Zirconia crowns have become a popular option in restorative dentistry, largely replacing traditional porcelain-fused-to-metal (PFM) restorations. This material, a form of zirconium dioxide, is a high-strength, metal-free ceramic known for its durability and aesthetic qualities. Its success stems from high flexural strength, often ranging from 900 to 1200 MPa for monolithic varieties, which allows for thinner crown walls and less tooth reduction. While offering significant advantages in strength and biocompatibility, zirconia presents specific challenges related to wear, clinical management, and long-term retention.
Concerns Regarding Wear on Opposing Teeth
The exceptional hardness of zirconia presents a paradoxical issue: the crown is highly resistant to wear, but the opposing natural tooth enamel may be subjected to increased abrasion. This problem is highly dependent on the crown’s surface finish, as a rough surface acts like sandpaper against the softer opposing enamel. The material’s high compressive strength, around 2000 MPa, increases the potential for wear on the natural dentition it contacts.
Proper surface treatment is mandatory to mitigate this abrasive effect. Studies suggest that a highly polished surface on monolithic zirconia crowns is more wear-friendly to opposing enamel than a glazed finish. The glaze, a thin ceramic layer applied for aesthetics, can wear off over time, exposing a rougher underlying surface that accelerates enamel loss. When chairside adjustments are made to the bite, the highly abrasive unpolished zirconia underneath must be corrected immediately.
Failure to re-polish an adjusted area can result in significant wear on the natural tooth; some studies show opposing enamel wear of 21 to 27 micrometers over six months against finished monolithic zirconia. This emphasizes that the crown’s behavior is dictated less by the material’s innate hardness and more by the dentist’s adherence to meticulous finishing protocols. Monolithic zirconia, which is a single solid block, generally causes less wear than older layered zirconia, where the outer porcelain veneer risked chipping or exposing a rough substructure.
Challenges in Clinical Adjustment and Removal
The same properties that grant zirconia impressive durability also create difficulties for dental professionals during chairside adjustments and future removal procedures. The material’s extreme density and hardness mean that routine modifications, such as polishing the margins or adjusting the bite, require specialized tools. Diamond burs with specific grits and copious water cooling must be used to prevent excessive heat generation that could damage the underlying tooth structure.
Removing a cemented zirconia crown, often necessary for root canals or replacement, is significantly more time-consuming and arduous than removing traditional metal or PFM crowns. The material resists conventional cutting, demanding the use of specialized, aggressive diamond rotary instruments. Dentists frequently cite restoration removal as a major disadvantage, noting that the process is lengthy and results in high wear on the specialized burs, sometimes requiring a bur change within a minute of cutting.
This extended chair time for the patient, coupled with increased procedural complexity, is a direct consequence of the material’s structural integrity. The dentist must exercise extreme care to avoid damaging the prepared tooth or generating too much heat. This clinical challenge highlights a trade-off between the crown’s long-term strength and its short-term clinical manageability.
Risks of Chipping and Retention Failure
While monolithic zirconia is known for high fracture toughness, chipping remains a concern, particularly with older or more aesthetic layered designs. Layered zirconia crowns feature a strong core covered with an aesthetic porcelain veneer, which is prone to fracture or chipping under chewing forces. This failure, known as veneering ceramic chipping, is generally attributed to a mismatch in the thermal expansion coefficients between the two materials or insufficient thickness of the porcelain layer.
Even solid monolithic crowns, though highly resistant to failure, can chip if the material thickness is inadequate or if subjected to excessive bite forces. Modern multi-layered zirconia, which blends strength and translucency, can have reduced fracture resistance in translucent areas containing a higher concentration of yttria. Clinicians must ensure sufficient tooth reduction during preparation to allow for adequate crown material thickness, especially in high-stress areas.
Retention failure, or the crown coming off, is another common problem with zirconia. Zirconia is chemically inert and does not form a strong bond with conventional cements, making successful retention highly dependent on strict bonding protocols. The crown’s inner surface must be meticulously cleaned and treated, typically through air abrasion with aluminum oxide particles, followed by the application of a specialized adhesive phosphate monomer (MDP-containing primer). Contamination from saliva or blood can severely compromise the bond strength, leading to debonding that necessitates re-cementation.