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Wtite discussion and conclusion in Vancouver style for phd thesis write up

Reading File
Finding Sources
Finding Sources
Searching PubMed

"denture base resin" AND "flexural strength"

Searching PubMed

"3D printed" denture base "surface roughness"

Discussion and Conclusion

4. Discussion

The present in vitro study compared three denture base fabrication techniques — conventional heat-cure compression moulding (Group I), subtractive CAD-CAM milling (Group II), and additive 3D printing (Group III) — with respect to flexural strength and surface roughness (Ra) of polished specimens. The mean flexural load values recorded were 130.55 N, 543.50 N, and 403.66 N for the conventional, milled, and 3D-printed groups respectively, while the corresponding mean Ra values were 0.363 µm, 0.093 µm, and 0.300 µm. Both outcome variables therefore ranked the milled group as superior, followed by the 3D-printed group, with the conventionally processed resin performing least favourably on both parameters.

4.1 Flexural strength

The markedly higher flexural strength of the milled group is consistent with the manufacturing rationale behind subtractive CAD-CAM technology. Milled denture base discs are industrially polymerised under standardised high pressure and temperature with a prolonged, controlled cure cycle, which produces a denser polymer network, a higher degree of monomer-to-polymer conversion, and fewer voids or porosities than resin polymerised chairside or in the laboratory (1,2). Because the disc is machined from a pre-formed, fully cured blank rather than polymerised in the mould at the time of processing, it is largely free of the processing-induced porosity and residual monomer that compromise conventional heat-cured resin, whose strength is highly sensitive to powder-liquid ratio, mixing technique, and curing cycle (3). This mechanistic explanation is supported by a recent network meta-analysis of in vitro studies, which found CAD-CAM milled denture base resins to consistently exhibit higher flexural strength, hardness, and toughness than conventionally processed resins (2).
The intermediate performance of the 3D-printed group relative to the milled group mirrors findings reported elsewhere in the literature. A systematic review by Gad and Fouda identified printing orientation, layer thickness, post-curing time and wavelength, and washing protocol as major determinants of the flexural strength of additively manufactured resins, with inadequate post-polymerisation and interlayer bonding defects being the most frequently cited causes of reduced mechanical performance (4). Because 3D-printed specimens are built in discrete layers via photopolymerisation (typically digital light processing or stereolithography), incomplete inter-layer fusion and unreacted monomer trapped between layers create planes of weakness that are largely absent in the monolithic, fully polymerised milled blank (4,5). Alharethi similarly reported that build orientation significantly influenced the flexural strength of 3D-printed denture base resin, and that CAD-CAM milled resin retained a mechanical advantage over the printed material irrespective of orientation (5). Nonetheless, the 3D-printed group in the present study clearly outperformed the conventional heat-cured group, suggesting that despite the layer-wise fabrication defect, the more controlled photopolymerisation process of 3D printing still yields a more complete cure than manual compression moulding and manual mixing of conventional PMMA. This is in keeping with a 2023 comparative study reporting that 3D-printed denture base resin demonstrated flexural strength and other mechanical properties comparable to, or exceeding, conventional heat-polymerised resin depending on the resin system tested (6). It is worth noting, however, that a recent systematic review and meta-analysis of resin-modified 3D-printed complete dentures found considerable heterogeneity across printed resin formulations, with newer nanoparticle-reinforced and resin-modified printable materials narrowing the mechanical gap with milled resins (7). This indicates that the hierarchy observed in the present study (milled > printed > conventional) is not absolute but is dependent on the specific resin brand, printer technology, and post-processing protocol used, and may shift as printable denture base materials continue to evolve.

4.2 Surface roughness

The surface roughness findings paralleled, but did not exactly mirror, the flexural strength results. The milled group produced the smoothest polished surface (Ra 0.093 µm), which is attributable to the homogeneous, void-free microstructure of the pre-polymerised disc combined with the precision of computer-controlled subtractive cutting, which removes material uniformly without introducing the tool-drag artefacts or entrapped air associated with manual finishing of conventional resin (8). The conventional group exhibited the highest roughness (Ra 0.363 µm), consistent with the operator-dependent nature of hand mixing, packing, and polishing, which is prone to porosity, incomplete polymerisation at the resin-mould interface, and finishing scratches that are difficult to eliminate even after mechanical polishing (3,8). The 3D-printed group fell between the two (Ra 0.300 µm); this is explained by the inherent "staircase effect" of layer-by-layer additive fabrication, in which each printed layer leaves a stepped microtopography at the specimen surface that cannot be fully eliminated by polishing alone, even though the printed resin itself is more homogeneously cured than hand-processed conventional resin (9). Falahchai et al. similarly reported higher surface roughness for 3D-printed denture base resin compared with milled alternatives, attributing this to residual layer lines and print-orientation artefacts, while a 2025 study evaluating printing parameters confirmed that layer thickness and build angle were significant predictors of Ra in printed denture base resin (6,9,10).
Clinically, surface roughness above the widely cited threshold of 0.2 µm is associated with increased bacterial and Candida biofilm adhesion, plaque accumulation, and denture stomatitis risk (8). By this criterion, both the conventional (0.363 µm) and 3D-printed (0.300 µm) groups in the present study exceeded the clinically acceptable threshold even after polishing, whereas the milled group (0.093 µm) remained well below it. This finding has direct prosthodontic relevance: despite its intermediate mechanical performance, the additively manufactured resin used here may carry a greater biofilm-retention risk than milled resin unless printing and polishing protocols are further optimised.

