Which type of restoration is not effective as a retainer for fixed dental prostheses (FDP)?

The reason why all-ceramic restorations are not typically effective as retainers for fixed dental prostheses (FDP) concerns their inherent material properties. While all-ceramic restorations are aesthetically pleasing and suitable for many applications, they have lower tensile and flexural strength when compared to metal-based restorations.

In the context of fixed dental prostheses, retainers are crucial components that need to withstand significant occlusal forces and provide stability. The metal materials used in cast metal, metal-ceramic, and porcelain fused to metal restorations possess superior strength and resilience, allowing them to maintain their structural integrity under functional load. All-ceramic materials, despite advances in their formulation, are generally more prone to fracture and wear over time, which can compromise the stability and longevity of the FDP.

Thus, while all-ceramic restorations have their indications, they are less preferred as retainers for fixed prostheses due to these concerns related to strength and durability.