Comparison of fracture resistance of endodontically treated maxillary first premolar with mesio-occlusal-distal access restored with composite resin, fiber post, and prefabricated metal posts restored with/without full-coverage metal crowns.

Background: A myriad of materials and procedures have been recommended for restoring the root-filled teeth with significant loss of tooth structure and the most common method being the use of "posts." Material and
Methods: A mesio-occluso-distal preparation was done on eighty intact maxillary first premolars, followed by access preparation and cleaning and shaping with rotary files. All samples were obturated and divided into four groups (n = 20). In Group 1, 3-mm gutta-percha was removed and restored using composite resin; Group 2 - Group 1 + crowns; Group 3 - prefabricated fiber post + crowns; and Group 4 - prefabricated metal post + crowns. All the groups were subdivided into eight groups (n = 10), fractured with 90° vertical and 45° oblique loading by a universal testing machine. Statistical Analysis: Fracture resistance was analyzed and compared by means of two-factorial two-way analysis of variance and Scheffe's post hoc test.
Results: Under 90° loading, the fracture load of teeth restored with the Nayyar's composite resin core and crown was greatest. Under 45° loading, the fracture load of teeth restored with prefabricated metallic posts was significantly (P < 0.05) less than that in other groups. Under 45° load, group with fiber post exhibited significantly (P < 0.05) more number of favorable fractures than other groups.
Conclusion: Under the conditions of vertical and oblique loadings, fiber post and composite resin core with a full-coverage crown is the most effective restorative modality for protecting the remaining tooth structure in pulpless teeth. Copyright:

INTRODUCTION

Composite resin is an excellent choice of material for restoring endodontically treated teeth (ETT) with minimal tooth structure loss. Due to its adhesive property, it allows minimal cavity preparation and intracoronal reinforcement.[1] Recent studies have indicated that routine use of full-coverage restorations may not be required if marginal ridges are sound. With the loss of marginal ridges, there is a dramatic decrease in the fracture resistance of teeth, making them more susceptible to fracture.[2]

Substantial loss of tooth structure requires a more complex restorative design to achieve the desired objectives. A variety of materials and techniques have been advocated such as full-coverage restorations and posts.[3] A series of studies analyzed the clinical performance of 1273 ETT and found that crown placement significantly improves the success rates of posterior teeth.[4]

The purpose of the present study is to evaluate the fracture resistance of maxillary first premolars which have lost both marginal ridges with different posts and crown combinations under 90° vertical and 45° oblique loading.

MATERIAL AND METHODS

Eighty orthodontically extracted maxillary first premolars were collected from patients between the ages of 18–24 years and divided into four (n = 20) groups. Teeth collected had no significant differences among groups in terms of buccopalatal (f = 2.7, P > 0.05), mesiodistal (f = 3.49, P > 0.05), and working length (f = 1.73, P > 0.05) dimension by means of Levene's test and analysis of variance (ANOVA) at 95% level of confidence.

All double-rooted teeth were examined under ×15 magnification (dental operating microscope, Labomed Prima, Labo America Inc.) for no visible root caries, restoration, signs of internal or external root resorption or calcification, and completely formed apex.

Standard mesio-occluso-distal cavity and access cavity preparations were made, as shown in Figure 1. Canal instrumentation with ProTaper rotary till #F2 was done with copious amount of 3% sodium hypochlorite irrigation. All teeth were obturated using AH Plus sealer (Dentsply Maillefer, Ballaigues, Switzerland) and continuous wave compaction (Calamus unit, Dentsply Maillefer, Ballaigues, Switzerland).

Figure 1

Dimensions of cavity design: Mesio-occluso-distal preparation on maxillary first premolar where B is intercuspal distance at isthmus, A is 1/3B. Gingival floor width is 2 mm. Access cavity preparation was done to gain straight-line access to the pulp space, C

In Group N (Nayyar's core with composite resin core without crown) and Group NC (Nayyar's core with composite resin core with crown), 3 mm of gutta-percha was removed with a heated spreader and restored with Xeno V (Dentsply Maillefer, Ballaigues, Switzerland) and nanohybrid resin composite (Tetric N-Ceram, Ivoclar Vivadent, Schaan, Liechtenstein).

The gutta-percha was removed in Groups FC (fiber post with composite resin core with crown) and MC (metal post with composite resin core with crown) to prepare post space leaving behind 5 mm of gutta-percha apically. Post space preparation was done using a low-speed Tenax drill, TE-A-11 (Coltene/Whaledent, UK) for fiber post (Tenax Fiber White post, TF#11, Coltene/Whaledent, UK) and ParaPost drill size 4.5 (Coltene/Whaledent, UK) for prefabricated metal post (ParaPost, stainless steel serrated post no. 3, Coltene/Whaledent, UK). The canals were coated with self-adhesive resin cement (Multilink, Ivoclar Vivadent, Schaan, Liechtenstein) posts placed and light cured (SmartLite PS, Dentsply Maillefer, Ballaigues, Switzerland) for 40 s while maintaining finger pressure. They were then restored as Groups N and NC. Crown preparation was done for Groups NC, FC, and MC with circumferential chamfer finish line. Metal crowns were fabricated and cemented. The specimens were stored in 100% humidity environment at 37°C for 24 h for thermocycling. Each group is now divided into two subgroups under 90° and 45° loading (n = 10).