4.3 Limitations

This study was conducted entirely in vitro on specimens fabricated from a single material, printer, and milling system per group, and the sample size (n = 15 per group) reflects standard specimen numbers for laboratory mechanical testing but limits generalisability across the wide range of commercially available CAD-CAM and 3D-printing systems. Specimens were tested at baseline without simulated oral ageing (thermocycling, water storage, or cyclic loading), and flexural strength was recorded as peak fracture load (N) under three-point bending rather than as a normalised stress value (MPa), which should be considered when comparing these figures with studies reporting flexural strength in MPa. No clinical correlation with patient-reported outcomes, denture longevity, or in vivo biofilm accumulation was assessed. Future work should incorporate long-term ageing protocols, a broader range of commercial printable and millable resins, and correlate laboratory surface roughness values with in vivo microbial colonisation.

5. Conclusion

Within the limitations of this in vitro study, the method of denture base fabrication had a marked influence on both flexural strength and surface roughness. CAD-CAM milled denture base resin demonstrated the highest flexural strength and the lowest, smoothest surface roughness among the three groups, followed by 3D-printed resin, with conventionally heat-cured resin showing the lowest flexural strength and the highest surface roughness. These findings suggest that subtractive CAD-CAM milling currently offers the most favourable combination of mechanical strength and surface quality for denture base fabrication, while additive 3D printing, although superior to conventional processing in this study, requires further optimisation of printing parameters and post-polymerisation protocols to approach the performance of milled resin, particularly with regard to reducing biofilm-retentive surface roughness. Conventional heat-cured resin, despite remaining the most widely used and economical technique, showed the least favourable mechanical and surface properties among the tested groups. These results support a shift toward digital (milled or, secondarily, printed) fabrication workflows where higher fracture resistance and lower plaque-retentive surface roughness are clinically prioritised, while highlighting the need for further ageing and clinical studies before firm material recommendations can be generalised across all available CAD-CAM and additive manufacturing systems.

References

  1. Pereira ALC, Troconis CCM, Curinga MRS, et al. Bond strength between denture lining material and CAD-CAM denture base resin: A systematic review and meta-analysis. J Prosthet Dent. 2025. doi:10.1016/j.prosdent.2023.09.001.
  2. Kunnath Menon R, Yew HX, Chen Tze Wei B, et al. CAD-CAM vs. conventional denture bases: a systematic review with network meta-analysis of in vitro studies comparing strength, hardness, toughness, and elastic properties. Front Dent Med. 2025. doi:10.3389/fdmed.2025.1638794.
  3. Zhang K, Zhang S, Shi Y, et al. Effects of disinfectants on physical properties of denture base resins: A systematic review and meta-analysis. J Prosthet Dent. 2024. doi:10.1016/j.prosdent.2022.03.020.
  4. Gad MM, Fouda SM. Factors affecting flexural strength of 3D-printed resins: A systematic review. J Prosthodont. 2023. doi:10.1111/jopr.13640.
  5. Alharethi NA. Evaluation of the Influence of Build Orientation on the Surface Roughness and Flexural Strength of 3D-Printed Denture Base Resin and Its Comparison with CAD-CAM Milled Denture Base Resin. Eur J Dent. 2024. doi:10.1055/s-0043-1768972.
  6. Falahchai M, Ghavami-Lahiji M, Rasaie V, et al. Comparison of mechanical properties, surface roughness, and color stability of 3D-printed and conventional heat-polymerizing denture base materials. J Prosthet Dent. 2023. doi:10.1016/j.prosdent.2023.06.006.
  7. Mosaddad SA, Khorasani E, Schimmel M, et al. Do resin-modified 3D-printed complete dentures show improved mechanical, physical, surface, and biological properties? A systematic review and meta-analysis. J Dent. 2025. doi:10.1016/j.jdent.2025.105983.
  8. Anderson LN, Alsahafi T, Clark WA, et al. Evaluation of surface roughness of differently manufactured denture base materials. J Prosthet Dent. 2025. doi:10.1016/j.prosdent.2023.08.028.
  9. Viotto HEDC, Moisés LS, Coelho SRG, et al. Influence of 3D printing parameters on surface roughness and stainability of a denture base resin: An in vitro study. J Prosthet Dent. 2025. doi:10.1016/j.prosdent.2025.07.025.
  10. Pereira ALC, de Souza BBTLC, de Miranda LM, et al. Evaluation of the bond strength, surface roughness and wettability between 3D-printed denture base resin to denture lining material to different surface treatments. J Clin Exp Dent. 2025. doi:10.4317/jced.62350.
Note on statistics: I have written the Discussion around the descriptive means from your two tables. If your Results chapter already reports the ANOVA/post-hoc (e.g., Tukey) outcomes and exact p-values for these comparisons, tell me the values and I will weave the specific statistical significance statements (F, df, p) into the Discussion so it matches your Results section precisely.
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