Introduction

Oral rehabilitation plays an important role in masticatory function and facial esthetics. Reconstruction of impaired tooth structure as a part of oral rehabilitation is one of the basic treatment plans, especially after root canal therapy of severely damaged tooth, in modern dentistry. Post and core is a procedure to rebuild a tooth in order to provide proper support for a crown, regain retention and function. Misfit of posts and cores can affect thickness of cement layer and it may emerge problems such as vertical root fracture, marginal discrepancy, and consequently caries and periodontal diseases which result in treatment failure
(1-4).

Polymers have been used in dentistry for many purposes ranging from dentures to veneers (1). 
Auto-polymerizing acrylic resins are one of the polymers which can be used in fabricating post and core patterns ,inlay patterns,  bite registration and soldering (5). These materials offer strength, rigidity and provide easy manipulation with rotary instruments with no fear of distortion(6) but like other resins, they have shrinkage because of  the polymerization procedure and this  might have an adverse impact on retention and resistance of final posts and cores (7, 8).

Dental prosthesis are exposed to  oral microbial flora such as viruses, bacteria, and fungi which  can make cross infection and put the dental personnel at risk of acquiring infections while working with them
(9, 10). Consequently, acrylic post and core patterns should be disinfected like every other materials used in a dental office before sending to the laboratory. Several studies have evaluated  disinfection solutions and storage environments and  their effect on dental materials like changing the color, hardness, roughness, dimensional stability, and flexural properties which may have an adverse influence on the clinical outcome
(11-13).

Disinfection is an inevitable procedure so, determining the best disinfection for storing APARPs without any dimensional changes is very important. Hence, this study was aimed to compare the effect of three types of storage environments on the dimensional stability of post and core patterns.

Materials and Methods

This in-vitro experimental study was conducted on two type of Duralay auto-polymerizing acrylic resins (Aria dent, Iran and Reliance, Dental Mfg. Co, USA)) to make post and core patterns. Based on a result of a pilot study, 30 pieces were determined for each group (N=60).

To fabricate the acrylic specimens, a two-piece metallic cubic mold with internal pattern was constructed to simulate post and core pattern for a mandibular canine, the core was 8mm in length and 4.5 mm in width, the post was 12.5 mm in length, 1.5 mm in apical and 2.7mm in coronal dimension (Fig. 1). Next  the polymer  and monomer were manipulated in a ratio of 1:1 according to the manufacturer's instructions and poured into the lubricated mold  .Then metallic mold were  placed on a vibrator at room temperature to reduce the risk of bubble formation on the acrylic pattern. Acrylic resin setting was accomplished after 10 minutes which was recommended by the manufacturers. Afterwards, the patterns were gently removed from the mold. Corono-apical length (CAL), Coronal (CD) and apical diameters (AD) were measured immediately by a digital caliper with a precision about 1 mm (Mitutoyo, Japan: 0.01 mm). The distance from apical point to coronal margin was considered as CAL; AD was defined as the mesiodistal distance from 1.5 mm above the apical point and CD was defined as the mesiodistal distance from 1.5 mm beyond the coronal margin of APARPs.

The fabricated APARPs were stored into following storage environments: deconex®53plus (Borer ChemieAG,Switzerland), Unident ® Impre
(USF Healthcare S.A,Switzerland) and water in 25°. Based on a computer-generated random sequence using http://random.org, a site of creating random numbers, each group contained 10 pieces of acrylic post and core patterns which were tested in three mentioned storage environments. The CAL, AD and CD of the acrylic post and core patterns were measured after one hour.

After one hour, mentioned measurements were compared with the base measurements and analyzed by Paired T-test and ANOVA using SPSS software version 17 at significant level of 0.05.

Results

Table-1 represents the means and standard deviations of measured CAL, AD and CD in three different environments after one hour.  Datasets were parametric based on Kolmogrove –Smirnov test with p value= 0.083. To determine the difference in each sample before and after disinfection protocol, paired t -test was used which is shown in table 1. The data showed the post and core patterns by Aria Dent, Iran acrylic resin type, which were stored in Unident ® Impre, contracted 0.015, 0.07 and 0.013 mm in CD, AD and CAL, respectively while post and core patterns by Dental Mfg Co, USA in same environment, contracted 0.052, 0.08 mm in CD and CAL and dilated 0.048 mm in AD.

Also data showed that the post and core patterns by Aria Dent, Iran  , which were stored inDeconex®53 plus, contracted 0.049,0.028 mm  in CD and CAL and dilated 0.035mm in AD while post and core patterns by Dental Mfg Co, USA acrylic resin type in same environment, dilated 0.041,0.57 mm in CD and AD and contracted 0.178 mm in CAL.

 The post and core patterns by Aria Dent, Iran acrylic resin type , which were stored in water, dilated 0.04 mm in AD without changing in CD and CAL while post and core patterns by Dental Mfg Co, USA in water, contracted 0.037,0.036 mm in AD and CAL and dilated 0.024 mm in CD.

Results showed that coronoapical length, coronal and apical dimensions of post and cores had no significant difference (p value> 0.05) based on  resins and  disinfection environment  type  with the exception of following points: significant dimensional change was seen in apical diameter and coronoapical  length with Dental Mfg Co, USA acrylic resin which were  disinfected by Deconex®53 plus in comparison with pre disinfection scores(Paired T test :p =0.023). Differences between groups based on ANOVA test are presented in table 2 and 3.This test revealed that there is no statically differences between disinfection environments.

Table 1. Dimensional changes (mm) and P values of paired T-test of acrylic posts and cores patterns in different environments

CD: coronal diameter, AD=Apical diameter, CA L=Coronoapical length

*= paired t-test

Table2. ANOVA results for effect of environment on dimensional changes of Aria dent, Iran pattern resin

Table3. ANOVA results for effect of environment on dimensional changes of Dental Mfg Co, Inlay pattern resin, USA