applied

sciences
Review
Comparison of CAD/CAM and Conventional Denture Base
Resins: A Systematic Review
Cindy Batisse 1,2, * and Emmanuel Nicolas 1,2

1 UFR d’Odontologie, Université Clermont Auvergne, CROC, F-63000 Clermont-Ferrand, France;

emmanuel.nicolas@uca.fr
2 CHU Clermont-Ferrand, Service d’Odontologie, F-63003 Clermont-Ferrand, France
* Correspondence: cindy.lance@uca.fr

Abstract: At present, complete dentures (CDs) remain the only treatment available for the majority
of edentulous patients. CDs are primarily fabricated using a conventional method using polymethyl-
methacrylate (PMMA) resin. The steps involved in PMMA polymerisation directly affect the quality
of the resin prosthetic base and any error reduces retention and occlusal accuracy of CDs. Further-
more, when using the conventional technique, the residual monomer alters the resin mechanical
properties and may cause mucosal reactions. Recently, computer aided design and computer aided
manufacture (CAD/CAM) techniques were increasingly used to fabricate CDs by machining resin
discs that have been manufactured under high pressure and temperature. This systematic review
compares CAD/CAM and conventional CDs according to their mechanical, physical and chemical
characteristics, as well as the clinical impact of any differences between them. A review was con-
ducted according to the preferred reporting items for systematic reviews and meta-analyses checklist
on 392 publications from both PubMed and backward research. Fifteen studies have been included.



Results showed that CAD/CAM resins had globally better physical and mechanical properties than

 conventional resins. The use of machined resin could improve the clinical performance, maintenance
Citation: Batisse, C.; Nicolas, E. and longevity of CDs. Further studies in clinical use would be required to complement these results.
Comparison of CAD/CAM and
Conventional Denture Base Resins: Keywords: denture base; complete denture; acrylic resin; CAD-CAM; milling resin
A Systematic Review. Appl. Sci. 2021,
11, 5990. https://doi.org/10.3390/
app11135990

1. Introduction
Academic Editor: Mitsuru Motoyoshi
Due to anatomical, physiological or financial restrictions, complete dentures (CDs)
still represent the only treatment available for the majority of edentulous patients. Since
Received: 4 June 2021
Accepted: 22 June 2021
the 1950s, complete dentures have been primarily designed and fabricated by hand, using
Published: 28 June 2021
conventional methods that follow well-defined clinical protocols and laboratory techniques.
To produce a set of complete dentures, edentulous patients must have at least five clinical
Publisher’s Note: MDPI stays neutral
appointments with their dentist to (1) make preliminary and (2) final impressions to make
with regard to jurisdictional claims in
a replica of the oral tissues (gypsum cast), (3) record the maxillomandibular relationship
published maps and institutional affil- and select the appropriate denture teeth, (4) test the trial wax prostheses and (5) insert the
iations. complete prostheses. Once the trial wax dentures are deemed appropriate, the laboratory
steps to make the final prostheses begin. The gypsum cast and the trial wax replica are
flasked in more gypsum to produce the external surface form. Once all the gypsum is
solidified, the trial wax is removed using hot water, producing a mold cavity. The resin to
Copyright: © 2021 by the authors.
fabricate the final denture is then introduced into the mold cavity within the flask by either
Licensee MDPI, Basel, Switzerland.
pressure or injection. Each technique has advantages and disadvantages. The quality of the
This article is an open access article
prosthetic resin material obtained is strongly influenced by the steps involved in handling
distributed under the terms and the resin.
conditions of the Creative Commons To date, acrylic resin polymethylmethacrylate (PMMA) is the most popular denture
Attribution (CC BY) license (https:// base material [1]. PMMA resin best meets all the specifications for an ideal denture base
creativecommons.org/licenses/by/ material. Its advantages range from easy repair to good appearance and reasonable cost. It
4.0/). has simple handling characteristics and polymerization is initiated by mixing polymethyl

Appl. Sci. 2021, 11, 5990. https://doi.org/10.3390/app11135990 https://www.mdpi.com/journal/applsci

Appl. Sci. 2021, 11, 5990 2 of 15

methacrylate (polymer) and methyl methacrylate (monomer) [2]. The polymerization of

