Restorative Modalities for Structurally Compromised Teeth with Thin-walled Roots: A Literature Review

Abstract Proper restoration and maintenance of endodontically treated teeth with thin-walled roots and immature apices is of a great concern due to the high prevalence of such cases. The aim of this study was to review the efficiency of different materials used for post–core systems in order to restore endodontically treated teeth. A literature review was conducted using electronic databases including PubMed, Scopus, Web of Science, and Google Scholar to find relevant articles. Randomized controlled trials that were related to different post–core techniques for restoring endodontically treated teeth and were in the English language were included from 2000 until 2022. Most articles concluded that cast posts have higher fracture resistance than fiber posts. All articles comparing stainless steel posts with other materials found that it had higher fracture strength. No particular difference between various types of fiber posts was identified, and all can reinforce the immature roots with thin dentinal walls.


Introduction
Thin-walled teeth can be a result of overinstrumentation and endodontic treatment, large dental caries, or immature root development. 1Fracture resistance of filled thin-walled roots appears to be lower than vital teeth.The remaining dentin thickness and the selection of restorative materials and post-core systems influence the tooth resistance. 2 Also in childhood, dental caries and traumatic accidents can cause severe coronal structure damage in permanent immature teeth.This can lead to necrosis of pulps; therefore, treating these immature teeth with thin walls and wide apices have always been a difficult challenge.Several techniques for healing immature apices are practiced; however, for preserving the roots walls and cervical region, reinforcement and the choice of restorative materials are essential. 3,4Several studies report different treat-ments for endodontically treated teeth (ETT) and the endurance of post systems.In some of them, optimal treatments with suitable techniques have been proposed; however, the long-term success of those treatments is not determined yet. 5,6In ETT, posts are placed to improve fracture resistance and the maintenance of coronal restorative treatments. 7In current restorative approaches, post and core systems are gradually developing toward being more corrosion resistant and stronger, and less invasive techniques that do not weaken the remaining root structure are recommended due to the likeliness of flared canals to fracture. 8Therefore, the remaining dentin should be retained as much as possible when preparing the root canal for post fabrication. 9enerally, two main types of post systems are advocated to restore root canal; cast posts and fiber-reinforced composite (FRC) posts.Cast posts include prefabricated and custom-made posts.They are used clinically to reinforce and rehabilitate ETT by distributing forces through the root structure.Prefabricated post systems are less pricey and may be less invasive in certain conditions. 8Several drawbacks are associated with cast posts including unsatisfactory aesthetics, lengthy treatment and laboratory procedures, highly reductive effects on teeth, and insufficient retention.Fiber posts are utilized as an alternative with some improved features and they are as well, used as prefabricated and custom-made.Different materials are used for fiber reinforcement including glass, carbon, and quartz fibers for the prefabricated FRC posts and polyethylene fibers (Ribbond) for customized fiber post systems.Their main proposed benefit was that they are more adaptable than metal posts and more adjustable to the root shape.They have nearly the same modulus of elasticity (stiffness) as dentin and when bonded in canal with resin cement, it is assumed that forces would be distributed more evenly through the root walls, resulting in fewer root fractures. 10ince restoring teeth with insufficient and weakened canal wall is a clinical challenge, the aim of this paper is to review various studies that utilized different materials and techniques for restoration of ETT.

Materials and Methods
An electronic search of PubMed, Scopus, Web of Science, and Google Scholar, limited to articles published from 2000 to 2022 was conducted.The following query keywords were included: Thin-walled teeth, Thin-walled roots, Weakened roots, Post and Core Techniques, "Dowels, Dental," Fracture resistance, Endodontically-treated teeth, Immature teeth, Structurally compromised teeth.Studies that were relevant to this study, written in English, and were conducted as a randomized controlled trial were chosen.For the initial article searching, titles and keywords were considered and 566 articles were found (PubMed: 247, Scopus: 234, Web of Science: 10, Google Scholar: 75).In the first study selection based on screening abstracts and titles and removing duplicate articles, 87 studies remained.In the second study selection based on full-text analysis, the studies were chosen if related to different post and cores application and methods for thin-walled and structurally compromised teeth, and the evaluation of post and dentin thickness and length.Eventually, 36 studies were chosen.Twenty studies compared different post-core materials.Eight studies compared weakened and nonweakened roots, five studies compared using composite resin or glass ionomer cement (GIC) or neither as restorative materials for reinforcing thin-walled roots, and three studies compared stainless steel posts with fiber posts and cast posts.

