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Emerging Technologies in Arthroplasty: Additive Manufacturing Samik Banerjee, MS (Orth), MRCS (Glasg)1
Gene Kulesha, MS2
1 Department of Orthopaedic Surgery, Center for Joint Preservation
and Replacement, Rubin Institute for Advanced Orthopedics, Sinai Hospital of Baltimore, Baltimore, Maryland 2 Stryker Orthopaedics, Mahwah, New Jersey
Mark Kester, PhD2
Michael A. Mont, MD1
Address for correspondence Michael A. Mont, MD, Center for Joint Preservation and Replacement, Rubin Institute for Advanced Orthopedics, 2401 West Belvedere Avenue, Baltimore, MD 21215 (e-mail: [email protected]).
Abstract
Keywords
► arthroplasty ► technology ► manufacturing
Additive manufacturing is an industrial technology whereby three-dimensional visual computer models are fabricated into physical components by selectively curing, depositing, or consolidating various materials in consecutive layers. Although initially developed for production of simulated models, the technology has undergone vast improvements and is currently increasingly being used for the production of end-use components in various aerospace, automotive, and biomedical specialties. The ability of this technology to be used for the manufacture of solid-mesh-foam monolithic and coated components of complex geometries previously considered unmanufacturable has attracted the attention of implant manufacturers, bioengineers, and orthopedic surgeons. Currently, there is a paucity of reports describing this fabrication method in the orthopedic literature. Therefore, we aimed to briefly describe this technology, some of the applications in other orthopedic subspecialties, its present use in hip and knee arthroplasty, and concerns with the present form of the technology. As there are few reports of clinical trials presently available, the true benefits of this technology can only be realized when studies evaluating the clinical and radiographic outcomes of cementless implants manufactured with additive manufacturing report durable fixation, less stress shielding, and better implant survivorship. Nevertheless, the authors believe that this technology holds great promise and may potentially change the conventional methods of casting, machining, and tooling for implant manufacturing in the future.
Additive manufacturing, previously known as rapid prototyping, is a generic description for several rapid manufacturing processes such as laser sintering and melting, fused deposition modeling, three-dimensional (3D) printing, electron beam melting, and stereo-lithography. Since its introduction almost two decades ago, this technology has undergone numerous software and hardware developments and has been used in various automotive, aerospace, and biomedical subspecialties. However, it is only recently that it has received increased attention among the orthopedic community and implant manufacturers due to its potential ability
to decrease costs and production times during manufacturing of highly engineered implants, scaffolds, custom guides, and simulation models. In addition, this technology is currently being used for the production of lower extremity arthroplasty implants with complex shapes, sizes, and surface geometries of varying porosity, strength, and stiffness that were previously considered not manufacturable.1 There has been a paucity of articles in the orthopedic literature that have elaborated on this novel technology and reviewed its current applications. Therefore, in this report, we aimed to briefly discuss the: (1) technology; (2) other
received February 21, 2014 accepted February 26, 2014 published online April 24, 2014
Copyright © 2014 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0034-1374810. ISSN 1538-8506.
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ancillary applications; (3) its application in hip and knee arthroplasty; and (4) the current concerns with this technology.
Technology Description Overview of the Technology Additive manufacturing technology was initially developed in the 1970s, although the first attempts can be traced back to the 1800s when it was initially used for topography and photosculpture.2,3 Marked developments and expansion of this technology has occurred in the past three decades, with increasing application in various industrial processes. The technology, which converts computer-aided design visual models into a manufacturing paradigm, is analogous to three-dimensional printing where a stack of layers are printed one after another to ultimately produce the desired object. Although this technology has been applied in the past for the generation of custom implants in orthopedic surgery, recently, this has been used for the production of lower extremity arthroplasty implants with three-dimensional geometries using computed tomography (CT) data from electronic skeletal database libraries such as the SOMA database (Stryker Orthopaedics Modeling and Analytics; Stryker Orthopaedics, Mahwah, NJ). Initially, computer-aided design (CAD) models of implants of specific three-dimensional geometry were converted to a surface tessellation file (STL), which essentially reduces the model to a set of triangular facets connected at the vertices, by tessellating it (►Table 1). A horizontal slicing software algorithm is applied to the computerized model of the implant to generate the manufacturing information for each layer of varying thickness. The components are then fabricated through selectively curing, depositing, and consolidating materials in an additive layered process instead of removing material, as commonly done with traditional machining (►Table 1). Various rapid manufacturing methods, such as powdered bed fusion, fused deposition modeling, direct energy deposition, vat photo polymerization, sheet lamination, binder jetting, and material jetting are then used to manufacture the final implant or part. These final manufacturing processes lead to the production of components which are exact replicas of the CAD model.