this type of PMMA resin is completed by application of heat. When this resin is used for
dental prostheses, it shows linear distortions by contraction with a theoretical shrinkage of
6%. When using the conventional technique, internal stresses are created during the final
heat application during polymerization. These stresses are released when the prosthesis is
removed from the gypsum mold and flask, resulting in twisting, bending and dimensional
variations of the denture bases compared to the cast. Clinically, these deformations result
in a loss of accuracy and retention of the complete dentures.
There is also a loss of precision in the occlusal relationship as compared to the wax
trial prosthesis [3]. Furthermore, the chemical reaction between the monomer and the
polymer is never complete. The residual monomer that remains after the polymerization
process can alter the mechanical properties of the resin [4]. The released monomer is also
suspected of being responsible for allergic or irritating chemical reactions such as burning
sensations in the mouth, stomatitis, edema or ulceration of the oral mucosa [5].
With the development of digital technology, computer aided design and manufacture
(CAD/CAM) techniques are increasingly popular in dentistry. Thirty years after the first ar-
ticles on digital removable prostheses, many systems now exist to both reduce the number
of patient appointments needed and the laboratory time needed for prothesis fabrica-
tion [6–9]. The process of manufacturing CDs with computer-assisted technology involves
the acquisition of clinical information and the digital design of prostheses on computer
software. CDs can be produced using an additive (3D printing) or subtractive (milling)
process. The most commonly used method at present is the subtractive method [10–13].
In the subtractive method, the prostheses are milled from proprietary resin discs that are
polymerized under high pressure and high temperature [14]. CAD/CAM techniques for
denture fabrication have many clinical and laboratory advantages [15–18]. The computer-
aided design and manufacture of complete prostheses simplifies the laboratory protocol,
stores data and reduces the number of appointments required. A literature review by Wang
et al. [19] studied the impact of this manufacturing process on the adaptation of CDs and
occlusion. Their review showed that all prostheses tested had a clinically acceptable fit.
The CAD/CAM CDs showed a similar adaptation to conventional prostheses, sometimes
even better. Recently, Baba et al. [20] reviewed currently available techniques to manu-
facture CAD/CAM complete dentures and concluded that their physical properties were
improved over those fabricated by conventional laboratory techniques. With these results
in mind, the present literature review is aimed at expanding the evaluation of CAD/CAM
dentures by comparing their mechanical, physical and chemical characteristics with those
of conventional dentures. For the first time, the clinical repercussions of the CAD/CAM
machining process were assessed and discussed.

2. Materials and Methods

A thorough search of the literature was conducted using the Medline database (via
PubMed). A search strategy developed for Medline via PubMed was employed using
the following keywords: (CAD/CAM complete denture base); (CAD/CAM complete
denture) NOT (implant); (CAD/CAM complete denture base) AND (resin properties). A
systematic review according to the PRISMA checklist was applied. A manual search of the
included articles was also performed to identify further eligible studies not detected by
keyword search and this strategy is outlined in Figure 1. In January 2021, 392 titles listed in
PubMed were identified. After screening the initial articles and backward searching, some
publications were excluded based on the following criteria: papers not written in English,
animal and in vivo studies, case reports, systematic reviews and when only abstracts
were available.
Appl. Sci. 2021,
Appl. 11, 11,
Sci. 2021, x FOR
5990 PEER REVIEW 3 of 15

Figure
Figure 1. PRISMA
1. PRISMA flowofchart
flow chart ofbased
records records based
on initial on initial
PubMed PubMed
database searchesdatabase
and back- search
searching.
ward searching.

Studies comparing the properties of CD resins manufactured conventionally to those

Studies
manufactured comparing
digitally thewere
by milling properties
targeted. of CD resins
Articles dealingmanufactured
with CDs digitally convent
manufactured
manufactured by 3Ddigitally by milling
printing methods were targeted.
were excluded, Articles
leaving only dealing with CD
the manufacturing
process by machining for comparison to the conventional manufacturing method. Studies
ufactured by 3D printing methods were excluded, leaving only the manufa
comparing the manufacturing techniques of complete removable prostheses based on
by machining
criteria other than thefor comparison
properties to the
of resins were also conventional manufacturing
excluded, as well as studies on implant-method
paring the manufacturing techniques of complete removable prostheses b
supported prostheses, immediate dentures, or partial removable prostheses.
Full-text analysis was performed independently by two reviewers. Finally, fifteen
other than the properties of resins were also excluded, as well as studies o
articles were included.
ported prostheses, immediate dentures, or partial removable prostheses.
Full-text analysis was performed independently by two reviewers. Fin
3. Results

ticles were included.

Three hundred and ninety-two titles were identified from the PubMed electronic
search. The PRISMA selection methodology resulted in the inclusion of fifteen studies is
summarized in Table 1.
3. Results
The objective of this literature review was to compare the characteristics of manufac-
tured CAD/CAM PMMA resins with those of PMMA resins produced by conventional
Three
laboratory hundred
techniques. For thisand ninety-two
purpose, titles
the results are wereaccording
presented identifiedto thefrom the Pub
mechan-
search.
ical, The
physical andPRISMA selection
chemical properties of methodology resulted
the resins. In all the studies, in
thethe inclusion of fi
conventional
summarized in Table 1.
resins were produced using clamping pressure or injection techniques, followed by poly-
merization by heat using common dental laboratory techniques. The CAD/CAM resins
The objective
were obtained of thismanufactured
from industrially literature review was
resin discs thatto compare
were polymerizedthe characterist
under
tured CAD/CAM PMMA resins with those of PMMA resins produced b
high pressure and high temperature.
laboratory techniques. For this purpose, the results are presented accord
chanical, physical and chemical properties of the resins. In all the studies, th
resins were produced using clamping pressure or injection technique
polymerization by heat using common dental laboratory techniques. The
ins were obtained from industrially manufactured resin discs that were p
der high pressure and high temperature.

3.1. Mechanical Properties

Seven articles have focused on the surface hardness of the PMMA re
Appl. Sci. 2021, 11, 5990 4 of 15

Table 1. Included articles (HP: Heat_polymerising; IM: Injected molding).

