Effect of Cervical Lesions on Fracture Resistance and Failure Mode of Maxillary Central Incisors Restored with Fiber Posts and Complete Crowns
To investigate the effect of a cervical cavity extending 1 mm apical to the cemento–enamel junction (CEJ) on fracture resistance and failure mode of maxillary central incisors that have been treated endodontically, present with complete and incomplete ferrules, and are restored with and without a fiber post. 50 intact human maxillary central incisors were divided into five groups (n=10): CG (control group) 6-mm fer-rule height, no cervical cavity, and without post; (CO) 6-mm ferrule height without post, with a cervical cavity (access to root canal and cervical cavity restored with composite resin), cervical cavity; and post with ferrule heights of 1 mm (CP1), 2 mm (CP2), and 6 mm (CP6) restored with fiberglass post and composite resin core. After complete metal crowns were cemented on all specimens, they were subjected to thermal cycling (6000 cycles, 5°C/55°C), followed by immediate testing of fracture resistance. After failure, the specimens were sectioned buccolingually to evaluate and identify the mode of failure. The data were analyzed with an analysis of variance (ANOVA) and the Student–Newman–Keuls multiple comparison tests (α =0.05). A 1-mm ferrule height (CP1) fracture resistance was significantly lower (531±125 N) compared to the 6-mm ferrule height (CP6) (769±175 N) (p<0.05). With respect to the groups with similar residual dentin, with and without a cervical cavity, CG (667±119 N) and CO (668±119 N), the presence of a post (CP6) increased the resistance to fracture, although no statistically significant difference was demonstrated. Partial decementation was observed in all specimens of CG and CP6, in nine of CP1 and CP2, and in three in CO. Root fractures occurred in 23 specimens. The root surface was exposed 2 mm below the CEJ to simulate bone level. Propagation of subosseous cracks occurred in four specimens in CG and CP2, in seven specimens in CP6, in two specimens in CP1, and in six specimens in CO. All were considered catastrophic failures. Within the limitations of this study it is suggested that, when restoring an endodontically treated maxillary central incisor that has a cervical lesion and needs to be restored with a complete crown, a fiber post is cemented to improve fracture resistance.SUMMARY
Purpose
Methods and Materials
Results
Conclusions
INTRODUCTION
Complete crown restoration of endodontically treated maxillary central incisors is challenging due to extensive loss of tooth structure, which may have been caused by caries, fracture, endodontic access, and instrumentation and preparation for post and crown.1–3 The greater the loss the more it negatively effects the resistance to fracture.4–8 Furthermore, anterior teeth are stressed by horizontal forces, exposing them to a higher risk of biomechanical failures.9,10 Quantity and location of the remaining dentin appears to be the most important factor in predicting the prognosis of a tooth.11,12 If there is enough remaining dentin for the preparation of a complete ferrule, the outcome will be more favorable.13 By definition, a circumferential ferrule is formed by four axial dentin walls that are 2-mm high from the cervical finish line to the coronal junction.14–19 Several publications reported on the protective effect this offers on the final restoration as it reduces the concentration of stresses, thus decreasing clinical failures such as root fracture and/or decementation of the post.20–22
However, for various reasons, such as caries, erosion, abrasion or fracture, a circumferential ferrule is not always possible.23 This affects the prognosis of a restoration.24 Furthermore, the superiority of one partial ferrule over another depends on the height, number, and location of the remaining walls.25–30 Because the ferrule effect is not entirely understood, it has been suggested that more studies are necessary.31 In particular, maxillary incisors with cervical lesions and endodontic treatment has received little attention.32 It is this type of defect that affects the thickness of the facial wall, which compromises the integrity of the ferrule.32 When a cervical lesion involves the root, it is commonly associated with gingival recession.33 As a result, the preparation of the cervical shoulder margin has to include the lesion on the root surface, which has less dentin thickness due to the conicity of the root.14,34
The thickness of the dentin wall plays an important role in the function of the ferrule, which most studies have not considered.32 It has been advised to keep enough facial dentin in maxillary anterior teeth to counteract horizontally directed forces, as at least a 1-mm thick dentin ferrule may resist functional loads and prevent fracture.35 However, there is insufficient research information available that supports the effectiveness of this ferrule. In reviewing the literature, the work of Tjan and Whang35 can be considered a pioneering study with reference to the thickness of the facial wall of remaining dentin in maxillary central incisors at the level of the cemento–enamel junction (CEJ). The thickness was 1, 2, and 3 mm in combination with cast posts, without ferrule effect (no cemented crown). The authors compared fracture resistance under horizontal force, showing that although there were no statistically significant differences, the 1-mm thick facial dentin wall was more prone to fracture than to decementation. It should be noted that since the samples had no ferrules, and no complete crowns were cemented, the results should be interpreted with caution. Joseph and Ramachandran36 demonstrated that more than 1-mm thickness is required in the facial dentin wall to prevent root fracture. Furthermore, a 2-mm high dentin ferrule improved fracture resistance. Recently, Fontana and others37 suggested that when a ferrule thickness is 1 mm, it is more appropriate to use a fiber post. Naumann and others38 observed that in excessively widened teeth it is not possible to establish a ferrule, because the preparation will cause even more loss of tooth structure, resulting in a very thin wall or no wall at all.
