Substrate Rigidity Effect on CAD/CAM Restorations at Different Thicknesses.

Current dental research has established that ceramic restorations are prone to mechanical failure under simulated oral conditions. Prior studies have shown that material thickness and substrate flexibility influence fracture resistance. However, the specific effects of substrate rigidity on adhesively cemented ceramic restorations remain unclear. No prior work had resolved how different base materials interact with ceramic thickness to affect stress distribution. This gap motivated a focused investigation into how substrate rigidity impacts the mechanical behavior of lithium disilicate restorations. Existing knowledge does not fully explain the relationship between substrate flexibility and fracture resistance in simplified restorations. The uncertainty around optimal substrate choices for different restoration thicknesses has driven recent experimental approaches. Researchers have already demonstrated that mechanical cycling and thermocycling are standard protocols for simulating long-term oral conditions. Yet, the interplay between restoration thickness and substrate rigidity has not been thoroughly explored.

The study aimed to evaluate how substrate rigidity affects the fracture resistance of adhesively cemented lithium disilicate restorations at different thicknesses. The specific problem addressed is the lack of clarity on whether rigid or flexible substrates better support ceramic restorations under simulated oral conditions. The motivation stems from the need to optimize restoration longevity by selecting appropriate base materials. The authors sought to determine if substrate rigidity influences stress distribution at the adhesive interface. They also wanted to compare the mechanical performance of restorations at two thicknesses. The study's goal was to inform clinical decisions about substrate selection for CAD/CAM restorations. The research focused on simplified restorations, which are common in modern dental practice. By identifying the most favorable substrate types, the study aimed to reduce the risk of ceramic fracture in clinical settings.

The study used precrystallized lithium disilicate ceramic blocks to fabricate disc-shaped specimens at two thicknesses: 0.5 mm and 1.0 mm. Each specimen was cemented onto one of three base substrates: dentin analogue (control), resin composite (RC), or glass ionomer cement (GIC). The specimens underwent mechanical cycling in a chewing simulator with a load of 100 N for 1 million cycles at 4 Hz. Following mechanical cycling, the specimens were thermocycled for 10,000 cycles between 5°C, 37°C, and 55°C with 30-second dwell times. After fatigue testing, the specimens were loaded until failure using a universal testing machine. The fracture load was recorded in Newtons. A finite element analysis was conducted to calculate the first principal stress at the center of the adhesive interface. The study design allowed for statistical comparison of fracture resistance across different substrate types and restoration thicknesses.

The results indicated that restoration thickness, substrate type, and their interaction had statistically significant effects on fracture resistance. Regardless of thickness, specimens cemented to dentin analogue showed the highest fracture load. Among the base materials, resin composite (RC) build-up provided the highest fracture load and lowest stress magnitude for both restoration thicknesses compared to glass ionomer cement (GIC). The 0.5-mm restorations exhibited higher stress peaks and lower fracture loads during compressive testing. Fracture resistance decreased with increased substrate flexibility. The 1.0-mm restorations showed better mechanical performance than the 0.5-mm ones when cemented to the same substrate. The finite element analysis confirmed higher stress concentrations in more flexible substrates. These findings suggest that substrate rigidity plays a critical role in the mechanical behavior of adhesively cemented restorations.

The authors concluded that more rigid substrates are associated with higher fracture resistance in adhesively cemented lithium disilicate restorations. The study found that resin composite build-ups outperformed glass ionomer cement in terms of mechanical performance. The results suggest that substrate rigidity influences stress distribution at the adhesive interface. The findings support the use of rigid base materials to reduce the risk of ceramic fracture. The 0.5-mm restorations showed greater vulnerability to mechanical failure regardless of substrate type. The interaction between restoration thickness and substrate rigidity was statistically significant. The study's implications are specific to simplified restorations in lithium disilicate ceramic. The authors propose that clinicians consider substrate rigidity when selecting materials for CAD/CAM restorations.