Conventional Samples of Studied Studied

Year Title Authors CAD/CAM Resins Properties
Resins Resins Characteristics
Adherence of Candida to
Complete Denture Surfaces
Surface roughness
in vitro: A Comparison of HP resin (Major Wieland Digital Dentures Resin discs
2017 Al-Fouzan et al. Adherence of Physical
Conventional and dental) (Ivoclar Vivadent) (10 × 3 mm)
Candida albicans
CAD/CAM Complete
Dentures [28]
Baltic denture System (Merz
Do CAD/CAM dentures dental GmbH) Vita VIONIC
really release less monomer (Vita Zahnfabrik) Whole You Residual monomer
2017 Steinmassl et al. HP resin (Candulor) Dentures Chemical
than conventional Nexteeth (Whole You Inc.) content
dentures? [14] Wieland Digital Dentures
(Ivoclar Vivadent)
The residual monomer
content and mechanical
properties of CAD\CAM Flexural strength,
Resin bar specimens Mechanical
2017 resins used in the fabrication Ayman et al. HP resin (Vertex RS) Polident (Polident d.o.o) Hardness, Residual
(65 × 10 × 3 mm) Chemical
of complete dentures as monomer content
compared to heat cured
resins [21]
AvaDent (Global Dental
Science) Baltic denture System
In Vitro Analysis of the
HP resin (Candulor) (Merz dental GmbH) Vita Toughness, Elastic
Fracture Resistance of Resin bar specimens Mechanical
2018 Steinmassl et al. Autopolymerising VIONIC (Vita Zahnfabrik) modulus, Surface
CAD/CAM Denture Base (39 × 8 × 4 mm) Physical
resin (Candulor) Wieland Digital Dentures roughness
Resins [29]
(Ivoclar Vivadent) Whole You
Nexteeth (Whole You Inc.)
AvaDent (AvaDent Global
Dental) Baltic denture System
Influence of CAD/CAM (Merz dental GmbH) Vita Dentures Resin
Surface roughness,
2018 fabrication on denture Steinmassl et al. HP resin (Candulor) VIONIC (Vita Zahnfabrik) bar specimens Physical
Hydrophobicity
surface properties [30] Whole You Nexteeth (Whole (39×8×4 mm)
You Inc.) Wieland Digital
Dentures (Ivoclar Vivadent)
Appl. Sci. 2021, 11, 5990 5 of 15

Table 1. Cont.

Conventional Samples of Studied Studied

Year Title Authors CAD/CAM Resins Properties
Resins Resins Characteristics
Resin coupon
CAD/CAM milled complete
specimens
removable dental prostheses:
(11 × 11 × 2 mm) Elastic modulus,
An in vitro evaluation of AvaDent (Global Dental Mechanical
2018 Srinivasan et al. HP resin (Candulor) Resin bar specimens Hardness, Toughness,
biocompatibility, mechanical Science) Physical
(65 × 10 × 3 mm) Surface roughness
properties, and surface
Resin bar specimens
roughness [24]
(20 × 20 × 1.5 mm)
Evaluation of Flexural
Strength and Surface
M-PM Disc (Merz Dental
Properties of Flexural strength,
GmbH) AvaDent (AvaDent Resin bar specimens Mechanical
2018 Prepolymerized CAD/CAM Arslan et al. HP resin (Promolux) Hydrophobicity,
Global Dental) Polident (64 × 10 × 3.3 mm) Physical
PMMA-based Polymers Surface roughness
(Polident d.o.o)
Used for Digital 3D
Complete Dentures [31]
In vitro evaluation of M-PM Disc (Merz Dental Hydrophobicity,
Adhesion of Candida albicans GmbH) AvaDent (AvaDent Resin discs Surface roughness,
2019 Murat et al. HP resin Physical
on CAD/CAM Global Dental) Pink (10 × 2 mm) Adherence of Candida
PMMA-Based Polymers [32] CAD/CAM disc albicans
A Comparison of the Surface
Properties of CAD/CAM AvaDent (Global Dental Resin coupon Hardness,
Mechanical
2019 and Conventional Al-Dwairi et al. HP resin (Meliodent) Science) Tizian Blank PMMA specimens Hydrophobicity,
Physical
Polymethylmethacrylate (Schütz Dental) (25 × 25 × 3 mm) Surface roughness
(PMMA) [22]
Resin bar specimens
Conventional (65 × 10 ×2.5 mm)
heat-activated pack (DD, PRO) and Resin
Mechanical Properties of Wieland Digital Dentures Flexural strength
en press PMMA bar specimens
2019 CAD/CAM Denture Base Pacquet et al. (Ivoclar Vivadent): Fracture toughness Mechanical
(Probase Hot) (40 × 4 × 2 mm)
Resins [25] Ivobase (DD) Hardness
(PRO) IM (CAP) Resin bar
resin (Ivocap)(CAP) specimens
(39 × 8 × 4 mm)
Appl. Sci. 2021, 11, 5990 6 of 15

Table 1. Cont.

