Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
3D printing in dentistry with emphasis on prosthetic rehabilitation and regenerative approaches
Introduction
The process of three-dimensional (3D) printing any object comprises a series of procedures that begin with scanning and obtaining several two-dimensional (2D) images of the organ. The images obtained are stacked together to form a structure, followed by a processing step with the aid of processing software, and, finally, built up layer-by-layer. The first references of 3D printing are found nearly 40 years ago, in 1981, in Japan by Hideo Kodama . Stereolithography ( SLA ) was introduced 2 years later by Charles Hull, describing his proposition as “successive sheets corresponding to successive cross-sectional layers dispensed and selectively exposed to synergistic stimulation and integrated with preceding layers to provide a substantially layer-by-layer build-up of the 3D object” .
Although dentistry is familiar to milling, the inherent deficiencies, such as material waste, slow speed, accuracy bounded by the object complexity, material properties, and the size of the cutting tools, have led to seeking alternatives to overcome these pitfalls. The so-called additive manufacturing ( AM ) or rapid prototyping has the opposite rationale to subtractive manufacturing or milling, which relies upon material removal for object formation. 3D printing offers the possibility to produce customized implants of prosthesis at a fraction of the time and cost originally entailed owing to the versatility of the printing process, introducing the advent of fully personalized treatment. In other words, 3D printing, through incremental vertical object buildup, may produce large objects passively with fine details and no material wastage . Dentistry has benefited from this technology to in its every field, predominantly in oral surgery, prosthodontics, orthodontics, endodontics, and periodontics, as shown by rising numbers in publications containing the term “3D printing,” although still at lower levels compared to Μedicine . This rising trend owes much to the expiration of the early patents, which have been related to the AM devices and processes. AM may be categorized based on the method of fabrication and this includes SLA, fused deposition modeling (FDM), binder jetting (BJ), electron beam melting (EBM), laser melting (LM), laser sintering (LS), digital light processing (DLP), and material or photopolymer jetting (PJ), which are well-known technologies of 3D printing.
Based on the nature of the material used, the techniques are divided into four categories: those using light-cured resin ( SLA , DLP, and PJ), those using powder binder (BJ), those using sintered powder ( SLS, SLM, DMLS , and EBM), and the one using thermoplastic materials ( FDM ). SLA is a 3D printing method, in which a scanning laser cures a light-sensitive polymer layer-by-layer in a vat of liquid. It is a rapid fabrication method, with low-cost material, that has the ability to create complex structures with high resolution. DLP uses a projector light source to cure liquid resin layer-by-layer, building the object upside down in an elevating platform. It provides high accuracy and smooth surfaces, but the materials used are expensive and the support must be removed at the end of the fabrication process. While both SLA and DLP use light as a polymerizing source, DLP provides significantly higher speed, as it has the ability to simultaneously photocure all the surface . Photopolymer jetting (PJ)/ inkjet printing/material jetting is the method that resembles the traditional household inkjet printer, but instead of the ink being absorbed on paper (2D), micrometer-sized droplets of an ink (liquid resin ) are selectively jetted out of hundreds of nozzles and polymerized with ultraviolet (UV) light. Multiple print nozzles can be used with various color options and different materials. They layer thickness of such an apparatus may be as thin as 15 μm. Selective laser melting ( SLM ) and selective laser sintering ( SLS ) belong to laser powder forming methods where a laser is directed at a layer of fine powder substrate, and this laser causes full melting (for SLM) or sintering (SLS) of the powder. As soon as the layer is complete, another powder layer is deposited . Among the two methods, SLM uses higher energy density; the whole procedure is carried out in closed chambers to avoid oxidation of metals and finally the fusion of the powder particles in this layer-by-layer buildup results in the 3D object . During SLS, if the metal powders used are mixed, the one with lower melting points is melted and it is consequently used as a binder; this particular occasion is called direct metal laser sintering ( DMLS ) . FDM is based on thermoplastic material extrusion through nozzle onto an ascending build platform. FDM results in low accuracy and can be used in limited materials, while support material must be removed at the end of the fabrication process . EBM uses scanning electron beam that sinters powder layer-by-layer on a descending build platform. While it exhibits high processing speed, it has several disadvantages, such as extremely costly technology, production of hazardous dust, and explosive risk .
