7 3D Models in Dentistry
We discussed how to get a 3D digital model from diagnostic images and
how to convert it into a physical model using 3D printing. Both the
digital model and the physical model are elements that can elevate the
quality of the therapies provided by the doctors, and at the same time
those are of great value for the clinical researcher, thanks to the
retrievable amount of data.
The fields of application are many and embrace the surgical disciplines,
education and dental procedures such as implantology, prosthetics and
conservative dentistry 103. We will discuss some of the most
relevant implementations of the procedures we have described.
The training path of the dentist and medical doctor is a combination of
theoretical lessons and practical activities, aimed at providing
in-depth medical knowledge and a method of clinical reasoning. The
student has to develop manual skills to perform interventions on the
patient. In addition, every student in the medical field studies the
human anatomy, and if once cadavers dissection were what provided the
students with a practical learning context, now few institutes offer
this possibility 67.
With the decrease in the use of cadavers, the use of plastic replicas of
parts of the body has increased as a practical complement to the
learning of anatomy. Several authors have recently begun to explore the
possibilities offered by modern medical modeling and 3D printing
techniques in the field of medical training.
Anatomical models for the study of anatomy and for the explanation of
operative procedures have been produced both in the medical and dental
field 66, 70. Heng 67 evaluated the
short-term improvement in an anatomical knowledge test for students,
where 3D printed heart models and real cadaver hearts were used. He
found a positive outcome in evaluating the experience with 3D models.
Lambrecht 69 produced models for oral surgery training by
means of a stereolithographic printer (SLA), to facilitate students to
learn complex surgical procedures.
Soares et al 71 produced models of teeth to instruct
students on cavity preparation techniques. Other
authors realized 3D models as practical support for preclinical courses,
for example Kroger et al 72 created models to train on
caries removals and provisional prosthesis; Reymus et al 73
printed replicas of teeth with endodontic cavities to simulate
endodontic preparation. The common outcome of these studies was the
overall positive evaluation of the 3D printed models by both the
students and the teachers. 3D printers are also welcomed by students and
university staff, as documented by Walker 74.
There are already several online libraries in which anatomical models
can be found, such as the one provided by the NIH 75 and
the one on the website Embodi3d 76. Moreover, the
possibilities offered by the described workflow allow to create original
anatomical models of complex cases or special procedures at reduced
cost. The initial effort in software learning is offset by the range of
possibilities offered by the workflow in question as an aid to dentistry
training.
Surgical planning is an important step in the patient’s treatment,
because it provides the surgical team with an in-depth knowledge of the
case under examination and allows to evaluate the best approach to
accomplish the surgery. The digital imaging techniques (NMR, CBCT etc…)
associated with 3D models have been experimented by several authors with
the aim of providing the surgeon with a real reference to plan the
intervention.
Models that replicate the anatomy of the region to be operated have been
created for different surgeries, from vascular surgery to orthopedic
surgery. The Medical Modeling book by Bibbs et al 1
collects a series of interesting clinical, surgical, dental and research
cases that show the use of medical modeling and rapid prototyping
techniques. The surgeon can simulate on the models the execution of the
osteotomies, simulate the new arrangement of the bone segments and
create surgical guides as an aid to the surgery 77,
78.
Wang 109 reported the use of 3D models to plan mandibular
orthognathic surgery and for surgical guides manufacturing, noting
greater speed and precision in the execution of the osteotomy and
positioning of the bone segments.
The Blender OrtogOnBlender plugin 64, 79 helps
the planning of orthognathic surgery, facilitating
the simulation of osteotomies and allowing to evaluate the consequences
of bone mobilization on the patient’s face. The patient’s face can be
scanned or detected with a series of photographs; OrtogOnBlender allows
you to derive 3D models through photogrammetry, and to use these models
in combination with the CT scans, enabling the use of combined models of
the surface and inner part of the organism. In this way it is possible
to simulate the consequences of the bone segments positioning on the
patient’s face 145.
