Evaluation of Retentive Strength of 50 Endodontically-Treated Single-Rooted Mandibular Second Premolars Restored with Cast Post Cores Using 5 Common Luting (Cement) Agents

Introduction

Use of metal and wood placed within the roots of teeth to retain a bridge dates to the year 1728 and the mid-nineteenth century [1]. Later modifications in the nineteenth century included the ‘Richmond crown’ (single-unit cast metal post and crown) [2] and custom cast post core (CPC). Survival rates of endodontically-treated teeth increased with placement of an intraradicular post [3,4], but some critics have stated that intraradicular posts do not reinforce tooth structure [5,6]. Concerns include further depletion of dentin after endodontic treatment [7], loss of water and collagen [8], post space preparation and post design [7], greater differences in modulus of elasticity between posts (fiber post are 45.7–54.8 gigapascals [Gpa], cast posts are 178.34–198.76 Gpa, and dentin is 18.6 Gpa) [9,10], increased stress within roots and poor esthetics (prefabricated metal and CPC) [11]. Post cores are not essential for all teeth that have received endodontic treatment [12], but are indicated in cases where the remaining coronal tooth structure cannot retain a restoration and all other means or retaining a core have been exhausted [13]. Any tooth that has lost substantial coronal tooth structure due to caries, previous and current restorations, or endodontic treatment preparations (access cavity and root canal space) is likely to be a candidate for a post core, depending on the remaining tooth structure (post crown preparation), tooth position, patient/dentist preference, and core/crown material [14,15]. Metal dowels have been reported to increase fracture resistance of teeth [16], and studies reporting greater risk of root fracture when using a post core have not used the crown (isolated post and core) [17,18]. Recent studies on survival rates of post cores have reported they last for 10.9 years [19] to 11.74 years [20], with a cumulative failure rate of 28%. Root fracture in post core crown teeth in these studies was associated with primary crown retained partial dentures (removable) within the first 5 years [20]. In another study, the annual failure rates of CPC and glass fiber posts were 1.2% and 1.7%, respectively, with the authors concluding that both post systems performed well [21]. The most common failures were, listed from higher to lower prevalence, bonding failure (37%), apical lesions (37%), and crown dislodgement (11%), with a root fracture being only 3% of total failures [22]. Root fracture and root perforation have been reported to be more associated with removal of an intra-canal post [23] than routine functioning. Other recognized failures associated with post cores include secondary caries, periodontitis, cement failure, metal corrosion, post fracture, and post distortion [21,23]. Studies have reported no significant differences between prefabricated and cast post in terms of survival rates [15,24], although factors like crown type, bone attachment, patient age, and material have shown significant influences on survival time [19,20]. Clinically and biomechanically, CPCs offer several advantages over prefabricated posts, including higher rotation resistance [12], better retrievability [15,20], superior success [21], superior and maximum canal adaptation [12,15,21,25], conservative (minimum dentin removal) [26], and passive fit [27], and can be prepared on any canal configuration [10,13,16,21]. Ideally, the post must retain the core without undesirable stress and must have several ideal properties, such as high tensile strength, high resistance to fatigue, and ability to distribute forces, and must be biocompatible and esthetic [28]. Factors that influence post retention include the post itself (length, shape, design, diameter, surface characteristic), cement type, and endodontic sealer [29,30]. The disagreement between authors is primarily on the timing of post space preparation after completion of endodontic obturation and its effect on post retention [30,31]. A specific structural design feature known as ‘ferrule’, which encapsulates restored tooth providing a collar effect (1–2 mm sound tooth preparation) enhances stability and increases survival rate (5–10%) [32].

