Smoothing and Rounding Preparation Angles—How to Prevent Ceramic Fractures

A sharp internal line angle in a preparation is an invisible trap. Ceramic designed over a sharp angle creates a stress concentration point there—exactly where the material will fracture under mastication. This is one of the most common reasons for returned cases to the lab: "the shipment arrived fractured" or "the patient came in after two months with a fracture." deltalabs.' experience with thousands of cases annually clearly shows: proper rounding of angles = no fractures. Conversely—sharp angles = increasing failure rate.

Why Sharp Angles Are a Problem—Materials Mechanics

Ceramics have high compressive strength (500–900 MPa) but low tensile strength (50–100 MPa). A sharp internal line angle under ceramic = bending under occlusal forces. When the patient chews:

Force perpendicular to the occlusal surface
Ceramic flexes slightly (material elasticity)
At the sharp angle, stress concentration occurs—a localized increase in stress
The material exceeds its tensile limit and fractures

Studies show: sharp angles can increase local stresses by 30–50% compared to rounded ones. This is not marginal.

Which Materials Are Most Sensitive?

Material	Sensitivity to Sharp Angles	Note
e.max CAD	High	Monolithic feldspathic ceramic—low tensile strength
e.max Press	High	Glass-ceramic—sensitive to stress concentration
3Y Zirconia	Low–Medium	Stabilized zirconia (ZrO2)—more durable, but sharp angles can still cause problems
Layered Ceramic / PFM	Very High	Least expensive—most sensitive to sharp angles

Practical rule: e.max and layered ceramics require particular attention to rounding. Zirconia is more forgiving, but never ignore smoothing.

Where Sharp Angles Most Commonly Occur

Technically, sharp angles always appear where two preparation surfaces meet at an angle less than ~120°. In practice, these are:

Proximo-occlusal line angle—where the axial wall meets the occlusal surface
Angle between the preparation floor and the axial wall—in the trunk area
Edges after milling—if not immediately smoothed
Transitions between zones (e.g., between cervical and axial walls)
Marginal preparation lines—if not rounded

These areas bear the greatest stresses during mastication. This is not a coincidence—it is material mechanics.

How to Properly Smooth and Round a Preparation

Procedure in the Clinic

After initial grinding—immediately after completing preliminary grinding
Instrument: ball or pear-shaped bur, grit size 100–150 μm
Speed: 40,000 rpm (high-speed), no pressure
Time: 30–60 seconds of work—5–10 seconds per area
Coverage: every internal angle—always rounded

Rounding Parameters

Parameter	Value
Minimum rounding radius	≥0.5 mm
Optimal rounding radius	0.8–1.0 mm
Bur grit size	100–150 μm (medium)
Rotational speed	40,000 rpm
Bur pressure	Light—no forcing

Note: A 0.5 mm radius is the minimum. It is better to round more than too little.

Control in Impression/Scan

After smoothing, check with an impression or photograph:

Are all internal angles rounded?
Are there no "flashes" (sharp light reflections on the edges)?
Are the transitions smooth?

If you see flashes—return to the bur.

Impact of Smoothing on 3D Scanning

3D scanners work based on light triangulation. Sharp angles create artifacts in the scanner—a black "shadow" around the corner, indistinct data. The result of neglecting smoothing in scans:

The model in CAD software has artifacts
The technician is forced to manually clean up data
Delay in production
Risk to margin accuracy

A well-smoothed preparation scans cleanly—no noise, no artifacts. This is an additional benefit for the laboratory.