The putative benefits of the additive manufacturing technology are its speed and ability to produce parts or components without the need to make molds. Thus, subtle variations in geometry and design changes can be made on the CAD design in an offline workstation which not only increases the speed of the design and development cycle but may also substantially reduce the manufacturing costs through obviation of molds and recycling of raw unused materials. In addition, the technology potentially allows precise manufacturing of implants with complex three-dimensional geometries, such as producing undercuts, channels through sections, tubes within tubes, and internal voids, without the need for tooling and machining. This may markedly eliminate some of the conventional manufacturing constraints and limitations. We briefly describe below two commonly used metal additive manufacturing processes for the production of arthroplasty implants.
Fusion Deposition Modeling The technology was developed initially in the 1980s for industrial production of components based on the principle of additive layering. During the manufacturing process, a metal filament or wire is passed through a heating element which converts the metal to molten state. This is then passed through a nozzle for depositing the metal on to the implant it is manufacturing in a layered process which is guided by the information provided by the slicing software. As the metal is deposited in its molten state, it fuses with the surrounding material that has already been deposited. Once one layer has formed, the nozzle head moves vertically to deposit another layer on top of the previous layer. This process is continued until the entire implant is manufactured.
Powder Bed Fusion The basic philosophy of all powder bed fusion processes involves the use of thermal energy to fuse powdered particles at specified locations of each layer. Selective laser sintering was one of the first commercially available powdered bed fusion manufacturing processes. Although initially developed for use in the production of polymers, this technology has been used for the manufacturing of ceramics and metals. However, due to limitations in controlling the pore size and
Table 1 Workflow in additive manufacturing process for development of lower extremity arthroplasty implants • Computed tomography scan data from digital database library or individual patient’s anatomy • Data segmented to generate three-dimensional anatomic models • Implant fitting and analytical tools used to devise three-dimensional implant models that have more conforming geometries to anatomic models • Three-dimensional implant model is converted to stereolithography file which is subsequently sliced using computer algorithm • Sliced data are used to provide the manufacturing information for layer-by-layer production of the final implant which represents an exact replica of the three-dimensional implant model • Production of the final implant or component through a rapid manufacturing process such as powdered bed fusion, fused deposition modeling, or direct energy deposition
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Fig. 1 Schematic diagram describing the selective laser melting process.
shape, and production of metals which are brittle and prone to crack propagation with the powder sintering process, three recent modifications of the original laser sintering process have been introduced which include laser melting, laser engineered net shaping, and electron beam melting.
Selective Laser Melting In the selective laser melting (SLM) process, the metal powder (powder particle diameter, 10–45 μm) is fed on to a substrate platform through either a hopper or nozzle delivery system to create a powder layer which is subsequently melted (►Fig. 1). The motion of this substrate is controlled through a computerized system which constantly moves it beneath the stationery high energy laser beam to create the thin cross-sectional geometry for each layer as predetermined by the slicing software. Following completion of each layer, the substrate platform is incrementally lowered in the z-axis and the entire process is repeated. As there are no processing agents or any material transfer during the SLM, the final product has the same composition as the metal powder. In contrast to laser sintering, this fabrication process has the ability to completely melt the metal powder rather than sintering the powder. In addition to the advantages of rapid production, this technology has the ability to process a wide variety of metals and manufacture components of high accuracy and surface finish. However, when fabricating large, bulky parts, this technology does have the disadvantage of potentially producing components with high internal residual stresses due to the presence of thermal gradients during the manufacturing process which necessitates a post-build heat treatment to improve the mechanical properties of the implants produced.
Moreover, the SLM process can be slow and may not be costeffective for the production of complex geometries or for large-scale production.
Laser Engineered Net Shaping In laser engineered net shaping, in contrast to laser melting technology, a low power laser (Nd:YAG laser, 250–300 W) is used to partially melt the surface of the metal powders.4 This leads to fusion of the individual surface melted powders at the particle interfaces, leaving some residual porosity between the particles. The porosity is controlled in this technique by changing the laser scan speed, which affects the particle interaction time. Various modifications, such as changing the deposition angle or optimizing the distance between the laser scans, and varying the thickness of the metal powders, may allow desired graduated control of the porosity in metal structures with this technique.