Conventional Samples of Studied Studied

Year Title Authors CAD/CAM Resins Properties
Resins Resins Characteristics
Flexural strength of denture
Compressed molding
base acrylic resins processed AvaDent (Global Resin bar specimens Flexural strength,
2020 Aguirre et al. resin (Microstone) IM Mechanical
by conventional and Dental Science) (64 × 10 × 3.3 mm) Flexural modulus
resin (Microstone)
CAD-CAM methods [33]
A Comparison of the
Flexural and Impact
Strengths and Flexural AvaDent (Global Dental Flexural strength,
Resin bar specimens
2020 Modulus of CAD/CAM and Al-Dwairi et al. HP resin (Meliodent) Science) Tizian Blank PMMA Flexural modulus, Mechanical
(65 × 10 × 3 mm)
Conventional Heat-Cured (Schütz Dental) impact strength
Polymethyl Methacrylate
(PMMA) [34]
Comparison of Mechanical
Interdent (Interdent d.o.o)
Properties of 3D-Printed, HP resin (ProBase
Polident (Polident d.o.o) Resin bar specimens Flexural strength
2020 CAD/CAM, and Prpić et al. Hot, Paladon 65, Mechanical
Wieland Digital Dentures (64 × 10 × 3.3 mm) Hardness
Conventional Denture Base Interacryl Hot)
(Ivoclar Vivadent)
Materials [23]
Influence of High-Pressure
Flexural strength,
Polymerization on HP resin (ProBase Wieland Digital Dentures Resin bar specimens
2021 Becerra et al. Elastic modulus Mechanical
Mechanical Properties of Hot) ± high pressure (Ivoclar Vivadent) (65 × 10 × 3.3 mm)
Hardness
Denture Base Resins [26]
Degos Dental L-Temp (Degos Resin bar specimens
Assessment of CAD-CAM Flexural strength,
HP resin (Paladon 65) Dental) Zirkonzahn Temp (65 × 10 × 3.2 mm)
polymers for digitally Microhardness,
2021 Perea-Lowery et al. Autopolymerizing Basic Tissue (Zirkonzahn SRL) Resin coupon Mechanical
fabricated complete dentures Nanohardness,
resin (Palapress) Wieland Digital Dentures specimens
[27] Elastic modulus
(Ivoclar Vivadent) (10 × 10 × 2 mm)
Appl. Sci. 2021, 11, 5990 7 of 15

3.1. Mechanical Properties

Seven articles have focused on the surface hardness of the PMMA resins. Hardness is
a measure of the resistance of a material to plastic deformation, induced by either abrasion
or mechanical indentation. The data are presented in Table 2. Two studies have shown
that resins for CAD/CAM prostheses have a higher microhardness than conventional
resins [21–23]. Others studies showed no significant difference in hardness between the
two types of resins when tested for nano-hardness [24,25]. Recently, however, one study
showed that the hardness of CAD/CAM resin was significantly lower than that of hot-
pressed conventional resin [26]. Perea–Lowery [27] revealed differences in material-related
hardness and microhardness between CAD/CAM and conventional resins

Table 2. Hardness test results.

Average Hardness (±Standard Average Hardness (±Standard

Deviation) of Conventional Resin Deviation) of CAD/CAM Resin
T: 19.80 (±1.08) *,a
Al-Dwairi et al. (2019) [22] (in mPa) 18.09 (±0.31)
A: 20.60 (±0.33) *,b
Ayman et al. (2017) [21] (in mPa) 13.22 (±0.88) P: 22.41 (±1.50) ***
Becerra et al. (2021) [26] (in (HVN) 23.1 (±1.9) 18.7 (±1.7)
Pacquet et al. (2019) [25] (in mPa) 19.46 (±0.4) 19.31 (±1.48)
Perea-Lowery et al. (2021) [27] (in mPa) NC NC
Prpić et al. (2020) [23] (in mPa) NC NC
Srinivasan et al. (2018) [24] (in mPa) (nano-hardness) 232 (±15) A: 221 (±14)
CAD/CAM resin discs: A: Avadent; P: Polident; T: Tizian-Schütz; NC: Graphic data: precise values not communicated. * Statistically
significant difference compared to conventional resin (p < 0.05); *** Statistically significant difference compared to conventional resin
(p < 0.001); a and b indicate a statistical difference between CAD/CAM resins.

The resistance to deformation was studied by three-point bending tests in nine stud-
ies [21,23,24,26,27,29,31,33,34]. Results showed that CAD/CAM resins have superior me-
chanical properties; higher elastic moduli [21,24,26,29,33], flexural strength [23,24,26,31,33,34],
impact strength [34] and yield strength [24,25] as compared to conventional resins. How-
ever, two studies [21,27] revealed that the mechanical properties of CAD/CAM resins were
not always superior to that of conventional resins. Arslan et al. showed that the flexural
strength of all the resins tested was significantly lower after thermocycling [31]. Most
CAD/CAM resins have a higher toughness than conventional resins [24,29].
Only one study focused on the analysis of the load at fracture of resins using a 3-point
bending test on rectangular resin samples (4 mm-thick). The maximum load at fracture
of six CAD/CAM resins was then compared with that of conventional heat-cured and
self-curing resins [29]. Table 3 shows the results of the fracture tests. The fracture resistance
of CAD/CAM resins was not always superior to that of conventional heat-cured or self-
curing resins. Indeed, fracture resistance is highly variable within CAD/CAM resins. The
average breaking loads ranged from 40.27 N ± 3.40 to 82.49 N ± 7.47 for CAD/CAM resins.
Of the six CAD/CAM resins tested, only the Wieland digital dentures (WWD) resin had
significantly higher values than conventional resins. This study also showed that there
was a positive correlation between fracture surface roughness and the maximum load at
fracture of the resin. For example, the WDD CAD/CAM resin had the roughest fracture
surface with the highest maximum breaking load.
Appl. Sci. 2021, 11, 5990 8 of 15

Table 3. Breaking load test results.

