The Effect of Joint Surface Preparations and Chemical Surface Treatment on the Transverse Strength of Repaired Denture Base Resin.

AIM: The aim of the study is to investigate the effect of joint surface preparations and chemical surface treatment given to prepared surface on the transverse strength of repaired denture base resin.
METHODOLOGY: Two hundred and twenty specimens (DPI India) were randomly allocated into eight groups (n = 10), according to the combination of joint surface preparation (Butt joint, 45° bevel, and rounded joint) and chemical surface treatment (methylene chloride, ethyl acetate, and methyl methacrylate) for 30 and 60 s each. Specimens, after surface treatment, were repaired with self-cured resin (DPI India) and then subjected to 3-point bending test to measure transverse strength. Fractured surfaces were viewed under stereomicroscope and subjected to AutoCAD analysis to determine type of failure (adhesive or cohesive) and measure their area. Measurement data were analyzed by one-way analysis of variance, paired t-test, Dunnett's D test, and multiple comparison Bonferroni t-test, with a 5% significance level.
RESULTS: Most of the comparisons between the test specimens were statistically significant. Specimens (methylene chloride for 60 s and rounded joint) show 60%-70% gain of strength.
CONCLUSIONS: For providing better bond strength to a repaired specimen, the segment of fractured specimens to be provided "Rounded joint" preparation with etching by methylene chloride for 60 s. Copyright:

INTRODUCTION

The loss of teeth is a matter of great concern, and their replacement by dentures is most wanting fact from psychological, social, and functional point of view. The material most commonly employed in the construction of dentures is the polymethyl methacrylate.[1] Several materials and techniques have been used for repair of the prosthesis. The success of repair relies upon the phenomenon of adhesion between the already cured resin and repair material.[2]

This study was aimed to investigate the effect of three more commonly employed joint surface preparations along with the surface treatment given to prepared surface by three chemicals and also the evaluation of the areas of adhesive/cohesive failure. The objectives were:

To study the effect of changing of design of prepared surfaces of fractured segments on the transverse strength of repaired acrylic denture resin (control group)

To study the effect of surface treatments given to prepared surfaces of fractured segments by 3 chemicals, namely ethyl acetate, methylene chloride, and methyl methacrylate on the transverse strength of repaired acrylic denture resin and its comparative evaluation in relation to the time of exposure

To evaluate the type of failures, namely adhesive, cohesive, and combination after fracture of the repaired specimens and deciding the percentage of adhesive/cohesive failure in repaired specimen using AutoCAD software.

METHODOLOGY

Two hundred and twenty specimens were prepared from heat-cured denture base resin (DPI India) of dimension, length = 65 mm, width = 20 mm, and height = 2.5 mm according to the ANSI/ADA specification no. 12.[3] A “Center line” was marked on the specimen. The samples were then cut 1 mm on either side of center using mechanical saw. The joint surface was provided the shape as (Butt joint, 45° bevel joint, and rounded joint). A “Repair index” was prepared using dental stone in which repair specimen will fit snugly. The sample and index were numbered on both the ends to allow realignment in the same position. Grouping of the specimens was done as under [Table 1]:

Table 1

Grouping of the materials

Group	Abbreviations	Full forms
Group 1	IS	Intact specimen
Group 2	NCT	Control group: Repaired specimen without chemical etching of specimen
Group 3	MC 30	Repaired after etching with methylene chloride for 30 s immersion
Group 4	EA 30	Repaired after etching with ethyl acetate for 30 s immersion
Group 5	MMA 30	Repaired after etching with methyl methacrylate for 30 s immersion
Group 6	MC 60	Repaired after etching with methylene chloride for 60 s immersion
Group 7	EA 60	Repaired after etching with ethyl acetate for 60 s immersion
Group 8	MMA 60	Repaired after etching with methyl methacrylate for 60 s immersion

Numerical number one will be superscript after Smith

Using “Sprinkle on” technique, the addition of autopolymerizing repair resin (DPI India) was done in the gap until the joint space was slightly overfilled to compensate polymerization shrinkage. After finishing and polishing, the specimens were stored in distilled water for 24 h before subjecting to 3-point bending test for the evaluation of transverse strength using compression testing machine with a crosshead speed of 5 mm/min. Transverse strength (S) was then calculated using formula – S = 3PL/2bd2, where P = Breaking load, L = Distance between support, b = Width of specimen, and d = Thickness of specimen.

