Common Crown Preparation Errors.
These are not exotic problems. They are the handful of recurring faults behind most remakes, and each is straightforward to catch before the record is taken.
Insufficient occlusal reduction
By a clear margin the most frequent. It usually arises from reducing until the preparation looks right rather than measuring, and from checking clearance only in maximum intercuspation.
Caught by an index or a bite registration record through the prepared area, checked in excursion. Corrected by further reduction, opposing enameloplasty, or a deliberate change of material to one that performs in thinner section.
Undercuts
Frequently created without being noticed, particularly on the lingual of posterior teeth and around existing restorations. They prevent seating entirely.
With a scan, review the insertion axis in the software rather than accepting the default view. With a conventional impression, examine the die along the intended path of draw. Corrected by removing the undercut, not by blocking it out in the laboratory, which sacrifices fit at exactly the point being blocked.
Indistinct or ragged margins
Produced by a worn or fine-grit bur used to finish, by changing bur angulation mid-arc, or by preparing into an existing restoration so the finish line runs partly on tooth and partly on old material.
The laboratory has to interpret an indistinct margin, and interpretation produces either an open margin or an overextension. Where the finish line crosses into an existing restoration, the honest options are to remove and replace the restoration or to extend the preparation to sound structure.
Over-taper
Usually cumulative rather than deliberate — a series of small angulation corrections, each reasonable, adding to a preparation with very little retention.
A restoration on an over-tapered preparation seats beautifully and comes off under function. If a preparation has ended up over-tapered, adding grooves or boxes restores resistance form, and the cement choice should reflect the geometry rather than being defaulted to.
Sharp internal line angles and unsupported enamel
Sharp angles concentrate stress and initiate ceramic fracture. Undermined, unsupported enamel at the margin fractures during impression removal, cementation, or shortly after — leaving a defect at the finish line of a restoration that fitted correctly when it was made.
Both are quick to address at preparation and impossible to address afterwards.
Preparing without a pre-operative record
A pre-operative impression, scan or photograph costs a minute and provides the reference for everything afterwards: a reduction guide, a provisional matrix, the original contour and contacts, and a record of what the tooth looked like before it was reduced.
Without one, the reduction has to be judged by eye, the provisional has to be shaped freehand, and the definitive restoration is designed to an idealized form rather than to what the patient had. For a badly broken-down tooth, or for anterior work of any kind, that reference is not optional.
Adjusting the preparation after the record was taken
This one causes disproportionate trouble because nothing about the case looks wrong. The impression or scan was accurate when it was captured; the preparation was subsequently refined — a contact opened, a sharp edge rounded, an undercut removed — and the restoration is now made to a tooth that no longer exists.
If you adjust after recording, retake the record or tell us exactly what changed. A restoration made to a superseded preparation typically fits nowhere properly, and the cause is very difficult to diagnose from the returned case.
Damaging the adjacent teeth
Iatrogenic damage to the proximal surfaces of adjacent teeth during preparation is common, frequently unnoticed, and consequential — the restoration's contact is then established against a scarred or notched surface, and the adjacent tooth is left more susceptible to caries.
A metal matrix band or a proximal guard protects the neighbour, and inspecting the adjacent surfaces after preparation catches damage while it can still be smoothed and polished.
Ignoring what the opposing arch is doing
The preparation is assessed against the opposing dentition, and if the opposing arch has supra-erupted, tilted or carries a plunger cusp, the space available is not what a standard reduction would produce.
Mounting study casts, or simply examining the occlusion carefully before starting, identifies these situations while there is still a choice about how to address them. Discovered after preparation, the options narrow to further reduction of an already short preparation, or a restoration built into an interference.
Anterior-specific faults
Anterior preparations fail in their own particular ways. Insufficient facial reduction is the commonest, and it produces either an overcontoured restoration that pushes the lip or a thin one with no room for internal character.
Insufficient incisal reduction is next, and it is often masked because the tooth looks obviously shortened. What matters is the clearance in protrusive and in lateral excursion, not the amount removed. A preparation with adequate incisal reduction at rest can still leave the restoration in contact during function.
The third is failing to break the contact fully, which leaves the technician unable to establish a proper contact area and unable to see the margin interproximally in either a scan or an impression.
A two-minute check before the record
Almost every fault described in this article is detectable in the two minutes before the impression or scan is taken, and almost none is detectable afterwards.
Look at the preparation from the intended path of insertion for undercuts. Check clearance with an index or bite material, in intercuspation and in excursion. Run an explorer around the finish line to confirm it is continuous and smooth. Confirm the margin sits on sound tooth structure throughout. Inspect the adjacent teeth for damage. Confirm any additional retention features you intended are actually cut.
Six checks, no equipment beyond what is already on the bracket table. The practices with the lowest remake rates are almost always the ones doing something like this consistently rather than the ones preparing teeth differently.
Failure to record what was intended
A prescription that does not state the material, the margin type, the shade with its guide, or anything unusual about the case leaves the laboratory reconstructing intent from a die.
Most of the errors above are visible to us and we will raise them. But a case arriving with a clear note — short preparation, limited clearance accepted, margin uncertain on the distal — is handled correctly the first time rather than after a phone call.
In short
- Insufficient occlusal reduction is the most common fault by a wide margin
- Review the insertion axis in scan software rather than accepting the default view
- Margins crossing into existing restorations should be extended to sound tooth
- Over-taper is cumulative; grooves and boxes restore resistance form
- Remove unsupported enamel at preparation, not after it fractures
More on preparation
Discuss a case
Where a case sits between options, a short conversation before preparation is usually quicker than resolving it afterwards.