A dental crown functions as a cap placed over a damaged, decayed, or weakened tooth, restoring its shape, size, strength, and appearance. While traditional materials like metal alloys and porcelain have long been used, zirconia has emerged as a modern, high-performance ceramic in restorative dentistry. Zirconia is valued for its unique combination of durability and tooth-colored aesthetics. This material is a strong contender for restoring teeth across the entire mouth, from front incisors to back molars.
Core Material Properties of Zirconia
Zirconia, or zirconium dioxide, is classified as an advanced ceramic and is often referred to as “ceramic steel” due to its exceptional mechanical resilience. Its crystalline structure, typically stabilized with yttrium oxide (Y-TZP), provides superior toughness and strength compared to other dental ceramics. This material exhibits flexural strengths of 900 to 1,200 megapascals (MPa).
The material’s remarkable fracture resistance is partly due to a mechanism called transformation toughening. When a microcrack starts to form, the local stress causes the crystalline structure to change from a tetragonal phase to a monoclinic phase, which involves a slight volume expansion. This expansion essentially compresses the crack, preventing it from growing. The inherent strength allows for the fabrication of thinner crowns, preserving more of the natural tooth structure during preparation.
Zirconia is also highly biocompatible, meaning it is well-tolerated by the human body with minimal risk of allergic reactions or soft tissue irritation. The material is available in several forms, including monolithic, or full-contour, zirconia, which maximizes strength for back teeth, and layered zirconia, where a highly aesthetic porcelain is fused over a zirconia core.
Aesthetic Outcomes and Clinical Fitting
The aesthetic quality of zirconia has advanced, moving past its early reputation for being too opaque. Modern high-translucency and multi-layered zirconia blocks are now available. These newer materials integrate layers of color and translucency, allowing the crown to blend more seamlessly with the adjacent natural teeth, particularly in the anterior region.
The clinical process of placing a zirconia crown is conservative regarding tooth preparation. Zirconia restorations typically require less aggressive tooth reduction compared to PFM crowns. However, zirconia requires a different approach to cementation than traditional crowns because its dense, non-reactive surface does not bond easily with conventional cements.
For optimal retention, dentists frequently rely on specialized resin-based cements and primers designed to chemically bond to the zirconia surface. Early concerns existed that the hardness of zirconia could cause abrasive wear on the opposing natural teeth, but this risk is mitigated by careful polishing and glazing techniques. Modern milling and finishing protocols ensure the crown surface is smooth.
Comparison to Other Crown Materials
Zirconia is often compared to Porcelain Fused to Metal (PFM) crowns. Zirconia significantly surpasses PFM in aesthetics by eliminating the dark or gray line that can appear at the gum line as gum tissue recedes from the metal margin.
Zirconia is superior to the porcelain layer of a PFM crown, which can sometimes chip or fracture under heavy load. PFM restorations often require more aggressive removal of healthy tooth structure to accommodate both the metal coping and the overlying porcelain layer.
Compared to E-max, the differences are primarily a trade-off between strength and visual clarity. E-max is renowned for its exceptional translucency and ability to disappear aesthetically into a smile, making it the preferred material for most single-unit restorations on front teeth. However, E-max has a flexural strength of around 500 MPa, making it less suitable for patients with heavy bite forces or for posterior restorations.
Zirconia is the material of choice for molars, bridges, or patients who grind their teeth (bruxism) due to its superior fracture resistance. E-max is reserved for cases where ultimate aesthetic detail is required in the front of the mouth.