Effect of Post Material on Fracture Resistance of Thin-Walled Teeth
Among studies that compared cast posts with fiber posts, most of them came to the same conclusion that thin-walled teeth with cast post-cores had the highest fracture resistance (►Table 1).Kivanç et al and Maccari et al 11,12 reported that cast posts had higher strength and there was no significant difference among various fiber post types.Also, Balkaya and Birdal 1 showed that parallel-sided cast posts have the highest fracture resistance among all, and cast posts with less diameter show higher resistance, whereas fiber posts with larger diameters are slightly more resistant.In Marchi et al 13 study, the fracture resistance was shown to decrease for custom cast posts, prefabricated metallic posts, and prefabricated carbon fiber posts.In contrast, Goncalves et al 14 stated that among the weakened roots, groups with metal CuAl cast post had the lowest resistance and there was no significant difference between roots filled with light-transmitting polymerization post and prefabricated titanium post with different resin types in each group.Also, Li et al 15 reported that groups with fiber posts showed more resistance compared with metal posts.Considering stress values, Khadar et al 5 observed higher stress concentration for teeth restored with cast metal posts, whereas it is more even for fiber post and composite posts.This may be due to the possible monoblock unit formation in the tooth structure.
Regarding the mode of fracture Vidya et al 16 found that groups with cast posts showed more catastrophic fractures leading to extraction compared with glass fiber and titanium posts.
Considering fiber posts with different materials, Ayad et al and Akkayan and Gülmez 17,18 concluded that quartz fiber posts required a higher mean load to fracture than titanium posts.Akkayan and Gülmez 18 reached the result that the fracture resistance from high to low was found for quartz fiber posts, glass fiber posts, zirconia posts, and titanium posts, respectively.While in Sharafeddin et al, Maccari et al, and Dikbas et al 12,19,20 studies, there was no statistical difference found between quartz and glass fiber posts.Dikbas et al 20 stated that using either glass fiber, quartz fiber, or zirconia posts can reinforce the root in immature teeth.Furthermore, application of multiple unidirectional FRC posts as an individual post leads to a higher fracture resistance comparing with one single FRC post. 21ccording to Newman et al, 8 stainless steel para post had the highest fracture resistance comparing with glass fiber posts and Ribbond woven polyethylene fibers.Also in Makade et al 22 study, it was found that the mean fracture resistance was lessened for the group with stainless steel posts and composite core, glass fiber post with composite resin, and cast posts, respectively.Amarnath et al 23 came to the same result that groups with stainless steel posts had higher mean failure load.Additionally, Zogheib et al 24 found that no statistical difference was seen in roots filled with glass fiber posts used with different techniques such as using composite resin with incremental technique, accessory fiber posts, and anatomic glass fiber posts.
Wu et al 26 found that using a layer of resin- Restorative Modalities for Structurally Compromised Teeth Azimi et al.Restorative Modalities for Structurally Compromised Teeth Azimi et al.Restorative Modalities for Structurally Compromised Teeth Azimi et al.
based composite (RBC) or GIC both lead to a better function than a cast post alone.Also, short FRCs have been found to be promising as a core for structurally compromised teeth compared with dual-cured core build-up composite. 27

Effect of the Thickness of Dentin and Post Length on the Fracture Resistance of Endodontically Treated Teeth
Regarding the remaining dentin thickness, generally, weakened roots have a lower fracture resistance than nonweakened roots. 24,28,29Kivanç et al 11 found that fracture resistance was highly influenced by the remaining dentin thickness in fiber post groups, although not significant.In contrast, in groups with cast metal posts, load failure was affected by axioproximal dimension of dentin walls and the cast metal posts with dentin thickness of 2.0 mm showed fewer values than that of 1.0 and 1.5 mm.Da Rosa et al 30 reported that favorable failures were most predominant in partially weakened teeth with a glass fiber post, followed by the groups with healthy roots and a glass fiber post, and groups with extensively weakened teeth and glass fiber post plus five accessory glass fiber posts.
In a study by Junqueira et al, 31 glass fiber posts with different lengths were utilized in nonweakened, medium weakened, and highly weakened roots and no significant alteration in fracture resistance due to neither post length nor root dentin thickness was observed, although the relation between length and dentin thickness were remarkably related.However, Amarnath et al 23 reported that as the post length increased, the fracture resistance was higher to an extent of two-thirds of the root length and core debonding occurred in short-length posts.In contrast, Seto et al 32 study demonstrated that the 3-mm post length had higher fracture resistance than 7 mm.
According to Newman et al, 8 there was no significant difference between flared or narrow canals and mean load fractures of the post systems used, except for the Ribbond standard groups.For the narrow canal roots, the highest mean load value required for fracture was for the glass fiber posts, and in the flared canal groups it was for both glass fiber posts and woven polyethylene fibers, respectively (►Table 2).