The large availability of devices makes it possible to expand the printable materials to a wide range, including resins, polymers, ceramics, and metals . Although promising, 3D printing in Dentistry is not devoid of limitations. The so-called “staircase effect” on the finished product due to the layer-by-layer manufacturing, the aesthetically poor porous ceramic products, the low reproducibility, the need for support structures in some of the manufacturing techniques, and the need for devices tailored to dental and orofacial applications are some paradigms highlighting the need for further improvements before incorporation of these technologies as state of the art in everyday practice .
In this chapter, the recent advancements of 3D printing technology related to dental and orofacial applications are thoroughly reviewed and discussed, while future perspectives in oral reconstruction and regenerative dentistry are further highlighted.
Oral surgery
Based on the high number of publications in the last decade, the first and widest field of 3D printing applications in Dentistry is that of oral surgery . This holds true since this is a tremendous tool in the surgeon's arsenal, helping to overcome “guesswork” of a third plane until the actual operation time, and providing insight of the surgical anatomy in a 3D environment imitating the natural case scenario. The armamentarium has been supplemented with virtual surgical planning, deriving from 3D scans (cone beam computed tomography, CBCT), where reconstruction of the missing tissues is made possible, by mirroring the contralateral nonaffected sides. Furthermore, for maxillomandibular reconstruction cases following tumor resection or trauma or orthognathic surgeries, it has been made possible to preview the final result. On top of these, the advancements in 3D imaging and planning offer options of simultaneous implant placement at the donor sites and precise transfer to the recipient site in oral cavity, minimizing the needed surgical procedures. By postoperational scanning and superimpositions, it is also possible to evaluate the final outcomes.
The procedure for maxillary or mandibular reconstruction proceeds as follows: In a computer software, data from the radiographic imaging (CBCT) of the area of interest are entered in a format called DICOM (digital imaging and communications in medicine). This allows for a complete representation of hard and soft tissue anatomy in 3D. Labor that can be implemented in this environment includes addition of the missing areas by insertion with the aid of stock mandibles manipulated for the reconstruction or even by reflection of the existing contralateral area so as to reproduce the missing anatomy, although a CBCT prior to tumor resection would be a more reliable source for the restoration. This derived model can be used as a template for prebending the titanium plates that will be used to anchor the added graft; they can also act as a template for the fabrication of osteotomy guides, with slots for the saw blade, which will be fixed to the donor site (e.g., the fibula) via monocortical screws. A second CT scan, of the donor site this time (lower extremity), is also inserted within the virtual environment to orientate the osteotomy guide production and in such a way the stents manufactured for this purpose may also serve as a drill guide for immediate implant placement at this site. The latter aid in the precise segmentation of the donor site in order to accurately fit in the area with the deficits . The mostly utilized method for this model production is SLA. In addition to the above, in the case of maxillary reconstruction, navigation may also be used in assistance, due to the immobile nature of the site but not for the mandible .
The benefits of the above techniques are numerous. In a case control study, Hanasono et al. compared the outcomes of microvascular free flap reconstruction of the mandible with the aid of CAD and rapid prototyping technique or by the conventional method, where bending the titanium plates was performed on the native mandible. The overall operative time was less for the rapid prototyping group (8.8 ± 1.0 h) than for the conventional group (10.5 ± 1.4 h). Furthermore, by measuring the mean change in postoperative length of bony landmarks of the mandible through postoperative CT scans and comparing that to the planned lengths, there was less deviation for the rapid prototyping group (4.11 ± 3.09 mm) than for the conventional group (6.92 ± 5.64 mm). Further to these, in another study, it was shown that operative time (666 ± 33.4 vs. 545 ± 12.6 min), as well as ischemia time (120 ± 19.8 vs. 73 ± 11.2 min), could be further reduced if virtual surgical planning was used in addition to the CT scans and stereolithic models for prebending the titanium plates and producing osteotomy guides. While in the cases where plates were prebent on stereolithic models (only serving as templates) but osteotomies were performed without cutting guides, the osteotomies were less accurate than virtual surgical planning cases and required the use of bone paste and grafts to replenish the gaps (27% vs. 2% of the cases, respectively).
