Abstract
-
Three-dimensional printing is a rapidly growing manufacturing method for orthopaedic implants and it is currently thriving in several other engineering industries. It enables the variation of implant design and the construction of complex structures which can be exploited in orthopaedics and other medical sectors.
-
In this review, we develop the vocabulary to characterise 3D printing in orthopaedics from terms defined by industries employing 3D printing, and by fully examining a 3D-printed off-the-shelf acetabular cup (Fig. 1). This is a commonly used 3D-printed implant in orthopaedics, and it exhibits a range of prominent features brought about by 3D printing.
-
The key features and defects of the porous and dense regions of the implant are clarified and discussed in depth to determine reliable definitions and a common understanding of characteristics of 3D printing between engineers and medical experts in orthopaedics.
-
Despite the extensive list of terminology derived here, it is clear significant gaps exist in the knowledge of this field. Therefore, it is necessary for continued investigations of unused implants, but perhaps more significantly, examining those in vivo and retrieved to understand their long-term impact on patients and the effects of certain features (e.g. surface-adhered particles).
-
Analyses of this kind will establish an understanding of 3D printing in orthopaedics and additionally it will help to update the regulatory approach to this new technology.
Introduction
Three-dimensional printing (also known as additive manufacturing (AM)) is rapidly being adopted in orthopaedic implant manufacturing; this market is projected to be worth approximately £3 billion by 2027 (1). The potential clinical benefits include the ability to manufacture intricate shapes to fit complex bony anatomies and more refined control over surface porosity for bone ingrowth (2).
Two prevalent methods for 3D-printed implants as defined by ISO/ASTM are laser powder bed fusion (PBF) using a laser beam (LB) or an electron beam (EB) (3, 4, 5). PBF utilises metal powders and powder beds to construct the part layer by layer, where thermal energy selectively fuses the powder particles using a laser or electron beam (3, 6). In engineering sectors, these methods are commonly referred to as selective laser melting (SLM) and electron beam melting (EBM) (5), respectively. They give rise to new features of the implant both at the surface, such as complex porous layers (6), and in the dense regions where defects may occur if powder particle consolidation is insufficient (7). These and other features will exhibit different characteristics depending on the 3D-printing method used; however, currently there is no universal agreement as to how these should be described.
The aim of this review is to describe the features of 3D-printed implants. Key terminology for characterisation of these features in orthopaedics will be defined, learning from literature regarding various industries that are successfully implementing this technique, presenting this paper as a resource for those involved in this field going forward. To achieve this, a 3D-printed acetabular cup will be analysed (Fig. 1). This implant displays a wide range of features, therefore allowing clear definitions, and a sufficient characterisation and understanding of the features, as well as identifying areas for further investigation. This type of implant has been selected as a suitable example as acetabular cups are the most commonly used type of 3D-printed implant in orthopaedics.
A selection of 3D-printed custom and off-the-shelf acetabular cups, from a range of manufacturers and 3D-printing methods. Certain features of the cups can start to be considered, such as the locations of the porous regions and the different types of porous structures.
Citation: EFORT Open Reviews 9, 9; 10.1530/EOR-23-0182
Features of 3D-printed implants
A key advantage of manufacturing implants via 3D-printing is the ability to incorporate areas of complex geometries during the construction of the implant (8). In an acetabular cup, this capability is utilised on the outer bone-facing side where a layer with a highly porous structure is fabricated (2). Therefore, this implant has both a porous and a dense region, where different features and defects exist.
Several industries utilising 3D printing have some defined and accepted terms, which will be applied to the context of orthopaedics to describe the features of a 3D-printed acetabular cup. However, given this is still a new technology across all sectors, there may be aspects known to other industries, aspects known to orthopaedics, and those which are completely unknown to all sectors and require investigation. For the same reasons, conflicting definitions exist for certain features between industries, and so definitions of correct terminology for use in orthopaedics will be clarified in this paper. The features that will be explored in this review are set out in Table 1.