Orthognathic surgery simulation with OrtogOnBlender 80
starts loading DICOM images into Blender, but an already processed .stl
model can also be used. The plugin facilitates the digital osteotomies
by creating cutting planes to be placed in the desired position. After
the digital osteotomies we can isolate the segments and move them in the
planned position.
The management of the pictures for the photogrammetry reconstruction is
fast: after importing the folder containing the pictures, the software
automatically creates the model of the patient’s face. Through a few
operations the scan of the face is aligned with the CT, enabling the
preview of the planned surgery of the jaws.
Rapid prototyping technologies have been used successfully for the
realization of a custom surgical obturator following the removal of an
upper maxillary carcinoma 81.
Ackland et al 82 rehabilitated a patient with Temporo
Mandibular Joint osteoarthritis (TMJ) by designing a customized
prosthesis, on which they performed Finite Element Analisys (FEA) to
optimize their position and fixation. The digital model was then printed
in Titanium 6Al4V with an SLS printer and implanted on the patient with
good results.
The integration of digital techniques and 3D printing in the surgical
workflow can be of considerable help to the surgeon for the surgery
planning, especially in cases of complex surgeries and in sensitive
areas of the organism (for example, near neurovascular bundles). These
technologies allow to customize any rehabilitation devices, such as
prostheses that adapt to the patient’s anatomy and biomechanics.
Multidisciplinary collaboration in surgical planning is a building block
of this rehabilitative approach focused on customization.
Digital imaging is fundamental in implantology for bone evaluation and
implant site selection, while modeling and rapid prototyping techniques
allow to quickly create customized surgical guides, which can be
sterilized and used for the insertion of the implants 83.
As shown by literature analysis, the surgical guide allows to operate with
greater precision than the manual insertion procedures. Van Assche
105 carried out a literature review, giving indications on
the use of surgical guides in implantology. The position of the implant
inserted with the guides is more predictable than the manual insertion,
and the guide in the implant insertion phase has a higher precision than
the guidance of the sole osteotomies, where only the site preparation is
guided, while the subsequent insertion of the fixture is manual. The
average error found with the guides is about 1mm in the entry position,
1.3mm at the apex with an inclination difference of about 4 degrees,
although with a wide variability between the analyzed studies.
Beretta 104 has found similar data in the literature, but in
his small series of 14 implant rehabilitations performed with surgical
guides, he found lower errors. The greatest precision is attributed to
some measures, such as the use of extraoral references for correct
anatomical positioning, the combined use of CT scans and optical scans
in positioning procedures, and intraoral fixation of the guide with mini
implants.
According to the analyzed reports the use of the surgical guides is a
valid aid to the implant insertion procedures, keeping in mind an
adequate margin of error of at least 2mm from sensitive areas
104. The accuracy in the production of the guides is
important, so we must try to reduce the error accumulated between the
scan of the arches, the design and manufacture of the guide.
In the field of implantology, the authors described anatomical shaped
implants to be inserted in post-extraction alveolus (Root Analog Implant - RAI). These implants are made with CAD/CAM techniques of additive or
subtractive manufacturing (milling), and replicate the morphology of the
dental element to be replaced.
The anatomy of the alveolus can be obtained through the use of a CBCT scan or with the optical scan of the
extracted tooth root. The optical scan requires operating in two steps,
the dentist needs to extract the tooth to scan it, create the digital
model, print the implant and reopen the surgical site to insert it. The
preoperative CBCT allows to plan the intervention, to create the
personalized implant and then to insert it immediately after the
extraction of the tooth, in a single session.
