Copy to clipboard 
These days, it’s a rarity to find a spine company that doesn’t market at least one implant that is manufactured via 3D printing. In fact, a review of FDA’s 510(k) database shows that the vast majority of orthopedic companies that are using additive manufacturing (AM) are doing so for spinal implants.
While plenty of other orthopedic verticals are using AM to some extent, spine seems to be ahead of the curve in terms of widespread adoption. There are several possible reasons for that, including the advantages that AM brings to creating spinal implants with complex geometries and devices that promote osseointegration at a level that isn’t possible through traditional manufacturing.
“With the additive manufacturing printing in layers, you are able to create porous or open structures, which cannot really be achieved with any other competencies for bone ingrowth,” said Nikaj Droop, Sales Manager for FMI Instrumed, a medical device contract manufacturer in The Netherlands. “The rough structure created by 3D printing is an advantage for osseointegration for the bone adhesion to cages. It fulfills the demand of high-end PEEK cages covered with a coating.”
To clarify, AM doesn’t — and cannot in its current capacity — replace traditional manufacturing completely. Device manufacturers still need traditional machines to thread screws, do laser marking and inspection, as well as other downstream processes. What changes with AM is the initial material and the initial build process.
“Additive provides a different solution than we’ve seen in the past,” said Brett Sobczak, Additive Manufacturing Engineering Manager at rms Company, a contract manufacturer based in Minnesota. “If you want to have multiple lattices, different support structures of strength, there are things that we haven’t been able to do in traditional subtractive machining that we can now incorporate into the design of an additively manufactured part.”
In some cases, the specific implant design that a company brings to the table dictates the use of AM, because traditional manufacturing can’t create the part. Furthermore, spine companies are able to speed up their processes and reduce supply chain risk by eliminating the need for tooling and post processing.
“AM is a new tool in our tool belt, another arrow in our quiver that has been helpful,” said David Hawkes, President of Nexus Spine. “When we need to create geometries that cannot be built through traditional manufacturing, it’s helpful there. But it also doesn’t require the tooling or programming of traditional manufacturing, and so it provides for much shorter lead times.”
Nexus Spine has found that its timeline for prototypes and research and development is compressed with AM, allowing for iteration and innovation to occur more quickly.
Christopher Cho, Staff Application Engineer for nTopology, an engineering design software company, agrees that AM is advantageous in spine R&D.
“AM wasn’t created to make spinal devices; AM unlocked a new way to create these spinal devices,” Cho said. “With AM, if you’re testing different kinds of osseointegrative structures and you don’t know which one is the best, you can print several different variations on the same build. When you’re doing traditional machining, that setup is primarily to run the same thing over and over and over again hundreds of times, and that’s not conducive to good design exploration.”
NuVasive uses AM to advance their vision of Advanced Materials Science®, which enhances the surface, structure and imaging properties of their spinal implants for optimal clinical outcomes that otherwise couldn’t be realized effectively through traditional manufacturing.
“The unique capabilities of AM elevates our ability to deliver outcome-driven innovation,” said Ryan Donahoe, Chief Technology Officer at NuVasive.
AM Adoption Drivers in Spine
AM has been around for decades, so why the surge now in spine?
“As AM technology continues to mature, it is not only helping to advance the clinical features of our implants but to also improve the economics as well,” Donahoe said. “The porous surface architecture of our Modulus® implants and the lattice structure optimized for strength, stiffness and radiolucency, provides clinical outcomes that give surgeons confidence in the care they are delivering to their patients. We’re also now seeing that advancements in AM technology can help enable cost reduction and lead times for spine companies through the manufacturing efficiencies gained.”
At the same time, it seems that design innovation has caught up to traditional manufacturing technology and in some cases, has surpassed it. Current devices are not providing the desired outcomes, and AM looks to be the key to creating better solutions.
“It’s largely the need for better bone growth surfaces, and geometries, and stiffnesses that are driving the adoption of AM in spine,” Hawkes said. “Traditional machining has not created geometries that perfectly foster bone healing and long-term stability. And because of the shortcomings of stiff interbodies, a market segment has emerged of really expensive supplemental biologics that have questionable value.”
That biologics market segment that’s trying to address the shortcomings of competitive devices now represents 20% of the cost of a fusion procedure, Hawkes added.
Software advancement has also been driving additive manufacturing in the spinal market.
