Traditional forms of metallic manufacturing by which many orthopaedic implants are created have relied on several processes: forging, welding, milling (subtractive manufacturing), or casting (formative manufacturing). In forging, a metal object is heated to a molten state and then shaped by compressive forces. In welding, a high heat source is similarly used to melt two metal parts together. Both of these techniques were used to create early orthopaedic implants particularly oncologic and hinged total knee prostheses. More modern arthroplasty implants are created through subtractive and formative manufacturing. In subtractive manufacturing, a component is shaped from a larger piece of metal using a cutting instrument such as a burr. Although useful, this technique wastes much of the metal substrate during the subtractive process, and designs may only be so complex. Moreover, milling with a high-speed burr generates heat which creates irregularity in the metal. Alternatively, casting or formative manufacturing creates objects by pouring liquid metal into a mold. Again, the design complexity is limited by the shape of the mold.1,2
In the mid 1970s, the problem of femoral stem loosening in total hip arthroplasty led to the development of noncemented implants which directly incorporated into host bone. This bony incorporation was accomplished by a roughened metallic surface with pore sizes of 100 to 400 µm (see below).3 Options to generate this porous surface topology have relied on postmanufacturing treatments. Bead and fiber mesh applications were two early surface treatments for bone “in-growth” whereby the bead or mesh would be sintered or diffusion bonded to the stem. However, such treatments require extreme heat and can induce fatigue in metal. Bone “on-growth” is another method of generating a roughened surface. This includes techniques such as grit blasting with high pressure aluminum oxide or plasma spray whereby molten metal is applied to a substrate. Although these techniques have been highly successful in creating durable orthopaedic implants, the microscopic detail generated is homogeneous and limited.
Metallic additive manufacturing (AM), also known as metal-based 3D printing, is a new fabrication process, which overcomes many of the problems with milling and casting, and can create geometrically complex monolithic structures. This process allows for the generation of custom metal components with intricate shapes and substructures. The advantages are numerous and include the ability to create custom patient-specific implants with improved chemical and mechanical properties, while reducing the cost of tooling, energy usage, and waste.4,5 There are multiple AM techniques which are differentiated by the type of heat source used to build up the base material, each discussed in this review. In addition, a table of basic terminology has been added for this review (Table 1).
Table 1 -
Additive Manufacturing Basic Terminology
||Process by which metal structures are created in a layered fashion based on a computer-aided design.
||An early surface treatment for bone “in-growth” whereby the bead would be sintered or diffusion bonded to the stem; requires extreme heat.
||A method of generating a roughened surface on an implant. Including such as grit blasting with high-pressure aluminum oxide spray or plasma spray.
|Casting (formative manufacturing)
||Creates objects by pouring liquid metal into a mold and allowing for the creation of uniform components
||3D model generated on a computer.
|Direct metal laser sintering (DMLS)
||A form of direct energy deposition for metallic additive manufacturing which works by using a lower energy laser to melt metal particles together.
|Electron beam melting (EBM)
||A form of direct energy deposition for metallic additive manufacturing which works by using electron beam to melt metal particles together.
|Fiber mesh application
||An early surface treatment for bone “in-growth” whereby the mesh would be sintered or diffusion bonded to the stem; requires extreme heat and can induce fatigue in metal.
||When a metal object is heated to a molten state and then shaped by compressive forces.
|Hot isostatic pressing (HIP)
||Where a heat treatment (800°C for 2 hr) and high pressures are applied to allow the object to solidify.
||Additive nanoparticles which act as a nucleus on which metallic crystals can grow.
|Milling (subtractive manufacturing)
||When component is shaped from a larger piece of metal using a cutting instrument such as a burr.
||Design with a repetitive pattern.
|Selective laser melting (SLM)
||A form of direct energy deposition for metallic additive manufacturing which works by using a high-energy laser to melt metal particles together.
||Design pattern which has random variation.
||When a high heat source is used to melt two metal parts together.
