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Fracture behaviour of teeth with conventional and mini-invasive access cavity designs
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Feature article
Fracture behaviour of teeth with conventional and mini-invasive access cavity designs Zdenˇek Chlup a,∗ , Radovan Zˇ iˇzka b , Jakub Kania b , Michal Pˇribyl b a b
CEITEC IPM, Institute of Physics of Materials AS CR, Brno, Czechia Institute of Dentistry and Oral Sciences, Palacky University Olomouc, Czechia
a r t i c l e
i n f o
Article history: Received 20 December 2016 Received in revised form 6 March 2017 Accepted 10 March 2017 Available online xxx Keywords: Fracture resistance Cavity Tooth Compressive test Fractography
a b s t r a c t Presented work is targeted toward fracture analysis of endodontically treated human teeth. Three sets of teeth were loaded by compression simulating natural loading conditions. For this purpose, each tooth was mounted into the resin in the axis angle declination of 30◦ and kept all the time in saline up to the moment of test to simulate the intraoral environment. Two access cavity designs – mini-invasive (conservative) and conventional (traditional), were analysed. Fracture behaviour of treated teeth with mini-invasive access was compared with conventional and with the intact set of teeth. Complex monitoring of the fracture process together with loading traces enables to characterise typical fracture features and crack propagation schemes. The extensive fractographic analysis reveals the effect of adhesive bonding on the crack propagation. Also, fracture initiation and damage accumulation were identified. The quality of newly developed mini-invasive design in comparison with the conventional one was proved. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction The investigation of fracture of composite materials and its mechanisms under mechanical loading are of scientific interest for decades due to their complicated structure [1]. The toughening mechanisms acting in the case of brittle matrix composites are beneficial for overall mechanical response and its reliability during application [2]. A similar situation can be observed in the case of natural materials where similar toughening mechanisms play an important role [3]. Rapidly developing research area deals with bio-inspired materials where the effort is dedicated to the manufacturing of artificial materials possessing similar behaviour as the natural one [4–6]. In the medicine, the combination of natural and artificial materials is very common when damaged or missing tissue reconstruction is needed. The behaviour of such a complex structure which is comprised of different composites is an important issue [7,8]. In the field of dentistry, the combination of natural composite material (tooth) and artificial man-made composite materials (restorative materials) is frequently clinically applied. In recent years, the concept of minimally invasive endodontics is gradually gaining acceptance in clinical dentistry even though there is only limited supporting scientific evidence [9,10]. The present
∗ Corresponding author. E-mail address: [email protected] (Z. Chlup).
concept of minimally invasive endodontics is strongly related to the preservation of sound tooth structure during access cavity opening under direct vision and instrumentation to smaller apical width and taper. Such preserved sound tooth tissue is capable of maintaining the mechanical stability of restored tooth [11,12]. The most important tooth structure responsible for long-term survival is supposed to be the pericervical dentin [13]. It is a dentin’s structure located 4 mm below and 4 mm above the alveolar crest [11], responsible for distribution of functional mechanical stresses inside tooth [14,15]. On contrary, in traditional endodontics, the removal of sound tooth structure according to the concept of extension for instrument fracture prevention was favoured. Traditional endodontic access cavities, i.e. hereafter abbreviated as TEC, are adjusted for risk reduction during root canal treatment. In TEC it is more probable to find aberrant anatomy, to discover fracture lines, to remove complete pulp from all pulp horns and to minimise a chance of the instrument breakage. In minimally invasive endodontics, it is also called minimally invasive, contracted [10] or conservative endodontic access cavity, i.e. hereafter abbreviated as CEC [16] is used state-of-the-art instrumentation to minimise the access cavity. In comparison to TEC, it prefers the removal of restorative materials to tooth structure of enamel to dentin and of occlusal tooth structure to cervical dentin. It preserves parts of the pulp chamber roof and pericervical dentin. Although the preserved tooth structure may offer a benefit of improved fracture resistance [17], the scientific evidence for CEC remains scarce. So
http://dx.doi.org/10.1016/j.jeurceramsoc.2017.03.025 0955-2219/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Z. Chlup, et al., Fracture behaviour of teeth with conventional and mini-invasive access cavity designs, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.03.025
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Fig. 1. Comparison of the TEC access cavity design (a), the CEC access cavity design (b), and the loading scheme (c).
