Chipping at the margin of a preparation when milling zirconia almost always comes from a combination of three causes: a dull or low quality bur, a feed rate that is too high when passing over the margins, and a workpiece that is not held firmly enough during cutting. A study published in August 2026 in the Journal of Advanced Prosthodontics shows that the marginal gap of a zirconia crown grows from 34.78 µm with new burs to 99.69 µm with burs at the end of their service life, used on the same tool set without replacement. The IPS e.max ZirCAD instructions for use from Ivoclar Vivadent are blunt on one point: coarse tungsten carbide burs used for finishing "may cause chipping", which rules them out on marginal areas.
Does a worn bur cut zirconia less cleanly?
A new bur shears the material; a dull one ploughs through it. Cutting pressure rises, local temperature climbs, and microcracks start exactly where the piece is thinnest: at the margin. That is what the study published in the Journal of Advanced Prosthodontics in August 2026 confirms. Its authors milled 56 monolithic zirconia crowns with a single set of tungsten carbide burs, split into four groups of 14 by manufacturing order. A software alert to replace the bur triggered as early as the 27th restoration, but milling continued without changing the burs in order to measure the effect of wear. The result: the vertical marginal gap rose from 34.78 ± 5.89 µm in the first group to 99.69 ± 21.66 µm in the last, with a strong correlation between wear and the decline in marginal fit, even though every measured gap stayed under the 120 µm clinical threshold the authors set.
On the shop floor, a trade article published on October 3, 2025 by Excellement puts the useful life of a 2 mm bur at 400 to 500 restorations on a medium hardness block, and only 250 to 300 on a harder one: the denser the material, the faster the tool wears, and the sooner the risk of margin chipping appears. A variance of just 0.02 mm in a 2 mm bur's diameter is already enough to degrade a restoration's internal fit.
Should feed rate and CAM parameters be adjusted near margins?
Yes. A technical article published on October 21, 2025 by BSM Dental recommends lowering the feed rate on the final passes that run along the margin, rather than keeping the same setting used for roughing out the bulk volume. Excessive cutting forces on an already thin margin geometry, overly aggressive CAM parameters that amplify dynamic loads, and a nesting orientation that exposes the margin to unfavorable toolpaths are among the causes that article identifies. It also recommends regular microscopic inspection of burs and following the manufacturer's stated tool life cycles, rather than deciding on replacement by eye when a bur still seems to cut properly.
On handling after milling, the IPS e.max ZirCAD instructions likewise require that any post processing be done in the green state, using light pressure only, never at high speed or with heavy pressure.
Does disc clamping and holding bar placement change the chipping risk?
How firmly the disc is held during milling matters as much as the bur's condition. The IPS e.max ZirCAD instructions require the disc's circumferential notch to be perfectly clean before it is fixed into the disc holder, and the clamping screws to be tightened evenly all the way round: uneven clamping introduces vibrations that carry through into the cut, particularly on thin margins.
The same instructions set precise rules for the holding bars that keep the piece attached to the disc before separation: a diameter of at least 2.0 mm, positioned at least 1.0 mm above the preparation edge, never in an interdental area. A bar placed too close to the margin, or too thin, concentrates separation stress exactly where the piece is most fragile. When separating the restoration from the disc, the same instructions flatly forbid using a cutting disc on bridge connectors: doing so creates a predetermined breaking point and weakens the ceramic piece's strength, a risk that adds to that of margin chipping.
What margin thickness limits the risk of chipping?
A margin that is too thin has less material to absorb cutting stress, which mechanically raises the risk of chipping as the bur passes. The IPS e.max ZirCAD instructions give a minimum monolithic crown thickness of 0.4 mm in the anterior region for the most translucent range, and 0.8 to 1.0 mm for the more resistant ranges, with a shoulder or chamfer at least 1.0 mm wide recommended on the preparation itself. They add a useful geometric rule: the radius of the preparation's edges, particularly in anterior teeth, must be at least as large as the radius of the smallest milling instrument used, otherwise the bur cannot follow the sharp angle without forcing.
The instructions from the manufacturer of the zirconia discs distributed by Dentev, which cover the ST, SHT and 3D Pro multilayer ranges, classified as class 5 type II under EN ISO 6872, along with the UT range in class 4, do not publish a margin thickness value specific to milling: they point to the instructions supplied with each disc for that point, and focus instead on sintering and coloring. Margin guidelines therefore have to come from the CAD design stage, before the bur ever touches the disc.
How can the green restoration be finished without creating new chips?
Separating and finishing in the green state is as risky a moment as milling itself. The IPS e.max ZirCAD instructions recommend fine tungsten carbide burs or fine diamond grinding instruments to detach the piece and smooth the holding bar attachment points: coarse burs "may cause chipping, among other things", they state. They also call for light pressure only, for avoiding rubber polishers, which condense the surface and contaminate it, and for cleaning zirconia dust with a soft brush followed by oil free compressed air rather than an ultrasonic bath or steam jet, both forbidden on an unsintered piece.
After sintering, if zirconia dust remains stuck to the piece, the same instructions allow sandblasting with a 50 to 110 µm abrasive at a maximum pressure of 1.5 bar: beyond that, blasting damages the surface instead of cleaning it. On safety, the instructions from the manufacturer of the Dentev discs note that milling zirconia generates dust and require a dust mask and protective goggles; they also state that each disc is single use, which avoids reusing a support already weakened by an earlier milling run.
- Marginal gap with new burs versus end of life burs: 34.78 µm versus 99.69 µm, across 56 zirconia crowns (Journal of Advanced Prosthodontics, August 2026)
- Clinical marginal gap threshold set by the study: 120 µm, never exceeded despite wear (Journal of Advanced Prosthodontics, August 2026)
- Bur replacement alert triggered: at the 27th restoration milled with the same tool set (Journal of Advanced Prosthodontics, August 2026)
- Service life of a 2 mm bur: 400 to 500 restorations on a medium hardness block, 250 to 300 on a hard block (Excellement, October 3, 2025)
- Minimum holding bar diameter: 2.0 mm, at least 1.0 mm above the preparation edge (IPS e.max ZirCAD instructions, Ivoclar Vivadent, 2019 revision)
- Allowed post sintering sandblasting: 50 to 110 µm grit, maximum pressure 1.5 bar (IPS e.max ZirCAD instructions, Ivoclar Vivadent, 2019 revision)
- Minimum monolithic crown thickness by range: from 0.4 mm to 1.0 mm (IPS e.max ZirCAD instructions, Ivoclar Vivadent, 2019 revision)
Sources
IPS e.max ZirCAD Instructions for Use, Ivoclar Vivadent, revised June 27, 2019 (Rev. 3), ivoclarvivadent.com.
Instructions for use from the manufacturer of the zirconia discs distributed by Dentev, summarized in the internal sintering and milling documentation (CE marking, notified body 1639, EN ISO 6872 standard).
"Influence of repeated milling bur use on the marginal adaptation of monolithic zirconia crowns", Journal of Advanced Prosthodontics, volume 18, issue 4, pages 337 to 346, August 2026.
"How Milling Bur Lifespan Affects Zirconia Crown Chipping", Excellement, excellement.com, October 3, 2025.
"Preventing Margin Chipping in Milled Zirconia Crowns", BSM Dental, bsmdental.com, October 21, 2025.
