Computerized occlusal forces analysis in complete dentures fabricated by additive and subtractive techniques

Abstract

BACKGROUND:

Fabrication of complete dentures by computer-aided designing and computer-aided manufacturing (CAD-CAM) techniques are now common. Subtractive and Additive are the two principal CAD-CAM techniques used for this purpose. However, studies that evaluated the occlusal forces by CDs manufactured by these techniques are lacking.

OBJECTIVES:

To compare the occlusal forces in complete dentures fabricated by additive, subtractive and conventional techniques with different occlusal schemes, using computerized occlusal force analysis system [Tech-Scan III (T-Scan III)].

METHODS:

Three groups (Gr) were made on the basis of techniques of fabrication of CDs: Conventional CDs (CCD), Subtractive CDs (SCD), and Additive CDs (ACD). Each group CDs were further divided into three sub groups based on occlusion schemes: bilateral balanced occlusion (BBO), lingualized occlusion (LO) and mono plane occlusion (MO). A total of 45 CDs were made: 15 in each group with 5 CDs of each occlusal scheme. For all samples, occlusal force analysis (percentage of occlusal force applied on the right and left sides of the arch, centralization of forces and percentage of maximum occlusal force) was done using computerized occlusal analysis system: T-Scan III. Univariate regression analysis and logistic regression analysis were used to find the effects of the technique of fabrication and occlusion scheme over the occlusal forces ( $p<$ 0.05).

RESULTS:

The intergroup comparison revealed statistically significant differences ( $p<$ 0.01) in right-left side force difference, maximum bite force in CDs fabricated by various techniques and with different occlusion schemes. Though the effect of occlusion scheme was more than the technique of fabrication (according to effect size estimation). The maximum force difference between right-left side was observed in combination of CCD technique and MO scheme (36.88 $\pm$ 2.82 N). Furthermore, the maximum bite force was observed for SCD technique (89.14 $\pm$ 6.08 N) and LO scheme (92.17 $\pm$ 3.22 N). In comparison to ACD, the chances of centre of force out of ellipse was 2.53 time more in CCS and 0.75 times less in SCD techniques and in comparison to MO, the chances of out of ellipse was 0.298 times less in BBO and 0.396 times less in LO schemes, though these chances were not statistically significant ( $p>$ 0.05).

CONCLUSIONS:

The digital CDs fabricated by subtractive technique were proved to be superior to additive technique in terms of occlusal force analysis on tested parameters. However, further research is needed on patients to determine the exact superiority of one technique over the other.

Keywords

Digital dentures T-Scan occlusal forces milling 3-D printing

1. Introduction

The growth in the digital dentistry has led us to a way where work efficiency and quality have increased immensely. The use of computer-aided design and computer-aided manufacturing (CAD-CAM) procedures has become an inseparable part of clinical and laboratory work [1]. After the introduction in dentistry in the 1970s, CAD/CAM has enhanced the way fixed prosthodontic restorations are made and recently its benefits were realised in removable dentures [2, 3, 4, 5, 6, 7]. The potential benefits of digital CD fabrication include: standardized fabrication by using high-value, quality-controlled materials, fewer clinical appointments, and overall improved cost-effectiveness [8]. With the use of CAD-CAM, the complete dentures can be fabricated using subtractive method viz. milling or additive method viz. 3-dimensional printing (3DP).

At present the subtractive method is more popular among clinicians and patients probably due to the reduced number of required patient visits compared to conventional complete dentures [9, 10]. Also, previous comparative studies showed that milled complete dentures illustrated similar or better fit of the tissue surfaces, biocompatibility, with superior mechanical properties [2, 11, 12, 13]. In the subtractive method, pre-polymerized resin blocks are used which release less monomer, no polymerization shrinkage, high degree of strength and accuracy but still this subtractive method has inherent limitation of machining tools resulting in the waste of a large amount of denture base material [9, 10, 13].

On the contrary the additive method, consists of segmenting the 3D model data (CAD design) into multislice images and by using simple layering technique usually 5 to 20 per milli-meter of material in a layer, the definitive shape is produced [14, 15, 16]. Moreover, amount of material used in 3-D printing technique is very less compared to milling method [17]. In addition, it could be economical as there is practically no loss of material and even unused material can be processed in the future also, multiple dentures can be printed at a time with fine detail reproduction [15]. Among the various 3-D printing techniques, stereolithography (SLA) [11, 17, 18] is used most often. It consists of polymerization of photosensitive liquid polymer layers with ultraviolet light beam, which results in fabrication of dentures. Advantages include high accuracy, smooth surface finish, fine building details [19] and enhanced patient satisfaction comparable with that for conventional complete dentures [20, 21].

