Analysis of stress relaxation in temporization materials in dentistry

Abstract

Objective

Although temporization is intended as an interim step, complexity of individual treatment situations may demand medium to longer term use of temporary appliances in clinical practice. The durability and integrity of these restorations for continued use to meet the treatment demands is therefore an important clinical problem. The goal of this study was to evaluate the short to medium term stability of these materials under controlled loading to study their stress relaxation behavior.

Methods

Acrylic resins (poly(methyl) and poly(ethyl) methacrylate) and bis-acryl composite resins were tested in vitro in this study. The stress decay data with time (under an applied constant strain) due to internal strain caused by molecular relaxation were systematically analyzed using important parameters derived from stress changes with time.

Results and Significance

The results showed significant differences in the stress relaxation behavior between different materials which may have significant bearing on their durability in medium to longer term interim clinical applications. Poly(ethyl) methacrylate (PEMA) resins subjected to applied constant strain over a period of time showed large time dependent decay of applied stress, indicating very high internal molecular relaxation effects, relative to those of poly(methyl) methacrylate (PMMA) and bis-acryl composites. The results showed that PMMA and composite resins were superior in their ability to maintain constant strain without excessive dissipation of applied stress than PEMA resin. This suggests that internal strain caused by molecular relaxation events may lead to excessive dimensional instability in PEMA.

Introduction

Temporization is now a routine procedure in dentistry for treatment involving fixed prosthodontics, implant dentistry, cosmetic dentistry or other similar procedures. Several temporization materials are used to fabricate interim appliances such as single unit crowns and fixed partial dentures (FPDs). Such appliances are traditionally meant to be used in fixed prosthodontics for a relatively short period such as a few weeks to a few months until a permanent appliance is fabricated to replace the interim device. However, with recent popularity of implant supported prosthodontics or more complex procedures requiring longer term treatment planning, it is often necessary for interim appliances to remain in the mouth for several months because of a longer period needed for safe, effective and robust treatment. The medium to longer term integrity of the temporization material is then of critical importance both for the patient and the dental practitioner. The major reasons for mechanical or functional failure of a temporization material are potential fracture and excessive dimensional change. The ability to resist dimensional change is typically achieved by increasing the material stiffness, and this often increases the brittleness of a material leading to enhanced risk of fracture. It is therefore necessary to design materials for interim prosthesis using trade-offs between stiffness and dimensional stability within an optimum range. Creep and stress relaxation studies are important tools to optimize both stiffness and dimensional stability of polymeric materials which are used in temporization. Creep is studied by measuring deformation under constant stress, while stress relaxation is measured by monitoring stress under constant strain. The changes in stress (or stress decay) that occur under constant strain in a stress relaxation test result from in situ molecular relaxation events that may add an additional time and temperature dependent strain to the initial mechanical strain produced immediately on application of the initial stress, and this increases the total strain on the material as a function of time and temperature. If the total strain is held constant, there is a corresponding decrease in the applied stress as a function of time and temperature. This decrease in stress results in time/temperature dependent changes in its transient modulus. The transient modulus is given by the ratio of stress at any time t to the constant strain applied, i.e., { σ ( t )/ ɛ 0 } where σ ( t ) is the stress at any time ( t ) during stress measurement and ɛ 0 is the constant strain used in the experiment. The transient modulus as a function of time at a selected temperature is referred to as stress relaxation modulus (or simply as relaxation modulus) to signify the relationship of the modulus changes to the relaxation events under stress at the selected temperature. The relaxation modulus changes that occur with time can be analyzed for some valuable information in the stress relaxation tests. They are:

  • (a)

    The initial relaxation modulus (IRM) that occurs immediately on application of stress is the elastic modulus ( σ (0)/ ɛ 0 ). The value of IRM is important because it represents the resistance to elastic deformation or initial stiffness in the material.

