Journal of Interdisciplinary Dentistry

ORIGINAL ARTICLE
Year
: 2019  |  Volume : 9  |  Issue : 2  |  Page : 66--72

A comparative evaluation of flexural strengths of two different chairside repair materials with and without modification of implant attachment housings: An in vitro study


Abhinav Agarwal, Manesh Lahori, Kushal Singh 
 Department of Prosthodontics and Crown and Bridge, K. D. Dental College and Hospital, Mathura, Uttar Pradesh, India

Correspondence Address:
Dr. Abhinav Agarwal
K. D. Dental College and Hospital, Mathura, Uttar Pradesh
India

Abstract

Statement of Problem: Implant overdentures become thinner and weaker after direct transfer of implant attachment housings. The introduction of a metal housing changes the character of the repair as denture has to be relieved to provide space for the housings; therefore, a strong method of repair is desirable to avoid prosthesis fracture. Purpose: The purpose of this study was to compare flexural strength of two different chairside repair materials with and without modification of implant attachment housings. Materials and Methods: Eighty 13 mm × 10 mm × 41 mm heat-polymerized acrylic resin blocks were processed, assessed for porosities, and polished. An 8.5-mm diameter hole was created to a depth of 5 mm in the center of each block. Two different attachment housings sandblasted and nonsandblasted were placed into the blocks with two different repair materials: autopolymerized acrylic resin (APAR) and light-polymerized acrylic resin. Later blocks were immersed in water for 7 days in an incubator. A three-point bend test was done in a universal testing machine, and load to fracture was recorded (MPa). Results were compared with one-way analysis of variance (α = 0.05). Results: The mean maximum strength of APAR groups was significantly higher than light-polymerized acrylic resin groups. Groups with sandblasted attachment housings showed significantly higher strength compared to nonsandblasted groups. Conclusions: The flexural strength of self-cured acrylic resin with and without sandblasting of attachment was significantly higher than light-cured acrylic resin with and without sandblasting of attachment housings. Sandblasting produced higher flexural strength in denture blocks repaired with self-cured acrylic.



How to cite this article:
Agarwal A, Lahori M, Singh K. A comparative evaluation of flexural strengths of two different chairside repair materials with and without modification of implant attachment housings: An in vitro study.J Interdiscip Dentistry 2019;9:66-72


How to cite this URL:
Agarwal A, Lahori M, Singh K. A comparative evaluation of flexural strengths of two different chairside repair materials with and without modification of implant attachment housings: An in vitro study. J Interdiscip Dentistry [serial online] 2019 [cited 2019 Oct 15 ];9:66-72
Available from: http://www.jidonline.com/text.asp?2019/9/2/66/268374


Full Text



 Clinical Relevance to Interdisciplinary Dentistry



Sand blasting of implant overdenture attachment housing can enhance the flexural strength in dentures repaired with self cure acrylic leading to less chances of prosthesis fracture.

 Introduction



Implant overdentures have become the standard of care for edentulous patients. One of the favored treatments is to pick up implant attachment housings into the denture using chairside techniques with self-cured repair resin.[1] The attachment housings on the denture will then snap onto the implant abutments and help with the retention of the prosthesis. The denture has to be relieved to provide enough space to accommodate these housings, resulting in a thin denture base. A lot of research has been done regarding denture base repair methods.[2] However, these data do not directly apply to repairs involving attachment pickup. Repair of implant overdentures for attachment housing pickup does not only entail optimal bonding of the denture base resin and the repair material but also the adhesion between the repair material and the attachment housing.[3] The introduction of the metal housing greatly changes the dynamic of the repair; thus, it is desirable for patient care to use the strongest method available to pickup attachments. This could possibly be an easy and cost-effective way to improve the strength of the denture after attachment pickup.[4]

