Michael Barbella, Managing Editor03.16.21
It’s a peculiar keepsake, even for the nerdiest of geeks.
The anatomical model sits on a dresser in Haley Kessinger’s home, prominently displayed beside a framed school portrait. The model, however, is freakishly flawed—so contorted and curved that it looks more like a slithering snake than a human spine.
“...dang, that’s so messed up,” Kessinger said, rotating the misshapen model around in her hands.
“Messed up” is a kind way of describing the model, a three -dimensional replica of Kessinger’s own spine before it was straightened through corrective surgery. The abnormal curve, molded by the genetic double-whammy of achondroplasia (Dwarfism) and kyphosis (hunchback), worsened as Kessinger grew, and ultimately limited her ability to walk.
“I was in a lot of pain and didn’t want to move much,” she recalled.
Fearing eventual paralysis, Kessinger consulted with Kent L. Walker, M.D., a Louisville, Ky.-based orthopedic surgeon with a special interest in pediatric spine conditions. Although he understood the criticality of Kessinger’s deteriorating condition, he was a bit daunted by the complexity of its remedy.
“Very complicated cases like this come one or two a year,” Walker said. “This was probably one of the most complicated cases in America that day. I went to conferences, I went to courses specifically to prepare for this case. It was intense. This was an extremely complex case. If left untreated, she [Haley] would have eventually become paralyzed from the waist down. When you’re dealing with the spine, you have to be very precise.”
Walker gained that precision through patient-specific pedicle screw navigation guides from Englewood, Colo.-based Mighty Oak Medical. The guides are created from digitized computed tomography (CT) scans that are 3D printed; the technology (FIREFLY) is designed to optimize pedicle screw size and trajectory during complex spinal procedures. The guides can be used with any spinal deformity correction system.
Along with the navigation guides, Mighty Oak’s FIREFLY technology produces an autoclavable 3D-printed spinal replica that surgeons can use to study and practice with before the procedure. The model proved invaluable to Walker as he prepared for Kessinger’s eight-hour surgery.
“It’s helpful to have an actual spine in your hand that is just like the spine you are operating on because you can’t pick up the patient and turn [them] around,” he noted, “but you can pick up the spine [model] and look at it as a three-dimensional component and then translate that to the patient you are operating on.”
Such translation has becoming increasingly common in the age of digital medicine. 3D modeling is revolutionizing surgical planning in orthopedics, helping clinicians precisely position implants and match bone geometry or screw angles. The latter advantage is particularly significant in spinal surgery, where pedicle screw violation rates range between 3 percent and 54.7 percent, and complication rates can run as high as 7 percent.1 Studies have shown improperly placed screws can damage nerves, proximal blood vessels, and nearby organs, and also lead to post-surgical complications like loosening, pullout, and breakage.
3D-printed surgical guides, however, can improve screw placement accuracy, helping reduce (or even eliminate) the possibility of complications. A 2019 clinical trial, for example, found a 96.4 percent “safe” screw placement rate using MySpine, a patient-specific pedicle screw placement guide from Medacta International SA, compared with an 82.9 percent rate for free-hand implementation. MySpine also limits X-ray exposure for patients and OR staff, the data showed.
While 3D-printed spinal surgical guides are mainly used for screw placement, other procedures have benefitted from the technology, including osteotomies and expansive open-door laminoplasty in cervical myelopathy.
“Additive manufacturing brings another tool to the tool belt in the [orthopedic] industry,” noted Adam M. Clark, CEO of Fairborn, Ohio-based Tangible Solutions Inc., a contract manufacturer of 3D-printed implants. “It’s not applicable to everything and it’s finding success in particular niches aligned with particular strategies. The biggest innovations are happening in software and ancillary/supporting technologies. There has been a significant amount of trial and error within the industry and we as a whole are starting to see where the technology applies. Once there is an application, it’s followed by accelerated use.”
Case in point: 3D-printed surgical guides not only are used in spinal applications, but in most other joint replacement procedures, too. Medacta offers guides for hip and shoulder implant placement, recently winning approval for the former in Australia, Japan, and the United States. 3D Systems’ template toolbox, contrarily, includes procedural planning solutions for craniomaxillofacial, pelvis, and extremities implants, while Zimmer Biomet Holdings Inc.’s navigational dossier features knee and shoulder options.
Many navigation guides are augmented by 3D-printed life-sized anatomical models (like the one of Kessinger’s spine) that clinicians can use to better understand patient pathologies and specific musculoskeletal conditions. The models also enable surgeons to draft preoperative planning strategies for improved outcomes.
Such was the case for Kessinger, who has grown more than an inch and a half taller since her surgery. “It used to hurt when I walked but now it just feels normal,” she said. “I was so scared that I would become paralyzed and now I don’t worry about that anymore.”
Happy endings like Kessinger’s are becoming more common as orthopedic implant manufacturers leverage 3D printing technology to produce customized solutions at the point of care. The global market for 3D-printed orthopedic devices is forecast to swell 26 percent annually through 2023 due to rising demand for longer-lasting, better performing, personalized implants, according to Technavio.
