Although, both milling and printing technique use a digital 3D-image file designed by CAD software to manufacture, the two 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, repeatedly layered on a support structure and polymerized by an ultraviolet 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 auto-polymerizing 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 latest 3D printers are using DLP technology and contraction of the material while constructing layers of minimum thickness, as well as further contraction and residual stress accumulation during the post curing process, occur during 3D printing in this technology. In addition, bond strength between printed denture base and relining material is weaker than milled base.
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. 5] Comparison of printed and milled denture adaptation; A, B, C: printed, D, E, F: Milled
[fig. 6] Color maps of Tooth movement
Denture teeth move during processing, with maxillary dentures distorting more than mandibular dentures. Tooth movement occurs during both the flasking and the processing stages of the pack and press fabrication technique.
A correlation between both the water to powder ratio of the investing gypsum and the amount of pressure applied during acrylic resin packing has been shown to affect the amount of denture tooth movement. Also, tooth movement during investing has been attributed to the setting expansion of the gypsum. In some separate studies demonstrated that fluid resin complete dentures underwent larger dimensional changes than pack-and-press. The injection molding technique produced a significantly smaller incisal pin opening than the standard compression molding techniques. Additionally, even minimal denture tooth movement occurring in both arches can have a significant effect on the occlusal vertical dimension.
The CAD/CAM monolithic denture demonstrated the best combination of accuracy and reproducibility, resulting in the least overall denture tooth movement.
Complete denture can be fabricated using several different processes. The goal of each technique is to produce a prosthesis that exhibits intricate mucosal adaption resulting in good retention, stability and support with minimal fabrication distortion. Today, milled or printed denture bases and teeth have been used to replace the conventional base and teeth. And the materials have different properties.
A body of scientific literature related to computer aided technology for complete dentures is emerging. Significant advancements in this technology have now resulted in their commercial availability with shorter clinical protocols. However, prospective clinical trials with true clinical endpoints are necessary to validate this technology. Both techniques still need further study for long term clinical results such as wear resistance, bond strength between denture base and teeth, and so forth. For reference, the feasible combinations of fabricating complete denture are attached.