In the realm of 3D printing in healthcare, a simple idea sits at the center of modern orthopaedic implant design: the best implant is one that the body forgets is there.
Not one that is the strongest, not one that is the stiffest, but one that restores function well enough that bone grows around it as if nothing happened. It is a deceptively simple goal for an engineering problem, but for designers working on 3D printed titanium implants, it has become their most frustrated aspiration.
The person making this case at our AMA: Healthcare 2025 online event was Matthew Shomper, Founder of Not a Robot Engineering, an engineer who has spent over 15 years in orthopaedics working “from the back of the head to the tip of the tailbone,” he said.
He has supported more than a few dozen 510(k) medical submissions to the US Food and Drug Administration (FDA) in the AM space and has advised numerous orthopedic companies, specifically in spine, on their 3D printed design direction.
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How Bone Biology Dictates Implant Success
The problem begins with biology. Bone responds to load. Julius Wolff articulated this in the nineteenth century, and Harold Frost expanded the idea in the 1960s into the mechanostat theorem.
Shomper summarised it as more nuanced than the version most people carry around. “It’s not necessarily just ‘load good, no load bad’,” he said. There is a range within which bone actively builds and outside of which it resorbs. The clinical trouble starts when an implant disrupts that range.
When a surgeon inserts a metal implant into a joint or spinal segment, it absorbs the mechanical load the bone would otherwise carry. The bone interprets this as a signal that it is no longer needed and begins to resorb.
“Even cyclic bone stresses as little as one percent can cause measurable differences over time,” Shomper said. “If the implant isn’t the exact same stiffness as the bone and you have slight changes, it’s going to have a profound effect on stress shielding.” The resorbed bone pulls away, micro-motion increases, and eventually the implant loosens. This is aseptic loosening, a very large and largely unsolved problem in hip, knee, and spinal surgery.
For a while, 3D printing in titanium seemed to offer a way out. Titanium is osteogenic. As Shomper put it, “Bone really likes it, it’ll grow into it, it’ll grow onto it, it’ll grow through it,” but solid titanium implants are extremely stiff, far stiffer than bone. PEEK plastics, which emerged in the early 2000s, sat closer to cortical bone in stiffness but lacked similar osteogenic properties.
A titanium lattice promised both: biological compatibility with tunable stiffness. Designers began producing trabecular structures claiming modulus values of 1-5 GPa, in the range of cortical and trabecular bone.
The problem is that those claims do not survive physical testing. “The lattice profile may indeed be that low of stiffness,” Shomper said, “but when you test the whole implant with all of the solid structures around it, they’re drastically stiffer.” The biology was not being served. The reason nobody could fix it, Shomper argued, came down to the regulatory framework.
When the Baseline Dates Back Fifty Years
The FDA clears devices through a process of demonstrating substantial equivalence to devices already on the market. The strength thresholds new devices must meet trace back, through an unbroken chain of predicates, to devices legally marketed prior to May 28, 1976.
“Basically,” Shomper said, “the initial standard for strength was for devices that were in the market fifty years ago.” Fifty years ago in spine surgery, the state of the art was placing metal spherical balls between vertebrae and calling it a day.
Median compressive force of submitted lumbar interbody devices sits at around 22,000 N. The clinically relevant lumbar load during normal walking is approximately 500 N, rising to perhaps 1,500 at peak. “You can see the difference,” Shomper said. “22,000 versus 500, forty times different.”
Now each new device must match or exceed its predecessor, which means more material, more stiffness, and worsening the very stress shielding the design was meant to address. “Designing these sort of functional structures is basically,” he said, “as you get stronger you’re going to get stiffer, that’s just physics.”
High stiffness correlates with subsidence and failed fusion, but insufficient strength leads to fracture. “You can’t go all the way to the low end of it,” he said. There is no clean exit from this bind.
What frustrates him most is that the clinical outcomes have not improved. “We say okay, we’ve got these better devices, we’re implementing patient-specific stuff now, we’re implementing robotic guides, surgical planning,” he said, “but the revision rates have largely maintained themselves over the last decade or so since the advent of this new technology.”
The first path Shomper outlined is talking to the FDA earlier, something many companies never think to do. “If you’ve got questions before you submit all your data to them, the FDA is very happy and usually really helpful,” he said, and pre-submission meetings are free.
Secondly, a company can walk in and get a read on whether a breakthrough device designation applies. Lastly, the hardest route is pushing back on test requirements that lack clinical grounding.
He described a recent appeal in which the FDA demanded strength metrics he and the company considered “pointless, [as] there’s no reason the device should be tested in this way because it’s not clinically relevant.” They won, but he was measured about what that meant. “The percentage of appeals that make it with the FDA are much less than what [is submitted],” he said.
The longer-term answer requires industry and regulators in the same room before submissions arrive. He pointed to efforts within the North American Spine Society (NASS) to build exactly that kind of forum.
Here, he noted that the FDA typically participates in standards committees to help shape emerging guidelines, while also actively soliciting industry recommendations and feedback on various topics. Shomper emphasized that while this communication channel exists, most companies simply choose not to take advantage of it.
Throughout all of this, Shomper was careful not to cast the FDA as an obstacle. “I really think the FDA in particular with orthopaedic devices has advanced the state of the art for 3D printing. They’ve done a good job.”
Shomper added, “They’re bound by red tape that has sort of restricted their ability to be quick and [adapt] to these new technologies.” The engineers understand what the bone needs. The regulators understand what the rules require. And somewhere in the space between those two positions, the implant that the body is supposed to forget keeps getting stiffer.
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