Electron Beam Melting Electron beam melting is a powder-based additive manufacturing fabrication process that uses focused highenergy electron beams to melt uniformly raked metal powder layers that are gravity fed from powder hoppers. The CAD software enables the electron beam to melt only select portions of the powder layer and the process is repeated in the building direction to manufacture complex three-dimensional geometries at very fast build speeds. The vacuum environment prevents oxidation of reactive metals, for example, titanium, and reduces the risk of contamination. In addition, the electron beam periodically heats the metal powder to avoid development of thermal gradients that The Journal of Knee Surgery
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increase the internal residual stresses within the metal components. Some of the advantages of this novel technology include the low energy required in the production process, the ability to produce implants with optimal mechanical properties and porosities,5 the capability to integrate porosity to solid substrates, and to manufacture components with low internal residual stresses. In addition, compared with conventional low-energy laser melting, EBM due to the greater energy density has the advantage of reduced build times and potentially lower manufacturing costs due to the greater amount of energy delivered by the electron beam.5
Applications of Additive Layered Manufacturing in Hip and Knee Arthroplasty Manufacturing of Cementless Implants There has been a constant evolution and vast expansion in the field of implant manufacturing for hip and knee arthroplasties in the past decade, due to the need for the development of highly porous metals which have greater porosity for bone ingrowth, better elastic properties to avoid stress shielding, and optimal stiffness to withstand physiological forces.6–8 With the advent of powdered bed fusion additive manufacturing technologies, hip and knee implants are being manufactured which have these desired properties.6 Recent studies have shown that materials produced through additive manufacturing technology may have improved fixation stability as a result of incorporation of innovative, complex, three-dimensional geometries, and optimal mechanical strength,5,6 with the potential for increased bone ingrowth compared with the current generation of porous implants produced through more conventional manufacturing methods.9 Murr et al reported on the fabrication of novel monolithic, functional, solid–mesh–foam titanium alloy (Ti-6Al-4V) tibial baseplates and Co-Cr-Mo alloy femoral components using electron beam melting technology, for cementless fixation in total knee arthroplasty.6,8 The authors also reported that using the additive layering process, femoral stems for cementless fixation may be fabricated that have a mesh–foam construct on the exterior surface and a solid core structure in the interior for optimal stiffness and bone ingrowth.6,8 This technology has also recently been used for the fabrication of a novel Triathlon Tritanium tibial baseplate (Stryker Orthopaedics).10 In this design, the pegs had three-dimensional cruciform geometric features with proximal porous bands incorporated to potentially account for minor inconsistencies of bone resection and morphology (►Fig. 2). In addition, using this technology, a differential surface architecture between the bone-facing side and the polyethylene-facing side of the patellar implant was produced (►Fig. 3).10 Although the clinical results of this new generation of cementless design, intended to promote biological fixation, are currently unavailable, it is encouraging to note that previously considered composition and geometric manufacturing constraints can be overcome with this technology. Bertollo et al, in an ovine study, compared the bone ingrowth potential and shear strength of implant dowels The Journal of Knee Surgery
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Fig. 2 Three-dimensional designs of pegs with band of porous coating in the proximal part of the peg manufactured through additive manufacturing technology.
manufactured with electron beam melting and titanium plasma spray technologies.9 The authors found that the dowel produced from electron beam melting technology had significantly higher surface roughness (p ¼ 0.002) and shear strength (p ¼ 0.03) at 12 weeks postoperatively, compared with the plasma-sprayed dowel.9 In addition, the authors also found that the bone ingrowth for EBM dowels were similar for press-fit, line-to-line fit, and 1-mm gap, whereas, with the plasma spray dowel, ingrowth was significantly lower for the 1-mm gap compared with line-to-line fit and press-fit scenarios.9
Fig. 3 Differential (a) polyethylene-facing and (b) bone-facing surfaces of the patellar button manufactured through electron beam melting technology.
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Manufacturing of Patient-Specific Guides One of the commonly used applications of the additive manufacturing technology has been the production of patient-specific guides for bone cuts during total knee arthroplasty11–13 and acetabular placement during total hip arthroplasty.14 These patient-specific instrumentations have been proposed to reduce the number surgical steps, intraoperative instrumentation, and potentially the overall costs of surgery. Although there is debate about the current status of patient-specific instrumentation in reducing radiographic alignment outliers,15–17 operating times,18,19 and fabrication costs, refinements in the present technology may improve the accuracy and cost-effectiveness of the current generation of patient-specific guides in the future.20
Preoperative Planning With the use of modern additive layering manufacturing processes, accurate three-dimensional models of the osseous or soft tissue anatomy can be rapidly fabricated to enable surgeons to perform preoperative planning on these solid models before performing the actual procedure.21–24 This may involve understanding the fracture patterns, preoperative contouring of plates, as well as planning the trajectory of screws and positioning of plates during trauma surgery, or positioning screws and implants during difficult hip and knee reconstructions.22,25 Although this technology has been used in the past in various specialties such as cardiothoracic surgery,24 neurosurgery,26 orthopedic trauma surgery,22 spinal surgery,27 dental surgery,28 and craniomaxillofacial surgery,29,30 it is only recently that its role has been evaluated in lower extremity total joint arthroplasty.25 Won et al, in a study of 21 complex primary total hip arthroplasties, evaluated the accuracy of preoperative planning with simulated models of the patients’ anatomy of the pelvis manufactured using additive layering technology.25 The authors found that in approximately 81% of cases (n ¼ 17), the acetabulum was sized accurately (within 2 mm of the predicted size). In addition, the authors found that all hips had ingrown femoral stems and acetabular cups
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at a mean follow-up of approximately 3 years (range, 2–7 years).
