**Linear Dimensional Change in Acrylic Denture Teeth Positions Factored by Different Processing Techniques and Occlusal Forms: An In Vitro Study**

**Mohammed E. Sayed 1,\* , Amit Porwal 1, Saurabh Jain 1,\* , Abdulkarim Hussain Alshehri 1, Nasser M. Alqahtani 2, Ashwaq Hadi Ali Hadadi 3, Rawan Abdulwadoud Zakri 3, Sahar Mahdi Musa Zeed 3, Saswan Ibrahim Nahari 3, Fatimah H. Alsurayyie 1, Hatem Alqarni 4, Saeed M. Alqahtani <sup>2</sup> and Saad Saleh AlResayes <sup>5</sup>**


**Abstract:** The current literature lacks substantial evidence for the effect of denture base processing techniques and posterior denture tooth forms on denture tooth shifts due to denture base resin polymerization. The aim of this study was to evaluate the combined effect of PMMA-based denture processing techniques (compression packing and injection molding) and posterior tooth forms (semianatomic and non-anatomic) on the linear dimensional shift of denture teeth following denture processing in both horizontal and vertical dimensions. Two different complete denture fabrication techniques were used to prepare forty ideal maxillary complete dentures using two different types of posterior tooth forms. The used fabrication techniques were conventional heat polymerized compression packing and injection molding. The posterior tooth forms used in the current study were non-anatomic tooth (0 degrees) and semi-anatomic tooth forms (approximately 20 degrees). Initial linear measurements (vertical and horizontal) were taken from pre-specified points for the central incisor and first molar. Specimens were randomly divided into four groups (*n* = 10), and denture processing was performed using the two techniques. Final linear measurements were recorded. The linear change in dimension for all six parameters was calculated by deducting the after values from the before values. Since the discrepancies were both positive and negative in magnitude, the absolute value of the difference was taken for further analysis. This value represents the dimensional change. *T*-tests were used to compare the mean dimensional changes. Furthermore, the mean dimensional changes for all the six parameters were compared using a two-way analysis of variance. The alpha error was set at 5%, and a *p*-value of less than 0.05 was considered statistically significant. The injection molding technique showed significantly fewer tooth movements in both the vertical and horizontal measurements as compared to the conventional compression packing technique. The non-anatomic tooth showed significantly fewer changes in tooth movement as compared to semianatomic teeth in both the compression and injection techniques. This study can guide the selection of a proper processing technique for a particular posterior tooth form, thus minimizing occlusal discrepancies and reducing occlusal corrections during laboratory and clinical remount procedures.

**Citation:** Sayed, M.E.; Porwal, A.; Jain, S.; Alshehri, A.H.; Alqahtani, N.M.; Hadadi, A.H.A.; Zakri, R.A.; Zeed, S.M.M.; Nahari, S.I.; Alsurayyie, F.H.; et al. Linear Dimensional Change in Acrylic Denture Teeth Positions Factored by Different Processing Techniques and Occlusal Forms: An In Vitro Study. *Appl. Sci.* **2022**, *12*, 7058. https:// doi.org/10.3390/app12147058

Academic Editor: Mary Anne Melo

Received: 18 June 2022 Accepted: 11 July 2022 Published: 13 July 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**Keywords:** PMMA; acrylic teeth; non-anatomic teeth; semi-anatomic teeth; injection molding technique; compression molding technique; denture processing; tooth movement; denture base resin; posterior tooth form; linear dimensional change; complete denture