Since its conception nearly 40 years ago, 3D printing—a.k.a., additive manufacturing (AM)—has become relatively widespread in various industries, revolutionizing the way tooling aids, functional prototypes, and finished components are made. Healthcare was among the technology’s earliest adopters, with dental implants and custom prosthetics routinely created in the late 1990s and early 2000s.
Ensuing applications have involved human tissue, blood vessels, and organs, but 3D printing’s predominant use lies in preoperative planning and production of anatomical models, surgical instruments, and implants. The technology has quickly gained momentum in orthopedics in recent years as surgeons sought a better understanding of bone structures, disease treatments, and clinical interventions.
“AM [additive manufacturing] offers the opportunity for better clinical outcomes. For Total Hips, a benefit of 3D-printed acetabular cups is initial fixation, which reduces incidences of malalignment and thus, reduced dislocation,” noted Brian McLaughlin, founder and CEO of Scarborough, Maine-based Amplify Additive, an additive manufacturing supply chain company for orthopedic implants. “For Total Knees, the reduction of the use of cement conserves bone. Should a patient need revision surgery and need a replacement, aside from the bone in-growth that occurs with the 3D scaffolds, we can print this onto implants. For oncology and trauma, we can deliver customized solutions for the most challenging of situations, and in many cases, save limbs and improve the quality of lives for patients. AM isn’t just a sexy new technology; it is a game-changer for the industry.”
Indeed, AM has changed the state of play in orthopedics.
The technology is routinely used to generate patient-specific anatomical models, surgical instruments, external fixators, and cutting guides; once reserved for only the most complex cases, AM is now a standard part of various orthopedic procedures, used for preoperative planning purposes in upper extremity trauma cases, deformity corrections, arthroplasties, foot/ankle surgeries, spinal fusions, hip and acetabulum procedures, hip and knee arthroplasty, and oncology (e.g., bone cancer).
The anatomical models used in these cases are just as diverse, made to mimic practically every bone and joint in the body. 3D Systems, whose co-founder Charles Hull invented stereolithography (3D printing), offers anatomic replicas in various styles and materials using U.S. Food and Drug Administration (FDA)-cleared, standalone modular software (D2P) designed to address and consolidate all 3D-modeling preparation steps.
The company also houses a Craniofacial Model Skull Library, a collection of pediatric craniomaxillofacial deformities created from an imaging archive established over a 20-year period (1983-2003) by two St. Louis doctors. The library is comprised of selected diagnosis-specific 3D CT datasets from the imaging archive; the images represent the most “characteristic dysmorphology” of the physical abnormalities before surgical intervention.
“3D Systems makes very realistic, lifelike models,” Stephen J. Snyder, M.D., a Southern California surgeon specializing in arthroscopy and shoulder reconstruction procedures, said in a company website video.
“[The] model of my favorite joint, the shoulder, [has] literally got everything I need to learn the anatomy, but also to be able to plan and discuss the various pathologies with the patient and with my students. It’s really amazing that they [3D Systems] can do this and 3D print the anatomy so clearly and so precisely. It’s going to be really good to help us with our teaching and education—for both doctors and patients.”
Greater knowledge also awaits both groups through Los Angeles-based Stratasys Ltd.’s anatomical models, which mimic porous bone structures, fibrotic tissues, and ligaments. Created with the company’s upgraded J750 Digital Anatomy 3D printer, the “biomechanically realistic” models behave like human bone and provide clinicians with haptic feedback when practicing screw insertion, drilling, or sawing through bone, according to the company.
Stratasys claims its J750 software and printer can create bones in various disease stages or densities—constructing, for example, healthy or degenerative intervertebral discs, and facet joints in varying degrees of stiffness. The models also can reflect the skull’s denser structure (compared to general bones), and differing amounts of marrow in long bones.
The models’ biomechanical properties are attained through several advanced materials with contrasting degrees of softness, flexibility, and density. GelMatrix, used for simulating both large and small vascular structures, has a pudding-like consistency while the more durable TissueMatrix is soft and flexible but can withstand suturing, cutting, inserting, and deploying devices. TissueMatrix is best suited for replicating fatty tissue, fibrotic tissue, soft organs, and tumors. BoneMatrix is the toughest of the substances, yet it is flexible and maintains its shape during trial discectomies, drilling, reaming, or sawing.
Study data on Statasys’ bone models indicate they have a similar haptic response to human cadaver bone in screw pull-out force. Moreover, the company’s 3D-printed lumbar vertebrae models accurately represent human spinal range of motion, other trial results show.
“The mechanical properties of bone are so fundamental to the ability of our skeletons to support movement, provide protection for our vital organs, and ultimately affect our quality of life,” Vice President Osnat Philipp, leader of Stratasys’ global healthcare team, said last December in announcing updates to the J750 Digital Anatomy 3D printer. “We believe that better preparation leads to better clinical outcomes. Being able to 3D print models that are biomechanically accurate and unique to each patient is critical to that preparation.”