Other Applications Manufacturing of Scaffolds A variety of synthetic bone scaffolds of graduated porosities are being manufactured through the additive layering process for intraoperative use in fracture healing, and the repair or reconstruction of bone defects. These scaffolds provide structural support and may also assist in osseointegration to native bone due to their highly porous microstructure.31–33 In an animal study of 36 healthy mature dogs, Huang et al evaluated the stability and fracture repair potential of a novel three-dimensional scaffold made of chitosan fiber and calcium phosphate ceramics manufactured through an additive manufacturing process for the treatment of comminuted radius fractures.32 The authors found early new bone formation at approximately 4 weeks postinjury and partial integration of the new bone with the scaffold at 8 weeks. At 12 weeks postsurgery, complete fracture healing was found to occur in the scaffold cohort, while no bone formation was found in the comparison control. In addition, the authors found that the mechanical strength of the scaffold containing radii was three times that of the comparison control at 12 weeks. In a study of 16 adult sheep, Yang et al evaluated the bone ingrowth and implant stability of a self-stabilizing artificial vertebral body made of highly porous titanium alloy fabricated by electron beam melting technology.33 The authors found significant improvements in the bone ingrowth with the highly porous vertebral body at 6 and 12 weeks postsurgery following attempted C4–C6 spinal fusion, compared with a control group (p ¼ 0.001). In addition, the authors found significant increases in bending and rotational stability in the artificial vertebral body group compared with the comparison controls (p < 0.05).33
Medical Education Additive manufacturing technology is projected to be a useful tool for the accurate production of simulated models of both normal and pathologic anatomy for enhancing medical education and training as well as satisfaction among medical students.34–36 Bustamante et al34 evaluated the accuracy of two rapid prototype generated models of tracheobronchial anatomy by comparing these to flexible fiberoptic bronchoscopic images obtained intraoperatively during lung isolation. The authors found substantial congruence between the patients’ intraoperative bronchial anatomy and the simulated model, which suggested that this technique can potentially become a valuable educational tool during medical training.34 Challoner and Erolin, in a questionnaire study conducted on 20 volunteers, compared the student and the tutor preferences toward virtual models and simulated physical models generated through additive layered manufacturing during medical education.35 The authors found that the majority of the volunteers rated the physical models as more user friendly (70 vs. 35%) compared with the virtual models and all of the volunteers believed that the ability to touch and feel the The Journal of Knee Surgery
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Massari et al, in a study of 72 patients who had undergone primary total hip arthroplasties in four centers, evaluated the radiographic and bone densitometric outcomes of highly porous titanium alloy acetabular shell produced with additive manufacturing technology.40 At a preliminary follow-up of 1 year, the authors found no evidence of radiographic loosening or osteolysis and marked improvements in the bone mineral density measurements in the periacetabular bone after an initial fall in the measurements between 1 week and 3 months. There is currently a paucity of mechanical and physiological testing data and clinical reports for some of these monolithic solid–mesh–foam components. However, it is exciting to realize that contrary to popular belief, monolithic solid and highly porous metal implants can potentially be manufactured with additive layering technology that may have the ideal porosity for ingrowth and the mechanical strength to withstand physiological forces.
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physical models was markedly helpful for the educational exercise. Currently, there is limited data on the use of this technology in the field of medical education. However, evidence is emerging that its use may improve learning and development of skills and may be a valuable tool in the classroom of the future.
Concerns with Additive Manufacturing Technology There have been some concerns about the accuracy of the three-dimensional models developed from CT scans which may ultimately affect the dimensional measurements of the component produced. A variety of factors can affect the quality of the CT data which directly influences the production of the CAD models, such as pitch, gantry tilt, tube current voltage, and slice thickness. Of these, slice thickness is a prime factor which can lead to a partial volume averaging error (or inter-slice volume averaging effect) from loss of sharp corners or edges during the production of the three-dimensional model. Moreover, a geometric stair-step error can occur, as each layer of the implant manufactured approximates a discrete representation of the cross-sectional geometry. Another source of the error occurs during conversion to the STL format where some loss of resolution occurs, as the format represents only triangles instead of arcs or splines.37 These geometric errors can then be incorporated into the fabricated implant. Although some of these errors can be reduced by reducing the size and increasing the number of the triangular facets, it occurs at the cost of data storage capacity. Nevertheless, the technology is constantly developing and has the potential to make profound impacts on the speed and cost of manufacturing more conforming, complex implant geometries.
Future Outlook
proportion of the overall fabrication of orthopedic implants. Nevertheless, additive manufacturing holds great promise and may have the potential to make a profound impact on the manufacturing of anatomically more conforming and complex implant geometries with graduated porosities at faster rates and lower costs.
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There has been great interest in the use of additive manufacturing technology for the manufacturing of lower extremity total joint arthroplasty implants. A variety of materials and compositions are being used by modern additive manufacturing process to produce implants of varying texture, graduated porosity, and mechanical strength for use in lower extremity total joint arthroplasty. In addition, it is exciting to note that there is potential application of this technology for the manufacturing of patient-specific implants in future.23,38,39 With the development of the additive manufacturing format (AMF), the conventional STL format may be effectively replaced in future. The AMF format allows inclusion of data about surface texture and porosity which may markedly improve the overall accuracy of the technology. This may potentially eliminate some of the inaccuracies of the current generation of additive manufacturing process.