  • (b)

    The difference between the IRM and the transient relaxation modulus at time t {RM( t )} is given by { σ (0) − σ ( t )}/ ɛ 0 or Δ σ ( t )/ ɛ 0 where Δ σ ( t ) = { σ (0) − σ ( t )}, and is designated in this study as ΔRM( t ). Physically this modulus change is determined by the dimensional change associated with time dependent deformation of the material due to in situ molecular relaxation events. The greater the value of the above time dependent deformation, the greater is the value of ΔRM( t ). The ratio of this modulus difference to the elastic modulus of the material {i.e., ΔRM( t )/IRM} thus varies with the ratio of the dimensional change due to relaxation events to that due to elastic deformation. This ratio has a value of 0 when a material is an ideal elastic material with no time dependent modulus changes during deformation, i.e.: σ ( t ) = σ (0) for any time t . In the above case, all deformation is elastic deformation generated by applied stress only with no relaxation effect with time. A mechanical analog is a spring. If, on the other hand, in a material with strong time dependent relaxation behavior, σ ( t ) may decrease with time to reach a value of 0 when a steady state is reached. In this case, the above ratio assumes a value of 1 indicating that strain due to relaxation events has replaced all initial mechanical strain. A dashpot with a Newtonian fluid is a mechanical analog for such a material. Typically, most biomedical polymeric materials exhibit varying levels of stress relaxation behavior between these two cases because they combine the instantaneous elastic response of a spring and time dependent relaxation of a dashpot containing a Newtonian liquid. The ratio ΔRM( t )/IRM can therefore be used as a numerical index that varies with time, and bears a functional relationship to the fraction of the initial dimensional change replaced by time dependent deformation due to molecular relaxation at time t . This ratio will be designated arbitrarily as Relaxation Index {RI( t )} in this study.

  • (c)

    Finally, the final relaxation modulus (FRM) at the end of the stress relaxation experiment also is an important parameter because it indicates how much stress the material continues to support in spite of relaxation effects.

Many authors have characterized various properties of materials used for temporization.

Several investigators have reported on mechanical properties such as compressive strength, flexural strength, fracture toughness, micro-hardness, etc. for selected materials . Other authors have focused on microstructures , marginal adaptation , color stability , surface roughness , monomer conversion , etc. There is very limited published work on the dimensional stability or time dependent modulus changes of these materials under stress. Pae et al. reported the overall dimensional changes of selected materials subjected to compressive stress of 4 MPa for 30 min, and showed significant differences in the dimensional stability between bis-acryl composite and mono-methacrylate polymer systems used to fabricate interim prosthetic appliances. Their analysis was limited to overall dimensional changes after compressive loading over 30 min, and did not monitor real time dimensional changes during loading. The stress relaxation behavior of selected interim restorative materials in oral surgery (such as zinc oxide eugenol and Cavit) was reported by Maerki et al. in 1979. To the best of our knowledge, no stress relaxation study on recent polymeric materials used for temporization has been published in the recent literature.

The objective of the current study was to determine and analyze the stress relaxation behavior of selected recent temporization materials used for fixed interim appliances. The null hypothesis is that there is no significant difference in the stress relaxation behavior of different types of polymeric materials used for temporization in prosthodontic clinical practice. The parameters used to analyze stress relaxation behavior included (a) IRM, (b) transient relaxation modulus as a function of time {RM( t )}, (c) transient relaxation modulus change from IRM with time {ΔRM( t )} and the corresponding Relaxation Index {RI( t )}, defined earlier and (d) FRM. The analysis was focused on selected materials currently used in temporization.

Materials and methods

A large number of temporization materials are commercially available, but they fall broadly under two types: powder-liquid acrylic resins and composites resins. The powder-liquid acrylic resin systems typically use either a methyl methacrylate or ethyl methacrylate monomer as liquid and its polymer as powder. The liquid and powder are mixed manually and self-cured. Most composite temporization materials in the market are based on bis-acrylate resins (BisGMA or urethane methacrylate) and they come in auto-mix cartridges which is more convenient for the clinician. This study was focused on selected acrylic resins and bis-acryl composite resins.

