Study models

The earliest application of 3-D printing was in the making of anatomic study models.^[7]

CBCT along with 3-D printing technology has transformed diagnosis and treatment planning in the field of dentistry.^[8], ^[9]

The CBCT provides volumetric data to the 3-D printer to produce a detailed model of the bone structure in question. This allows the clinician to gain a clear idea of unusual, or unfamiliar anatomy to draw up the most favorable treatment plan.

The ability to clearly understand the anatomy and to produce suitable cutting guides using the technology has paved the way for newer surgical approaches which are less invasive and have better predictable outcomes. In reconstructive surgeries, 3-D printing has been used to prepare customized reconstruction plates and prostheses. This can also be used as an alternative to invasive autologous bone grafting procedures.^[10]

Applications in designing prosthesis

The use of intraoral optical scanners enables the development of precise virtual models of implant positions, prepared tooth surfaces, and even the complete dental arches. For prosthodontics rehabilitation, prosthesis and restoration can be designed using CAD software. The data of the designed prosthesis can be milled using CAD-CAM software or can be 3-D printed.^[11]

Fabrication of metallic structures using 3-D printing can be achieved by either indirect methods like printing in burn-out resins or waxes for a lost-wax process or directly in metal alloys.^[12]

Printing in resin/wax and then using a traditional casting approach is beneficial as there is a reduced need for post-processing procedures when compared to that in the direct 3-D printing of metal casting alloys.^[13]

Printing directly in metals is found to be more expensive as they require the use of costly technologies^[14] and also demand elaborate post-processing before components may be ready for use.

Though there is some amount of material wastage, milling is advantageous over printing as the material used is intrinsically homogeneous and unaffected by operating conditions. Milling requires little post-processing, and the equipment is by far less expensive.^[11]

Applications in restorative dentistry

Intraoral scanners are increasingly being used to scan the prepared tooth surfaces. This has necessitated the employment of 3-D printers to make physical models from the scanned data.

Though it is not always necessary to prepare physical models¹⁶ for fabrication of restorations, the models required when veneering materials have to be added. The models are also needed for the display of the restorations even when these restorations and digitally fabricated.^[11]

Orthognathic surgery

The success of orthognathic surgery hinges on accurate diagnosis and treatment planning.

Conventionally orthognathic surgery planning involves 2 stages- radiographic simulation with cephalometric planning and manipulation of physical dental models in the desired direction. This process is essential to establish a postoperative occlusion which is then used to fabricate a surgical wafer. 3-D imaging technologies like CT and CBCT provide accurate data to develop study models, both digital and physical which are required for treatment planning.

Simulated repositioning of jaws using digital study models can be performed and surgical wafers can be accurately fabricated using 3-D printing technology.

This eliminates the arduous process of plaster model making and facebow recording and in turn minimizes the errors and imprecisions.

The wafer fabrication stage involves virtual filling in the void between the upper and lower digital dental models in their final occlusal positions. After the approval of the digital surgical wafer by the technician and clinician, the surgical wafers can be 3-D printed prior to the surgery.^[15]

Scanners and printers

New intraoral scanners are rapidly being introduced into clinical practice.^[24] Though the detailed description of all the available technologies is beyond the scope of this article, a brief listing of the available scanning and 3-D printing technologies is listed in [Table 1].

Review of recent literature involving 3-D printing technology incorporated into orthodontics

In a study conducted to compare 5-axis machining and 3-D printing it was observed that 5-axis machining can achieve the 0.01–0.02 mm accuracy level that is required by the dental industry standard. However, a 5-axis milling machine can fabricate only one model at once. Conversely, 3-D printing though less accurate (about 0.03–0.05 mm) had the advantage of multiple simultaneous productions.^[25]

A study was conducted to evaluate the accuracy of a model made using the computer-aided design/computer-aided manufacture (CAD/CAM) milling method and 3-D printing method and to confirm its applicability as a working model for dental prosthesis production. This study concluded that the accuracy of the 3-D printing method is superior to that of the milling method.^[26]

A study was conducted to determine the possibility of implementing a fully digital workflow clinically with individually designed and three-dimensional printed (3D-printed) brackets. This study concluded that it is possible to perform treatment with an individualized 3D-printed brackets system by using the proposed fully digital workflow.^[27]

Digital workflow

How to set up a digital laboratory?