far, only mandibular premolars have been studied [16], but not maxillary premolars. Maxillary premolars have a shape which facilitates the fracture of cusps under occlusal loads [18,19] and thus can be observed in the higher incidence of cusp fractures of upper premolars in the oral cavity [20,21]. Additionally, not only the shape of access cavity could play an important role but also the conditioning of the dentin before application of filling can affect (positively or negatively) the overall fracture resistance as was reported [22–24]. However, a direct comparison of fracture behaviour of CEC, TEC and intact premolars under loading was not yet reported. The objective of this study was to assess the influence of CEC and TEC on the fracture resistance of lower (mandibular) and upper (maxillary) premolars which were reconstructed according to the concept of minimally invasive dentistry and characterisation of fracture process using fractographic approach. 2. Experimental procedures 2.1. Material Thirty maxillary and thirty mandibular previously extracted human non-carious, mature, intact premolars were used. Subsequently, both maxillary and mandibular premolars were randomly divided into three groups – CEC group, TEC group and negative control group (n = 10 teeth/group). Teeth in CEC groups were drilled by long shank diamond bur (FGSL H.1.316.010, Komet USA, Rock Hill, USA) and were accessed by 1 mm buccal to central occlusal fossa. Access cavities were extended apically, maintaining part of the chamber roof [16]. Teeth in TEC groups were prepared according to generally accepted suggestions of traditional endodontic access cavity [25]. Root canals were negotiated using ISO 10 K-file (Flexofile; Dentsply Maillefer, Balleigues, Switzerland) all the way to the anatomical foramen. The working length was established as 0.5 mm coronally according to the anatomical foramen and root canals were shaped with primary WaveOne reciprocating instruments (Dentsply Maillefer, Balleigues, Switzerland). Canals were irrigated by 5% sodium hypochlorite using 30G irrigation needles (NaviTip, Ultradent, South Jorda, UT, USA). Shaped and cleaned root canals were filled with gutta percha and epoxy resin (AdSeal, Meta Biomed, Chungbuk, South Korea) using warm vertical compaction where fillings ended 1 mm apically to the orifice of the root canal. After that pulp chamber was cleaned with 96% ethanol and teeth were adhesively restored using selective etch adhesive system Singlebond Universal (3 M ESPE, St. Paul, MN, USA) and dual-cured resin composite (Dentocore, Spofadental, Jiˇcín, Czech Republic) with Filtek ultimate (3 M ESPE, St. Paul, MN, USA).
All teeth were mounted to a self-curing resin (Dentacryl, Spofadental, Jiˇcín, Czech Republic) at the angle of 30◦ from the tooth long axis and up to 2 mm apical to cementoenamel junction (see Fig. 1(c)). The comparison of access cavity of traditional (TEC) and mini-invasive (CEC) approach is demonstrated in Fig. 1(a, b).
2.2. Characterization All specimen teeth embedded in the resin were mounted to the cylindrical holder and placed to the micrometric positioning table mounted on the base of an Instron Universal Testing Machine (Instron, Canton, MA, USA), loading was carried by a stainless steel sphere with a diameter of 3/16” fixed to the loading bar. The loading scheme is shown in Fig. 1(c). The diameter of the loading stainless steel ball was determined according to the 3D reconstruction using a laser confocal microscope Lext OLX 3100 (Olympus, Japan) of the typical tooth as is shown in Fig. 2(a). The maximal central profile was extracted and diameter of the ball was adjusted to simulate an optimal contact with the tooth surfaces (see Fig. 2(b)) with the aim to maximise stresses in the cavity location. Therefore, prior testing, each specimen was preciously positioned to obtain perfect two-point contact with the loading stainless steel sphere as shown Fig. 1(c) and fixed in this position. The crosshead speed of 0.5 mm/min was used until final fracture occurred. The acoustic emission signal was monitored continuously during loading using an IPL Analyser (Dakel, Czech Republic). A digital 2MPX USB microscope for visual monitoring of the fracture process was used. The loading curves with both the acoustic emission signal and the video captured were synchronised to allow understanding of the damage development. A scanning electron microscope Lyra XML3 (Tescan, Czech Republic) for fractographic observation of the tooth fracture surface and analysis of the surface damage in the tooth-ball contact areas was employed. The fracture forces leading to the fatal damage of the individual tooth were determined from the loading curves on the basis of video and acoustic emission signal. The statistical evaluation of obtained results was conducted using statistical software Statgraphics Centurion XV (StatPoint, USA). Average load to fracture values were calculated for each group and data were compared among particular groups with Kruskal-Wallis tests. In the present study, no simulation of the periodontal ligament was performed. In the majority of published articles on fracture resistance of postendodontic treated endodontic treated teeth, there was no simulation of periodontal ligament [26]. Although such simulation is appreciated, standardised model for the simulation of the periodontal ligament has not been introduced yet [26].
Please cite this article in press as: Z. Chlup, et al., Fracture behaviour of teeth with conventional and mini-invasive access cavity designs, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.03.025
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Fig. 2. 3D surface reconstruction of the tooth (a), and corresponding tooth profile with the designed diameter of the loading stainless steel ball (b).
