Conventional Denture Process Techniques - Paper Example

Introduction

Over the years, there has been the development of a complete denture molding process to resolve the limitations of traditional methods. Some of the traditional methods include compression molding technique, and injection mold technique. Compression molding technique involves using a compression mold to generate the dentures. Also called the pack and press method, it is the most popular method that has been in use for many years. The definitive cast used and the wax dentures are put in the flasks to ensure that the cast fits appropriately. As this method developed at the height of the "Age of thermoplastics," the Vulcanite was replaced with polymethylmethacrylate (PMMA) as the material of choice.27

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Compression molding technique, while being a pioneering methodology used by dentists, requires considerable attention to time and details to realize accurate results, especially when compared to injection molding. Even though the capacity to customize teeth position and verify every stage delivery of the dentures is a substantial advantage of compression molding technique, there are many demerits to be deliberated when fabricating complete dentures. These entail; period for both dentist and patient because of numerous appointments needed and the lab costs and time are unpredictable. Additionally, polymerisation shrinkage from denture processing can result in poor adaptation between the PMMA denture base and the underlying tissue. The injection molding produces a substantially smaller incisal pin opening compared to compression molding technique. Additionally, the injection molding technique utilizing polymethyl methacrylate is a more accurate model for processing dentures17.

Complete dentures can be fabricated utilizing various processes. The objective of each technique is to generate a prosthesis that shows intricate mucosal adaptation leading to good retention, stability, and reinforcement with minimal fabrication. Compression molding in a conventional flask has been utilized for decades and is the most prominent technique even nowadays. The injection molding technique was first introduced in 1942 by Prior and commercialized by Ivoclar in 1970. Injection molding technique incorporates features of pack and press as well as pour processing, integrating the advantages of heat processing with the reduced time of the pour model. Injection molding technique led to a reduction in cost and increased preciseness and stability of the denture bases21.

Injection molding combined the characteristics of both the compression method and the press and pour method. The technique entails the use of a thermoplastic material in an electric cartridge furnace at a specific temperature. The flexible denture base material is in the form of granules and is thermoplastic. They are heated at high pressure before injecting the material into a flask. McLaughlin, Ramos & Dickinson, (2017) highlights that injection molds are preferable for patients having ovoid-shallow arches. Sekar, Sendhilnathan, Lakshmi, Abraham & Annapoorni, (2015) found that injection molded compact dentures were associated with better palatal adaptation as compared to dentures made using the compression technique3,5. The main merits of the method are that it combined the benefits of heat processing of the compression method and the decreased time of development from the injection molding approach. A merit of this approach is that there was a decrease in the cost of development of the dentures, improved accuracy and better stability of the denture bases2.3.

Today, PMMAs remain the preferred material of choice for fabricating complete dentures. Their main advantage is the low cost, they are relatively easy to use, and they only need the use of simple equipment for processing. They have replaced other materials used previously such as polystyrene, and polycarbonates among others2,6.

Why the CAD/CAM Became Popular

Nowadays, the CAD/CAM model has become very much popular. As seen above, before the adoption of CAD/CAM technology into removable prosthodontics, the main impediment was that there was no congruence between the denture base and denture bearing tissues which were mainly caused by the shrinkages resulting from polymerization. These shrinkages results in distortions hence have an adverse effect on fit and retention5. The development of CAD/CAM technologies was influenced by the need to find alternatives that could overcome some of these issues. Manufacturers sought to develop alternative materials that would be aesthetically appealing, bond well, have biocompatibility features, take less time and also entail fewer clinical visits by patients.6,7Many of the research conducted in the area before that hinted that modern technology would yield more accurate results25. The popularity of the digital CAD/CAM technologies was also influenced by the fact that there are reduced instances of clinical mistakes owing to the high levels of accuracy brought about by the technologies8.

Introduction to Two Types of CAD/CAM

McLaughlin, Ramos & Dickinson, (2017) suggest that CAD/CAM improved patient outcomes, especially when compared to traditional techniques such as compression-molded dentures. They further agree that CAD/CAM has the most accurate adaptation and low instances of a misfit. There are two main CAD/CAM techniques used:

Milling/subtractive manufacturing/computer-aided manufacturing: in this technique, the complete denture is milled from a solid block of material and by the 3D design generated by the software. Since the completed dentures are generated from prepolymerized acrylic resins, and the process involves high pressure and heat, chances of polymerization shrinkages are reduced, porosity is nearly eliminated, and the adherence of Candida albicans to the denture base is diminished8-12.