The authors found no studies in the literature regarding the effect of cervical lesions on fracture resistance in the maxillary endodontically treated central incisor restored with a complete crown, except for a single study using the direct resin composite technique with or without a fiber post.39 The purpose of this study was to evaluate the effect of the cervical cavity extending 1-mm apical to the CEJ, with variations in height of the remaining coronal structure on fracture resistance and the mode of failure of endodontically treated maxillary central incisors and restored with a fiber post. The null hypothesis was that the placement of a fiber post in maxillary crowned central incisors and in the presence of a restored facial cervical lesion will not affect the fracture resistance.
METHODS AND MATERIALS
Specimens and Preparation of Root Canal
Fifty intact human maxillary central incisor teeth recently extracted for periodontal reasons were stored in a 0.9% physiologic saline solution (Laboratorios ALFA, Santo Domingo, Dominican Republic) with 0.1% Chloramine T (Sigma Aldrich, St. Louis, MO, USA) at room temperature. The teeth were cleaned to remove soft tissue and debris and were examined under 3.5× magnification (Double lens head-wearing, Gerleek, YTOM, China) for defects. X-rays were taken to verify the absence of fractures, internal root resorption, and obstructions or endodontic filling material. The teeth were stored at room temperature and were used within 3 months post-extraction.
The root lengths (13.8±0.9 mm) and the buccolingual diameters 6.6±0.4 mm) at the CEJ were measured with an electronic caliper (Mitutoyo Absolute 573–751, America Corp, Aurora, lII, USA). The teeth were divided into five experimental groups of 10 teeth each (n=10). The power test on the data was “Power of performed test with α=0.050: 0.998.” Therefore, the sample size was sufficient. The nature of variability existing in all tissue is such that data must vary, and standard deviations will be relatively large.
The preparation of the endodontic access cavity was made with a round bur (No. 6801.31.014, Brasseler, Savannah, GA, USA), outlining a triangular cavity. All teeth received conventional endodontic treatment using manual instrumentation with files up to #40 (Dentsply Sirona, York, PA, USA), irrigation with 3% sodium hypochlorite (ChloridCid; Ultradent Products Inc., South Jordan, UT, USA), lateral condensation with gutta-percha (Dentsply Sirona), and use of a sealing cement (AH 26, Dentsply Sirona).