Edentulous patients seek denture treatment to restore function and aesthetics in the most comfortable fashion. One of the most important factor to achieve comfort in CD is correct occlusion, which should provide not only stability to dentures but also sufficient force to chew the food with uniform force distribution throughout the denture foundation. This stands true not just for conventional CDs but also for digital CDs. Presence of deflective occlusal contacts, poor occlusal force summation and unseen collection of unbalanced forces may result in torque and dislodgement of complete dentures affecting stability, comfort and patient acceptance [22, 23, 24].

Various occlusal scheme has been recommended for CDs mainly bilateral balanced occlusion (BBO), lingualized occlusion (LO) and neutral-centric or mono plane occlusion (MP). These schemes’ efficacy has been investigated previously for conventional dentures in terms of patient satisfaction for mastication, stability, aesthetics etc. Varied results were obtained on different aspects for each scheme in conventional CDs [24, 25, 26, 27, 28, 29, 30, 31]. However, no study has yet analysed the occlusal forces in digital dentures and its comparison with conventional CDs, which would be of critical interest and effect on patients due to its variation in the method of teeth attachment to denture bases, the use of newly developed materials and most importantly the techniques of fabrication.

Thus, this study was conducted to compare the occlusal forces in complete dentures fabricated by additive, subtractive and conventional techniques with different occlusal schemes, using computerized occlusal force analysis system (T-Scan III). The null hypothesis formulated was that no difference would be found in the occlusal forces of complete dentures fabricated by either technique.

Computerized occlusal analysis system as T-scan, have digital sensors that gather the required data with the help of associated software and allows simultaneous registration and imaging of the distribution of forces in relation to the maximum force exerted and the occlusal contact time sequences. All the contacts are accompanied by a percentage number, which shows the force distribution along the dental arch and thus provides a quantitative estimation of occlusal contact pressure [32, 33, 34].

2. Materials and methods

The present cross-sectional study was conducted at the Department of Prosthodontics, College of Dentistry, King Khalid University, Abha, KSA, and was approved by the institute’s ethical committee (IRB/KKUCOD/ETH/2019-20/069). To standardize the study protocol, a synchronized flowchart was prepared for fabrication of various complete dentures (Fig. 1).

Figure 1.

Flowchart of the study design.

Three groups (Gr) were made on the basis of techniques of fabrication of CDs-Conventional technique CDs (CCD), Subtractive/Milled CDs (SCD), and Additive/3D printed CDs (ACD) technique. Each group’s CDs were further divided into three sub groups based on occlusion schemes: bilateral balanced occlusion (BBO), lingualized occlusion (LO) and neutral-centric or mono plane occlusion (MO). A total of 45 CDs were made: 15 in each group with 5 CDs of each occlusal scheme.

To start, a completely edentulous normal patient with good neuromuscular control was selected from the outpatient department with well-formed, high rounded, completely healed maxillary and mandibular ridges with class I relation, in accordance with the American College of Prosthodontists Type A classification of residual ridge morphology [35]. The patient received information about the study and written informed consent was obtained. From primary impressions to jaw relation using Face Bow (Hanau Spring-Bow; Whipmix, Louisville, KY, USA) and inter-occlusal bite (polyether bite registration material; RAMITEC, 3M ESPE, St. Paul, MN, USA) records were clinically obtained from the patient in a conventional manner of CD fabrication [29].

Following this the secondary casts and wax occlusal rims with the recorded jaw relationship were scanned using desktop laboratory scanner (D800, 3Shape A/S., Copenhagen, Denmark). The laboratory scanner software has a self-alignment setting which joins the multiple scans of various sections and produce a complete virtual image. The data of scan was stored as a standard tessellation language (STL) format for further use.

Master dies of maxillary and mandibular secondary casts were made using their scans and casting them in metal (cobalt-chromium; Wirobond C; Bego Gmbh, Germany) after printing it with castable resin. The metal master dies so prepared were duplicated and study casts were poured with type IV die stone (GC Fujirock EP; GC Europe). In total 15 casts were made.

For fabrication of samples in CCD gr, at the beginning the occlusal rims were mounted using type I dental plaster (Kalabhai Karsol Pvt. Ltd, India) mixed with antiexpansion solution (Mp Sai Enterprises, Mumbai, India) in the semi adjustable articulator (Whipmix 2000 Serise-2240, Louisville, KY, USA), using clinically obtained data from the patient. Then, the occlusal rims were duplicated on the light cure temporary denture base (fabricated on previously duplicated study cast) using customised jig. This methodology provided consistent base thicknesses and occlusal rim positions on it. On each occlusal rim the teeth arrangement was done following the basic principles of particular occlusion scheme (BBO, LO, MO), with the help of jig. A total of 5 CDs of each scheme were fabricated using heat cured denture base acrylic resin (Dentsply Trubyte, USA) and long curing cycle (9 hours in a water bath at 73 ${}^{\circ}$ C $\pm$ 1 ${}^{\circ}$ C followed by 1/2 hour in boiling water as recommended by the manufacturer) following manufacturers’ instructions and basic guidelines of compression molding technique [36, 37, 38]. After retrieval of dentures only primary finishing was done and no polishing procedures were carried out. The fitting of each CD was verified on the mounted secondary cast on which the teeth arrangements were done.