Discussion
The main purpose of post and cores is to contribute to retaining endodontically treated and thin-walled teeth.Numerous materials and techniques have been recommended for using posts for ETT due to the high demand for appropriate treatment.Metallic cast posts, zirconia posts, titanium and stainless steel posts, and nonmetallic dowels such as carbon, glass, quartz, or polyethylene fiber-reinforced post systems are utilized.Also, in case of immature teeth with open apices, large caries or trauma lead to thin and weak dentinal walls, thereby intraradicular reinforcement of immature teeth after apexification is a concept that has been found to improve the tooth's function and maintenance. 33he aim of this review was to compare and evaluate studies that examined at least two different post materials, in order to select materials that may improve the retention in root canals.
According to the findings in this study, cast posts and cores are more rigid than fiber posts and are able to resist higher loads.An explanation might be that custom cast cores comprise homogenous structures with a high modulus of elasticity For the cast posts it was higher with 1.3 mm diameter than for 1.7-mm posts whereas, for the fiber posts it increased as the diameter also increased.The teeth restored with composite resin and fiber posts demonstrated higher fracture resistance than those with fiber posts alone (F1.3, F1.7).Also, no significant difference was found between roots with and without space enlargement and their interaction with post diameters.The parallel-sided cast posts (C4 group) had the highest fracture resistance Seto et al 32  Restorative Modalities for Structurally Compromised Teeth Azimi et al.
that imitates the contour of the inner root wall; therefore, they have higher load-bearing capacity. 9Somehow, in some cases they have been linked to root/crown fractures and also endodontic failures due to their elastic modulus mismatch. 7verall, in a retrospective study it was found that after an average of 4-year follow-up, postendodontic treatments with Kurer anchor system metal posts had high survival rates. 34iber posts have increased the operation of adhesive dentistry due to their benefits such as better bonding to canal surface, simpler placement, and removal during a single office visit, having the same elastic modulus as dentin, and better aesthetic appearance. 29As stated in the monoblock concept, in order to increase the rigidity of weakened roots after endodontic instrumentation and immature teeth with open apices, a mechanically homogenous unit is required.Therefore, a strong bond between the reinforcement material and root canal as well as a similar modulus of elasticity of the material with the bonding surface will reduce the stress concentration. 35It has been suggested that post materials with similar elastic modulus to dentin and carbon fiber posts accomplish this aim. 36egarding composite cores, some authors have asserted that restorations with fiber posts and composite resin cores result in a higher fracture resistance in comparison to cast metal post cores due to their elasticity and their applied luting cement. 37,38It was explained in a study that in contrast to cast posts, between the fiber-reinforced resin posts and the dentin walls, a gap is present.This gap is filled with resin cement that can balance the stress instead of directly transferring it to the root. 12On the contrary, another study have stated that a fiber post in a wide and flared canal such as immature teeth will fit improperly and the thick layer of resin cement might cause air bubbles entrapment, leading to debonding. 5Generally, the most frequent failures are due to adhesion when adhesively luted posts are used, which can be related to problems in using techniques.Other causes might be root structure fractures or defects and lack of the ferrule effect.Additionally, root cracks and fractures are by themselves associated with the elastic modulus of posts according to some studies. 39onsidering the mode of fracture, based on the articles included in this study and a systematic review, in comparison to metal posts, fiber posts have considerably demonstrated less severe root fractures. 10mong different post materials, in a systematic review, it has been stated that carbon fiber posts with resin matrix show significantly fewer failure outcomes than cast posts with precious alloys. 40The fracture resistance of carbon fiber posts may depend on appropriate mechanical retention, and hence a proper bond of adhesive materials to the root canal. 13mong different alloys used in posts, titanium alloys have the most corrosion resistance and high fracture resistance, whereas alloys including brass show less strength corrosion resistance. 9,41In comparison with fiber posts, a systematic review concluded that titanium and fiber posts show similar fracture resistance. 10lass fiber resin post systems that are formed from unidirectional glass fibers embedded in a resin matrix are reported to lower the risk of fractures of restored roots. 42lasticity of dentin is 20 Gpa, and glass fiber posts with a 40 Gpa modulus of elasticity would be an ideal post material. 16chmitter et al 43 found that glass fiber posts are significantly better than metal screw posts.On the contrary, in a study after performing a 3-year follow-up it was concluded that the survival rate of glass fiber and cast metal posts was similar. 