Clinical analysis of CAD-CAM milled and printed complete dentures using computerized occlusal force analyser

Abstract

BACKGROUND:

Digital complete dentures (CDs) by computer-aided designing and computer-aided manufacturing (CAD-CAM) techniques (milling and three-dimensional (3-D) printing) have been evaluated clinically and provided satisfactory results. But clinical studies assessing occlusal forces by digital dentures are lacking.

OBJECTIVES:

To compare the occlusal force parameters in complete dentures (CDs) fabricated by milling, 3-D printing and conventional techniques having 3 commonly used occlusal schemes, using computerized occlusal force analysis system (Tech-Scan III- T-Scan III).

METHODS:

A total of 45 CDs were fabricated for 5 patients. Nine sets of CDs were made for each patient and were divided into 3 groups: Conventional CDs (CCD), Milled CDs (MCD), and 3-D printed CDs (3-DP CD). The CDs in each group were further divided into 3 sub-groups based on occlusion schemes – bilateral balanced (BBO), lingualized (LO) and mono plane (MP). Occlusal force analysis [percentage (%) of occlusal force applied on the right and left sides of the arch difference between them, centralization of forces and % of maximum occlusal/bite force] was done using computerized occlusal analysis system (T-Scan III) at the time of denture insertion. Univariate regression analysis and logistic regression analysis were performed ( $p<$ 0.05).

RESULTS:

The intergroup comparison of force distribution on right and left side in CDs fabricated by various techniques showed insignificant differences ( $p>$ 0.05) but statistically significant differences ( $p<$ 0.01) were found in right-left side force difference, maximum bite force % and centralization of forces. The maximum force difference on right and left side was observed CCD with MO (37.48 $\pm$ 1.03 N) and maximum occlusal-bite force % was observed for 3-DPCD with LO (95.40 $\pm$ 1.30 N). In comparison to 3-DP CD, the chances of centre of force out of ellipse (centralization of forces) was 3.36 and 2.15 times more in CCD and MCD techniques made CDs respectively.

CONCLUSIONS:

The occlusal parameters in CDs were affected by the fabrication techniques and occlusal schemes of CDs. The digital CDs retain adjusted occlusal schemes better and 3-DP CDs with BBO and LO occlusal schemes provided centralization of forces, better distribution and high maximum occlusal force % respectively.

Keywords

Digital dentures T-Scan occlusal forces subtractive technique additive technique milling CAD-CAM

1. Introduction

Computer-aided design/computer-assisted manufacture (CAD/CAM) has evolved as a game changer in almost every aspect of life. In dentistry, it was first introduced in 1970s by Duret and Preston [1]. Initially, its use was limited to fixed dental prosthesis but with expanding horizons of digital dentistry it is now frequently used in complete dentures (CDs) fabrication [2, 3]. Steps involved in fabrication of these digital CDs include: Acquisition of data (scanning), computer-aided design (CAD), and computer-assisted manufacture (CAM). The CAM part basically has two methods: Milling (Subtractive method) and three-dimensional printing (3-DP) (Additive method) [4, 5]. In the milling method, resin blocks are pre polymerized and minimal monomer is released, while in 3-DP method, polymerization of photosensitive liquid polymer layers (usually 5 to 20 per milli-meter of material in a layer) with ultraviolet light beam is done to produce definitive CD [6, 7]. These CDs have potential benefits over CDs fabricated by conventional methods: standardized fabrication techniques, high accuracy, high-value, quality-controlled materials, with superior mechanical properties, biocompatibility, smooth surface finish, fewer clinical appointments, similar or better fit of the tissue surfaces [2, 3, 4, 5, 6, 7, 8, 9, 10], overall improved cost-effectiveness [6] and enhanced patient satisfaction [6, 11].

In the present dental market, milling method is more popular than 3-DP, but soon this might reverse as subtractive method has inherent limitation of machining tools resulting in wastage of a large amount of denture base material [6, 7, 10]. In addition, 3-DP could be more 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 [4, 5].