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Emerging Technologies in Arthroplasty: Additive Manufacturing Samik Banerjee, MS (Orth), MRCS (Glasg)1
Gene Kulesha, MS2
1 Department of Orthopaedic Surgery, Center for Joint Preservation
and Replacement, Rubin Institute for Advanced Orthopedics, Sinai Hospital of Baltimore, Baltimore, Maryland 2 Stryker Orthopaedics, Mahwah, New Jersey
Mark Kester, PhD2
Michael A. Mont, MD1
Address for correspondence Michael A. Mont, MD, Center for Joint Preservation and Replacement, Rubin Institute for Advanced Orthopedics, 2401 West Belvedere Avenue, Baltimore, MD 21215 (e-mail: [email protected]).
Abstract
Keywords
► arthroplasty ► technology ► manufacturing
Additive manufacturing is an industrial technology whereby three-dimensional visual computer models are fabricated into physical components by selectively curing, depositing, or consolidating various materials in consecutive layers. Although initially developed for production of simulated models, the technology has undergone vast improvements and is currently increasingly being used for the production of end-use components in various aerospace, automotive, and biomedical specialties. The ability of this technology to be used for the manufacture of solid-mesh-foam monolithic and coated components of complex geometries previously considered unmanufacturable has attracted the attention of implant manufacturers, bioengineers, and orthopedic surgeons. Currently, there is a paucity of reports describing this fabrication method in the orthopedic literature. Therefore, we aimed to briefly describe this technology, some of the applications in other orthopedic subspecialties, its present use in hip and knee arthroplasty, and concerns with the present form of the technology. As there are few reports of clinical trials presently available, the true benefits of this technology can only be realized when studies evaluating the clinical and radiographic outcomes of cementless implants manufactured with additive manufacturing report durable fixation, less stress shielding, and better implant survivorship. Nevertheless, the authors believe that this technology holds great promise and may potentially change the conventional methods of casting, machining, and tooling for implant manufacturing in the future.
Additive manufacturing, previously known as rapid prototyping, is a generic description for several rapid manufacturing processes such as laser sintering and melting, fused deposition modeling, three-dimensional (3D) printing, electron beam melting, and stereo-lithography. Since its introduction almost two decades ago, this technology has undergone numerous software and hardware developments and has been used in various automotive, aerospace, and biomedical subspecialties. However, it is only recently that it has received increased attention among the orthopedic community and implant manufacturers due to its potential ability
to decrease costs and production times during manufacturing of highly engineered implants, scaffolds, custom guides, and simulation models. In addition, this technology is currently being used for the production of lower extremity arthroplasty implants with complex shapes, sizes, and surface geometries of varying porosity, strength, and stiffness that were previously considered not manufacturable.1 There has been a paucity of articles in the orthopedic literature that have elaborated on this novel technology and reviewed its current applications. Therefore, in this report, we aimed to briefly discuss the: (1) technology; (2) other
received February 21, 2014 accepted February 26, 2014 published online April 24, 2014
Copyright © 2014 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662.
DOI http://dx.doi.org/ 10.1055/s-0034-1374810. ISSN 1538-8506.
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ancillary applications; (3) its application in hip and knee arthroplasty; and (4) the current concerns with this technology.
Technology Description Overview of the Technology Additive manufacturing technology was initially developed in the 1970s, although the first attempts can be traced back to the 1800s when it was initially used for topography and photosculpture.2,3 Marked developments and expansion of this technology has occurred in the past three decades, with increasing application in various industrial processes. The technology, which converts computer-aided design visual models into a manufacturing paradigm, is analogous to three-dimensional printing where a stack of layers are printed one after another to ultimately produce the desired object. Although this technology has been applied in the past for the generation of custom implants in orthopedic surgery, recently, this has been used for the production of lower extremity arthroplasty implants with three-dimensional geometries using computed tomography (CT) data from electronic skeletal database libraries such as the SOMA database (Stryker Orthopaedics Modeling and Analytics; Stryker Orthopaedics, Mahwah, NJ). Initially, computer-aided design (CAD) models of implants of specific three-dimensional geometry were converted to a surface tessellation file (STL), which essentially reduces the model to a set of triangular facets connected at the vertices, by tessellating it (►Table 1). A horizontal slicing software algorithm is applied to the computerized model of the implant to generate the manufacturing information for each layer of varying thickness. The components are then fabricated through selectively curing, depositing, and consolidating materials in an additive layered process instead of removing material, as commonly done with traditional machining (►Table 1). Various rapid manufacturing methods, such as powdered bed fusion, fused deposition modeling, direct energy deposition, vat photo polymerization, sheet lamination, binder jetting, and material jetting are then used to manufacture the final implant or part. These final manufacturing processes lead to the production of components which are exact replicas of the CAD model.
