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3 Questions on 3D Printing

As 3D printing or additive manufacturing of orthopedic implants and instruments evolves, so too will the technology used to design and manufacture products. We reached out to experts, each with a specialized focus, to ask them three questions about how orthopedic professionals should think about advancements in the space.

The topics and experts included:

Design - Matt Shomper, Director of Engineering, Tangible Solutions
Software - Christopher Cho, Senior Application Engineer, nTopology
Materials - Radhakrishnan Manjeri, Manager – Strategic Marketing, and Brent Marini, Strategic Business Developer - Medical, Carpenter Technology
Post-Processing - Mukesh Kumar, Ph.D., Global Technology Director, Lincotek Medical

3D-Printed Designs Enhance Orthopedic Implants

Interviewee: Matt Shomper, Director of Engineering, Tangible Solutions

How have lattice structures advanced? AM Structures Tangible Solutions

3D structures for medical devices have undergone a rapid shift within the past several years. The two most prevalent structures have been ordered lattices (lattices repeating themselves in all three axes with a predefined strut and pore sizes) and stochastic lattices (randomly oriented struts connected at nodes with targeted pore and strut sizes). As additive manufacturing has become more ubiquitous, the focus has been on what I like to call “biologically-relevant” structures—basically, what does the local anatomy need to allow for successful implantation and fusion?

This is being done by:

  • Functional grading: Varying the lattice’s pore or strut size with respect to a distance or feature, which changes its density and mechanical properties. This allows for precise control of the performance of the device.
  • Digital roughening: Add various types of surface roughnesses utilizing the printing process. These roughnesses can be used to bolster the implant’s ability to withstand expulsion or loosening and provide an advantageous surface for osseointegration.
  • Bio-inspired structures: “New” TPMS (sheet-like structure) lattices with novel mechanical properties, shape-grading a structure to adapt to loading conditions, or mimicking biological structures found in nature.

What specific changes to 3D-printing technology have spurred these advancements?

This shift appears to be tied to two things:

  1. Better software capabilities: New and improved software is being developed that is allowing for groundbreaking structure development, with nTopology’s Platform software being one of the most powerful. This type of software allows for unprecedented creation of novel lattices, design of custom surface roughnesses and integration of finite element analysis (FEA) tools to assess mechanical performance and adapt accordingly.
  2. Rapidly improving processes: The metal additive manufacturing processes have continually improved, allowing for better resolution of features. This allows designers and engineers to create digitally and have their complex designs realized in 3D spaces exactly as intended. As processes become more intentional, features can be realized on the macro and micro scale.

As these things continue to advance, engineers will continually be pushing the bar for what constitutes a biologically relevant structure, and I think that these will continue to draw their design cues from nature.

What must orthopedic companies consider when it comes to advanced features for their additively manufactured implants?

Not all structures are created equal! Engineers should learn that a cool look does not equal biological significance, nor does a high degree of surface roughness (either process or design-based). Stochastic structures sound nice, and you’ve probably heard “it’s like trabecular bone” (hint: it’s not). The best type of advanced features are ones that are well-researched and backed by data, and of course the icing on the cake is that they can look cool.

As 3D-printed devices become pervasive in the marketplace, the landscape will become ever more competitive. Companies with me-too products might fall by the wayside in favor of devices that have data backing their significance. If the implants are not biologically-significant, the pendulum of the market will swing to the next novel technology that provides the promise of the perfect fusion.


Software Provides Flexibility for Orthopedic Engineers

Interviewee: Christopher Cho, Senior Application Engineer, nTopology

How has software enhanced the design of 3D-printed orthopedic implants?

I would not say that software has enhanced the state of additively manufactured orthopedic implant design. Rather, it is a functional prerequisite for it.
Today, additive manufacturing offers a way to create these lattice structures in a highly repeatable fashion, despite their mesoscopic scale. Not only can the structures themselves be defined and controlled, but having a digital representation of your target geometry is actually a requirement so that the “toolpath” for them can even be defined. A lot of different software solutions have sprung up over the past few years that attempted to adapt their current state of design technology to something that would be useful for additive manufacturing. However, nTopology has taken a different approach entirely, and pursued implicit modeling as the backbone of our software because it has proven itself to be faster and more efficient.

How is implicit modeling different from traditional boundary-representative CAD modeling?

CAD tools are the cornerstone of digital design. Parametric solid modeling and surfacing have been the standard for as long as many engineers can remember, and have done quite a fine job propelling the user far beyond the limitations of drafting by hand. In the past decade, however, 3D-printing has enabled the opportunity to explore design in a way we’ve never seen before. Complex geometry such as organic shapes and lattice structures were unthinkable in a time when manufacturability was defined by only what the cutting tool could see.