Various aspects of the digital and conventional CDs have been evaluated and reported in previous in vivo and in vitro studies viz. retention [9], trueness [4, 8], patient and dentist satisfaction [6, 11], ease of fabrication [4, 6], material properties and even mechanical behaviour [4, 10, 12] but occlusion was not much reviewed, which in no doubt should be highly precise for successful digital CDs. The presence of deflective occlusal contacts, poor occlusal force summation and unseen collection of unbalanced forces might result in torque and dislodgement of complete dentures affecting stability, comfort and patient acceptance [13, 14, 15].

Several occlusal scheme has been recommended for CDs mainly bilateral balanced occlusion (BBO), lingualized occlusion (LO) and neutral-centric or mono plane occlusion (MO). These schemes had been extensively assessed for conventional dentures for mastication, stability, esthetics, satisfaction etc. [16, 17, 18, 19, 20]. However, no clinical study has been performed till date for occlusal force analysis in digital dentures, which is essential as there is variation in the techniques of fabrication, use of modified varied materials and most of the time there are differences in the mode of teeth attachment to denture bases of digital dentures. Thus, this in vivo study was conducted to compare the occlusal force parameters (maximum bite force %, % of force on right and left side, difference between forces on right and left side and centralisation of forces) in CDs fabricated by milling, 3-DP and conventional techniques having 3 commonly used occlusal schemes (BBO, LO, MO), using computerized occlusal force analysis system (T-Scan III). The null hypothesis formulated was that there would be no difference in the occlusal forces parameters of CDs fabricated by either technique with any occlusal schemes inconsideration.

A computerized occlusal analysis system such as the T-Scan has enormous benefits over the articulating paper. It provides an intuitive graphic that identifies the position of contacts and the corresponding occlusal pressure intensity in percentage and shows distribution of the force bilaterally in the arch with the help of digital sensors. It also helps in recording the first contact, maximum intercuspation, applied maximum force moment, last contact, etc., against the time. This digital way of quantitative estimation of occlusal parameters makes T-Scan III a very effective tool in research and clinical practice [21, 22, 23, 24].

2. Materials and methods

The present cross-sectional study was conducted in 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.

The participants were recruited from the outpatient department, reporting for the complete denture prosthetic treatment. All participants were explained about the study and written consent was obtained before inclusion in the study. Participants were screened by a single examiner with the following inclusion criteria: good neuromuscular control, high well 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 [25] and completely edentulous for at least 3 months. Exclusion criteria were uncontrolled systemic disease, mental problems, muscular dystrophy and temporomandibular disorders and any history of fractured jaw.

Total 45 CDs were fabricated in the study for 5 patients. For each patient, 9 sets of CDs were fabricated and were divided into 3 groups based on method of fabrication of CDs: Conventional techniques CDs (CCD), Milled/Subtractive CDs (MCD), and 3-D printed/Additive CDs (3-DP CD) techniques. Each Group CDs were further divided into 3 sub groups based on occlusion schemes-bilateral balanced occlusion (BBO), lingualized occlusion (LO) and mono plane occlusion (MO). During fabrication of CDs clinical steps were performed by chief researcher of the study and laboratory work was done by a single technician (blinded from the study) under the supervision of chief researcher. All researchers and involved technicians were trained and calibrated for conventional and digital CDs. The lottery system was used to decide which type of CDs with which type of occlusion had to be fabricated first for the patient