Better clinical outcomes is actually one of the main innovation drivers in 3D-printed orthopedic implants. Besides its preoperative planning perks, the technology allows for simpler procedures and improved biocompatibility.
Many reconstructive procedures can now be completed faster and less invasively with AM implants due to the pre-surgical insight gained from anatomical models and cutting/drilling guides. Surgeons that map out their strategies beforehand spend less time in the operating room, thereby reducing overall procedure time and expediting their patients’ recovery rates. Studies have shown that AM surgical guides can reduce OR time by an average 23 minutes, and anatomic models can trim 62 minutes off procedure times.2
“In addition to the spine market, oncology and trauma are both market segments that can certainly leverage AM for better patient outcomes,” Amplify Additive’s McLaughlin tells Orthopedic Design & Technology. “Total joints—knees, hips, and shoulders also leverage the technology to promote better initial fixation and better overall results. That is the key—AM provides better clinical outcomes.”
Those outcomes, however, would likely be impracticable without material advancements. Titanium has long been the go-to substance for 3D-printed implants, favored for its exemplary bioinert properties, bone-bonding ability, and strength-to-weight ratio. Most, if not all, AM developers offer Ti6AI4V and other titanium alloys for orthopedic use.
Carpenter Technology Corporation offers 3D-printed joint reconstruction, spinal, and trauma implants in both Ti6AI4V and cobalt chrome molybdenum (CCM), a material traditionally prone to fragility and cracking under additive manufactured processes. The company, though, overcame this challenge by developing a CCM powder suitable for AM purposes.
“The availability of AM optimized non-titanium powders is a positive step to enable widespread adoption of AM beyond the regular use of titanium alloys,” noted Gaurav Lalwani, medical applications development engineer for the Philadelphia-based global developer of high-performance specialty alloy-based materials. “Cobalt chrome molybdenum (CCM) alloy is widely used in conventional manufacturing of implants.
However, the standard chemistry of CCM results in brittleness and cracking in the AM process. To meet this challenge, Carpenter Technology identified and optimized the trace elemental composition within the ASTM-approved limits and developed a CCM powder optimized for 3D printing. Another example of Carpenter’s materials innovation is Ti Grade 23+ powder, which provides a 15 percent to 20 percent improvement in mechanical properties vs. standard Ti Grade 23, which enables the design engineers to realize ultra-fine and complex device features that typically fail with regular Ti Grade 23 powder. Additionally, availability of next-generation materials for AM such as BioDur 108 (cobalt and nickel-free FDA-approved implantable alloy) and nitinol powders will enable the translation of new applications to AM.”
“[AM] technologies have provided the design freedom to enable OEM’s selection of more biocompatible materials,” Lalwani continued. “For example, many spine OEMs are now using titanium materials vs. PEEK for interbody devices due to the ability to print cavitation and porous structures that allow for better implant integration without the drawbacks of PEEK (i.e., development of scar tissue around the implant).”
Despite its drawbacks, PEEK (polyether ether ketone) has nonetheless become a frequent material choice for AM implants due to healthcare’s ongoing penchant for biologically-friendly products. Biocompatible implants for various orthopedic applications are available from numerous AM specialists, including Evonik Industries AG, which offers the PEEK-based VESTAKEEP i4 3DF, a high viscosity, biocompatible, biostable resin; and 3D Printlife’s biocompatible, non-toxic bone replacement filament made from polyamide polyolefin and cellulose (the material is reserved for patient-specific anatomical models).
Austrian AM systems developer Lithoz GmbH offers bioresorbable ceramic implants made of beta-tricalcium phosphate (ß-TCP), hydroxyapatite, or zirconia using lithography-based ceramic manufacturing (LCM), a digital light processing technique. LCM produces dense and highly precise ceramic parts via a photocurable ceramic suspension that is hardened in a photolithographic process.
Last year, Lithoz debuted multi-material 3D printing technology for the creation of multi-functional components. Its LED-driven CeraFab Multi 2M30 printer can generate parts containing combinations of ceramics, polymers, and metals, enabling the company to produce implants with gradual changes in material composition and microstructure. Using the new machine, Lithoz printed a jawbone implant that mimicked the dense exteriors and lighter porous interiors of human bone. The company used a high-strength zirconia for the mandibular cage’s outer shell and tricalcium phosphate for the interior.
Although significant, Lithoz’s multi-material printing technology could soon be forced into early retirement. Australian scientists have developed a ceramic-based ink that might potentially enable surgeons to generate 3D-printed bone parts with actual living cells. The ink, according to published reports, is comprised of calcium phosphate and combines with a collagenous substance containing living cells to create (in situ) bone-like tissues.
The scientists deemed their printing technique ceramic omnidirectional bioprinting in cell-suspensions (COBICS). Clearly, more testing is needed to determine whether the living cells will continue growing after being implanted in existing bone tissue, but the development is significant because it would allow clinicians to customize and build better-fitting parts on the spot.
