CAD-CAM Block Materials
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Direct answer
A CAD-CAM block arrives industrially overqualified: machine-milled from factory-standardised, defect-free material, it carries fewer pores and processing flaws than anything stacked by hand in a laboratory. The block ladder mirrors ceramic classification — feldspathic blocks (about 120-160 MPa flexural, maximally translucent, for veneers and anterior work), leucite-reinforced blocks in a similar strength band, lithium disilicate blocks milled in a partially crystallised soft "blue" state and then crystallised near 840-850°C to reach about 360-400 MPa, and zirconia milled pre-sintered and shrinking about 20-25 per cent during sintering at 1350-1500°C toward 900-1200 MPa. Hybrid blocks combine a ceramic network infiltrated with polymer for machinability and toughness. Bonding follows microstructure: hydrofluoric acid and silane for glass ceramics, sandblasting and MDP primer for zirconia.
What you must remember
- The industrial advantage: standardised composition, absence of laboratory processing porosity and defects — the core reason milled restorations report consistent strength at the quoted values.
- Feldspathic blocks: high translucency, flexural strength in the low hundreds of megapascals — etchable and bondable, indicated for veneers and low-stress anterior crowns.
- Leucite-reinforced blocks: dispersion-strengthened glass-ceramic, strength band roughly 120-160 MPa, reliable chairside aesthetics.
- Lithium disilicate blocks: milled in a partially crystallised, whitish-blue soft state, then crystallised at about 840-850°C for around 20-30 minutes; final flexural strength about 360-400 MPa — the workhorse single-crown block.
- Zirconia workflows: pre-sintered blanks milled oversized with software compensating the roughly 20-25 per cent linear sintering shrinkage at 1350-1500°C; alternatively fully sintered blanks milled slowly with bur wear.
- Hybrid blocks: polymer-infiltrated ceramic networks (for example, a resin-infiltrated feldspathic network) with flexural strength near 150 MPa, easy machining, kind finishing and repairability — used for chairside crowns on prepared teeth.
- Milling mechanics: glass ceramics milled wet with diamond tools; zirconia dry-milled; bur wear and milling strategy affect marginal fit.
- Bonding by class: glass-containing blocks — hydrofluoric acid etch plus silane and resin cement; zirconia — airborne-particle abrasion plus phosphate (MDP) primer; hybrids bond through their resin network.
Choosing one block for one chairside crown
A single chairside crown appointment forces the choice out loud. A premolar with average function and an aesthetic zone boundary: lithium disilicate — milled soft, crystallised in the furnace in about half an hour, delivering 360-400 MPa flexural strength, then etched, silanised and adhesively bonded for fracture resistance that also reinforces remaining tooth structure. An upper incisor veneer candidacy leans the other way: feldspathic or leucite-reinforced, whose translucency lets the underlying tooth shine through and whose etched bond to enamel is the strongest interface in adhesive dentistry — strength comes from the bond, not the block.
A heavy bruxist's first molar exits the chairside conversation entirely: the monolithic zirconia crown is planned in the laboratory, milled pre-sintered, sintered with its 20-odd per cent shrinkage exactly compensated in the software, and delivered — its opacity irrelevant at the back of the mouth and its 900-plus megapascals exactly relevant under night-time loads; because the material is that strong, a conventional luting agent is acceptable where bonding is impractical. Between the extremes sits the hybrid block for a minimally prepared or sensitive patient, machined quickly, finished without furnace firing in some workflows, and repaired intraorally like a composite — its lower strength honestly balanced by handling and margin friendliness.
Blocks versus layered porcelain
The viva asks why milled blocks outperform layered powder ceramics, and the full answer is microstructural: hand-stacked porcelain carries entrapped air, condensation variability and multiple firings; an industrial block is dense, uniform and optimally fired once. The MCQ set attaches to numbers and names: the crystallisation temperature of lithium disilicate blocks (about 840-850°C), the sintering shrinkage of zirconia (about 20-25 per cent), the meaning of the blue partially crystallised state, and the identity of the polymer-infiltrated hybrid class. A trap question asks whether zirconia can be etched with hydrofluoric acid — no, and the block choice predetermines the cementation protocol. Chairside systems since the mid-1980s (the Cerec concept) are fair game as history one-liners.
Frequently asked questions
Why are CAD-CAM blocks stronger than laboratory-layered ceramics?
Industrial standardisation produces dense, pore-free, uniformly fired material without the entrapped air and variability of hand-layered, multiple-fired porcelain.
What happens to a lithium disilicate block after milling?
It is crystallised at roughly 840-850°C for about 20-30 minutes, transforming from the soft partially crystallised state to the final strong, translucent glass-ceramic.
How is zirconia's sintering shrinkage handled?
The pre-sintered blank is milled oversized by design and the software compensates the roughly 20-25 per cent linear shrinkage that occurs during sintering at 1350-1500°C.
What is a polymer-infiltrated hybrid ceramic block?
A porous ceramic network infiltrated and reinforced with polymer, combining ceramic-like aesthetics and wear behaviour with composite-like machinability and repairability at moderate strength.
Which bonding protocol follows for a glass-ceramic versus a zirconia block?
Glass ceramics are etched with hydrofluoric acid and silanised for adhesive cementation; zirconia is airborne-particle abraded and primed with an MDP phosphate monomer.