When Lithium Disilicate Restorations No Longer Fit After Firing

In the accompanying customer picture, two anterior restorations are shown on a working model from several angles. The customer reported a fit problem after firing. The footage provides context for the complaint, although it does not establish when or why the discrepancy developed.

For a dental laboratory, a restoration that cannot seat accurately means additional investigation, adjustment, or a remake. For a manufacturer, it is an opportunity to examine how material performance and processing conditions interact. The first step is to establish what changed and at which stage.

1. Understand What Happens During Firing

For lithium disilicate CAD/CAM blocks supplied in a partially crystallized state, the heat treatment after milling is more accurately called crystallization firing. During this process, the crystalline structure develops within the glass matrix, producing the intended final properties.

This article focuses on that workflow. Fully crystallized blocks and pressable lithium disilicate materials have different processing requirements, so instructions should always be checked for the exact product.

Small dimensional changes can occur during crystallization. Research has measured changes in marginal and internal fit, but measurable change does not automatically make a restoration unacceptable. In one study, the tested crowns remained within the authors’ fit criterion after crystallization despite statistically significant changes.

A crown that no longer seats requires investigation to distinguish dimensional change from other sources of interference.

2. Establish Whether Deformation Actually Occurred

In the customer video, the restorations are viewed from different angles. This helps identify areas requiring closer inspection, but a clear comparison of fit before and after firing is missing.

Incomplete seating can have several explanations. Residual firing paste or debris inside the restoration may obstruct placement. Accidental glaze contamination, tight proximal contacts, insufficient internal clearance, or inaccuracies in the model may also need checking.

Where the product permits assessment before crystallization, document the fit on the working die, the model replica of the prepared tooth. Repeat the assessment after the prescribed cooling and cleaning, using the same die and a comparable seating method.

Record whether the restoration rocks, binds internally, or shows an open margin. These observations help determine which checks to prioritize.

3. Review the Firing Program and Furnace

Lithium disilicate contains both crystalline and glass phases. Its response to heat depends on its composition, structure, and thermal history. Excessively high firing temperatures can cause plastic deformation, potentially affecting contours and margins.

Begin with the selected program. A setting labelled “lithium disilicate” may have been created for another manufacturer’s material. Similar shade names or advertised strength values do not establish that two products share the same firing requirements.

Check all specified stages, including heating rates, peak temperatures, holding times, vacuum settings, and cooling. Verify furnace calibration according to the furnace manufacturer’s procedure. The displayed temperature alone does not confirm that the restoration experienced the intended conditions.

Tray selection, loading arrangement, and any additional glaze or correction firings should also be recorded. The complete processing history gives a more useful basis for investigation than the peak temperature alone.

4. Check Support, Design, and Machining

Correct firing support helps stabilize the restoration during heat treatment. Depending on the system, the manufacturer may specify a particular tray, support pin, firing paste, or combination.

Follow the instructions for the product and restoration type. Check stability and positioning, particularly around delicate margins. When a pin-and-paste system is required, observe the specified clearances and avoid placing unwanted pressure on the ceramic.

Restoration geometry also deserves attention. Excessively thin areas, abrupt changes in thickness, or substantial grinding before firing should prompt a design review. Minimum thickness requirements depend on the material and indication; a single value cannot cover every lithium disilicate restoration.

Review CAD cement-space settings, milling-tool condition, and internal features that may be difficult to machine accurately. Limited clearance can make small dimensional changes more noticeable during seating, while machining errors can create a discrepancy before the restoration reaches the furnace.

5. Investigate With Controlled Comparisons

A useful investigation preserves evidence before extensive adjustment. The following sequence can help:

• Inspect and clean. Examine the internal surface and margins under magnification, removing residues according to the product instructions before reassessing fit.
• Separate contact interference from internal binding. Compare seating on the isolated die and in the full model. A difference between them warrants checking the proximal contacts and model assembly.
• Identify the processing stage. When crystallization and glazing are separate, assess fit between them to determine when the discrepancy first becomes detectable.
• Compare the pattern. Several affected restorations in one firing suggest reviewing shared conditions. A single affected unit directs attention toward its individual design, support, or seating surfaces.
• Test one variable at a time. Use matched test restorations and documented conditions. If a material batch remains suspect, compare it with a reference batch under the same validated workflow.

These patterns guide troubleshooting; they do not establish the cause alone. Keep the CAD file, material batch number, firing record, support details, and fit observations together.

Extensive grinding can remove evidence and compromise thickness or marginal accuracy. Where confirmed distortion cannot be corrected within the product’s requirements, remaking the restoration is the appropriate course.

6. The ICERA Perspective: Evaluate the Finished Result

Manufacturers have a role in understanding dimensional performance. Glass composition, manufacturing consistency, and controlled crystallization influence how a material behaves during processing. Evaluation should therefore consider fit after the recommended workflow alongside strength, shade, and translucency.

At ICERA, we believe a discussion about lithium disilicate quality should include the accuracy of the finished restoration. A customer feedback video is valuable because it connects material evaluation with a laboratory’s practical experience.

For laboratories considering ICERA lithium disilicate, we encourage documented trials using representative designs, the recommended processing instructions, and consistent fit assessments. This provides a practical basis for discussing performance and identifying where further investigation is needed.

The case shown here highlights the value of locating the stage at which fit became unacceptable. Establishing that with evidence helps manufacturers and technicians work toward fewer remakes and more predictable results.

Connect with ICERA to discuss lithium disilicate processing and the fit challenges your laboratory encounters.