Patients who received implant-supported dentures reported improved function and satisfaction, showing a diet with more fiber.[5] However, this may also suggest an increase in forces generated with masticatory function. Patients with a mandibular implant-supported denture have a maximum bite force of 60%–200% greater than patients with conventional dentures. Studies done by Haraldson et al. (1988) and Fontijn-Tekamp et al. (1998, 2000) have shown that maximum bite force increases as a result of implants.[6] To be able to withstand functional and parafunctional masticatory forces, the denture base of the prosthesis must be strong.[7] This is especially important with implant-supported overdentures, wherein relief of acrylic is needed to provide space for the attachment housings, resulting in a thinner denture base. A number of overdenture fractures have been demonstrated in areas where copings are present, which is due to the increased forces and thinning of acrylic bases brought by accommodation of implant components and tissue bars.[8] Thus, this aspect of denture fracture and repair should be taken into consideration and should be given more focus. To endure repeated masticatory loads and resist plastic deformation, acrylic resin materials should exhibit fatigue resistance and a high proportional limit. The ability of a material to resist catastrophic failure under flexural load is called flexural strength. Denture success greatly depends on high flexural strength since alveolar resorption is a continuing, irregular process which leads to tissue-borne prosthesis being unevenly supported.[2] The ability of the repair material to sustain higher flexure in combination with high resistance to cyclic loading may render acrylic less prone to clinical failure.[2]

A variety of materials have been used in dental practice to repair fractured acrylic resin dentures. These include autopolymerized acrylic and visible light-polymerized acrylic resins.[4],[8] Due to simplicity and quick repair, the use of autopolymerizing acrylic resins is popular; however, it often re-fractures at the repair site, which usually occur at the interface junction of the original base and repair materials;[4] thus, bond strength produced between the base and repair material is important. To improve bond strength, mechanical modifications such as grinding with burs, airborne-particle abrasion, and laser treatment to increase surface area or chemical pretreatments of acrylic resins using methyl methacrylate or organic solvents such as acetone, chloroform, and methylene chloride may be used.[4]

In a study done by Stipho and Talic, triad visible light-polymerized (VLP) reline material produced the greatest bond with the triad VLP denture base resin; however, when used with polymethyl methacrylate (PMMA) acrylic resin groups, it produced a very low after-repair tensile and shear bond strengths.[9] In 1995, Lewinstein et al. reported that there was no difference in bond strength when heat-cured resin specimens were repaired with self-cured resin or triad visible light-cured resin if the specimens were pretreated with monomer.[10]

Successful denture repair of implant overdentures depends on the phenomenon of adhesion[9] not only between the denture base resin and the repair material but also adhesion between the attachment housing and the repair material as well.

The purpose of this study is to compare flexural strength of two different chairside repair materials with and without modification of implant attachment housings.

The null hypothesis in this study that there is no difference in the flexural strength of acrylic blocks after direct pickup of attachment housings using these four different methods was rejected.

 Materials and Methods



A steel die was made for investing silicone putty patterns [Figure 1] and [Figure 2]. Eighty heat-cured PMMA denture blocks were prepared by investing putty patterns in conventional denture investment flasks [Figure 3]. The lower half of the flask was filled with high-viscosity silicone material, and after it had set, another layer of silicone impression material was added to cover the patterns and the mold. Subsequently, after setting, the upper half of the flask was further supported entirely by gypsum material.{Figure 1}{Figure 2}{Figure 3}

After the invested material had set, the flasks were separated, and the putty patterns were removed from the silicone mold. DPI (The Bombay Burmah Trading Corporation, Ltd.) heat-cured denture base resin was proportioned and mixed following manufacturer's instructions. The resin was packed and polymerized in a water tank at 170°F for 9 h. Long-curing cycle was followed to completely cure the blocks and avoid porosities. After processing, all specimens were bench-cooled for 30 min.[8],[11]

All specimens were trimmed to 14 mm × 11 mm × 42 mm and were assessed for porosities. They were finished in a polishing machine using 80, 100, and 240 grit sandpaper for 20 s on each of the four sides. Finish polishing was done using 1200 grit sandpaper for 10 s per side followed by a polishing rag and an alumina-oxide slurry solution for 10 s per surface. All specimens were having final dimension of 13 mm × 10 mm × 41 mm measured with digital Vernier caliper.[8]

The titanium housings (Alpha Dent, Alpha Dent Implants Ltd., Germany) that were used had a dimension of 5 mm diameter × 3.2 mm height. These are composed of Ti-6Al-4V, which meets the specification of the American Society for Testing and Materials (ASTM) F136. Each housing was composed entirely of the titanium alloy that was subjected to an electrolytic passivation process called anodization, to control the oxidized layer formation on the Ti-alloy surface. This, in turn, gives the housing its gold color. The thickness of an anodized oxide layer ranges from 500 to 1000 angstroms.[12]

