Impact of progressive diamond bur wear on the trueness of subtractively-manufactured lithium disilicate crowns

Objectives: The trueness of lithium disilicate restorations produced via subtractive manufacturing is essential for long-term success, which can be related to bur degradation. This study assessed how progressive diamond bur wear affects manufacturing trueness. Additionally, the performance of various subtractive manufacturing machines and their respective milling strategies was compared. Methods: Maxillary first premolar crowns were subtractively-manufactured using lithium disilicate (IPS e.max CAD) by two machines (inLab MC X5 and PrograMill PM7). Manufacturing processes continued until diamond burs were at manufacturer end life, resulting in two groups for each milling unit (n = 27/group from MC X5 and n = 75/group from PM7). The crown trueness was evaluated using root-mean-square values and color deviation maps. Bur degradation was indicated by remaining percentages of bur life, color deviation maps, surface roughness, and scanning electron microscopy. Statistical comparisons were performed both between and within machines. Results: Most crown regions machined by the PM7 exhibited significantly higher overall trueness than MC X5 (p < 0.05). Deterioration rates of each bur within the same set and across machines varied significantly due to different milling strategies. The PM7 exhibited lower bur wear rates, allowing more restorations to be fabricated with repetitive use of diamond burs. Bur deterioration significantly reduced trueness in most crown regions analyzed. Conclusions: Progressive bur wear is associated with a reduction in the trueness of lithium disilicate crowns. The trueness of crowns fabricated using different subtractive manufacturing machines showed significant differences. Clinical significance: The size and retention of diamond particles, along with milling strategies, influence the bur degradation rate, which coincides with decreased trueness of fabricated restorations and clinical complications.