“This has the potential to radically change current practice, reducing patient suffering, and ultimately saving lives,” Dr. Iman Roohani, a University of New South Wales (UNSW) scientist working on the project, told the press earlier this year. “I imagine a day where a patient needing a bone graft can walk into a clinic where the anatomical structure of their bone is imaged, translated to a 3D printer, and directly printed into the cavity with their own cells.”
Until that day arrives, though, patients will have to partly rely on good, old-fashioned 3D-printed implants to fix their ailing joints and bones. Ostensibly primitive compared to the breakthrough bioprinting developed by UNSW scientists, AM implants nonetheless are exceptional in their own right, largely for the design freedom and geometric complexity they provide to product engineers. 3D-printed devices can be more easily molded into anatomical shapes than conventionally manufactured products, and they also can be designed with porous surface structures that facilitate faster bone-implant integration.
South Korean medical firm Mantiz, for example, manufactures a 3D-printed line of interbody fusion cages with complex mesh-like structures, and Draper, Utah-based Nexus Spine engineers its Tranquil interbody fusion devices to mimic spinal bone on a microscopic level via specific pore size, shape, volume, and flexibility. The company claims its Tranquil portfolio provides “substantially more” bone through-growth than predicate devices.
“Additive manufacturing has allowed for the creation of designs that were not previously possible,” Nexus Spine product engineer Peter Halverson said. “With past techniques, we were limited to geometries that we could reach with physical tooling. With additive manufacturing, we can create structures and features internal to the part. These features can increase the function and performance of the part without a proportional increase in production costs. Additionally, additive manufactured titanium has been shown to be very bone-friendly, producing the reactions necessary to facilitate bony attachment and in-growth.”
Optimal osseointegration, however, is dependent on pore structure. For titanium implants, pore interconnectivity, size, and shape (geometric arrangement) are crucial ingredients in cell ongrowth success.
Ideal interconnectivity requires neighboring pores to open both inwardly and outwardly, with the pores responsible for the connection itself large enough to accommodate proper body fluid circulation.
“[AM technology] increases the possibility of developing new solutions to address unmet requests, such as devices with mechanical tailored performances, differentiated point-to-point, along with biocompatibility and anti-allergic or anti-infection features,” said Francesco Robotti, technology business development manager for Lincotek Medical, a fully-integrated, global contract manufacturer for the orthopedic market and a recognized expert in casting, machining, plasma spray coatings, and additive manufacturing technologies in the medical field. “[AM] enables the manufacturing of devices with unprecedented complexity in shape and surface topology.”
Those shapes can take many different forms, though lattice structures seem to offer the best opportunity for bone biointegration. Studies have found that porous lattice structures actually foster osseointegration better than smooth or roughened surfaces because they provide a scaffold for fostering cell ongrowth and ingrowth.3
Generating these lattice structures is possible through advanced software like nTopology’s nTop Platform and 3DXpert from 3D Systems. The former, used by companies like Amplify Additive, Nexxt Spine, Renishaw, and Tangible Solutions, offers various lattice structures that can be customized to individual patients. The software generates lattices through organized workflows, combining CAD (computer-aided design) modeling with analysis data and incorporating Engineering Notebooks to help teams document and share knowledge.
“Clearly certain software packages have improved the ability to convert customer-specific CT scan data into specific CAD, upon which an implant can be modeled with greater precision, and to create new optimized designs, including integrated lattice structures,” noted Erik Poulsen, medical market segment manager for GF Machining Solutions Management S.A., a Swiss provider of machine tools, diverse technical solutions, and services to manufacturers of precision molds and tooling of tight tolerance, precision-machined components. “This is certainly the case with 3DXpert, and has helped open the door for better, more accurate product designs as well as lowering costs by reducing scrap rates. When you include an integrated approach on the CAM side from additive to subtractive technologies seamlessly, the end result is more accurate products.”
3DXpert is ideal for defining lattice cell structures, creating radial lattices to fit circular parts, and applying variable lattice thickness to components based on FEA stress analysis. The software’s latest iteration, Lattice QuickSlice, features efficient and fast slicing for any-sized lattice structure; the tool, according to 3D Systems, can significantly shorten the time required to slice enormous lattice structures while maintaining optimal quality.
“There are several software systems on the market but 3DXpert is state-of-the-art additive design and build preparation software. It allows you to design and optimize lattice structures, support structures, and hatch strategies as well as simulate builds all in one suite,” explained Dr. Gautam Gupta, vice president and general manager, Medical Devices, at 3D Systems. “You can make changes on the fly in that software suite, and shorten the product development time from months into weeks. With the advanced application tools, our customers can create several iterations of the design in a very short time. That kind of speed to observe several different designs is unparalleled. The combination of hardware and software is creating a tremendous sandbox for engineers to design products.”
References
The anatomical model sits on a dresser in Haley Kessinger’s home, prominently displayed beside a framed school portrait. The model, however, is freakishly flawed—so contorted and curved that it looks more like a slithering snake than a human spine.
