An In Vitro Evaluation of Fracture Resistance of Endodontically Treated Maxillary Central Incisor Restored with Custom-Made Cast Post and Core with Uniform and Nonuniform Core Ferrule Heights.

AIM: The aim of this study was to evaluate in vitro fracture resistance of endodontically treated teeth restored with custom-made cast post and core having uniform and nonuniform core ferrule heights.
MATERIALS AND METHODS: Thirty-five freshly extracted human maxillary central incisors were included in this study. All teeth were subjected to standard root canal treatment. The teeth were randomly divided into five groups-Group 1: uniform ferrule (2 mm buccal, lingual, and proximal), Group 2: uniform ferrule (3 mm buccal, lingual, and proximal), Group 3: nonuniform ferrule (2 mm buccal, 3 mm lingual), Group 4: nonuniform ferrule (2 mm buccal, 4 mm lingual), and Group 5: no ferrule. The teeth were sectioned horizontally 4 mm above cementoenamel junction and post space preparation was performed maintaining 4 mm of apical gutta-percha. Ferrule was prepared according to dimension designated for each group. Custom-made cast post and core were fabricated and luted using zinc phosphate cement. Testing was conducted using universal testing machine with application of static load (Newton), and failure load was recorded. Data were analyzed by one-way analysis of variance and Tukey test. The mode of fracture was noted by visual inspection for all specimens. RESULT: Significant differences (P < 0.001) were found among mean fracture forces of test groups. Group 1: 1181.66 ± 68.29, Group 2: 1455.58 ± 173.11, Group 3: 1019.00 ± 52.55, Group 4: 971.58 ± 66.52, and Group 5: 888.00 ± 60.56. The presence of nonuniform ferrule height resulted in a significant decrease (P < 0.0001) in mean fracture strength compared to uniform 2- and 3-mm core ferrule height.
CONCLUSION: The central incisors restored with cast post and core and crowns with 3-mm uniform core ferrule were more fracture resistant compared to central incisors with nonuniform core ferrule height. Both the uniform and nonuniform core ferrule groups were more fracture resistant than the group that lacked ferrule.

INTRODUCTION

Root-filled anterior teeth with extensive loss of tooth structure often requires a post and core due to lateral and shearing force acting on it, and presence of smaller pulp chambers as compared to molars.[1]

The post is a relatively rigid material extending approximately two-thirds of the length of the root canal of the pulpless teeth to provide reinforcement and retention. The core may be the coronal extension of the dowel, which replaces the missing coronal tooth structure. Retention for the core is provided by dowel, and the core provides retention for the final restoration and replaces the lost coronal tooth structure.[2]

Ferrule is the crucial factor in tooth preparation when a post and core is indicated.[3] The word “ferrule” is originated from the Latin word, ferrum, iron and viriola, meaning a bracelet that is an encircling band of cast metal around the coronal surface of the tooth providing protective reinforcement to endodontically treated teeth.[4]

The concept of ferrule was first proposed by Rosen in 1961. In the literature, various names for ferrule were given by various authors such as extra coronal brace, metal band, metal collar, and subgingival collar and subgingival apron of gold. Earlier, ferrule was defined as an encircling band of cast metal.[4] Sorensen and Engelman[5] modified the definition of ferrule as “360degree metal collar of the crown surrounding the parallel walls of the dentin extending coronal to the shoulder of the preparation.”

Incorporation of the ferrule in the preparation reduces the wedging effect of the tapered dowel, resists the functional lever forces and the lateral forces exerted during dowel insertion, and improves resistance to dynamic occlusal loading.[6]

Majority of studies have investigated the effect of crown ferrule[789] (parallel walls of dentin surrounding cervical portion of crown) on fracture resistance of tooth; however, very few studies have investigated the effect of core ferrule on fracture resistance on endodontically treated tooth. Circumferential uniform core ferrule design is not always possible in clinical situation due to variations in the loss of coronal tooth structure. This has led to the incorporation of nonuniform core ferrule design with the available coronal tooth structure in endodontically treated teeth. Therefore, we had designed a study to investigate the same.

Aim

The aim of this study was to evaluate the fracture resistance of the endodontically treated maxillary central incisors restored with custom-made cast post and core with uniform and nonuniform core ferrule height.

Objective

The objective of the study was to find out whether:

incorporation of ferrule increases the fracture resistance of the endodontically treated teeth, and

there is a difference in fracture resistance of teeth restored with cast post and core with uniform ferrule and nonuniform ferrule height.