2.1 Fabrication of CDs

2.1.1 Conventional CDs (CCD) techniques

The conventional CDs were fabricated following the routine procedure. Since for each patient 3 CCDs had to made in 3 different occlusal schemes so, master casts of each patient were duplicated using duplicating material and secondary casts were poured with type IV die stone (GC Fujirock EP; GC Europe), to be used in further steps. Clinical steps involved were- primary impressions border molding with definitive impressions jaw relations using face bow (Hanau Spring-Bow; Whipmix, Louisville, KY, USA) and inter-occlusal bite record by polyether bite registration material (RAMITEC, 3M ESPE, St. Paul, MN, USA) teeth selection, wax trial placement, denture adjustment (expect occlusal surface adjustments) and insertion. Laboratory steps were-master cast and special tray fabrication, mounting on a semi-adjustable articulator (Whipmix 2000 Serise-2240, Louisville, KY, USA), teeth arrangement into predecided occlusal schemes (by single technician with the help of jig) and fabrication of CDs with conventional compression molding technique, lost wax technique and long polymerization 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) using heat-polymerizing acrylic resin (Dentsply Trubyte International, Inc., New York, PA, USA). The heat-polymerized CDs were finished and polished before insertion [26]. During teeth arrangement the recommended guidelines of each occlusal scheme was meticulously followed. The commercially available cross-linked teeth (Basic Kulzer Denture teeth 33 ${}^{\circ}$ and 0 ${}^{\circ}$ , Kulzer GmbH, Hanau, Germany) were modified to fit the particular occlusal scheme and occlusal adjustments were made both in the laboratory and in the clinical setting based on the following considerations: BBO: bilateral and simultaneous contact of posterior teeth in centric and eccentric positions. LO: articulation of the maxillary palatal cusps with the opposing mandibular occlusal surfaces while maxillary buccal cusps did not contact the mandibular teeth during centric or eccentric movements. MO: articulation of the maxillary and mandibular teeth with each other in a flat occlusal plane parallel to the mean denture base foundation. Thus, the teeth were not inclined to form compensatory curves in this scheme [16, 17, 18, 19]:

Milled CDs (MCD) and 3-D printed CDs (3-DP CD) techniques [2, 3, 4, 27, 28, 29] (Fig. 2).

Figure 2.

Schematic illustration of digital denture fabrication. (a) Scanned images of maxillary master cast. (b) Scanned images of mandibular master cast. (c) Occlusal rims with jaw relation record. (d) Virtual teeth arrangement with the help of CAD software. (e) CAM fabrication of complete dentures by 3-D printing method. (f) CAM fabrication of complete dentures by milling method. (g) and (h) Representative completed denture.

In the fabrication of digital CDs by milled or 3-D printing technique, the basic initial steps remained same i.e., acquisition of data (scanning) and computer-aided design (CAD). The difference was at manufacturing step. For data acquisition, the records obtained from the previously performed clinical steps for conventional techniques CDs (from primary impressions to jaw relation) were used and the secondary cast and occlusal rims with jaw relation record were scanned for digital dentures (if CCD was fabricated first, then same data was scanned before proceeding to next step of Conventional CD processing).

The procedure involved for scanning secondary casts, occlusal rims with jaw relation record was done by using desktop laboratory scanner (D800, 3Shape, Copenhagen, Denmark). This scanner had a high resolution and was calibrated to a precision of 8 $\mu$ m, with a manufacturer-specified nominal point spacing of 6 to 8 $\mu$ m and a repeatability of 10 $\mu$ m at an accuracy of 20 $\mu$ m. The modified teeth (commercially available cross-linked teeth; Basic Kulzer Denture teeth 33 ${}^{\circ}$ and 0 ${}^{\circ}$ , Kulzer GmbH, Hanau, Germany) for each occlusal scheme were scanned using an intraoral scanner (TRIOS 3; 3Shape, Copenhagen, Denmark). Then, virtual point cloud data of all scans were stored as a standard tessellation language (STL) format. These STL files were transferred to the software (Meshmixer Software – version 3.5, Autodesk, Inc., USA) and teeth arrangement was done digitally following the basic guidelines of each scheme. The excursions of the mandible was 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. The software adapted the denture teeth automatically to the base with a defined gap to the alveolar ridge. The esthetic try-in in digital dentures were not performed and consent was obtained from each patient. The complete teeth arrangement CAM files were subsequently transferred to the respective software for fabrication of MCDs and 3-DP CDs.

For MCD group, the advanced CAM software (3-Shape Dental System – Complete Restorative – Complete denture module, Copenhagen, Denmark) was used to mill the samples. The denture bases were milled from highly cross-linked, industrially polymerized, monomer-free PMMA (Polymethyl-methacrylate) blank (Vipi Block Gum CAD/CAM Blank, $\phi$ 98.5 $\times$ 25 mm, Dark Pink, Veined, São Paulo, Brazil) and the teeth were milled on commercially available teeth blanks (Vipi block CAD/CAM Blank Monolayer A2, Hight 16 mm, São Paulo, Brazil) using the milling machine. The teeth were milled as a single unit and 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.