Assessment of type of failure by stereomicroscopic study

The fractured surfaces were then viewed under the stereomicroscope using ×8 magnification to evaluate and determine the nature of failure, i.e., adhesive and/or cohesive type or combination failure.

Computation of area of heat-cured and self-cured acrylic resin using AutoCAD photograph

All specimen groups were photographed for evaluation using “AutoCAD software”

Based on this, the “Percentage evaluation” of adhesive and/cohesive failure was determined for specimens of all groups [Figures 1–3a and b].

Figure 1

(a) Stereomicroscopic picture showing cohesive failure. (b) AutoCAD photograph showing adhesive failure

Figure 3

(a) Stereomicroscopic picture showing combination failure. (b) AutoCAD photograph showing combination failure

(a) Stereomicroscopic picture showing adhesive failure. (b) AutoCAD photograph showing cohesive failure

STATISTICAL ANALYSIS AND RESULTS

Measurement data were analyzed by one way ANOVA, Paired t- test, Dunnet D test and multiple comparison Bonferroni t- test, with a 5%significance level.

Most of the comparisons between the test specimens were statistically significant. Specimens (Methylene chloride for 60 sec and rounded joint) show 60-70% gain of strength.

Shows that transverse strength of all the test groups were much less than the strength of intact specimen. Lowest for no chemical treatment and butt joint increasing progressively to methylene chloride and highest with rounded joint and methylene chloride 60 sec treatment specimens. All the comparisons were statistically significant.

DISCUSSION

Since the early 1940s, “Polymethyl methacrylate,” introduction, it has became the most[4] commonly used denture base material because of its many desirable properties. As reported 1 by Smith, one of the most common drawbacks of denture base resin is its tendency to fracture. To deal with the problem of fracture, the temporary or definitive repair most commonly using autopolymerizing resin is the immediate measure to be undertaken.[56] Specimen repaired with autopolymerizing resin has approximately 60% to 65% of the original strength of the denture, while the strength of heat polymerized acrylic resin repairs range from 75% to 80% of the original bulk material.[578] To overcome the problems of strength of a repaired specimen, various measures have been advocated.[914]

This study was undertaken to evaluate the effect of providing surface treatments by methylene chloride, ethyl acetate, and methyl methacrylate with immersion time of 30 and 60 s and with three types of joint surface preparations on the transverse strength of the repaired specimens. The routine laboratory procedure was used to repair the specimens. After waiting for 48 h after repair, the specimens were subjected to “3-point bending test” using compression testing machine. The values so recorded were then compared with the values obtained from control group specimens and critically analyzed with intra and intergroup values for suggesting better chemical etchant, time of immersion, and joint preparation.

The result of this study is in agreement with the previous studies where plasma spray, chloroform, ethyl acetate, alcohol, methylene chloride, methyl methacrylate, and few more[1518] materials were used for surface treatment. The comparison between Groups 1 and 2 has underlined the fact that repaired specimens have just 1/3rd strength [Table 2] as compared to intact specimens. In Group 2, the repair specimen with rounded joint surface preparation proved to be superior over other two joint preparations. Out of the three chemical etchants used in the study, the methylene chloride has exhibited excellent results over ethyl acetate and methyl methacrylate. The values of transverse strength with chemical treatment group are almost one and half time more as compared with no treatment group [Graph 1].