3D Printing (Additive manufacturing): the process involves layering to replicate the 3D model generated by the CAD software physically. Once the model is generated, it is segmented and continuously layered until completion9. 3D printing exists in various technologies.

Stereolithography (SLA); An SLA printer utilizes mirrors referred to as galvanometers, with one situated on the X-axis and another on the Y-axis. These galvanometers quickly aim a laser beam across a vat of resin, selectively treating and solidifying a cross-section of the denture, building it up layer by layer15.

Digital Light Processing (DLP); DLP uses a digital light projector to flash a single image of each layer all at once. Since the projector is a digital screen, the image of each layer is comprised square pixels, leading to a layer of formed from small rectangular blocks referred to as voxels. Light is projected onto the resin using light-emitting diode screens that are directed to the denture by a digital micromirror device15.

Development of CAD/CAM Denture

Computer-aided design and manufacturing across different sectors emerged around the 1980s. After its inception and amidst the rapidly growing adoption, it was only a matter of time before it was applied in the development of removable complete dentures13. The integration of CAD/CAM technology into complete denture design and fabrication assists in the improvement of consistency of complete dentures and improves the fit when the base is milled from pre-polymerized resin. CAD/CAM involves layering manufacturing, and which are critical in the creation of models that have no voids and mistakes. As CAD/CAM is a subtractive process, there is no polymerization shrinkage, an issue that was characteristic of traditional approaches. CAD/CAM makes it easy to manufacture prostheses using denture data from patients. The CAD/CAM technology has been extended to the fabrication of complete dentures of the commonly called 'digital dentures14-19. The main merit and variation is the use of the of the three dimensional computer software to model the dentures using anatomical landmarks as reference. Digital dentures technique establishes an advanced accuracy for the intaglio surface of the dentures, thereby improving retention and intimate accurate fit to the denture bearing anatomy. 41

Review of Milled and Printing Denture

The iTero and the iTero element are regarded as the most accurate as they both use OrthoCAD software. However, the software relies on a physical bite registration that must be shifted from the mouth of the patient to the computer software. Accurateness of digital impressions for complete dentures is better than that for palatal soft tissues. The accuracy of digital impressions for palatal soft tissues is better than for complete dentures. Subtractive manufacturing technique is the most efficient way to fabricate complete dentures. The 3Shape Dental System 2013, WIELAND ZENOTEC TI milling machine and the CAM of WIELAND V2.0.049 are demonstrated as the most suitable design and fabrication tools for complete denture.41 CAD/CAM systems are made up of three main components:

Wieland Digital Denture: This system utilizes subtractive manufacturing for the fabrication of complete dentures. The medical reports can be acquired by replicating existing dentures and sending them to the manufacturer, using denture rims that are digitally designed and milled or using digitally configured and customized impression integrated with bite plates.

Software- software for the design of virtual restorations is used which makes it easy it easy to compute the milling parameters. CAD software used include 3Shape Dental System and AvaDent digital dentures.

A computerized milling device- this machine is used for restoration from a solid block of material through the milling process or additive practices such as 3D printing20.

CAD-CAM technology permits fabrication of complete dentures. In this technology, the dentures are milled from a prepolymerized acrylic resin puck, which is generated under high pressure and heat. Polymerization shrinkage does not appear, and the observance of Candida albicans to the denture base is reduced. The absence of polymerization shrinkage linked to milled dentures leads to a highly accurate denture fit and increased retention22-24.

The advanced fit could explain the retention and exceptional suction effect observed in CAD-CAM technology. This virtual design process permits the dentist to define the minimal thickness of the denture bases and to add a stippled configuration and anatomic characteristic on the polished surfaces of the dentures. The CAM technology, also called subtractive manufacturing, makes use of the computerized numerical control (CNC) machining. The baseplates and the dentitions are fabricated differently, with close adherence to the teeth sizes and dimensions24.

Regarding printing, after the patient's denture data is collected using the CAD/CAM technology, the STL files are processed and analyzed to generate final prototypes. The final models are fed into a 3D printer which commences the printing process. After that, the teeth are arranged and tested25-27.

In their study seeking to compare compression techniques such as injection to non-compression methods such as CAD/CAM techniques, Janeva, Kovacevska & Janev et al., (2018) found that complete dentures processed under CAD/CAM was stronger, had least shrinkages and had better orientation when fixed (15). Dentistry has had a long link with subtractive prototyping, usually referred to as 'milling.' CAD-CAM for the milling of the crown and fixed partial denture systems is identical with existing dental technology. The five-axis milling machines nowadays are the most versatile as they can generate precise details which some multifaceted restorations need. A study by Janeva et al. (2018), inferred that the CAD-CAM framework could finis...