“Powder bed fusion has been around for a while, and now we’re seeing different software that can help,” Sobczak said. “Advances in software are helping facilitate how we adopt different technologies and it’s making it easier to generate randomized structures or porous lattice gyroid shapes.”
Nexus Spine’s Stable-C is part of the company’s additive manufacturing portfolio.
Standing Out in the Spine Market
Spine has seen waves in the use of materials and manufacturing practices: from titanium to PEEK, back to titanium, and now 3D printing of titanium. Companies have to differentiate themselves in stronger ways than merely being another company that offers additive manufacturing simply for the sake of doing so.
“There are two camps here. On one side there are those who are not differentiating themselves at all. They’re just basically taking their part and finding a way to justify putting a 3D-printed structure on it, because a lot of the game is ‘catching up’ for them. But then, there is a camp of, ‘What is 3D printing able to enable?’” Cho said.
The latter camp of companies is carving a niche for themselves within the spine market through innovative uses of AM.
“AM on its own is simply a manufacturing technology and process. To convert a subtractive manufactured implant to AM inherently provides no incremental clinical value, yet we see a considerable number of companies touting ‘3D printing’ as part of their implant value proposition.” Donahoe said. “In our early days of AM implant development (around 2015), we partnered with leaders in biomaterial research and clinical practice to understand how to leverage AM to unlock clinical and economical value. We focused on the ideal AM implant architecture, creating a proprietary optimization algorithm that balances porosity strength, stiffness and radiolucency, all guided and supported by clinical science. This, coupled with our unique procedural solutions, results in intelligent advancements in patient care with the right economic profile.”
NuVasive’s understanding of the clinical requirements of spine surgery and how implant features drive fulfillment of them has led to Modulus being a differentiated implant portfolio on the market, Donahoe said.
“While AM has enabled next-generation implant designs, to truly deliver transformative care we place a strong continued emphasis on our procedural solutions that they’re integrated with,” he added. “These seamless solutions, optimized for outcomes, are really the key to unlocking and delivering the value in AM implants. ”
Nexus Spine is taking a completely different approach from other spine companies. As an expert in a new field of engineering called compliant mechanisms, Nexus achieves motion with their spinal implants by bending materials rather than using traditional sliding joints.
“By bending materials, we can get titanium to behave like human tissues so that we can restore the function of human tissues with synthetic devices,” Hawkes said. “We have 11 FDA-cleared products that address the major market segments of spinal hardware in the United States, and we’re using additive manufacturing for all of those segments.”
Nexus’ pedicle screw system departs from the use of set screws to get stability. By eliminating set screws through the use of elastically stretching titanium, the Nexus system is used for interbody devices, providing stability while reducing bulk to about one-fourth the size of a traditional system.
According to Wolff’s Law, excessive stiffness can prevent rapid bone healing, leading to slower healing and less stability, which can be painful for patients, Hawkes explained. Nexus’ interbody device exactly matches the stiffness of spinal cancellous bone.
“Other devices can be anywhere from eight to 40 times stiffer than spinal cancellous bone,” he said. “Traditional-looking interbody devices or competitive devices, even those that are additively manufactured, are too stiff to allow bone to heal, so they take months to maybe a year for scar tissue to form to provide stability that relieves pain.”
By exactly matching the stiffness of spinal cancellous bone, Nexus’s pedicle screw system can get stability in weeks with rapid bone on-growth and through-growth without the use of supplemental biologics.
“We can now make metal flex like human tissues do,” Hawkes said. “By manipulating geometry, we use a long proven bone-friendly alloy, titanium, to have the stiffness of any host bone.”
Ideal pore structure on a device is also vital for optimal healing. Pores need to be large enough for capillary beds to fill the device to provide blood supply to human tissue. However, if pores are too large, the bone cannot bridge the structure. But it’s not just the hole size that’s important; it’s also how much metal is between the holes.
“You want to have a hole of ideal size, and then a very thin web of material, and then another hole of ideal size,” Hawkes said. “Through compliant mechanisms, we can get that thin material between the pores of ideal size, and we can engineer that thin material to always be within its elastic limit. You can load it and it won’t permanently bend; it will rebound to its original shape like a human tissue will.