The putative benefits of the additive manufacturing technology are its speed and ability to produce parts or components without the need to make molds. Thus, subtle variations in geometry and design changes can be made on the CAD design in an offline workstation which not only increases the speed of the design and development cycle but may also substantially reduce the manufacturing costs through obviation of molds and recycling of raw unused materials. In addition, the technology potentially allows precise manufacturing of implants with complex three-dimensional geometries, such as producing undercuts, channels through sections, tubes within tubes, and internal voids, without the need for tooling and machining. This may markedly eliminate some of the conventional manufacturing constraints and limitations. We briefly describe below two commonly used metal additive manufacturing processes for the production of arthroplasty implants.
Fusion Deposition Modeling The technology was developed initially in the 1980s for industrial production of components based on the principle of additive layering. During the manufacturing process, a metal filament or wire is passed through a heating element which converts the metal to molten state. This is then passed through a nozzle for depositing the metal on to the implant it is manufacturing in a layered process which is guided by the information provided by the slicing software. As the metal is deposited in its molten state, it fuses with the surrounding material that has already been deposited. Once one layer has formed, the nozzle head moves vertically to deposit another layer on top of the previous layer. This process is continued until the entire implant is manufactured.
Powder Bed Fusion The basic philosophy of all powder bed fusion processes involves the use of thermal energy to fuse powdered particles at specified locations of each layer. Selective laser sintering was one of the first commercially available powdered bed fusion manufacturing processes. Although initially developed for use in the production of polymers, this technology has been used for the manufacturing of ceramics and metals. However, due to limitations in controlling the pore size and
Table 1 Workflow in additive manufacturing process for development of lower extremity arthroplasty implants • Computed tomography scan data from digital database library or individual patient’s anatomy • Data segmented to generate three-dimensional anatomic models • Implant fitting and analytical tools used to devise three-dimensional implant models that have more conforming geometries to anatomic models • Three-dimensional implant model is converted to stereolithography file which is subsequently sliced using computer algorithm • Sliced data are used to provide the manufacturing information for layer-by-layer production of the final implant which represents an exact replica of the three-dimensional implant model • Production of the final implant or component through a rapid manufacturing process such as powdered bed fusion, fused deposition modeling, or direct energy deposition
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Fig. 1 Schematic diagram describing the selective laser melting process.
shape, and production of metals which are brittle and prone to crack propagation with the powder sintering process, three recent modifications of the original laser sintering process have been introduced which include laser melting, laser engineered net shaping, and electron beam melting.
Selective Laser Melting In the selective laser melting (SLM) process, the metal powder (powder particle diameter, 10–45 μm) is fed on to a substrate platform through either a hopper or nozzle delivery system to create a powder layer which is subsequently melted (►Fig. 1). The motion of this substrate is controlled through a computerized system which constantly moves it beneath the stationery high energy laser beam to create the thin cross-sectional geometry for each layer as predetermined by the slicing software. Following completion of each layer, the substrate platform is incrementally lowered in the z-axis and the entire process is repeated. As there are no processing agents or any material transfer during the SLM, the final product has the same composition as the metal powder. In contrast to laser sintering, this fabrication process has the ability to completely melt the metal powder rather than sintering the powder. In addition to the advantages of rapid production, this technology has the ability to process a wide variety of metals and manufacture components of high accuracy and surface finish. However, when fabricating large, bulky parts, this technology does have the disadvantage of potentially producing components with high internal residual stresses due to the presence of thermal gradients during the manufacturing process which necessitates a post-build heat treatment to improve the mechanical properties of the implants produced.
Moreover, the SLM process can be slow and may not be costeffective for the production of complex geometries or for large-scale production.
Laser Engineered Net Shaping In laser engineered net shaping, in contrast to laser melting technology, a low power laser (Nd:YAG laser, 250–300 W) is used to partially melt the surface of the metal powders.4 This leads to fusion of the individual surface melted powders at the particle interfaces, leaving some residual porosity between the particles. The porosity is controlled in this technique by changing the laser scan speed, which affects the particle interaction time. Various modifications, such as changing the deposition angle or optimizing the distance between the laser scans, and varying the thickness of the metal powders, may allow desired graduated control of the porosity in metal structures with this technique.