Basics of Additive Manufacturing
The term AM refers to the process of sequentially fusing an alloy feedstock (powder or wire) by a heat source (laser or electron beam) in a layer-by-layer fashion based on a digital model to fabricate an object. In this way, a solid mass with a predetermined design can be generated in an array of shapes ranging from simple to extremely complex. No tooling is necessary as in subtractive manufacturing, and thus, the design complexity is limitless without a concordant increase in cost. AM is particularly powerful at producing complex lattice structures which are not feasible with traditional manufacturing methods. Moreover, lightweight (hollow or honeycomb) structures are easily produced because the components need not be cast from a solid metal. Different structural motifs such as solid cores and porous surfaces may be combined into a single component for increased functionality. The revolution of AM components use in orthopaedic surgery has been driven by technological advances in other industries such as aviation and has allowed the science to expand in a relatively short period.4
AM begins with the blueprint of a computer-aided design (CAD) model. A CAD model may be reverse engineered from human images such as computed tomography (CT) scans and thus allows macroanatomic and microanatomic reproduction. This 3D computer model is then digitally sliced into 2D layers which have micron thickness and serve as the template for each successive layer in the welding process. Depending on the structural geometry, a support lattice is also generated which acts as a platform to joist the component preventing any structural distortion by its own weight and also dissipates heat. This lattice is later removed during postprocessing as described below.
There are several different techniques in metal AM including binder jetting, powder bed fusion, and directed energy deposition. While similar conceptually, each process uses specific raw materials, slice thicknesses, and energy sources and thus have slightly varying applications.6
Powder bed fusion works by using a heat source such as a laser or electron beam to selectively weld metal powder particles (spherical beads approximately 15-45 μm in size) together in a layered fashion. Laser powder bed fusion may be done by either selective laser melting—where a higher energy laser fully melts metal particles together or direct metal laser sintering which uses a lower energy laser to consolidate particles by sintering. The use of an electron beam as the heat source is known as electron beam melting.
The melting process occurs in a build chamber filled with inert gas to prevent the incorporation of unwanted elements and combustion of the metal particles. A thin layer of metal powder is spread over the base of the build chamber on to an area known as the build plate. The laser or electron beam rapidly burns the cross section of the 2D CAD slice into the bed of metal particles. Thus, the CAD section is rasterized into the metal powder. Another coating of powder is spread across the chamber floor, and the process repeats moving to the next slice of the CAD model and building on the previous layer. Powder bed fusion is the most commonly used technique in orthopaedic AM today (Figure 1).
Other AM processes include binder jetting in which the metal powder particles are fused in a targeted fashion according to the CAD model with an adhesive binder material (typically a polymer and wax) creating a “green” component. This component is then sintered to remove the adhesive and create a pure metallic component.
Finally, in direct energy deposition (also known as laser direct metal deposition, a high energy heat source melts the metal powder in an injected stream while simultaneously depositing it in a layered fashion onto the substrate. The metal is thus deposited in the form of a cross-section CAD model. This form of AM is less commonly used in orthopaedics but maybe useful for the repair of implants or on-demand fabrication of structures.7
The most commonly used alloys in AM are titanium alloy (TiAl6V4v) and cobalt chromium alloy because they are the easily weldable. Moreover, they are generally corrosion-resistant, nontoxic, immunologically inert, and noncarcinogenic while maintaining a high degree of wear resistance, hardness, and stiffness.6
Titanium alloy which is difficult to machine with traditional manufacturing methods due to its high tensile strength and low ductility is ideal for AM as it is available in powder form. Moreover, it has a high strength to density ratio and is corrosion resistant secondary to the formation of an oxide layer on its outer surface. In addition, titanium alloy has very low tissue concentrations after implantation because of a low leeching rate.8 In general, components created from these alloys using AM have material properties similar to or stronger than the original alloy.2
There are some technological challenges using other alloys in AM. One is that the layered manufacturing process creates temperature gradients in the metal which can put stress on the alloy. Moreover, certain alloys exist as either a liquid or gas at the temperature required for 3D printing which can result in the formation of cracks. Recently, the use of inoculants (additive nanoparticles which act as a nucleus on which metallic crystals can grow) has been proposed to solve this problem. This advance has the potential for an expanded range of alloys to be used in AM which would allow for the creation of a diverse range of components with varying strengths, toughness, and fatigability9
Finally, almost all 3D-printed structures require some form of postprocessing which includes internal stress relief, removal of the support lattice, washing to remove excess metallic particles, and surface treatment to further alter the surface properties of the object. The first step is generally a heat treatment which helps remove the internal stressors and imperfections which inherently develop as part of the rapid heating and cooling during the manufacturing process. This is particularly important for objects created using SLS which may exhibit notable roughness, heterogeneity, and residual stress and thus have decreased mechanical strength.10 One such stress relief process is hot isostatic pressing where a heat treatment (800°C for 2 hours) and high pressures are applied to allow the object to solidify.