The digital laboratory unlike the conventional laboratory does not require a stone dispenser, investment vibrator, model trimmer, large sink with plaster trap, or designated countertop space for poured casts. Instead, it requires space for a computer workstation with CAD/CAM software, the 3-D printers and resin cartridges, and the post-processing procedures.^[16] Appropriate CAD and printing technology should be purchased as per use. The lab should be well ventilated, free from dust and disturbances. For the working of the thermoplastic machine compressed air is necessary. Compressed air is also required for cleaning freshly printed models. The working area depends on the type of printing technology used. Post-processing phases are different for different technology used. Appropriate equipment and a washing area have to be constructed.

Advantages

Adopting a digital workflow has numerous benefits. For instance, it takes a smaller number of appointments with shorter appointment time with increased patient comfort.^[16] This improves patient compliance. The traditional plaster models are predisposed to clinical and procedural errors due to damage and distortion of impression. Technical errors like improper mixing of materials resulting in thin casts, porosities, chalky surface, improper trimming of the margins can occur it is time-consuming to manually prepare good plaster models. These models are susceptible to damage during transport and processing.

All the above problems can be eliminated by 3-D printing. 3-D printed models are accurate, stable, durable, and tolerates processing procedures of appliance fabrication well. These models can be easily reprinted if required as the data is digitally stored. The digital workflow allows modifications in the data set of the archived impression as per the clinical needs. Ex. Tooth movement can be simulated in aligner therapy before fabrication of the appliance. Brackets and bands can be digitally removed from the data before the fabrication of any appliance or retainers. The use of digital workflow saves time and expenditures due to outsourcing for appliance fabrication.

Disadvantages

Digital in-office lab has a high cost of maintenance. The printers require an annual subscription to CAD/CAM software.

Soldering of the appliances while placed on a resin model results in model distortion and there arises a need to replicate the model. To avoid this laser welding is advised over soldering, this is an add-on expense. Special training of the staff to familiarize them with 3-D software and hardware is required. Adequate training and knowledge are required in order to prepare digital models without printing and post-processing errors. Uncured resins and cleaning solvents can be irritating to the skin.^[29] All health and safety guidelines should be followed during the handling and storage of these materials.

The costs and benefits of establishing an in-house digital lab should be carefully assessed and outsourcing should be considered if found economical.

3-D printing during COVID-19 pandemic

On 31^st December 2019, 27 cases of pneumonia of unknown etiology were identified in Wuhan City, Hubei province in China. ^[31]

The Chinese Centre for Disease Control and Prevention (CCDC) analyzed the throat swab cultures and identified the causative agent which was subsequently named Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2).

On 30th January 2020, the WHO declared the outbreak of COVID-19 to be a Public Health Emergency of International Concern. ^[32]

Following this various guidelines for prevention and control of this disease were issued by multiple public health agencies and local health authorities.

These guidelines included social distancing, use of frequent hand hygiene, and masks.

Orthodontic patients are required to have routine follow-up visitations to the dental office.

With multiple travel restrictions imposed and apprehension of contacting the infection, patients often find regular dental visitations challenging. Many patients happen to miss their appointments, leading to unwanted treatment results and delayed conclusion of the treatment.^[33]

Though clear aligner therapy are advantageous in the COVID-19 era, orthodontists must be fully aware of the limitations of aligners over tooth movement control.^[34]

The following are some of the advantages offered by aligner therapy^[35], ^[36], ^[37], ^[38], ^[39], ^[40]

Aesthetically pleasing

Better oral hygiene

Removable

Better in terms of comfort

Can be used for a variety of cases

Invisible attachments

Beneficial for intrusion and expansion

By delivering complete aligner sets upfront, the orthodontist can monitor the treatment progress virtually and recall the patient to the dental office only when required.

Use of in-house aligners may prove to be more economical over the long run, and also ensure that the patients' treatment goes on as planned with predictable tooth movements. ^[34]

The use of an in-house 3-D printer minimizes the need for frequent patient visitations as the data once scanned are archived with which appliances and retainers can be rapidly fabricated.