Another field of interest where AM finds use in craniofacial operations is that of orthognathic surgery. Conventional methods of data acquisition rely on 2D radiographic imaging, as well as planning on plaster dental models mounted on semiadjustable articulators through face bow transfer. This poses obstacles in cases of facial asymmetries of the ears or eyes rendering face bow transfers inaccurate and difficult. As opposed to the above, 3D radiographic imaging, photography, dental model scanners, virtual planning, and 3D printing may assist in resolving these issues and most importantly it provides insight of the final outcome in terms of facial appearance preoperatively. A comparative study between 3D virtual model surgery with SLA versus the conventional model surgery showed that both methods had discrepancies in the predicted and the actual location of designated landmarks, but mean discrepancy was lower in the virtual model than in the conventional model (0.95 mm with SD of 0–3.2 and 1.17 mm with SD of 0–3.6 mm, respectively) .
Orthodontics precede and proceed the surgical procedure, initially to decompensate malocclusion, coordinate the width of the arch and rigidly stabilize the arches prior to surgery, while handling the final occlusion, and provide retention postsurgery. In between, 3D data and virtual planning aid in splint fabrication through 3D printing . In this intermediate step, the surfaces of the dental casts of the maxilla and mandible are laser scanned and “mounted” on 3D CBCT scan in centric relation, the maxillomandibular readjustments are made in virtual space, and splints to guide the actual surgical practice are 3D printed using SLA . In another study concerning virtual planning, it was shown that only 4% of the orthognathic virtual plans were abandoned, while 85% were completely adhered to and the rest were partially adhered to. Reasons for abandoning a virtual plan were poor communication between surgeons and engineers, misplacement of the condyle preoperatively, rapid tumor progression, and poor evaluation of the anatomy of the area preoperatively . This stresses that the data acquisition needs to be obtained at a time frame close to the actual operation, so that deviations will be kept to the minimum.
Besides the oral cavity, maxillofacial surgery and dental implants seem to find purpose in cosmetic surgery, using AM to print auricular, orbital, or even nasal prostheses out of silicone. Existing contralateral segments of the respective areas are laser scanned and digitized, and their mirror images are 3D printed. They are held in place through magnetic or mechanical attachments between the dental implants and the prostheses . Based on the above, 3D planning and printing are regarded as invaluable for tackling facets of craniofacial surgery and improving the quality of life of patients with serious deficiencies with improved predictability compared to conventional methods.
Prosthodontics
Fixed prosthodontics
Current progress in 3D printing technologies offers limitless possibilities in the field of prosthetic dentistry. Even though integration of 3D printing in the clinical workflow is still limited, it has already been applied in almost all aspects of prosthodontic treatment, from dental casts, metal frameworks, and removable complete and partial dentures to obturators for maxillofacial defects. The wide variety of materials used in prosthodontics necessitates the use of different AM techniques.
There are several parameters that must be taken into consideration when fabricating a dental prosthesis with the aid of 3D printing, such as dimensional accuracy, mechanical and physical properties, cost, and time. Those parameters are closely associated with factors that are defined and affected by the manufacturing technique, such as layer thickness, build orientation, and support structure . More particularly, build orientation, which is defined as the orientation of printing, has been found to affect dimensional accuracy, as well as surface properties and overall fabrication time . In addition, selection of build orientation affects the extent of manifestation of the “staircase effect,” which further affects dimensional accuracy of the printed object . As a result, it becomes apparent that slight modifications of those dominant factors can directly affect the accuracy of the restoration and by extension marginal and internal fit.