A summary of the features of a 3D-printed acetabular cup.
| Region | Feature | Brief physical description |
|---|---|---|
| Porous layer | Porosity | The total volume of free space in the porous region of the implant (9). Often determined by both the strut thickness and pore size of the porous structure (Fig. 2) |
| Strut thickness | The thickness of the connecting structures in the porous region of the implant (11) (Fig. 2) | |
| Pore size | Often referred to in terms of the diameter of the pores within the porous region of the implant (9, 11) (Fig. 2) | |
| Surface adhered particles | These are partially consolidated powder particles that are often found attached to the struts of the porous structure (2). ( Figs 5 and 6) | |
| Dense region | Diameter and wall thickness | Internal diameter of the acetabular cup. The total thickness of the wall, the sum of the thickness of the dense and porous regions of the implant (Fig. 7) |
| Surface area and volume | The surface area of the internal surface of the implant. The total surface area (or volume) of the implant refers to the sum of the area (or volume) of the internal surface in addition to the surface area (or volume) of the struts of the porous region. | |
| Internal surface roughness and roundness | The roughness and roundness of the surface of the implant that will be in contact with the liner (Fig. 8) | |
| Voids | Defects that exist within the dense region of the implant. Cavities that have the potential to compromise the mechanical properties of the implant (13) (Fig. 9) |
Porosity and features of the porous layer
Porosity
Porosity is the percentage of void spaces over the total volume (9). In the case of medical implants and orthopaedics, porosity helps to characterise the highly porous outer layer of an implant (e.g. acetabular cup (2)). This layer is often made up of a complex lattice with a regular or irregular structure to facilitate bone ingrowth (2), and it simulates the open-porous structure and mechanical properties of human trabecular bone. This decreases the risk of problems in vivo including stress shielding, by significantly reducing the disparity in stiffness values between the bone tissue and implant (2, 9).
The porosity of human trabecular (or cancellous) bone is in the region of 50–90%, with pore sizes around 300–500 μm and strut thickness in the order of hundreds of microns (9, 10). Therefore, the level of porosity of the porous layer is dependent on structural dimensions such as pore size and strut thickness (11), as indicated in Fig. 2.
(A) A 3D-printed off-the-shelf acetabular cup. An area of the porous layer has been enlarged, displaying its structure. (i) Pore size and (ii) strut thickness are indicated. The diameter of the pore is calculated as the diameter of a circle of equal area to the shape indicated in red. (B) A mesh structure from a 3D-printed cup rendered in analysis software (Simpleware, Synopsys, Exeter, UK), where the colours indicate variability in strut thickness in the porous layer.
Citation: EFORT Open Reviews 9, 9; 10.1530/EOR-23-0182
Many different 3D-printed acetabular cup designs are currently commercially available from various manufacturers, and therefore a range of porous lattice structures exist. The aim is to manufacture an open-pore (lattice or cellular) structure to mimic that of highly porous cancellous bone and promote osseointegration, to prevent loosening of the implant later in service (2).
The efficacy of the porous layer for fixation is significantly influenced by its geometry (hexagonal, cuboidal, cylindrical, etc.), which depends on the fabrication approach and design of the implant. When considering the design of the porous layer, a compromise is made to maintain the mechanical strength of the structure whilst still providing a suitable pore size for bone ingrowth (2). This leads to a range of different porous architectures amongst implant manufacturers and designers. Using 3D printing facilitates the production of any kind of porous shape, both regular (repeat unit cells) and irregular (random structure). Repeat unit cells tend to have a specific geometry (hexagonal or cuboidal, etc.) that is repeated and remains unchanged throughout the structure. Often, CAD tools are used in combination with computed tomography (CT) or magnetic resonance imaging (MRI) and mathematical modelling to determine the pore shape and optimise the topology (2). Some porous structures from a variety of manufacturers can be observed in Fig. 3.
(A) SEM images of a regular or cellular and an irregular 3D-printed porous structure and the corresponding manufacture method (electron beam melting (EBM) and selective laser melting (SLM)). (B) Computer-rendered meshes formed using micro-CT data from 3D-printed acetabular cups from four separate manufacturers with varying porous structures. Comparisons between porous structures available from different manufacturers can be made.