3. Results and discussion Six groups of human non-carious, mature, premolars were prepared for investigation of the effect of access cavity preparation method using static loading to the fracture simulating severe conditions with respect to possible axial tooth fracture. The typical loading curve of selected mandibular premolar treated by CEC technique is shown in Fig. 3. The mandibular premolar exhibiting the highest fracture force was selected to demonstrate all damage stages and ongoing fracture processes taking place during static loading. Three stages labelled A, B and C are marked on the loading curve and corresponding video frames are shown. Similar fracture stages were observed in all tooth groups under investigation, however, the terminal damage was usually identified as a fracture of protrusions (cusps) not the crack propagating trough the restored access cavity. The overview of a typical fracture behaviour through all groups under investigation is shown in Fig. 4 where final stages of fracture are presented by video frames captured during loading with crack paths marked by (red) arrows. The first damage of the tooth starts at relatively low load (i.e. usually between 100 and 200 N – see the loading curve in Fig. 3) with the enamel micro-cracking in the tooth – stainless steel ball contact areas due to high compressive stresses developed. An example of such area after the test is shown in Fig. 5(a). This kind of enamel micro-cracking was not detected by the AE sensor placed on the ball holder due to pre-set lower sensitivity to be able to detect the macro cracking of the tooth which produces a strong signal. The contact area is significantly plastically deformed and from this area the short cracks (usually assumed as median cracks) propagate radially. They frequently stop their propagation when the stress field intensity dropps. However, they can propagate further when their orientation is convenient for the stress field applied and in some rare cases cracks propagating axially up to the tooth root were observed. The point B on the loading curve indicates the first macroscopically visible damage (from the recorded video) when protrusions are fractured by shear stresses (see Fig. 3). This kind of tooth damage was found as very frequent and is not critical from the treatment point of view with a good chance to be successfully repaired. Additionally, no severe damage to the part in the vicinity of the cavity was observed. In this particular case, the protrusion fracture was not significant for the load transfer and allows continuation of loading. The increasing loading causes propagation of previously formed short median cracks in the enamel into the dentin. The cross-section of such median crack with typical fea-
tures of toughening mechanism acting in dentin, i.e. crack bridging and deflecting due to its composite nature is shown in Fig. 5(b). Further loading up to the final fracture is marked as the point C in the loading curve. The corresponding macro view of visible main cracks going through the cavity to the dentin – adhesive layer – filling interface is visible in the optical image in Fig. 3, point C, and additionally in Fig. 6(a) as a SEM micrograph accompanied by identification of individual materials. The detailed view of the polymeric adhesive layer (dark) and dual cure resin composite fillings (light) of the cavity in Fig. 6(b) shows the presence of polymeric adhesive pre-treatment and composite resin filling. This fatal fracture was rather an exception and most of the loading ended with a complete shear fracture of protrusions followed by rearrangement of tooth – ball contact points to the location of the cavity or on the newly formed fracture surfaces in places at protrusions. The results from all mechanical experiments conducted divided into three groups of treatment as well as for two tooth types represented by average values and corresponding standard deviations are summarised in Table 1. The average load at fracture was found lower in maxillary premolars than in mandibular one. Even though the average load at fracture for CEC was generally higher (20% higher for maxillary and 12.3% for mandibular premolars) than for TEC and was almost the same as for the control group, all groups did not differ significantly for maxillary (P = 0.4373) as well as mandibular premolars (P = 0.4335). The results of this in in-vitro study tend to support the null hypothesis since CEC a TEC groups did not significantly differ in affecting the fracture resistance of endodontically treated premolars. Here the dentine conditioning was kept to be the same for all specimens, therefore, its effect was not possible to clearly determine. The fracture observed within this study showed not statistically significant difference between groups of both cavity access methods: CEC and TEC. However, a statistically significant difference was found between maxillary and mandibular tooth type. The mandibular teeth show in both cases, i.e., CEC and TEC methods, approx. 25% and 35% higher resistance than maxillary teeth, respectively. The difference in the average Table 1 Average fracture forces in Newton for all tested groups (mean ± standard deviation). Tooth type
Maxillary Mandibular
Cavity access method CEC
TEC
Control
860.0 ± 206.8 1079.0 ± 383.2
687.4 ± 279.4 946.6 ± 384.1
745.0 ± 418.6 1171.8 ± 568.0
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Fig. 3. Typical loading curve from the compressive test of a mandibular premolar treated by CEC technique with an acoustic emission signal trace and images documenting damage at given points of the loading curve marked as A–C.
Fig. 4. Typical fracture patterns for maxillary (a–f), and mandibular (d–f) premolars treated by the CEC (a, d), and the TEC (b, e) techniques in comparison with the control group (c, f). Note each fracture path is indicating by (red) arrows for convenience. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. SEM images of selected tooth contact area with stainless steel ball (a), and a detailed view of the cross-section of a crack propagating trough the enamel to the dentine – not visible here (b).