## **1. Introduction**

Complete edentulism, as defined by a loss of all permanent dentition, is a global health condition that was reported to affect a range of 7–69% of adult populations worldwide [1]. Even though implant dentistry has changed the approach of treatment planning for completely edentulous cases, conventional complete dentures remain a valid and sometimes first option, especially when implants are contraindicated in such cases. Balanced complete denture occlusion is crucial for the stability and retention of complete dentures in patients with moderate to severe alveolar ridge resorption [2]. Excessive denture tooth movements due to processing may compromise the form, esthetics and occlusion and, therefore, mean that dentures need to be remade [3]. Several factors have been reported to affect the positional stability of a denture tooth during processing. These factors include the investment method [4], investment material [5], flasking and polymerization technique [6,7], denture base thickness [8], flask closure and cooling method [9], palatal form [10] and box preparation on the heel of the master cast [11]. In addition, Goodacre et al. [12] studied the effect of the denture processing technique on denture tooth movement and found that CAD–CAM technology was the most accurate and reproducible in comparison to conventional packing and injection techniques. The study also reported that the packing technique resulted in higher vertical tooth movement. However, CAD–CAM technology for denture fabrication was only implemented in 10% or less of the cases treated in US dental schools [13]. Therefore, the focus should be directed toward perfecting the techniques that are commonly used in complete denture practice. Jackson and colleagues [14] reported that the presence of acrylic teeth has a significant effect on the accuracy of denture bases fabricated using compression and injection techniques. Several studies have indicated that the injection technique is more accurate with respect to changing vertical dimension and occlusal errors [15,16]. The case-specific selection approach of the posterior tooth form is important for maintaining masticatory efficiency and the success of denture treatment [17]. The available evidence is lacking with regard to the effect of posterior denture tooth form (i.e., cuspal height) on denture tooth movement due to acrylic resin processing. Hence, the goal of this study was to evaluate the combined effect of processing techniques (compression and injection molding) and posterior tooth forms (semi-anatomic and non-anatomic) on linear dimension changes (horizontal and vertical dimensions) in denture tooth positions following processing. The null hypothesis of the study was that the variation in posterior tooth forms and complete denture processing technique has no effect on the dimensional position of anterior and posterior teeth following processing.

## **2. Materials and Methods**

The research protocol was approved by the research board at the College of Dentistry, Jazan University (reference number: CODJU-2003I).

#### *2.1. Materials*

In this study, two different complete denture fabrication techniques were used to prepare forty ideal maxillary dentures using two different types of posterior tooth forms. The fabrication techniques used were: conventional heat polymerized compression molding and injection molding. The posterior tooth forms used in the current study were nonanatomic tooth (0 degrees) and semi-anatomic tooth forms (approx. 20 degree). Details of the materials used in the fabrication of maxillary complete dentures are listed in Table 1.


**Table 1.** Materials used in the study.

## *2.2. Pouring of the Cast and the Arrangement of Teeth*

To standardize the reference points for measurements, a round bur (# 001/027, Quattro, DFS-Diamon GmbH, Ländenstraße, Riedenburg, Germany) was used to make holes on the land area of the ideal edentulous maxillary cast, corresponding to the disto-buccal cusps of the right and left first molars and maxillary right central incisor. The silicone mold made using this reference cast was used to pour forty ideal casts with grooves in the land area (Figure 1).

**Figure 1.** Steps of duplicating cast and teeth setup: (**A**,**B**) round holes made on cast corresponding to maxillary right central incisor and disto-buccal cusps of the right and left first molars; (**C**) ideal edentulous cast with orientation grooves; (**D**) custom edentulous silicone mold.

Two sheets of modelling wax were adapted on the cast, and ideal teeth arrangements for complete dentures were performed. The arrangements of the teeth on all the casts were obtained by the duplication of the ideal teeth arrangement from the initial cast, i.e., twenty sets of semi-anatomical teeth and twenty non-anatomic teeth. Following this, vertical grooves were placed on the right central incisor incisal edge (tooth number 1.1) (bur (# 114/023, Quattro, DFS-Diamon GmbH, Ländenstraße, Riedenburg, Germany)) and the round holes on the disto-buccal cusps or right and left first molars (tooth number 1.6 and 2.6) (bur (# 001/023, Quattro, DFS-Diamon GmbH, Ländenstraße, Riedenburg, Germany)) (Figure 2).

**Figure 2.** Dentures before processing: (**A**) vertical groove placed on the right central incisor incisal edge; (**B**) round holes on the disto-buccal cusps or right and left first molars.
