Clinical failures of all-ceramic fixed partial dentures (FPDs) have been a major problem for clinicians and dental ceramists. Fracture surface topography of clinically failed FPDs can be characterized with a single number called the fractal dimensional increment (D*) and D* is related to fracture toughness, K_c (K_c ~ D*^1/2). Objectives: To measure the fractal dimensional increment (D*) of clinically failed all-ceramic fixed partial dentures, and to estimate the fracture toughness (K_c) using fractal analysis. Methods: Three clinically fractured FPDs made of a yttria stabilized zirconia-based core ceramic and a glass-based veneer were retrieved and analyzed. A modified slit island technique was used to measure the D* of glass-based veneer. The fracture toughness (K_c) values were estimated using equation; K_c=E a₀^1/2 D*^1/2 where E is the Elastic modulus and a₀ is a characteristic length (1.5 nm). Failure stresses of specimens were calculated using a fracture mechanics equation. Results: Fracture origins of two clinically failed FPDs were located on the veneer surface. However, fracture origin was missing in one specimen due to local chipping at the crack initiation site. The fractal dimensional increment (D*), critical flaw size (c) and fracture toughness (K_c) and failure stress (σ_f) values of each specimen are as follows:

Specimen number	D*	c (μm)	σf (MPa)	Kc (MPa·m1/2)
1	0.08	253	38	0.6
2	0.07	324	34	0.6
3	0.15	Not identifiable	------	0.8

Conclusion: The fractal dimensional increment (D*) and fracture toughness (K_c) of two clinically failed veneers correlated well with previous studies. However, one specimen had greater D* and K_c values which may be due to compressive residual stresses.

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