Insights Into In-Office Milling

Technology is always evolving, and so it is no surprise the capabilities of in-office scanning, milling and printing systems are greatly enhanced from what was available five years ago. To gain a better handle on what’s changed in terms of materials, software and hardware, we asked Fadi AL Farawati, DDS, MS, MClinDent, an assistant professor in the Department of Restorative Sciences at the Dental College of Georgia at Augusta University, to share perspective on the current state of digital dental technologies.

Please describe today’s in-office milling technology in terms of recent developments in systems, mills and software.

Investing in an in-office milling setup is a big decision, so clinicians should do their own research into what suits their budget and specific plans for the mill. Today’s technology has reached a point where most setups are hybrid systems in which there is usually a fee for using the mill, the software or both. The traditional closed system (meaning, a proprietary system from one manufacturer) offers the advantages of having a more reliable workflow and easier access to customer support (meaning, the dentist calls one place with any questions, whether they have to do with scanning, milling, software or materials). On the other hand, a closed system is more expensive and the practice is limited to the materials available from that manufacturer. An open system provides access to more materials, but this also means contacting multiple call centers for support, which can be frustrating and time consuming.

In terms of capabilities, in-office mills are more efficient and reliable than they were five years ago. Although the milling process has not really changed, the variety of materials and applications the new systems can handle has increased, which reduces dependence on dental labs. For example, today’s in-office mills allow fabrication of screw-retained single implant crowns or four-unit zirconium fixed dental prostheses. Similarly, three-dimensional (3D) printers can be used to produce accurate surgical implant guides.

Five years ago, milled crowns were made from fragile glass ceramics. Although these restorations were highly translucent, they were associated with high fracture rates. Recently, a block of glass ceramic that can withstand a biaxial force of 700 MPa was introduced — an innovation that helps strike a balance between efficiency, esthetics and strength for in-house milled crowns. Zirconia is another material that has evolved dramatically in the last few years. The mono­chromatic and opaque earlier generations have become more translucent and multilayered, which reduces the need to cut back and layer the restoration to provide better esthetics. These monolithic zirconia materials do not require the same occlusal clearance as porcelain-fused-to-zirconia crowns, which supports more conservative treatment. 

Is offering “same-day dentistry” a significant practice builder? If so, how can dentists best leverage this competitive advantage?

Yes, it is an important addition to any dental office. The biggest advantage is reduced chairtime, and that is only possible by normalizing the process to introduce a productive digital workflow. 

What does normalizing the process mean? Think about a productive practice where everyone is busy and all dental operatories are occupied. This practice will not realize the true benefits of adopting chairside milling without adding digitally trained assistants. 

The process typically starts with acquiring an intraoral scan that is sent to special design software (which is usually incorporated in the acquisition scanner). After the clinician traces the margins and designs the crown, the resulting file is sent to the in-house mill, followed by staining, glazing and sintering. This process might take hours, and the dentist definitely should not be doing all of that. Rather, the dentist’s role should be limited to preparing the tooth, quality control during the scan, design and fabrication processes, and then delivering the prosthesis. Optimally, a trained dental assistant or in-house lab technician does the heavy lifting. One thing to keep in mind, however, is the possibility of regulatory limits from state dental boards regarding how much a dental assistant can be involved in this process.    

Additional benefits of in-house fabrication involve the patient’s chairside experience and perception

What are some of the benefits of bringing fabrication in-house?

Beyond the benefits of reducing chairtime by implementing a normalized digital workflow, another advantage is data storage, as well as ready access to this data. An example would be a crown the dentist delivered seven years ago, and now the patient presents with a fracture on that crown. With chairside milling, that same patient can walk out with a new crown in 60 to 90 minutes. That’s priceless compared to the traditional process of remaking the impression, placing a provisional, sending the case out for lab fabrication, and bringing the patient back for delivery of the final prosthesis.

Additional benefits of in-house fabrication involve the patient’s chairside experience and perception. In this sense, the process helps build confidence and strengthens the patient-provider relationship. Another consideration is cost, as in-office milling delivers significant savings compared to lab-fabricated restorations.

What kinds of restorations and which types of materials are suitable in-office milling? Conversely, in which situations should fabrication be delegated to a lab?

While in-house milling allows use of a variety of materials for same-day deliveries, most chairside units utilize wet-milling technology, which, in some cases, limits the amount of materials available for treatment planning. These materials evolve on a daily basis, and can be summarized as zirconia monolithic blocks, glass porcelain, reinforced glass ceramics, porcelain-resin hybrid materials, resin composites, titanium, and polymethyl methacrylate. A dental lab can work with all the above-mentioned materials, in addition to others, such as multilayered zirconia, wax, castable resins, chrome cobalt, and high performance plastics (such as Trinia, PEEK and Pekkton). 

Another consideration is the difference in software and hardware between in-house setups and dental labs. In-house software may offer various modules that include treatment planning, smile design, crown and bridge design, and implant design. With these modules, dentists can fabricate single- and multiunit restorations, provisionals, single copings, inlays/onlays, veneers, surgical guides, models, and custom abutments. 

