Zirconia is often talked about as if it were one material, but the zirconia used for crowns today comes in several grades with different recipes. The grades are usually labelled 3Y, 4Y and 5Y, after the amount of yttria (yttrium oxide) mixed into them. As the yttria goes up, the material lets more light through and looks less opaque, but it also becomes less strong and less tough. Choosing a grade is largely a trade between those properties.
What the numbers mean
The number refers to the percentage of yttria, measured in mol%. Yttria is added as a stabiliser for zirconia's crystal structure, and raising the yttria content stabilises more of the material in what is called the cubic phase. According to a 2024 review of 15 years of zirconia research by Cesar and colleagues (Dental Materials), the current approach to making zirconia more translucent is to lower the alumina content and raise the yttria content, and the resulting 4Y and 5Y materials may contain more than half of their crystals in the cubic phase, with a decrease in mechanical properties.
The reason is partly optical. A 2024 systematic review by Yousry and colleagues (BMC Oral Health) explains that cubic zirconia is optically uniform in every direction, so light passes through it more easily. The tetragonal crystals that make up most of the original 3Y zirconia are not uniform in this way, so they scatter more light. A 2026 review of 58 laboratory studies by Bakitian (BMC Oral Health) explains that the tetragonal phase is mainly what makes 3Y zirconia tough, because under stress it can change form in a way that resists cracks spreading. Cubic-rich zirconia lacks this toughening mechanism, which reduces its resistance to cracks, and across many studies higher yttria content improved translucency at the expense of strength and toughness.
Four generations in about twenty years
The Yousry review sets out how the grades arrived. The strength figures below are the laboratory ranges quoted in that review. They are ranges rather than exact values, and Yousry and colleagues found that different zirconias of the same generation differ in translucency.
| Generation | Introduced | What changed | Flexural strength quoted |
|---|---|---|---|
| First: 3Y-TZP | Late 1990s | Strong but opaque; used as a framework covered with porcelain, which could chip | About 900 to 1,200 MPa |
| Second: more translucent 3Y | 2011 | Less alumina and changed firing, so more light passes through; made as full-contour (monolithic) restorations without porcelain | Biaxial strength reduced from 1,150 to 900 MPa |
| Third: 5Y | 2014 | More yttria and more cubic phase, so noticeably more translucent | 700 to 800 MPa |
| Fourth: 4Y | 2018 | Falls between high-strength 3Y and high-translucency 5Y | 600 to 900 MPa |
The review also describes a fifth generation, a still more translucent 6Y grade introduced more recently, with lower mechanical properties again. For comparison, Zhang and Lawn (Journal of Dental Research, 2018) described zirconias as the strongest of the dental ceramics, but noted that conventional Y-TZP zirconia (the 3Y type) lacks the appearance of competing glass-ceramics, which has somewhat restricted its use at the front of the mouth.
Layered blocks that change from bottom to top
Many zirconia blocks are now made in layers. Cesar and colleagues describe the market trend as discs with tougher 3Y zirconia in the bottom layers and more translucent 5Y at the top, so a crown milled from the disc can be stronger near the gum and more translucent near the biting edge, a little like a natural tooth. Other multilayer blocks change only in colour from layer to layer, not in composition.
Layers bring their own questions. Cesar and colleagues note that flaws between layers could be a problem, although newer blocks appear to have addressed this with improved manufacturing. The Bakitian review found that the strength of multilayer zirconia depends on the yttria gradient, how the restoration is positioned in the disc, its design, how it is fired and how its surface is treated, and that no layering approach was clearly superior.
How the translucent grades have done in patients
Laboratory figures do not translate directly to the mouth, and clinical studies of the more translucent grades are still short and fairly small.
- Kern and colleagues (Journal of Dentistry, 2026) followed 65 back-tooth restorations in 38 patients, including 32 crowns and 16 partial coverage restorations in 5Y zirconia. After a median of 60 months none had been replaced. Most complications were biological or related to the bite or contacts, and one partial coverage restoration had a small ceramic fracture that was polished. The authors note that clinical evidence for back-tooth 5Y restorations remains limited. Solventum, a dental materials company, supplied the materials and financially supported the study, although the authors state it had no role in the design, analysis or writing.
- Güth and colleagues (Clinical Oral Investigations, 2024) placed 22 three-unit bridges of shade-graded 5Y zirconia on back teeth. At three years all were still in place with no mechanical complications. This was a pilot study, and two of the authors declare that they give scientific lectures for the zirconia's manufacturer.
- Gardell and colleagues (International Journal of Prosthodontics, 2021) randomised 60 back-tooth crowns in 44 patients to translucent zirconia or lithium disilicate in two public clinics. After a mean of 40 months no crown had fractured or chipped. Survival was 93.3% for zirconia and 100% for lithium disilicate, and there was no significant difference between the materials.
Very thin crowns are one area where laboratory testing has shown differences between materials. In a 2024 laboratory study by Benalcazar Jalkh and colleagues (Journal of Esthetic and Restorative Dentistry), lower molar crowns only 0.5 mm thick were loaded repeatedly. For 100,000 cycles at 200 N, the estimated reliability was 82% for lithium disilicate, 54% for 3Y zirconia and 20% for 5Y zirconia; lithium disilicate was significantly more reliable than 5Y at this load, but not significantly different from 3Y. At 300 N the reliability of all three dropped markedly, and the authors advised careful thought before using such thin crowns in high-load areas.
