment is available [15]. It is a simple and conservative way to restore the appearance of a fractured crown [17,18]. This method is aesthetically pleasing because it maintains the original tooth shape, color, and surface texture, and the crown has a good long-term prognosis due to the similar wear rate between its incisal surface and that of the adjacent teeth [15,17,18]. In addition, this method compensates for the patient’s psychological loss after the trauma of the fracture because it uses the original tooth fragments; it is considered economical, involving lower costs and reduced treatment times compared to other procedures [19].
Toshihiro et al. [11] have established that among L*, a*, and b*, the L* value significantly influences changes in tooth color. The L* value is closely related to the moisture content of the tooth’s surface [20]. Dehydrated teeth can exhibit increased enamel opacity, which makes them appear whiter [11]. This is because the water around the enamel prisms is replaced by air [21]. This change alters the enamel’s refractive index, causing the dehydrated teeth to reflect more light than hydrated teeth, thus appearing brighter to observers [22,23].
In this study, color measurements were conducted at seven time points, from before reproducing tooth fracture to 3 weeks. Similar to previous research, the L* value was highest at the point of dehydration (T1) and gradually returned to its original value through rehydration. At the T2 time point, CPP-ACP demonstrated a more rapid recovery to the original value compared to other groups. Previous studies have indicated that calcium, a primary component of hydroxyapatite, plays a crucial role in light scattering on the enamel surface [24]. The high calcium content in CPP-ACP is likely to have contributed to the accelerated recovery of brightness.
The ΔE*ab value is primarily used to judge color similarity. If the ΔE*ab value is less than 1.00, it indicates that even an expert cannot recognize the difference when comparing two colors side by side, and if it is less than 3.3, no significant difference will be noticeable to the average viewer [25]. In this study, a reference of 3.3 was set as the criterion for determining the recovery to the original color.
The ΔE*ab values measured 20 minutes after the reattachment of the fractured fragment showed a significant difference in all experimental groups compared to the control group. Previous studies have also shown similar results, indicating that dehydrated teeth return to their original color after 20 minutes of rehydration [21]. When comparing the tooth fragments three weeks after reattachment to the original teeth, all groups except the control group exhibited no significant color difference to the naked eye. This implies that the hydrated fragment is expected to exhibit no significant color difference from its original state to the patients within three weeks regard-less of the storage solution used.
Particularly, the CPP-ACP group was the first to reach a ΔE*ab value below 3.3 at the T3 time point, indicating that the color had recovered to a shade indistinguishable to the naked eye. CPP-ACP can provide short-term esthetic and functional restoration after fragment reattachment in clinical settings, offering satisfaction to pediatric trauma patients and their parents who experience distress from the injury. In our study, considering both L* and ΔE*ab values, all experimental groups appeared to be effective compared to the control group, with the CPP-ACP group being particularly useful for fragment reattachment due to color recovery, making it beneficial in clinical dentistry.
Farik et al. [26] emphasized that fracture resistance significantly decreases if tooth fragments are dried for more than an hour, highlighting the critical role of moisture content in successful reattachment. Similarly, Shirani et al. [27] demonstrated that simple rehydration without additional preparation, like beveling, effectively restores the fracture strength. In our study, the fractured fragments were reattached through rehydration alone, and the results showed clinically acceptable fracture resistance.
Under the conditions of this study, no significant differences in fracture resistance were found among the experimental groups. This indicates that rehydration over three weeks post-reattachment created a uniform, orallike environment, leading to comparable fracture resistance across all groups. While previous studies, such as those by Sharmin et al. [28] and Shirani et al. [29], suggest certain storage solutions may enhance bond strength, our findings demonstrate that any hydration method is sufficient, as long as it maintains moisture content.
The limitation of this study is that each group was stored in different storage media for 20 minutes and then maintained in artificial saliva to simulate clinical conditions. As a result, the measurement of fracture resistance was conducted after storing the specimens in artificial saliva for 3 weeks to observe color changes as well. This could have diluted the differences that might have been present due to the original storage media.
Conclusion
This study investigated the effects of different storage media and a control group on the color change and fracture resistance of reattached anterior tooth fragments after trauma. The CPP-ACP group demonstrated more effective color stability and short-term color recovery than the other groups after 20 minutes of rehydration. Additionally, other groups also showed noticeable color recovery by the end of the 3 weeks follow-up period. No significant differences were observed in fracture resistance tests between the six groups, suggesting all teeth were adequately rehydrated in artificial saliva after reattachment in 3 weeks.
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