A Comparison of the Denture Base Adaptations of Different Fabrication Techniques

Complete edentulism affects quality of life and health, as well as social and personal relationships. There are various treatment options for complete edentulism, and each option has its advantages and drawbacks. The treatment for the edentulous patient can be divided into fixed and removable prostheses. A fixed prosthesis involves four or more implants, while a removable treatment consists of conventional complete dentures and overdentures supported by teeth, implant attachments, or implant-supported bars. Dental implants have revolutionized treatment for completely and partially edentulous patients, but not everyone is a candidate or can afford implants due to their socioeconomic background or health condition for surgery. Therefore, removable complete dentures will continue to be a necessary treatment modality in dentistry.

Complete dentures are an effective treatment for edentulous patients. Denture fit or adaptation is one of the most critical factors determining the quality of complete dentures. Well-fitting dentures provide comfort with fewer traumatic ulcers and greater denture retention and stability.

Polymethylmethacrylate (PMMA) combined with the compression molding technique has been commonly used for complete denture fabrication. However, this technique has a volumetric shrinkage of approximately 7% and linear shrinkage of 0.45%~0.9%. This distortion decreases the adaptation of the denture base to the underlying mucosa.

Injection molding was introduced to overcome this problem, with the volumetric shrinkage being reduced by directional control of the polymerization process through the sprues and the application of continuous pressure to compensate for polymerization shrinkage, resulting in a smaller linear shrinkage of 0.65%.

The fabrication of complete dentures by computer-aided design and computer-aided manufacturing (CAD-CAM) methods has become popular in both clinical and laboratory practices in recent years. Two CADCAM techniques – a computerized numeric control subtractive milling process and a system of rapid prototyping (RP) that is commonly known as 3D printing, an additive manufacturing process –
are available to fabricate CAD-CAM complete dentures.

CAD-CAM milled denture bases are produced by machining a cylinder of acrylic resin that had been pre-polymerized under high pressure and heat, thus preventing polymerization shrinkage during processing. The highly condensed nature of the material is reported to result in less residual monomer, and less porosity.

Complete dentures fabricated using the RP technique have also elicited patient satisfaction comparable with that for conventional complete dentures. RP has been further used in complete denture fabrication for the precise reproduction of denture bases and printed wax patterns.

The purpose of this article is to compare the denture base adaptation of CAD-CAM milled, 3D printed, injection, and compression techniques for fabricating complete dentures.

AlHelal et al. (2017) studied the differences of retention between maxillary milled and conventional denture bases. The study fabricated 20 milled and 20 conventional denture bases and attached a stainless steel hook to the center of the denture bases to apply vertical pulling force.

 

(a) Center of Denture Base

(b) Stainless steel hook

(c) Force transmission device

[Fig. 1] Test specimens and device for retention force of
denture base

 

In the result, the average retention for the milled denture base was 74.14 ±32.56 N, and the average retention for the conventional denture base was 54.23 ±27.36 N. A significant increase of 19.91 N was noted in the retention for the milled denture base group compared with the conventional polymerizing method.

This study revealed higher retention for maxillary digital denture bases. This is most likely because the lack of polymerization shrinkage associated with milled denture bases results in an improved fit, thereby improving retention.

Kalberer et al. (2019) evaluated trueness of CAD-CAM milled and 3D printed denture bases. The best-fit 3D superimposition color mapping and analysis of the differences were performed. A completely edentulous maxillary cobalt-chromium model used in a previous experiment served as the master reference model. All the complete denture specimens were fabricated using the scan of this reference model. A total of 20 complete dentures were fabricated and all the intaglio surfaces were scanned. The scan file of the master reference model was inverted, and the intaglio surface scans were superimposed with a best-fit alignment. The software computed the distances between the superimpositions.

The fabrication of complete dentures by subtractive milling or by additive RP is a recent development. Although both the techniques use a digital 3D-image file designed by CAD software to manufacture the complete dentures, the 2 modes of fabrication are entirely different.

In the milling method, the complete denture is fabricated at a milling station using a pre-polymerized PMMA puck manufactured under high pressure. The RP technique uses photosensitive liquid resins, repetitively layered on a support structure and polymerized by an ultraviolet or a visible light source. Distinct advantages and disadvantages for each of the 2 techniques exist.