Advances in digital dentistry have brought a wide range of new materials and techniques in relation to both prefabricated and CPC systems and luting agents. Traditionally, the various materials that have been used for posts include alloys (eg, gold, titanium, base metal) [10], ceramics (eg, zirconium oxide) [12,22], and composites (eg, glass fiber-reinforced) [12,29]. The materials have been customized using traditional techniques like casting [12,21], computer-aided diagnosis, computer-assisted machining (CADCAM) [33–35], and 3D printing [33,36]. Milled posts have been reported to have better accuracy of fit than CPC in terms of feasibility of impression-taking [37]. However, Hendi et al [38] reported that CPC had better retention than half and full digitally fabricated milled post cores (mean force CPC – 171.6 N, half digital – 91.8 N). Modern alloys containing base metal nickel and chromium have also been reported to cause less post bending, which enhances stress distribution to the roots [39]. There have also been advances in luting agents, and with the introduction of adhesive cements many of the drawbacks of various post systems have been either eliminated or minimized. Earlier studies reported higher retention of CPCs with zinc phosphate [40] and glass ionomer cements [41]. CPC retention was also found to be higher with resin cements (Panavia F) using ultrasonic decementation [42]. Currently, there are a vast range of both adhesive and non-adhesive cements that are available to a dental practitioner that are indicated for either CPC or prefabricated posts (metal-, zirconia-, fiber-reinforced). Resin cements provide additional advantages of being insoluble in fluids and have superior mechanical strength [43]. Within adhesive systems, there are cements that are self-adhesive and do no require tooth etching. However, they are also technique-sensitive and result in development of secondary caries, besides having restricted working time and polymerization sensitivity [44]. Some of these drawbacks have been overcome by resin-modified glass ionomer cements, which have the combined advantages and properties of glass ionomer and resin cement. Amelia et al [45] compared glass ionomer with self-adhesive resin cement but did not find any significant differences in the tensile bond strength of the CPC. Hesarkhani et al [46] found highest retention with resin cement (Panavia SA) as compared to zinc phosphate and glass ionomer. Other studies on CPC retention using zinc phosphate and glass ionomer cement showed significantly higher retention with zinc phosphate cements [47]. Hybrid cements like resin-modified glass ionomer have been compared to zinc phosphate and glass ionomer cement for CPC retention and were found to have significantly higher mean retentive strengths (310.9 N) [48]. Selection of a luting agent can also be based whether it is for short- or long-term use, and certain cements are better for long-term use [49].

The cement retention of CPC is difficult to determine in the oral cavity, so traditional methods of evaluating cement retention have been used in vitro studies. The samples are cemented on extracted teeth that are prepared to simulate endodontically-treated teeth, after which they are thermocycled to represent oral conditions. Retentive strength is then tested using tensile tests. Our investigation of the literature found that the retentive strength of hybrid cement has not been compared with the resin and polycarboxylate cement, which would elucidate the properties of luting agents and improve clinicians’ choices regarding luting options for CPC. Therefore, this study aimed to evaluate the retentive strength of cast post core restorations in 50 endodontically-treated single-rooted mandibular second premolars using 5 common luting (cement) agents. The objective of the study was to determine which cements are currently able to provide the minimum required retentive strength for cast post cores. Assessing differences in retentive strength would also improve understanding of the cement strength of cast post cores. We hypothesized that retentive strengths vary between different cements, the basis of which is their ingredients. Alternately, the null hypothesis was that there is no difference in retention between various cements.

Material and Methods

ETHICS:

This research was a part of the postgraduate program and was approved by the college and university ethics committee (university registration number SVSUSDCH/Staff-E000204-Pr). All materials used in this study are regulated and approved by the Central Drugs Standard Control Organisation (CDSCO) (Ministry of Health, India).

STUDY DESIGN:

This in vitro experimental study followed the comparative approach and was conducted in different stages. The primary stage involved designing of the post and core samples, while the secondary stage involved cementation in the cast post and testing of the specimens.

OPERATIONAL DEFINITIONS:

“CPC post” was operationally defined as a single-piece foundation restoration comprising post that fits within a previously endodontically-treated tooth of a root canal, while the core is the portion of the CPC that replaced the missing coronal tooth structure and would allow tooth preparation for retention and supporting the crown [50]. “Luting agent” was operationally defined as a material used to cement the desired indirect restoration (in this case, CPC) to prepare post space within the root canal of the natural tooth. The term “retention” was defined as the ability of CPC to resist dislodgment in the vertical direction or along the path of placement [50]. The term “failure stress” was defined as the bond failure at an interface between 2 materials because of tensile force. Higher failure stress contextually indicates specimens failed at higher stress, while lower failure stress indicates a specimen failing at lower tensile stress. This term was specifically denoted as adhesion failure and is used interchangeably here [51].

SAMPLE SIZE ESTIMATION: The guide for the sample size was taken from earlier studies [46,47], which was verified by determination of sample size using G power software (V3.1.9.7; Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany). An effect size of 0.7 with alpha error probability of 0.05 and power of 0.90 yielded a total sample size of 11 for each group (actual power of 0.911) [52]. Five specimens were kept as supplementary to replace specimens that had an error during fabrication or testing.

SAMPLE SELECTION AND PREPARATION: A comprehensive list of various materials and instruments used in the study is presented in Table 1. All experimental procedures were conducted on extracted single-rooted mandibular second premolar human teeth (Figure 1A). The inclusion/exclusion criteria included no evidence of decay/caries/restoration extending apically, adequate root length (minimum mean length 15±1 mm) and diameter, uniform diametrically, no evidence of root surface cracks/fracture/other continuity breaches, and no history of endodontic treatment. Fifty-five similar natural teeth meeting these criteria were cleaned of debris and then chemically sterilized in 5.25% sodium hypochlorite, and then were stored in diluted formalin solution (10%) for 1 day before storing them in saline [53]. Light microscope (10×) magnification ruled out surface cracks and crazing. All teeth were also radiographed to exclude those with extra canals, internal resorption, calcifications, or any abnormal anatomical aberration. A line was marked on the surface of the tooth from labial to lingual cemento-enamel junction traversing the proximal surfaces on each side to guide the axis of decoronation. The teeth were then decoronated at 1 mm above the cemento-enamel junction, using a disc (Carborundum) (Dentorium, New York, NY, USA) on a high-speed handpiece with coolant (compressed air/distilled water), leaving the root length of 15 mm (10 mm for post and 5 mm for apical seal).

