Polymethyl methacrylate (PMMA) remains the denture base material of choice, although being introduced in 1936. ¹ , ² Several types of PMMA denture base resin available today are similar in composition but small variations lead to different physical properties and processing methods. ³ Dentures made by light-activated demonstrate lower mechanical properties over heat-activated resins. ⁴ The polymerization shrinkage of PMMA by ordinary compression molding method prompts inaccurate adaptation of the base material to the dental replacement bearing tissues, bringing about a poor border seal. To minimize the dimensional inaccuracies of the compression molding technique, Pryor (1942) developed the injection molding technique as an alternative. In 1970, Ivoclar company introduced a special resin for injection molding. ⁵

Computer-aided design/computer-aided manufacturing (CAD/CAM) was introduced in the late 1950s with the introduction of PRONTO, a numerical control programming tool. Dr. Patrick J. Hanratty developed the first CAM software system ⁶ followed by Dr. Moermann and Dr. Andersson, with CEREC® and Procera® systems for clinical dental restorations. ⁷ CAD/CAM technology scans the denture base morphology and records the tooth positions. Data are then imported into a virtual tooth arrangement program where teeth can be articulated followed by exposing it to milling device for the fabrication of the complete dentures. ⁸ Prepolymerized resin pucks are used for milling the denture bases to provide superior strength and fit with an additional advantage of reduced bacterial adhesion and cost-effectiveness to patients and clinicians. ⁹

During mastication for several years, denture bases undergo repeated flexing leading to fatigue failure, thereby demanding a high fatigue strength property of denture base resins. ¹⁰ The highest stress in a material is experienced at its moment of rupture represented by flexural or transverse strength which is required high to prevent catastrophic failure under load for denture success. With respect to extrinsic factors, the American Dental Association Standard No. 139 in accordance with ISO 20795-1, time and temperature during polymerization and testing affect physical and mechanical denture base properties. ¹¹ Thermocycling negatively affects flexural strength (Fs) ¹² hence, conditioning the resin before testing is necessary.

Literature reports numerous studies on denture handling techniques and variation in flexure strength. ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹ , ²⁰ with compression and injection molding being the most popular methods. Ivocap High impact (Ivoclar Vivadent), in combination with injection molding results in improved accuracy of fit. ² Studies also report the effectiveness of CAD/CAM dentures being superior over conventionally prepared dentures. ²¹ , ²² Since the comparison of dentures with different processing methods is essential for the acknowledgment of the denture with the highest Fs, the present study was conducted to compare the Fs of denture base resins fabricated by compression molding, injection molding, and CAD/CAM milling technique. The null hypothesis was, there is no significant difference in the Fs among the three techniques.

In this in vitro study. Forty-five denture base resin specimens (n = 15, each group) (compression molding [Group A], injection molding [Group B], and CAD CA [Group C]) with specified dimensions were fabricated with standardization done using, metal strips of 65 mm × 10 mm × 3 mm Figure 1according to the American Standards for Testing and Material. Figure 1

Metal strips.

Descriptive data on mean flexure strength in all three groups are presented in [Graph 1] [Figure 17]. Flexure strength was highest with CAD/CAM (97.46 ± 9.93) followed by injection molding (84.42 ± 10.42) and compression molding (71.72 ± 11.58) technique fabrication as shown in Table 1.[INLINE:2]{Table 1}

The one-way ANOVA is used to determine whether there are any statistically significant differences between the means of three or more independent (unrelated) groups.

[Graph 2] and Table 2presents the statistical difference in flexure strength between the groups using one-way ANOVA. Since the P from the one-way ANOVA test is 0.00, which is less than the significance level of 0.05, the null hypothesis is rejected.[INLINE:3]{Table 2}

At this point, it is important to realize that the one-way ANOVA is an omnibus test statistic and cannot tell you which specific groups were statistically significantly different from each other, only that at least two groups were. To determine which specific groups differed from each other, we used the post hoc Tukey test.

Post hoc tests are an integral part of ANOVA. When you use ANOVA to test the equality of at least three group means, statistically significant results indicate that not all of the group means are equal. However, ANOVA results do not identify which particular differences between pairs of means are significant. Post hoc (”after this” in Latin) tests are used to uncover specific differences between three or more group means when an ANOVA F test is significant.

The Post hoc Tukey comparison table shows that all three manufacturing groups depicted different letters in the grouping when the test was conducted. The Post hoc Tukey comparison [Graph 3] shows that no interval contains 0. Both these findings conclude that data are statistically significant between any two (and all three) groups.[INLINE:4]

Fs comprises three mechanical properties, namely compressive strength, tensile strength, and shear strength. Therefore, the Fs becomes a key factor to be analyzed for the success of a denture base. In the present study, Fs of the denture base resins produced through the CAD/CAM milled technique was significantly higher than the ones obtained through the compression molding technique and injection molding technique. The denture base was able to succeed simulation intraorally to high functional loads during parafunction and mastication when subjected to a 3-point bend test. ¹⁹ , ²⁰ , ²³ The 3-point flexural test is commonly used for measuring flexural properties. The acrylic denture base resins should have at least 65 MPa Fs according to the ISO standards. ¹⁷ By taking these criteria into consideration, all the groups in this study were suitable for clinical use.

