Failure Load Resistance of Different Repaired CAD/ CAM Ceramic Crowns Following Endodontic Access Preparation

Aims: This study aimed to evaluate the fracture resistance of different CAD/CAM ceramic crowns following standardized endodontic access preparation and repair using two restorative protocols.

Materials and Methods: Ninety extracted human maxillary premolars were prepared for full-coverage crowns and restored with lithium disilicate (LD), yttria-stabilized tetragonal zirconia polycrystal (ZR), or polymer-infiltrated ceramic (VE) (n=30 per group). Each group was subdivided into three subgroups (n=10): intact crowns, crowns with access preparation repaired using a porcelain repair kit (GC), and crowns repaired with composite resin (Filtek Z350 XT). Endodontic access cavities were standardized using a custom template and prepared with diamond burs under water cooling. Specimens underwent thermomechanical loading, followed by fracture testing in a universal testing machine. Data were analyzed using one-way ANOVA and Duncan multiple range tests (α=0.05).

Results: ZR crowns exhibited the highest fracture resistance across all conditions (mean intact: 3887.3 N; GC repair: 3371.7 N; composite repair: 2955.3 N), followed by LD (2742 N; 2281.5 N; 1960.2 N) and VE (1849.4 N; 1397.8 N; 1281.2 N). Access preparation significantly reduced fracture resistance in all materials (p<0.05). Repair using GC yielded significantly higher fracture resistance than composite in ZR and LD, but not in VE. Most failures (60%) were unfavorable, with oblique cusp fractures predominating.

Conclusion:  Endodontic access preparation significantly weakens CAD/CAM crowns regardless of material or repair method, although all tested crowns (ZR, LD, and VE) maintained fracture resistance above maximum reported masticatory forces (500–700 N). Zirconia crowns demonstrated superior mechanical performance, and GC-based repair provided better strength recovery in ZR and LD.