Bond Strengths of Custom Cast and Prefabricated Posts Luted with Two Cements

Quintessence International

Bond strengths of custom cast and prefabricated posts luted with two cements

Khalil Ibrahim Aleisa, BDS, MSc1

Objective: This in vitro study evaluated the bond strength of custom cast and prefabricated posts luted with resin or zinc phosphate cements into unobturated canals of extracted teeth. Method and Materials: Forty-eight custom cast and prefabricated posts were placed into extracted single-rooted human teeth. Post-cavity preparation was 1.5 mm in diameter and 10 mm in depth. Specimens were randomly divided into 4 groups of 12 each. Two of the groups were then luted with resin cement, while the other two groups were luted with zinc phosphate cement. A pull-out bond strength evaluation was performed using a universal testing machine. The Kolmogorov-Smirnov test was used to prove normal distribution. Data were statistically analyzed using two-way ANOVA and the Student t test (α = .05). Results: For both luting agents, the prefabricated posts group exhibited significantly less bond strength than the custom cast posts group (P = .0001). There were statistically significant differences in mean bond strength for the prefabricated posts group luted with resin cement vs the group cemented with zinc phosphate cement (P = .002). There was no significant difference between the mean bond strength values of custom cast posts luted with resin cement or zinc phosphate cement. Conclusion: Custom cast posts showed significantly greater bond strength than prefabricated posts when luted with either resin or zinc phosphate cements. The type of cement had less significance on the retention of custom cast posts. (Quintessence Int 2011;42:e31–e38)

Key words: adhesive resin cement, bond strength, cast post, prefabricated Parapost, zinc phosphate cement

Posts are often used to retain a core in end- merizing resin that in turn is cast in either odontically treated teeth that have minimal precious or nonprecious metal alloys.14 remaining tooth structure.1 Retention of the Prefabricated posts are either tapered or post in the root is one important consid- cylindrical in shape and made of metallic or eration.1 Length,2–4 diameter,3,5,6 geometric nonmetallic materials.1,14 design, and surface configuration of posts2–4,6; Laboratory studies showed that pas- coronal preparation after cementation7–9; type sive smooth tapered posts are the least of root canal sealers used for obturation10–12; retentive.2–5 However, on the other hand, and type of luting agents used3–13 are some several clinical studies have shown an factors that affect the retention of posts. excellent survival rate of custom cast Generally, posts can be classified as posts and cores.15–17 Gluskin et al16 found custom cast or prefabricated. Custom cast that the traditional custom cast post was posts reproduce the morphology of the less likely to result in a catastrophic root prepared canal by using wax or autopoly- fracture. In addition, cast posts had better retention than prefabricated posts.16 Moreover, Balkenhol et al17 analyzed 802 1 Assistant Professor, Department of Prosthetic Dental Sciences, custom fabricated cast posts and cores College of Dentistry, King Saud University, Riyadh, Saudi Arabia. using a standardized technique and Correspondence: Dr Khalil Aleisa, PO Box 60169, Riyadh 11545, reported a cumulative failure of 11.2% Saudi Arabia. Email: [email protected], drimplant_ [email protected] over 10 years.