“The compliant mechanism technology and additive manufacturing, when they get paired together, allow us to create those fine little webs of material to get the right stiffness while getting the right pore size, while getting the right amount of material between pores, and then providing a very large surface area so that the bone-to-implant contact is spread out like a snow shoe on snow so that our device doesn’t penetrate up into the bone and lead to subsidence.”
NuVasive’s Modulus portfolio offers a breadth of implants.
Current Limitations and the Future of AM in Spine
AM is still considered a relatively new technology, in terms of adoption, in orthopedics. There’s room to grow and advance in AM to lead to even greater innovation and technological adoption. Current challenges include reliability and accuracy issues.
The transfer from a 3D model to an actual built part is precise, but it’s not entirely accurate. A modeled part, when built, may be a little larger or smaller than the design, but it will be exactly the same size each time it is printed. On the other hand, traditional machining typically is more accurate to the 3D the first time, but it can vary as time passes in the manufacturing process.
With AM, material is printed layer by layer. The laser is at the top, and as latent heat builds up as the part is built, accuracy may be affected. The amount of latent heat in a pool of powder is different from the amount of heat of solids in the powder, so it is not an isotropic process — the solid does not expand equally in all directions. A part printed at the bottom of a machine or on the corner of a machine may be slightly different than the same part printed in the same orientation on another corner of the machine, or at the different heights of the part as it is printed.
“The failure rate is still higher than it should be,” Cho said. “If you design a part and run it past validation and get it approved and you’re ready to go to market, the issue here is that there’s a very tight fence around what comes off the machine to sell. And if it even so much as breathes outside that fence, you scrap it.”
Material properties themselves can also be compromised. In AM, metal is liquified to take on a certain shape, and as the metal cools and hardens, thermal dissipation can have an effect on it, causing deformation. So, manufacturers must consider build orientation and thermal density because of this.
“For a small component like a spinal cage, not a lot of people print more than one or two layers,” Cho said. “That’s a genuine issue with the variability of the process. So, the process can see improvement in actually being able to take advantage of an entire build chamber. The manufacturing process needs to improve in terms of what it offers from a reliability and a repeatability standpoint.”
AM also tends to have a limit on feature size due to the beam width and localized powder heterogeneity. If printed paths are too close together, the part cannot be printed accurately.
“The size of the laser and the size of the granules are determining the size of the features that we can create,” Hawkes said. “I think as time passes, that minimum limit on feature size will get smaller, but right now that’s a bit of a problem for us at the bottom end of how small features can get. And I feel like at Nexus, we’re always right at the limit. We want to go a little bit farther, but additive manufacturing has not yet provided for all of the things that we’d currently like to do.”
Sobczak agreed that advancements in lasers, in addition to jetting processes, will help make AM processes more efficient and faster.
“It’s really going to be how do we get the implants to the surgeons, and ultimately the patient, faster, as well as maintaining that bony ingrowth porosity, and all the design fatigue considerations we have within the design,” Sobczak said. “Additional lasers will speed up the process, resulting in decreased build times, having the ability to make parts faster and get them into patients. But some of the challenges include what having those additional lasers means for the end product and how the FDA and governing boards are looking at strength requirements, fatigue requirements, and how that directly relates back to the printing process itself.”
However, advancements are constantly being made, and there’s reason to be optimistic that these known challenges are being addressed.
“Additive manufacturing and the surrounding technology has really improved in recent years,” Donahoe said. “Innovations in design tools, manufacturing equipment and post-processing have enabled AM to evolve from a prototyping tool to a viable production technology that is advantageous in some cases. With any new technology and process though, new risks materialize that our industry is acutely focused on addressing. As we work through these opportunities, I believe we will see increasing adoption of AM technology to deliver exciting new innovations.”
The momentum has already started to improve AM’s ability to fill gaps in traditional manufacturing technologies.
“Today, we are in the middle of the change of that landscape in spine and it’ll probably continue for the next few years adding other materials,” Droop said. “3D-printed spinal implants will be a big share in the total number of implants and that is quite rapidly growing for replacing titanium implants from raw material. The amount of 3D-produced implants, the base of 3D printing, is increasing quite rapidly.”
As more companies adopt AM, the compressed timelines and reduced costs it provides allows for more innovation to take place, offering the possibility for faster improvements in the process.
“AM opens a world of opportunities,” Donahoe said. “Harnessing that potential will look different for each medical device specialty and company as we figure out how to leverage it to enhance technology value.”