Preparation and Ferrule
In all teeth, except the control group (CG), the cervical cavity was prepared as follows. A round diamond bur (no. 6801.31.014, Brassaler) was used to prepare the cavity characterized by a gingival wall parallel to the CEJ and extending 1 mm apical to it, mesial and distal walls parallel to the long axis of the tooth, and an incisal wall following the contour of the incisal edge.40 Subsequently, with a cylindrical carbide bur (SSW no. FG-558, SSW, New Jersey, NY, USA), the dimensions of the cavity were defined: height = 3 mm, width = 3 mm, and depth = 2 mm. Crown preparation was performed with a super coarse round-end taper FG diamond bur (no. 5856.31.018 Brasseler). Preparations were done under copious water cooling. The reduction of the crown was performed according to the height of the pre-established remaining walls in the experimental groups, following the contour of the CEJ as a reference.41 The location of the facial cervical finish line of the preparation followed the contour of the cervical cavity and was located 1 mm apical to the CEJ, while it was located at the level of the CEJ in the remainder of the tooth. The finish line served as a reference for the linear measurement of the remaining coronal heights in each specimen. While the CG had no cervical cavity, the location of the finish line was similar to the other groups. Thus, the following five experimental groups were established (Fig. 1): CG (6-mm ferrule height, no cervical cavity, and no post), CO (6-mm ferrule height, with a cervical cavity), CP1 (1-mm ferrule height, cervical cavity, and post), CP2 (2-mm ferrule height, cervical cavity, and post), and CP6 (6-mm ferrule height, cervical cavity, and post). Table 1 presents the materials, composition, and protocols used in this study.
Citation: Operative Dentistry 46, 6; 10.2341/20-164-L
Post Groups Preparation
In the CP1, CP2, and CP6 groups, the Unicore Post and Drill System was used, which consisted of a radiopaque fiber post of conical design and a drill for the preparation of the post space. The post is characterized by prestressed quartz fibers encased in a bondable resin matrix that creates a monoblock when the core material is placed. The post selected in the study was size 3 (blue), with an apical diameter of 1.17 mm and coronal diameter of 1.56 mm (UniCore UP, Ultradent Products). For the post preparation in CP1, a Peeso No. 2 reamer (Union Broach Co, York, PA, USA) was used to remove the gutta-percha to a depth of 9 mm measured with a stop from the remaining coronal structure. In the groups, CP2 (2 mm) and CP6 (6 mm), the stop was placed at 10 and 14 mm, respectively. As a result, the length of the post space was consistent at 8 mm for all the groups. The post space was then gradually widened using a Peeso reamer No. 3 and 4 (Union Broach Co). For the final preparation of the post space, a blue helicoidal bur size 3 (UniCore UP 7123, Ultradent Products) was used to a depth of 8 mm, which more precisely defined the shape of the post space to the matching post.
The posts were cemented with a self-curing resin cement (Multilink N, Ivoclar Vivadent), following the manufacturer’s instructions. The surface of the post was cleaned with an alcohol-saturated cotton swab, then Monobond N (Ivoclar Vivadent) was brushed on the surface of the post, allowing it to react for 60 seconds before air dispersing with dry clean air. The post space was irrigated with distilled water and dried with paper points (Coltene Whaledent, Altsta¨tten, Switzerland). Primer A and B (Multilink N, Ivoclar Vivadent) were mixed with a microbrush and introduced into the post space. After 15 seconds, excess material was removed with paper points (Coltene). The cement mix was coated on the fiber post, and the post was seated using light repetitive torsional movements to reduce hydraulic pressure and prevent air entrapment. Digital pressure consistent with clinical practice was applied followed by visible light irradiation for 20 seconds (VALO Cordless, Ultradent Products). The LED light activation produces a higher adhesion strength of the resin cement to the intraradicular dentin in the coronal region.42 The remaining coronal walls of groups CP1, CP2, and CP6, as well as the root canal access and cervical cavity of the groups CG and CO were etched with 37% phosphoric acid (Prime Dental) for 15 seconds, rinsed with a triple air–water syringe and then gently dried. With a microbrush, a dentin adhesive was applied (Single Bond Universal, 3M Oral Care, Seefeld, Germany), which was light cured for 40 seconds (VALO Cordless; Ultradent Products) because when a long photoactivation time is used in adhesive systems, the level of polymerization increases.43,44 The VALO light is high intensity and had an output of 1000 mW/cm2. Then, the cervical cavity and core were built in increments with composite resin (Z350 Filtex, 3M Oral Care), each increment light cured for 20 seconds.
With a diamond high speed bur (no. 5856.31.018; Brasseler), under copious irrigation, the preparations were finalized to meet the following parameters: a height of 6 mm, a rounded shoulder finish line extending apically to the cervical cavity and in the other areas at the CEJ with a reduction of 1.0 mm on the facial, 1.0 mm interproximally and lingually, and an incisal reduction of 2 mm.
