Since the 1930‘s dental patients needing teeth replacement, whether partials, bridges, or complete removable dental prostheses, had only one option—milled dentures. Milled dentures are traditionally made out of a product called polymethyl methacrylate (PMMA). It can be used for any dental appliance, especially prosthetics, such as complete dentures and partials, implants, or denture teeth. The denture base was fabricated from this material, a clear pourable acrylic resin that comes in a puck-like form milled or carved out by a machine. Eventually, artificial teeth were chosen, added one by one, and then adhered to the denture base resins.
Manufacturing these prosthetics took a significant amount of time and several visits by the patient to the dental office. The length of time chairside for the dental practitioner or clinician was also a time investment. That is until the introduction of computer-aided design and manufacturing and 3D printing of a denture base and teeth. Since the integration of modern technology into dentistry, there have been improvements in:
- Time in the chair time for the patient and time chairside for the dentist
- Number of appointments
- Quicker denture delivery
- Software improvements to archive digital scanned impressions for future reference
- Denture replacements that are easily replicated
- Retention of fabricated prosthetics
- Clinical and patient improved results and satisfaction
However, before discussing the advancements in the digital side of denture fabrication, the subject of impressions must be explored. After all, they are an integral part of the milled denture journey. Any comparison between milled and digital dentistry must involve the history of this process.
Making a Good Impression
Milled dentures were initially completely analog. Conventional dentures were created from a plaster mold made from the patient’s bite impression and then injected with high-impact acrylic resin. The process was messy, with the patient enduring a first impression of the existing teeth using alginate or polyethers, polyvinyl siloxanes, and hybrids.
There is some dispute over the best material used for impressions. Most dentists will use alginate, the go-to material in the dental world, due to cost, speed, and ease of use. Others feel that silicone impression material is much better because of the detail it can capture. Some dental practitioners believe that the ability to record more nuanced renditions of teeth with a non-alginate substance makes it a better choice for crowns, bridges, and restorations.
The difficulty in using alginate is that it is known to distort easily and requires casting to be done quickly. This method’s problem depends on how fast the dental office sends the impressions or casts to the dental laboratory. Silicone, on the other hand, has a longer shelf life. It has increased stability, and the dimensions can last longer without distortions. Still, there are problems with silicone impression material.
Though there are various types of impression material, not all have common viscosity in their makeup. For example, silicone is thinner to pick up the characteristics of the teeth, spaces, and grooves. However, the consistency is unpleasant for the patient, especially when it tends to slide to the back of their throat. It can be problematic, if they have an intense gag reflex. Also, though the runny material makes thorough impressions, it has a tendency to shrink when curing. That means that it cannot be used for the entire mouth.
When making the initial impression, the material is placed in stock trays which come in generic sizes of small, medium, large, and extra-large. The quandary with this method is that every oral cavity is unique, and stock trays do not reflect the variety of possible mouth shapes. Additionally, if there were mistakes in the first impression, the process must be repeated, attempting to replicate the structure of the teeth, palate, and gums.
From the initial impression, the dentist fabricates a Custom Tray (essentially, a custom-made impression tray) for the second round of impressions. These are referred to as “Wash Impressions” or “Border Molding.” During this phase, dentists will use a “Light Body” or “Heavy Body” Wash impression on the customer tray. This Wash is meant to capture patient muscle movement and the anatomy of their mouth.
The second impression is intended to be more precise by surrounding all anatomical features of the mouth. The purpose is to ensure a higher accuracy than the initial impression. It will be sent to the lab to make an accurate denture incorporating space for muscle movement. However, that is not the conclusion to the process. The lab will use the Wash impression to fabricate Occlusal Wax Rims and send them back to the dentist’s office for comparison to the patient. This is not only a long process. It can be an unpleasant experience for the patient.
Unfortunately, impression material capabilities cannot always meet the specific patient’s needs. For instance, the future complete denture wearer requires a top and bottom model. If a patient possesses an edentulous arch–completely without teeth–the impression material of choice in the past was light body polysulfide or light body vinyl polysiloxane.
To this day, doctors and technicians are going through the same process of heating metal spatulas, shaping wax by hand, converting that wax into acrylic, creating a plaster mold, boiling out that plaster mold, and packing the plaster mold again without a guarantee of complete accuracy. Does that feel overwhelming? It should.
The truth is that for the past 45 years, little has changed in prosthodontics regarding materials and techniques. It is time-consuming and manual labor. That was the case until the introduction of digital dental scans. Now, with intraoral scans (IOS), the technology of the scanner and the associated software provides detailed high-resolution data images that can be performed in minutes, giving patients less time in the chair and less time chairside for the dentist. The imaging can be highly effective with selected filters that have multiple option levels to show how filtered images are different from one another. However, the scans are dependent on the type of scanner and the associated technology and software.