Cold cure acrylic resin (DPI-RR) block of dimensions 25 mm × 20 mm × 20 mm in a stainless steel cylinder was prepared and the root of each specimen was embedded in it, up to 2 mm below the cementoenamel junction (CEJ) and with the long axis of the root at the right angle to the base of the block. Polysiloxane impression material (Dentsply Maillefer, Germany) was injected into the acrylic resin root cylinder. Dental roots were thus covered to mimic periodontal ligament. The specimens were kept undisturbed for 24 h to allow curing of the acrylic resin. Teeth were mounted in a customized fixture and subjected to 90° and 45° load. The load was applied with a sphere ball of diameter 3 mm at a crosshead speed of 0.5 mm/min using a universal testing machine (INSTRON, Norwood, Massachusetts, USA) until fracture. The vertical load was applied to the center of the occlusal surface, whereas the oblique load was applied to the center of the palatal cusp. The fracture load was recorded and the mode of fracture was inspected visually under magnification and transillumination.

Fracture loads of all the groups were noted onto Microsoft Excel 2010 (Microsoft®, Redmond, Wash) and analyzed using the SPSS Statistics version 21. 0 (SPSS IBM Inc., Chicago, Ill., USA). Fracture resistance was analyzed by means of two-factorial two-way ANOVA and Scheffe's post hoc test.

RESULTS

The minimum, maximum, and mean fracture resistance (Newton) with standard deviation for each of the four interventional groups under 90° and 45° loading are summarized in Table 1. The intragroup comparison of mean fracture loads at 90° and 45° is shown in Table 2. Under 90° loading, Group N exhibited significantly lower fracture resistance than Groups NC, FC, and MC. There was no statistically significant difference in fracture resistance between the different groups with crowns. Under 45° loading, the lowest fracture resistance was observed in Group N, followed by Groups MC, FC, and NC, and this result was statistically significant. Group MC exhibited statistically significantly lower fracture resistance as compared to Group FC.

Table 1

Fracture load (newton) with standard deviation of pulpless teeth under 90° and 45° loading

	90° loading				45° loading
	Maximum	Minimum	Mean	SD	Maximum	Minimum	Mean	SD
Group N	1100	644	851	138.05	845	366	606	148.4
Group NC	2678	1876	2240	280.6	366	1096	1597.9	294
Group FC	2398	1590	1883	306.8	2034	990	1585	536.8
Group MC	2563	1202	2001	439.8	1510	762	1077	261

SD: Standard deviation, N: Nayyar’s core with composite resin core without crown, NC: Nayyar’s core with composite resin core with crown, FC: Fiber post with composite resin core with crown, MC: Metal post with composite resin core with crown

Table 2

Two-factorial two-way analysis of variance with Scheffe’s post hoc test for fracture load of pulpless teeth under 90° and 45° loading

I	J	90° loading		45° loading
		Mean difference (I-J)	P	Mean difference (I-J)	P
Group N	Group NC	−1389.00	0.000*	−1191.40	0.000*
	Group FC	−1031.50	0.000*	−1028.40	0.000*
	Group MC	−1149.80	0.000*	−521.20	0.001*
Group NC	Group N	1389.00	0.000*	1191.40	0.000*
	Group FC	357.50	0.104	162.90	0.602
	Group MC	239.20	0.409	670.20	0.000
Group FC	Group N	1031.50	0.000*	1028.40	0.000*
	Group NC	−357.50	0.104	−162.20	0.602
	Group MC	−118.30	0866	−507.20	0.002*
Group MC	Group N	1149.80	0.000*	521.20	0.001*
	Group NC	−239.20	0.409	−670.20	0.000*
	Group FC	118.30	0.866	−507.20	0.002*

*Statistically significant (P<0.05). N: Nayyar’s core with composite resin core without crown, NC: Nayyar’s core with composite resin core with crown, FC: Fiber post with composite resin core with crown, MC: Metal post with composite resin core with crown

Under 45° loading, all groups fractured at a significantly lower load (P < 0.05) when compared to 90° loading. This study demonstrates that under 90° load, all the groups exhibited unfavorable fracture. Under 45° load, fiber post with composite resin core and crown (70%) exhibited significantly (P < 0.05) more number of favorable fractures than other groups.

DISCUSSION

ETT are believed to be more brittle because of moisture depletion, loss of collagen cross-linking, impaired blood supply,[5] and use of irrigating solutions.[6] Sedgley and Messer[6] tested the biomechanical properties of dentin from 23 ETT with their contralateral “vital” pairs 10 years of posttreatment and found comparable properties. Their study did not validate the inference that ETT are more brittle.