Fabrication and Cementation Crowns
One specimen was coated with 1 layer of die spacer (Die Spacer, Keystone Industries, Gibbstown, NJ, USA), and a complete contour wax pattern was fabricated. This pattern measured 8-mm high, with a circumferential thickness of, approximately, 1–1.5 mm. A rectangular-shaped stop with a central concavity was added 2-mm apical to the incisal edge on the lingual surface. This stop facilitated loading during the fracture resistance tests. An impression in vinyl polysiloxane (Putty Normal Set, Elite HD, Zhermack, Badia, Polesine, Italy) was made of the wax pattern, which served as a mold for replicas of all other samples. Final adjustment of the wax replicas was carried out for each individual sample to obtain good marginal adaptation.
The wax patterns were cast in nickel–chrome alloy (VeraBond II, AalbaDent, Inc, Fairfield, CA, USA). After finishing, a resin-reinforced glass ionomer cement was mixed, according to the manufacturer’s instructions (Fuji Plus, GC, Alsip, IL, USA), and used for cementation. The castings were held under finger pressure until initial set had occurred.
Fracture Load Test
After 24 hour storage in water at room temperature, the specimens were thermocycled in distilled water (6000 cycles; 5°C/55°C, 2 minutes each cycle) to mimic clinical conditions.45 The roots were embedded in self-curing translucent acrylic resin that had a curing time of 5–8 minutes (SamplKwich Kit, Buehler, Lake Bluff, IL, USA), leaving 2 mm of the root surface exposed while following the contour of the CEJ to mimic bone support.
Each sample was subjected to a fracture resistance test using a universal testing machine (5567, Instron, Norwood, MA, USA). A compression force was applied to the stop on the lingual surface of the crowns by means of a spherical tip with a 3-mm diameter. The force was directed at an angle of 45° to the longitudinal axis of the tooth at a crosshead speed of 0.5 mm/min, until fracture. This angle was based on the natural occlusal forces on maxillary incisors in a Class I occlusion41 (Figure 2).
Citation: Operative Dentistry 46, 6; 10.2341/20-164-L
Failure Mode Analysis
The mode of failure was evaluated by embedding the specimens in self-curing acrylic resin (SamplKwich, Buehler). Two sections were made in a bucco-lingual direction, using a low-speed diamond blade under water irrigation (IsoMet 5000, Buehler). They were photographed with a digital microscope (VHX-1000, Keyence, Osaka, Japan) at x20× magnification. The failure mode, based on Fokkinga and others46, was characterized as follows: complete decementation of the post-and-core and crown; partial decementation of the post-and-core and/or crown; fracture of resin composite foundation–coronal dentin; dislodgement of lingual cervical margin of the crown; root fracture trajectory (oblique or horizontal); fracture propagation (subosseous, supraosseous); favorable failure (failure allows restoration of the tooth); and unfavorable failure (failure does not allow predictable restoration).
Data Analysis
The data were analyzed by analysis of variance (ANOVA) and the Student–Newman–Keuls (SNK) multiple comparison statistical test (α=0.05) (Sigma-Plot 14; Sytat Software, San Jose, CA, USA).
RESULTS
The fracture resistance and standard deviation data are presented in Table 2. The CP6 group presented the highest fracture load resistance (769±175 N), the CP1 group being the lowest (531±125 N). An ANOVA analysis showed a statistically significant difference between these two groups (p<0.05). With respect to the groups with 6-mm high dentinal axial walls, without and with a cervical cavity, fracture resistance was similar CG (667±119 N) and CO (668±119 N). In the CP6 group with cervical cavity and 6-mm-height remaining walls, the placement of the post increased fracture resistance (769±175 N), although this was not statistically significant in relation to the CG and CO groups. In the CP2 group with 2-mm-high remaining walls with cervical cavity and with a post, the fracture resistance was 608 ± 192 N, close to the CG and CO group values.