One thing that must be addressed is that the quality of the impression aids in the manufacturing techniques and leads to the success of the dental prosthesis, removable or otherwise, and the patient’s comfort in its use. The last thing any dental professional wants is for the patient to experience pain and the dental tissue to become irritated, inflamed, or break down the jawbone due to ill-fitting dentures, partials, or bridges.
Milled Denture Composition
The conventional method of making standard dentures from a solid block disc of polymethyl methacrylate (PMMA) milled in dental machines has been used in the dental industry for almost 100 years. The synthetic resin is durable and known for its strength. Not only is it used in the manufacturing and repair of artificial teeth, denture bases, and other dental prostheses. PMMA is found in shatterproof windows, skylights, and aircraft canopies. Its strength is necessary when considering the average human bite strength is 162 pounds per square inch (psi), with the second molars exerting a bite force between 1,100 and 1,300 Newtons, more remarkable than some primates. However, patients may lose 70% of their bite force with traditional dentures. So, anatomy and fit matters.
Once the wax impression has been returned to the lab, it is prepared for the final process. Each manufacturer has its methodology. The standard process as prescribed by the Foundation for Oral-Facial Rehabilitation is that the dentures are removed from the articulator mountings and inspected for any damage. Next, they are placed in water and then positioned in a flask. The flask is partially filled with stone. The positioning within the flask must land the cast and plaster at the same level as the edge of the flask. Then, the wax is replaced with polymerized acrylic resin. Simple? Actually, the entire process is complicated, and the teeth have yet to be added.
Artificial teeth have been used for centuries. Early artificial teeth were not artificial teeth at all. They were human. There have also been ivory, porcelain, and even a rubber base from a substance called Vulcanite made by Charles Goodyear in the late 1800’s. In the 1930’s and 1940s dentures and partials with metal occlusals were designed to rest on the mandibular denture opposite slightly concave maxillary posterior teeth. The metal used consisted of everything from gold to chrome cobalt alloys.
Modern milled dentures typically have two types of artificial teeth: carded or milled. They were chosen to be compatible with the patient’s structure of face, parfunction, previous dentures or partials, and jaw relationship. Carded teeth, named for the identification card they were placed on, are identified by mold, shape, and shade of the teeth (versions A, D, C, E, and F). Most carded teeth consist of anterior or posterior tooth selection guide with both upper and lower teeth.
They are manufactured in an extrusion molding process in a continuous strip. Manufacturers can also include a kit with a facial meter that measures facial features for distance to correlate cards with a variety of different sized upper teeth helping the dentist and patient evaluate and select teeth to be placed in the base. Some of the most commonly used are Ivoclar DCL teeth, which are a highly durable nanohybrid composite version of acrylic resin, or Dentsply Portrait interpenetrating polymer network (IPN) teeth. Both are premium teeth known for long-lasting wear, and resistance to cracks and chips. Still there are versions of artificial teeth in a more economical porcelain that are durable depending on the bite strength of the individual.
The strength of the denture does not mean they are not imperishable. The care of all denture appliances contribute to their endurance. Avoidance of hot water is at the top of the list as it can warp the denture base. Whitening, bleaching, and abrasive cleaning is not advisable. Patients are cautioned to handle dentures or partials with care, so as not to bend the plastic or clasps. Cleaning them after eating by removing them, rinsing, and placing them on something soft like a towel or washcloth is important. Brushing them daily and seeing a dentist regularly is highly advised. The moment a dental appliance feels loose or begins to irritate the patient, they need to bring them to their dental practitioner to avoid complications.
Regarding the choice of milled dentures with either acrylic or porcelain teeth, it comes down to durability, esthetics, and costs. There can be bonding issues if there are errors in the impression or manufacturing. Given that each tooth or group of teeth is fitted individually, there is a greater chance of tooth loss. Without proper fit or consistent wear of the complete denture, there can be bone loss because the gums and jawbone are not being stimulated for continued growth. Additionally, improper oral hygiene can lead to the deterioration of the dentures and staining to any dental prosthetic.
Enter Tech-Aided Digital Dentistry
It was not until the 1980s that computer aided design or manufacturing (CAD/CAM) was introduced to help manufacture dental prosthetics. It was not widely accepted initially, identifying it as a passing fad or too cumbersome to learn. Over time, CAD/CAM became more of a part of the dentist’s workflow and dental lab work.
Denture materials, like modern milled denture bases such as the Ivoclar Ivobase CAD milling system produces high-impact acrylic that can replicate fibered “veins” for aesthetic enhancement. While they were pleasing to the eye, the fibrous appearance incurred a higher fee. Still, the idea of 3D digital printed complete dentures was an unrealized idea. It was more of the software that was developed for the milling machine that moved prosthetic dentistry into the digital age.