Manufacturing complete dentures from a pre-polymerized PMMA puck may eliminate the shrinkage and porosities caused by the packing and polymerization process. Also, the dentures should contain lower levels of residual monomer  and have better material properties. However, the residual monomer content of the milled complete dentures has been reported to be not markedly reduced when compared with conventional heat-polymerized complete dentures and significantly lower than that of complete dentures manufactured from autopolymerizing resin.

The RP technique uses unpolymerized resins for manufacturing complete dentures, and once processed, it requires an additional final light-polymerization step to complete the process. During the RP workflow, polymerization shrinkage is theoretically possible, as complete dentures are not completely polymerized before the final light-polymerization procedure. A deformation of the prostheses can occur when demounting the partially polymerized complete denture from the build platform. Furthermore, a residual layer of unpolymerized resin invariably remains on the finished prostheses, which must be eliminated by thorough rinsing with a suitable solvent. The claimed advantages of an additive manufacturing process include higher accuracy, less material wastage, and low infrastructure costs; however, these have not yet been scientifically proven with regard to complete denture fabrication.

 

[Fig. 2] A, B, C: 3D printed base D,E,F: milled base

 

Theoretically, the accuracy of the fabricated complete dentures should be different with the different manufacturing processes, but both the techniques have been documented to be clinically acceptable and better than conventional methods. The results of this in-vitro study demonstrated that the trueness of the CAD-CAM milled complete dentures was statistically better than that of the rapidly prototyped complete dentures, both for the entire intaglio surface and specific regions of interest. Whether this difference in trueness is clinically relevant is unclear as studies have demonstrated that the accuracy of rapidly prototyped complete dentures has clinically acceptable levels of precision and have also reported good patient and clinician satisfaction.

Hsu et al. (2020) evaluated the denture base adaptation of CAD-CAM milled, printed, and conventional denture bases. The purpose of this in-vitro study was to compare the denture base adaptation of CAD-CAM milled (CCM), 3D printed (3DP), injection molded (IM), and compression molded (CM) techniques for fabricating complete dentures.

The maxillary and mandibular reference models were mounted on a universal testing machine. All denture bases were hydrated for 24 hours, and a standardized portion of low-viscosity polyvinyl siloxane impression material was coated on each denture base in each group and then placed on the reference models under a 49N axial load. After polymerization, the silicone film was removed from the model and measured by using a digital thickness gauge.

In addition, the intaglio surface of each denture base and the tissue surface of each reference model were scanned. The STL file of each denture base was superimposed on the reference model by using a surface matching software program. After superimposition with best-fit alignment, the distance between the intaglio surfaces of the superimposed denture base and the scanned reference model was measured.

According to silicone thickness measurement, the value in the CCM group was less than 0.7 mm, which was significantly lower than that in the other groups.

Regarding the digital superimposition analysis, The CCM  group demonstrated the most uniform trueness. Dentures processed by using CM techniques showed greater changes in horizontal and diagonal directions than those processed using IM techniques. Shrinkage of PMMA has been reported to be 0.9% for CM techniques and 0.65% for IM techniques in the cross-arch dimension. However, in this study, no significant difference in denture base adaptation was found between the IM and CM groups in either the maxilla or the mandible or in either the silicone thickness measurement or the digital superimposition analysis.

Lee et al. (2019) evaluated accuracy of denture bases fabricated by the injection molding, CAD/CAM milling, and rapid prototyping methods. The mean value of discrepancies was the lowest in the RP method, followed by that in the milling method and the injection molding method. They concluded that CAD/CAM milling and 3D printing techniques for fabricating a complete denture may obtain clinically acceptable fit accuracy and retention.

Within the limitations of these clinical studies, the following conclusion can be drawn:

  1. The retention offered by milled pre-polymerized PMMA complete denture bases was significantly higher than that of conventional heat-polymerized denture bases.
  2. The CAD-CAM milled technique had the best denture adaptation, whether using the silicone thickness measurement or digital superimposition analysis.
  3. 3D-printing had better fit than others in some in-vitro studies. But the research recommended further study in regard to various factors such as light intensity, shrinkage between layers, and post-curing procedure, all of which influence the accuracy of 3D printing.

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