In vitro comparative evaluation of mechanical properties of temporary restorative materials used in fixed partial denture.

INTRODUCTION: Materials used to fabricate provisional restorations can be classified as acrylics or resin composites. Provisional crows can be either prefabricated or custom made. ACRYLICS: These materials have been used to fabricate provisional restorations since the 1930s and usually available as powder and liquid. They are the most commonly used materials today for both single-unit and multiple-unit restorations. In general, their popularity is due to their low cost, acceptable esthetics, and versatility. COMPOSITES: Composite provisional materials use bis-acryl resin, a hydrophobic material that is similar to bis-GMA. Composites are available as auto-polymerized, dualpolymerized and visible light polymerized. PREFORMED CROWNS: Preformed provisional crowns or matrices usually consist of tooth-shaped shells of plastic, cellulose acetate or metal. They are commercially available in various tooth sizes and are usually selected for a particular tooth anatomy. They are commonly relined with acrylic resin to provide a more custom fit before cementation, but the plastic and metal crown shells can also be cemented directly onto prepared teeth. AIMS AND
OBJECTIVES: The aim of this study is to choose a material to serve as a better interim prosthesis and to compare three different properties - flexural strength, compressive strength, and color stability.
MATERIALS AND METHODS: The samples were made with three different provisional materials (Revotek LC, Protemp 4, TemSpan). RESULT: It was inferred from the study that no one material was superior in all three tested parameters.

Provisionals have to fulfill important functions within the timeframe between preparation of a tooth and until fitting the final fixed metal or ceramic restoration. A well-made provisional fixed partial denture should provide a preview of the future prosthesis and enhance the health of the abutments and periodontium.[1] To reduce tissue toxicity and thermal irritation of the conventional resin systems, new interim restorative materials that contain no methyl methacrylate have been introduced such as visible light cure resin, bis-acrylic composite resins, and visible light and chemical cure (dual cure) resins. Controlled prospective clinical trials on temporary crowns and fixed partial dentures do not exist in the dental literature.[2] These provisional-fixed prostheses must fulfill biologic, mechanical, and esthetic requirements to be considered successful. Resistance to functional loads and removal forces is “mechanical factors” that must be considered when choosing a provisional restorative material for clinical use.

Aims and objectives

The aim of this study is to choose a material to serve as a better interim prosthesis and to compare three different properties – flexural strength, compressive strength, and color stability.

The materials for the comparative study are as follow:

Revotek LC

Protemp 4

TemSpan.

Materials and Methods

The samples were made with three different provisional materials (Revotek LC, Protemp 4, TemSpan) as mentioned above to compare the mechanical properties such as flexural strength, compressive strength, and color stability.

Revotek LC (GC Corporation, Japan): Light-cured single component composite resin. Group 1 - Thirty samples made, ten samples each for three properties

Protemp 4 (3M ESPE, USA): Chemically cured two component systems. Group 2 - Thirty samples made, ten samples each for three properties

TemSpan (Pentron Clinical Technologies, LLC): Dual-cure resin system. Group 3 - Thirty samples made, ten samples each for three properties.

The specimens were fabricated for each material with use of stainless steel molds [Figures 1 and 2]. The materials were mixed according to the manufacturers recommendations and loaded into the mold. Another glass slab with a plastic matrix was later placed on top of the molds and wiched between two glass slabs.

Figure 1

Stainless steel mold for flexural strength and compressive strength specimens

Figure 2

Stainless steel mold for color stability specimens

Specimen A: According to the ADA specification number 27, size 25 mm × 2 mm × 2 mm samples for comparing flexural and compressive strength values using universal testing machine Instron [Figure 3].

Figure 3

Specimen A samples

Specimen B: Size 20 mm diameter circle, 2 mm thick samples for comparing color stability values using spectrophotometer [Figure 4].