Citation: EFORT Open Reviews 9, 9; 10.1530/EOR-23-0182
In other industries, such as aerospace, the term ‘porosity’ refers to an unwanted defect in a dense component that is often produced by 3D printing (12). This is similar to what is termed a void in orthopaedics and is also a defect found in the dense region of the implant (13). In orthopaedics, the definition of porosity helps characterise the porous coating of an implant and is a feature that is intentional and beneficial for bone fixation (14), versus being considered an undesirable defect in the dense region (a void).
Strut thickness
Strut thickness is the dimension of the single metal parts that form the framework of the porous layer (11). This feature is limited in its dimension by the particular 3D-printing technique that is used and its respective minimum feature size. Defining the thickness of the struts is influenced by the trabecular (strut) thickness of cancellous bone, which is in the range of hundreds of microns (11). It has been reported that the minimum feature size for SLM parts is in the range of 40–200 µm, and approximately 0.5 mm for EBM (11), and changing this dimension influences the level of intricacy and complexity of the structure of the porous coating, which in turn will impact the extent of osseointegration into the porous layer. This contrast in strut thickness can be observed in Fig. 3, where two implants have been manufactured using SLM and EBM, respectively. Additionally, the difference in the surface features of the internal struts observed in Fig. 3 (surface-adhered particles) could also raise the question of which process is more suitable for this application.
Pore size
Pore size, like the level of porosity, represents the available space for bone ingrowth into the porous layer and is characterised by calculating the equivalent diameter of the pores (9, 11). Similarly, with strut thickness, pore size is influenced by the manufacturing method used, and an increased pore size results in a higher porosity or level of porosity in the porous layer. This parameter is critical in effective bone ingrowth, and the dimensions of this feature in 3D-printed parts are influenced by the pore sizes found in cancellous bone (300–500 μm (10)). Pore size and the interconnectivity of the pores directly affect the biological performance, including adhesion, osteogenic behaviour, and ultimately the fixation and improved ability of the implant (11). This is because pore size determines the type of cell that will proliferate and colonise within the porous structure.
Current technologies apply pore sizes of approximately 100–500 μm, consistent with pore sizes found in trabecular bone and with positive results of in vivo studies to date using these dimensions (11). 3D printing is beneficial in providing manufacturers with some control over the pore size of the lattice structure, particularly as the optimal pore size is yet to be defined (15).
There are several ways of characterising pore sizes in porous structures that have been discussed, depending on the regularity of the structure and the size of the pores. Going forward, a method involving fitting and modelling a sphere of the largest diameter in the pore of interest will be adopted when characterising pores in regular structures (16) (Fig. 4). Then, completing this for a sufficient number of pores (~10) in the porous layer across several different regions will allow computation of values and for comparison to determine if variability of pore size exists between different regions of the porous layer. The pore size measurements obtained are then averaged, and this is often the value (or range of values) for pore size that is quoted by manufacturers in marketing material (17, 18). It should be clarified that this method can be applied to pores in regular lattice structures with repeating unit cells, but a suitable method to examine pores in irregular structures is yet to be determined.
(A) A render of a custom 3D-printed acetabular cup created using Micro-CT data and imported into analysis software (Simpleware, Synopsys, Exeter, UK). From this data, (B) the porous layer is examined and isolated, and (C) a mesh model of the structure is generated. (D) A single mesh unit can then be extracted, followed by (E) best fit modelling with a sphere, to assist in calculating the porosity of the porous layer.
Citation: EFORT Open Reviews 9, 9; 10.1530/EOR-23-0182
Surface-adhered particles
Surface-adhered particles (SAP) are a by-product of metal powder-based 3D printing processes and are defined as solid metal powder particles that are partially fused to the surface of the as-built part (2). The extent of their attachment and their frequency depend on the type of 3D printing method used and its respective processing parameters.
In Fig. 5, surface-adhered powder particles can be observed on the struts of the lattice structures produced by several manufacturers using different 3D printing methods, depicting the variability of porous structures currently available. This is a common result of powder-based 3D printing methods, and these particles are easily peeled off after implantation (19), which poses many doubts about their effects in the human environment. In addition to the processing parameters, other factors affect their presence such as the powder particle size and the type and angle of the heat source (20), explaining the variability between different manufacturers and designs of implants.