All specimens were having a depth hole of 8.5 mm × 5 mm which was created itself in the putty patterns by the extension provided in the upper part of the split die. The manufacturer suggested that at least 1.5 mm of clearance is needed around and above each housing for maximum retention in the denture base.[8] Attachment housings were placed in the center of each hole in the blocks by inverting blocks over the housings using two different repair materials: self-cured acrylic (DPI; The Bombay Burmah Trading Corporation, Ltd., Mumbai, India) for Group 1, light-cured acrylic (Triad Gel; Dentsply Trubyte, Pennsylvania, USA) for Group 2, self-cured acrylic (DPI; The Bombay Burmah Trading Corporation, Ltd., Mumbai, India) with sandblasted attachment housing for Group 3, and light-cured acrylic (Triad Gel; Dentsply Trubyte, Pennsylvania, USA) with sandblasted attachment housing for Group 4. Specimens were randomized to the four groups using a randomly permuted block assignment scheme. Titanium housings for Groups 3 and 4 were sandblasted using the Begokorostar (Bego, BEGO GmbH and Co. KG, Bremen, Germany). All materials were used following manufacturer's instructions.

Group 1: After cleaning and drying of the drilled PMMA surface, liquid methyl methacrylate monomer was brushed on the exposed surface for 180 s to enhance adhesion of the repair material [Figure 4] and the denture base resin. Self-cured acrylic was applied using the “salt-and-pepper” technique. Once the resin filled three-fourth of the hole, block was inverted and placed over the titanium housing secured in the center of glass slab-simulating clinical attachment pick-up [Figure 5]. The block was pressed against a glass slab for 10 min. Using “salt-and-pepper” technique, additional self-cured resin was placed on the repaired side to fill any voids. Once set, the repaired surface with the attachment housing was repolished.{Figure 4}{Figure 5}

Group 2: Triad-bonding agent was applied after cleaning and drying of the drilled area. It was left to settle for 2 min and then cured for another 2 min in the triad-curing unit (Triad 2000; Dentsply Trubyte, Pennsylvania, USA). Flowable resin was subsequently used to fill three-fourth of the hole, and the denture block was inverted to pick up the titanium housing, simulating clinical attachment pickup [Figure 6] and [Figure 7]. The block was pressed against a glass slab and the resin was cured for 4 min while inverted, using a portable light-curing unit (Cotlux; Coltène/Whaledent Private Ltd., Mumbai, India) [Figure 8]. Additional flowable resin was added around the housing to fill any voids and was initially cured for 4 min using a portable light-curing unit. Air barrier coating was applied on the light-cured resin to prevent inhibition of polymerization by oxygen, and each block underwent final curing in the Triad curing unit for another 8 min. The repaired surface was then repolished.{Figure 6}{Figure 7}{Figure 8}

Group 3: Using Begokorostar, an abrasive blasting system, the titanium housing was sandblasted with silica-modified 30 μm aluminum oxide (Korox 250). A blast pressure of 2.8 bar was used for sandblasting [Figure 9]. This was done to assure an adequate high level of energy to create the triboplasma. The surface was sandblasted at right angles from a distance of 1 cm for 15 s all around the titanium housing. Pickup was accomplished using self-cured acrylic and with the same protocol as Group A. Once set, the repaired surface was repolished.{Figure 9}

Group 4: Using Begokorostar, an abrasive blasting system, the titanium housing was sandblasted with silica-modified 30 μm aluminum oxide (Korox 250) under a blast pressure of 2.8 bar. This was done to assure an adequate high level of energy to create the triboplasma. The surface was sandblasted at right angles from a distance of 1 cm for 15 s all around the titanium housing. Attachment pickup was accomplished using light-cured acrylic and with the same protocol as Group B. Once set, the repaired surface was repolished.

All specimens were immersed in 37.6°C distilled water for a minimum of 7 days for saturation in a 37°C incubation chamber. Final dimensions were recorded using a digital Vernier caliper (Mukesh Trading Co., India). The flexural strength of the repaired denture blocks with the attachment housings was measured during a three-point bending test using a digital testing machine (Narmada Engineering and Calibration Services, Delhi, India). The three-point bending jig was set to have a span of 30 mm, and a 5000 N Instron load cell was used. The testing jig that was used was made of two parallel rods, with a third rod centered above, parallel and between the first two, giving a three-point load to the denture block.[8] Force was applied at the center of each block above the repaired area [Figure 10] and [Figure 11]. The test was conducted at a crosshead speed of 5 mm/min until failure occurred [Figure 12]. The formula used for determining the flexural strength is σ = 6M/bh2.{Figure 10}{Figure 11}{Figure 12}

Where σ is the bending stress, M is the moment about the neutral axis, b is the width of the specimen, and h is the depth of the specimen. Calculations of the mean and standard deviations were done. The study utilized a 2 × 2 factorial design, with factors defined by type of repair material (light-cured or self-cured) and sandblasted (sandblasted or not). Results were analyzed using one-way analysis of variance.

