Full contour zirconia, like the above, is increasingly replacing lithium disilicate as the restoration of choice, even in anteriors. However, according to many dental lab owners, zirconia preparation guidelines provided by manufacturers lost during clinical procedures can place even zirconia at risk of failure. How important is meeting reduction criteria and margin design? In some ways a lot, while in other ways, not as much. Even researchers don’t always agree.
Most often, margins with a chamfer or a rounded shoulder are suggested, with occlusal reduction of 1.5 mm and axial reduction of at least 1.0 mm. But what are the tolerances when we don’t meet these guidelines?
A study by Beuer et al on zirconia axial thickness of 0.4 mm, showed preparation design differences led to significant variations in fracture strength. Vult von Steyern reported that load to fracture for zirconia three unit bridges was much higher with shoulder margins than when a deep chamfer was used. According to some, the data is less clear for single unit monolithic zirconia crowns.
According to Kobayashi et al, “A limiting factor could be the aging of Y-TZP, due to its potential sensitivity to Low Temperature Degradation (LTD).” When Y-TZP is subject to low temperature degradation (LTD) in the presence of water it undergoes a phase change from tetragonal to a weaker monoclinic structure. This can decrease strength and alter the surface.
A study was conducted to determine the combined effects of margin design and LTD. Three designs were tested with .8 mm occlusal clearance: a shoulderless margin for control was stored dry, a .4 mm chamfer underwent 5,000 TC and mechanically loaded (1,200,000 at 50N), and a .8 chamfer that experienced LTD from 3 hours of autoclave to simulate 10 years at body temperature. Full zirconia crowns with identical contours and uniform 50 µm cement layer thickness were designed for each preparation.
All crowns were subjected to airborne-particle abrasion with 50 µm aluminum oxide using 0.4 MPa pressure, cleaned with steam and 70% alcohol, then cemented to metal dies with RMGI (KetacCem, 3M ESPE) under a static load of 50 N for 10 minutes.
For the above: Chart Link
The results showed margin design and LTD affect flexural strength. However, although some research has indicated that .5 mm occlusal thickness is adequate, the amount of tooth reduction was substantially less than manufacturer recommendations of 1 mm axially and 1.5 mm occlusally. Minimal reduction can have a negative effect.
In the above study, the shoulderless margin showed highest fracture loads. However, this margin design has been reported to be detrimental to gingival health, since feather style margins always terminate at a point of zero reduction, making them over-contoured. It is advisable, based on the study, to use a small chamfer instead of a feather edge margin.
Not all zirconia is the same. Some have impurities, internal voids, or inconsistent compaction. The affects of LTD will depend on which full contour zirconia is used, prep design, and material management in the lab and chair side. LTD can cause functional wear, and/or lead to micro fractures that may or may not close. This can be especially problematic for weakened zirconia crowns placed in high occlusal stress areas that are made too thin or drilled/adjusted after sintering.
The best thing we can do for patients when using zirconia, is to make sure we follow manufacturer guidelines, refrain from using lower strength “anterior” products in posterior regions, and use smooth diamonds with copious water when adjusting zirconia surfaces.