After the heat treatment, the supportive struts must be removed carefully such that no damage is done to the main structure. These struts are generally created in a hollow fashion such that they do not require notable force to remove and can be cut off using a high-speed burr or drill. This is typically done before further postprocessing or sterilization.11
As many 3D-printed products have a fine porous structure, a final cleaning to remove excess base material left in the crevices is essential. Laser-sintered products are generally cleaned with air-blasting followed by sonication in ethanol or a chemical treatment.12,13 Finally, finishes may be applied to the structure to increase porosity or other surface properties, and the product must be sterilized. Currently, the FDA requires the same postmanufacture quality assurance process as any other metallic implant12 (Figure 1).
Specific Advantages of Additive Manufacturing in Orthopaedics
In orthopaedics, there are two categorical advantages for AM-fabricated implants to traditionally manufactured implants. The first is the creation of implants with precisely engineered surface and intrinsic mechanical properties which allow for more rapid and durable microincorporation into host bone. The second is the creation of patient-specific designs which are able to accurately account for bony defects and recreate patient anatomy and mechanics eliminating issues of sizing. Moreover, custom screw placement and trajectories which are preplanned based on patient anatomy are easily incorporated allowing for improved macrofixation.
Surface Technology and Microscopic Implant Design
Implant surface roughness is a critical factor for implant osteointegration into local bone through the osteoblastic response. Microscopic characteristics of the implant surface such as the distance between peaks and curvature of the troughs affect the level of osteoblastic response (differentiation and local factor production).14 A pore size of >100 µm has been shown to enhance bony ingrowth.15 Moreover, appropriately microstructured titanium can induce the differentiation of mesenchymal stem cells into osteoblast lineages.16 Cellular structures may either be stochastic (having random variation) or nonstochastic (having repetition of the same pattern). Nonstochastic structures are preferred in AM because of more uniform mechanical properties and easier removal of unused powder.6 Past methods used to generate surface topology and porosity have relied on chemical or mechanical etching.17 However, these techniques are limited in that they cannot accurately control the shape size or porosity in comparison with AM. Total hip and knee prostheses have attempted to achieve biologically compatible surfaces through porous coating methods such as bead sintering or grit blasting. Although these methods have been very successful in the past, they are not optimized for surface integration as they tend to provide shallow troughs with limited porosity. By contrast, additive manufactured implants may be fashioned to specific surface patterns which more readily stimulate bony ingrowth.18
Even in situations where there is good bony ingrowth of an implant to host bone, differences in the modulus of elasticity between metal implants and native bone result in stress shielding causing loss of cortical bone mass and implant failure or periprosthetic fracture.19 This occurs as the host bone attempts to adapt to the implant through the Wolff law, whereby the portion of bone in contact with the implant experiences less stress and thus undergoes atrophy.20 AM can address this issue by using human trabecular bone as a template to design the entire structure of the prosthesis, more ideally modulating the stiffness to match host bone.21,22 Through AM, engineers are able to use micro-CT scans of trabecular bone to create CAD templates which can be translated into models more biomechanically compatible to human bone. Wieding et al22 showed AM-manufactured titanium scaffolds in various densities had excellent mechanical loading in a large segmental femoral bone defect model. Moreover, Arabnejad et al23 showed that femoral stems created with an optimized microstructure could reduce the amount of stress shielding by 75% compared with a solid implant. Finally, in the spine, topology-optimized lumbar interbody fusion cages may be AM fabricated based on micro-CT scans of cortical cancellous bone to more closely match the modulus host bone and improve incorporation.24
Macroscopic Patient-Specific Implant Design
The creation of custom arthroplasty components is perhaps one of the first instances where AM was used in orthopaedics. The first implanted AM-printed acetabular cup was designed to seamlessly integrate the metal ingrowth outer face with the socket component to theoretically promote efficient load transfer through the component to bone. These cups have demonstrated high survivorship.25,26 In addition, 3D unicompartmental knee prostheses have been created from proprietary CT scanning software which maps the defects in the articular surface of the host to create a design which can correct the mechanical axis of the lower limb.27 Thus, AM is able to create arthroplasty components which not only incorporate multiple functionalities but also recreate normal patient-specific anatomy and mechanical alignment. Although a recent systematic review did not find any added benefit to custom implants in regard to postoperative limb alignment, functionality, or operating room efficiency, this may come to change with improving technology.28,29
3D implants are also useful in instances of large bony defects, particularly in pelvic discontinuity or oncologic procedures.30 Colen et al31 reported on the use of a modified custom-made triflanged acetabular reconstruction ring in 6 patients with severe acetabular bone defects with acceptable to good results in all. None of their reconstructions required revision for failure at a minimum of 10-month follow-up. This device was constructed using data from a 3D CT model and contained a titanium porous structure to encourage bony ingrowth on the surfaces in contact with the native pelvis. In addition, this implant allowed for custom screw trajectories based on preoperative planning. Thus, AM implants are able not only to account for specific anatomic defects but also allow for the strategic deployment of fixation devices.
