Case Analysis: Why Your Temporary Crown Fractured – The Critical Role of Choosing the Right PMMA Disc

Case Analysis of Temporary Crown Fracture: The Importance of Selecting the Right PMMA Disc

In digital dental workflows, temporary crowns serve as an essential transitional restoration between prepared teeth and final prosthetics. However, material fracture and chipping remain common challenges in dental laboratories, especially during CNC milling processes.

This article analyzes real-world fracture cases and explains why proper PMMA disc selection is critical for improving restoration stability.

Typical Cases of Temporary Crown Fracture

Temporary crown failures in dental labs usually occur in three stages:

 1. Weak structural design in CAD stage

Insufficient marginal thickness can create stress concentration zones, increasing the risk of microcracks during milling.

2. Material response during CNC milling

Materials with inconsistent internal structure may chip or fracture under high-speed cutting forces.

3. Stress release during finishing and try-in

Localized pressure during adjustment or clinical try-in can trigger crack propagation in weak areas.

 How Material Properties Affect Stability

From a materials engineering perspective, temporary crown stability depends on three key PMMA parameters:

• Flexural strength ≥100 MPa (ensures structural integrity under masticatory simulation, source: PMMA data page PMMA-02)
• Elastic modulus 2800–3000 MPa (maintains dimensional accuracy and fit, source: PMMA data page PMMA-03)
• Hardness 220–250 MPa (improves surface durability and scratch resistance, source: PMMA data page PMMA-01)

These parameters collectively determine fracture resistance during processing.

Selection Guide to Reduce Fracture Risk

1. Choose PMMA discs with uniform internal structure

Homogeneous polymer distribution helps reduce internal stress accumulation.

2. Match disc thickness with restoration type

• Single crowns: 10–12mm
• Multi-unit bridges: 16–18mm
  Proper thickness reduces weak-point failure.

3. Ensure machining compatibility

Standard disc sizes (98mm / 120mm) ensure stable CNC milling performance.

4. Evaluate elastic modulus stability

Stable modulus ensures predictable deformation behavior during milling.

Industry Insight: From “Fracture Issues” to “Material Selection Strategy”

Dental laboratories in Europe and North America are increasingly shifting from viewing crown fracture as a processing issue to a material selection challenge. High-performance PMMA discs help ensure consistent behavior across milling and clinical stages.

This reflects a broader trend where material science plays a central role in digital dentistry efficiency.

Conclusion

Temporary crown fracture is not a single-factor issue but the result of design, processing, and material performance interaction. By selecting PMMA discs with stable mechanical properties and optimizing milling conditions, dental labs can significantly reduce chipping risks and improve restoration reliability.