For fabrication of samples for digital dentures (SCD Gr and ACD Gr), the STL files of scanned secondary casts and occlusal rims with jaw relation record were used. The advanced CAD-CAM software (3-Shape Dental System – Complete Restorative – Complete denture module) for digital denture fabrication was used which was characterised by the consistency and seamless linking of all required components. With the help of software (Meshmixer Software – version 3.5, Autodesk, Inc., USA), a digital model analysis and the integrated setting up of teeth in pre-decided occlusal schemes (BBO, LO, MO) were done. For each scheme, commercially available cross-linked teeth (Basic Kulzer Denture teeth 33 ${}^{\circ}$ and 0 ${}^{\circ}$ , Kulzer GmbH, Hanau, Germany) were customized individually (following the guidelines of each scheme) [25, 26, 27, 28, 29, 30, 31] and scanned using an intraoral scanner (TRIOS 3; 3Shape, Inc., USA) and stored in STL format. These teeth STL files were used during designing and teeth set-up for CDs. The excursions of the mandible were simulated in the virtual articulator. This allowed any interfering areas of the teeth in protrusion, laterotrusion and retrusion to be visualised and corrected according to the occlusal scheme. The dynamic occlusion was adjusted by moving the teeth directly in the software. To meet particular scheme requirements the anterior and posterior teeth set-up was customised individually. The software adapted the denture teeth automatically to the base with a defined gap to the alveolar ridge.

Figure 2.

Schematic illustration of digital denture fabrication (CAD-CAM). (a) Scanned images of maxillary and mandibular master cast. (b) Occlusal rims with jaw relation record. (c) and (d) Virtual teeth arrangement with the help of CAD software. (e) and (f) CAM fabrication of complete dentures by subtractive method. (g) and (h) CAM fabrication of complete dentures by additive method. (i) Representative completed denture.

Using these designed CDs by CAD software (Fig. 2a–d), the samples for SCD Gr were milled. The denture bases were milled from highly cross-linked, industrially polymerized, monomer-free PMMA (Polymethyl-methacrylate) blank (Vipi block CAD/CAM Blank Monolayer A2, Hight 16 mm, São Paulo, Brazil) and the teeth were milled on commercially available teeth blanks (Vipi block CAD/CAM Blank Monolayer A2, Height 16 mm) using the milling machine. Following which the milled teeth were attached to the recess of milled denture bases by using fast polymerizing acrylic resin (Jet Repair; Lang Dental Mfg Co, Inc, USA) following the manufacturer’s instructions (Fig. 2e and f)

Similarly, using same STL files of designed CDs, samples for ACD Gr were printed. For ACD Gr, the designed CDs STL files were transferred to a software (Asiga Composer Software) for print preparation and then sent to a 3D printer with Digital Light Processing technology (Asiga, 3D printer Max series, Alexandria, Australia) with build volume of 119 $\times$ 67 $\times$ 75 mm. The denture bases (Asiga Denta Base) and denture teeth (Asiga Denat Tooth) were fabricated by printing a biocompatible photopolymer with a layering thickness of 80 $\mu$ m per layer and the maximum laser speed of 5000 mm/s with the position and quantity of supports being designed with the help of software, avoiding the cup areas and all support pins located on CD tissue surface area. After printing, the complete dentures were separated from the build platform, and the supports were removed. The printed denture bases and teeth were washed using Isopropyl alcohol, dried and post-cured in Asiga Flash for 20 minutes (as recommended by the manufacturer). Then the teeth were placed into denture base and were bonded with each other using light cure bonding agent at room temperature (Fig. 2g–i).

Once all samples of each group were fabricated, occlusal force analysis was done for each sample using computerized occlusal analysis system: T-Scan III (Tekscan Inc. South Boston, MA, USA). The T-Scan III system can be used to evaluate the pre-adjusted occlusal force imbalance and the contact timing characteristics of the complete dentures at insertion. In the present study, the occlusal force parameters of CDs were assessed immediately after processing without laboratory remount and finishing or polishing.

Figure 3.

Placement of T-Scan III handpiece between the dentures on the mounted semi-adjustable articulator for recording occlusal parameters. (a) Without load. (b) With 30 N loads. (c) 1 kilogram (kg) weight $\sim$ 9.80665 newtons (N).

Figure 4.

Representative image of T-Scan III desktop data of a complete denture sample illustrating. (a) Percentage of forces on left and right side (46% left–54% right), 3-D column view. (b) 2-D contour view – housing center of force (COF) trajectory, summation diamond-shaped icon and COF ellipse inside. (c) Zoom graph. (d) Force versus time graph with black curving line showing percentage of maximum occlusal force (99.5%).