Mean Load at Fracture (±Standard Mean Load at Fracture (±Standard
Deviation) of Conventional Resin (N) Deviation) of CAD/CAM Resin (N)
Conventional heat-cured
A: 50.26 (±4.02) **
resin: 61.66 (±5.60)
BDS: 49.37 (±4.91) **
Steinmassl et al. (2018) [29] VV: 40.27 (±3.40) **
Self-curing resin: 53.51 (±4.07) DD: 82.49 (±7.47) **
WNu: 62.35 (±2.44) **
WNc: 63.44 (±4.91) **
CAD/CAM resin discs: A: Avadent; BDS: Baltic denture system; DD: Wieland digital denture; VV: Vita Vionic; WNu: Whole You Nexteeth,
dentures without the customary full surface coating, WNc: Whole You Nexteeth, dentures with the customary full surface coating;
** Statistically significant difference compared to conventional resin (p < 0.01); The different subscript letters indicate a statistical difference
between CAD/CAM resins.

The study by Steinmassl et al. [29] highlighted the differences in mechanical properties
between CAD/CAM resins.

3.2. Physical Properties

Seven studies compared the surface roughness of CAD/CAM and conventional
resins [22,24,28–32]. Surface roughness was measured using laser or contact profilometers.
Three of the seven studies showed that conventional resins had significantly higher surface
roughness than CAD/CAM resins [22,28,32]. Another study also showed that conventional
resins had a higher roughness, except for one brand of CAD/CAM resin that showed a
slightly lower roughness that was not statistically significant [30]. The study by Steinmassl
et al. showed that two of the five CAD/CAM resins tested had lower roughness than
conventional resins [29]. One study showed that surface roughness was significantly higher
for CAD/CAM resins [24]. The study by Arslan et al. showed that the resins did not exhibit
different roughness values before and after thermal cycling [31]. The data are summarized
in Table 4.
Four studies have focused on the hydrophobicity of resins using the sessile drop
technique with calculation of the contact angle [22,30–32]. The results of the studies are pre-
sented in Table 5. Two studies showed that most CAD/CAM resins were more hydrophobic
than conventional resins [22,31], while another study showed a higher hydrophobicity
rate for conventional resins [30]. All the studies highlighted variations in hydrophobicity
according to the CAD/CAM resin used [22,30–32].
One study used thermocycling to simulate the clinical aging process and found
that conventional resins become more hydrophobic and CAD/CAM resins become more
hydrophilic after aging [31].

3.3. Chemical Properties

Two studies have investigated the content of residual monomer in industrially poly-
merized CAD/CAM resins [14,21]. The results are presented in Table 6. Steinmassl et al.
showed that CAD/CAM resins release very little free monomer after milling. However,
their residual monomer content was not statistically lower than that of conventional
laboratory-polymerized resins, regardless of the CAD/CAM process tested (BalticDen-
ture, Whole You Nexteeth, Wieland Digital Dentures or Vita) for the manufacture of CDs.
Another study showed lower free monomer content in industrially polymerized resin
specimens, compared to conventionally polymerized resins [21].
Appl. Sci. 2021, 11, 5990 9 of 15

Table 4. Surface roughness test results.

Average Surface Roughness Average Surface Roughness
(±Standard Deviation) of (±Standard Deviation) of CAD/CAM
Conventional Resin (µm) Resin (µm)
A: 0.16 (±0.03) ** a
Al-Dwairi et al. (2019) [22] 0.22 (±0.07)
T: 0.12 (±0.02) *** b
Al-Fouzan et al. (2017) [28] 0.073 (±0.015) WDD: 0.037 (±0.001) *
Without thermocycling: Without thermocycling:
0.22 (±0.07) a A: 0.22 (±0.06) a
M: 0.21 (±0.07) b
P: 0.26 (±0.09) a
Arslan et al. (2018) [31]
With thermocycling: With thermocycling:
0.29 (±0.09) b A: 0.24 (±0.04) c
M: 0.18 (±0.04) d
P: 0.32 (±0.08) c
M: 0.18 (±0.04) *,a
Murat et al. (2018) [32] 0.34 (±0.06) P: 0.21 (±0.04) *,a
A: 0.20 (±0.05) *,a
Srinivasan et al. (2018) [24] 0.12 (±0.29) 0.37 (±0.03) ***
Conventional heat-cured resin: A: 1.11 (±0.38) *
3.47 (±0.10) BDS: 2.04 (±0.42)
Steinmassl et al. (2018) [29] VV: 3.23 (±0.31)
Self-curing resin: WDD: 6.25 (±2.74)
2.42 (±0.79) WN: 0.65 (±0.11)
A: 0.28 (±0.16) **
BDS: 0.44 (±0.13)
Steinmassl et al. (2017) [14] 0.55(±0.14) VV: 0.28 (±0.07) **
WN: 0.04 (±0.01) **
DD: 0.30 (±0.10) **
CAD/CAM resin discs: A: Avadent; BDS: Baltic denture system; DD: Wieland digital denture; M: M-PM; P: Polident; T: Tizian-Schütz;
VV: Vita Vionic; WN: Whole You Nexteeth; * Statistically significant difference compared to conventional resin (p < 0.05); ** Statistically
significant difference compared to conventional resin (p < 0.01); *** Statistically significant difference compared to conventional resin
(p < 0.001). a, b, c, d indicate a statistical difference between CAD/CAM resins.