Table 2

Mean values of transverse strength of specimens (MPa)

Group	Abbreviations	Butt joint	45° bevel	Rounded joint
Group 1	IS		74.96
Group 2	NCT	15.03	18.56	24.14
Group 3	MC 30	23.96	35.61	34.94
Group 4	EA 30	25.34	25.56	27.10
Group 5	MMA 30	20.39	22.61	24.63
Group 6	MC 60	43.41	46.09	50.61
Group 7	EA 60	37.12	37.83	39.60
Group 8	MMA 60	29.85	24.14	35.01

EA- Ethyl Acetate, MMA- Methyl methaacrylate, IS- intact specimen, NCT- No chemical treatment, MC- Methylene chloride

Graph 1

Bar diagram showing comparative relationship of the mean values of transverse strength of all groups after chemical treatment for 30 s and 60 s

The values of transverse strength of the specimen repaired after chemical treatment for 30 s have shown superior results with all three chemicals and all types of joint preparation, but the results with the repaired specimens after chemical etching for 60 s are far superior with specimens of all groups, and the values are nearly double. In order of values, the rounded joint preparations have established its superiority over Butt joint and 45° bevel joint preparation. The fact that the rounded joint was superior to other supports the general principle that sharp angled surfaces promote stress concentrations and that the amount of stress concentration is directly related to the degree and abruptness of surface change.

Time interval for chemical etching for 60 s produced better results over 30 s immersion. It is due to more time provided for dissolution of polymer hence better bonding. The chemical etching by methylene chloride with rounded joint preparation is proved to be better combination over other.

AutoCAD software was used in this study to evaluate the percentage failure. Rounded joint preparation with methylene chloride chemical etching for 60 s demonstrated “No adhesive failure” in any specimen indicative of the perfect union between heat-cured resin with the repair resin. The cohesive failure noticed in 12 specimens out of 30 and combination failure noticed in 18 samples out of 30. With 45° bevel joint also shown “No adhesive failure,” but the combination failure was more (21/30) as compared to cohesive failure (9/30).

Among the joint preparations, Butt joint demonstrated all types of failures. It is worthy to note that Butt joint demonstrated adhesive failure in all 10 specimens of no chemical treatment group, hence the Butt joint proved to be weak the weakest of all joint preparation in providing adequate strength to repaired specimen [Table 3].

Table 3

Type of failure and percentage evaluation of adhesive failure in mixed type using auto computer-aided design software

Group	Butt joint			45° bevel joint			Rounded joint
	A	C	M	A	C	M	A	C	M
No t/t	10	-	-	8	-	2 (70)*	7	-	3 (61)*
Methylene chloride for 30 s	6	-	4 (70.65)*	1	2	7 (44)*	1	4	5 (61)*
Ethyl acetate for 30 s	7	-	3 (40.13)*	2	2	6 (42.24)*	1	3	6 (50.69)*
Methyl methacrylate for 30 s	8	-	2 (42.84)*	2	2	6 (50.6)	1	2	7 (31.41)*
Methylene chloride for 60 s	3	2	5 (32.69)*	-	4	6 (26.79)*	-	5	5 (26.33)*
Ethyl acetate for 60 s	4	1	5 (35.81)*	-	3	7 (24.5)*	-	4	6 (24.25)*
Methyl methacrylate for 60 s	4	2	4 (41.27)*	-	2	8 (42.84)*	-	3	7 (23.37)*

*Percentage (%). A: Adhesive failure, C: Cohesive failure, M: Mixed failure

SUMMARY AND CONCLUSIONS

Chemical treatment given to prepared fracture surfaces of segments provide better strength to repaired specimens

Surface treatment by methylene chloride with rounded joint has established its significance over the ethyl acetate and methyl methacrylate treatment

Chemical treatment for 60 s yielded better values as compared to 30 s etching in all groups

More adhesive failure noticed in all groups with Butt joint preparation; hence “Butt joint” to be avoided while preparing the fractured segments. 45° Bevel joint preparation ranked 2nd best in order

AutoCAD software was used successfully to count percentage failure in combination (mixed) failure in repaired specimens

For providing better bond strength to a repaired specimen, the segment of fractured specimens to be provided “Rounded joint” preparation with etching by methylene chloride for 60 s.

Further exploration as regards to the effectiveness of other chemicals and utilization of AutoCAD software is required.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.