Margin Design.
Margin type is a genuine clinical decision, but it is second to margin clarity. A well-chosen margin the laboratory cannot read unambiguously will not produce a well-fitting restoration.
Matching the margin to the material
A chamfer of roughly half a millimetre suits metal-ceramic copings and works well with zirconia, which retains strength in thin section and does not need bulk at the finish line to survive handling.
A shoulder or rounded shoulder of around a millimetre suits lithium disilicate and all-ceramic restorations generally, providing the material thickness those restorations need at their most vulnerable point and giving a defined seat.
A knife edge or feather margin is appropriate for cast metal, which can be finished to a fine edge and burnished. It is a poor choice under ceramic, where it produces either an unacceptably thin ceramic margin or an overcontoured one.
Definition is the governing requirement
The laboratory has to identify the margin around the full circumference without guessing. Where the finish line is indistinct, ragged, or disappears into an existing restoration, the margin has to be interpreted — and an interpreted margin is how open and overextended margins occur.
A continuous, smooth finish line is worth more than a theoretically ideal margin type executed inconsistently. Scalloped or stepped margins created by changing bur angulation are difficult to read on a die and worse on a scan, because reconstruction algorithms smooth exactly the detail that needs preserving.
Placement and the periodontium
Supragingival margins are the kindest to the tissues, the easiest to impress, the easiest to clean and the easiest to maintain. Where aesthetics permit, they are the right answer.
Equigingival and subgingival placement is often unavoidable — existing restorations, caries extending apically, or aesthetic demands in the anterior. The constraint is the attachment: a margin placed too close to the alveolar crest impinges on the supracrestal attached tissues and produces chronic inflammation that no amount of laboratory accuracy will resolve. Where the required margin position would encroach, crown lengthening is the answer rather than deeper placement.
Subgingival margins also compound every impression difficulty at once, which is where tissue management stops being optional.
Capturing the margin
The margin must be visible before it is recorded. Retraction and haemostasis are not preparatory niceties — a margin obscured by tissue or fluid is recorded as obscured, whether the record is a wash impression or a scan.
Conventional and digital capture fail differently. Impression material tears or drags at a fine margin and traps sulcular fluid as a void. A scanner cannot see through blood or tissue at all and will reconstruct a plausible surface where the real margin was hidden, which is more dangerous because it looks complete on screen.
In both cases the fix is clinical and happens before the record is taken.
Cement space and internal fit
A restoration that fits the die perfectly with no relief will not seat clinically, because cement needs somewhere to go. Die spacer, or its digital equivalent, creates that space — typically applied to the axial walls while stopping short of the margin so the finish line itself remains closely adapted.
How much relief is appropriate depends on the cement. A filled resin cement needs more room than a thin-film luting cement, and a restoration relieved for one and cemented with the other will either sit high or fit loosely.
This is usually left to the laboratory, and reasonably so. But if you have a strong preference — a particular cement you always use, or a case where you want an especially close fit accepting a more difficult seat — say so, because otherwise we are applying our standard.
Margins on post-restored and previously crowned teeth
Re-preparing an existing crown preparation is common and carries a specific risk: the new finish line is often placed at or apical to the old one, progressively deepening with each replacement until it encroaches on the attachment.
Before deepening, ask whether the existing margin position was actually the problem. Frequently the failure was secondary caries on an accessible surface, or a margin that was never well adapted, and reproducing the same depth with better execution is the better answer than going further apically.
Where the margin must be extended, extend it to sound structure and assess whether the remaining ferrule still justifies restoring the tooth.
Mark it if you can see it
You prepared the tooth and you know where the finish line is. The technician is reading a die or a mesh. Where a system allows margin marking at the chair, using it transfers knowledge that would otherwise be lost.
On conventional cases the equivalent is marking the die, which is standard laboratory practice, but a note identifying an ambiguous surface — the margin is clear except on the distolingual, where it sits just beneath the tissue — does the same job and takes one line.
Where the finish line genuinely cannot be identified on part of its circumference, that is not a marking problem. It is a record problem, and the answer is to expose the margin properly and record it again.
Margins and the provisional
A provisional that is short of the margin, overextended, or rough at the finish line will produce tissue inflammation within days, and the tissue you carefully retracted at the impression appointment will not be the tissue present at the seat appointment.
This is a recurring and avoidable source of difficulty at delivery: the definitive restoration is correct, but the tissue has become swollen and friable around a poorly finished provisional, and the margin can no longer be assessed or the restoration cleanly cemented. Time spent finishing and polishing the provisional margin protects the case it is holding.
What to state on the prescription
Name the margin type you have prepared and the material intended, so the laboratory can confirm they are compatible before fabricating. If you want a specific die relief or a particular margin finish — a slight bevel, a porcelain labial margin, a metal collar of a stated dimension — write it down.
Where the margin is ambiguous on part of the arc, marking the die or noting which surface is uncertain is far more useful than leaving the laboratory to choose.
In short
- Chamfer for metal-ceramic and zirconia; shoulder for lithium disilicate
- Knife edge suits cast metal and is a poor choice under ceramic
- An unreadable margin is interpreted, and interpretation causes misfit
- Respect the supracrestal attachment; crown lengthen rather than go deeper
- Scanners reconstruct plausible surfaces over hidden margins — clear the field first
More on preparation
Discuss a case
Where a case sits between options, a short conversation before preparation is usually quicker than resolving it afterwards.