Mangano 84 used CBCT reconstructions to make the customized
implants by means of DLMF (Direct Laser Metal Forming) printing
technology, which uses a laser to sinter titanium particle at layers
height of 0.2 mm. Final ceramic crown was
inserted on the RAI and the maintenance of peri-implant tissues was
noted at the annual inspection. Mangano 85 then performed a
study of 15 patients, using root analog implants. Although further
studies are needed, the work showed how RAI made with DLMS (Direct
Laser Metal Sintering) can be a treatment option for post-extractive
rehabilitation cases of dental elements where atraumatic avulsion is
possible, so to keep intact corticals.
Pirker 86 modified the root of the extracted tooth with the
addition of composite macro-retentions on the distal and proximal
portion, leaving the vestibular surface and the lingual surface of the
root unaltered. The root thus modified was scanned with an optical
scanner, and the obtained digital model was slightly reduced in the
diameter of the vestibular and lingual regions (between 0.1 and 0.3 mm)
to limit the risk of fracture of the alveolar corticals. The implant was
then produced in zirconia using a CAD-CAM milling machine and implanted
in the alveolus. Primary stability was optimal,
thanks to the use of interdental macroretention. At the 2 year follow-up
there were no signs of bone resorption or gingival retraction, sign of a
correct distribution of stress on the alveolus wall.
The same author then made a comparison between different topographies of
zirconia RAI made with CAD-CAM technology in two group of patients
87. A group of patients was rehabilitated with RAI
characterized by a rough surface created by sandblasting, while the
second group was treated with sandblasted RAI on which macroretentions
on the interdental surfaces were present. The group of sandblasted
implants showed a success rate of 0%, with all 6 implants inserted that
failed before the application of the final crown. The group treated with
macro-retention RAI showed a success rate of 92% at two years, with only
one implant lost on 12 inserted. The failure of the sandblasted implants
was attributed to the uniform pressure exerted by the implant on the
alveolus walls, instead in case of RAI with interproximal macroretention
the distribution of the load in defined areas allowed to reduce the
stress on the bone, favoring the osseointegration of the RAI.
Patankar 88 replicated the zirconia RAI with interdental
macroretentions of Pirker for the rehabilitation of a lower premolar,
with a positive result.
Moin 89 used the Digital Light Processing (DLP) additive
manufacturing technique to realize a zirconia replica of a dental
element. The author then digitally compared
the CAD model of the replica, the scan of the printed zirconia replica
and the scan of the original tooth, noting the adequate precision of the
DLP technology for the additive manufacturing of zirconia products.
The production of ceramic objects is currently also possible using the
extrusion printing technique, as documented by Nötzel 97.
The process used by Nötzel consists in the production of a filament
composed of paraffin, LDPE and Al2O3 particles; the filament is molded
by extrusion to form the object, which is then subjected to chemical and
thermal treatment for the removal of the medium in which the Al2O3
particles are dispersed, and finally sintered to give the final product.
The extrusion printing of ceramic products is still to be evaluated in
the manufacturing of dental products.
These reports show us that, using digital techniques, the production of
Root Analog Implants is now possible in a precise manner and with the
use of biomaterials that promote osteointegration and aesthetic and
functional rehabilitation.
The correct evaluation of the alveolus and RAI morphology, and of the
distribution of the masticatory forces on the alveolar bone is important
in this perspective. The atraumatic avulsion of the tooth is at the
basis of rehabilitation with RAI, because any trauma to the alveolus
results in bone resorption and gingival retractions, with consequent
degradation of the aesthetic characteristics of the prosthetic
rehabilitation. The distribution of the forces on the alveolus walls is
also to be precisely assessed.
With the RAI we seek primary stability by means of the dimensional
congruity between the alveolus and the Rot Analog; it has been shown
that excessive stress on the alveolus walls causes implant failure,
probably due to reduction of the blood supply to the implant site and to
the surrounding bone, which undergoes resorption. Further studies are
necessary to asses the safety and standardization of the procedures here
presented, but the anatomical implants could allow, in selected cases, a
functional and aesthetic solution to the patient problem, and facilitate
the resolution of post-extractive implants treatments maintaining high
aesthetic standards.