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Even though custom-made metallic Method and Materials posts have several advantages, there are now many concerns in regard to the asso- Tooth preparation ciated inhomogenous stress distribution, Forty-eight single-rooted mandibular sec- biologic adverse effects due to microleak- ond premolars extracted for orthodontic age and corrosion, and the influence of their reasons were selected for this study. Teeth dark color under all-ceramic restorations.4,18 were subjected to radiographic and micro- The introduction of nonmetallic fiber- scopic examination and were stored in a reinforced composite (FRC) posts helped dark container in 0.5% Chloramines T at to improve the stress distribution issue 4°C and used within 6 weeks of extraction. as their elastic modulus is similar to that Teeth were decoronated 2 mm coronal to of dentin.19–22 Although fiber posts proved the most incisal point of the cementoenamel effective in withstanding compressive loads junction (CEJ) using a low-speed diamond in posterior teeth,23,24 they behave differently saw (Isomet 2000, Buehler) under copious in anterior teeth where nonaxial forces pre- water cooling. vail and flexural behavior plays an important Endodontic treatment of the root canals role.25 was performed following the conventional In fixed prosthodontics, the cement procedure of using hand instrumentation functions as a luting and intermediate with K-files (LD Caulk Division, Dentsply) agent, where the stresses are distributed sequentially from size 15 through 50. Each evenly throughout the root surface.26 A file was used five times and then discarded. number of studies have investigated the The root canals were left unobturated. The effects of cement on post retention.3,5,27–34 preparation for post spaces was performed Early studies reported no significant dif- with Peeso reamers (Pulpdent) sequentially ferences in post retention with zinc phos- from no. 1 through 5 to a depth of 10 mm. phate, polycarboxylate, or glass-ionomer A new Peeso reamer was used for every 10 cements.3,5,13 teeth. A standard dimension of post spaces Recently, the use of resin-based lut- was prepared using a no. 6 parallel-sided ing agents has become widely accept- Parapost twist drill (Parapost Black P-42, ed.27–34 Several studies have reported a Whaledent) at low speed. The resultant significant increase in bond strength with post space was 1.5 mm in diameter and resin cements compared to conventional 10 mm in depth. Water irrigation was used luting cements.27–30 Sen et al27 showed that during instrumentation to clean debris from Paraposts (Whaledent) cemented with zinc the canal. Radiographs of the specimens phosphate cement had significantly less were taken mesiodistally and buccolingual- bond strength than those luted with adhe- ly to ensure a minimum of 1 mm of tooth sive resin cements. However, other reports structure thickness around the post space. have not confirmed these findings.6,12,13,31–34 Throughout all root canal therapy and post Turner13 compared the retention of cast space preparations, teeth were held in a posts cemented with different luting agents gauze sponge soaked in saline to maintain and found that posts luted with zinc phos- moistness. phate cement had the highest retention. Leary et al32 found that there was no significant difference in bond strength between Post preparation zinc phosphate cement and resin cement. After preparation, the teeth were randomly The purposes of this study were (1) to divided into 2 equal groups of 24 each investigate the effect of two types of den- (which were both later split into two groups tal cement materials on the bond strength of 12). In group 1, a no. 6 parallel-sided, of two metal post systems (cast post and vented prefabricated post (Parapost EP one prefabricated Paraposts) and (2) to 44-6-12, Whaledent) was used. All posts compare the bond strength of custom cast were passively fit when completely seated posts with prefabricated Paraposts. in their respective canals before luting. In group 2, Parapost serrated, cylindrical, vented, plastic burnout post patterns (Black

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Table 1 Study materials

Material Description Dual-cure adhesive resin Bisphenol-A-diglycidylether dimethacrylate (BisGMA) and triethylene glycol cement (Rely X ARC, 3M ESPE) dimethacrylate (TEGDMA) polymer Zinc phosphate cement Zinc oxide (powder) and buffered solution of phosphoric acid (liquid) (Kleio, Lascod) Prefabricated stainless steel No. 6 parallel-sided, vented stainless steel post; not sandblasted post (Parapost 44-6-12, Whaledent) Plastic post (Parapost Black Serrated, cylindrical, vented, plastic burnout post patterns P-50-6, Whaledent) Base-metal alloy Nickel 63.4%, chromium 21.6%, molybdenum 8.4%, niobium 4.2%, iron (Bellabond, Bego) 2.2%, and aluminum 0.2%

P-50-6, Whaledent) were positioned in their Post application respective prepared canals. Posts were Materials tested in this study are summa- sprued, invested using phosphate-bonded rized in Table 1. The specimens in each investment material (Bellavest T, Bego), group were further randomly divided equal- and then cast in fresh base-metal alloy ly into two subgroups of 12 each. The posts (Bellabond, Bego) using an induction cast- in the first group were cemented with zinc ing machine (Fornax 35 EUM, Bego). The phosphate cement (Kleio, Lascod) and in castings were examined under 10× mag- the second group with a dual-cured adhe- nifications to detect casting defects. Each sive resin cement (Rely X ARC, 3M ESPE). casting was placed on the respective tooth The canal surface preparation and mixing to verify its fit and adjusted if needed using and handling of the cements were both a no. ½ round carbide bur with a high-speed accurately performed according to the man- handpiece. Finally, only cast posts were ufacturers’ instructions. Light curing was sandblasted with 50 μm aluminum oxide. carried out using a halogen light-polymerizing unit (Astralis 10, Ivoclar Vivadent) at Sample preparation 750 mW/cm2. The cement was then placed To ensure the specimens stayed in the in the canals with a lentulo spiral, and the acrylic blocks during testing, the roots were posts were also coated with cement. The notched and a 0.7-mm-diameter hard steel posts were seated to place in the canals wire was looped through a transverse hole with finger pressure and held in position for drilled near the apex of each root. The root 10 minutes. The specimens were left undis- was embedded into a matrix (25 mm in turbed on the bench for an additional 15 diameter and 40 mm in length) filled with minutes prior to storage in distilled water at self-curing resin (Ortho Resin, Dentsply) up 37°C for 24 hours in an incubator (Imperial to 2 mm below the CEJ. A dental surveyor IV, Lab Line Instruments). (JM Ney) was used when mounting the specimens to ensure that posts would be Bond strength testing procedure subjected to an axially directed withdrawal To test the bond strength of posts, a uni- force during testing. versal testing machine (Model 8500 Plus