Loss of tooth structure associated with dental caries or existing restorations, endodontic procedures,[1] and increased cuspal deflection during function[7] has been associated with increased probability of fracture in ETT.[8] Mondelli et al.[9] concluded that fracture strength was inversely proportional to the amount of tooth structure removed. It has been reported that when the intercuspal distance is increased from one-third to half, there is a reduction in fracture strength of 75% and 89%, respectively.[1011]

ETT maxillary premolars have a high incidence of fracture.[1213] In comparison to molars, it is more likely for premolars to be subjected to lateral forces during mastication.[14] It has been reported that smaller functional cusps of maxillary premolars show a greater incidence of fracture in comparison to their counterparts.[1314] Cusp elongation due to cavity preparation may be a major factor in fracture susceptibility, primarily in root canal-treated maxillary premolars whose anatomy tends to separate the buccal and palatal cusps under occlusal load.[15]

Nayyar et al.[16] described a technique called coronoradicular reinforcement, wherein a retentive core is produced by removing 2–4 mm of gutta-percha from the canal orifice and slightly undercutting the pulp chamber. Currently, the material of choice for core buildup is bonded composite resin restoration.[17] Bonded restoration improves the fracture resistance of premolars, and cuspal coverage with composite can avoid cuspal elongation in consequence to the wedging effect.[11]

Numerous in vitro studies,[418] epidemiological studies,[19] and retrospective clinical studies[4] have reinforced the importance of full cuspal coverage to resist fracture in endodontically treated posterior teeth. Various authors have also ascertained a significant association between the placement of crown and the survival probability of ETT.[420] Sorensen and Martinoff[21] reviewed ETT that had been restored from 1 to 25 years and demonstrated that coronal coverage significantly improves the clinical survival of posterior teeth. In a 10-year retrospective study of ETT, Aquilino and Caplan[20] found that ETT without crown coverage were lost six times more often than teeth that received crowns post obturation. Similarly, in the present study, Group N recorded the least fracture resistance under 90° and 45° loading.

The purpose of using a post is to provide anchorage and retention for a core so as to retain the coronal restoration in teeth with excessive tooth structure loss.[2223] In the present study, parallel-sided metal and fiber post were used and a statistically insignificant difference was found in fracture resistance within the groups with cuspal coverage under 90° loading. Fracture resistance was significantly less for groups at 45° than under 90° loading. This reduction in fracture resistance can be explained by unfavorable stress distribution.[23] Difference in modulus of elasticity between the rigid metal post and the less rigid dentin results in concentration of stresses at the post tip. Modulus of elasticity of fiber post measures closely to that of dentin. This can be concurred in our study where we found a significantly lower fracture resistance for Group MC when compared to other groups with cuspal coverage under 45° loading.

Several retrospective studies have demonstrated that ETT restored using fiber-reinforced posts have reported a success rate of approximately 95%.[24] In the present study, we have found that the cuspal coverage was a more important determinant, as increased fracture resistance was observed with Nayyar's core and full cuspal coverage under 90° and 45° loading as compared to the groups with posts. The group without cuspal coverage exhibited significantly lower fracture resistance as compared to the groups with cuspal coverage.

Some investigators have emphasized that fiber posts increase the fracture strength of ETT, whereas others believe that fiber posts do not reinforce the teeth, but they can only reduce the occurrence of catastrophic fracture modes,[624] making these teeth more likely to be restorable. Many studies postulated favorable failure modes with fiber posts than with metal posts.[25] We also found more favorable fracture, i.e., above CEJ (P < 0.05) under oblique loading for Group FC, when compared to all the other groups. Under vertical loading in all groups, unfavorable fractures were seen as the cracks propagated in the middle and apical third. This was probably because of buckling of the root dentin. Our study reinforced the fact that ETT restored with fiber-reinforced posts allow the teeth to be re-restorable in the event of catastrophic failure.

Over the years, studies have demonstrated that research design influences the final result. One of the major drawbacks of in vitro studies is that the static load does not always simulate the dynamic load in oral conditions during mastication. The present study was designed to evaluate the fracture resistance of ETT restored with different postendodontic restoration with or without full coverage. This study highlights the fact that posts are required only for core retention and their presence does not improve the fracture resistance in ETT. Fiber posts do not concentrate stresses and also lead to favorable fracture under oblique load. On the contrary, metal post and crown causes unfavorable tooth fracture under vertical and oblique loads. Groups with full-coverage restoration without post exhibited a higher fracture resistance at 90° and 45° loading as compared to groups restored with posts and crowns. Cuspal coverage is a better determinant of fracture resistance than the presence or absence of posts, and the present study supports the same.

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Nil.

Conflicts of interest

There are no conflicts of interest.