Electron Beam Melting Electron beam melting is a powder-based additive manufacturing fabrication process that uses focused highenergy electron beams to melt uniformly raked metal powder layers that are gravity fed from powder hoppers. The CAD software enables the electron beam to melt only select portions of the powder layer and the process is repeated in the building direction to manufacture complex three-dimensional geometries at very fast build speeds. The vacuum environment prevents oxidation of reactive metals, for example, titanium, and reduces the risk of contamination. In addition, the electron beam periodically heats the metal powder to avoid development of thermal gradients that The Journal of Knee Surgery
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increase the internal residual stresses within the metal components. Some of the advantages of this novel technology include the low energy required in the production process, the ability to produce implants with optimal mechanical properties and porosities,5 the capability to integrate porosity to solid substrates, and to manufacture components with low internal residual stresses. In addition, compared with conventional low-energy laser melting, EBM due to the greater energy density has the advantage of reduced build times and potentially lower manufacturing costs due to the greater amount of energy delivered by the electron beam.5
Applications of Additive Layered Manufacturing in Hip and Knee Arthroplasty Manufacturing of Cementless Implants There has been a constant evolution and vast expansion in the field of implant manufacturing for hip and knee arthroplasties in the past decade, due to the need for the development of highly porous metals which have greater porosity for bone ingrowth, better elastic properties to avoid stress shielding, and optimal stiffness to withstand physiological forces.6–8 With the advent of powdered bed fusion additive manufacturing technologies, hip and knee implants are being manufactured which have these desired properties.6 Recent studies have shown that materials produced through additive manufacturing technology may have improved fixation stability as a result of incorporation of innovative, complex, three-dimensional geometries, and optimal mechanical strength,5,6 with the potential for increased bone ingrowth compared with the current generation of porous implants produced through more conventional manufacturing methods.9 Murr et al reported on the fabrication of novel monolithic, functional, solid–mesh–foam titanium alloy (Ti-6Al-4V) tibial baseplates and Co-Cr-Mo alloy femoral components using electron beam melting technology, for cementless fixation in total knee arthroplasty.6,8 The authors also reported that using the additive layering process, femoral stems for cementless fixation may be fabricated that have a mesh–foam construct on the exterior surface and a solid core structure in the interior for optimal stiffness and bone ingrowth.6,8 This technology has also recently been used for the fabrication of a novel Triathlon Tritanium tibial baseplate (Stryker Orthopaedics).10 In this design, the pegs had three-dimensional cruciform geometric features with proximal porous bands incorporated to potentially account for minor inconsistencies of bone resection and morphology (►Fig. 2). In addition, using this technology, a differential surface architecture between the bone-facing side and the polyethylene-facing side of the patellar implant was produced (►Fig. 3).10 Although the clinical results of this new generation of cementless design, intended to promote biological fixation, are currently unavailable, it is encouraging to note that previously considered composition and geometric manufacturing constraints can be overcome with this technology. Bertollo et al, in an ovine study, compared the bone ingrowth potential and shear strength of implant dowels The Journal of Knee Surgery
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Fig. 2 Three-dimensional designs of pegs with band of porous coating in the proximal part of the peg manufactured through additive manufacturing technology.
manufactured with electron beam melting and titanium plasma spray technologies.9 The authors found that the dowel produced from electron beam melting technology had significantly higher surface roughness (p ¼ 0.002) and shear strength (p ¼ 0.03) at 12 weeks postoperatively, compared with the plasma-sprayed dowel.9 In addition, the authors also found that the bone ingrowth for EBM dowels were similar for press-fit, line-to-line fit, and 1-mm gap, whereas, with the plasma spray dowel, ingrowth was significantly lower for the 1-mm gap compared with line-to-line fit and press-fit scenarios.9
Fig. 3 Differential (a) polyethylene-facing and (b) bone-facing surfaces of the patellar button manufactured through electron beam melting technology.
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Manufacturing of Patient-Specific Guides One of the commonly used applications of the additive manufacturing technology has been the production of patient-specific guides for bone cuts during total knee arthroplasty11–13 and acetabular placement during total hip arthroplasty.14 These patient-specific instrumentations have been proposed to reduce the number surgical steps, intraoperative instrumentation, and potentially the overall costs of surgery. Although there is debate about the current status of patient-specific instrumentation in reducing radiographic alignment outliers,15–17 operating times,18,19 and fabrication costs, refinements in the present technology may improve the accuracy and cost-effectiveness of the current generation of patient-specific guides in the future.20
Preoperative Planning With the use of modern additive layering manufacturing processes, accurate three-dimensional models of the osseous or soft tissue anatomy can be rapidly fabricated to enable surgeons to perform preoperative planning on these solid models before performing the actual procedure.21–24 This may involve understanding the fracture patterns, preoperative contouring of plates, as well as planning the trajectory of screws and positioning of plates during trauma surgery, or positioning screws and implants during difficult hip and knee reconstructions.22,25 Although this technology has been used in the past in various specialties such as cardiothoracic surgery,24 neurosurgery,26 orthopedic trauma surgery,22 spinal surgery,27 dental surgery,28 and craniomaxillofacial surgery,29,30 it is only recently that its role has been evaluated in lower extremity total joint arthroplasty.25 Won et al, in a study of 21 complex primary total hip arthroplasties, evaluated the accuracy of preoperative planning with simulated models of the patients’ anatomy of the pelvis manufactured using additive layering technology.25 The authors found that in approximately 81% of cases (n ¼ 17), the acetabulum was sized accurately (within 2 mm of the predicted size). In addition, the authors found that all hips had ingrown femoral stems and acetabular cups
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at a mean follow-up of approximately 3 years (range, 2–7 years).
