Exploring Residual Porosity in Removable Partial Dentures Using Computed Tomography.
Advancements in dental prosthesis manufacturing technologies and materials have transformed removable partial dentures (RPDs) fabrication and performances.
Cobalt-chrome and titanium remain popular material candidates for RPD frameworks due to their excellent mechanical properties and biocompatibility. However, the manufacturing process can significantly affect the quality of the final product, particularly in terms of internal structure and RPD component functionality.
Residual porosity, which can affect the denture's strength, durability, and fit, is a critical aspect to consider.
This is also a risk consideration when addressing the amount of porosity available in partial denture framework claps. Indeed, no technician, dentist, or patient wants to experience clasp rupture in their daily activities.
A previous study from J. Schweiger et al. [1] has set the scene by providing useful information about how 3D printing performs by producing a metal partial framework.
What can we expect today? Given the technological advancements of 3D printing over the past five years, the expectations for quality should be elevated.
A fruitful collaboration
A collaboration between the companies maXerial and Swiss m4m Center recently undertook a comparative study utilizing computer tomography (μCT) to evaluate residual porosity in casted cobalt-chrome, 3D-printed cobalt-chrome, and 3D-printed titanium RPD frameworks.
This exploration project sheds light on the internal structure of different manufacturing techniques and offers valuable insights for improving dental prosthesis quality and patient outcomes.
Understanding Residual Porosity and Its Implications
Residual porosity refers to tiny voids within the material structure that arise during manufacturing. These voids can compromise the material's mechanical properties, potentially leading to premature failure or structural weakness in dental applications.
For RPD frameworks, minimizing residual porosity is crucial to ensure optimal patient performance, comfort, and longevity.
Computer Tomography: A Powerful Tool for Porosity Analysis
CT scanning provides a non-destructive testing (NDT) method to visualize and measure the internal structure of materials with high precision. By capturing radiographic images, CT scanning enables the precise 3D reconstruction of a denture's internal architecture. This process reveals even microscopic porosities that may be undetectable through conventional inspection methods, providing a comprehensive evaluation of the material's internal integrity. This capability makes CT an ideal tool for evaluating the internal quality of both casted and 3D-printed dental materials.
For this project, we used Waygate v|tome|x M300 with a Dynamic 41|200 CsI scintillation detector.
Due to the high X-ray absorption and long penetration lengths of the steel alloys, high-power μCT scans were conducted using voxel sizes ranging from 9 μm to 35 μm, and power settings between 10.5 W and 45 W. Pore analyses were carried out using Volume Graphics software.
Comparing Manufacturing Methods: Casted vs. 3D-Printed Alloys
1.? Casted Cobalt-Chrome RPD Frameworks
Traditional casting methods have long been used in the dental industry to produce cobalt-chrome frameworks. However, the casting process can introduce porosities due to shrinkage, gas entrapment, or inclusions during the cooling phase.
Our μCT analysis showed that while casted cobalt chrome still meets clinical requirements, it generally exhibits higher levels of residual porosity compared to 3D-printed alternatives. This may induce a risk of device failure, especially if large pores appear in critical places such as the base to the RPD clasp.
Despite the highly absorbent CoCr material, a good signal-to-noise ratio was achieved for accurate surface determination. A μCT scan with 35 μm voxel resolution, radiograph, and 3D renderings of the dental CoCrW part have been used.
The analysis reveals an overall porosity of 0.101 %. The evaluation shows that the mean value of the equivalent pore diameter is 0.22 mm, and the largest pore has a diameter of 0.87 mm.
2.? 3D-Printed Cobalt-Chrome RPD Frameworks
Additive manufacturing, or 3D printing, has brought precision and consistency to the production of dental frameworks. Selective laser melting (SLM) for cobalt-chrome alloys enables layer-by-layer construction, which can reduce porosity by offering better control over the solidification process.
The CT scans revealed a significant reduction in porosity levels in 3D-printed cobalt-chrome frameworks compared to traditional casting. The improved microstructure potentially contributes to better mechanical performance and durability. Cobalt Chrome exhibited the lowest levels of residual porosity among the three materials.
Especially, the clasp regions have been assessed. We could demonstrate that porosity is neglectable in this area, ensuring the good function of the clasp activation process and longevity of the denture over time.
A μCT scan with 35 μm voxel resolution, radiography and 3D renderings of the dental CoCr part have been used. maXerial's multi-material scanning technique was used to minimize beam hardening and artifacts. This method reduced artifacts, allowing for robust porosity analysis and high metrological accuracy. The minimum detectable volume is 0.000186 mm3. The total porosity is 0.00103 %, two orders of magnitude lower than for casted cobalt-chrome RPD frameworks.
