By default, the CEREC instrument management interface often shows only 8 primary burs. Many new operators are unaware that the software contains two additional dedicated slots reserved exclusively for Combination 5—the Extra Fine (EF) Grinding configuration.

These burs—the Cylinder Bur 12 EF and Cylinder Pointed Bur 12 EF—only appear on the tool selection overlay when the "Extra Fine Grinding" option is manually toggled in the advanced processing options.

1. What Are 12 EF Burs? Technical Design Principles

Compared to standard 12S series grinding burs, 12 EF instruments are electroplated with an ultra-fine grit distribution of industrial diamond particles.

• Technical Advantages: When processing ultra-thin non-prep veneers or highly intricate anterior crowns using feldspathic or glass-ceramics, the extremely fine diamond grit executes micro-scale ablation at a minimal depth-of-cut per rotation. This minimizes stress concentrations at fine margin interfaces, allowing the machine to accurately reproduce sharp anatomical fissures and micro-thin bevels without chipping.

2. Technical Documentation Realities: The Production Tradeoff

While extra-fine margins sound ideal, clinical operators and lab managers must weigh this feature against a major workflow drawback explicitly noted in the technical manuals: Production time will be significantly longer when using EF grinding.

Because EF burs utilize an ultra-fine grit profile, they lack the material-removal efficiency required for bulk roughing phases. Utilizing Combination 5 requires the spindle motor to drastically reduce its axial feed speed. Consequently, the milling time per crown can double or triple, presenting a substantial time investment for high-throughput chairside clinics.

3. Understanding Sirona’s Official "95% Rule"

To guide clinicians on whether to use EF mode routinely, Sirona provided an objective assessment within their official technical whitepapers:

"Sirona estimates that 95% of cases should not need to utilize the EF option."

The underlying logic is that the standard Step Bur 12S configuration (Combination 1), when paired with modern software pathing algorithms, generates marginal adaptation and surface smoothness that comfortably exceed acceptable clinical thresholds. The 12S combination delivers high processing speeds while maximizing standard tool lifespan.

4. When Should a Clinic Enable the 12 EF Mode?

Based on these manufacturing facts, EF mode should not be used as a default setting. Combination 5 provides genuine clinical and economic value only under specific circumstances:

• Ultra-Thin No-Prep Veneers: When marginal thickness approaches physical limits (around $0.3\text{ mm}$), where a standard 12S bur might introduce micro-fractures along the friable margins.

• Complex Anterior Characterization: When an anterior case demands exact mechanical replication of intricate micro-textures, mamelons, or deep developmental grooves, and the patient is willing to accept extended chairside waiting times.

Reserving 95% of routine cases for the high-efficiency 12S setup and saving the 12 EF configuration for the top 5% of demanding aesthetic cases balances clinical efficiency with marginal precision.