Similarly, for 3-DP CD group, same STL files of designed CDs were used, to print the samples. The CAM STL files of designed CDs were transferred to a software (PreForm, Formlabs Inc., USA) for print preparation and then send them to a 3-D printer (Form 2 $\sim$ 3-D printer, Formlabs Inc., Somerville, MA, USA) with build platform of 14.5 $\times$ 14.5 cm. The denture bases (Formlabs Denture Base Resin, Dentca Inc., USA) and denture teeth (Formlabs Denture Teeth, Dentca Inc., USA) were fabricated by printing a biocompatible photopolymer with a layering thickness of 100 $\mu$ m per layer and the maximum laser speed of 5000 mm/s with the location and number of supports being designed by the software, avoiding the cusp areas and all support pins located on CD tissue surface area. After printing, the denture bases and teeth were separated from the build platform, and the supports were removed. The printed denture bases and teeth were washed using Form Wash (96% ethanol solution in an ultrasonic bath) and dried. Then the teeth were placed into denture base and post-cured in Form Cure followed by this teeth and denture base were bonded with each other (light-cured bonding agent; Dentca Inc., USA) at room temperature. After being post-cured, the CDs were sterilized using gamma-ray sterilization.

After fabrication of digital CDs, the finishing and polishing was carried-out but laboratory/clinical remount procedures were not performed either for CCD or MCD, 3-DP CD groups. At the denture insertion appointment, stability and retention, over-extension, lack of frenal relief, any areas of soft tissue pressure or hard tissue problems related to dentures were solved. No occlusal corrections were done, and patients were informed about possible occlusal discrepancies. After that occlusal force analysis was done for each patient using computerized occlusal analysis system – T-Scan III (Tekscan Inc., South Boston, MA, USA). T-Scan III system was used to evaluate the pre-adjusted occlusal force imbalance. In the present study, the occlusal force parameters of CDs were assessed immediately after processing without laboratory/clinical remount and after finishing of study the occlusal adjustments were performed following basic protocols of remounting to satisfy the patients and make CDs comfortable.

2.2 T-Scan registration [20, 21]

The chief researcher and one of the co-researchers were trained for T-Scan III use and calibrated before the start of study for data collection. After denture insertion patients were asked to sit in upright position and digital registration of occlusal contacts was performed with 100 $\mu$ m-thick sensors of the T-Scan III HD system and using the software v8.1 (Tekscan Inc., South Boston, MA, USA).

The T-Scan sensor was inserted between the occlusal surface of the denture teeth intraorally by resting the T-Scan sensor support within the facial central incisor embrasure of the maxillary denture’s central incisor teeth (Fig. 3). Then recording was activated and force levels were recorded starting with zero loads and ending with the maximum load. The patient was asked to firmly intercuspate into the sensor using their complete denture’s occlusion and hold their teeth together for 1–3 s once maximum intercuspation is reached. This procedure was repeated three times, considering the first two trials as accommodation attempts, only the last recording was used for statistical analysis. The T-Scan III provided the occlusal contact time-sequence from 1st tooth contact through to complete intercuspation and 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. Using this feature, the total amount of percentage of occlusal force of CDs of different groups were observed and centralization of forces were obtained by viewing the location of diamond icon inside or outside the eclipse.

Figure 3.

Placement of T-Scan III handpiece in the patient mouth between the dentures for recording occlusal parameters.

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%) (Fig. 4). The data so obtained was tabulated under headings of percentage of maximum bite fore, % of force on right and left side 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 effects of 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.