MATERIALS AND METHODS

Materials

The materials included 35 extracted human maxillary central incisors, airoter handpiece (NSK Technologies, Japan), contra-angle handpiece (NSK Technologies), endo access bur (SS White, Lakewood, New Jersy), 0.9% normal saline (Claris Otsuka, Ahmedabad, Gujarat), 3% sodium hypochlorite (Prevest Denpro), Carborundum disc, K-Files (Mani, Japan), spreaders (Mani, Japan), 2% gutta-percha (Meta Biomed, Chungcheongbuk-do, South Korea), absorbent points (Meta Biomed), zinc phosphate cement (D Tech Dental Technologies, India), cement mixing glass slab, cement spatula (stainless steel), auto polymerizing resin, peeso reamer, diamond bur (Swiss diameds), pattern resin (GC Technologies, Tokyo, Japan), orthodontic wire (21 gauge), phosphate-bonded investment material, casting ring, induction casting machine (BEGO, USA), nickel-chromium alloy, polyvinylsiloxane impression material, instron testing machine (model 3382), and digital Vernier caliper.

Methodology

Thirty-five human single-rooted maxillary central incisors extracted for periodontal reasons were used for the study. The teeth were selected such that they had had at least 13-mm root length and similar buccolingual dimensions. They were examined under microscope (×20 magnification) to rule out any cracks or caries craze lines. The teeth were also radiographed to determine the presence of a single canal. Teeth with more than one canal, immature root apices, with craze lines or fracture, and with root length less than 13 mm or thin curved roots were excluded.

Root canal treatment was performed in all the teeth that were included in study. The teeth were sectioned with a diamond disk horizontally 4 mm above the incisal edge to leave the crown height of 6–7 mm of cementoenamel junction (CEJ). The post space preparation was conducted with removal of gutta-percha using heated plugger followed by canal enlargement using peeso reamer of size 3 preserving 4 mm of apical gutta-percha. Preparation of the post space was maintained parallel. Antirotational features were not incorporated in the root canal preparation to confirm that only the ferrule effects were tested.

All the teeth were randomly assigned to five groups of seven teeth each. Group 1: uniform ferrule (2 mm buccal, lingual, and proximal) luted with zinc phosphate cement, Group 2: uniform ferrule (3 mm buccal, lingual, and proximal) luted with zinc phosphate cement, Group 3: nonuniform ferrule (2 mm buccal, 3 mm lingual) luted with zinc phosphate cement, Group 4: nonuniform ferrule (2 mm buccal, 4 mm lingual) luted with zinc phosphate cement, and Group 5: no ferrule (control group) luted with zinc phosphate cement.

Ferrule was prepared using flat end tapered diamond bur. Height of the ferrule was measured using a digital Vernier caliper. Depth of the ferrule was maintained to 0.6 mm with 3′–5′ taper on either side of the preparation. Post and core pattern were fabricated by direct method using pattern resin.

Each assembled specimens (tooth with acrylic resin block) was positioned in a custom-made zig that allowed the force from the instron universal testing machine to be applied 3 mm below the incisal edge of the palatal surface at an angle of 135° to the long axis of the tooth. A separate custom-made stainless-steel rod was attached to the universal instron testing machine and was used to transfer the forces directly on the lingual surface of the tooth in a way that resembled the articulation of the anterior teeth at maximum intercuspation. The specimens were subjected to static loading measured in newton (N) with a crosshead speed of 1 mm/min until fracture occurred. The point of fracture was determined by the sudden drop in force in the strain—stress diagram that was displayed in the monitor during the test. Fracture was defined as the point at which the loading force reached the maximum value. Failure load or fracture resistance was recorded from a force deflection curve.

One-way analysis of variance (ANOVA) was used to compare the mean failure load for each group.

RESULTS

The results of this study were subjected to statistical analysis to interpret the difference among uniform ferrule, nonuniform ferrule and no ferrule groups, and between the subgroups. One-way ANOVA, post hoc Tukey honestly significant difference (HSD) test were used for the statistical analysis in this study.

One-way ANOVA was used to study the overall variance within the groups. However, it is not possible to identify the difference between the various subgroups with the help of P-value obtained from ANOVA [Table 1]. Therefore, a specific statistical test was used for intragroup comparison. Hence Tukey test [Table 2] was performed to determine the group variation among each other.