A panel of SEM images depicts surface-adhered particles (indicated) in the porous layer of 3D-printed acetabular cups, and the variability of the particles with 3D-printing methods; electron beam melting (EBM) and selective laser melting (SLM). All images at ×200 magnification.
Citation: EFORT Open Reviews 9, 9; 10.1530/EOR-23-0182
There is some confusion surrounding the definition of surface-adhered powder particles and how they should be referred to. In some literature, they are termed partially melted particles, unmelted particles (19, 20) or non-melted powder grains (21), but these phrases can be easily misunderstood for loose unmelted metal powder particles, which is a separate defect. Unmelted particles are free, loose starting powder that has been trapped in the lattice structure or porosity. This concept should not be confused with SAP, which are fixed to the structure and require chemical or mechanical post-processing for removal. Also, the interchangeability of the term powder particle with ‘bead’ introduces ambiguity (11, 22), as this could imply a single powder particle or multiple particles that have formed an agglomeration, and so should be avoided.
SAP vary in their appearance between more consolidated, hemispherical particles to more complete particles that are only just attached to the strut surface. The extent of consolidation depends on the 3D-printing method used, its respective particle size, and the processing conditions. These extremes can be observed in Fig. 6.
SEM micrographs showing the extremes of surface-adhered powder particles within the porous layer: (A) Almost completely consolidated particles protruding from the strut surface compared with (B) almost completely unmelted particles only just attached to the strut surface. Images at 200× magnification.
Citation: EFORT Open Reviews 9, 9; 10.1530/EOR-23-0182
A recent study reported that there is increased bacterial growth when SAP are present in a porous structure, constructed by SLM. Adhesion of bacteria is affected by the chemical and physical properties of the implant material and is a significant concern surrounding orthopaedic implants. It has been established that 3D-printed porous implants provide improved bone ingrowth, but this study investigated the impact of surface-adhered particles present within the porous structure (23). It has been suggested that a surface with topological dimensions similar to that of bacteria provides an ideal contact area on the implant, leading to increased bacterial adhesion (24). Bacterial adhesion was therefore increased in the porous samples with SAP (versus treated non-particle samples), which simultaneously inhibited osteogenic activity and adhesion as a result. This, therefore, indicates that SAP have an inhibitive effect on osseointegration. This also introduces the possibility that these particles could induce inflammation in the periprosthetic tissue, which could lead to bone resorption and ultimately aseptic loosening of the implant. Additionally, this study concluded that in order to prevent implant-related infection, the depth of the porous layer should not exceed the point of bone ingrowth, as bone integration must occur to prevent bacterial adhesion (23). This observation could assist in determining the ideal depth of the porous layer, which is yet to be defined.
This study further demonstrates the necessity of post-processing to remove SAP. A homogeneous surface is difficult to achieve by line-of-sight mechanical methods or heat-related post-processing due to the intricacy of the porous structure and the requirement to maintain the porosity (versus removal) (25, 26). Therefore, methods such as chemical etching or electrochemical polishing have been examined for the removal of SAP from the porous layer (19, 25), as these methods can penetrate the structure through the interconnected pores (21).
Chemical etching has indicated effective removal in experimental settings on similar porous structures to that of a 3D-printed acetabular cup (e.g. stents) (19, 25). With an increase in the length of time, the size and number of particles removed increased (19). This process has also been combined with electrochemical polishing and demonstrated promising results in reducing surface roughness and the presence of SAP on the struts of the structure (21). However, care must be taken with these techniques, as prolonged etching or polishing can decrease the strut thickness, potentially compromising the mechanical properties (21).
The dense region
This region of the implant is most similar to the features described in the literature relating to 3D printing in other sectors, as the dense section of the implant has similar requirements to 3D-printed components for other industries such as aerospace (27). These consist of producing a fully dense component with minimal post-processing, and comparable useful properties to traditional manufacturing methods (27).