To record the occlusal forces of CDs using T-Scan III, initially the vertical pin of the articulator (Whipmix 2000 Serise-2240) was removed, and CD samples were placed on the mounted articulator one by one groupwise. Then the examiner inserted the handpiece of T-Scan III with a flexible sensor sheet and the device software was activated and calibrated to meet high sensitivity parameter. The force levels were recorded for 10 seconds starting with zero loads and ending with the maximum load [30 newtons (N) $\sim$ 3 kilograms (kg) comparable to maximum occlusal load applied by a CDs wearer] [39] (Fig. 3a–c). The T-Scan III provided the percentage of occlusal force applied on the right and left sides of the arch producing a quantitative analysis of the distribution of occlusal contacts (% of force on right and left side). Using this feature, the amount of percentage of occlusal force of CDs of different groups were observed (maximum occlusal force %). To verify the possible changes, the percentage of the right side of the arch was selected considering that a possible reduction in percentage would reflect an increase on the contralateral side, because they were complementary (the total of the sides results in 100%). The recordings were made three times, and the average was calculated (Fig. 4). All records were made by a single calibrated operator who was trained to use the equipment. The data so obtained was tabulated under headings of percentage of maximum bite fore, % of force on right and left side and difference between them and position of diamond icon inside or outside the eclipse, coded as 1 and 2 respectively and statistical analysis was done using statistical analysis software (IBM SPSS Statistics for Windows v22; IBM Corp., Armonk, NY, USA). The results were analysed using descriptive statistics (mean and SD) and intergroup comparisons were performed. Univariate regression analysis was used to find the effects of the technique of fabrication and occlusion scheme over the occlusal parameters. 95% confidence limits were applied to see the significantly higher or lower values of occlusal forces for various pairs of technique of fabrication and occlusion schemes. Logistic regression analysis was used to show the relationship of centre of force location with various techniques of fabrication and occlusion schemes. The $p$ -value was taken significant when less than 0.05 ( $p<$ 0.05) and confidence interval of 95% was taken.

Figure 5.

(a) Intergroup comparison of right and left forces techniques of fabrication and occlusion schemes. (b) Intergroup comparison of force differences (right-left) for various techniques of fabrication and occlusion schemes. (c) Intergroup comparison of maximum force for various techniques of fabrication and occlusion schemes. (d) Intergroup comparison of center of force for various techniques of fabrication and occlusion schemes.

Figure 5.

Continued.

3. Results

The intergroup comparison of right and left side forces in CDs fabricated by various techniques and with different occlusion schemes showed insignificant differences ( $p>$ 0.05). Furthermore, no significant interaction effect was found between right and left forces ( $p>$ 0.05), though the effect of occlusion scheme was more than the technique of fabrication (according to effect size estimation) (Fig. 5a and Table 1).

Table 1
Intergroup comparison of right and left forces for various techniques of fabrication and occlusion schemes (univariate regression analysis)

In vitro	Force – right			Force – left
	$F$ -value	$p$ -value	Effect size	$F$ -value	$p$ -value	Effect size
Intercept	1197.96	$<$ 0.001	0.971	1112.35	0.000	0.969
Technique of fabrication (T)	0.15	0.863	0.008	0.15	0.863	0.008
Occlusal scheme (OS)	1.25	0.299	0.065	1.25	0.299	0.065
T+ OS- interaction	0.19	0.942	0.021	0.19	0.942	0.021

Table 2

Intergroup comparison of force differences (right-left) for various techniques of fabrication and occlusion schemes – statistics

In vitro	Force – difference
	$F$ -value	$p$ -value	Partial eta squared
Intercept	167.85	$<$ 0.001	0.823
Technique of fabrication (T)	7.53	0.002	0.295
Occlusal scheme (OS)	21.62	$<$ 0.001	0.546
T+ OS- interaction	2.32	0.076	0.205

Table 3

Intergroup comparison of maximum force for various techniques of fabrication and occlusion schemes

In vitro	Force – maximum
	$F$ -value	$p$ -value	Effect size
Intercept	310028.78	$<$ 0.001	1.000
Technique of fabrication (T)	120.70	$<$ 0.001	0.870
Occlusal scheme (OS)	558.39	$<$ 0.001	0.969
T+ OS- interaction	3.10	0.027	0.256

Table 4

Logistic regression analysis to show the relationship of center of force condition with various techniques of fabrication and occlusion schemes

Dependent center of force
Explanatory variable	B	SE	$p$ -value	OR
Technique			0.300
CCD	0.927	0.803	0.248	2.53
SCD	$-$ 0.285	0.756	0.706	0.75
ACD	Ref.
Occlusal scheme			0.300
BBO	$-$ 1.212	0.805	0.132	0.298
LO	$-$ 0.927	0.803	0.248	0.396
MO
Constant	0.862	0.731	0.238	2.367

${}^{*}$ B-Beta coefficient, SE-Standard error, OR-Odds ratio, CCD-Conventional Complete denture, SCD-Subtractive Complete denture, and ACD-Additive Complete denture, BBO-Bilateral balanced occlusion, LO-Lingualized occlusion, MO-Mono plane occlusion.