Table 1

Distribution of mean fracture resistance among five groups using one-way analysis of variance

Groups	Minimum	Maximum	Mean	Standard deviation
Uniform ferrule
2 mm (Group 1)	1098.54 N	1286.73 N	1181.66 N	68.30
3 mm (Group 2)	1249.18 N	1714.57 N	1455.58 N	173.12
Nonuniform ferrule
Labial 2 mm, palatal 3 mm (Group 3)	929.76 N	1091.07 N	1019.00 N	52.56
Labial 2 mm, palatal 4 mm (Group 4)	873.34 N	1085.16 N	971.59 N	66.52
No ferrule
Group 5	826.46 N	974.67 N	888.00 N	60.57

Table 2

Multiple comparisons among the five groups using Tukey HSD test

(I) Group	(J) Group	Mean difference (I − J)	P value
Group 1	Group 2	273.91714	0.000
	Group 3	162.66143	0.025
	Group 4	210.07286	0.002
	Group 5	293.66143	0.000
Group 2	Group 1	273.91714	0.000
	Group 3	436.57857	0.000
	Group 4	483.99000	0.000
	Group 5	567.57857	0.000
Group 3	Group 1	162.66143	0.025
	Group 2	436.57857	0.000
	Group 4	47.41143	0.883
	Group 5	131.00000	0.102
Group 4	Group 1	210.07286	0.002
	Group 2	483.99000	0.000
	Group 3	47.41143	0.883
	Group 5	83.58857	0.485
Group 5	Group 1	293.66143	0.000
	Group 2	567.57857	0.000
	Group 3	131.00000	0.102
	Group 4	83.58857	0.485

DISCUSSION

Endodontically treated teeth restored with post, core, and a crown can be considered as a multicomponent engineering structure of complex geometry. Distribution of stress in this complex system is mainly reliant on the materials of post, core and the crown, rigidity of supporting structures, and the magnitude and direction of applied forces.[10]

Post is indicated when there is insufficient coronal tooth structure to retain a core for an artificial crown.[11] Majority of single-rooted pulpless teeth are restored with post and cores.[12] The custom-made cast post and core has always been regarded as the gold standard in post and core restoration. Cast post and core is indicated when there is moderate-to-severe tooth loss and when the number of visits is not an issue. It has been suggested that a well-adapted cast post and core restoration will be more retentive than a prefabricated post that does not match canal shape precisely.[13]

The long-term success of the post and core primarily depends on retention and resistance form incorporated into the endodontically treated teeth. Ferrule effect primarily provides resistance form and has greater influence on the longevity of the endodontically treated teeth. Ferrule maintains the structural integrity of root under functional loading, homogenous transmission of functional forces along the dowel.

Preservation of the tooth structure is an important factor and maximizing the residual amount of permanent tooth structure will increase the fracture resistance of an endodontically treated teeth.[14] The limitation of the earlier studies includes natural variation among the teeth that lack periodontal ligament, and ferrule preparation was placed at constant height but in in vivo situations ferrule height usually differs around the circumference of the tooth. Therefore, this study investigated the effect of nonuniform ferrule heights with uniform ferrule height.

Different height and forms of the ferrule have been studied in the literature. The height and form are essential for the success of the ferrule effect. When possible, encompassing 2.0 mm of intact tooth structure around the entire circumference of a core creates an optimally effective crown ferrule. Ferrule effectiveness is enhanced by grasping larger amounts of tooth structure. The amount of tooth structure engaged by the overlying crown appears to be more important than the length of the post in increasing a tooth’s resistance to fracture.[14]

The factor investigated in this study was the effect of uniform and nonuniform core ferrule lengths on the fracture resistance of custom-made cast post and core in the endodontically treated maxillary central incisors.

There are conflicting results regarding the ferrule effect on fracture resistance of the endodontically treated teeth. Libman and Nicholls[15] suggested that the maximum benefit of ferrule effect is achieved when there is a minimum of 1.5 mm of ferrule height with parallel dentin walls which is completely encircling the tooth and ending on a sound tooth structure. Akkayan[16] reported that 2-mm ferrule significantly improves the fracture resistance. All the teeth used in this study were reduced from the incisal edge by 4 mm by sectioning using a carborundum disk. This was performed to simulate a near-fractured tooth at the incisal edge and also to leave a remaining height of 6–7 mm from the CEJ. This was done to have enough tooth structure for core and ferrule preparation.

The first relevant finding in this study revealed that custom-made cast post and core with uniform ferrule significantly improve the fracture resistance of endodontically treated teeth. This is in agreement with the results of the studies conducted by Lu Zhi-Yue et al.[17] and Martínez-Insua et al.[18] There was a statistically significant difference between the uniform ferrule and nonuniform ferrule group studied (P <0.0001). The mean fracture resistance of custom-made cast post and core with 3-mm uniform ferrule (group 2), 1455.58 N. was significantly higher than all the experimental groups’ mean fracture resistance of 2-mm uniform ferrule group, which was 1181 N. The results also indicated that the tooth with no ferrule showed the minimum fracture resistance (mean value 888.00 N). This is the first study in which the core ferrule height was increased up to 3 mm to verify whether the effect of increasing the height of core ferrule can enhance the fracture resistance of endodontically treated tooth.