Diameter and wall thickness
The diameter and wall thickness of this region are indicated in Fig. 7. The diameter is often engraved on the acetabular cup, and both dimensions can be determined by micro-computed tomography (micro-CT) scanning. The total thickness of the implant can also be determined using a coordinate measuring machine (CMM).
A 3D-printed implant (e.g. (A) an acetabular cup). A micro-CT scan of the implant can assist in determining (B) the diameter of the implant and provide (C) isolated slices of the internal structure for measurement of (D) the thickness of the porous region, (E) the dense region, and (F) the total thickness.
Citation: EFORT Open Reviews 9, 9; 10.1530/EOR-23-0182
Three-dimensional printing also allows for the design of implants to reflect more closely the original biomechanics of the specific joint. In the case of both off-the-shelf and custom acetabular implants, this method allows a thinner wall thickness for the same specific cup diameter, sparing additional bone stock and accommodating a larger femoral head, and closely aligning to the biomechanics of the hip as a result (10).
However, a smaller wall thickness raises the question of the mechanical integrity of the implant, and if deformation is more likely, additionally, with a component that has known evidence of voids.
Surface area and volume
A recent study using morphometric analysis found that the total surface area of 3D-printed cups is greater than the conventionally manufactured equivalent (15), which could be explained by the high surface area of the porous regions. Both total surface area and volume of the implant can be characterised by micro-CT, combined with software packages such as Simpleware (Synopsys, Exeter, UK) and Vision Graphics (Heidelberg, Germany). This technique could also help quantify the metal surface area that patients are exposed to and help evaluate how much metal is used for the print.
Internal surface roundness and roughness
These features are important to consider as they influence the seating of the cup liner (9). They are determined by the particular 3D printing process used, resulting in dimensional and topological differences. However, this variability is often eradicated through post-manufacture machining of the internal cup surface, which is completed to avoid dimensional mismatches with the corresponding liner. Incorrect seating of the liner in the cup can have adverse effects when implanted, such as accelerated wear or fracture (9).
With several 3D printing processes, the layer-by-layer printing approach leads to variability in the surface roughness, where a larger layer thickness can result in an uneven curvature of the part. This build error is sometimes referred to as the ‘stair-stepping’ or ‘staircase’ effect, and a recent study has found that inclined surfaces (e.g. the internal surface of a cup) are more susceptible (28). Also, as previously discussed, powder-based 3D-printing methods leave an inherently poor surface finish due to SAP that remains on the surface, and both errors are usually rectified through post-processing (29).
For high-quality 3D-printed components, surface roughness has been specified to be < 1 μm (30). This is to prevent reducing the contact area between the liner and the cup, which can cause increased mechanical wear and corrosion (9). The roughness of the internal surface of an acetabular cup is characterised by a surface profilometer, and roundness is evaluated using a coordinate measuring machine (CMM) (Fig. 8).
(A) Obtaining a roundness measurement using a coordinate measuring machine (CMM). (B) Obtaining a measurement of surface roughness using a surface profilometer.
Citation: EFORT Open Reviews 9, 9; 10.1530/EOR-23-0182
Voids
Structural voids are a key defect that can be found in the dense region of the implant. They can be observed in Fig. 9 and are a flaw in the 3D-printing process compared to conventional manufacturing. They are cavities that act as stress concentration sites and compromise the mechanical and fatigue properties of the part (12). In a recent study, it was found that voids usually occurred in areas of the implant transitioning between design features, for example, a body–flange connection point in a custom acetabular cup, indicating variability in print quality (13). It should be acknowledged that in other industries (e.g. aerospace) this feature is often referred to as a ‘pore’ or ‘porosity’ instead of a void (31), and also as a structural cavity (22). Going forward, this feature will be termed a void.
(A) Image generated from micro-CT data to show voids in the dense region of a 3D-printed acetabular cup. From these images, void location and frequency can be analysed. This will be followed by (B) void size and shape evaluation.
