Summary: the real question about CAD/CAM is not “which brand of machine” but “how much error, and measured where”. This article gathers the reported figures for marginal fit of CAD/CAM zirconia restorations, the clinical acceptability threshold used as a benchmark, and which stages of the digital chain actually determine the final error.

Where the 120 µm threshold comes from

The most widely cited benchmark is the proposal by McLean and colleagues: a marginal discrepancy below 120 µm is considered clinically acceptable for ceramic restorations. That figure sits within the 4–174 µm range that systematic reviews report for ceramic crowns generally.

It should be read correctly. It is an acceptability threshold, not a target. A well-controlled workflow produces considerably smaller gaps, and most of the clinical value of narrowing the gap lies in reducing cement dissolution over time rather than in the number recorded at cementation.

Milled zirconia: the reported figures

For zirconia copings produced by CAD/CAM, studies report marginal discrepancies from about 10 µm to 160 µm, with most below 80 µm.
For monolithic zirconia crowns, some studies report a narrower band, from 11 µm to 58 µm.
A scoping review of 3D printing and CAD/CAM milling in prosthodontics records a mean marginal gap for milled zirconia of 123.89 ± 56.89 µm — and the size of that standard deviation is itself the point: variation between systems and workflows remains considerable.

Such a wide spread across studies is not a contradiction. It reflects reality: results depend on the CAD/CAM system, the measurement method, the number of measurement points, the material, and the preparation design — the last of which lies entirely on the clinical side.

Milling or 3D printing

For zirconia, milling remains the standard process in fixed prosthodontics. A study comparing 3D-printed with milled zirconia crowns found no significant difference in internal fit and marginal adaptation between the groups, but did find significant differences in trueness in the occlusal, axial and internal areas.

In other words: the two technologies can seat comparably, while their overall geometry is not identical. For cases demanding high morphological accuracy, that difference matters.

Where error actually accumulates

The mill is one link. Final error is the sum of the chain, and the first three stages are usually larger contributors than machining.

1. Input data

A finish line that is not clearly readable on the impression or scan file is the largest single source of error, and no downstream stage can compensate for it. An in vitro study comparing extraoral with intraoral scanning examines exactly this stage and its effect on the fit of milled and printed zirconia crowns.

2. Design

The die spacer set in software determines whether the restoration seats fully. Too tight and it binds, producing a marginal gap even though the internal fit is close; too generous and the cement layer is thick and prone to marginal dissolution over time.

3. Sintering

Zirconia is milled pre-sintered and shrinks on firing. The shrinkage factor is entered per blank batch; entering the wrong factor or mixing batches produces a global deviation that no earlier stage detects.

4. Machining and finishing

Worn burs round off fine detail, particularly at the margin. This is why a tool-replacement schedule based on running hours matters as much as machine selection.

What this changes for the clinic

Preparation design affects fit as much as equipment does. A clear, square, continuous margin outperforms a thin bevel — see preparation for crowns and veneers.
Ask the laboratory about cement space when one type of error recurs. It is a parameter the clinic never sees but which acts directly on the result.
Material dictates the thickness threshold, and thickness dictates the reduction required. The materials comparison and 3Y-TZP vs 5Y-PSZ zirconia set out the groups.

FAQ

What marginal gap is good?

Below the widely cited 120 µm threshold is acceptable. Many studies of monolithic zirconia crowns report considerably lower values. Consistency between cases matters more than the absolute number.

How is marginal gap measured?

In research, typically by microscopy or sectioning with measurements at multiple points. Clinically, probing and bitewing radiographs are the practical tools, though far less sensitive.

Does a more expensive machine give better results?

Not automatically. The literature lists the CAD/CAM system as one factor among several, alongside material, measurement method and preparation design. A well-controlled workflow on a mid-range machine is usually more consistent than a loose workflow on a high-end one.

Is 3D-printed zirconia ready?

Studies show fit comparable to milled crowns, but differences in geometric trueness are still reported. For routine fixed prosthodontics, milling has the deeper evidence base.

References

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