The intergroup comparison of right-left side force difference revealed statistically significant differences ( $p<$ 0.01). But no significant interaction effect was present in force-difference ( $p>$ 0.05), though the effect of occlusion scheme was more than the technique of fabrication (according to effect size estimation). Furthermore, the maximum force difference was observed in combination of CCD technique and MO scheme (36.88 $\pm$ 2.82 N). The force differences for the pairs CCD-MO, CCD overall and MO overall were significantly more than other pairs (these were lying above 95% UCL) while the force differences for the pairs CCD-BBO, SCD-BBO, SCD-LO, SCD overall, ACD-BBO, BBO overall were significantly less than other pairs (these were lying below 95% LCL) (Fig. 5b, Table 2).

The intergroup comparison of maximum bite force showed highly significant differences according to the technique of fabrication ( $p<$ 0.001), occlusion schemes ( $p<$ 0.001) and their interaction effect ( $p=$ 0.027). The effect of occlusion scheme was slightly more than the technique of fabrication (according to effect size estimation). The maximum of maximum bite force was observed for SCD technique (89.14 $\pm$ 6.08 N) and LO scheme (92.17 $\pm$ 3.22 N) (Fig. 5c, Table 3). The odds ratio (OR) explained that in comparison to ACD, the chances of centre of force out of ellipse was more in CCS (OR $=$ 2.53) and less in SCD (OR $=$ 0.75) techniques and in comparison to MO, the chances of out of ellipse was less in BBO (OR $=$ 0.298) and LO (OR $=$ 0.396) schemes, though these chances were not statistically significant ( $p>$ 0.05) (Fig. 5d, Table 4).

4. Discussion

The CAD-CAM technology has changed the way dentistry works. The wide variety of materials compatible with this technology has made it available to use it even in complete dentures. Previous studies have evaluated digital CDs trueness [11, 34], patient and dentist satisfaction [9], retention [12], ease of fabrication [3, 8, 9], material properties and even mechanical behaviour [13, 21, 34], but occlusion was not much reviewed, which without a doubt should be highly precise for successful digital CDs. Thus, the present study was conducted to compare the occlusal forces generated by conventionally and digitally (by additive and subtractive techniques) made CDs with different BBO, LO, MO occlusal schemes, using computerized occlusal force analysis system (T-Scan III). The results of our study rejected the null hypothesis, as there was better occlusal force equalisation and loading in CDs made by subtractive technique compared to additive having BBO and LO occlusal schemes compared to MO and with any occlusal schemes in CCDs.

In our study, digital CDs emerged as superior over CCDs in terms of occlusal parameters similar to previous studies reporting improved denture retention and fit [12], better mechanical and surface properties of materials, higher patient satisfaction [9], better clinician acceptance [9], and reduced procedure time and costs [40] in digital dentures. In addition, there is reduction in clinical and laboratory time which is of most importance to dentists and patient [40]. The CDs that provide esthetics with efficient function are considered successful. The functional aspect is mostly affected by the occlusal parameters thus in this study we assessed the center of force, percentage of force on right and left side of arch, difference between them and total maximum force percentage generated in complete occlusion, this was evaluated immediately after processing so as to avoid manual intervention or subsequent finishing and polishing procedural effect on CDs.

The occlusal parameters were analysed digitally using computerised method viz. T-Scan III. It is considered as suitable, precise and reliable for the occlusal analysis. The T-Scan III system proved to be beneficial as it is a rapid and accurate system in identifying the distribution of the loads, regions of excessive force the uneven force’s concentration and occlusal force summation which would not be possible with the conventional way of occlusal assessment like articulating paper [32, 33].