“...dang, that’s so messed up,” Kessinger said, rotating the misshapen model around in her hands.
“Messed up” is a kind way of describing the model, a three -dimensional replica of Kessinger’s own spine before it was straightened through corrective surgery. The abnormal curve, molded by the genetic double-whammy of achondroplasia (Dwarfism) and kyphosis (hunchback), worsened as Kessinger grew, and ultimately limited her ability to walk.
“I was in a lot of pain and didn’t want to move much,” she recalled.
Fearing eventual paralysis, Kessinger consulted with Kent L. Walker, M.D., a Louisville, Ky.-based orthopedic surgeon with a special interest in pediatric spine conditions. Although he understood the criticality of Kessinger’s deteriorating condition, he was a bit daunted by the complexity of its remedy.
“Very complicated cases like this come one or two a year,” Walker said. “This was probably one of the most complicated cases in America that day. I went to conferences, I went to courses specifically to prepare for this case. It was intense. This was an extremely complex case. If left untreated, she [Haley] would have eventually become paralyzed from the waist down. When you’re dealing with the spine, you have to be very precise.”
Walker gained that precision through patient-specific pedicle screw navigation guides from Englewood, Colo.-based Mighty Oak Medical. The guides are created from digitized computed tomography (CT) scans that are 3D printed; the technology (FIREFLY) is designed to optimize pedicle screw size and trajectory during complex spinal procedures. The guides can be used with any spinal deformity correction system.
Along with the navigation guides, Mighty Oak’s FIREFLY technology produces an autoclavable 3D-printed spinal replica that surgeons can use to study and practice with before the procedure. The model proved invaluable to Walker as he prepared for Kessinger’s eight-hour surgery.
“It’s helpful to have an actual spine in your hand that is just like the spine you are operating on because you can’t pick up the patient and turn [them] around,” he noted, “but you can pick up the spine [model] and look at it as a three-dimensional component and then translate that to the patient you are operating on.”
Such translation has becoming increasingly common in the age of digital medicine. 3D modeling is revolutionizing surgical planning in orthopedics, helping clinicians precisely position implants and match bone geometry or screw angles. The latter advantage is particularly significant in spinal surgery, where pedicle screw violation rates range between 3 percent and 54.7 percent, and complication rates can run as high as 7 percent.1 Studies have shown improperly placed screws can damage nerves, proximal blood vessels, and nearby organs, and also lead to post-surgical complications like loosening, pullout, and breakage.
3D-printed surgical guides, however, can improve screw placement accuracy, helping reduce (or even eliminate) the possibility of complications. A 2019 clinical trial, for example, found a 96.4 percent “safe” screw placement rate using MySpine, a patient-specific pedicle screw placement guide from Medacta International SA, compared with an 82.9 percent rate for free-hand implementation. MySpine also limits X-ray exposure for patients and OR staff, the data showed.
While 3D-printed spinal surgical guides are mainly used for screw placement, other procedures have benefitted from the technology, including osteotomies and expansive open-door laminoplasty in cervical myelopathy.
“Additive manufacturing brings another tool to the tool belt in the [orthopedic] industry,” noted Adam M. Clark, CEO of Fairborn, Ohio-based Tangible Solutions Inc., a contract manufacturer of 3D-printed implants. “It’s not applicable to everything and it’s finding success in particular niches aligned with particular strategies. The biggest innovations are happening in software and ancillary/supporting technologies. There has been a significant amount of trial and error within the industry and we as a whole are starting to see where the technology applies. Once there is an application, it’s followed by accelerated use.”
Case in point: 3D-printed surgical guides not only are used in spinal applications, but in most other joint replacement procedures, too. Medacta offers guides for hip and shoulder implant placement, recently winning approval for the former in Australia, Japan, and the United States. 3D Systems’ template toolbox, contrarily, includes procedural planning solutions for craniomaxillofacial, pelvis, and extremities implants, while Zimmer Biomet Holdings Inc.’s navigational dossier features knee and shoulder options.
Many navigation guides are augmented by 3D-printed life-sized anatomical models (like the one of Kessinger’s spine) that clinicians can use to better understand patient pathologies and specific musculoskeletal conditions. The models also enable surgeons to draft preoperative planning strategies for improved outcomes.
Such was the case for Kessinger, who has grown more than an inch and a half taller since her surgery. “It used to hurt when I walked but now it just feels normal,” she said. “I was so scared that I would become paralyzed and now I don’t worry about that anymore.”
Happy endings like Kessinger’s are becoming more common as orthopedic implant manufacturers leverage 3D printing technology to produce customized solutions at the point of care. The global market for 3D-printed orthopedic devices is forecast to swell 26 percent annually through 2023 due to rising demand for longer-lasting, better performing, personalized implants, according to Technavio.
