Three-Dimensional Printing of Dentures Using Fused Deposition Modeling

So how does a FDM 3D printer work? Beginning with computer-aided design (CAD) or scan data, the 3D printer extrudes and deposits molten thermoplastic polycarbonate in layers to build parts from the bottom up (Figure 2 and Figure 3). Each layer of molten plastic is deposited on top of the previous one and flattened slightly by the extrusion head. The layers instantly fuse to one another, thus making very complex parts easy to produce. The final appearance of the finished plastic has a characteristic surface that is appropriate for use with many common materials that can be added by the technician, such as acrylic and wax. Or, the material can be polished smooth on a bench lathe just like acrylic.

Despite the promise of fused deposition modeling, there are still no consistently reliable substitutes for proven and established methods of full denture fabrication by the laboratory technician. This is because established methods boast a fully formed protocol complete with checks and balances that ensure the accuracy and functionality of each removable restoration. For example, it is already understood in the laboratory industry that the dentist must provide accurate records for the technician to use in order to process a usable removable prosthesis as ordered. It is also necessary for the dentist and technician to recognize that they are cooperating in a team effort to yield the best possible denture result.

This paper will demonstrate the benefits of 3D printing when used in conjunction with proper clinician/laboratory protocols to enhance success.

Creating Removable Prosthetics Using 3D Printing Production Processes

Below are the detailed steps that must be taken to create a 3D printed removable prosthetic.

Study Models

There are two ways to create study models for working in a virtual environment. The first involves scanning the impression and digitalizing for transfer into a software program. The second is to take the impression with a stock or semi-custom tray and pour the model in stone. The stone model can then be scanned or used directly in the procedure protocol. If necessary, the study model may be duplicated conventionally with duplicating hydrocolloid or printed from a good quality scan.

Custom Impressions

Custom trays may be fabricated from computer scans of preliminary impressions/models and printed, or can be made directly with readily available materials. Thermoplastic, light-cured, or self-cured acrylic trays can be created in
the laboratory.

One of the most demanding procedures involves obtaining a final impression that features anatomic landmarks and variations of the hard and soft tissues without defects or distortion. If a prior denture has been scanned, this scan may be used to print a denture copy, which would then act as a highly individualized custom impression tray (Figure 4). Once obtained, the impression and/or master model may be scanned. A scanned digital record of either a final custom impression or of the poured model would be very helpful should a duplicate model be needed for a second denture set-up.

Custom Bite Registration Rims

Denture record measurements must be clearly defined, reproducible, and transferable to and from bite rims. Vertical dimension, smile line, midline, corners, lip support, etc., must be carefully measured, and this information must be considered in the wax set-up, try-in, and final denture. Tissue contact portions of rims may be printed directly from archived scans of models or from scans of prior dentures using biocompatible materials. These may actually function as preliminary baseplates if the software is capable of reducing or removing undercuts before printing the baseplate design into a solid material. Also, a prior denture scan can be printed and the denture tooth portion removed to produce a baseplate. The technician may then add wax tooth-bearing occlusal rims at the bench (Figure 5).

Wax Set-Up for Try-In

Adding teeth to the arrangement as prescribed or indicated is a significant step. If correctly scanned, a wax set-up can be printed and may be tried in the mouth as an actual trial denture. Having a 3D printed trial denture allows the patient to test the function, comfort, esthetics, and fit of the removable prosthetic for several hours or even days prior to processing a final denture. This is contrary to other try-ins, which often last only a few minutes and do not provide a definitive trial run for the patient. Printed dentures may be characterized with stains that are used with conventional acrylics (Figure 6), making them suitable for extended trial periods. In addition, actual denture teeth may be placed into a printed denture base to allow for an even longer extended trial period for the patient (Figure 7 and Figure 8). A 3D printed denture created for try-in may also be modified and used for patient demonstration (Figure 9 through Figure 11). These modifications may help direct the use of different shades or molds and be repeated (or eliminated) in the finished final denture according to the patient’s desires. This is valuable in that the patient, dentist, and laboratory are not committed to finalizing any questionable denture aspects of a short wax try-in visit that may otherwise later appear problematic in the final processed appliance.

Processing and Finishing

Regardless of whether dentures are processed by conventional packing/pressing or by injection molding, preservation and reproduction of all details in the wax set-up are critical for denture success. Any displacement of teeth or distortion created in the finalized denture base may compromise the patient’s ability to wear and/or function with the denture.

A printed baseplate can be incorporated in the wax set-up and removed with the wax in the usual boil-out process. Regular processing may then take place. Any archived scans of the wax set-up or denture may be recalled at anytime to help recreate the original denture for duplication.