Occlusal harmony is important for CDs comfort. The most commonly accepted and reported occlusal schemes (BBO, LO, MO) for CCDs were included in this study and evaluated for digital dentures. Frequently BBO has been recommended for CCDs and LO as occlusion of choice for patients with severe ridge resorption [30, 31]. Greater chewing ability has been reported for patients with LO or BBO schemes compared to MO. LO resulted in significantly lower pain score than the MO and patients reported fewer eating problems with BBO compared to MO [41]. Meanwhile while there were no differences between LO and BBO. In a crossover randomized clinical trial, Clough et al. [42] found that LO was superior to monoplane occlusion in terms of chewing ability and patient comfort. Patients reported greater denture retention satisfaction with LO than with BBO [24, 25, 26, 27, 28, 29, 30, 31]. Similar to previous clinical studies [25, 26, 27, 28, 29, 30, 31, 41, 42], in the present in vitro study, in CDs with BBO and LO schemes occlusal force was distributed more evenly than the MO and also there was better centralization of force with these schemes in each type CDs. This was evident by significant differences noted in intra and inter group comparison of tested parameters (Center of force, percentage of force on right and left side of arch) of these occlusal schemes in CCD, SCD and ACD groups. The total maximum force percentage of LO (92.17 $\pm$ 3.22) was maximum followed by BBO (88.86 $\pm$ 2.94) and least for MO (79.82 $\pm$ 1.93), this was in association with study by Khamis et al. [41] who found patient preference for the BBO and LO as opposed to monoplane occlusion by their improved ability to chew hard foods with no differences between these two (BBO, LO). Similar conclusions were reached for better masticatory efficiency for LO and BBO than monoplane occlusion [30, 31]. However, Kydd [43] found no differences between complete dentures when assessed for masticatory efficiency for these three occlusal schemes. These results depict that LO and BBO occlusal schemes would provide better ability to crush the food, masticatory efficiency, retention with uniform distribution of forces and overall better patient satisfaction with digital dentures compared to CCDs, as all values of tested parameters were significantly higher in SCD and ACD groups respectively.

Data analysis revealed the effect of fabrication techniques on occlusal parameters. The inter group comparison showed clearly that SCD group had better Centralisation of forces, equal percentage of force on right and left side of arch and higher total maximum force percentage than ACD and CCD groups.

The reasons for it can be ascertained by the facts that in SCD group (milling technique), the complete dentures were fabricated at a milling station using a pre-polymerized PMMA blanks manufactured under high pressure, which resolved the problem of uncontrolled shrinkage during polymerization. Highly defined computerized controlled milling of recesses in the denture bases and corresponding tooth would had resulted in accurate fitting and bonding of teeth with base thus, maintaining the occlusal scheme in a precise way [3, 4, 5, 6, 7, 8, 9, 10, 11]. Likewise, in ACD group (3-D printing technique) fabrication of CDs takes place from a photosensitive liquid resin, repetitively layered on a support technique structure and polymerized by an ultraviolet or a visible light source [5, 8, 14, 18, 19, 34]. Later teeth and bases were bonded with each other in a precise manner. Thus, occlusion in both techniques was not subject to polymerization shrinkage either, thus it might had retained adjusted occlusal schemes in better way. On the contrary in CCD group, because of presence of inherent drawbacks of the technique like shrinkage during polymerization, release of stress of modelling waxes, high residual monomer content, packing pressure and uncontrolled expansion of dental plaster resulted in movement of the teeth [36, 37, 38], thus altering the established occlusal scheme which was clearly evident by the results of the present study. This was even proved by the fact that most authors recommend the laboratory remount for CCDs for removing the processing errors [44, 45].

The SCD group had better values than ACD group which was in association to another in vitro study by Kalberer et al. [34] that demonstrated the trueness of the CAD-CAM milled complete dentures was statistically better than that of the rapid prototyped complete dentures. These differences between SCD than ACD groups could be accounted to the uses of unpolymerized resins for manufacturing complete dentures in 3-D printing, and once processed, it requires an additional final light-polymerization step to complete the process. During the 3-D printing, polymerization shrinkage is theoretically possible, as complete dentures were not completely polymerized before the final light-polymerization procedure, and also a deformation of the prostheses could occur when demounting the partially polymerized CDs from the build platform, but still it would be very minimal compared to CCDs as verified in the results [8, 14, 18, 19].

Although both techniques use CAD files to manufacture the CDs, they are completely different in fabrication methods. These techniques have been considered as superior over conventional methods for CDs fabrication and same has been proved in the present in vitro study and also reported by Kattadiyil et al. [9]. Presently the commercial market is captured by the CDs fabricated by subtractive technique but surely in coming years additive CDs would be a better alternative to SCD owing to its obvious benefit viz. less material wastage, low infrastructure costs; comparable accuracy, biocompatibility, mechanical properties and patient satisfaction over milled techniques, however, further studies are needed to quantify these findings clinically and objectively with regard to CDs.

4.1 Limitation

This was an in vitro study with static occlusion assessment on articulator, so effect of recorded occlusal discrepancy in the study needs clinical assessment. In the future, patient-centred research is needed on the dynamic occlusion taking into account the resiliency of mucosa, effect of stomatognathic system various occlusal parameters and/or schemes to determine the superiority of one technique over the other with regard to fabrication of CDs by CAD-CAM techniques. Also, the effect of different machines and different available software were not assessed in this study.