Fig. 6. An overview of the final fracture with description of visible materials (a), and the detailed view (magnified yellow rectangle) of the adhesive layer with composite filling (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
fracture forces in case of intact teeth is reaching nearly 60%. From this comparison, it can be stated that the difference is decreased for CEC due to a rise of the fracture resistance of maxillary teeth. This effect can be ascribed to the blunting of initially sharp and deep natural “notch” between protrusions lowering the stress concentration at this critical place. The strength of protrusions remains unaffected by the formation of the access cavity independently on its size caused by CEC and TEC approach. Protrusions are predominantly fractured in the shear plane being normal to the tooth – ball contact plane. Additionally, a presence of filling in the access cavity consisting of a polymeric composite system can lower the stress concentration in the edge of the cavity and can be beneficial from the fracture point of view [27–30]. This can apply only when a sufficient adhesion between the dentine and composite filling is present as was demonstrated here. Statistical representation of the fracture behaviour using a statistical box plot can be found for convenience in Fig. 7. The diagram describes broadly previously discussed average values of fracture forces. There is observable evidence of decreasing fracture resistance of premolars with extended loss of dental tissue reported in literature [31–33]. The loss of dental tissue necessary for the access cavity should reduce the relative tooth rigidity by 5% only [33]. The more important factor which influences the relative rigidity is preservation of marginal ridges, because the loss of their integrity
leads to almost half reduction of tooth rigidity [33,34]. In general, the values of mean forces at fracture demonstrate a standard deviation ranging from 24 to 56%. High standard deviation is common problem which was described also in other studies [32,34–37] and is connected with divisibility of human teeth. The possible explanations are due to natural variations in teeth morphology as one can find in Fig. 4(a–f) or in literature [34] such as teeth dimensions, intercuspal angle and differences in physical properties of dental hard tissue [35]. It is also difficult to duplicate exactly access cavity designs counting described natural variation of each tooth [34]. Obtained results taken together with described uncertainty exhibit a difference in used access cavity method with slight preference of the CEC method, but not on the statistically significant level. The important finding is also that within tested groups only treated teeth exhibited catastrophic fracture (i.e. the fracture going to the tooth root area through pulp region). We have observed between 10 and 20% of such fracture for CEC and TEC groups. Another factor that influences the fracture resistance is the choice of material which is used for the restoration of access cavity. The most recommended material for restoration of posterior teeth with conservative access cavities is a resin composite [38] used also here. We are able to state that used composite system acts rather positively especially for the maxillary premolars treated by CEC in comparison with control group (see Fig. 7a). Although it has
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Fig. 7. A Box and Whisker plot representing obtained fracture resistance data for both maxillary (a), and mandibular (b) premolars. Note the box indicates upper and lower quartile with horizontal marking of median, whiskers indicate variability outside the upper and lower quartiles ant the ends represents the upper and lower extreme. Individual stars are used for outliers.
been emphasized that when occlusal wear, heavy forces or parafunctional habits are present in the mouth, more clinical data are required [38]. We must point out that during static loading the force was applied slowly with crosshead speed of 0.5 mm/min. This corresponds to the load in a parafunction rather than to a chewing type load or an impact type load [35]. We have elaborated also with a cycling loading which should better correspond to the natural loading during chewing but we have found severe damage in the contact place leading to the penetration of a loading ball in to the tooth without typical fracture observed in the clinical dentistry. According to observed fracture patterns applied static loading corresponds more realistically to such fatal situations. Because there was no significant difference between all (CEC, TEC and control) groups and the fracture resistance was found much higher than 400 N which is considered as threshold for premolars [39], we can assume that CEC will not have a significant effect on the long-term survival of treated mandibular premolars. On the other hand the CEC can make easier reconstruction of the access cavity and possess positive overlook for maxillary premolars.
4. Conclusions Within limitations of the current study, it can be stated that there is no statistically significant difference between TEC and CEC in maxillary and mandibular premolars, respectively, although the average loads at fracture for CEC were generally higher. A favourable prognosis for maxillary premolars treated by CEC method can be expected. The preservation of sound tooth tissue together with the favourable effect of polymeric composite filling increases the fracture resistance of teeth. The largest reduction in the fracture resistance results from traditional preparation, especially the loss of marginal ridges. The observed fracture behaviour and formed fracture patterns well correspond to those found in the clinical dentistry.
Acknowledgements The authors are grateful for financial support to Institutional Research Plan No. 68081723. This work was financially supported by Czech Science Foundation projects No. 14-11234S. This work was realised in CEITEC − Central European Institute of Technology with research infrastructure supported by the project
CZ.1.05/1.1.00/02.0068 financed from European Regional Development Fund.