A partial post-core and/or crown decementation was identified in all CG and CP6 specimens, in nine CP1 and CP2 specimens, and in three CO specimens. The dislodgement of the cervical margin on the lingual of the crowns occurred in seven CG specimens and in all CO, CP1, CP2, and CP6 specimens. Fracture of the composite resin–dentin core occurred in seven CP1 specimens, in five CP2 specimens, and in four specimens of groups CG, CO, and CP6. Horizontal and oblique fractures occurred in few specimens, mostly in CP6. The location and trajectory were more frequently at the level of the cervical and middle-thirds of the root, and at the apical limit of the post. The propagation of subosseous cracks occurred in approximately half of all the specimens, slightly below the margin of the acrylic resin that simulated the level of the bone crest. Failures were considered favorable when there was sufficient remaining dentin to allow for a preparation of a new cervical finish line. While unfavorable failures compromised the integrity of the tooth, making it unrestorable (Table 3). Figure 3 shows specimens with failures.
Citation: Operative Dentistry 46, 6; 10.2341/20-164-L
DISCUSSION
The reason for this laboratory research was based on published arguments by Jotkowitz & Samet.32 The authors stated: “The evidence relating to restoring the endodontic treated tooth with extensive destruction is deficient.” To the best of our knowledge, this was the first study to investigate the effect of facial cervical lesions on fracture resistance in the endodontically treated maxillary central incisor restored with a complete crown. While this does not occur frequently,39 it is still of clinical importance. The fracture resistance of the CO group (668 N) with dentin walls 6 mm in height with a cervical cavity and without a post was similar to the CG group (667 N) that had a ferrule of 6 mm in height without a cervical cavity and post. While in the CP6 group (769 N) with 6-mm-high walls and a cervical cavity and a post, the fracture resistance increased, although there was no statistically significant difference. According to these results, the null hypothesis that placement of a fiber post would not affect the fracture resistance of maxillary central incisors restored with a complete crown and a facial cervical lesion was rejected.
The samples that had endodontic treatment only (CG) recorded no difference in fracture load compared to the teeth that had endodontic treatment and a cervical lesion (CO). This may be due to a combination of facts, such as the large amount of the remaining coronal structure, height of dentinal walls, and the protective effect of the complete crown, all contributing to a better distribution of stress.20 This suggests that the loss of dentin caused by a cervical lesion did not affect the fracture resistance between these two groups.
Groups CP6 and CO had similar-height walls in dentin and cervical lesions. It was observed that the placement of a fiber post created a positive effect, because it increased the fracture resistance CP6 (769 N) compared to the CO group (668 N) without a post, albeit it was not statistically significant. This finding suggests that a fiber post and resin cement contribute to the strengthening of the tooth, which is in accordance with other studies.7,9 Posts also play a role in absorbing energy47 and reduce stress on the tooth due to the distribution of the occlusal load within the root dentin.48 This is consistent with the previous reports showing that a post increases fracture resistance when compared to a CG without post with the same remaining wall height.17,41,49 In addition, the reduction of tooth structure due to preparation and endodontic treatment causes an increase in the deflection of the tooth, exposing it to more stress.48 In that sense, the fiber post increases the load values and decreases the deflection of the compromised tooth structure.50 Another factor that could explain the findings mentioned above is the location of the defect in the tooth. A study has shown that there was no difference in fracture resistance compared to indirect restoration with a ceramic crown when the defect is a Class III (mesial and distal restored with direct composite resin).3 This indicates that whether or not a fiber post is present, the fracture resistance is not affected.3,12 On the other hand, if a cervical cavity is present in the facial, a post exerts a positive effect, as was demonstrated by an increase in fracture resistance.39 This difference between cervical cavity and class III defects can be explained, as the endodontically restored tooth is subject to more stress in the cervical region,16,19,31 while the presence of a post reduces stress.19 According to a finite element study, a decrease in root dentin thickness in the cervical area contributes to high levels of tension within the root canal.15
