Why should internal line angles be rounded?

• A. Improve marginal seal
• B. Aesthetic enhancement
• C. Reduce stress concentration
• D. Increase retention

Answer: (C)

Rounding internal line angles helps prevent failures by reducing stress concentration in the tooth-restoration complex. Amalgam is strong in compression but relatively brittle, so chewing forces create tensile stresses at sharp corners. These sharp internal angles act as stress risers where cracks can start and propagate, leading to possible fractures of the tooth structure or the restoration and even marginal breakdown over time. By rounding the angles, the load is distributed more evenly along the walls and floor, lowering the peak stress at those points and making cracks less likely to initiate or grow. This choice directly supports the durability and longevity of the restoration. Aesthetic considerations and increasing retention aren’t driven by internal line-angle shape, which is why they’re not the primary justification.