Moreover, each of the manufacturing techniques used for 3D printing bear its own characteristics that can influence processing accuracy. The accuracy of SLA is considered one of the highest among the different AM techniques . The x-y planes are mainly related to the accuracy of SLA; however, z plane, which can be affected by many factors, can significantly influence printing accuracy . SLA has been widely used to produce dental models. Several studies assessed trueness and precision of SLA-fabricated dental models in comparison to the original stone models , , . Keating et al. found that despite the fact that the mean difference between the measurements made on the stone models and those made on the SLA models was not statistically significant, the mean difference of the measurements on the z plane reached a statistically significant level . In a similar study, Jin et al. compared the accuracy of dental models from two different AM techniques, SLA and PJ, to the stone models , . In terms of trueness, there were no significant differences among the three different manufacturing processes, while in terms of precision, SLA- and PJ-fabricated models exhibited higher precision compared to the stone models .
In LM or LS techniques, such as SLS, SLM and DMLS, the characteristics of the particles, such as melting temperature, shape, and size, affect surface properties of the fabricated object . Deviations in melting temperature can result in distortion and increased surface roughness, which have been associated with poor adaptation of SLM-fabricated frameworks , .
LM or sintering techniques have been applied for the production of metal frameworks for fixed partial dentures (FPD) with marginal fit comparable to frameworks fabricated with the conventional workflow and within clinically acceptable values , , , . More specifically, in vitro studies using AM techniques for the fabrication of frameworks have had some very promising results, with improved marginal fit. Pompa et al. compared the marginal fit of 3-unit FPD, where the reported marginal gap was 43.9 μm for the SLM-fabricated cobalt-chromium (Co-Cr) frameworks and 47.5 μm for the lost-wax-fabricated nickel-chromium (Ni-Cr) frameworks . However, internal adaptation was better for the Ni-Cr frameworks (54 μm) compared to the SLM-Co-Cr frameworks (58.7), though the difference was not statistically significant . In a similar study, Örtorp et al. showed that DMLS-fabricated 3-unit FPD frameworks had significantly better marginal fit (84 μm) compared to conventional lost-wax-fabricated frameworks (133 μm) and milled frameworks (166 μm) . Furthermore, in the study by Ucar et al., DMLS-fabricated crowns exhibited similar internal gap with cast frameworks . Similarly, DMLS Co-Cr frameworks for implant-supported restoration exhibited lower values of marginal gap compared to conventionally fabricated cast Co-Cr frameworks . On the other hand, Kim et al. found that the marginal and internal gap of Co-Cr FPD frameworks produced with DMLS was significantly larger compared to the conventionally fabricated cast frameworks . In a clinical study, Quante et al. assessed the marginal fit of single crowns fabricated using SLM either from base metal or from precious alloy, where no statistically significant difference was found between the two alloys, with values ranging from 74 to 99 μm . In addition, Huang et al. compared the marginal fit of SLM-fabricated Co-Cr crowns and cast Au-Pt crowns and found that there was no significant difference among the two types of crowns , . Furthermore, clinical assessment over a period of 47 months revealed that laser-sintered crowns exhibited a cumulative failure rate of 1.7%, with the main causes of failure being extraction of abutment teeth or need for endodontic treatment . While most of the studies assessing the accuracy of 3D printed frameworks were in vitro , the superiority of SLS/SLM/DMLS-fabricated frameworks over the conventional cast or milled frameworks was revealed in terms of marginal adaptation.
Regarding post and core fabrication, only one study assessed the fracture resistance of DMLS-fabricated post and core compared to the ones produced with conventional casting and milled cast . DMLS-fabricated post and core exhibited similar fracture resistance to the ones from conventional casting and lower compared to the milled cast post and core .