On the other hand, lab-based software can fabricate all the previously mentioned restorations and much more, such as telescopic crowns and wax-ups — and in a more comprehensive way. The lab technician can fabricate implant frameworks, bars, complete dentures, partial dentures and other restorative solutions. Additionally, labs typically utilize dynamic occlusion software that allows virtual articulation analysis.

Another difference between lab and office systems is the milling unit. The chief distinction is the number of milling axes: X, Y and Z planes, and the A and B rotational axes (A rotates around the X plane, and B rotates around the Y plane). Chairside machines typically have four axes (X, Y, Z and A), while lab machines utilize five axes. The fifth axis provides a lot of flexibility — a good example is the milling of bars and fixed dental prostheses with undercuts. Additionally, units with five axes can mill materials in blocks and pucks (i.e., discs), while four-axis units usually only mill blocks. 

When first implementing digital workflows, practitioners may wish to consider a hybrid approach

Does in-office milling offer the same quality as lab-crafted restorations? What does the evidence say?

The comparison between in-office and lab-fabricated restorations should be evaluated from multiple angles. A 2016 study by Kollmuss et al¹ demonstrates chairside milling was able to accurately mimic the details of a preoperative crown when compared to lab fabrication. On the other hand, the lab-fabricated crown had more occlusal points, resulting in better occlusion. When it comes to crown fit, the studies show either comparable results between chairside- and lab-fabricated prostheses, according to Brecht et al² — or, per Bosch et al,³ better adaption of crowns fabricated with five-axis mills compared to four-axis mills (due to greater precision allowed by five-axis technology). 

How can dentists ensure this technology is successfully incorporated into practice? 

It is understood that switching from traditional methods can be challenging; however, clinicians should not ignore the significant benefits associated with digital dentistry in regard to the patient experience, data storage, accuracy and cost effectiveness. In terms of restorative care, it is inevitable that patients will draw comparisons between the need for multiple appointments in one practice with an office that can take a 60-second oral scan and deliver a definitive crown the same day.

When first implementing digital workflows, practitioners may wish to consider a hybrid approach. For example, they may start with an intraoral scanner and begin sending digital files to the lab instead of physically shipping their cases, or buying a desktop 3D printer and fabricating surgical guides in-house based on data from cone beam scans. From there, they can transition to in-office milling processes.

These decisions require time and financial commitments, as well as an investment in training so the dental team is proficient in these new digital workflows and can assist the dentist in meaningful ways that will increase efficiency and reduce chairtime.

Practically speaking, what’s involved in adopting this technology? For example, beyond the initial capital investment, what can dentists expect in terms of costs?

Clinicians tend to look at the initial price of the scanner and milling unit, which typically ranges around $120,000, but they forget about the true cost of implementing digital workflows, which includes setting up an in-office lab with a ceramic furnace, lab motor, staining kit, burs, and milling blocks of different materials, shades and sizes (which might add $15,000 to $20,000). Then there are ongoing costs, which include licensing/maintenance fees and updates, which often range from $3000 to $4000 per year. Some companies might require a usage fee for every file exported or restoration milled. Of course, financial planning should extend to tech support, data storage, and any computer updates that might be required.

Finally, beyond considering hardware depreciation and return-on-investment calculations, providers should carefully assess whether in-office fabrication technologies will improve the quality of care while simultaneously increasing productivity.

Do you have any final thoughts on in-office fabrication?

It is not a matter of whether digital dentistry is here to stay; rather, the question is how clinicians can best embrace this wave without falling further behind. This is already changing in the way dental schools teach restorative care, how new dentists equip their start-up practices, and how established offices are successfully transitioning from traditional care to the advantages of digital practice. 

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Key Takeaways

• Within the last five years, technological advances have enhanced the capabilities of intraoral scanners and in-office fabrication systems.
• In addition, the variety of materials and applications these new systems can handle has increased, which reduces dependence on dental labs.
• An important benefit of in-house fabrication involves the patient’s chairside experience and perception. In this sense, the process helps build confidence and strengthens the patient-provider relationship. 
• Another consideration is cost, as in-office milling delivers significant savings compared to lab-fabricated restorations.
• Milling materials suited for in-office fabrication evolve on a daily basis, and can be summarized as zirconia monolithic blocks, glass porcelain, reinforced glass ceramics, porcelain-resin hybrid materials, resin composites, titanium, and polymethyl methacrylate.
• Ultimately, patients presenting for restorative care will draw comparisons between the need for multiple appointments in one practice with an office that can take a 60-second oral scan and deliver a definitive crown the same day.

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References

1. Kollmuss M, Kist S, Goeke JE, Hickel R, Huth KC. Comparison of chair side and laboratory CAD/CAM to conventional produced all-ceramic crowns regarding morphology, occlusion, and aesthetics. Clin Oral Investig. 2016;20:791–797. 
2. Brecht LE, Pizzi P, Prestipino VJ. Chairside vs. laboratory milling: What factors into a clinician’s decision? Compendium. 2019;40:642–645. 
3. Bosch G, Ender A, Mehl A. A 3-dimensional accuracy analysis of chairside CAD/CAM milling processes. J Prosthet Dent. 2014;112:1425–1431.

From Decisions in Dentistry. October 2021;7(10):8-10.