Since its conception nearly 40 years ago, 3D printing—a.k.a., additive manufacturing (AM)—has become relatively widespread in various industries, revolutionizing the way tooling aids, functional prototypes, and finished components are made. Healthcare was among the technology’s earliest adopters, with dental implants and custom prosthetics routinely created in the late 1990s and early 2000s.
Ensuing applications have involved human tissue, blood vessels, and organs, but 3D printing’s predominant use lies in preoperative planning and production of anatomical models, surgical instruments, and implants. The technology has quickly gained momentum in orthopedics in recent years as surgeons sought a better understanding of bone structures, disease treatments, and clinical interventions.
“AM [additive manufacturing] offers the opportunity for better clinical outcomes. For Total Hips, a benefit of 3D-printed acetabular cups is initial fixation, which reduces incidences of malalignment and thus, reduced dislocation,” noted Brian McLaughlin, founder and CEO of Scarborough, Maine-based Amplify Additive, an additive manufacturing supply chain company for orthopedic implants. “For Total Knees, the reduction of the use of cement conserves bone. Should a patient need revision surgery and need a replacement, aside from the bone in-growth that occurs with the 3D scaffolds, we can print this onto implants. For oncology and trauma, we can deliver customized solutions for the most challenging of situations, and in many cases, save limbs and improve the quality of lives for patients. AM isn’t just a sexy new technology; it is a game-changer for the industry.”
Indeed, AM has changed the state of play in orthopedics.
The technology is routinely used to generate patient-specific anatomical models, surgical instruments, external fixators, and cutting guides; once reserved for only the most complex cases, AM is now a standard part of various orthopedic procedures, used for preoperative planning purposes in upper extremity trauma cases, deformity corrections, arthroplasties, foot/ankle surgeries, spinal fusions, hip and acetabulum procedures, hip and knee arthroplasty, and oncology (e.g., bone cancer).
The anatomical models used in these cases are just as diverse, made to mimic practically every bone and joint in the body. 3D Systems, whose co-founder Charles Hull invented stereolithography (3D printing), offers anatomic replicas in various styles and materials using U.S. Food and Drug Administration (FDA)-cleared, standalone modular software (D2P) designed to address and consolidate all 3D-modeling preparation steps.
The company also houses a Craniofacial Model Skull Library, a collection of pediatric craniomaxillofacial deformities created from an imaging archive established over a 20-year period (1983-2003) by two St. Louis doctors. The library is comprised of selected diagnosis-specific 3D CT datasets from the imaging archive; the images represent the most “characteristic dysmorphology” of the physical abnormalities before surgical intervention.
“3D Systems makes very realistic, lifelike models,” Stephen J. Snyder, M.D., a Southern California surgeon specializing in arthroscopy and shoulder reconstruction procedures, said in a company website video.
“[The] model of my favorite joint, the shoulder, [has] literally got everything I need to learn the anatomy, but also to be able to plan and discuss the various pathologies with the patient and with my students. It’s really amazing that they [3D Systems] can do this and 3D print the anatomy so clearly and so precisely. It’s going to be really good to help us with our teaching and education—for both doctors and patients.”
Greater knowledge also awaits both groups through Los Angeles-based Stratasys Ltd.’s anatomical models, which mimic porous bone structures, fibrotic tissues, and ligaments. Created with the company’s upgraded J750 Digital Anatomy 3D printer, the “biomechanically realistic” models behave like human bone and provide clinicians with haptic feedback when practicing screw insertion, drilling, or sawing through bone, according to the company.
Stratasys claims its J750 software and printer can create bones in various disease stages or densities—constructing, for example, healthy or degenerative intervertebral discs, and facet joints in varying degrees of stiffness. The models also can reflect the skull’s denser structure (compared to general bones), and differing amounts of marrow in long bones.
The models’ biomechanical properties are attained through several advanced materials with contrasting degrees of softness, flexibility, and density. GelMatrix, used for simulating both large and small vascular structures, has a pudding-like consistency while the more durable TissueMatrix is soft and flexible but can withstand suturing, cutting, inserting, and deploying devices. TissueMatrix is best suited for replicating fatty tissue, fibrotic tissue, soft organs, and tumors. BoneMatrix is the toughest of the substances, yet it is flexible and maintains its shape during trial discectomies, drilling, reaming, or sawing.
Study data on Statasys’ bone models indicate they have a similar haptic response to human cadaver bone in screw pull-out force. Moreover, the company’s 3D-printed lumbar vertebrae models accurately represent human spinal range of motion, other trial results show.
“The mechanical properties of bone are so fundamental to the ability of our skeletons to support movement, provide protection for our vital organs, and ultimately affect our quality of life,” Vice President Osnat Philipp, leader of Stratasys’ global healthcare team, said last December in announcing updates to the J750 Digital Anatomy 3D printer. “We believe that better preparation leads to better clinical outcomes. Being able to 3D print models that are biomechanically accurate and unique to each patient is critical to that preparation.”
Better clinical outcomes is actually one of the main innovation drivers in 3D-printed orthopedic implants. Besides its preoperative planning perks, the technology allows for simpler procedures and improved biocompatibility.