3.? 3D-Printed Titanium RPD Frameworks
Titanium is known for its biocompatibility and lightweight properties, making it an attractive alternative for RPDs. Using SLM to produce titanium frameworks also benefits from precise control over the material's microstructure. This material is also a newcomer in the field of RPD 3D printing.
The analysis indicated that 3D-printed titanium exhibited as well a low level of residual porosity, leading to superior fatigue resistance, which is particularly beneficial for patients who require durable, long-lasting dental solutions.
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μCT scans with a voxel resolution of 35 μm were used. Optimized scanning parameters allowed clear visualization of the internal structure and provided robust pore analysis. The minimum detectable volume is 0.000186 mm3. The total porosity is 0.003729 %. This is a factor some 25 lower than in casted chrome-cobalt RPD frameworks. The evaluation shows that the mean value of the equivalent pore diameter is 0.056 mm. The largest pore has a diameter of 0.134 mm.
Key Findings and Implications for Dental Practice
Our study found that 3D printing, with cobalt-chrome and titanium, provides a notable advantage in reducing residual porosity in RPD frameworks. While casted cobalt-chrome remains a viable and widely-used option, the advancements in additive manufacturing techniques enable the production of dental prostheses with more consistent internal quality, this minimizing the risk of framework rupture.
High-resolution scans enabled us to thoroughly examine the clasps of cobalt-chrome and titanium 3D-printed frameworks.
In titanium, the minimum detectable volume is 0.000006 mm3. The total porosity is 0.0446 %. In Cobalt Chrome, the total porosity of 0.000933 % is lower than that of the Ti model. The minimum detectable volume is 0.000006 mm3.
In summary, we got the following porosities on the complete metal framework.
In general, it has been demonstrated that 3D printing offers at least 25 times less residual porosity than casted cobalt chrome, where 3D printed Cobalt Chrome has 3 times less residual porosity than 3D printed Titanium alloy. Overall, the residual porosity of 3D-printed cobalt chrome is two orders of magnitude lower than casted cobalt chrome.
These findings support the growing trend of adopting 3D printing technologies in dental laboratories. Lower porosity can translate to longer-lasting prostheses and reduced need for adjustments or replacements, ultimately improving patient satisfaction and clinical outcomes.
Limitations
This analysis has been made on a few samples, more CT scans will be needed to confirm the statements made above.
Conclusion
As dental technology continues to evolve, so does the need for reliable and thorough quality assurance methods. Computed tomography, as a quantitative and non-destructive method, plays a pivotal role in assessing the internal quality of dental materials, helping clinicians and technicians make general informed decisions on material selection and manufacturing methods.
By embracing 3D printing techniques and leveraging advanced imaging technology, we can enhance the quality of dental prosthetics, ensuring that patients receive the best possible care.
This study is a step forward in our journey toward better dental solutions, but we encourage further research and collaboration to continue refining these techniques. Together, we can shape the future of dental prosthetics and improve patient experiences worldwide.
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About maXerial
maXerial AG, headquartered in Vaduz, Liechtenstein, is a leader in industrial computed tomography (CT) and X-ray analytics. The company leverages cutting-edge imaging techniques and artificial intelligence to unlock the complexities of materials, uncover hidden patterns, and deliver deep insights into material behavior.
Founded by Roger Herger, Patrick Bleiziffer, and Thorsten Wiege, maXerial brings together a wealth of expertise in material science, physics, and engineering. This strong foundation enables the company to provide advanced solutions for non-destructive testing, process optimization, and quality assurance across a range of industries.
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About Swiss m4m Center
The Swiss m4m Center, located in Bettlach, Switzerland, is dedicated to advancing additive manufacturing (3D printing) for the medical and dental sectors. The company provides comprehensive services, including prototyping, 3D printing from personalized to pilot production series, development support for innovative medical devices, as well as training.
With a mission to make additive manufacturing accessible and affordable for small and medium-sized MedTech companies, the Swiss m4m Center aims to enhance patient care through cutting-edge technology and innovative solutions.
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Feel free to reach out to maXerial or the Swiss m4m Center if you are interested in collaborating on similar projects or exploring how these findings can be applied to your practice.
At the Swiss m4m Center, we are committed to driving innovation in the dental field, and our ongoing research into material properties and manufacturing processes aims to set new standards for dental excellence.
Let's make the future of dentistry brighter, one RPD at a time.
References
[1] J. Schweiger et al. In-vitro evaluation of mechanical quality of casted/laser-sintered clasps for removable, Dental Materials, 33, 2017
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