ROOT CANAL FILLING:

After extirpation of the dead pulp (barbed broach [Dentsply, Switzerland]) and cleaning of the root canal. Patency of each canal was determined using conventional endodontic files (#10K) followed by determination of canal length. The working length of endodontic treatment was standardized for all specimens using rubber stops. A jig fixed to the work table helped to align each tooth vertically, with any discrepancy between the tooth and the jig filled by laboratory putty-consistency elastomer (Reprosil, Dentsply/Caulk; USA). Biomechanical endodontic root canal preparation was accomplished by using NiTi (nickel titanium) rotary files (ProTaper next, Dentsply-Maillefer) attached to the corresponding endo-motor (contra angle, Endo-Mate DT, Japan). The preparation was standardized by keeping a constant torque (200 gcm), speed (300 rpm), irrigating solution (5.25% sodium hypochlorite) (Chloraxid, Poland), distilled water rinsing, and paper points (Pro Taper next size X3) (Dentsply-Maillefer) drying. For all teeth, the smear layer from within the root canal was accomplished using a 17% ethylene diamine tetra-acetic acid (EDTA, PD, Vevey, Switzerland). A standard clinical procedure for biomechanical preparation was followed for instrumentation until Pro Taper next file size X3. Obturation of the canals was standardized by filling with gutta percha (Meta Biomed; South Korea) and root canal sealer (AD; Meta Biomed; South Korea) with thermal (cold) lateral compaction technique. Once obturation was completed, the canal orifice was sealed with a temporary restoration (light cure; Temp. It, Korea), which prevented seepage of setting sealer. All samples were then stored at 37°C with 100% relative humidity for 2 weeks.

CAST POST CORE PREPARATION/CEMENTATION: The temporary restoration over the canal orifice was dismantled with a round (initial penetration) and straight fissure bur (bulk) in a high-speed air rotor handpiece under air/water coolant. The visible gutta percha was then pierced through with a Gates Glidden drill (Dentsply-Maillefer, Ballaigues, Vallorbe, Switzerland) used from size #1 to #5 to remove the gutta percha using a slow-speed contra-angled handpiece. The post space was then prepared using a Pesso reamer (size 1 to 5) (Dentsply-Maillefer, Ballaigues, Vallorbe, Switzerland) up to the root canal length of 10 mm. The diameter of the post space was standardized by preparing each post up to size 4 of the Pesso reamer, followed by final enlargement by size 5 up to 10 rotations at the minimum speed of the control box. The canals were then cleaned by rinsing with a syringe containing distilled water. A ferrule 2 mm high and 2 mm wide was kept at the coronal end by preparing the tooth using a high-speed round bur. Another standard feature incorporated was anti-rotational groove towards the cervical portion of the post space preparation using a tapered bur. For all specimens, the posts and cores were fabricated by direct technique using autopolymerizing resin (Duralay; Reliance, Dental Mfg. Co., USA). Layers of pattern resin were carried on the corrugated plastic pin provided by the manufacturer. The direct pattern within the canal was further extended by 1 mm from the height of the ferrule, after which it was further extended by 4 mm towards the coronal direction (Figure 1B, 1C). A loop was prepared extending 2 mm above the canal orifice to assist in engaging the key during testing. The entire pattern was provided with a sprue so that it could be cast directly without any addition of material (Figure 1C). After completing the pattern, the roots were filled with dry sterile cotton within the canal while the orifice was sealed temporarily until post fabrication and testing. All patterns were then invested using a phosphate-bonded investment (Bellasun, Bego, Bremer Goldschlägerei Wilh Herbst GmbH & Co) manipulated according to the manufacturer’s recommendations. All the specimens were cast using nickel-chromium alloy (Wiron 99, BEGO) in an induction casting machine. Once the investment material cooled, the CPCs were retrieved and the investment was cleaned from the casting using sandblasting (alumina 200 μm). The fit of the dowel in the respective root preparation was evaluated with light finger pressure. Verification of the fit was confirmed by evaluation of the margins at the cervical end. Castings that were not accurate in fit were rejected and the process repeated. The portion of the sprue inside the loop was cut off so that only the loop for testing would remain (Figure 1D). Each casting was then finished from the outside leaving the root canal portion untouched. Before cementation of CPC for each group, the root canals of each tooth were washed with triple-distilled water and dried with paper absorbent points. Cement manipulation was performed according to each individual cement manufacturer’s instruction using recommended instruments and parameters (mixing time, working time, excess removal) (Table 1). A load of 5 kg was applied during the setting of the cement until the dowel was stable. Wherever desired, the cement was carried into the canal using the clinically recommended Lentulo spiral (Mani Co., Tokyo, Japan). All cemented CPCs were thus divided into 5 groups: zinc phosphate (Gp ZP), glass ionomer (Gp GI), resin cement (Gp RC), polycarboxylate (Gp PC), and resin-bonded glass ionomer (Gp RGI). To simulate clinical conditions, the samples were then stored in an incubator (37°C, 100% humidity, 1 week duration).