Rapid prototyping has been used both in fixed and removable prostheses,
for the realization temporary restorations, aesthetic guides and
mockups. Various additive manufacturing technologies and various
materials have been used.
Tahayeri 98 tested various features of SLA printing with
specific dental resins (NextDent); he evaluated the influence of some
printing parameters on printing precision, mechanical properties and
degree of conversion of the resin. The printed specimens proved to be in
the precision range required for clinical use, as were the mechanical
properties of the samples themselves. The author showed differences in
the printing properties between various dental resins made by the same
manufacturer, as well as different intensities of the printer’s laser
during the polymerization of the resins, depending on the darkness of
the resins color and the relative light absorption rate. A selection of
resins and printers optimized for combined use could further improve
print accuracy; better results may be obtained from the fine adjustment
of the printing parameters.
To be considered that, according to the manufacturer’s indications, the
resin would have had to undergo a second polymerization step after
printing, which the authors did not perform to accelerate the eventual
production process of the temporary restoration. Nevertheless, the
mechanical properties of the non-post-polymerized resin have proved to
be adequate to resist intraoral loads.
Katreva 99 realized a workflow that integrate the use of 3D
printed working models, the 3D printing of the temporary restoration and
the printing of the final prosthesis for the pressed ceramic
conversion.
Revilla-León 100 used a digital workflow for the scanning of
impressions, the creation of the diagnostic wax-up, the printing of a
guide for the realization of the temporary restoration and finally for
the production of the veneers, useful for aesthetic and functional
rehabilitation of anterior sector of the maxillary arch. The guide for
the construction of the temporary prosthesis was made using the DLP 3D
printing technique, while the final lithium disilicate veneers were
CAD-CAM milled.
Alharbi 101 evaluated the possibility of making prosthetic
crowns with SLA printing technique. The author measured the accuracy of
the printed crown at various angles with respect to the horizontal plane
and with the use of different size of supports. The same research group
then evaluated the crown printing accuracy using the DLP 102
printing technique. Both technologies proved to be accurate, with SLA
printing which showed greater accuracy in replicating the morphology of
the digital model. Both technologies are interesting for use in
dentistry, but it is necessary to carry out further studies on the
influence of the printing parameters on the final model and better
explore the properties of the dental resins.
Ebert printed zirconia crowns with a custom printer using the ink-jet
technique. To make the crown, a zirconia-containing solution was
deposited layer by layer to form the product, which was then sintered. This study has shown that zirconia crowns with
good mechanical properties and precision for clinical use can be
achieved by means of an additive ink-jet manufacturing process
106.
Alharbi 107 evaluated the use of additive manufacturing in
prosthetics, analyzing various reports and studies focusing on the fixed
prosthesis and the partial and total removable prosthesis. The author
has reported that the metal framework realized with the SLS (Selective
Laser Sintering) method are equivalent or more precise than the
classical melting procedures, with respect to the gap between the metal
framework and the dental abutment; in the same way the mechanical
properties of the SLS products were equivalent or better than those
realized with the classical procedures. Metal framework for the creation
of removable partial dentures were produced by means of additive
manufacturing either directly or indirectly. Direct manufacturing
consists in printing the design by means of SLS processes; Indirect
manufacturing consists in the realization of the castable resin product
by means of SLA or DLP printing, the integration of the calcinable model
in refractory material and the subsequent casting of the metal or alloy
for the realization of the metallic framework. Both techniques for
building removable prosthesis frameworks have shown acceptable accuracy,
although the results are primarily derived from in vitro and case report
studies.