Citation: EFORT Open Reviews 9, 9; 10.1530/EOR-23-0182
Voids can form during the manufacture of a 3D-printed part. This could be due to poor-quality powder feedstock, inadequate processing conditions, or gas in the build chamber that becomes trapped in the melt pool, resulting in spherical voids (22, 30). Lack of fusion, where the combination of parameters has insufficient energy to consolidate the powder particles, is also a possible source of voids that are more elongated and irregular in shape (2). Conversely, where the parameters generate excessive energy, keyholing (i.e. vaporisation of the material) can occur, leaving behind almost-spherical voids in the final part (2).
The impact of these voids is significant on the mechanical properties of the dense 3D-printed part, and there are factors that contribute to the magnitude of their effect. Voids concentrate stress under loading, leading to premature failure, particularly under fatigue conditions (31). In order of descending importance, the location, size, and shape of the voids impact the fatigue life of the respective component (13).
The location of the void is the most influential factor in crack initiation. It was found that voids located within 0.4 mm of the surface acted as crack initiation sites. The secondary factor is the size of the void, where, unsurprisingly, a larger void (> 0.5 mm) increases the risk of fracture and reduces the fatigue properties of the implant. Finally, the shape or sphericity also influences the properties, where the more irregular-shaped voids were likely stress risers, compromising the mechanical performance compared to smaller and more spherical voids (13).
Therefore, voids become most concerning when they are large, irregularly shaped, and close to the surface, alongside sufficient cyclic loading to exceed the fatigue limit of the implant (13). This type of loading is often experienced by components in aerospace, which forms the basis of many investigations into 3D-printed parts. The lower level of cyclic fatigue loading in biomedical applications helps explain the absence of fractures in 3D-printed implants, despite the discovery of voids in the dense regions (13). It should be considered that in some instances voids can become interconnected, creating a major defect with potentially detrimental effects on the mechanical properties, particularly where the structural integrity of the implant is significant, for example, at the body–flange connection.
Therefore, measures are taken during manufacture for the prevention of voids in the final part, such as optimising the 3D printing process parameters (12). Post-processing methods have also been investigated in attempts to close up voids, such as heat treatments and hot isostatic pressing (HIP) (32). It has been reported that employing HIP can effectively reduce the size of the void and also that parts made by SLM can reach fatigue strengths comparable to conventionally processed titanium alloys (31).
Discussion
The features of a 3D-printed implant have been defined by combining knowledge from several industries and existing literature in orthopaedics. However, many areas which require further investigation have been highlighted by this review, such as the long-term clinical impact of these implants. Such investigations could also resolve the question of whether SLM or EBM is more suitable for manufacturing 3D-printed implants and if this should be recommended to manufacturers from a clinical safety perspective.
Several regulatory bodies, including the U.S. Food and Drug Administration (FDA) (33), and the Medicines and Healthcare products Regulatory Agency (MHRA) (34), have identified ISO/ASTM standards that should be followed by manufacturers when producing 3D-printed implants and instruments (3, 35, 36, 37, 38). Due to the novelty of 3D printing and, more importantly using metal source powder, there is a lag in regulation. This is apparent in the guidance provided, which sets out recommendations for manufacture and testing irrespective of material (plastic vs metal) and device type (surgical guide vs implant) (39). When discussing post-processing, the methods suggested are unsuitable for the porous characteristics produced by SLM and EBM and lack the detail to sufficiently guide manufacturers at this stage of production.
In the most recent guidance on Technical Consideration for AM Medical Devices, published in 2017 by the FDA, it states that ‘it is anticipated that AM devices will generally follow the same regulatory requirements and submission expectations as the classification and/or regulation to which non-AM device of the same type is subject’ (39). Over 7 years later, there has been no significant update in this advice.
Similarly, the guidance provided by the EU Medical Device Regulations (MDR) has also been criticised for its lack of clarity and harshness. A survey involving employees of medical device manufacturers in Germany was conducted by the EU MDR, before and after changes to reflect 3D printing in medical devices (in 2021 and 2023, respectively) (40). The results of these surveys have recently been scrutinised as they indicate that the respondents’ knowledge surrounding these MDR changes had not improved. Additionally, several companies do not view these changes as an improvement, and the over-administration is leading to the downsizing of product portfolios and withdrawals of devices from the EU market (41). Looking forward, it is possible this approach to MDR could stunt competitiveness and ultimately innovation within medical devices.