Table 5. Hydrophobicity test results.

Average Contact Angle (±Standard Average Contact Angle (±Standard
Deviation) of Conventional Resin (◦ ) Deviation) of CAD/CAM Resin (◦ )
Without thermocycling: Without thermocycling:
73.97 (±3.53) A: 92.95 (±2.65) ***,a
M: 81.03 (±3.29) ***,b
Arslan et al. (2018) [31] P: 82.39 (±3) ***,b
With thermocycling: Without thermocycling:
83.35 (±4.37) A: 83.52 (±2.6) c
M: 77.21 (±3.01) ***,d
P: 76.16 (±3.3) ***,d
A: 72.87 (±4.83◦ ) ***
Al-Dwairi et al. (2019) [22] 65.97 (±4.67◦ )
T: 69.53 (±3.87◦ )
Without saliva: Without saliva:
73.43 (±17.82) A: 69.72 (±10.57) *,a
M: 71.31 (±6.94) *,a
P: 69.63 (±4.85) *,a
Murat et al. (2018) [32]
With saliva: With saliva:
86.51 (±11.07) A: 74.98 (±4.28) *,b
M: 79.56 (±5.06) *,c
P: 86.19 (±5.82) d
Appl. Sci. 2021, 11, 5990 10 of 15

Table 5. Cont.
Average Contact Angle (±Standard Average Contact Angle (±Standard
Deviation) of Conventional Resin (◦ ) Deviation) of CAD/CAM Resin (◦ )
A: 70.35 (±8.99)*
BDS: 75 (±5.42) *
Steinmassl et al.(2018) [30] 82.50 (±3.44) VV: 74.4 (±2.32)***
WN: 77.7 (±9.87)
DD: 77.5 (±3.34)
CAD/CAM resin discs: A: Avadent; BDS: Baltic Denture System; DD: Wieland Digital denture; M: M-PM; P: Polident; T: Tizian-Schütz;
VV: Vita Vionic; WN: Whole You Nexteeth; * Statistically significant difference compared to conventional resin (p < 0.05); *** Statistically
significant difference compared to conventional resin (p < 0.001); a, b, c, d indicate a statistical difference between CAD/CAM resins.

Table 6. Residual monomer content test results.

Average Residual Monomer (±SD) of Average Residual Monomer (±SD) of
Conventional Resin (ppm) CAD/CAM Resin (ppm)
At baseline: 17.10 (±0.38) At baseline: P: 1.61 (±0.05) ***
Ayman et al. (2017) [21] At two-day: 16.04 (±0.13) At two-day: P: 1.03 (±0.06) ***
At seven-day: 13.54 (±0.46) At seven-day: P: 0.90 (±0.01) ***
BDS: 0.6 (±0.4)
Steinmassl et al. (2017) [14] 1.5 (±1.6)
WN: 6 (±2.7) **
CAD/CAM resin discs: BDS: Baltic denture system; P: Polident; WN: Whole You Nexteeth; ** Statistically significant difference compared
to conventional resin (p < 0.01); *** Statistically significant difference compared to conventional resin (p < 0.001).

4. Discussion
The objective of this literature review was to compare the characteristics of CAD/CAM
resins with those of conventional resins in order to assess the impact of CAD/CAM
resins on the CDs’ clinical performance and durability. Results showed that switching
to digital technology for the manufacturing process of resins led to different properties
between conventional and CAD/CAM resins. Throughout this discussion, the effect of the
physical property differences of these resins on the success of clinical denture treatment
will be highlighted.

4.1. Retention of Denture Bases

The clinical performance of CDs is partly determined by their retention within the oral
cavity. Lack of retention and instability are two of the main complaints from people wearing
CDs. Retention refers to the amount of vertical force needed to resist the dislodgement of
the prosthesis away from its supporting structures, similar to the force needed to remove
a suction cup. As with a suction cup, wettability of the resins influences the prostheses’
retention to the supporting mucosa and has to be taken into account in the manufacturing
process. Wettability indicates whether saliva is able to spread more or less easily on a
surface and reflects whether fluids aid retention of the denture surface to oral mucosa,
thus affecting retention. CDs with good wettability therefore optimize retention. The
wettability of a material is defined by observing the angle of contact between the material
and a drop of water: the greater the angle, the lower the wettability and therefore, the more
hydrophobic the material will be. In the studies included in the review, hydrophobicity
varied according to the type of resin used and could be explained by several hypotheses.
Hydrophobicity could be correlated with the rate of monomer released by the resins during
polymerization [31]. Another hypothesis could call into question the additives contained
in the resins. Indeed, dentures are not made of pure PMMA. They contain numerous
additives such as polymerization initiators, cross-linking agents, fillers and dyes, which
can all influence the hydrophobicity of the resins [31]. The origin of these variations in
hydrophobicity value in dental resins, as well as the effect of aging on resin hydrophobicity,
should be investigated.
On the one hand, these results on hydrophobicity do not clearly explain why some
studies show that digital CDs are more retentive than conventional CDs. On the other
hand, one study has shown through adhesion tests that the retention of digital CDs is
Appl. Sci. 2021, 11, 5990 11 of 15

better [35]. If the resins’ physical properties do not play a major role in CAD/CAM denture
retention, then the manufacturing process itself could favor the precision of adaptation
and the adhesion of digital CDs. Still, the precision of the manufacturing process of these
resins favors adaptation and the adhesion of digital CDs and has shown that the retention
of digital CDs is better than that of conventional prostheses [35]. The review by Wang
et al. [19] on the accuracy of digital full dentures showed that digital CDs have a similar or
better fit than conventionally manufactured CDs. The manufacture of CDs using digital
techniques helps to meet the three biomechanical requirements to ensure CD success;
namely retention, stability and support (accuracy of fit to the oral tissues) as defined in
Housset’s triad.

