Ceramics & Zirconia Types – Optical/Mechanical Properties Relevant to Milling

Overview

Dental ceramics are inorganic, non-metallic materials widely used for ceramic restorations due to their aesthetic qualities, biocompatibility, and mechanical performance. Within the category of dental ceramics, zirconia (zirconium dioxide, ZrO₂) has emerged as one of the most important CAD/CAM machinable materials because of its high strength, fracture resistance, and versatility in optical properties. Different ceramic types are selected based on clinical demands for translucency, toughness, and milling efficiency. 

Classification of Dental Ceramics

Dental ceramics can be grouped by composition and structure, which influence their milling performance and final properties: 

Glass-Based Ceramics

• Feldspathic porcelain – mainly glass matrix; high translucency and excellent aesthetics but relatively low strength.
• Leucite-reinforced ceramics – glass ceramics with leucite crystals for improved strength.
• Lithium disilicate ceramics – glass-ceramics with needle-like crystals that balance strength and aesthetics.
  Glass-based ceramics typically exhibit higher translucency and are often favoured for anterior restorations, veneers, or cosmetic crowns. 

Polycrystalline Ceramics

• Zirconia – predominantly crystalline with no glass phase, resulting in high flexural strength and fracture toughness, but lower translucency compared to glass ceramics.
• Alumina and zirconia-toughened alumina (ZTA) – offer enhanced mechanical properties through composite microstructures.
  Polycrystalline ceramics are ideal for high-stress areas such as posterior crowns, bridges, and implant abutments due to their superior mechanical performance. 

Zirconia Generations & Properties

Zirconia used in dentistry is stabilised with yttria (Y₂O₃) to maintain specific crystal phases that influence its optical and mechanical characteristics: 

First-Generation (3Y-TZP)

• Contains ~3 mol% yttria; mainly tetragonal phase.
• Exceptional strength and toughness, with flexural strength typically 900–1200 MPa and high fracture resistance.
• Lower translucency, often requiring veneering for aesthetic restorations.
  Ideal for long-span bridges and posterior load-bearing restorations where mechanical durability is critical. 

Second-Generation (4Y/5Y Zirconia)

• Increased cubic phase content improves translucency and colour match.
• Slightly reduced mechanical strength compared with 3Y-TZP but enhanced aesthetics.
  Used for restorations where appearance is important, such as anterior crowns or monolithic restorations. 

Multilayered & Gradient Zirconia

• Built with graded translucency and colour across the block (e.g., esthetic cervical to incisal transitions).
• Combines strength in functional zones with aesthetics where light transmission is desirable.
  This design enables monolithic crowns that closely mimic natural dentition. 

Mechanical Properties Relevant to Milling

Mechanical behaviour determines milling performance, restoration longevity, and clinical indication: 

• Flexural Strength – Ability to resist bending forces; highest in zirconia, followed by lithium disilicate, leucite-reinforced, and feldspathic ceramics.
• Fracture Toughness – Resistance to crack propagation; zirconia’s transformation toughening mechanism confers superior toughness compared to glass ceramics.
• Wear Resistance – Critical for opposing teeth contact; ceramics show excellent wear compatibility relative to metals.
  High mechanical values contribute to robust, long-lasting restorations but can also increase tool wear during milling of fully sintered blocks. 

Optical Properties

Optical behaviour affects aesthetics and shade matching:

• Translucency – Feldspathic ceramics exhibit the highest translucency, glass ceramics moderate, while zirconia generally shows lower translucency unless modified for aesthetic purposes.
• Fluorescence & Opalescence – Features that mimic natural enamel and dentin under various lighting, influenced by ceramic composition.

Enhancing translucency in zirconia often involves increasing cubic phase content or using multilayered blocks to balance aesthetic appeal with mechanical integrity. 

Implications for CAD/CAM Milling

The selection of ceramic and zirconia types affects milling strategy and final outcomes: 

• Pre-sintered (soft) blocks – Easier to mill but require controlled sintering post-milling to achieve final properties.
• Fully sintered (hard) blocks – Provide high precision without shrinkage but increase milling tool wear and processing time.

Adaptations in software compensation and choice of machines/tools are critical in maintaining accuracy, surface finish, and structural integrity across different materials. 

Clinical Relevance

Understanding ceramic and zirconia types enables clinicians and technicians to choose materials that balance aesthetic demands, mechanical requirements, and manufacturability for indications ranging from veneers to long-span frameworks in both anterior and posterior regions.