MEASURES, DATA COLLECTION, EVALUATION, AND ANALYSIS: The specimens of each group were then placed in a hollow rectangular metal block (3/2/2 centimeter) (Figure 2A) as preparation for testing. A mixture of autopolymerizing acrylic resin was poured into these metal blocks leaving 5 mm of root exposed to the outside to hold the specimens during testing (Figure 2B). A cast surveyor was used to hold the exposed portion of the sprue and embed the specimens perpendicular to the base of the blocks to ensure that tensile loading occurred along the long axis of the root canal during testing (Figure 2C). After proper alignment of each CPC assembly, the clamp of the machine was placed into the loops of each sample. Positive contact of the loop with the machine was obtained with the help of locking pliers. The samples were finally tested (pilot test) by connecting the assembly to a calibrated universal testing machine (Instron, model 4511, Brucks, United Kingdom) using a crosshead speed of 1 mm/min until cement failure. The force required for cement failure or dislodgment of CPC was automatically recorded by the machine in kilograms.

STATISTICAL ANALYSIS:

Data obtained for individual specimens in each group were entered in Microsoft Excel, where it was refined, cleaned, and then coded to be analyzed using SPSS (Chicago, IL, USA, V25.0). For each specimen within each group, the mean values and standard deviations were calculated. Differences between the groups were determined using one-way analysis of variance (ANOVA) with the P value being considered significant at less than or equal to 0.05. The independent t test was performed to identify 2 independent experimental groups that were significantly different and served as a multipurpose comparison test. Significant differences were determined at P less than 0.05.

Results

RETENTIVE STRENGTH:

The mean force (tensile) that caused an adhesive failure between CPC restoration and the root canal surface within the natural tooth is presented in Table 2 along with the overall differences between various investigated groups. CPCs specimens in Gp RGI showed the highest forces (m=21.46) required to dislodge the CPC restoration, indicating that CPC cemented with resin cement is more retentive than other cements. The lowest forces to dislodge CPCs were observed in Gp PC (m=13.63), indicating that the lowest retention for CPC to tooth was with polycarboxylate cement.

GROUP DIFFERENCES:

A statistically significant difference in the mean adhesive failure values existed between different cement groups (P≤0.05) (Table 2). A multiple-group comparison of adhesive failure between CPC and natural root surface between different groups is presented in Table 3 and graphically depicted in Figure 3. The differences between the groups were statistically determined at 2 different levels – (significant (P≤0.05) and highly significant (P≤0.01) – to differentiate the levels of differences between 2 different cements. No difference in means was observed in groups cemented with zinc phosphate and glass ionomer, although both had higher retentive values than polycarboxylate cement. Significant differences were observed in 2 group comparisons (Gp ZP vs Gp PC and Gp PC vs Gp GI) (P≤0.05), while all other group differences were highly significant (Figure 3).

Discussion

STRENGTH AND LIMITATIONS OF THE STUDY:

The main strength of the study includes investigating a wide variety of chemically different luting agents that are commonly used for cementation of CPC. The limitation of the study is related to in vitro design which cannot be the exact replica of a clinical situation. The influence of periodontal ligament to bear some of the tensile force is also lacking in such studies. There are also many manufacturers of a particular cement which have not been included in this study. In vivo bonding failure occurs in different clinical conditions which are difficult to replicate in laboratory studies. A CPC may undergo bonding failure after serving for a long period of time during which the influence of other variables plays a role. Our study replicated that condition for only a week which may be considered less of a CPC that serves 4 years.

Conclusions

We found that the cements investigated had wide variations in mean retentive strengths (range 13.63 to 21.46 kg), with resin cement showing the highest and polycarboxylate cement showing the lowest strengths. Significant differences in retentive strength of cements existed among these 5 cements for CPC cementation. These values may be considered sufficient for a CPC, although the issue of having a cutoff standard needs attention. Clinically, our results suggest that if for some reason the clinician feels that the endodontic treatment may require intervention in the future, then polycarboxylate cement should be used for CPC cementation. For long-term, trouble-free cementation, resin cement shows higher retentive strengths, but other factors must be weighed to determine its feasibility.