4.2. Resistance to Deformation and Breakage

When worn, complete dentures are subjected to high stresses during chewing. This
creates cyclic deformation of the denture polymer, which in turn can lead to crack formation
and possible fatigue fracture of the denture [36]. Therefore, good resistance of the resins to
deformation is very important. The results of this literature review showed that CAD/CAM
resins have superior mechanical properties to conventional resins.
First of all, conventional resins had a lower modulus of elasticity than CAD/CAM
resins. The elastic modulus is an important property in characterizing the material’s rigidity.
Resins with a higher modulus of elasticity will be more resistant to elastic deformation.
A high elastic modulus may make it possible to fabricate prostheses with reduced thick-
ness without compromising the mechanical properties, an important attribute for patient
comfort and acceptance of the prosthesis. In addition, a deformation-resistant prosthe-
sis will have a more stable occlusion. These differences in rigidity between CAD/CAM
and conventional resins are probably due to the manufacturing process. The modulus
of elasticity would be related to the residual monomer content. It has been shown that a
high monomer content reduces the glass transition temperature, making the resin more
flexible. The high-temperature, high-pressure polymerization of CAD/CAM resins leads
to higher monomer conversion with lower residual monomer values, thus resulting in
a more rigid resin. High-pressure and high-temperature polymerization also allows a
different arrangement of the polymer chains. Previous studies had shown that the use of
high pressure during polymerization increased crosslinks between polymer chains [15,37].
Secondly, fracture resistance characterizes the maximum stress a material can with-
stand before breaking. Results have shown that fracture resistance varied within CAD/CAM
resins. For example, in the study by Steinmassl et al. [29], the Wieland digital denture
(WDD) resin exhibited higher loads to fracture but had a lower elastic modulus than the
others. Thus, it had lower rigidity and more susceptible to plastic deformation. It would
therefore be preferable not to reduce the bases thickness too much to compensate for the
risk of plastic deformation. Of the six CAD/CAM resins tested, only the WWD resin
had significantly higher yield strength values compared to conventional resins. WDD
also exhibited the roughest fracture surface. It was the only resin with fine grains on the
fracture surface. This difference in resin surfaces is thought to be due to the geometry,
size and distribution of the polymer powders used to make the resins. In the study by
Steinmassl et al. [29] the measured maximum breaking loads were all greater than 40 N.
However, the maximum bite force was reduced to 40 N in complete prosthesis wearers
with high resorption of the mandibular alveolar bone [38]. Based on these results, some
manufacturers encourage the fabrication of full dentures with reduced resin thickness. It is
imperative to remain skeptical regarding this commercial argument, especially since in the
study, the tests were performed on 4 mm thick resin samples, which is much thicker than
a prosthetic base. Furthermore, CDs are subjected to much more complex stresses in the
mouth, involving different types of forces in different directions (such as chewing). For
a valid prediction of clinical fatigue resistance, further studies on cyclic loading behavior
and fracture toughness are necessary.
Appl. Sci. 2021, 11, 5990 12 of 15

The results highlighted differences in mechanical properties between CAD/CAM

resins. More information on the composition and manufacturing processes of CAD/CAM
resins would help to better understand some of the results.

