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Inside Dental Technology
September 2020
Volume 11, Issue 8

Printing vs Milling Dentures

One laboratory’s detailed evaluation of the advantages and disadvantages of each fabrication method

Conrad J. Rensburg, ND, NHD

Over the last two decades, the dental industry has seen the rise of subtractive manufacturing technology. CAD software programs have proven predictable and are now also within the financial reach of most dental laboratories. However, a new challenger has emerged to test milling's technological supremacy: additive manufacturing, also known as 3D printing.

Although this technological evolution was first observed in the fixed prosthetic landscape, there is now an undeniable shift toward the removable arena. Although slow-paced at first, the growing demand for a feasible digital solution in removables is now driving manufacturers to find the next great material and technology combination. Lately, heavy emphasis has been placed on finding that solution for additive manufacturing.

The gold standard for many decades has been the hand-processed, high-impact denture. The technology, either additive or subtractive, that proves capable of upsetting this trusted champion will be the one that offers the most viable combination of scalability, esthetics, cost, and strength. Initially, limited material options restricted manufacturers to subtractive solutions, but with the advancement in accuracy of 3D printers and the development of new printable polymers, the next generation of solutions holds a lot of promise.

Every laboratory has different needs and priorities. In this article, the author will discuss the effectiveness of both techniques for his laboratory—with four locations and more than 100 employees—by comparing esthetics, strength, fabrication techniques, and economics. The costs and production times referenced in this article were compiled from real-world historical data generated by the author's laboratory. Production times and processing costs are unique to individual laboratories. To report more universally, numbers in this article are reported in comparative percentages.

Esthetics

Esthetics once were seen as the biggest inconsistency between a milled and printed denture due to the quality of initial printing materials. First-generation printing materials were translucent and brittle, which resulted in a weak and esthetically poor prosthesis. The biggest challenge was the struggle to hide the underlying tooth structures in the cementoenamal junction (CEJ) areas. Most of these early printed dentures required the addition of arduously hand-applied external stains. With the introduction of the latest printable polymers, the playing field is once again level. Figure 1 and Figure 2 show printed (left) and milled (right) dentures from the author's laboratory. Neither of the options require external staining to enhance esthetics.

Labor Time and Cost

One of the foremost advantages to transitioning into the digital world is increased efficiency. With this comes the ability to scale, while maintaining a more consistent quality product. As is the case in most industries, labor is not only our greatest expense but also—and more importantly—our most valuable asset. Even in this ever-changing digital landscape, artistry remains the great differentiator between a premium product and an average product. Digital workflows will never replace true artistry, but it does offer the dental laboratory of the 21st century the means to scale and magnify its effect on business.

In the author's laboratory, milling proved to require 38% less hand-processing time when compared with analog methods, equating to a 39% reduction in labor cost per denture. Printing proved to require 47% less time hand processing than analog methods, equating to a 49% reduction in labor cost per denture. The major difference is that the author's milling protocol requires additional tooth structure milling, which causes an interrupted manufacturing cycle. The author's printing protocol incorporates the use of pre-manufactured denture teeth, with a more efficient luting process.

Even though very similar, printing required 13% less hand-processing time, which equated to an 18% savings in labor cost when compared with milling a denture.

Equipment Processing Time

Most additive technologies have always offered a distinct production efficiency advantage over subtractive processing. In order to translate this efficiency to return on investment, it is vital to consider the effect equipment cost will have on the overall product cost. Hard cost contribution to product cost is influenced by two factors: equipment cost (hard cost) and number of units the equipment can produce in a specified period.

The hard cost of equipment can only be offset by production capacity. Therefore, the equation of number of units produced per day/equipment cost plays an integral part in calculating your potential return on investment.

Over an 18-month period, data showed a daily minimum production requirement of four units to deliver a similar product cost compared with a hand-processed denture.

The reported numbers are based on milling a denture base in a PrograMill® PM7 (Ivoclar Vivadent) vs printing a denture base in an M2 printer (Carbon®). Hard cost contribution was calculated by dividing the equipment cost by the number of units produced per day. The time frame used for these calculations was one working day (480 minutes) and the equipment costs were broken down into a 5-day working week.

