Effect of Internal Surface on Cusp Strength in Machined Cavities

Objective: To assess the effect of sharp internal line angles on the tendency of weakened cusps to fracture, compared to preparations with rounded internal surfaces. Methods: 27 premolar teeth were prepared to leave buccal and lingual cusps of a standardised height and thickness. Cavity preparation was carried out using a computer controlled milling machine mounting either a flat ended fissure bur (27 cusps) or a round ended fissure bur (28 cusps). The roots of the tooth were embedded in light curing acrylic. A locating jig was placed within the cavity to locate the cradle of a load testing machine 3mm above the cavity base. Both cusps were tested to fracture by pulling, and the necessary stress recorded. Result: There was no statistically significant difference between fracture strength of cusps surrounding cavities with sharp internal angles (mean = 45.1N) and those surrounding cavities with rounded internal angles (mean = 47.03 N). Conclusion: The creation of sharp internal angles within a dental cavity has no deleterious effect on the fracture strength of the surrounding cusps.