The implicit modeling technology behind the nTop Platform is not just different, but is a uniquely efficient approach to designing and managing the complex geometry that has dominated the manufacturing industry, especially in medical devices. Where previously designers had to parametrically manage splines and surfaces and triangles, our implicit modeling approach is able to define the geometry through a lightweight, volumetric representation. This greatly accelerates the design process because the burden of coping with all those traditional design elements is no longer a computational bottleneck that design engineers have to endure.

Illustration nTopology Software

With traditional boundary representations, the system must manage the existence and location of all the geometric subcomponents such as surfaces, edges, and vertices. With nTopology's Implicit representation, the geometry is defined by a single volumetric equation that dictates where the object is and where the object isn't.

How is new software bringing reliability and repeatability to the design phase?

There are no rules for using design software. The design process in traditional CAD is a serialized set of operations performed on a 3D model so that your end result looks the way you want. If you were to dissect a CAD model today, it is highly likely that you would find a confusing amalgamation of design operations, some of which might even be completely pointless. Is this bad? Not necessarily. As long as you had a perfectly functional 3D model at the end of the day, you were golden. But if you think about it, arbitrary design steps that may or may not do anything are conflicting with what reliability and repeatability are all about. Their existence implies that the design process as a whole is not fundamentally sound. And even though the end intent is achieved, it is not necessarily translatable or reusable across a part family of similar designs.

Applying an identical design workflow from one part to another can result in a regeneration failure due to shortcomings in the modeling technology or flaws in the design process. If this is considered “repeatable,” it is at best repeatable at a scale of one. When a user designs in the nTop Platform, each step is taken intentionally and created with forethought. There is just as much value and importance on the design process as there is on the end result. Each design operation is changeable and replaceable, but ultimately traceable. Once complete, the design workflow can be reused and reapplied to any other part to achieve similar results. Because everything is clearly tracked, nTop Platform doesn’t just bring reliability and repeatability to the design phase; reliability and repeatability are fundamental to how it works.


Material Choice Expands for 3D-Printing in Orthopedics 

Interviewees: Radhakrishnan Manjeri, Manager – Strategic Marketing, and Brent Marini, Strategic Business Developer - Medical, Carpenter Technology

Orthopedic companies have started to use materials other than titanium for their additively manufactured implants. What different materials are companies using or considering, and why?

Manjeri and Marini:
Orthopedic companies are exploring or already using alloys such as cobalt-chrome and select variants of stainless steel in the additive manufacturing of implants for various applications.

1. Biodur 108: Stainless steel implants are widely used for trauma applications. However, their nickel content is becoming more critical as patient sensitivities to materials continue to rise. Exposure to nickel ions released from the normal wear of medical implants can lead to adverse side effects, such as local inflammation, aseptic loosening and device failure. BioDur 108 (ASTM F2229, UNS S29108) is an essentially nickel and cobalt-free stainless alloy used in FDA-cleared designs for implantable medical devices and high-strength surgical instruments. Its corrosion-resistance, strength, fatigue and non-magnetic properties are advantageous to the medical design community and it exhibits improved mechanical properties over 316LS or 734 stainless alloys.

2. Cobalt-Chromium-Molybdenum (CCM) biocompatible alloy (ASTM F3213, UNS R31537) has been adopted in additive manufacturing for components that are subject to wear and for applications that require higher strength and fatigue properties. Because conventional machining of CCM is difficult, additive manufacturing provides an opportunity to significantly reduce that challenge by offering production at near net shape.

3. Nitinol, a nickel-titanium alloy, is being investigated for additive applications, as it offers shape memory and superelastic properties that are beneficial for certain orthopedic applications. The design freedom, potential yield improvements and smaller manufacturing runs that are supported using additive manufacturing technology drive these evaluations.

What opportunities and challenges do these other materials present for 3D-printed orthopedic devices?

Manjeri and Marini: Additive manufacturing offers design and production advantages over established subtractive manufacturing that designers are eager to capitalize on, but material performance cannot be sacrificed. The additive manufacturing of BioDur 108 presents a unique opportunity to achieve enhanced mechanical properties not possible through using traditional stainless steel alloys, such as 316LS or 17-4. Carpenter Additive has developed a combination of optimized powder chemistry and tailored printing parameter sets to achieve strength properties meeting 20% cold-worked BioDur 108, compared to 48% cold-worked 316L in an additively manufactured component. This far exceeds ASTM F3184 AM minimum requirements. When comparing additive manufacturing material properties, the essentially nickel- and cobalt-free BioDur 108 offers a more than 50% increase in strength over 316L while demonstrating superior corrosion resistance and an expected increase in fatigue strength. In the as-built or stress relieved condition, it also has a comparable ultimate and yield strength to 50% cold-worked 316L properties.