39olyethylene woven fibers are materials with woven network that permit wetting of the fibers and resin infusion into the fibers.Therefore, the network effectively transfers the stress through the interface of enamel and adhesive materials to increase the fracture strength of restorative materials. 44Ribbond materials can be used as individually shaped dowels by placing the pieces into the canal with a technique described by Erkut et al. 45 They are a kind of polyethylene woven fibers that seem to show better maintenance and continuity at the adhesion interface compared with glass fiber or quartz fiber reinforced dowels. 45They are composed of Leno-woven, which is a special triaxial pattern of crosslinked threads.This lace-like structure improves its durability and adaption to teeth contours.It also increases mechanical interlocking of composite resins to surfaces. 46Plastic light-transmitting posts were developed aiming to transmit light to the surrounding composite resin as a matrix around metallic posts and reinforcing the remaining flared root canals. 47Frater et al 21 compared the use of multiple long unidirectional FRC posts and short fiber-reinforced composite (SFRC) post applied with bioblock technique.In the bioblock technique, SFRC post, either packable or flowable, is luted directly as a bulk into the canal, eliminating the use of luting cement.They came to the result that multiple long unidirectional FRC posts have better fracture behavior than SFRC posts.Also, multiple fiber post technique in comparison to a single FRC posts has shown better results. 6,21ost diameter has a great influence on post stiffness, but its effect on fracture resistance is yet to be researched.According to some studies, posts with larger diameters are recommended since they have demonstrated higher resistance than less diameter posts, but this is not as important as saving the tooth structure. 9Also, in some studies evaluating post lengths, no significant difference in fracture resistance between glass fiber posts of different lengths was found. 31,48entin thickness is as well an effective factor.As much tooth tissue as possible should be preserved while restorative procedures since it has been reported that 1 mm of dentin thickness has less fracture resistance under horizontal pressure than 2 to 3 mm dentin walls. 26,49Another approach with regard to post diameter is the proportional approach where post width should not be larger than one-third of the canal width. 41Therefore, the dentin tissue around the post impacts fracture strength, mostly when cast metal posts are used. 30Although during the preparation of root canal for cast posts, more dentin is removed and root walls may be weakened, there is no consensus whether this fact is solely a weak point for cast posts compared with fibers posts.Several other factors such as post material and its modulus of elasticity, diameter, type of cement and post adaptation must be considered. 50onsidering the intensity of the failure, in most studies, nonrepairable fracture patterns have been seen in teeth filled with cast metal posts, whereas repairable fractures are mostly observed in roots filled with fiber posts.A reason can be that cast posts apply stress directly on the root. 7,31n many studies, the ferrule effect is suggested to enhance the fracture resistance against functional forces and wedge effect of posts and also reduces possible microleakage of cement area in teeth with enough remaining supragingival structure. 51,52However, in a recent systematic review it was assumed that no conclusive evidence suggesting that ferrule effect significantly reduce fractures is available. 53In many clinical conditions, the teeth might be obliquely broken or have lost the coronal walls.Methods such as crown lengthening and orthodontic forced eruption methods are performed to provide more tooth structure in this cases.However, it is unclear whether it is also useful in case of oblique fractures.Meng et al found that these methods decreased the fracture resistance of teeth with oblique fractures. 54Further clinical studies comparing teeth with or without ferrule is needed.
RBCs increase the thickness of the dentin walls.Also, it is assumed that having the same elastic modulus as dentin for these reinforcing materials as well as posts can help the tooth withstand functional forces. 26In composite resin restorations, a bond is formed with the dentin tubules, resulting in production of a hybrid layer and resin tags, which can improve micromechanical retention. 17Amin et al 55 found that relining the post with composite resin increases the resistance compared with using a bulk of luting cement.GICs also can improve fracture resistance of root canaled teeth with thin-walled roots. 26Failure in resistance might be ascribed to inconvenient application or fracture of the composite resin layer. 12he studies that we reviewed has some limitations such as not studying full crowns or the absence of the periodontal ligament that may affect the failure modes and further investigations are needed to clarify the effects of these conditions.Another limitation concerns the fact that in most studies tests were performed on single-rooted and different anterior teeth.Specific post preparations and similar methods of compressive pressure were used but in various angles.This might differ from the natural occlusion forces on teeth.