Many reconstructive procedures can now be completed faster and less invasively with AM implants due to the pre-surgical insight gained from anatomical models and cutting/drilling guides. Surgeons that map out their strategies beforehand spend less time in the operating room, thereby reducing overall procedure time and expediting their patients’ recovery rates. Studies have shown that AM surgical guides can reduce OR time by an average 23 minutes, and anatomic models can trim 62 minutes off procedure times.2
“In addition to the spine market, oncology and trauma are both market segments that can certainly leverage AM for better patient outcomes,” Amplify Additive’s McLaughlin tells Orthopedic Design & Technology. “Total joints—knees, hips, and shoulders also leverage the technology to promote better initial fixation and better overall results. That is the key—AM provides better clinical outcomes.”
Those outcomes, however, would likely be impracticable without material advancements. Titanium has long been the go-to substance for 3D-printed implants, favored for its exemplary bioinert properties, bone-bonding ability, and strength-to-weight ratio. Most, if not all, AM developers offer Ti6AI4V and other titanium alloys for orthopedic use.
Carpenter Technology Corporation offers 3D-printed joint reconstruction, spinal, and trauma implants in both Ti6AI4V and cobalt chrome molybdenum (CCM), a material traditionally prone to fragility and cracking under additive manufactured processes. The company, though, overcame this challenge by developing a CCM powder suitable for AM purposes.
“The availability of AM optimized non-titanium powders is a positive step to enable widespread adoption of AM beyond the regular use of titanium alloys,” noted Gaurav Lalwani, medical applications development engineer for the Philadelphia-based global developer of high-performance specialty alloy-based materials. “Cobalt chrome molybdenum (CCM) alloy is widely used in conventional manufacturing of implants.
However, the standard chemistry of CCM results in brittleness and cracking in the AM process. To meet this challenge, Carpenter Technology identified and optimized the trace elemental composition within the ASTM-approved limits and developed a CCM powder optimized for 3D printing. Another example of Carpenter’s materials innovation is Ti Grade 23+ powder, which provides a 15 percent to 20 percent improvement in mechanical properties vs. standard Ti Grade 23, which enables the design engineers to realize ultra-fine and complex device features that typically fail with regular Ti Grade 23 powder. Additionally, availability of next-generation materials for AM such as BioDur 108 (cobalt and nickel-free FDA-approved implantable alloy) and nitinol powders will enable the translation of new applications to AM.”
“[AM] technologies have provided the design freedom to enable OEM’s selection of more biocompatible materials,” Lalwani continued. “For example, many spine OEMs are now using titanium materials vs. PEEK for interbody devices due to the ability to print cavitation and porous structures that allow for better implant integration without the drawbacks of PEEK (i.e., development of scar tissue around the implant).”
Despite its drawbacks, PEEK (polyether ether ketone) has nonetheless become a frequent material choice for AM implants due to healthcare’s ongoing penchant for biologically-friendly products. Biocompatible implants for various orthopedic applications are available from numerous AM specialists, including Evonik Industries AG, which offers the PEEK-based VESTAKEEP i4 3DF, a high viscosity, biocompatible, biostable resin; and 3D Printlife’s biocompatible, non-toxic bone replacement filament made from polyamide polyolefin and cellulose (the material is reserved for patient-specific anatomical models).
Austrian AM systems developer Lithoz GmbH offers bioresorbable ceramic implants made of beta-tricalcium phosphate (ß-TCP), hydroxyapatite, or zirconia using lithography-based ceramic manufacturing (LCM), a digital light processing technique. LCM produces dense and highly precise ceramic parts via a photocurable ceramic suspension that is hardened in a photolithographic process.
Last year, Lithoz debuted multi-material 3D printing technology for the creation of multi-functional components. Its LED-driven CeraFab Multi 2M30 printer can generate parts containing combinations of ceramics, polymers, and metals, enabling the company to produce implants with gradual changes in material composition and microstructure. Using the new machine, Lithoz printed a jawbone implant that mimicked the dense exteriors and lighter porous interiors of human bone. The company used a high-strength zirconia for the mandibular cage’s outer shell and tricalcium phosphate for the interior.
Although significant, Lithoz’s multi-material printing technology could soon be forced into early retirement. Australian scientists have developed a ceramic-based ink that might potentially enable surgeons to generate 3D-printed bone parts with actual living cells. The ink, according to published reports, is comprised of calcium phosphate and combines with a collagenous substance containing living cells to create (in situ) bone-like tissues.
The scientists deemed their printing technique ceramic omnidirectional bioprinting in cell-suspensions (COBICS). Clearly, more testing is needed to determine whether the living cells will continue growing after being implanted in existing bone tissue, but the development is significant because it would allow clinicians to customize and build better-fitting parts on the spot.
“This has the potential to radically change current practice, reducing patient suffering, and ultimately saving lives,” Dr. Iman Roohani, a University of New South Wales (UNSW) scientist working on the project, told the press earlier this year. “I imagine a day where a patient needing a bone graft can walk into a clinic where the anatomical structure of their bone is imaged, translated to a 3D printer, and directly printed into the cavity with their own cells.”