4.3. Adhesion of Microorganisms

Preventing the adhesion of microorganisms to dentures helps preserve the health of
tissues in the oral cavity. The prevalence of denture stomatitis among denture wearers
has been shown average around 28% [39]. Microorganisms can cause pathology in the
mucosa supporting the prosthesis, such as candidiasis (induced mainly by Candida albicans).
Microorganisms accumulate more easily on prostheses than on natural tissue. The surfaces
of removable maxillary prostheses are the main reservoir of Candida albicans [40,41]. The
physical properties of the prosthetic resin influence this adhesion, including surface rough-
ness and hardness. The results of this literature review showed that CAD/CAM machined
resins had globally more advantageous physical properties than conventional resins.
Surface roughness is the most important factor for Candida albicans’ adhesion to the
prosthesis surface. Optical and electron microscopy studies revealed that initial adhesion
of microorganisms usually begins in the microscopic pits and cracks of rough surfaces.
This surface roughness provides protection against removal (shear) forces and gives time
for microbes to irreversibly adhere to the surface [42–45]. A smoother surface might reduce
Candida albicans adhesion, which ultimately lowers the risk of prosthetic stomatitis. It has
been demonstrated that CAD/CAM resins have significantly lower surface roughness
than conventional resins [22,28,29,32]. The polishing technique and the operator’s ability
to polish both affect the roughness of the resins’ cameo (external) surface [46]. However,
the prosthetic intaglio (internal surface in contact with the supporting mucosa) is never
polished, and only the polymerization of the resins would determine surface roughness.
Therefore, irregularities in conventionally fabricated dentures would be due to the evapora-
tion of monomers during polymerization. This can be avoided by applying high-pressure
polymerization, as seen when utilizing proprietary CAD/CAM resins [32]. In case of
conventional resin polymerization, the amount of pressure applied is limited to avoid
fracturing the gypsum mold in which resins are polymerized. Conversely, machined
resins for CAD/CAM prosthetics are made from resin discs that are polymerized at very
high pressure, which could explain their lower roughness [30]. Furthermore, in order to
avoid the accumulation and colonization of microorganisms on the prostheses’ surface,
roughness of the resins should not exceed 0.2 µm [47]. In this review, most of the resins
had roughness above 0.2 µm. This characteristic should be improved in the future, taking
into consideration that the surface profile of the prosthetic intaglio surface always exhibits
striations due to milling process used for CAD/CAM resins.
In addition to the biofilm adhesion onto the resins, surface roughness also tends to
induce halitosis and mucosal inflammation [48], and reduces patient comfort. Thus, milling
resins could reduce these nuisances and facilitate denture maintenance.
Hardness is another factor influencing bacterial adhesion to the prostheses over time.
Surface hardness is defined as the ability of the material to resist penetration or permanent
indentation related to the wear and scratches appearing as the dentures are worn and
cleaned. A prosthesis should have a hard surface to reduce denture deterioration and
therefore limit the adhesion of microorganisms [37,49]. Included studies focused on the
microhardness as well as the nanohardness of resins. For the most part, surface micro-
hardness was higher in CAD/CAM prosthetic resin blocks and no significant difference
was found between nanohardness values. These results could be related to the polymer-
ization process because the polymerization conditions of the CAD/CAM resin discs (high
temperature and pressure) and the addition of inorganic substances promote the formation
of longer polymer chains and limit the dimensional shrinkage of polymerization [50]. This
high-temperature, high-pressure polymerization leads to higher monomer conversion,
minimizing the plasticizing effect of the residual monomer [51,52]. Thus, CAD/CAM
Appl. Sci. 2021, 11, 5990 13 of 15

resins seem to be harder than conventional resins, which could help prevent bacterial
adhesion and therefore increase the long-term comfort of the dentures.

4.4. Residual Monomer Content

The residual monomer alters the resins’ mechanical properties. It is also suspected of
being responsible for allergic or irritating chemical reactions such as burning sensations
in the mouth, stomatitis, edema or ulcerations of the oral mucosa. A certain amount
of residual monomer is unavoidable due to the monomer-polymer balance required for
the radical polymerization of resins [53]. Although the ISO 20795-1 norm (2013) allows
a maximum residual monomer content of 2.2% by weight [54], the presence of residual
monomer in the denture base resins should be reduced to a least amount possible.
In the study by Steinmassl et al. [14], the amount of residual monomer in industrially-
polymerized and then machined CAD/CAM resins was not statistically different from that
of conventional resins. Four CAD/CAM resins were tested (BalticDenture, Whole You
Nexteeth, Wieland digital dentures and Vita). The BalticDenture process exhibited the least
residual monomer of the resins tested. In the BalticDenture system, the prosthetic teeth are
incorporated into the base during polymerization. In the other systems, the prosthetic teeth
are bonded separately to the base after milling, using a PMMA bonding agent. These results
suggest that the bonding agents used to attach the teeth to the milled resin base are a source
of methacrylate monomer release. Thus, industrially polymerized resins would contain
less residual monomer than laboratory-cured resins [21]. PMMA discs are polymerized
industrially at high temperature and high pressure. This type of polymerization promotes
the formation of longer polymer chains and would therefore lead to a higher level of
monomer conversion with lower residual monomer values [37]. Assembling the prosthetic
teeth to the machined bases during the CD manufacturing process would explain why
the residual monomer content of some machined CDs is not that different from that of
conventional CDs. Further studies need to be conducted to confirm this hypothesis, using,
for example, Ivotion (Ivoclar’s newest monolithic CD material).

5. Conclusions
This review showed that CAD/CAM resins have similar or even superior character-
istics to conventional resins, particularly in terms of mechanical properties. However, it
should be noted that the variabilities of the characteristics are not only due to the manu-
facturing techniques but could also be due to the composition of resins. This entails that
manufacturers should provide more complete information in terms of resin composition,
and their manufacturing process, as it often remains confidential. CAD/CAM resins exhibit
properties that can improve patient satisfaction and clinical success of CDs. The longevity
of CDs should also be investigated in future studies in clinical use by investigating the
resins’ aging process and fatigue resistance.
Using digital techniques to manufacture CDs would help meet the three biomechanical
requirements to ensure CDs balance, namely retention, stability and support (Housset’s
triad). Ultimately, this literature review is encouraging for more frequent utilization of
CAD/CAM resins in the future, to become the new standard protocol.

Author Contributions: Conceptualization, C.B. and E.N.; methodology, C.B.; validation, C.B. and
E.N.; formal analysis, C.B.; investigation, C.B.; original draft preparation, C.B.; writing—review and
editing, C.B and E.N. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: The authors thank Caroline Eschevins for her valuable technical support, Isabelle
Denry and Julie A Holloway for their help in revising the manuscript.
Appl. Sci. 2021, 11, 5990 14 of 15

Conflicts of Interest: The authors declare no conflict of interest.

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