Current anterior spinal reconstruction implants such as expandable titanium cages and titanium mesh cages may cause end plate fracture and collapse.32,33 AM may overcome these issues as they are able to improve host integration while also closely matching the stiffness of bone (see above). As AM is particularly useful in recreating complex geometries, they are able to be used in areas where current implants are ill suited to replace the local anatomy such as the atlantoaxial region.34,35 In a sheep model, Yang et al36 showed that a Ti6Al4V electron beam melting–fabricated C4 vertebrae facilitated bone ingrowth and maintained cervical spine stability without subsidence. Mobbs et al used AM to create a custom anterior prosthesis for the odontoid and posterior fixation device for the treatment of a chordoma. They were also able to create a unique anterior lumbar interbody device which accounted for a congenital deficiency of the end plate.35 AM is particularly useful for the design of custom complex spinal prostheses where major bone replacement is necessary such as cases of infection and tumor and, thus, may have many future applications in spine surgery.37
Metal AM carry specific dangers which limit its use to industrial locations as opposed to polymer AM which may be done almost anywhere. First, metal powders and lasers may cause damage to the eyes, and thus, chambers must be shielded, and workers must wear protective eyewear. Metallic powders may also injure the respiratory system, and thus, workers handling the raw materials must wear respirators, clothing, and gloves to prevent contact. Furthermore, the metal powers are highly flammable and thus are processed in chambers containing inert gas and by workers wearing antistatic clothing.38
Although a description of the complete federal regulatory process is beyond the scope of this review, in brief to come to market, 3D-printed devices must pass regulatory standards based on the class of device (class I, II, or III). Most implantable devices belong to the class III category. Custom-manufactured prosthetics generally fall under the same regulatory class to similar orthopaedic implants regardless of manufacturing method and are governed by the FDA's Center for Device and Radiological Health. For custom devices, the FDA may offer exemptions under section 520(b) of the Food, Drug, and Cosmetic Act (Fd & C Act) which allows for the distribution of devices which are “not generally available,” “modified to fit the order of the physician,” and related to “special need” or for “sufficiently rare conditions or unique pathology.”39 Under this program called the “humanitarian Use Device” program, inventors are encouraged to develop products to treat rare conditions (<4,000 cases in the United States per year), although they must still meet the rigorous standards set forth for traditional devices. For example, devices must still meet requirements for Quality System Regulation, Medical Device Reporting, Labeling Corrections, and Removal and Registration and Listing. Through this program, the inventor must submit information on the rationale behind the device and its intended use and are limited to the use of “no more than 5 units per year.”12 Moreover, these devices may not be marketed to the general public. Currently, the software used to design patient-specific implants falls under the regulatory umbrella of the FDA, and the manufacturing systems and facilities used to produce additive products must be validated by the FDA. In 2014, the FDA held a public working group that summarized best practices for quality control and safety in AM.40 In summary, those who chose to manufacture high-risk devices are held to the same standards as every other manufacturer of metallic devices.
AM is an exciting technology which is primed to revolutionize patient-specific orthopaedic implants. Through AM, we are not only able to create implants with improved surface and mechanical properties ideally suited for osseous integration and mechanical loading but also able to design prosthesis which match patient anatomy perfectly. AM allows the use of creative fixation strategies which overcome bony defects and improve fit in areas where traditional implants are ill suited. Thus, AM will allow the orthopaedic surgeon to expand the range of pathology which current implants are unable to treat.
References printed in bold type are those published within the past 5 years.
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