Influence of repetitive diamond bur use on the trueness and surface roughness of subtractively manufactured PICN restorations.

OBJECTIVES

Progressive diamond bur degradation during the subtractive manufacturing (SM) of polymer-infiltrated ceramic networks (PICNs) compromises restorative clinical quality. To evaluate the degradation extent across two SM systems and investigate its effects on the trueness and surface roughness of PICN crowns.

MATERIALS AND

METHODS

A total of 121 premolar crowns were fabricated from PICN blocks (Vita Enamic) using two SM machines (PrograMill PM7 and inLab MC X5). Crown trueness was quantified by superimposing crown scans onto the original digital design using three-dimensional inspection software (Geomagic Control X) and reported as the root-mean-square (RMS) ± standard deviation (µm). Surface roughness (Ra, Sa, Sq) was measured using confocal microscopy. Bur degradation was assessed after every five crown fabrications using machine-reported lifespan, surface roughness, and color deviation maps. Diamond bur tips, before and after use, were examined using scanning electron microscopy to characterize wear patterns. Statistical comparisons were performed both between and within machines using Welch's Analysis of Variance (ANOVA) (p < 0.05).

RESULTS

Significant differences were observed in regional trueness (p < 0.0001): the PM7 yielded more accurate intaglio (inner) surfaces (22.21 ± 4.31 µm), while the MC X5 achieved higher external surface trueness (17.89 ± 2.76 µm). Bur degradation led to distinct error modes - whereas the PM7 exhibited a significant increase in negative deviations (overmilling), the MC X5 predominantly resulted in under-milled surfaces but with less impact from bur degradation. Crown surface roughness also differed between the two SM systems. While a consistent decreasing trend in surface roughness was observed in the MC X5 with repetitive milling cycles, the PM7 showed an initial decline followed by an accelerated increase after the 55th crown.

CONCLUSIONS

The rate and extent of bur degradation varied by systems, causing distinct impacts on crown quality. Degradation in the PM7 predominantly resulted in adverse over-milling errors, whereas the MC X5 was more likely to cause inadequate material removal. System-specific degradation mechanisms substantially affect the ultimate quality of restorations. Surface roughness initially decreased in both SM systems; however, the systems followed divergent late-stage trajectories - one maintained a downward trend while the other exhibited a U-shaped recovery.