3. Results

The intergroup comparison of force distribution on right and left side in CDs fabricated by various techniques showed insignificant differences ( $p>$ 0.05) but with different occlusion schemes it was significant ( $p<$ 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 vivo	Force-right			Force-left
	F-value	$p$ -value	Effect size	F-value	$p$ -value	Effect size
Intercept	1171.75	0.000	0.970	915.43	0.000	0.962
Technique of fabrication (T)	0.09	0.913	0.005	0.09	0.917	0.005
Occlusal scheme (OS)	5.84	0.006	0.245	5.79	0.007	0.243
T+ OS-interaction	0.09	0.985	0.010	0.09	0.986	0.010

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 (66.1% left–33.9% right), 3-D column view; (b) 2-D contour view, housing center of force (COF) trajectory, summation diamond-shaped icon and COF ellipse, outside; (c) Zoom graph; (d) force versus time graph with black curving line showing percentage of maximum occlusal force (93.7%).

The intergroup comparison of right-left side force difference in CDs fabricated by various techniques ( $p=$ 0.009) and with different occlusion schemes ( $p<$ 0.01) revealed statistically significant differences. 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). The maximum force difference was observed in CDs made by CCD with MO scheme (37.48 $\pm$ 1.03 N). The force differences for CDs in CCD-MO, MCD-MO, 3-DP CD-MO and MO overall were significantly more than other pairs (these were lying above 95% UCL-upper control limit) while the force differences for CDs in CCD-BBO, MCD-BBO, 3-DP CD-BBO and BBO overall were significantly less than other pairs (these were lying below 95% LCL-lower control limit) (Fig. 5b, Table 2).

The intergroup comparison of maximum bite force % showed highly significant differences according to technique of fabrication, occlusion schemes and their interaction effect ( $p<$ 0.001). The effect of occlusion scheme was more than the technique of fabrication (according to effect size estimation). Overall and individually, the highest maximum-bite force % was observed for 3-DP CD technique and specifically with LO scheme (95.40 $\pm$ 1.30 N) (Fig. 5c, Table 3).

Table 2

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

In vivo	Force – difference
	F-value	$p$ -value	Partial eta squared
Intercept	367.25	0.000	0.911
Technique of fabrication (T)	5.42	0.009	0.231
Occlusal scheme (OS)	59.24	$<$ 0.001	0.767
T+ OS-interaction	1.58	0.201	0.149

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.

Table 3

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

In vivo	Force-maximum
	F-value	$p$ -value	Effect size
Intercept	371009.33	$<$ 0.001	1.000
Technique of fabrication (T)	41.40	$<$ 0.001	0.697
Occlusal scheme (OS)	387.82	$<$ 0.001	0.956
T+ OS-interaction	10.87	$<$ 0.001	0.547

Figure 5.

Continued, (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.

In comparison to 3-DP CD, the chances of centre of force out of ellipse was more in CCD (OR $=$ 3.36) and in MCD (OR $=$ 2.15) techniques but in comparison to MO, the chances of centre of force out of ellipse was less in BBO (OR $=$ 0.015) and LO (OR $=$ 0.186) schemes, though these chances were not statistically significant ( $p>$ 0.05) except between BBO and MO ( $p=$ 0.015) (Fig. 5d, Table 4).

Table 4

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

Center of force
In vivo	B	SE	$p$ -value	OR
Technique			0.412
CCD	1.212	0.934	0.195	3.360
MCD	0.766	0.889	0.389	2.150
3-DP CD	Ref.
Occlusal scheme			0.038
BBO	2.894	1.186	0.015	0.055
LO	1.680	1.206	0.163	0.186
MO	Ref.
Constant	2.090	1.105	0.059	8.084

*B-Beta coefficient, SE- Standard error, OR- Odds ratio, CCD- Conventional Complete denture, MCD- Milled Complete denture, 3-DP CD- 3-Dimentional Printing Complete denture, BBO- Bilateral balanced occlusion, LO- Lingualized occlusion, MO- Mono plane occlusion.