Lin 108 demonstrated a technique for the realization of
provisional total prostheses through a digital protocol that provides
optical scanning, digital diagnostic waxing of the prosthesis, printing
of the prosthetic base and respective dental arch in separate phases,
with resins of color and suitable characteristics; finally the union of
arch and prosthetic base is achieved. The study has not tested its use on the patient and there are no further
reports of the clinical performance of total provisional removable
prostheses made with this procedure and these materials. This concept
could be explored, evaluating the accuracy compared to the other
available manufacturing methods and the duration of the prosthesis over
time, both from the perspective of the color and wear of the resin.
The available data on digital treatment planning techniques and the use of additive manufacturing technologies show promising prospects in dentistry, both for the production of temporaries 125 and of removable prostheses 107. Further studies and evaluations with longer follow-up are still necessary before extensive clinical application of the technology. Furthermore, the possibility of producing definitive ceramic or zirconia prostheses, theoretically achievable with technologies such as SLS or inkjet printing, remains unexplored.
Orthodontics is one of the branches of dentistry that more can gain
advantage from the new possibilities offered by digital treatment
planning and additive manufacturing technologies. Orthodontic therapy is
classically planned by means of teleradiography, plaster models in the
articulator and set of photos of the patient, in addition to the
fundamental clinical and functional evaluation. The complex
relationships between the bones of the skull involved in the oral
function are difficult to analyze adequately on two-dimensional
radiography and plaster models, especially if the treatment involves a
surgical phase.
Digital orthodontic therapy can use the ability to print custom brackets
115 and surgical guides for the insertion of orthodontic
implants 116. 3D printing also
facilitates the production of guides for brackets positioning in the
patient 127, which favor a fast and precise positioning of
the brackets, and the design and manufacturing of personalized auxiliary
tools 126. The custom brackets were made by Krey through
FreeCAD software and a DLP printing process. With
the same technique a positioning splint was virtually designed and
printed, allowing quick and precise positioning of the brackets.
Intraoral scans at time intervals were performed and compared in MeshLab
to verify the orthodontic movement of the teeth; the post treatment
retention splint was also virtually designed and 3D printed.
This was a test study with edgewise brackets printed with
light-curable resin; the authors suggested the possibility of revising
part of the design to optimize the mechanical resistance of the
brackets. The report is positive and opens up the possibility of a
radical change in the way in which orthodontics with fixed devices can
be performed in the dental office, with the transition from the use of
generic brackets to custom brackets achievable in the clinic.
In modern dental practice, the use of plaster models is often
accompanied by digital scans and 3D printing of the model. Several
authors have evaluated the accuracy of models made with additive
manufacturing techniques, with different results. Dietrich
112 evaluated the accuracy and accuracy of dental models
made with polyjet and SLA techniques. The printed models were scanned
and evaluated via software to asses the discrepancy with the original
model. Both methods of printing proved to be capable of printing
accuracy suitable for orthodontic use, with a maximum detected error of
about .
Wan Hassan 113 manually compared, by means of a caliber,
dental arched models of patients, made of plaster and model printed with
SLA technology. The author assessed that the printed models are not
suitable for orthodontic use due to a discrepancy of about 1mm compared
to the plaster one. The author reports that the digital scan of the
plaster models resulted in a loss of detail, moreover the measurements
with gauge were sometimes not easy to carry out due to the overcrowding.
Both of these conditions may have contributed to the reported error.
Removable devices can also be manufactured with 3D printing
111. In addition, 3D printed surgical guides have been used
to perform cortical osteotomies in order to accelerate orthodontic
movements 114.
Notable is the integration with biomechanics allowed by digital
orthodontics. The use of optical scans and CBCT give detailed
information on the patient’s structure. Orthodontic movements are based
on biological and physical principles, where controlled forces are used
to activate the bone remodeling process, which allows the movement of
the tooth. These factors could be investigated by integrating the
anatomical data (bones, muscles, ligaments, organs) and functional data
(masticatory force, mechanical characteristics of the tissues and
materials involved, range of mandibular movement …) to simulate in
silico treatment, providing to the patient a personalized treatment
according to its biological features and treatment response
110, 140.
