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In-Office Milling vs. Lab Fabrication: Making Case Decisions

In-Office Milling vs. Lab Fabrication: Making Case Decisions

The incorporation of intraoral scanning in the modern dental practice offers numerous benefits, including improved accuracy, reduced chair time, and a more comfortable patient experience, as well as a faster turnaround time and more precise fit for the restoration.1-3 To further shorten the restorative timeline, in-office mills allow practitioners to produce crowns and other indirect restorations, such as inlays and onlays, within their offices. Dentists can now offer same-day dentistry, which is highly appealing to patients, who value the opportunity to receive a restoration in a single visit.

Inevitably, dentists who have added these technologies to their practice are faced with a decision: Which cases should be milled in-office, and which should instead be submitted to the lab for design and fabrication? In cases where the diseased or damaged portion of the tooth allows for a conservative preparation to support an inlay or onlay, in-office milling is almost always the best option. For patients who require a crown, each case must be evaluated individually to determine the optimal means of fabrication.

Dentists who have added these technologies to their practice are faced with a decision: Which cases should be milled in-office, and which should instead be submitted to the lab for design and fabrication?

Five Criteria for Deciding Between In-Office Milling and Lab Fabrication

1. Location in the Mouth — When the tooth is in the esthetic zone, the digital impression is usually submitted to a dental lab for fine-tuning of the crown contours and shade to meet the esthetic needs of the case.

2. Opposing Dentition and Previous Restorations in Vicinity — The characterization of surrounding teeth may make it challenging to mill a crown in-office that blends well with the neighboring dentition. If there are existing crowns in the area of the tooth in question, the dental lab should fabricate the new restoration to ensure as close a match as possible.

3. Parafunctional Habits — Patients who exhibit bruxism or other parafunctional habits require the most durable restoration possible. When there is evidence of bruxing, grinding or a heavy bite, a lab-fabricated maximum-strength crown, such as BruxZir® Full-Strength Solid Zirconia or full-cast metal, may be needed to ensure a long-lasting restoration.

4. Stump Shade — The shade of the tooth can determine which material is ideal for the restoration. In cases where the stump shade is dark, a lab-fabricated crown produced from BruxZir Solid Zirconia or a layered material is often needed to mask the underlying tooth structure. If the stump of the tooth is a natural dentin shade, an in-office mill can be used to fabricate a lifelike restoration from a glass ceramic like Obsidian® lithium silicate (Glidewell Direct; Irvine, Calif.).

5. Time Requirement — For in-office milling, there must be time allotted in the patient’s and the practice’s schedule to fabricate, prep and deliver the restoration. From the time the patient arrives at the office, it typically takes 90–120 minutes to deliver a crown milled in-office. For an inlay or onlay, 70–90 minutes are required. If either the doctor or patient cannot accommodate this schedule, lab fabrication is more efficient. Further, when multiple restorations are needed, the time required to mill in-office is often prohibitive.

The decision-making process for in-office milling vs. lab fabrication is best explained by presenting an example of each. The following cases illustrate both how this determination is made and the step-by-step restorative workflow involved in these two approaches to digital dentistry.

Case Report No. 1: Lab Fabrication

The patient visited my office after receiving endodontic treatment for tooth #20 that necessitated crown replacement. A lab-fabricated restoration was chosen, as I was concerned about the dark shade of the underlying tooth and wanted a restoration with a more opaque substructure. There was also color variation on the adjacent tooth, requiring customization better-suited to the skills of a lab technician. After removing the patient’s existing crown, the tooth was prepared, and the impression was made with an intraoral scanner. The case was digitally submitted to the lab, and a crown was fabricated with a zirconia core and overlying porcelain. The final esthetics of the restoration were very pleasing and matched the color and characterization of the neighboring teeth quite well.

Case Report No. 2: In-Office Milling

The patient sought treatment for a broken crown on her maxillary left first premolar. The previous restoration and a majority of the core buildup were missing, and the patient desired same-day crown replacement. Producing the crown in-office was also ideal because it would have been challenging to create a temporary restoration without fabricating a matrix from the existing crown. In addition, there was minimal tooth structure, and by bonding an Obsidian lithium silicate restoration to the preparation, excellent bond strength could be achieved.

The tooth was a shade A3, and did not require extensive characterization. The tooth was prepped, a digital impression was taken, and an Obsidian restoration was fabricated using the TS150™ in-office mill (Glidewell Dental; Irvine, Calif.). The crown was delivered less than an hour after intraoral scanning was performed, and the patient was extremely happy with the final result.

A chairside-milled Obsidian crown was selected, as the lithium silicate material met the esthetic requirements of the case and allowed for a high-strength bond for the restoration.

Conclusion

Whether the restoration is produced by the lab or milled in-office, intraoral scanning offers improved efficiency compared to traditional impression-taking. It has been proven that digital impressions reduce or eliminate issues with fit, margins and occlusion.4 In addition to delivering a predictable, highly accurate restoration to the patient in less time, the return on investment for these technologies is substantial due to cost savings on the fabrication of indirect restorations. By following some straightforward criteria, practitioners who choose to embrace these digital tools can determine the correct method of fabrication with confidence, while enjoying the tremendous marketing opportunity offered by same-day dentistry.

The crown proposal generated by the FastDesign software was well-suited to the case and did not require any occlusal adjustments.

References

  1. Ng J, Ruse D, Wyatt C. A comparison of the marginal fit of crowns fabricated with digital and conventional methods. J Prosthet Dent. 2014 Sep;112(3):555-60.
  2. Zarauz C, Valverde A, Martinez-Rus F, Hassan B, Pradies G. Clinical evaluation comparing the fit of all-ceramic crowns obtained from silicone and digital intraoral impressions. Clin Oral Investig. 2016 May;20(4):799-806.
  3. Yuzbasioglu E, Kurt H, Turunc R, Bilir H. Comparison of digital and conventional impression techniques: evaluation of patients’ perception, treatment comfort, effectiveness and clinical outcomes. BMC Oral Health. 2014 Jan 30;14:10.
  4. Pradíes G, Zarauz C, Valverde A, Ferreiroa A, Martínez-Rus F. Clinical evaluation comparing the fit of all-ceramic crowns obtained from silicone and digital intraoral impressions based on wavefront sampling technology. J Dent. 2015 Feb;43(2):201-8.
Chairside Magazine: Volume 12, Issue 2

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