Until that day arrives, though, patients will have to partly rely on good, old-fashioned 3D-printed implants to fix their ailing joints and bones. Ostensibly primitive compared to the breakthrough bioprinting developed by UNSW scientists, AM implants nonetheless are exceptional in their own right, largely for the design freedom and geometric complexity they provide to product engineers. 3D-printed devices can be more easily molded into anatomical shapes than conventionally manufactured products, and they also can be designed with porous surface structures that facilitate faster bone-implant integration.
South Korean medical firm Mantiz, for example, manufactures a 3D-printed line of interbody fusion cages with complex mesh-like structures, and Draper, Utah-based Nexus Spine engineers its Tranquil interbody fusion devices to mimic spinal bone on a microscopic level via specific pore size, shape, volume, and flexibility. The company claims its Tranquil portfolio provides “substantially more” bone through-growth than predicate devices.
“Additive manufacturing has allowed for the creation of designs that were not previously possible,” Nexus Spine product engineer Peter Halverson said. “With past techniques, we were limited to geometries that we could reach with physical tooling. With additive manufacturing, we can create structures and features internal to the part. These features can increase the function and performance of the part without a proportional increase in production costs. Additionally, additive manufactured titanium has been shown to be very bone-friendly, producing the reactions necessary to facilitate bony attachment and in-growth.”
Optimal osseointegration, however, is dependent on pore structure. For titanium implants, pore interconnectivity, size, and shape (geometric arrangement) are crucial ingredients in cell ongrowth success.
Ideal interconnectivity requires neighboring pores to open both inwardly and outwardly, with the pores responsible for the connection itself large enough to accommodate proper body fluid circulation.
“[AM technology] increases the possibility of developing new solutions to address unmet requests, such as devices with mechanical tailored performances, differentiated point-to-point, along with biocompatibility and anti-allergic or anti-infection features,” said Francesco Robotti, technology business development manager for Lincotek Medical, a fully-integrated, global contract manufacturer for the orthopedic market and a recognized expert in casting, machining, plasma spray coatings, and additive manufacturing technologies in the medical field. “[AM] enables the manufacturing of devices with unprecedented complexity in shape and surface topology.”
Those shapes can take many different forms, though lattice structures seem to offer the best opportunity for bone biointegration. Studies have found that porous lattice structures actually foster osseointegration better than smooth or roughened surfaces because they provide a scaffold for fostering cell ongrowth and ingrowth.3
Generating these lattice structures is possible through advanced software like nTopology’s nTop Platform and 3DXpert from 3D Systems. The former, used by companies like Amplify Additive, Nexxt Spine, Renishaw, and Tangible Solutions, offers various lattice structures that can be customized to individual patients. The software generates lattices through organized workflows, combining CAD (computer-aided design) modeling with analysis data and incorporating Engineering Notebooks to help teams document and share knowledge.
“Clearly certain software packages have improved the ability to convert customer-specific CT scan data into specific CAD, upon which an implant can be modeled with greater precision, and to create new optimized designs, including integrated lattice structures,” noted Erik Poulsen, medical market segment manager for GF Machining Solutions Management S.A., a Swiss provider of machine tools, diverse technical solutions, and services to manufacturers of precision molds and tooling of tight tolerance, precision-machined components. “This is certainly the case with 3DXpert, and has helped open the door for better, more accurate product designs as well as lowering costs by reducing scrap rates. When you include an integrated approach on the CAM side from additive to subtractive technologies seamlessly, the end result is more accurate products.”
3DXpert is ideal for defining lattice cell structures, creating radial lattices to fit circular parts, and applying variable lattice thickness to components based on FEA stress analysis. The software’s latest iteration, Lattice QuickSlice, features efficient and fast slicing for any-sized lattice structure; the tool, according to 3D Systems, can significantly shorten the time required to slice enormous lattice structures while maintaining optimal quality.
“There are several software systems on the market but 3DXpert is state-of-the-art additive design and build preparation software. It allows you to design and optimize lattice structures, support structures, and hatch strategies as well as simulate builds all in one suite,” explained Dr. Gautam Gupta, vice president and general manager, Medical Devices, at 3D Systems. “You can make changes on the fly in that software suite, and shorten the product development time from months into weeks. With the advanced application tools, our customers can create several iterations of the design in a very short time. That kind of speed to observe several different designs is unparalleled. The combination of hardware and software is creating a tremendous sandbox for engineers to design products.”
References
- https://aoj.amegroups.com/article/view/4993/html
- https://www.academicradiology.org/article/S1076-6332(19)30418-0/pdf
- Simmons CA, Valiquette N, Pilliar RM. Osseointegration of sintered porous-surfaced and plasma spray-coated implants: An animal model study of early postimplantation healing response and mechanical stability. Journal of Biomedical Materials Research. 1999; 47(2): 127-138.
