Effect of margin design on fracture load of zirconia crowns

Abstract

Zirconia‐based restorations are showing an increase as the clinicians’ preferred choice at posterior sites because of the strength and esthetic properties of such restorations. However, all‐ceramic restorations fracture at higher rates than do metal‐based restorations. Margin design is one of several factors that can affect the fracture strength of all‐ceramic restorations. The aim of this study was to assess the effect of preparation and crown margin design on fracture resistance. Four groups of bilayer zirconia crowns (with 10 crowns in each group) were produced by hard‐ or soft‐machining technique, with the following four different margin designs: chamfer preparation (control); slice preparation; slice preparation with an additional cervical collar of 0.7 mm thickness; and reduced occlusal thickness (to 0.4 mm) on slice preparation with an additional cervical collar of 0.7 mm thickness. Additionally, 10 hard‐machined crowns with slice preparation were veneered and glazed with feldspathic porcelain. In total, 90 crowns were loaded centrally in the occlusal fossa until fracture. The load at fracture was higher than clinically relevant mastication loads for all preparation and margin designs. The crowns on a chamfer preparation fractured at higher loads compared with crowns on a slice preparation. An additional cervical collar increased load at fracture for hard‐machined crowns.

Effect of Reduced Occlusal Thickness with Two Margin Designs on Fracture Resistance of Monolithic Zirconia Crowns

“…15,16 These favorable results were related with the manner of stress distribution through increasing the load on the crown in the shoulderless margin design as this force would be transmitted to the axial walls rather than the margin of the supporting die, resulting in stress concentration on the occlusal surface of the crown rather than the margin area as the fracture mode and the fractographic analysis revealed. 16 This result showed disagreement with another study 6 ; this discrepancy could be related to the use of epoxy dies and the type of cement in their study. In contrast, the slight chamfer margin design carried the occlusal stresses which led to stress concentration on a small area of finish line rather than a wide area of occlusal surface which may lead to early failure of restoration as the mode of fracture and the fractographic analysis revealed.…”

“…5 The recommendation of the margin design for highstrength ceramic materials, such as zirconia, is not evident, as the clinical recommendations are still based on that for all-ceramic and metal-ceramic crowns. 6 The monolithic zirconia restorations can be used successfully in clinical situations especially in patients with limited interocclusal distance and in patients with high occlusal loads. 7,8 Thus, it is possible to reduce the invasive preparation of teeth by the use of monolithic high-strength ceramics.…”

Monolithic and semi-monolithic translucent zirconium-dioxide restorations : aspects on design, material and strength

“…Paper IV of this thesis raises some important points for discussion regarding the effect of framework design on stress distribution within partially veneered semi-monolithic FDPs using 3D-FEA. 93…”

“…[19][20][21][22][23]90 Several investigations show that the chosen design plays an essential role in the high load-bearing capacity and expected high clinical success for all-ceramic restorations made of zirconia materials. [91][92][93][94] It is significant for any design to be able to maintain the well-known mechanical properties of zirconia while achieving optimal aesthetics, taking into account the common probable sites for clinical failures such as the veneer, the connector and the cervical margin. [91][92][93][94]…”

“…119,120 Load to fracture mechanical test combined with artificial aging procedures are valuable testing methods for analysing the external fracture behaviour and for determining the fracture resistance of restorations. 25,91,93,94,101,112,114 This kind of test is not only supposed to provide the limits of strength where a restoration fails, but is also crucial for providing expected behaviour during normal function, for understanding material-design relationships, and tooth response to stress and strain. A good laboratory test is one that mimics clinical failures of dental restorations.…”