Regarding the type of coronal restoration, it has been suggested that the mechanical properties of the crown affects the resistance to fracture,7 and the level of stress and strain along the dentin–cement–post interface.9 Although the use of ceramic crowns has increased considerably in the anterior region due to its esthetic qualities and advantage to be adhesively cemented,26 metal–ceramic crowns continue to be indicated in clinical practice.7 The aim of the present study was to compare the influence of the facial cervical cavity and a fiber post on the resistance to fracture. For this purpose, the use of a metal crown is preferable, because it is more suitable for evaluating the ferrule effect, as it allows stresses to concentrate more on the remaining coronal dentin, according to a finite element study.16
The microscopic evaluation of the failure mode of the sections of the selected specimens allowed precise detection of the type of failure. The CP6 group presented a greater number of catastrophic failures, identifying seven specimens, in relation to CO and CG with 6 and 4 catastrophic failures, respectively. This result can be attributed to the presence of a fiber post, as it serves as a distributor of stress and load to the core and crown within the root canal.9,19 In the absence of a post, the concentration of stress in the canal was insignificant.19 That could explain the horizontal and oblique root fractures located in the middle-third and at the apical limit of the post in the CP6 group. Subosseous propagation of crack also occurred in the majority of the CP6 group and was characterized to a greater extent on the surface of the dentin. The results of the present study show 70% nonrepairable failures in CP6, while another study showed 100% and has questioned the use of a fiber post.2
The fracture resistance of CP1 with a cervical cavity, a post and 1-mm-high ferrule (531 N) was inferior compared to CP6 that had a cervical cavity, a post and 6-mm-high ferrule (769 N); the difference was statistically significant (p<0.05). This showed that tooth resistance is directly related to the amount of remaining dentin, which corroborates the findings of other studies.17,27 However, the CP1 group had an unfavorable failure in only two specimens compared to the CP6 group with seven specimens. Due to the minimal coronal structure, high stresses are concentrated in the cervical area,6 which causes fracture at the level of the resin composite–dentin foundation, which was characterized as a favorable failure. With an increase in coronal structure, stresses are transmitted to the apical area,6,22 causing a greater number of root fractures, as evidenced in CP6.
Despite the intrinsic limitations of a static load, it is a frequently applied methodology in laboratory studies to evaluate the ability to resist fracture, because it is an efficient method due to its low cost while being less time consuming. Reproducing exact oral conditions in laboratory studies is not possible. However, in this study, the specimens were subjected to thermocycling to mimic clinical conditions and simulate aging of the restoration before a load was applied. This study observed that facial cervical lesions affect the configuration of the ferrule and influence fracture resistance in maxillary central incisors under static load. More studies are needed to assess the significance of the thickness of remaining dentin in the cervical lesion after the ferrule preparation. Furthermore, the performance of different types of fiber posts, cementation protocols, and luting agents and the composition of crown materials need to be examined after applying dynamic load testing.
CONCLUSIONS
Within the limitations of this study it is suggested that, when an endodontically treated maxillary central incisor that has a cervical lesion needs to be restored with a complete crown, a fiber post is cemented to improve fracture resistance.
The experimental groups showing the cervical cavity, preparation of the ferrule, fiber post-and-core and crown. (A): specimens of the control group (CG) 6-mm ferrule height, without cervical cavity and no post. (B): specimens of Group (CO) 6-mm ferrule height and only with cervical cavity. (C): specimens of Group (CP1) 1-mm-height ferrule with cervical cavity and fiber post. (D): specimens of Group (CP2) 2-mm-height ferrule with cervical cavity and fiber post. (E): specimens of Group (CP6) 6-mm-height ferrule with cervical cavity and fiber post (F): schematic design illustrating the dimensions of the preparation, post, and crown.
Schematic representation of the tooth under static load.
Representative samples of failures observed using digital microscopy. A: Group CG specimen with coronal dentin fracture and dislodgement of lingual cervical crown margin; B, Group CO lingual supraosseous fracture; C, Group CP1 dislodgement of lingual cervical crown margin and core displacement; D, Group CP2 horizontal fracture of coronal dentin; E, Group CP6 oblique root fracture.
Contributor Notes
Clinical Relevance
In the presence of a cervical cavity, placement of a fiber post is recommended to strengthen a maxillary central incisor. This increases the fracture resistance of the tooth, leading to a more favorable prognosis of the completed restoration.