Difference calculated by subtracting the post-processing values from the pre-processing values; \* significant (*p* < 0.005); ΔVI, mean linear dimensional change in the position of the incisor in the vertical direction; ΔVLM, mean linear dimensional change in the position of the left molar in the vertical direction; ΔVRM, mean linear dimensional change in the position of the right molar in the vertical direction; ΔHI, mean linear dimensional change in the position of the incisor in the horizontal direction; ΔHLM, mean linear dimensional change in the position of the left molar in the horizontal direction; ΔHRM, mean linear dimensional change in the position of the right molar in the horizontal direction.

When the linear dimensional changes in the positions of the molar in the horizontal direction were assessed, it was observed that complete dentures with semi-anatomic teeth that were processed using the conventional compression molding technique showed the maximum movement (0.65 ± 0.79 and 0.78 ± 0.63). The least movement was observed in complete dentures with non-anatomic teeth that were processed using the injection molding technique (0.02 ± 0.49 and 0.02 ± 0.54). The maximum linear dimensional changes in the positions of the incisor in the horizontal direction were observed in complete dentures with semi-anatomic teeth that were processed using the conventional compression molding technique (0.62 ± 0.46), whereas complete dentures with non-anatomic teeth that were processed using the injection molding technique showed the smallest changes (0.004 ± 0.32).

Table 3 depicts the two-way ANOVA comparison among the four groups with respect to dimensional change. The dimensional change was found to be greater in the conventional technique as compared to the injection molding technique. Additionally, there was a statistically significant difference in the non-anatomic teeth as compared to the semianatomic teeth.


**Table 3.** Two-way ANOVA comparing the tooth forms and processing techniques.

Dependent variable: dimensional change; <sup>a</sup> 167 MS (technique) + 0.833 MS (error); <sup>b</sup> MS (error).

#### **4. Discussion**

With respect to their impact on dimensional changes, there was a significant interaction between the components, i.e., the posterior tooth forms and processing techniques applied. The statistical analysis revealed that the results were rather pronounced, with the injection molding technique exhibiting fewer changes during the transition between the waxing stage and post-processing. Another observed pattern was that the non-anatomic teeth

positional change was less than that for the semi-anatomic teeth, regardless of the dimension examined and the processing technique used. The outcomes of the study revealed that changes in the positions of the molar were the highest in the horizontal dimension as compared to other dimensions tested for all the groups, when semi-anatomic teeth were processed using the conventional molding technique. However, the smallest differences were observed in the non-anatomic group, when the injection processing technique was used. The significant positional changes in semi-anatomic teeth can be explained by the complex interlock between the denture teeth and the investment material, which may create excessive stresses during acrylic polymerization that lead to significant teeth movement. The results of this study negate the proposed null hypothesis.

In the annals of contemporary dentistry, the introduction of acrylic resins in 1937 was a watershed moment. Their adoption and acknowledgment in prosthodontics are quite remarkable, as they are proven to be more esthetic and simpler to manipulate to be used in the laboratory as well as in clinics [18]. Methacrylates, particularly methyl methacrylate (MMA) acrylic resin, have been used in dentistry as a denture foundation material since their inception and have been widely employed, since they are regarded as suitable substances for use in the oral environment [19,20]. Recently, the usage of pour-type (fluid) resins in the manufacturing of denture bases has expanded dramatically [21].

The dimensional shift that happens during polymerization shrinkage is crucial to the preservation and stability of the complete denture [22]. The movement of teeth occurs during and after the production of complete dentures, according to research published in the literature [9,23]. A careful understanding and consideration of this may allow for the creation of functioning complete dentures, which require fewer occlusal modifications on the articulator and minimal corrections in the patient's mouth. Previous research has found that the highest amount of tooth movement occurs in the posterior teeth [24]; however, this study found that the maximal teeth movement occurs solely in the compression molding semi-anatomic teeth group for this study and also for the molar teeth. Final measurements were performed after the dentures were recovered from flasking but without removal from the casts. The observed dimensional changes were greater, as the dentures were not immersed in water, as suggested previously in the literature [24]. The dimensional changes produced by water sorption cause expansion, presumably due to the entrance of water between polymethyl methacrylate molecules [25]. The movement of teeth during the production of a complete denture is also influenced by a variety of other factors. The influence of base thickness [8], geometric palatal form [10] and closing flask pressure [26] has been studied. The flask-closing procedure may cause tooth displacements, and the post-pressing time association was partially acknowledged. Denture imperfections, including base distortion and false tooth displacement, were addressed through meticulous measurements in the current study.

The literature includes many studies [16,27–29] related to the comparison of the conventional and injection molding processing techniques. Although, in recent decades, the conventional technique has shown promising results and is still widely used, there is still continuous ongoing research related to the development of other processing techniques. The recently introduced injection molding technique has shown promising results, with a minimal incisal pin opening and reduced adjustments required later. However, the movement of the teeth was in both the horizontal and vertical directions; it showed significant differences in the vertical dimension when the compression molding technique was used with PMMA as the material in both the techniques [16]. Bahra et al. [30] studied both linear and volumetric dimensional changes in six injection molding PMMA denture base resins and found that the IvoBase system was more accurate as compared to other resins processed by other curing techniques. Strohaver [29] compared compression and injection molded complete dentures and found fewer vertical dimension changes in the injection molding technique. When a comparison was conducted for the accuracy of the linear measurements of chemically distinct injection molding materials (PMMA, nylon and styrene) to that of compression molding acrylic resin (PMMA) by Parvizi et al. [27], they concluded

that nylon had the greatest overall distortion and that styrene had the smallest. According to Sykora [28], the injection molding technique's higher dimensional accuracy compared to the conventional method may be due to smaller resin particles, lower polymerization temperature, the lack of resin layer formation between the flask counterparts and the lack of the movement of the two halves of the flask during resin packing.