Conclusion
Within the limitations of this study, the following can be concluded: • It seems that cast posts, despite their need for more root canal dentin removal, show higher fracture strength when used for ETT treatment.• Quartz fiber posts, glass fiber posts, and Ribbond customized posts can preferably be used as an alternative to zirconia and titanium posts due to their advantages.• Stainless steel posts show the highest fracture resistance in studies that compared them to other materials including fiber posts and cast posts.
• Applying a layer of composite and resin tag formation through the root canal can increase the fracture resistance.
It is still a matter of controversy over which post length is more effective.

Table 1
Effect of post material on fracture resistance of thin-walled teeth European Dental Research and Biomaterials Journal Vol. 3 No. 1-2/2022 © 2023.The Author(s).Restorative Modalities for Structurally Compromised Teeth Azimi et al. 5

Table 1 (
Continued) 59türk et al59/2018 Maxillary canines 4 groups (N ¼ 80): Group 1: No post-core, composite resin Group 2: Cast post-core Group 3: Glass fiber-reinforced epoxy post system (Radix fiber post), composite resin core Group 4: Glass fiber-reinforced epoxy post system (I-TFC ipost), composite resin core The highest fracture resistance was observed for the group 2, group 3, group 4, and control group, respectively.With the highest rate of nonrestorable fractures for group 2. In the Groups 3,4 fracture mode would allow restoring the tooth Amarnath et al 23 /2015 Mandibular premolars 6 groups (N ¼ 60): Group 1,2,3: Stainless-steel (SS) Group 4,5,6: Glass fiber post (FP) The highest mean strength was obtained for stainless-steel posts, and the lowest was obtained for glass fiber posts Cauwel et al 60 /2014 Weakened bovine incisors 4 groups (N ¼ 75), Group 1: (n ¼ 20), control group, Unfilled teeth Group 2: (n ¼ 17), filled with mineral trioxide aggregate (MTA) Group 3: (n ¼ 18), filled with calcium phosphate bone cement (CPBC) Group 4: (n ¼ 20), filled with fiber reinforced composite (FRC) posts The results denoted the highest mean initial fracture load for the FRC group and the lowest for the control group.The control group (95%), The FRC (90%), and CPBC (89%) groups showed most favorable fractures, respectively.All fractures were at the cervical area of the palatal part of the root Sharafeddin et al 19 /2014 Mandibular premolar roots 4 groups (N ¼ 40), n ¼ 10 Group EX: Exacto glass fiber post Group EXR: Exacto glass fiber post þ 2 Reforpin accessories Group DT: D.T. Light quartz fiber post Group DTF: D.T. Light quartz fiber post þ 2 Fibercone accessories There were no statistically significant differences in fracture resistance among teeth.The fracture resistance order was as follows: EX > DT > EXR > DTF Wandscher et al 61 /2014 -2 groups (N ¼ 80): Weakened teeth (n ¼ 50): 5 subgroups n ¼ 10: CPC-gold (cast post and core made of gold alloy) CPC-Ni (cast post and core made of Ni-Cr alloy) FP (glass fiber posts) FP-W (glass fiber posts with a wider coronal diameter) In the survival analysis, group FP-W demonstrated the highest survival rate.Cast post groups had higher fracture loads than fiber post groups.No differences were observed among groups with nonweakened roots European Dental Research and Biomaterials Journal Vol. 3 No. 1-2/2022 © 2023.The Author(s).

Table 2
Effect of the thickness of dentin and post length on the fracture resistance of endodontically treated teeth Restorative Modalities for Structurally Compromised Teeth Azimi et al. 11 European Dental Research and Biomaterials Journal Vol. 3 No. 1-2/2022 © 2023.The Author(s).