When 3D printable prosthetics arrived on the scene several years ago, in all honesty, they were not equal to their milled counterparts. The imaging aided the construction with additive manufacturing, moving it further away from analog construction. Still, the 3D construction aesthetics had not reached the same quality level as milled dentures and teeth. Then technology upped its game and caught up to milled prosthetics.
Modern milled dentures and partials had an undeniable aesthetic edge over early 3D printable prosthetics. The milled dentures looked more realistic, especially the gum or soft tissue. Some of the appearance of the milled denture is subjectively dependent on the dentist. If they like the look of the milled denture base or the patient has a “gummy” smile, the look of the base is irrelevant. There were also issues with the durability of the printed teeth in the first-generation printed prosthetics.
In recent years, however, the technology has matched the printable teeth capability, and the second and third generations of teeth have improved. What is now commonplace is CAD milled base and teeth. However, what the digital process has over the milled process is the time expense. Rapid prototyping of the denture removes weeks from the process. Additionally, studies are finding that “Digital immediate and definitive complete dentures produced from intraoral scans of periodontally compromised teeth bring a significant advantage for the treatment planning and delivery of outstanding complete dentures.”
Another study found on PubMed aimed to compare the trueness of one type of CAD/CAM milled complete removable dental prostheses (CRDPs) with injection-molding and conventionally manufactured CRDPs. Thirty-three CRDPs were fabricated by three different manufacturing techniques. The first group was CAD/CAM utilizing the manufacturer AvaDent. The second group utilized injection molding with Ivocap. The last group was a traditionally milled flask-pack-press. All used a single master reference model and incubated in artificial saliva for 21 days. They sought to find the trueness of the entire intaglio surface along with five specific regions of interest: vestibular flange, palate, tuberosities, alveolar crest, and post-dam areas. Each was compared with non-parametric tests at a level of significance set at p < 0.05. The study found “there was no difference in the trueness of the total intaglio surfaces between the groups.” The perception of milled denture superiority over digital dentistry and CAD/CAM dentures seems to be demystified.
A Matter of Time
Once a model of the teeth was made, that was not the end of the journey for a milled denture patient. Dentists will use the first impression to make a cast and even a second impression leading to more visits. The typical process requires multiple appointments. Usually, there are a minimum of five in-chair visits with possible follow-up consultations for adjustments.
The amount of time is considerable compared to the intraoral scanner and digital dentistry. From the initial scan, depending on the practitioner’s skill, the quality of the scanner, and the sophistication of the accompanying software, the patient can be done in two visits. Yes, two visits. The only mess to clean up for the individual clinician in the dental laboratory is the 3D printer. Gone are the days of goo and hot spatulas.
As technology advances with each iteration of scanners and software, the detail of the scans produces better images that are saved and easily reproducible. Then, there is new manufacturing material on the horizon.
A recent study examined the dimensional accuracy, and the surface topography of a custom-designed, 3D-printed zirconia dental implant, and the mechanical properties of printed zirconia discs. The process included a custom-designed implant that was 3D-printed in zirconia using the digital light processing technique (DLP). The dimensional accuracy was assessed using the digital-subtraction technique. The result was that “the printed implant was dimensionally accurate with a root mean square (RMSE) value of 0.1mm.” With advancements in materials will there be an elimination of issues such as shrinkage of partials and removable complete dentures?
As for partial dentures, it is a whole different game, and everything about complete dentures does not apply. For the most part, there are twofold impediments to 3D partials. Material-wise, partial dentures will fall into either metal frameworks and mechanical properties–like chrome, cobalt, or titanium–or patients will have flexible partials. Flexible partials have fundamentally a different chemistry that makes them not conducive to printing. However, progress may get the industry there soon.
This article is not really about milled versus printed dentures. It is about the digital process that really matters. The industry, studies and manufacturers are learning to combine strengths to improve dental prosthetics such as the creation of the digital denture process. With researchers such as Nicole Kalberer, Med. dent., MAS, and C.J. Goodacre, DDS, MSD, and Murali Srinivasan, perhaps dentistry will improve processes in partials, dentures, and implants relatively soon. It is also about additives, avoiding subtractives, and digital dentistry improving the industry. It is about innovation in materials that improve the trueness, structure, and flexural properties of dental prosthetics. What has been found is that with the advent and innovation of digital dentistry and 3D printed digital dentures, dentists need very little alginate or other impression material due to intraoral scans that are being perfected more and more. There is better accuracy in the design software leading to less time in the chair and rarely repeat visits for realignment, with the possibility of eliminating try-ins altogether.
Digital dentistry and printable prosthetics are creating a brand new patient experience that produces word-of-mouth marketing and draws in new patients while retaining the current patient base. It is also about improving dental office digital workflows to increase the volume of patients and cutting down on chair time by about 15 to 20%. With a one-person fabrication process, the dental lab overhead is significantly reduced. The question is with continuing advancement, will CAM complete dentures and 3D printing surpass the analog method of denture manufacturing?