The reason for increased fracture resistance would be that uniform increase in the ferrule height covers the remaining portion of the tooth core, leading to even distribution of the stress, which can increase the load to failure. It has also been revealed in some studies that ferrule effect decreases the occurrence of fracture in endodontically treated teeth by redistributing the forces acting on it.

This study demonstrates the positive effect of ferrule design incorporated in the preparation of teeth with post and core. Core ferrule encircles the sound tooth structure at 360°, serving as a reinforcing ring to protect tooth structure from vertical fracture.[4]

The results of this study showed that increasing the ferrule height of 3-mm uniform ferrule increases the fracture resistance compared to ferrule height of 2-mm uniform ferrule. The literature suggests that a nonuniform ferrule is still superior to no ferrule at all. Al-Wahadni et al.[19] in 2002 studied the presence of a partial ferule on anterior teeth. They compared having no ferrule to having 3 mm or more height of ferrule on the buccal surface alone and concluded that teeth with retained buccal dentine of 3-mm height, but no other dentine walls remaining, had significantly higher resistance to fracture compared to the control.

This study also showed similar results that higher fracture resistance in nonuniform ferrule (Group 3: 1019 N, Group 4: 971.59 N) compared to the control group (Group 5: 888.00 N). There was a statistically significant difference between Groups 3 and 5.

The results of this study indicate that the presence of remaining coronal structure did influence the fracture resistance of the teeth. The findings of this study are in agreement with those of Sorensen and Engelman, who found that even 1 mm of remaining coronal tooth structure was able to resist compressive load.

Ichim et al.[20] demonstrated that the presence of the ferrule increases the mechanical resistance of a post, core and crown restoration, and decreases the likelihood for displacement (labial and axial rotation) and compressive stresses within labial dentine and the canal wall. The results suggest that the ferrule height should be determined individually for each case based on the buccolingual cervical diameter of the root. As a drawback, the presence of the ferrule creates a larger area of palatal dentine under tensile stress, which may be a favorable condition for a crack to develop on the palatal aspect of the root eventually leading to an oblique root fracture. By comparison, a restoration without ferrule is prone to failure primarily by debonding and subsequently by root fracture through the lever action of the loose post. Maxillary central incisors are exposed to repeated oblique stresses due to their position in the dental arch. These stresses lead to tension on the cervical region of the buccal aspect and compression on the cervical region of the palatal aspect, as in the case with Angles Class-1 occlusion. In this study, the results revealed that the presence of the ferrule in the remaining coronal tooth structure acts by decreasing the stresses on the restoration in most tooth structure, which is more evident when the ferrule height is greater (group 2).

In both the core and crown ferrule studies,[789] the tooth’s resistance to fracture was increased when a substantive amount of tooth structure was engaged (2 mm in the core ferrule studies and 1–2 mm in the crown ferrule studies. Pereira et al.[9] compared the effect of no crown ferrule with 1-, 2-, and 3-mm crown ferrules. They found that the 3-mm crown ferrule significantly increased the fracture resistance of endodontically treated teeth compared with the 2-mm crown ferrule.

The result of this study revealed that Group 3 showed better fracture resistance compared to Group 4. This proves that nonuniform ferrule is also effective in resisting fracture and helps in uniform load distribution, whereas in Group 4 the fracture resistance was more than that of group 5. The palatal axial wall was as effective as a 360° circumference in providing fracture resistance. However, in this study increase in core ferrule height on the palatal aspect does not improve the fracture resistance of the endodontically treated teeth. The results also highlighted that fracture resistance of the nonuniform core ferrule group is higher than that of no ferrule group.

Further studies in this regard with in vivo simulation and dynamic loading will help in the understanding of fracture resistance of endodontically treated teeth.

CONCLUSION

Within the limitations of this study, it may be concluded that:

Uniform core ferrule with 3 mm is more effective than uniform core ferrule with 2 mm.

Uniform ferrule is more effective than nonuniform ferrule.

Core with ferrule is better than core without ferrule.

Hence, ferrule plays an important role in the fracture resistance of custom-made cast post and core restored teeth, and incorporation of ferrule is a must in designing post endodontic restoration of badly broken down teeth.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.