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Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Feature article
Fracture behaviour of teeth with conventional and mini-invasive access cavity designs Zdenˇek Chlup a,∗ , Radovan Zˇ iˇzka b , Jakub Kania b , Michal Pˇribyl b a b
CEITEC IPM, Institute of Physics of Materials AS CR, Brno, Czechia Institute of Dentistry and Oral Sciences, Palacky University Olomouc, Czechia
a r t i c l e
i n f o
Article history: Received 20 December 2016 Received in revised form 6 March 2017 Accepted 10 March 2017 Available online xxx Keywords: Fracture resistance Cavity Tooth Compressive test Fractography
a b s t r a c t Presented work is targeted toward fracture analysis of endodontically treated human teeth. Three sets of teeth were loaded by compression simulating natural loading conditions. For this purpose, each tooth was mounted into the resin in the axis angle declination of 30◦ and kept all the time in saline up to the moment of test to simulate the intraoral environment. Two access cavity designs – mini-invasive (conservative) and conventional (traditional), were analysed. Fracture behaviour of treated teeth with mini-invasive access was compared with conventional and with the intact set of teeth. Complex monitoring of the fracture process together with loading traces enables to characterise typical fracture features and crack propagation schemes. The extensive fractographic analysis reveals the effect of adhesive bonding on the crack propagation. Also, fracture initiation and damage accumulation were identified. The quality of newly developed mini-invasive design in comparison with the conventional one was proved. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction The investigation of fracture of composite materials and its mechanisms under mechanical loading are of scientific interest for decades due to their complicated structure [1]. The toughening mechanisms acting in the case of brittle matrix composites are beneficial for overall mechanical response and its reliability during application [2]. A similar situation can be observed in the case of natural materials where similar toughening mechanisms play an important role [3]. Rapidly developing research area deals with bio-inspired materials where the effort is dedicated to the manufacturing of artificial materials possessing similar behaviour as the natural one [4–6]. In the medicine, the combination of natural and artificial materials is very common when damaged or missing tissue reconstruction is needed. The behaviour of such a complex structure which is comprised of different composites is an important issue [7,8]. In the field of dentistry, the combination of natural composite material (tooth) and artificial man-made composite materials (restorative materials) is frequently clinically applied. In recent years, the concept of minimally invasive endodontics is gradually gaining acceptance in clinical dentistry even though there is only limited supporting scientific evidence [9,10]. The present
∗ Corresponding author. E-mail address: [email protected] (Z. Chlup).
concept of minimally invasive endodontics is strongly related to the preservation of sound tooth structure during access cavity opening under direct vision and instrumentation to smaller apical width and taper. Such preserved sound tooth tissue is capable of maintaining the mechanical stability of restored tooth [11,12]. The most important tooth structure responsible for long-term survival is supposed to be the pericervical dentin [13]. It is a dentin’s structure located 4 mm below and 4 mm above the alveolar crest [11], responsible for distribution of functional mechanical stresses inside tooth [14,15]. On contrary, in traditional endodontics, the removal of sound tooth structure according to the concept of extension for instrument fracture prevention was favoured. Traditional endodontic access cavities, i.e. hereafter abbreviated as TEC, are adjusted for risk reduction during root canal treatment. In TEC it is more probable to find aberrant anatomy, to discover fracture lines, to remove complete pulp from all pulp horns and to minimise a chance of the instrument breakage. In minimally invasive endodontics, it is also called minimally invasive, contracted [10] or conservative endodontic access cavity, i.e. hereafter abbreviated as CEC [16] is used state-of-the-art instrumentation to minimise the access cavity. In comparison to TEC, it prefers the removal of restorative materials to tooth structure of enamel to dentin and of occlusal tooth structure to cervical dentin. It preserves parts of the pulp chamber roof and pericervical dentin. Although the preserved tooth structure may offer a benefit of improved fracture resistance [17], the scientific evidence for CEC remains scarce. So
http://dx.doi.org/10.1016/j.jeurceramsoc.2017.03.025 0955-2219/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Z. Chlup, et al., Fracture behaviour of teeth with conventional and mini-invasive access cavity designs, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2017.03.025
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Fig. 1. Comparison of the TEC access cavity design (a), the CEC access cavity design (b), and the loading scheme (c).