Table 1 lists the materials studied with information on short designation for brand, manufacturer name and location, type of resin system and curing method. Alike (ALK) and TRIM II (TRM) are mono-methacrylate resin systems, Luxatemp (LXT), TMP (Temphase) and VRS (Versa) are bis-acryl composite resin systems. The systems included manual mix (ALK and TRM) or auto-mix (LXT, TMP and VRS) chemical cure formulations. Manufacturer recommended curing conditions were used in all cases. The specimens were 45 mm × 20 mm × 10 mm rectangular bars prepared in a stainless steel mold. After curing for 30 min, excess flash was trimmed off and the specimens were stored in a humidity chamber at 37 °C before testing after 24 h. The stress relaxation tests were performed isothermally at 32, 37 and 42 °C by programmed scanning under constant strain over a pre-optimized time span in a Dynamic Mechanical Analyzer model 2980 (TA Instruments, New Castle, DE). The constant strain of 0.2% was used for all the tests in a dual cantilever clamp. As pre-optimized in preliminary studies using stress decay with time to a steady state level, the isothermal relaxation data for each temperature was collected for 600 s by which period the relaxation profiles had reached a quasi-steady state with very little further change with time. Tests were performed using TA Advantage Instrument Control software. The stress data was collected as a function of time by TA Advantage data acquisition program and processed by the TA Instruments Universal Analysis program to generate the transient relaxation modulus {RM( t )} profiles. Additional calculations were made to determine ΔRM( t ) and RI( t ) from the sampled data using Microsoft Excel. The sample size was determined by power analysis of pilot data generated prior to the commencement of the study. The largest difference between group means of initial relaxation modulus data in pilot study was approximately four times the between-groups pooled standard deviation (SD). Therefore we assumed a large effect size of 0.75 for the a priori sample size calculation using g-power version 3.1.7. The total sample size for five groups corresponding to this effect size at 90% power and significance level of α = 0.05 was found to be 25. We used a total sample size of 30 (6 per group), giving a 94% overall power.

Table 1
Temporization materials studied.
Brand Code Supplier Compositional type Dispensing and cure type Resin ingredients listed by product manufacturer
TRIM II TRM Bosworth, Skokie, IL Acrylic resin, (mono-methacrylate) Manual mix, self-cure Polyethyl methacrylate (powder) and isobutyl methacrylate monomer (liquid)
Alike ALK GC International, Alsip, IL Acrylic resin (mono-methacrylate) Manual mix, self-cure Polymethyl methacrylate (powder) and methyl methacrylate monomer (liquid)
Luxa-temp LXT DMG America, Englewood, NJ Bis-acryl composite, dimethacrylate system Automix, self-cure Base catalyst system, contains urethane dimethacrylate, aromatic dimethacrylate, glycol dimethacrylate, filler, curing agents
Versa-temp VRS Sultan Chemicals, Englewood, NJ Bis-acryl composite (dimethacrylate resins) Automix, self-cure Base-catalyst system, contains BisGMA, filler, curing agents
Tem-phase TMP Kerr Manufacturing, Orange, CA Bis-acryl composite (dimethacrylate resins) Auto mix, self-cure Base-catalyst system, contains methacrylate ester monomers, filler and curing ingredients

The glass transition temperature of each temporization material was also determined using a dynamic mechanical analysis scan of each material. The oscillatory frequency used was 1 Hz and oscillation amplitude 15 μm. The temperature scanning rate was 5 °C/min over the temperature range of 40–150 °C. The Tan(delta) values were plotted by TA Universal software and the transition peaks determined.

All statistical analysis of stress relaxation data was done using JMP version 10 (SAS Statistical Institute, Corey, NC). The analysis consisted of testing for normality of distribution and SD homogeneity for data of each test group, and two-way analysis of variance and pairwise Tukey contrast, both at a significance level of α = 0.05. The leverage effects of main variables {material type ( M ) and temperature ( T )} on modulus related data and the corresponding interactive effects ( M * T ) were analyzed using Tukey HSD multiple comparisons of least square modulus means as a function of respective variables.

Materials and methods

A large number of temporization materials are commercially available, but they fall broadly under two types: powder-liquid acrylic resins and composites resins. The powder-liquid acrylic resin systems typically use either a methyl methacrylate or ethyl methacrylate monomer as liquid and its polymer as powder. The liquid and powder are mixed manually and self-cured. Most composite temporization materials in the market are based on bis-acrylate resins (BisGMA or urethane methacrylate) and they come in auto-mix cartridges which is more convenient for the clinician. This study was focused on selected acrylic resins and bis-acryl composite resins.

