Which factor can lead to the fracture of all-ceramic restorations?

The correct choice emphasizes the significant impact of sharp line angles on the structural integrity of all-ceramic restorations. Sharp line angles can create stress concentrations during mastication or other functional forces. This concentration of stress can lead to a wedging effect, which may initiate cracks or fractures in the ceramic material.

Ceramic materials, while aesthetically pleasing and capable of mimicking natural teeth, can be relatively brittle. They rely on a design that minimizes stress points and distributes forces evenly. Therefore, the presence of sharp angles does not allow for this even distribution of stress, making the restoration more susceptible to fractures under load.

The other factors can influence the outcome of restorations, but they do not have the same direct impact on cracking as sharp line angles do. For example, margin width and cusp reduction generally focus on ensuring adequate strength and retention, while excessive cement thickness can affect adhesive properties but is less likely to create immediate structural weaknesses in the ceramic.