far, only mandibular premolars have been studied [16], but not maxillary premolars. Maxillary premolars have a shape which facilitates the fracture of cusps under occlusal loads [18,19] and thus can be observed in the higher incidence of cusp fractures of upper premolars in the oral cavity [20,21]. Additionally, not only the shape of access cavity could play an important role but also the conditioning of the dentin before application of filling can affect (positively or negatively) the overall fracture resistance as was reported [22–24]. However, a direct comparison of fracture behaviour of CEC, TEC and intact premolars under loading was not yet reported. The objective of this study was to assess the influence of CEC and TEC on the fracture resistance of lower (mandibular) and upper (maxillary) premolars which were reconstructed according to the concept of minimally invasive dentistry and characterisation of fracture process using fractographic approach. 2. Experimental procedures 2.1. Material Thirty maxillary and thirty mandibular previously extracted human non-carious, mature, intact premolars were used. Subsequently, both maxillary and mandibular premolars were randomly divided into three groups – CEC group, TEC group and negative control group (n = 10 teeth/group). Teeth in CEC groups were drilled by long shank diamond bur (FGSL H.1.316.010, Komet USA, Rock Hill, USA) and were accessed by 1 mm buccal to central occlusal fossa. Access cavities were extended apically, maintaining part of the chamber roof [16]. Teeth in TEC groups were prepared according to generally accepted suggestions of traditional endodontic access cavity [25]. Root canals were negotiated using ISO 10 K-file (Flexofile; Dentsply Maillefer, Balleigues, Switzerland) all the way to the anatomical foramen. The working length was established as 0.5 mm coronally according to the anatomical foramen and root canals were shaped with primary WaveOne reciprocating instruments (Dentsply Maillefer, Balleigues, Switzerland). Canals were irrigated by 5% sodium hypochlorite using 30G irrigation needles (NaviTip, Ultradent, South Jorda, UT, USA). Shaped and cleaned root canals were filled with gutta percha and epoxy resin (AdSeal, Meta Biomed, Chungbuk, South Korea) using warm vertical compaction where fillings ended 1 mm apically to the orifice of the root canal. After that pulp chamber was cleaned with 96% ethanol and teeth were adhesively restored using selective etch adhesive system Singlebond Universal (3 M ESPE, St. Paul, MN, USA) and dual-cured resin composite (Dentocore, Spofadental, Jiˇcín, Czech Republic) with Filtek ultimate (3 M ESPE, St. Paul, MN, USA).
All teeth were mounted to a self-curing resin (Dentacryl, Spofadental, Jiˇcín, Czech Republic) at the angle of 30◦ from the tooth long axis and up to 2 mm apical to cementoenamel junction (see Fig. 1(c)). The comparison of access cavity of traditional (TEC) and mini-invasive (CEC) approach is demonstrated in Fig. 1(a, b).
2.2. Characterization All specimen teeth embedded in the resin were mounted to the cylindrical holder and placed to the micrometric positioning table mounted on the base of an Instron Universal Testing Machine (Instron, Canton, MA, USA), loading was carried by a stainless steel sphere with a diameter of 3/16” fixed to the loading bar. The loading scheme is shown in Fig. 1(c). The diameter of the loading stainless steel ball was determined according to the 3D reconstruction using a laser confocal microscope Lext OLX 3100 (Olympus, Japan) of the typical tooth as is shown in Fig. 2(a). The maximal central profile was extracted and diameter of the ball was adjusted to simulate an optimal contact with the tooth surfaces (see Fig. 2(b)) with the aim to maximise stresses in the cavity location. Therefore, prior testing, each specimen was preciously positioned to obtain perfect two-point contact with the loading stainless steel sphere as shown Fig. 1(c) and fixed in this position. The crosshead speed of 0.5 mm/min was used until final fracture occurred. The acoustic emission signal was monitored continuously during loading using an IPL Analyser (Dakel, Czech Republic). A digital 2MPX USB microscope for visual monitoring of the fracture process was used. The loading curves with both the acoustic emission signal and the video captured were synchronised to allow understanding of the damage development. A scanning electron microscope Lyra XML3 (Tescan, Czech Republic) for fractographic observation of the tooth fracture surface and analysis of the surface damage in the tooth-ball contact areas was employed. The fracture forces leading to the fatal damage of the individual tooth were determined from the loading curves on the basis of video and acoustic emission signal. The statistical evaluation of obtained results was conducted using statistical software Statgraphics Centurion XV (StatPoint, USA). Average load to fracture values were calculated for each group and data were compared among particular groups with Kruskal-Wallis tests. In the present study, no simulation of the periodontal ligament was performed. In the majority of published articles on fracture resistance of postendodontic treated endodontic treated teeth, there was no simulation of periodontal ligament [26]. Although such simulation is appreciated, standardised model for the simulation of the periodontal ligament has not been introduced yet [26].
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Fig. 2. 3D surface reconstruction of the tooth (a), and corresponding tooth profile with the designed diameter of the loading stainless steel ball (b).