5. Conclusion

Within the limitations of this study, it can be concluded that dentures made by additive or subtractive technique are timesaving and retain the designed occlusion better compared to the conventional dentures. No evident differences were found in the center of force and occlusal force summation of CDs made by digital techniques. The occlusal force equalization can be achieved better in BBO and total COF occlusal force summation is better in LO, than MO irrespective of type of CDs fabrication methods T-Scan III helps in visualization of occlusal discrepancies and later it can also help in its adjustment during denture insertion.

Footnotes

Acknowledgments

The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through General Research Project under grant number (G.R.P.-125-41). The authors are also very grateful to the Dentaly Dental Clinic and Research Center and Advanced Dental Lab Abha, KSA for CAD/CAM support.

Conflict of interest

None to report.

References

Baba

AlRumaih

Goodacre

. Current techniques in CAD/CAM denture fabrication. Gen Dent 2016; 64: 23-8.

Goodacre

Garbacea

Naylor

Daher

Marchack

Lowry

. CAD/CAM fabricated complete dentures: concepts and clinical methods of obtaining required morphological data. J Prosthet Dent 2012; 107: 34-46.

Maeda

Minoura

Tsutsumi

Okada

Nokubi

. A CAD/CAM system for removable denture. Part I: fabrication of complete dentures. Int J Prosthodont 1994; 7: 17-21.

Kawahata

Ono

Nishi

Hamano

Nagaoka

. Trial of duplication procedure for complete dentures by CAD/CAM. J Oral Rehabil 1997; 24: 540-8.

Sun

Luü

Wang

. Study on CAD & RP for removable complete denture. Comput Methods Programs Biomed 2009; 93: 266-72.

Kanazawa

Inokoshi

Minakuchi

Ohbayashi

. Trial of a CAD/CAM system for fabricating complete dentures. Dent Mater J 2011; 30: 93-6.

Inokoshi

Kanazawa

Minakuchi

. Evaluation of a complete denture trial method applying rapid prototyping. Dent Mater J 2012; 31: 40-6.

Schwindling

Stober

. A comparison of two digital techniques for the fabrication of complete removable dental prostheses: a pilot clinical study. J Prosthet Dent 2016; 116: 756-63.

Kattadiyil

Jekki

Goodacre

Baba

. Comparison of treatment outcomes in digital and conventional complete removable dental prosthesis fabrications in a predoctoral setting. J Prosthet Dent 2015; 114: 818-25.

10.

Bidra

Farrell

Burnham

Dhingra

Taylor

Kuo

. Prospective cohort pilot study of 2-visit CAD/CAM monolithic complete dentures and implant-retained overdentures: clinical and patient-centered outcomes. J Prosthet Dent 2016; 115: 578-86.

11.

Srinivasan

Cantin

Mehl

Gjengedal

Müller

Schimmel

. CAD/CAM milled removable complete dentures: an in vitro evaluation of trueness. Clin Oral Investig 2017; 21: 2007-19.

12.

Steinmassl

Dumfahrt

Grunert

Steinmassl

. CAD/CAM produces dentures with improved fit. Clin Oral Investig 2018; 22: 2829-35.

13.

Steinmassl

Wiedemair

Huck

Klaunzer

Steinmassl

Grunert

, et al. Do CAD/CAM dentures really release less monomer than conventional dentures? Clin Oral Investig 2017; 21: 1697-705.

14.

Chaturvedi

Alqahtani

Addas

Alfarsi

. Marginal and internal fit of provisional crowns fabricated using 3D printing technology [published online ahead of print, 2020 Apr 10]. Technol Health Care. 2020. doi: 10.3233/THC-191964.

15.

Abduo

Lyons

Bennamoun

. Trends in computer-aided manufacturing in prosthodontics: a review of the available streams. Int J Dent 2014; 78: 39-48.

16.

Keating

Knox

Bibb

Zhurov

. A comparison of plaster, digital and reconstructed study model accuracy. J Orthod 2008; 35: 191-201.

17.

Van Noort

. The future of dental devices is digital. Dent Mater 2012; 28: 3-12.

18.

Sun

Zhang

. The application of rapid prototyping in prosthodontics. J Prosthodont 2012; 21: 641-4.

19.

Liu

Leu

Schmitt

. Rapid prototyping in dentistry: technology and application. Int J Adv Manuf Technol 2006; 29: 317-35.

20.

Pereyra

Marano

Subramanian

Quek

Leff

. Comparison of patient satisfaction in the fabrication of conventional dentures vs. DENTCA (CAD/CAM) dentures: a case report. J N J Dent Assoc 2015; 86: 26-33.

21.

Ucar

Akova

Aysan

. Mechanical properties of polyamide versus different PMMA denture base materials. J Prosthodont 2012; 21: 173-6.

22.

Pero

Scavassin

Policastro

, et al. Masticatory function in complete denture wearers varying degree of mandibular bone resorption and occlusion concept: canine-guided occlusion versus bilateral balanced occlusion in a cross-over trial. J Prosthodont Res 2019; 63: 421-7.