Fs, impact strength, and flexural modulus were observed to be significantly improved in CAD/CAM resin over the conventional heat-cured group. ²³ In the present study, injection-molded denture base resin showed higher Fs than the compression-molded technique resins. These results are similar with Gharechahi et al. study, with similar methodology. ²⁰ Nandal et al. reviewed various advancements in the field of denture base resins and stated that the Fs, impact strength, and flexural modulus were observed to be significantly improved in CAD/CAM resin as compared to the conventional heat cure resins. ²⁴ , ²⁵ Thus, it can be stated that CAD/CAM denture base resin has superior mechanical properties compared to the injection molding technique and compression molding technique. This rejects the null hypothesis of no significant difference in the Fs among the groups.

However, a contrast was observed with the Aguirre study reporting high Fs by compression molding technique over injection molding technique. This can be attributed to different methods used for polymerization wherein polymerization degree affects the Fs. ²⁶ The Fs of CAD/CAM was observed to be higher than the conventional methods, supporting the same reason of variation in results. The conventional compression molding and injection molding techniques have a lower degree of polymerization while CAD/CAM milled denture resins exhibit a superior level of mechanical properties when compared to conventional methods. ²⁷

The rigidity of the material is demonstrated by the flexural modulus. ²⁰ Results from this study demonstrated that all three types of denture resins which were tested met the standards according to the standards for ISO-20795-1. Clinically, subclinical deformation of the denture would be observed rather than a fractured resin under load greater than the calculated mean Fs according to this finding. Fs obtained was different from each method because it was statistically significant. The denture bases fabricated from each method have different limits to bear the maximum load. Denture bases fabricated with CAD/CAM method are superior because they have the highest Fs of the three fabrication techniques. Injection-molded denture bases are inferior to CAD/CAM fabricated denture bases; but better than compression-molded denture bases, because compression-molded denture bases exhibited the least Fs. The CAD/CAM and injection-molded resins had higher flexural modulus which led to minimal-to-no deformation before fracture. Although the Fs of the compression specimens was the lowest and demonstrated significant plastic deformation before failure, the product remains a suitable denture base material. Thus, the techniques, advantages, and disadvantages of CAD/CAM conclude that the current innovations and technological developments allow the digital planning and manufacturing of removable dentures from start to finish. Thus, decreasing the chairside and working time for patients and dentists provide superior or satisfactory functional and esthetic outcomes. However, long-term clinical research and additional material-related aspects are required to reach definitive conclusions.

There are certain limitations influencing the outcome; first, the tested specimens did not resemble the shape of an actual denture. Second, the absence of a longer period of thermocycling, cyclic loading; third, manual preparation of samples leading to human error during the preparation and finishing stage leading to slight inaccuracy in the readings.

The mean obtained from CAD/CAM, injection molding and compression molding manufacturing techniques are 97.46, 84.42, and 71.72, respectively. The standard deviation obtained from CAD/CAM, injection molding and compression molding manufacturing techniques are 9.93, 10.42, and 11.58, respectively. We had done one-way ANOVA and Post hoc Tukey method. The level of significance was set to 95%. The P obtained was 0.00 which is <0.05; hence data are statistically significant. Post hoc Tukey method was used to determine that every method was statistically different from each other.

Within the limitations of the study, it can be concluded that, Fs of the denture base resin produced by CAD/CAM milled technique was observed to be higher than the denture base resins obtained through compression molding and injection molding, which are the two regular processing methods. The compression-molded resin depicted prominent deformation before fracture with a lower flexural modulus, whereas the injection-molded, and CAD/CAM milled denture resins fractured with minimal to no plastic deformation. Thus, prepolymerized CAD/CAM milled denture bases can be a replacement to conventionally processed denture bases (compression molded and injection molded) where higher bending forces are expected. However, furthermore, extensive research needs to be done as a continuation of the present study to evaluate interferences in vivo in patient's oral cavity to compare other mechanical properties such as the flexural modulus, impact strength, flexural fatigue, and fracture toughness of denture resins with larger sample size, better simulation of oral conditions, and long-term clinical trials. The specimens to be tested should be fabricated resembling the shape of an actual denture which may influence the results in future studies and cyclic loading can be used to simulate stress fatigue within the specimens to mimic intraoral conditions.

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The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or non-financial in this article.