4. Discussion

In the present study, digital CDs were superior in each tested occlusal parameter compared to CCDs. Thus, the proposed null hypothesis was rejected. The outcomes of the study revealed statistically significant difference in mean maximum force %, in centralization of forces and also in difference between forces on right and left side of the arch between the dentures fabricated by various techniques having different occlusal schemes. This was in association with the different previous studies on CD occlusal trueness [14, 30, 31, 32, 33], reporting a discrepancy in digital CDs with a maximum value of 0.64 mm in Milled CDs, whereas in the conventional injection molding method the maximum value was 1 mm [32]. Tooth displacement is inevitable in conventional processing method of CDs fabrication as reported in many studies [14, 34, 35, 36]. An increase in occlusal vertical dimension up to 1 mm was considered acceptable in CDs as was reported in a study investigating the increase in the vertical dimension of occlusion (OVD) in CDs after processing with a conventional method [14, 37].

With the advancement in CAD-CAM technology , the frequency of use of digital CDs has increased, leading to the increase in number of studies evaluating various aspects of digital CDs retention [9], adaptation of digitally made denture bases (ranged between 0.058 mm [9, 14] and 0.29 mm [14] for digital methods and between 0.105 mm [9, 14] and 0.30 mm [14] for conventional technologies), trueness [8, 4], patient and dentist satisfaction [6], ease of fabrication [6], and material properties and even mechanical behaviour [4, 10, 12] but occlusion was not much reviewed. In the present study, occlusal force parameters of conventionally and digitally (by milling and 3-DP techniques) fabricated CDs with different BBO, LO, MO occlusal schemes, were evaluated using computerized occlusal force analysis system (T-Scan III). The T-Scan III system proved to be beneficial as it could accurately evaluate occlusal contact distribution (occlusal force %) in maximum intercuspation with sequence of occlusal time. The method was precise reliable and repeatable and had been used in previous studies. It provided a quantitative assessment of occlusal contacts pressure which would be impossible with the conventional methods of detecting occlusal discrepancies like articulating paper [21, 22, 23].

In the present study, we tested the centralisation of forces, % of force on right and left side, difference between forces on right and left side of the arch and total maximum force % generated in complete occlusion, this was evaluated immediately after processing so as to assess the effect of fabrication techniques of CDs and to avoided any manual intervention or subsequent procedural effect on CDs occlusion. The values of each tested parameters were better for digital CDs compared to CCDs and differences were statistically significant. The force distribution on right and left side of arch in CDs fabricated by various techniques showed insignificant differences ( $p>$ 0.05), the reasons may be the resiliency of the mucosa. It is well documented fact that the tolerance of mucosal tissue displacement under CDs is large because of the dynamic movement of the mucosa and had been reported specifically that compressive mucosal displacement would be in the range of 0.375 to 0.5 mm [38]. Satoshi et al. [39] described that after CD insertion, the denture base sinks almost 0.3 mm from the occlusal force resulting in the deformation of the oral mucosa, thus making the base and mucosa fit better and ensuring a good effect on border sealing. Because of this mucosal effect, the force distribution on right and left side of arch in CDs had not shown statistical differences in CDs fabricated with milled, 3-DP or conventional techniques. This aspect of result of present study was in contrast to previous studies reporting that the digital method of fabrication of CDs are more precise, although the properties tested were denture retention and fit [9], clinician acceptance and patient satisfaction [6], procedure time and costs and other mechanical and surface properties of materials [4, 10, 12].

On analysing, the tested occlusal schemes irrespective of CDs fabrication techniques, BBO and LO appeared to be better that MO. In, BBO the distribution of the occlusal forces on either side of the arch was better than LO and MO while the maximum force percentage of LO was more than BBO and MO, which was in complete association of several in vitro and in vivo studies on occlusion of CDs [13, 16, 17, 18, 19, 20, 21, 22]. Results showed that 3-DP CDs with BBO had better centralization and bilateral distribution of forces followed by MCDs with BBO, 3-DP CDs with LO MCDs with LO, CCD with BBO and CCD with LO schemes. While maximum bite force % of 3-DP CD was highest overall, followed by MCDs and CCDs and specifically with LO schemes. This difference may be attributed to the basic difference in the technique of milling and 3-D printing.

In the milling technique, the CDs were fabricated at a milling station using a pre-polymerized PMMA blanks manufactured under high pressure, consequently no shrinkage during polymerization unlike in processing of CDs in conventional technique. 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 [4, 5, 6, 7, 8, 9, 10, 11, 12, 14].