The equipment cost for a milling unit (3-year purchase) amounted to $95 per day. For a printing unit (5-year lease), it was $125 per day. Processing time when milling was 210 minutes: rough mill (base) 30 minutes, rough mill (teeth) 30 minutes, and fine milling after luting 150 minutes. Thus, production capability is 2.2 milled dentures with tooth structures per 8-hour day. Meanwhile, the total printing time for 6 arches (reduced platform, 10+ arches full platform) was 130 minutes per run, so the production capability was 3.6 cycles or 21 denture bases per 8-hour day.

The inherent limitations of a subtractive system are highlighted even more when comparing the processing times with those of an additive solution. Table 1 demonstrates the effect that equipment cost distribution has on product costing, especially when scalability is limited because of the time required to process. The costing reported in the table took into consideration all labor processes, equipment and component/material cost, and scalability for each solution. Milling cost $43.18 per denture base and tooth structure, while printing cost $5.95 per denture base. In the table, a hand-processed denture was used as the standard base costing, and both milling and printing costs were reported in relation to that. Scalability was judged on the capacity a single piece of equipment had to produce denture bases in an 8-hour day.

In this study, the milling process requires an interrupted milling cycle, which severely limits the production capabilities. This workflow interruption is caused by the need to lute the base and tooth structures before the final milling process can be completed.

Before the 2020 COVID shutdown, one-piece milling pucks (monolithic tooth and base structure) with uninterrupted milling cycles were introduced. These uninterrupted milling cycles will allow the dental laboratory the ability to run the milling unit overnight. This will have an impact on the overall equipment capabilities and influence the overall cost contribution.

Ultimately, material choice is not the differentiating factor in processing capability. In terms of efficiency, additive technology continually overshadows the subtractive methodology.

Equipment Leasing vs Purchasing

Leasing vs purchasing equipment has become a hot topic among laboratories. Opinions seem to ignore the one paramount and extremely important consideration: return on investment. No matter which camp they are in, most laboratory owners agree that equipment is ultimately nothing more than a depreciating resource.

If equipment is continuously creating revenue, ownership or rental should not play a part in any decision. If equipment can produce a good return on the outlaid investment, the only other metrics truly important are the need, quality, scalability, and esthetics of the products produced.

Ultimately, cost (leasing or purchasing) is offset by the products the equipment produces.

It is important to remember that neither of these financing options produce an asset, but both avenues ultimately produce a sellable product. Leasing vs purchasing is ultimately a personal decision. 

Tooth Options: Milling vs Printing

The author's laboratory used Dentsply Sirona Lucitone® additive workflow, which only offered the option to use prefabricated IPN 3D carded teeth. Therefore, the use of this workflow was limited to cases with adequate clearance where no adjustments to the intaglio surfaces of the teeth are required.

The subtractive workflow offers multiple tooth structure options in combination with the denture base puck. Prefabricated and custom tooth structure solutions allow for the use of the subtractive workflow even in cases with severely reduced vertical clearance.

Conclusion

Upon evaluating the costing data, workflow, material properties, and other relevant criteria, it becomes more apparent that this is not an apples-to-apples comparison. Both systems share most of the positive product characteristics, but the technological weaknesses can mean differences in process efficiency.

Both systems have the ability to create products with a full digital workflow, producing strong and esthetic prosthetics, while neither requires extensive hand processing.

Because of multiple tooth structure solutions available, the subtractive workflow allows for fabrication of a milled denture to suit almost all edentulous patients. Having different tooth structure options allows the technician multiple solutions to satisfy many clinical considerations.

The inherent limitations of a subtractive process severely restrict the equipment's production capabilities. This inability to produce in volume unfortunately remains true even if a monolithic tooth-and-base puck combination is compared to a printable solution.

The above limitation means producing larger volumes will require multiple expensive milling units. This ultimately leads to products that require a higher price point to maintain profitability.

The printing workflow, meanwhile, has limits due to the current unavailability of a customizable tooth structure and more options of prefabricated teeth. One tooth, even a premium one, does not satisfy all clinicians' esthetic demands.

Ultimately, the advantage an additive solution offers over a subtractive option is its immense scalability and cost savings that accompany higher volume production. 

Every laboratory is unique and can prioritize criteria based on different metrics. These two technologies are vastly different yet produce a strikingly similar product. The question remains: Which one is right for you? 

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