Fracture Resistance of A Newly Proposed Occlusal Veneer Design Using Two Different CAD/CAM Ceramic Materials

Statement of the Problem: Management of severe occlusal erosive lesions constitutes a challenging therapeutic approach to the restorative dentist. Bonded occlusal veneers have been introduced as a conservative solution to traditional onlays or complete coverage crowns. Still, there is lack of information and conflict in the data concerning the mechanical performance of the occlusal veneer materials and designs.
Purpose: The purpose of this in vitro investigation was to evaluate and compare the fracture resistance and failure mode of a newly proposed occlusal veneer design constructed of zirconia reinforced lithium silicate and hybrid ceramic materials with the conventional design restored with the same tested materials.
Materials and Methods: Two mandibular first molars of a typodont were chosen to perform the master preparations; one for the conventional planar occlusal veneer design, and the second for the proposed modified design which consisted of occlusal veneer preparation with circumferential chamfer finish line and two shallow proximal slots. Ten elastomeric impressions were taken for each design using the putty and light polysiloxane addition silicone impression material. The twenty impressions were poured using type IV dental stone to produce twenty stone casts, ten for each design. Each group of stone casts was subdivided randomly into two equal subgroups according to the ceramic material to be tested; the zirconia reinforced lithium silicate “Celtra Duo” and the hybrid resin nano ceramic “Lava Ultimate”. All the occlusal veneers were fabricated using the CAD/CAM Cerec InLab SW 4.2.5 System. The veneers were then checked on both the stone casts and the master preparation on the typodont for perfect seating, after the Celtra veneers were crystallized and glazed and the Lava veneers being subjected to finishing and polishing. The preparations were duplicated to obtain twenty epoxy casts over which the occlusal veneers were adhesively cemented using a standard bonding protocol. The samples were then subjected to thermal cycling program. All the samples were individually mounted on the Instron testing machine with a loadcell of 5 KN and fracture test was done by compressive mode of load applied occlusally using a metallic rod with spherical tip (5.6 mm diameter) at cross-head speed of 1mm/min. The load required to fracture was recorded in Newton and tabulated to be statistically analyzed. The fractured specimens were examined to detect the mode of failure. Results: Regardless of the ceramic type, the modified occlusal veneer design showed statistically significant higher fracture resistance mean value (1106.0 N) than conventional design (957.6 N) at P ≤ 0.05. Irrespective of the design, the Lava Ultimate revealed higher statistically significant fracture resistance mean value (1114.1 N) than Celtra (949.5 N) at P ≤ 0.05. The Celtra veneers showed higher fracture resistance mean value (1016.9N) with the conventional design than the modified design (882.2 N), the difference was statistically significant at P ≤ 0.05. While the lava veneers presented higher statistically significant fracture resistance mean value with the modified design (1329.9 N) than the conventional design (898.3 N) at P ≤ 0.05. All the specimens exhibited cracks limited to the veneer material, except one revealed catastrophic failure.
Conclusions: 1- The modified occlusal veneer design offered promising results as far as fracture resistance is concerned particularly with the nano-hybrid ceramic which presented the highest statistically significant fracture resistance mean values.
2- The conventional planar occlusal veneer design proved to be superior with zirconia lithium silicate ceramic (ZLS) as it presented highly significant fracture resistance mean values than with the modified design.
3- Both the conventional and modified occlusal veneer designs presented fracture resistance mean values with the two tested materials “Celtra and Lava Ultimate” that exceed the range of clinical acceptability.
4- Rating of the tested occlusal veneers according to their fracture resistance mean values is as follows: the highest for the Lava Ultimate with the modified design, followed by Celtra Duo with conventional design, then Lava Ultimate with conventional design and Celtra Duo with modified design.
5- It can be recommended that in cases with increased occlusal stresses, Lava Ultimate occlusal veneer with the modified design be the first choice, followed by the Celtra Duo with the conventional design as a second choice.