 Results



The mean flexural strength values at failure per group were as follows: autopolymerized acrylic resin (APAR) was 2.82 MPa, light-polymerized acrylic resin was 2.43 MPa, APAR with sandblasted attachment housing was 3.32 MPa, and light-polymerized acrylic resin with sandblasted attachment housing was 2.53 MPa [Table 1] and [Graph 1].{Table 1}[INLINE:1]

The results of this study showed that the flexural strength of denture blocks repaired with self-cured acrylic with sandblasted attachment housing had the highest value compared with using self-cured acrylic alone, light-cured acrylic alone, and light-cured acrylic with sandblasted attachment housing. Furthermore, when the mean flexural strengths of both self-cured acrylic groups were compared with the two light-cured acrylic groups, self-cured acrylic groups rendered higher flexural strength values [Table 2] and [Graph 2].{Table 2}[INLINE:2]

 Discussion



In this study, blocks made of heat-processed denture base material were repaired using four different techniques: self-cured acrylic, light-cured acrylic, self-cured acrylic with sandblasted attachment housing, and light-cured acrylic with sandblasted attachment housing. Begokorostar sandblasting unit was used because the manufacturer recommended using 30 μm grain size of the carrier aluminum oxide for surfaces that are highly susceptible to abrasion. According to Pfeiffer in 1993, the use of 30 μm grain size produces the same adhesive strength as the 110 μm grain size but is less abrasive. Since the titanium housings that were used in this study had an anodized layer, which ranges from 500 to 1000 angstroms, the 30 μm grain size of silica-modified aluminum oxide was used.[8],[12]

All specimen denture blocks were immersed in water for at least 7 days in a 37°C incubation chamber to allow water saturation as denture base material undergoes changes due to continued water uptake. Later, a three-point bending test was used to measure flexural strength of each repair method. The test was conducted at a crosshead speed of 5 mm/min.[1],[2],[11],[13]

The null hypothesis in this study that there is no difference in the flexural strength of acrylic blocks after direct pickup of attachment housings using these four different methods was rejected.

According to studies done by Dar-Odeh et al.[14] and Vojdani et al.[11] which evaluated self-cured and visible light-cured acrylic as denture repair materials. Specimens repaired with self-cured acrylic showed greater values of modulus of rupture and higher transverse strength values than those repaired with light-cured acrylic. As denture repair materials, self-cured acrylic produces higher transverse strength values and modulus of rupture probably because it produces a greater bond with the denture base material since self-cured acrylic and heat-cured acrylic are both PMMA acrylic resins. Materials with the same composition have better bonding.

The data obtained from this study showed that sandblasting of the attachment housing with self-cured repaired blocks increased the bonding of the attachment housing with the self-cured acrylic, rendering a stronger construct. When 30-μm silica-modified aluminum oxide was sandblasted on the attachment housing, a roughened surface was produced which helped in better bonding between titanium and acrylic resin.

In contrast to this, sandblasting did not produce an increase in flexural strength of the denture block repaired with light-cured acrylic. A very weak bond formed between the two materials could have caused this failure, or it is also possible that there was an incomplete polymerization of the light-cured acrylic since there is only a certain depth that the curing light can reach.[8]

The strength of all denture base materials should be sufficient enough to withstand masticatory forces after direct attachment pickup to prevent fracture. Thus, with the results of this study, it is recommended to pretreat the attachment housing with sandblasting before pickup with self-cured acrylic for better bonding between resin and implant attachment housing.

 Conclusions



Within the limitations of this study, the following conclusions were drawn:

Flexural strength of self-cured acrylic resin with a sandblasted attachment housing was significantly higher than other groupsFlexural strength of self-cured acrylic resin with and without sandblasting of attachment housing was significantly higher than the light-cured acrylic groupsSandblasting produced higher flexural strength in denture blocks repaired with self-cured acrylic.

Further clinical investigation will be necessary to confirm the results of the presentin vitro study.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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