Other Applications Manufacturing of Scaffolds A variety of synthetic bone scaffolds of graduated porosities are being manufactured through the additive layering process for intraoperative use in fracture healing, and the repair or reconstruction of bone defects. These scaffolds provide structural support and may also assist in osseointegration to native bone due to their highly porous microstructure.31–33 In an animal study of 36 healthy mature dogs, Huang et al evaluated the stability and fracture repair potential of a novel three-dimensional scaffold made of chitosan fiber and calcium phosphate ceramics manufactured through an additive manufacturing process for the treatment of comminuted radius fractures.32 The authors found early new bone formation at approximately 4 weeks postinjury and partial integration of the new bone with the scaffold at 8 weeks. At 12 weeks postsurgery, complete fracture healing was found to occur in the scaffold cohort, while no bone formation was found in the comparison control. In addition, the authors found that the mechanical strength of the scaffold containing radii was three times that of the comparison control at 12 weeks. In a study of 16 adult sheep, Yang et al evaluated the bone ingrowth and implant stability of a self-stabilizing artificial vertebral body made of highly porous titanium alloy fabricated by electron beam melting technology.33 The authors found significant improvements in the bone ingrowth with the highly porous vertebral body at 6 and 12 weeks postsurgery following attempted C4–C6 spinal fusion, compared with a control group (p ¼ 0.001). In addition, the authors found significant increases in bending and rotational stability in the artificial vertebral body group compared with the comparison controls (p < 0.05).33
Medical Education Additive manufacturing technology is projected to be a useful tool for the accurate production of simulated models of both normal and pathologic anatomy for enhancing medical education and training as well as satisfaction among medical students.34–36 Bustamante et al34 evaluated the accuracy of two rapid prototype generated models of tracheobronchial anatomy by comparing these to flexible fiberoptic bronchoscopic images obtained intraoperatively during lung isolation. The authors found substantial congruence between the patients’ intraoperative bronchial anatomy and the simulated model, which suggested that this technique can potentially become a valuable educational tool during medical training.34 Challoner and Erolin, in a questionnaire study conducted on 20 volunteers, compared the student and the tutor preferences toward virtual models and simulated physical models generated through additive layered manufacturing during medical education.35 The authors found that the majority of the volunteers rated the physical models as more user friendly (70 vs. 35%) compared with the virtual models and all of the volunteers believed that the ability to touch and feel the The Journal of Knee Surgery
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Massari et al, in a study of 72 patients who had undergone primary total hip arthroplasties in four centers, evaluated the radiographic and bone densitometric outcomes of highly porous titanium alloy acetabular shell produced with additive manufacturing technology.40 At a preliminary follow-up of 1 year, the authors found no evidence of radiographic loosening or osteolysis and marked improvements in the bone mineral density measurements in the periacetabular bone after an initial fall in the measurements between 1 week and 3 months. There is currently a paucity of mechanical and physiological testing data and clinical reports for some of these monolithic solid–mesh–foam components. However, it is exciting to realize that contrary to popular belief, monolithic solid and highly porous metal implants can potentially be manufactured with additive layering technology that may have the ideal porosity for ingrowth and the mechanical strength to withstand physiological forces.
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physical models was markedly helpful for the educational exercise. Currently, there is limited data on the use of this technology in the field of medical education. However, evidence is emerging that its use may improve learning and development of skills and may be a valuable tool in the classroom of the future.
Concerns with Additive Manufacturing Technology There have been some concerns about the accuracy of the three-dimensional models developed from CT scans which may ultimately affect the dimensional measurements of the component produced. A variety of factors can affect the quality of the CT data which directly influences the production of the CAD models, such as pitch, gantry tilt, tube current voltage, and slice thickness. Of these, slice thickness is a prime factor which can lead to a partial volume averaging error (or inter-slice volume averaging effect) from loss of sharp corners or edges during the production of the three-dimensional model. Moreover, a geometric stair-step error can occur, as each layer of the implant manufactured approximates a discrete representation of the cross-sectional geometry. Another source of the error occurs during conversion to the STL format where some loss of resolution occurs, as the format represents only triangles instead of arcs or splines.37 These geometric errors can then be incorporated into the fabricated implant. Although some of these errors can be reduced by reducing the size and increasing the number of the triangular facets, it occurs at the cost of data storage capacity. Nevertheless, the technology is constantly developing and has the potential to make profound impacts on the speed and cost of manufacturing more conforming, complex implant geometries.
Future Outlook
proportion of the overall fabrication of orthopedic implants. Nevertheless, additive manufacturing holds great promise and may have the potential to make a profound impact on the manufacturing of anatomically more conforming and complex implant geometries with graduated porosities at faster rates and lower costs.
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There has been great interest in the use of additive manufacturing technology for the manufacturing of lower extremity total joint arthroplasty implants. A variety of materials and compositions are being used by modern additive manufacturing process to produce implants of varying texture, graduated porosity, and mechanical strength for use in lower extremity total joint arthroplasty. In addition, it is exciting to note that there is potential application of this technology for the manufacturing of patient-specific implants in future.23,38,39 With the development of the additive manufacturing format (AMF), the conventional STL format may be effectively replaced in future. The AMF format allows inclusion of data about surface texture and porosity which may markedly improve the overall accuracy of the technology. This may potentially eliminate some of the inaccuracies of the current generation of additive manufacturing process.
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