While 3-D printing technique fabrication of CDs takes place from a photosensitive liquid resin, repetitively layered on a support structure and polymerized by an ultraviolet or a visible light source [3, 4, 5, 14, 27, 28, 31]. Later teeth and bases were bonded with each other in a precise manner. Thus, occlusion in both of these techniques was not subject to polymerization shrinkage either, thus it might had retained adjusted occlusal scheme in a better way than CCD.

The conventional technique of CDs fabrication has inherent drawbacks of the technique itself like shrinkage during polymerization, release of stresses incorporated in the modelling waxes during manipulation, high content of residual monomer, high pressure of packing and uncontrolled expansion of dental plaster resulting in movement of the teeth [34, 35, 36, 37]. These procedural errors alter the established occlusal scheme and the same was clearly evident by the results of the present study.

In our clinical study, 3-DP CD group had better occlusal parameter values than MCD group which was in contrast to an in vitro study by Kalberer et al. [4] that demonstrated the trueness of the CAD-CAM milled complete dentures was statistically better than that of the rapidly prototyped complete dentures. These differences in occlusal parameter values might be attributed to the milling of teeth and denture base separately for MCD group and later bonding together. Recently introduced single bi-coloured dual disk (Ivoclar Vivadent) for milling might be helpful in reducing these differences. It was also reported that during the 3-D printing, polymerization shrinkage is practically negligible, and no trimming of the teeth and denture bases was done, compared to MCDs and CCDs as verified in the results [3, 4, 5, 14, 27, 28, 31].

Although the same CAD files were used in both milling and printing techniques, they differ in method of fabrication. At present the use of printing method is commonly used for try-in CDs and milling technique is preferred for production of definitive CDs, but soon the printing CDs would be used as an alternative to MCD owing to its obvious benefit viz. economical, better accuracy, biocompatibility, mechanical properties and patient satisfaction and overall low material wastage over milled techniques, however, further studies are needed to quantify these findings clinically and objectively with regard to CDs.

Overall, the study upheld the importance of one of the most basic steps of CDs, the laboratory and clinical remounting. In the present study also, patients were not satisfied even with digital CDs due to minute occlusal discrepancies, so clinical remount was done in CD of patient choice and later delivered to the patients, who showed complete satisfaction in follow up visits. This is even proved by the fact that most of the authors recommend the laboratory/clinical remount for CDs to remove the processing errors [4, 41].

4.1 Limitations

The computerized occlusion analysis system T-Scan III provided only relative value of the occlusal force, the absolute values were not possible. The occlusal parameters were tested immediately after processing, so effect of oral adaptation of CDs fabricated with different techniques were not assessed. The patients were asked to apply the maximum occlusal force instead of regular masticatory forces with CDs, so it is recommended to evaluate the CDs with regular masticatory forces. Also, effect of different machines and different available software were not assessed in this study. Finally, future studies are recommended with bi-coloured dual disk (for milled CDs) randomized clinical trials with a larger sample size and longer observation period to further assess the effect of fabrication techniques on occlusion of CDs.

5. Conclusions

Based on the findings of this clinical study, the following conclusions were drawn: the occlusal parameters were affected by the fabrication techniques and occlusal schemes of CDs. The digital CDs retain adjusted occlusal schemes better the conventionally fabricated CDs. 3-DP CDs with BBO and LO occlusal schemes provided centralization of forces, better distribution with total high maximum occlusal force percentage.

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 very grateful to Dentaly Dental Clinic and Research Center, Advanced Dental Lab and Mr. Mahmoud Rashed, Laboratory Chief Technician, Abha, KSA for CAD/CAM support.

Conflict of interest

None to report.

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Clinical analysis of CAD-CAM milled and printed complete dentures using computerized occlusal force analyser

Abstract

BACKGROUND:

OBJECTIVES:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Materials and methods

2.1.1 Conventional CDs (CCD) techniques

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

4.1 Limitations

5. Conclusions

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)