23.

Wang

Shi

Xie

. Accuracy of digital complete dentures: A systematic review of in vitro studies [published online ahead of print, 2020 Feb 27]. J Prosthet Dent. 2020. S0022-3913(20)30047-0.

24.

Abduo

. Occlusal schemes for complete dentures: a systematic review Int. J Prosthodont 2013; 26: 26-33.

25.

Rangarajan

Gajapathi

Yogesh

Ibrahim

Kumar

Karthik

. Concepts of occlusion in prosthodontics: a literature review, part I. J Indian Prosthodont Soc 2015; 15: 200.

26.

Rangarajan

Yogesh

Gajapathi

Ibrahim

Kumar

Karthik

. Concepts of occlusion in prosthodontics: a literature review, part II. J Indian Prosthodont Soc 2016; 16: 8.

27.

Żmudzki

Chladek

Kasperski

. Biomechanical factors related to occlusal load transfer in removable complete dentures. Biomech Model Mechanobi 2015; 14: 679-91.

28.

Farias-Neto

Carreiro Ada

. Complete denture occlusion: an evidence-based approach. J Prosthodont 2013; 22: 94-7.

29.

Zarb

Hobkirk

Eckert

Jacob

. Prosthodontic Treatment for Edentulous Patients-E-Book: Complete Dentures and Implant-Supported Prostheses: Elsevier Health Sciences; 2013 (ed 12). Mosby: St. Louis. p. 184, 225–6.

30.

Phoenix

Engelmeier

. Lingualized occlusion revisited. J Prosthet Dent 2010; 104: 342-6.

31.

Kimoto

Gunji

Yamakawa

, et al. Prospective clinical trial comparing lingualized occlusion to bilateral balanced occlusion in complete dentures: a pilot study. Int J Prosthodont 2006; 19: 103-9.

32.

Dias

RAB

Rodrigues

MJP

Messias

Guerra

FADA

Manfredini

. Comparison between conventional and computerised methods in the assessment of an occlusal scheme. J Oral Rehabil 2020; 47: 221-8.

33.

Kerstein

Thumati

Padmaja

. Force finishing and centering to balance a removable complete denture prosthesis using the T-Scan III computerized occlusal analysis system. J Indian Prosthodont Soc 2013; 13: 184-8.

34.

Kalberer

Mehl

Schimmel

Müller

Srinivasan

. CAD-CAM milled versus rapidly prototyped (3D-printed) complete dentures: an in vitro evaluation of trueness. J Prosthet Dent 2019; 121: 637-43.

35.

McGarry

Nimmo

Skiba

Ahlstrom

Smith

Koumjian

. Classification system for complete edentulism. J Prosthodont 1999; 8: 27-39.

36.

Mahler

. Inarticulation of complete dentures processed by the compression molding technique. J Prosthet Dent 1951; 5: 551-9.

37.

Atkinson

Grant

. An investigation into tooth movement during the packing and polymerizing of acrylic resin denture base materials. Aust Dent J 1962; 7: 101-8.

38.

Perlowski

. Investment changes during flasking as a factor of complete denture malocclusion. J Prosthet Dent 1953; 3: 497-9.

39.

Shala

Tmava

Dula

, et al. Evaluation of maximum bite force in patients with complete dentures. Open Access Maced J Med Sci 2018; 6: 559-63.

40.

Srinivasan

Schimmel

Naharro

Neill

McKenna

Muller

. CAD/CAM milled removable complete dentures: time and cost estimation study. J Dent 2019; 80: 75-9.

41.

Khamis

Zaki

Rudy

. A comparison of the effect of different occlusal forms in mandibular implant overdentures. J Prosthet Dent 1998; 79: 422-9.

42.

Clough

Knodle

Leeper

, et al. A comparison of lingualized occlusion and monoplane occlusion in complete dentures. J Prosthet Dent 1983; 50: 176-9.

43.

Kydd

. The comminuting efficiency of varied occlusal tooth form and the associated deformation of the complete denture base. J Am Dent Assoc 1960; 61: 465-71.

44.

Badel

Ćeleć

Kraljević

Pandurić

Dulčić

. Complete denture remounting. Acta Stomat Croat 2001; 35: 381-7.

45.

Shigli

Angadi

Hegde

. The effect of remount procedures on patient comfort for complete denture treatment. J Prosthet Dent 2008; 99: 66-72.

Computerized occlusal forces analysis in complete dentures fabricated by additive and subtractive techniques

Abstract

BACKGROUND:

OBJECTIVES:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Materials and methods

Table 1 Intergroup comparison of right and left forces for various techniques of fabrication and occlusion schemes (univariate regression analysis)

4.1 Limitation

5. Conclusion

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Intergroup comparison of right and left forces for various techniques of fabrication and occlusion schemes (univariate regression analysis)