Interim prosthesis
To protect the prepared abutments against microbial attacks and the underlaying pulp from stimuli entering the oral cavity, interim prostheses need to be manufactured and worn by the patient to serve functional, esthetic, and space maintenance purposes. An alternative to the conventionally prepared provisional restorations, by means of heat pressure, may be assembled through 3D printing. Here, casts of the prepared teeth or the teeth directly need to be scanned and retrieved within the design software. The interim restorations are then designed to restore initial anatomy of the respective intact tooth/teeth and saved into an STL file, this is then exported to the printer where individual printer parameters are set . Important parameters to assess are the orientation of printing, the printer itself, the technology of 3D printing, the color of the material, and the laser intensity among others.
Each millimeter of material to be printed corresponds to 15–20 layers of material to be laid and fused so that the final shape is delivered . The procedure of printing of a simple restoration, such as a single crown, takes approximately 20 min, which is a significant benefit, allowing the dentist/dental technician to perform other tasks and increasing the overall productivity while the restorations are being produced .
The materials for this purpose seem to follow a similar classification as to those employed for conventional provisional restorations, namely those based on monomethacrylates or acrylic resins and dimethacrylate or bis-acryl/composite resins, according to their chemical composition . The technique mostly utilized for this purpose is SLA and various studies have used it to study aspects of these restorations, such as their marginal fit. In a study by , it was found that the marginal fit produced by fused deposition using PLA (polylactic acid) was 122.89 μm (SD 26.06 μm), which is slightly above the acceptable marginal gap of 120 μm proposed by in literature . Addressing the same issue of marginal discrepancy, Lee et. al. showed that 3D printed provisional crowns had lower values than those produced by milling (mean discrepancy and standard deviation for two 3D printed methods and milling, respectively, 149.1 (65.9) μm, 91.1 (36.4) μm, and 171.6 (97.4) μm ). It was also mentioned that milling caused exhaustion of the materials and was more time-consuming. Other published work also consider that AM enables the production of more refined geometries that may even be impossible to be produced by milling, where dimensions are limited by the bur size .
An aspect to keep in mind is the direction of the structure to be printed. It seems that when specimens are printed vertically, so that the testing load is directed perpendicular to the printed layers, the compressive strength of the material is higher than in the case of horizontal printing (297 ± 34 MPa and 257 ± 41 MPa, respectively) . This is also beneficial because vertical printed structures require fewer supporting structures, thus there is a smaller contact area between the support structure and the printed object, subsequently less time is needed for finishing and polishing. In support of this statement, another study mentioned most accurate dimensional readings of width and length for specimens printed at 90 degrees (vertical to the build platform), and although the thickness accuracy was best for the horizontal orientation, it was thought that the ideal printing orientation was the vertical, since this would again demand less support structures . In the same study, 3D printed samples were compared to commercially available materials (“Jet” and “Integrity”). Seemingly, the 3D printed samples had higher degree of conversion than conventional methods, as deduced by FTIR spectroscopy and comparable elastic modulus to “Jet” and peak stress similar to “Integrity” proving to be equally sufficient in terms of mechanical properties. A limitation was, however, that the specimens for both the abovementioned studies were cylindrical. To simulate clinical conditions more closely, Alharbi et al. fabricated crown-shaped restorations with different angulations to the support structures and with two different versions of support (thick or thin) . It was deducted that the ideal configuration was when the angle between the transverse axis of the crown was at 120 degrees to the support platform with the lingual surface facing toward the support for both support occasions, as the deviation resulting from the superimposition of the printed crown compared to the scan of the reference model (designed crown) was as low as 0.031 and 0.029 mm for the thick and thin platform, respectively .
Another property where 3D printed interim restorations overshadow those produced conventionally or by milling is that of microhardness, as measured at 32.77 for 3D printing, 25.33 for milling, and 27.36 for conventional in Knoop hardness numbers ).
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here