3. Results and discussion Six groups of human non-carious, mature, premolars were prepared for investigation of the effect of access cavity preparation method using static loading to the fracture simulating severe conditions with respect to possible axial tooth fracture. The typical loading curve of selected mandibular premolar treated by CEC technique is shown in Fig. 3. The mandibular premolar exhibiting the highest fracture force was selected to demonstrate all damage stages and ongoing fracture processes taking place during static loading. Three stages labelled A, B and C are marked on the loading curve and corresponding video frames are shown. Similar fracture stages were observed in all tooth groups under investigation, however, the terminal damage was usually identified as a fracture of protrusions (cusps) not the crack propagating trough the restored access cavity. The overview of a typical fracture behaviour through all groups under investigation is shown in Fig. 4 where final stages of fracture are presented by video frames captured during loading with crack paths marked by (red) arrows. The first damage of the tooth starts at relatively low load (i.e. usually between 100 and 200 N – see the loading curve in Fig. 3) with the enamel micro-cracking in the tooth – stainless steel ball contact areas due to high compressive stresses developed. An example of such area after the test is shown in Fig. 5(a). This kind of enamel micro-cracking was not detected by the AE sensor placed on the ball holder due to pre-set lower sensitivity to be able to detect the macro cracking of the tooth which produces a strong signal. The contact area is significantly plastically deformed and from this area the short cracks (usually assumed as median cracks) propagate radially. They frequently stop their propagation when the stress field intensity dropps. However, they can propagate further when their orientation is convenient for the stress field applied and in some rare cases cracks propagating axially up to the tooth root were observed. The point B on the loading curve indicates the first macroscopically visible damage (from the recorded video) when protrusions are fractured by shear stresses (see Fig. 3). This kind of tooth damage was found as very frequent and is not critical from the treatment point of view with a good chance to be successfully repaired. Additionally, no severe damage to the part in the vicinity of the cavity was observed. In this particular case, the protrusion fracture was not significant for the load transfer and allows continuation of loading. The increasing loading causes propagation of previously formed short median cracks in the enamel into the dentin. The cross-section of such median crack with typical fea-
tures of toughening mechanism acting in dentin, i.e. crack bridging and deflecting due to its composite nature is shown in Fig. 5(b). Further loading up to the final fracture is marked as the point C in the loading curve. The corresponding macro view of visible main cracks going through the cavity to the dentin – adhesive layer – filling interface is visible in the optical image in Fig. 3, point C, and additionally in Fig. 6(a) as a SEM micrograph accompanied by identification of individual materials. The detailed view of the polymeric adhesive layer (dark) and dual cure resin composite fillings (light) of the cavity in Fig. 6(b) shows the presence of polymeric adhesive pre-treatment and composite resin filling. This fatal fracture was rather an exception and most of the loading ended with a complete shear fracture of protrusions followed by rearrangement of tooth – ball contact points to the location of the cavity or on the newly formed fracture surfaces in places at protrusions. The results from all mechanical experiments conducted divided into three groups of treatment as well as for two tooth types represented by average values and corresponding standard deviations are summarised in Table 1. The average load at fracture was found lower in maxillary premolars than in mandibular one. Even though the average load at fracture for CEC was generally higher (20% higher for maxillary and 12.3% for mandibular premolars) than for TEC and was almost the same as for the control group, all groups did not differ significantly for maxillary (P = 0.4373) as well as mandibular premolars (P = 0.4335). The results of this in in-vitro study tend to support the null hypothesis since CEC a TEC groups did not significantly differ in affecting the fracture resistance of endodontically treated premolars. Here the dentine conditioning was kept to be the same for all specimens, therefore, its effect was not possible to clearly determine. The fracture observed within this study showed not statistically significant difference between groups of both cavity access methods: CEC and TEC. However, a statistically significant difference was found between maxillary and mandibular tooth type. The mandibular teeth show in both cases, i.e., CEC and TEC methods, approx. 25% and 35% higher resistance than maxillary teeth, respectively. The difference in the average Table 1 Average fracture forces in Newton for all tested groups (mean ± standard deviation). Tooth type
Maxillary Mandibular
Cavity access method CEC
TEC
Control
860.0 ± 206.8 1079.0 ± 383.2
687.4 ± 279.4 946.6 ± 384.1
745.0 ± 418.6 1171.8 ± 568.0
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Fig. 3. Typical loading curve from the compressive test of a mandibular premolar treated by CEC technique with an acoustic emission signal trace and images documenting damage at given points of the loading curve marked as A–C.
Fig. 4. Typical fracture patterns for maxillary (a–f), and mandibular (d–f) premolars treated by the CEC (a, d), and the TEC (b, e) techniques in comparison with the control group (c, f). Note each fracture path is indicating by (red) arrows for convenience. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. SEM images of selected tooth contact area with stainless steel ball (a), and a detailed view of the cross-section of a crack propagating trough the enamel to the dentine – not visible here (b).