Delivery with Adjustments

This important step is as significant as the beginning of denture rehabilitation and satisfaction of the patient. The final denture delivery is mostly a confirmation of denture record integrity as related to the final fit, comfort, and function of the prosthetic. Once properly adjusted, the dentures should be scanned and digitally stored for future recall. A spare duplicate denture for emergency use may also be printed in a timely manner and kept in a safe place for possible later use. Consider that a denture printed from a scan preserves the original lip support, vertical dimension, tooth size, etc., and will help the dentist and technician recreate the desired features in any future replacement denture.

At every stage of denture fabrication, attention to the finest detail is imperative for success. No less can be said for the use of 3D printing in helping to create dentures. While time-savings and the recall of valuable scanned records are now possible, special factors involved in 3D printing with FDM materials must also be addressed.

Accuracy and Resolution

Two of the many important factors in 3D printing are the accuracy and resolution of the process used in creating a printed denture. Please note that while the terms “accuracy” and “resolution” are sometimes used interchangeably, they are not the same.

One simple way to visualize the distinction between these two concepts is to imagine that two measuring sticks of differing lengths are both marked as 3 inches long, yet the second stick is actually more than 0.5 inches shorter (Figure 12). The first stick is divided into 1/8-inch increments, and its true length is verified at precisely 3 inches. Even though the shorter stick reads “3 inches”, it is verified to be only 2 3/8-inches long. But this shorter measuring stick is divided into 1/32-inch increments, which is four times the resolution of the 3 inch measuring stick. The 3 inch ruler with 1/8-inch increments exhibits lower resolution but has high ultimate accuracy. The 2 3/8-inch ruler with the finer increments does the opposite: It exhibits high resolution but low absolute
accuracy. Resolution does not, in fact, translate directly to a system’s overall accuracy.

The same reasoning applies for 3D printing of dentures. Accuracy and resolution are both necessary in order to properly capture denture form (Figure 13). Furthermore, the ability to reproduce a detailed denture scan consistently, with minimal variation, is also important.

Consistency of Reproduction and Dimension

Consistency, from print-to-print and between 3D printing machines, is critical when recreating dentures. Without process control, dimensional variance will yield unacceptable results from the same scan data.

To help evaluate consistent accuracy of the FDM process between different printers, the FDM printer manufacturer conducted
two studies that analyzed thousands of dimensions over hundreds of non-dental test items manufactured on multiple FDM systems. One study showed that production-oriented FDM machinery had a standard deviation of just 0.0017-inch (0.043 mm), and that 99.5% of all dimensions of a printed item were within +/- 0.005-inch (0.13 mm).1 The other separate study showed that related FDM machinery produced items at 95.4% of all dimensions within +/- 0.005-inch (0.13 mm), for a standard deviation of 0.0027-inch (0.069 mm).2 Thus, the high reliability of FDM systems results in better reproduction of desired dimensions.

It is worth noting that no automated scanning or printing methods, regardless of accuracy or resolution, will guarantee exact or predictable expectation of final denture fit, function, or comfort. Any unforeseen discrepancy may easily manifest itself into the process at any stage of denture fabrication. However, there are important aspects that have been considered in FDM to help reduce this possibility. Measures should be developed by the laboratory and routinely applied in checking printed dentures and their original counterparts when possible (Figure 14).

It is obvious that, over time, new related technological advances, such as FDM, will become available to removable dental laboratory technology. It is the responsibility of technicians to stay adequately informed on these advances. However, regardless of the technological advances for FDM printing, it is critical to recognize that these advances only supplement and can not replace proven and reliable traditional techniques used in denture fabrication. Similarly, the information presented here invites participation so as to complement the irreplaceable talents, skills, and knowledge of the dental technician.

Interactive learning opportunities on 3D printing are now available at no cost. Laboratory technicians can obtain the basic tools needed to view 3D scans used in printing for free.

Technicians can go to www.solidworks.com, www.solidview.com, www.capvidia.com,
www.materialise.com or any other free 3D viewer software website. Download and install a basic 3D viewer for the popular .stl file format. Keep the program available to view any future .stl files. Be sure that your computer meets the necessary graphic and file storage requirements.

Technicians looking to incorporate 3D printing into the laboratory do not necessarily need to set aside additional laboratory space or a large capital investment, as it is possible to outsource the actual 3D printing itself. Many laboratories that already use 3D printers will take on outsourced work, and will print removable prosthetics from emailed scanned data for a fee. The printed prosthetic will then be sent back to the original laboratory.

Gregory S. Jacob, DDS
Owner
North Glen Dentistry
Glenview, IL