148
required for 3D printed dentures. A similar study done by Davda et al. (2017) found
that 3D printed acrylic templates reproduce the original denture with significantly
greater accuracy and precision compared to conventional techniques.
On the other hand, polymeric resins still have limitations in terms of their inability to be used for permanent and durable restorations due to their bacterial contamination, color changes, low fatigue resistance and strength (Alhareb et al. 2017), and
shrinkage (Anadioti et al. 2018). In order to overcome these disadvantages, Totu
et al. (2017b) developed photocurable PMMA loaded with various nanofillers in
order to improve the antimicrobial and mechanical properties of these 3D printed
materials. For example, Totu et al. (2017b) obtained a prototype RP complete denture with antibacterial properties against Candida scotti by incorporating 0.4 wt.%
of TiO 2 NPs into commercially available PMMA matrices for DLP. These materials
have been shown to have good biocompatibility (Totu et al. 2018a). Furthermore,
the maxillary and mandibular dentures obtained by 3D printing of these polymer
composites showed satisfactory results and performed without major clinical complications after 18 months of continuous use (Totu et al. 2017a; Pantazi et al. 2018;
Totu et al. 2018b,c; Cristache et al. 2019). Cristache et al. (2019) also evaluated
under clinical conditions and for one year the color stability of denture teeth for ten
complete maxillary prostheses obtained by 3D printing. These authors reported a
color difference (ΔE) between 1.63 and 5.24, measured in all patients using dentures made from TiO 2 NP-loaded PMMA (Cristache et al. 2019). These values were
below the maximum acceptability threshold (ΔE = 5.5) (Fontes et al. 2009) and
lower compared to unmodified PMMA, stored during the same period of time,
under ideal conditions (ΔE = 17.65). Cristache et al. (2018b) also concluded that
color stability increased with doping of PMMA with 0.4% wt.% of TiO 2 NPs.
Another approach to improve the antimicrobial properties of these materials is
the functionalization of polymers with positively charged functional groups such as
quaternary amino (Timofeeva and Kleshcheva 2011; Vigliotta et al. 2012), or less
toxic functional groups against mammalian cells such as phosphonium groups
(Popa et al. 2003), in both cases these functional polymers interact with the negatively charged bacterial cell wall, for both Gram-positive (Staphylococcus aureus)
and negative (Escherichia coli) bacteria (Ward et al. 2006; Mi and Jiang 2014;
González-Henríquez et al. 2019a).
Mechanical parameters of 3D printed materials such as flexural strength, hardness and impact strength are also of great importance when intended for the design
of bridge or crown frameworks, complete dentures, temporary polymer restorations,
or for patients exhibiting parafunctions. With this in mind, Alharbi et al. (2016a)
evaluated in vitro the effect of printing orientation on the mechanical properties of a
photocurable polymer for clinical use (Temporis, shade A1, Thiene, Italy). These
authors found that vertically printed material, with the load perpendicular to the
orientation of the layer, exhibited a higher compressive strength compared to horizontally printed material (Alharbi et al. 2016a). However, in order to increase the
stiffness of polymeric resins, it is common to introduce a hard filler into the soft
polymer matrix (Chen et al. 2018a, b).
C. M. Cristache and E. E. Totu
required for 3D printed dentures. A similar study done by Davda et al. (2017) found
that 3D printed acrylic templates reproduce the original denture with significantly
greater accuracy and precision compared to conventional techniques.
On the other hand, polymeric resins still have limitations in terms of their inability to be used for permanent and durable restorations due to their bacterial contamination, color changes, low fatigue resistance and strength (Alhareb et al. 2017), and
shrinkage (Anadioti et al. 2018). In order to overcome these disadvantages, Totu
et al. (2017b) developed photocurable PMMA loaded with various nanofillers in
order to improve the antimicrobial and mechanical properties of these 3D printed
materials. For example, Totu et al. (2017b) obtained a prototype RP complete denture with antibacterial properties against Candida scotti by incorporating 0.4 wt.%
of TiO 2 NPs into commercially available PMMA matrices for DLP. These materials
have been shown to have good biocompatibility (Totu et al. 2018a). Furthermore,
the maxillary and mandibular dentures obtained by 3D printing of these polymer
composites showed satisfactory results and performed without major clinical complications after 18 months of continuous use (Totu et al. 2017a; Pantazi et al. 2018;
Totu et al. 2018b,c; Cristache et al. 2019). Cristache et al. (2019) also evaluated
under clinical conditions and for one year the color stability of denture teeth for ten
complete maxillary prostheses obtained by 3D printing. These authors reported a
color difference (ΔE) between 1.63 and 5.24, measured in all patients using dentures made from TiO 2 NP-loaded PMMA (Cristache et al. 2019). These values were
below the maximum acceptability threshold (ΔE = 5.5) (Fontes et al. 2009) and
lower compared to unmodified PMMA, stored during the same period of time,
under ideal conditions (ΔE = 17.65). Cristache et al. (2018b) also concluded that
color stability increased with doping of PMMA with 0.4% wt.% of TiO 2 NPs.
Another approach to improve the antimicrobial properties of these materials is
the functionalization of polymers with positively charged functional groups such as
quaternary amino (Timofeeva and Kleshcheva 2011; Vigliotta et al. 2012), or less
toxic functional groups against mammalian cells such as phosphonium groups
(Popa et al. 2003), in both cases these functional polymers interact with the negatively charged bacterial cell wall, for both Gram-positive (Staphylococcus aureus)
and negative (Escherichia coli) bacteria (Ward et al. 2006; Mi and Jiang 2014;
González-Henríquez et al. 2019a).
Mechanical parameters of 3D printed materials such as flexural strength, hardness and impact strength are also of great importance when intended for the design
of bridge or crown frameworks, complete dentures, temporary polymer restorations,
or for patients exhibiting parafunctions. With this in mind, Alharbi et al. (2016a)
evaluated in vitro the effect of printing orientation on the mechanical properties of a
photocurable polymer for clinical use (Temporis, shade A1, Thiene, Italy). These
authors found that vertically printed material, with the load perpendicular to the
orientation of the layer, exhibited a higher compressive strength compared to horizontally printed material (Alharbi et al. 2016a). However, in order to increase the
stiffness of polymeric resins, it is common to introduce a hard filler into the soft
polymer matrix (Chen et al. 2018a, b).
C. M. Cristache and E. E. Totu