Table 1 lists the materials studied with information on short designation for brand, manufacturer name and location, type of resin system and curing method. Alike (ALK) and TRIM II (TRM) are mono-methacrylate resin systems, Luxatemp (LXT), TMP (Temphase) and VRS (Versa) are bis-acryl composite resin systems. The systems included manual mix (ALK and TRM) or auto-mix (LXT, TMP and VRS) chemical cure formulations. Manufacturer recommended curing conditions were used in all cases. The specimens were 45 mm × 20 mm × 10 mm rectangular bars prepared in a stainless steel mold. After curing for 30 min, excess flash was trimmed off and the specimens were stored in a humidity chamber at 37 °C before testing after 24 h. The stress relaxation tests were performed isothermally at 32, 37 and 42 °C by programmed scanning under constant strain over a pre-optimized time span in a Dynamic Mechanical Analyzer model 2980 (TA Instruments, New Castle, DE). The constant strain of 0.2% was used for all the tests in a dual cantilever clamp. As pre-optimized in preliminary studies using stress decay with time to a steady state level, the isothermal relaxation data for each temperature was collected for 600 s by which period the relaxation profiles had reached a quasi-steady state with very little further change with time. Tests were performed using TA Advantage Instrument Control software. The stress data was collected as a function of time by TA Advantage data acquisition program and processed by the TA Instruments Universal Analysis program to generate the transient relaxation modulus {RM( t )} profiles. Additional calculations were made to determine ΔRM( t ) and RI( t ) from the sampled data using Microsoft Excel. The sample size was determined by power analysis of pilot data generated prior to the commencement of the study. The largest difference between group means of initial relaxation modulus data in pilot study was approximately four times the between-groups pooled standard deviation (SD). Therefore we assumed a large effect size of 0.75 for the a priori sample size calculation using g-power version 3.1.7. The total sample size for five groups corresponding to this effect size at 90% power and significance level of α = 0.05 was found to be 25. We used a total sample size of 30 (6 per group), giving a 94% overall power.

Table 1
Temporization materials studied.
Brand Code Supplier Compositional type Dispensing and cure type Resin ingredients listed by product manufacturer
TRIM II TRM Bosworth, Skokie, IL Acrylic resin, (mono-methacrylate) Manual mix, self-cure Polyethyl methacrylate (powder) and isobutyl methacrylate monomer (liquid)
Alike ALK GC International, Alsip, IL Acrylic resin (mono-methacrylate) Manual mix, self-cure Polymethyl methacrylate (powder) and methyl methacrylate monomer (liquid)
Luxa-temp LXT DMG America, Englewood, NJ Bis-acryl composite, dimethacrylate system Automix, self-cure Base catalyst system, contains urethane dimethacrylate, aromatic dimethacrylate, glycol dimethacrylate, filler, curing agents
Versa-temp VRS Sultan Chemicals, Englewood, NJ Bis-acryl composite (dimethacrylate resins) Automix, self-cure Base-catalyst system, contains BisGMA, filler, curing agents
Tem-phase TMP Kerr Manufacturing, Orange, CA Bis-acryl composite (dimethacrylate resins) Auto mix, self-cure Base-catalyst system, contains methacrylate ester monomers, filler and curing ingredients

The glass transition temperature of each temporization material was also determined using a dynamic mechanical analysis scan of each material. The oscillatory frequency used was 1 Hz and oscillation amplitude 15 μm. The temperature scanning rate was 5 °C/min over the temperature range of 40–150 °C. The Tan(delta) values were plotted by TA Universal software and the transition peaks determined.

All statistical analysis of stress relaxation data was done using JMP version 10 (SAS Statistical Institute, Corey, NC). The analysis consisted of testing for normality of distribution and SD homogeneity for data of each test group, and two-way analysis of variance and pairwise Tukey contrast, both at a significance level of α = 0.05. The leverage effects of main variables {material type ( M ) and temperature ( T )} on modulus related data and the corresponding interactive effects ( M * T ) were analyzed using Tukey HSD multiple comparisons of least square modulus means as a function of respective variables.

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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Analysis of stress relaxation in temporization materials in dentistry
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