A separate question many people ask is whether harder crowns wear the teeth they bite against. In a two-year trial by Woraganjanaboon and colleagues (Journal of Prosthetic Dentistry, 2025), polished 5Y zirconia molar crowns did not wear the opposing enamel more than enamel against enamel. The wider evidence is covered in does a zirconia crown wear the opposite tooth.
Matching the grade to the tooth
The research above does not give a simple rule. Laboratory findings point towards the stronger, lower-yttria grades where bite forces are high and towards the more translucent grades where appearance matters more, while early clinical studies of 5Y on back teeth have so far reported few problems. Bakitian summarises one laboratory study recommending that back-tooth restorations be milled from the middle or bottom of a layered disc, and front-tooth restorations through the full thickness of the disc. Even so, Yousry and colleagues conclude that achieving an ideal appearance with monolithic zirconia remains challenging, and that more research is needed on the long-term performance of the newer grades.
Zirconia is not the only option. Lithium disilicate (e.max) and porcelain are compared in e.max vs zirconia vs feldspathic veneers and ceramic vs zirconia crowns, and what a front crown involves is covered in a crown on a front tooth. If you are curious about how a crown is designed and made, see inside the dental laboratory.
At Lumi Dental we use e.max (lithium disilicate) and offer zirconia as a choice. Which material suits a particular tooth is discussed at an examination, and more information is on our crowns and bridges page.
For dentists: know which layer the stress lands in
With composition-gradient discs, the position of the restoration in the disc affects which layer, and so which grade, carries the load. The Bakitian review cites a recommendation to mill posterior restorations from the middle or bottom of the disc and anterior restorations through its full thickness, and notes that bridge connectors placed in cubic-rich incisal layers may have reduced crack resistance because those layers lack the toughening mechanism of the tetragonal phase. The same review notes that aggressive sandblasting in the laboratory or heavy chairside adjustment can introduce microcracks, particularly in cubic-rich layers. Most of the included laboratory studies were rated at moderate risk of bias, and the review calls for cautious interpretation and long-term clinical studies. Cesar and colleagues add that the range of zirconias on the market can cause confusion and that following each manufacturer's instructions matters.
What patients ask about zirconia grades
Is 5Y zirconia strong enough for a back tooth?
Early clinical studies of 5Y crowns and bridges on back teeth have reported no replacements over three to five years, but they are small and some have industry links. In one laboratory study, very thin (0.5 mm) 5Y crowns had lower estimated reliability than lithium disilicate at a moderate load.
Does more translucent mean better looking?
More translucency can help a crown blend in, but appearance also depends on thickness, the cement, the surface finish and the tooth underneath. Yousry and colleagues found that achieving an ideal appearance with monolithic zirconia is still challenging.
Why does the type of zirconia matter if the crown is the same shape?
Two crowns can look identical but behave differently because their crystal make-up differs. The higher-yttria grades trade some strength and crack resistance for translucency.
Sources
- Cesar PF and colleagues. Recent advances in dental zirconia: 15 years of material and processing evolution. Dental Materials, 2024. PubMed 38521694
- Yousry M and colleagues. Translucency of recent zirconia materials and material-related variables affecting their translucency: a systematic review and meta-analysis. BMC Oral Health, 2024. PubMed 38443872
- Bakitian F. Influence of manufacturer, laboratory, clinical, and aging-related factors on the mechanical properties of multilayered zirconia: a systematic review of laboratory studies. BMC Oral Health, 2026. PubMed 41965706
- Zhang Y, Lawn BR. Novel zirconia materials in dentistry. Journal of Dental Research, 2018. PubMed 29035694
- Kern JS and colleagues. Marginal fit and five-year outcomes of posterior monolithic zirconia restorations: a randomized paired and observational clinical study. Journal of Dentistry, 2026. PubMed 42457117
- Güth JF and colleagues. Three-unit posterior monolithic fixed dental prostheses made from high-translucent shade-graded zirconia: 3-year results from a prospective clinical pilot study. Clinical Oral Investigations, 2024. PubMed 39681754
- Gardell E, Larsson C, von Steyern PV. Translucent zirconium dioxide and lithium disilicate: a 3-year follow-up of a prospective, practice-based randomized controlled trial on posterior monolithic crowns. International Journal of Prosthodontics, 2021. PubMed 33882563
- Benalcazar Jalkh EB and colleagues. Ultrathin lithium disilicate and translucent zirconia crowns for posterior teeth: survival and failure modes. Journal of Esthetic and Restorative Dentistry, 2024. PubMed 37676053
- Woraganjanaboon P and colleagues. A two-year clinical trial of enamel wear opposing 5Y-TZP and lithium disilicate crowns. Journal of Prosthetic Dentistry, 2025. PubMed 39138093
This article is general information only and is not a substitute for an in-person assessment by a dental professional.