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Results

The mean forces required to remove the posts and standard deviations (SD) obtained with the two luting cements are shown in Table 2. The highest mean bond strength was recorded for the custom cast posts group luted with adhesive resin cement (465.3 N), while the lowest bond strength was recorded for prefabricated posts luted with zinc phosphate cement (259.7 N). The two-way ANOVA showed that the type of cement material significantly influenced the bond strength (P = .026) (Table 3). In addition, the type of posts, whether custom cast or prefabricated, demonstrated a statistically significant difference in post retention (P = .000) (Table 3). The Student t test showed no significant differences in bond strength of custom cast posts between zinc phosphate and adhesive Fig 1 Specimen mounted in the universal testing resin cements (P = .450) (Table 2). Within the machine using a customized self-aligning testing prefabricated post groups, the adhesive resin assembly. cement group was significantly different from the zinc phosphate cement group (P = .002) (Table 2). Moreover, the Student t test for both Dynamic Testing System, Instron) was luting agents confirmed that custom cast used. A customized, self-aligning testing posts exhibited significantly higher bond assembly was used (Fig 1). It includes a strength compared to the prefabricated U-shaped stainless steel rod with an open- posts (P = .0001) (Table 2). ing at each end; a horizontal rod passed through a channel prepared through the lower part of the acrylic block along with the openings of the U-shaped rod. The hook of Discussion the mandibular jaw of the testing machine held the curve part of the U-shaped rod, In this study, the choice of parallel-sided post while the maxillary jaw clamped the post. design was made to achieve better standard- A pull-out was applied at a rate of 0.5 mm/ ization of specimens through the use of pre- minute until the post separated from the fabricated post patterns and Paraposts rather root. A sudden decrease in the load curve than tapered or individually-shaped posts. on the chart was recorded as the bond The influence of post design and surface strength to dislodge posts. structure on retention has been demonstrated in several studies.3,33,34 These studies report Statistical analysis superior retention of parallel-sided posts com- The Kolmogorov-Smirnov test was used to pared to tapered posts.3,33,34 Additionally, sur- prove the normal distribution. It showed that face roughness of the post may influence P value was .825, which confirmed the nor- the bonding strength.4,35 Studies showed that mality distribution. The two-way analysis of rough surfaces have a better retention than variance (ANOVA) and Student t tests were polished ones.3,35,36 Accordingly, in this study, used to determine the statistical significance cast posts were air-particle abraded, which of the mean differences among groups. All increased both the roughness and surface statistical analyses were performed at .05 area and consequently increased the bond level of significance using SPSS 10 (SPSS). strength values of the post. Disadvantages

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Table 2 Means and standard deviations (SD) of dislodging force in Newtons (n = 12)

Zinc phosphate Composite resin Cast Prefabricated Cast Prefabricated Mean 435.4a 259.7b 465.3a 331.3c SD 90.1 50.8 100.2 51.54

Different letters indicate significant difference (P < .05); a, b, and c are significantly different from each other.

Table 3 Two-way ANOVA

Sum of squares df Mean F P Cement material 30,878.337 1 30,878.337 5.279 .026 Post 287,779.435 1 287,779.435 49.199 .000 Cement × post 5,178.377 1 5,178.377 0.885 .352 Error 257,369.357 44 5,849.304 Total 7,256,891.181 48

Statistical significance P < .05.

of the cast post and core are that it requires cement; in this way, stresses are evenly dis- two visits, provisionalization, and a laboratory tributed throughout the remaining root struc- fee.37 In addition, the development of hydro- ture, which increases post retention. Coating static pressure during cementation of properly only the post gives an incomplete cement film fitting posts may lead to tooth fracture.38 and disrupts the dissipation of functional and/ Root canal filling materials used for canal or parafunctional stresses.40 For this reason, in obturation may affect the retention of cement- this study, the cement was placed into canals ed posts.1,10–12 Therefore, in this study, to with a lentulo spiral and the post was also eliminate a variable that may have effected coated with the cement. This procedure was on the results, the root canals were not obtu- also used in previous studies.6,7,43 rated.10–12 In addition, most root canal sealers Due to the difficulties of transmitting light may require more time to completely set after to the apical region of the post space, a dual- obturation, which may also influence the bond polymerizing dentin bonding agent with a strengths of the posts.39 dual-polymerized resin luting agent was used. The method of placing the cement into In the present study, although the pull- the canal may also affect the retention of the out bond strength evaluation test was used post.40–42 Cement can be placed into the canal to determine the retentive forces required to with a lentulo spiral, paper point, periodontal dislodge the posts from their canals, this type probe, or endodontic explorer. According to of experimental test may not directly reflect the several studies,13,40–42 the use of lentulo spirals clinical situation. However, this test was cho- to introduce the cement into the root canal sen because it is widely accepted and used during cementation procedures is essential by several investigators.7–9,27–30 to achieve a uniform, bubble-free layer of

