Finish with the Red Bur: What Your Prep Bur Colour Is Doing to Your Margins
An in vitro study of 50 molar teeth finds that finishing CAD-CAM crown preparations with a fine-grit red diamond bur after coarser preparation cuts the mean marginal gap roughly in half, from 64 µm down to 28 µm.
Source Paper
Effect of bur grit size on the marginal adaptation of glass ceramic restorations
There is a small habit that nearly every clinician develops but rarely writes down: once the bulk of tooth reduction is done with the workhorse bur (the black band, the green band, whatever takes off material fastest), you switch to something finer for the last pass and just clean it up. Sümbül et al., in “Effect of bur grit size on the marginal adaptation of glass ceramic restorations” (Journal of Prosthodontics, 2026), have done the actual work of proving this instinct correct and quantifying how much it matters for CAD-CAM crown fit.
The Data Anchor
Fifty extracted, caries-free human third molars were divided into five groups of n = 10, each testing a different finishing protocol with ISO-coded diamond chamfer burs: neutral band only (107 µm), black band only (181 µm), green band only (151 µm), black band followed by red band finishing (46 µm), and green band followed by red band finishing. Preparations were performed by a single operator at 300,000–350,000 rpm with standardised convergence of 6–8 degrees. Teeth were scanned with a Virtuo Vivo intraoral scanner (Dental Wings), crowns designed in Exocad V 2.2, and restorations milled from IPS e.max CAD blocks (Ivoclar Vivadent) on a Camcube M10 chairside unit. Marginal gap was measured with a Leica MZ16 stereomicroscope at 20x, capturing 32 equidistant points per specimen across four surfaces (1,600 data points total).
Key Findings
- Finishing with the red band bur roughly halved the marginal gap: Groups 4 and 5 achieved mean gaps of 34 µm and 28 µm respectively, compared to 63–64 µm for single-bur groups (p < 0.05 for all pairwise comparisons).
- The choice of coarse bur for bulk reduction did not matter on its own: neutral, black, and green band alone produced statistically indistinguishable gaps of 63, 64, and 64 µm (p > 0.05).
- The green-plus-red combination edged ahead (28 µm vs 34 µm, p = 0.012 on buccal and palatal surfaces), suggesting the starting grit is not entirely irrelevant once you commit to fine finishing.
- All groups remained within the accepted 120 µm clinical threshold; the authors note the ~20 µm difference reached statistical but not necessarily clinical significance, a genuinely honest caveat.
- Limitations: isolated extracted teeth without adjacent contacts, extracted tooth reflectivity differs from in vivo conditions, and only lithium disilicate was tested.
💡 The Clinical Bottom Line
Add a red band finishing step to every crown preparation before you scan, regardless of which bur you used for reduction. The coarse bur gets you the shape; the fine bur gets you the margin. Intraoral scanners and CAD-CAM milling machines are only as accurate as the surface you give them to interpret. For a workload built on same-day digital crowns, that is a protocol change worth making permanent.
Dr Samuel Rosehill is a general dentist with a prosthodontic focus, practising at Ethical Dental in Coffs Harbour, NSW. He holds a BDSc (Hons) from the University of Queensland, an MBA, an MMktg, and an MClinDent in Fixed & Removable Prosthodontics (Distinction) from King’s College London.
Clinical Relevance
Always finish crown preparations with a fine-grit (red band) diamond bur before scanning, regardless of which coarser bur was used for bulk reduction — this single step approximately halves the marginal gap in CAD-CAM lithium disilicate restorations.
Disclosure: The author has no financial conflicts of interest related to the products or topics discussed in this review. This is an independent summary prepared for educational purposes.
Continue the conversation
This review is also published on Substack, where you can leave comments and join the discussion.
Read on Substack →