Manufacturing with CCM has traditionally been a challenge because of its machining difficulty. Powder chemistry plays a significant role in successfully utilizing this material for additive manufacturing and, as such, Carpenter Additive developed a specific chemistry, CCM-MC, tailored for use in powder bed fusion (PBF). The unique chemical composition developed specifically for additive manufacturing has increased weldability in laser and electron beam processes. In the as-built condition, CCM-MC exhibits strength properties exceeding the minimum standards for warm-worked bar (ASTM F1537), and the HIP/Solution condition far exceeds standards for as-cast components (ASTM F75).

As Nitinol exhibits work hardening in conventional manufacturing processes, additive manufacturing presents unique opportunities in the production of orthopedic implants. Additive manufacturing provides OEMs an opportunity for design freedom, yield improvements and the enablement of more economical small manufacturing runs for devices made from Nitinol. Nitinol medical device properties are extremely sensitive to composition and production thermal gradients. Subtle changes in chemistry and heat treatment can lead to large variations in austenite finish (Af) temperatures of the finished component. Effective translation of material properties from wrought to additive manufacturing is essential to enable volume production of 3D-printed Nitinol devices.

Carpenter Additive offers EIGA (Electrode Induction Gas Atomization) Nitinol powder that maintains high nickel levels during atomization and is used in-house by our additive experts who tailor the process to targeted properties. Shape memory strain recovery of up to 6% has been demonstrated in additively manufactured Nitinol.

What should orthopedic companies know when considering a material other than titanium for their 3D-printed product?

Manjeri and Marini:
While titanium has certainly provided many companies with an opportunity to explore and implement additive manufacturing technologies and launch products, it is not a one-size-fits-all solution. Materials such as Carpenter CCM-MC and Biodur 108 provide increased mechanical properties compared to Ti 6-4 ELI with existing applications, such as articulating components and trauma products. These materials have been proven to be printable with relative ease of production, and studies of their efficacy are available. Additionally, traditional manufacturing of Nitinol is complicated and utilizing Nitinol for additive manufacturing presents an even bigger challenge. Carpenter Additive has successfully addressed the many challenges in translation of material properties from wrought to additive manufacturing.

It is important for companies to know that additive manufacturing at production scale of both traditional and newer alloys requires an in-depth understanding of metallurgical and material properties in addition to additive manufacturing expertise. As few companies have such a range of expertise in-house, it’s collaboration between such orthopedic companies and companies like Carpenter Additive, with both material and additive production under one roof, that leads to faster product development timeline and superior material properties.


Post-Processing an Integral Step in 3D-Printing Process

Interviewee: Mukesh Kumar, Ph.D., Global Technology Director, Lincotek Medical

What is the difference between Hot Isostatic Pressing (HIP) and High Temperature Vacuum (HTV) treatments?

Dr. Kumar: Both HIP and HTV use a high temperature process to reduce or eliminate internal stress on the additive manufactured parts and effect microstructural changes. However, in addition to high temperatures, HIP also makes use of concurrent high pressure from inert gas. At elevated temperatures, this pressure compresses the part and helps “squeeze out” residual porosity.

Why choose one treatment over the other?

Dr. Kumar: HIP is used to eliminate residual porosity. The presence of residual porosity, especially at critical areas, will substantially reduce mechanical properties of the additive manufactured parts. If the additive manufacturing process is robust, the instance of residual porosity can be reduced. With additive manufactured parts, especially those that have a porous structure, there is the possibility that loose powder will become trapped in the porous network. To counter this, many proponents of HIP argue that during HIP, loose powder will be sintered and will not fall off. Though this is fundamentally correct, the problem is that this approach reduces the porosity of the implant and may affect bone integration. Since these arguments are in favor of HIP, I suggest that the additive manufacturing process be correctly identified (after proper research) so that there is little or no residual porosity, and that a powder evacuation method be identified so that porosity is not reduced. HTV is simpler to set up in-house and is much cheaper to manage and operate.

What must we not forget about post-processing?

Dr. Kumar: Post-processing is based on the application. But if there is a need for machining, make sure that the cleaning process is designed to remove oils from a tortuous porous network.

Photos courtesy of Tangible Solutions and nTopology

Patrick McGuire is an ORTHOWORLD Contributor.