Figure 4

Specimen B samples

Measuring flexural strength

After fabrication and finishing, the specimens were soaked in artificial saliva at 37° for 10 days. Later, all specimens were placed on top of the platform of the universal testing machine to undergo a three-point bend test. A load of 10 kN load cell at a crosshead speed of 0.75 mm/min was applied. The force at fracture was recorded in Newton and calculated in MPa with the use of testing machine software [Figure 5].[3]

Figure 5

Sample under flexural load

Measuring compressive strength

The specimens were fabricated as similar as mentioned above and then placed on top of the platform of the universal testing machine. A load of 10 kN load cell at a crosshead speed of 0.75 mm/min was applied. The force the sample could withstand till the start of deformation was recorded in Newton and calculated in MPa with the use of testing machine software [Figure 6].

Figure 6

Sample under compressive load

Evaluating color stability

To evaluate the color stability of coffee solution, ten specimens of each group were immersed in coffee solution for 2 days. Color measurements were made using spectrophotometer before immersion (i.e., the baseline measurements), 7 days (T7) and 10 days (T10) after immersion [Figure 7].[45]

Figure 7

Sample evaluation using spectrophotometer

The following equation was used to measure color stability:[67]

ΔE = (Δ L*2 + a*2 + b*2)½

where ΔL*, Δa*, Δb* are the differences in L*, a*, and b* values before (T0) and after immersion at each time interval (T7, T10). Student's paired t-test was used to calculate the P value.

Results

Ten samples each from Groups 1 and 2 of specimen A underwent a three-point bending test using a universal testing machine Instron for measurement of flexural and compressive strength. The results obtained were tabulated and graphs were made [Tables 1–3 and Graphs 1 and 2]. Ten samples each from Groups 1 and 2 of specimen B underwent color analysis testing using spectrophotometer at different immersion periods. The results obtained were tabulated and graphs were made [Tables 1–3 and Graph 3].

Table 1

Readings for flexural strength testing

Table 3

Readings for color stability testing

Graph 1

Color graphic representation of the mean of the flexural strength value changes of three groups at different time interval measurements

Graph 2

Color graphic representation of the mean of the compressive strength value changes of three groups at different time interval measurements

Graph 3

Color graphic representation of the mean of the color changes of three group different time interval measurements

After statistical analysis, the following inference was obtained:

Group 2 has more flexural strength and compressive strength values compared to Group 1

Group 1 showed least color changes, and hence, it is more color stable compared to Group 2.

Discussion

An ideal provisional must fulfill biological, mechanical, morphological, psychological, and esthetic requirements, with the introduction of composite-based materials, which may be chemical, light, or dual cured acrylic resins have lost their popularity.[89] Diaz-Arnold in their study demonstrated that bis-acrylic-type composite resin exhibited higher microhardness and greater surface integrity than the methacrylate resin materials.[10] Polymerization shrinkage plays a major role in the fit of provisional restoration. Volumetric shrinkage was 6% for polymethyl methacrylate and 1.0–1.7% for composites. Hence, composites allow better marginal fit than polymethyl methacrylate because of less contraction due to polymerization.[11]

Haselton et al. in their study on testing the flexural strength of five methyl methacrylate-based resins and eight bis-acrylic-based resins for provisional crowns and fixed partial dentures demonstrated that flexural strengths vary greatly among provisional materials.[5] 10 samples from each of the two groups were subjected to three different property testings. From the results obtained:

Group 2 - Self-cure material – Protemp 4 has more flexural strength and compressive strength

Group 1 - Light cure material – Revotek LC is more color stable.

When provisional restoration is to be given in the esthetic region, then urethane dimethacrylate-based material (Revotek LC) can be used

When the provisional restoration has to be placed for a longer span of time, then chemically cured bis-GMA-based material (Protemp 4) can be used

If a provisional long span bridge has to be placed, then chemically cured Bis-GMA-based material (Protemp 4) can be used.

Conclusion

It was inferred from the study that no one material was superior in all three tested parameters. Although these products are made from similar materials, variation in formulation, including the cross-linking agents, appears to have resulted in variations in the performance. Further investigation is required to elucidate the nature of product differences and the way in which these materials respond to the oral environment.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

Table 2

Readings for compressive strength testing