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Discussion of study results Although custom cast posts luted with adhesive resin cement had higher bond The results of this study showed that the bond strength than the ones cemented with zinc strength of the prefabricated Parapost was phosphate cement, the bond strength was not significantly influenced by the type of cement significantly different. These results were in used. A similar conclusion was reported by agreement with previous studies.9,32 Hill et al48 Rosin et al29 using a prefabricated titanium found no significant difference between etched post. Forces required to dislodge prefabricat- base-metal posts luted with resin cement and ed Paraposts cemented with adhesive resin gold posts luted with zinc phosphate cement. cement were significantly greater than those However, other studies were in contrast with of zinc phosphate cement. The explanation of the results of the present study.13,31,33,47 this finding may be that bond strength is large- In the present investigation, zinc phos- ly determined by the adherence of the cement phate cement compared well with the adhe- to post or dentin, depending on which is sive resin cement, especially with custom cast weaker.44 The mechanical interlocking of resin posts. This is consistent with earlier studies cement in the post irregularities and its better that recommended zinc phosphate cement bonding to root dentin may also explain the for luting custom cast post.1,12,30,47 In addition, highest bond strength values obtained when adhesive resin cements are technique-sensi- compared to zinc phosphate cement. This is tive and more difficult to manipulate than zinc in agreement with previous studies that dem- phosphate cement.1,12,29,30,47 onstrate that resin cement has a higher bond For custom cast posts, the mean bond strength than zinc phosphate cement.10,27–30 strength for zinc phosphate cement found Utter et al30 found that Paraposts luted with in this study was within the range reported Panavia (Kuraray America) resin cement had in the literature,7,9,29 but is greater than the significantly higher bond strength than the one values reported by other studies.28,30,49 On the luted with zinc phosphate cement. However, other hand, the mean bond strength values of other studies reported results that were in con- adhesive resin cement (Rely X ARC) is very trast with the results of the present study.6,31 similar to that reported with a different brand Gallo et al6 found that the stainless steel posts of resin cement.8,9,46 The explanation of these luted with zinc phosphate cement were signifi- variations in reported findings is due to the cantly more retentive than the ones luted with lack of standardization in testing methods and resin cements. The differences in the results materials (eg, differences in storage duration, obtained from the previous studies could be methods of bonding, technique of testing, explained by the use of a variety of post sys- types of luting agents used, types of posts tems and post lengths with different brands of used, and types of root canal sealers). resin and zinc phosphate cements. The results of the present study showed Adhesive resin cements used with the that custom cast posts had significantly high- total-etch technique showed good results in er bond strength values than prefabricated bonding to dentin due to resin tag and hybrid Paraposts luted with either adhesive resin or layer formation.45 Additionally, they have a zinc phosphate cements. These differences better bonding ability to stainless steel6,46 than may be explained by differences in surface zinc phosphate cement, which could account energy characteristics between the posts and for the recorded high bond strengths of the the resin cements and by speculation that the Paraposts. surface energy characteristics of certain posts The mean forces required to dislodge match better with one of the resin cements prefabricated Paraposts luted with adhesive than with others.50 Another explanation could resin cement (Rely X ARC) were in the range be due to the relatively superior fitting of cast of the findings of Al-Ali8 (342 N) and higher posts compared to prefabricated Paraposts, than the ones reported by Utter et al30 (179 N) which might increase the friction level between and Mendoza and Eakle47 (240 N). The bond the post and tooth structure.17 Furthermore, strength values obtained for Paraposts luted sandblasting might have increased sur- with zinc phosphate cement in this study sup- face roughness of cast posts and therefore ports the finding of Ruemping et al,4 but are increased the bond strength values. Several higher than the findings of Utter et al.30 studies demonstrated that cast posts have a higher bond strength and should be recom-

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mended.13,15–17 Weine et al15 showed a 94% References success rate with no retention problems of

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