When lingualized balanced occlusion was compared with conventional balanced occlusion, the increase in the vertical dimension was found to be similar [31]. In 2004, Parvizi [27] performed a study on dimensional changes between compression and injection molding techniques using different denture base materials. All the materials used in the study responded differently with different dimensional changes in antero-posterior and cross arch measurements. The results of the current study are also in accordance with this study, showing different changes in both the horizontal and vertical dimensions for both the type of teeth and the processing technique used. However, in the complete dentures with non-anatomic teeth that were processed using injection molding technology, the lowest linear dimensional changes in the incisal and molar positions were found, whereas the highest dimensional changes were found in the semi-anatomic teeth with the conventional processing technique.

Venus [32] compared the denture base resins, processing methods and the denture bases with or without teeth and concluded that the processing technique is a more dominant variable than denture base resins. Peyton and Craig [33] reported difficulty in controlling the changes in the vertical dimension and identified pressure during flask closure as the most significant factor. Other mentioned factors that may be related are due to varied temperatures of the flask, dough consistency, stone mold strength and other factors. Additionally, Zakhari mentioned that teeth are under considerable pressure during packing and curing in the conventional curing technique [34]. Shippee [35] concluded from his experiment that, when the volume of acrylic used during final closure is increased, the teeth are more likely to be intruded; however, it can be reduced if proper venting is provided for acrylic. Atkinson and Grant (1962) [36] found in their investigation that the movement of the teeth is mainly due to a change in the dimension of the mold and also due to the pressure of the acrylic on the teeth and mold. They also proposed that teeth in the mold are carried to a different position due to the reconstitution of the gypsum on heating, cooling and compression. Additionally, the short curing technique has shown an increase in the vertical dimension and a lower number of occlusal contacts [37]. However, in the present study, utmost precaution was taken to maintain the same amount of acrylic used for each denture, standardized pressure during flask closure and a long curing cycle to reduce the effect of these factors on the movement of teeth. However, in the injection molding processing technique, the continuous flow of the non-polymerized material from the reservoir sprue would have compensated for the polymerization shrinkage, as also mentioned by Anderson [38].

The literature has shown that maximum changes after polymerization and the release of internal strains occur after polishing and water storage for 24 h. Additionally, it was shown by Venus [32] that intermolar width increases after water storage and decreases by shrinkage, suggesting that the fit of the denture improves after storage in water. Additionally, Chuchulska [39] studied four thermoplastic injection molding materials (Bre.flex 2nd edition, Vertex ThermoSens, Perflex Biosens and Polyan IC) and concluded that shrinkage is present in all the materials tested for the injection molding technique and that storage in water affects all the materials. In the current study, the post-processing measurements were performed after the removal of the dentures from the flasks but without removal from the casts and without any storage in water, which may be attributed to the more dimensional changes that were recorded. Complete dentures with non-anatomic teeth that were manufactured using the injection molding technique showed the smallest changes. However, dentures with semi-anatomic teeth that were processed using the conventional compression molding technique showed the greatest linear dimensional changes in the incisor and molar positions in both the horizontal and vertical directions in the present study.

The prevailing literature lacks studies related to the effects of posterior tooth forms and processing techniques on linear dimensional changes in the teeth. Zakhari [34] used non-anatomic acrylic teeth and established a combination of plaster and stone to reduce the change in the vertical dimension. However, this study also suggested stone with a lower water–powder ratio as an investing medium in order to control expanding acrylic resin. Woelfel et al. [40] compared different denture base materials in their study and noted that the sharper defined anatomy of Pilkington–Turner teeth seemed to coincide with more tooth movement. Carr et al. compared 0-degree teeth and 33-degree teeth and found less vertical tooth movement in the 0-degree teeth as compared to the 33-degree teeth, when plaster of Paris was used as compared to stone as an investing medium [5]. They attributed this to the greater setting expansion of plaster of Paris pushing the teeth into the wax followed by intrusion back in the mold due to the hydraulic pressure of the acrylic. In the current study, non-anatomic teeth also showed less tooth movement in both the vertical and horizontal dimensions as compared to the semi-anatomic teeth. This may be attributed to the sharper occlusal anatomy of the semi-anatomic teeth as compared to the uniform occlusal surface of the non-anatomic teeth, and it may also be related to the pressure during acrylic packing and flask closure.

Although there are continuous advancements in the field of polymers, the acrylization process and the digitization of procedures, the conventional compression molding technique is still an easier and cost-friendly substitute that is favored and practiced by a majority of laboratories. The effect observed in this in vitro study should be checked in a clinical setup for its clinical relevance. This study did not compare cross arch measurements, palatal adaptation measurements, the effect of different investing media, etc. Therefore, further studies should be planned with an increased sample size and various other parameters to check the effects of different tooth forms on denture tooth movement and their clinical relevancy. This may help in the selection of a proper processing technique for a particular tooth form, thus minimizing occlusal discrepancies and reducing occlusal corrections during laboratory and clinical remount procedures.