Fig. 6. An overview of the final fracture with description of visible materials (a), and the detailed view (magnified yellow rectangle) of the adhesive layer with composite filling (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
fracture forces in case of intact teeth is reaching nearly 60%. From this comparison, it can be stated that the difference is decreased for CEC due to a rise of the fracture resistance of maxillary teeth. This effect can be ascribed to the blunting of initially sharp and deep natural “notch” between protrusions lowering the stress concentration at this critical place. The strength of protrusions remains unaffected by the formation of the access cavity independently on its size caused by CEC and TEC approach. Protrusions are predominantly fractured in the shear plane being normal to the tooth – ball contact plane. Additionally, a presence of filling in the access cavity consisting of a polymeric composite system can lower the stress concentration in the edge of the cavity and can be beneficial from the fracture point of view [27–30]. This can apply only when a sufficient adhesion between the dentine and composite filling is present as was demonstrated here. Statistical representation of the fracture behaviour using a statistical box plot can be found for convenience in Fig. 7. The diagram describes broadly previously discussed average values of fracture forces. There is observable evidence of decreasing fracture resistance of premolars with extended loss of dental tissue reported in literature [31–33]. The loss of dental tissue necessary for the access cavity should reduce the relative tooth rigidity by 5% only [33]. The more important factor which influences the relative rigidity is preservation of marginal ridges, because the loss of their integrity
leads to almost half reduction of tooth rigidity [33,34]. In general, the values of mean forces at fracture demonstrate a standard deviation ranging from 24 to 56%. High standard deviation is common problem which was described also in other studies [32,34–37] and is connected with divisibility of human teeth. The possible explanations are due to natural variations in teeth morphology as one can find in Fig. 4(a–f) or in literature [34] such as teeth dimensions, intercuspal angle and differences in physical properties of dental hard tissue [35]. It is also difficult to duplicate exactly access cavity designs counting described natural variation of each tooth [34]. Obtained results taken together with described uncertainty exhibit a difference in used access cavity method with slight preference of the CEC method, but not on the statistically significant level. The important finding is also that within tested groups only treated teeth exhibited catastrophic fracture (i.e. the fracture going to the tooth root area through pulp region). We have observed between 10 and 20% of such fracture for CEC and TEC groups. Another factor that influences the fracture resistance is the choice of material which is used for the restoration of access cavity. The most recommended material for restoration of posterior teeth with conservative access cavities is a resin composite [38] used also here. We are able to state that used composite system acts rather positively especially for the maxillary premolars treated by CEC in comparison with control group (see Fig. 7a). Although it has
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Fig. 7. A Box and Whisker plot representing obtained fracture resistance data for both maxillary (a), and mandibular (b) premolars. Note the box indicates upper and lower quartile with horizontal marking of median, whiskers indicate variability outside the upper and lower quartiles ant the ends represents the upper and lower extreme. Individual stars are used for outliers.
been emphasized that when occlusal wear, heavy forces or parafunctional habits are present in the mouth, more clinical data are required [38]. We must point out that during static loading the force was applied slowly with crosshead speed of 0.5 mm/min. This corresponds to the load in a parafunction rather than to a chewing type load or an impact type load [35]. We have elaborated also with a cycling loading which should better correspond to the natural loading during chewing but we have found severe damage in the contact place leading to the penetration of a loading ball in to the tooth without typical fracture observed in the clinical dentistry. According to observed fracture patterns applied static loading corresponds more realistically to such fatal situations. Because there was no significant difference between all (CEC, TEC and control) groups and the fracture resistance was found much higher than 400 N which is considered as threshold for premolars [39], we can assume that CEC will not have a significant effect on the long-term survival of treated mandibular premolars. On the other hand the CEC can make easier reconstruction of the access cavity and possess positive overlook for maxillary premolars.
4. Conclusions Within limitations of the current study, it can be stated that there is no statistically significant difference between TEC and CEC in maxillary and mandibular premolars, respectively, although the average loads at fracture for CEC were generally higher. A favourable prognosis for maxillary premolars treated by CEC method can be expected. The preservation of sound tooth tissue together with the favourable effect of polymeric composite filling increases the fracture resistance of teeth. The largest reduction in the fracture resistance results from traditional preparation, especially the loss of marginal ridges. The observed fracture behaviour and formed fracture patterns well correspond to those found in the clinical dentistry.
Acknowledgements The authors are grateful for financial support to Institutional Research Plan No. 68081723. This work was financially supported by Czech Science Foundation projects No. 14-11234S. This work was realised in CEITEC − Central European Institute of Technology with research infrastructure supported by the project
CZ.1.05/1.1.00/02.0068 financed from European Regional Development Fund.
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