From these findings, the advice provided remains insufficient and requires updating to encompass the types of 3D-printed devices and processes available. To help close this regulation gap, the ASTM F42 Committee meetings, discussing advances in Additive Manufacturing (AM) Technologies, are held biannually to revise existing standards and develop new guidance (42).
The 3D-printing method is also challenging from a regulatory perspective because it introduces the flexibility to adapt the initial design file for customisation. As a result, manufacturers can easily incorporate small design changes, making it difficult for regulators to manage, and so often 3D-printed customised designs require approval on a case-by-case basis. By comparison, the standard manufacture of conventionally cast components ensures repeatability of part quality and consistency with designs and devices, using a well-accepted manufacturing method, and ultimately allowing for uncomplicated regulation. This is a key advantage of standard manufacturing processes versus 3D printing and is appreciated by regulators (10).
Many analysis techniques have also been investigated for evaluating the features of 3D-printed implants. These methods are used depending on the type, size, and location of the feature in the implant. A challenge with certain techniques is the inability to sufficiently characterise a surface due to issues with ‘line-of-sight’ (e.g. SEM and CMM). This was discussed by Carter et al. (43), identifying limitations of these methods, including the failure to quantify regions on the surface of a 3D-printed part that are hidden by overhangs and SAP. This is specifically applicable to 3D-printed orthopaedic implants due to the intricacy of the porous layer, which necessitates a technique that can analyse the topology of internal struts (e.g. the presence of SAP) as well as the visible top surface, and, if possible internal features or defects (e.g. voids). This review demonstrates the need for the development of these techniques and, for the same reasons, highlights the challenges faced by post-processing methods for SAP removal.
Micro-CT has been identified as a promising technique to overcome some of these challenges. It has demonstrated reliability when evaluating both the dense and porous regions of orthopaedic implants, characterising features which would be difficult to observe via methods relying on ‘line-of-sight’, whilst also preventing destructive testing (44).
To build on these terms, we should consider the features of retrieved implants in addition to an unused implant (Fig. 10). These could include the depth of bone ingrowth into the porous layer, the total bone contact area, the wear or reduction in thickness of the porous layer, and other features resulting from contact in vivo. Correct identification and definitions of such features will require further investigation, as well as the techniques by which they should be evaluated. SAP on the struts of the porous layer presents the potential issue of an increased surface area of contact in vivo, as well as the possibility of particles breaking off into the patient. Dedicated analysis of the features of retrieved implants and acknowledging the effects of SAP could provide an explanation for elevated blood titanium levels in patients with 3D-printed acetabular cups when compared to conventional implants (45, 46). Additional studies could include observing levels of blood titanium and other elements in vivo to monitor the condition of the implant and if higher levels might indicate particle breakages or increased diffusion due to a higher surface area, and any associated risks to patient safety.
(A) A retrieved 3D-printed custom acetabular cup. (B) A retrieved 3D-printed off-the-shelf acetabular cup. (C) An SEM image of the surface of a retrieved 3D-printed implant where tissue has integrated into the porous structure. (D) An image of the surface of a retrieved implant indicating (i) strut thickness, (ii) pore size, and (iii) strut separation; features that can be compared with an unused implant.
Citation: EFORT Open Reviews 9, 9; 10.1530/EOR-23-0182
Another important definition to consider in orthopaedics is that of a defect. It is often described as an imperfection whose size, shape, orientation, or location could cause the failure of the part in which it occurs (47). For orthopaedics, in addition to this definition, a defect is an adverse feature present in the final part not by design or intention, such as voids in the dense region or SAP in the porous region.
Longevity of these implants is another area of interest, and currently, they are exhibiting positive results. Several 3D-printed off-the-shelf acetabular cups, including the Lima Delta TT and Stryker Trident II, have been successfully implanted for several years with an ODEP (Orthopaedic Data Evaluation Panel) rating range of 3A–10A* (48). The ‘A’ indicates high strength evidence of the implant collected over a number of years (3–9), with the star denoting a < 5% revision rate at 10 years (49). As the choice of this type of implant becomes more widespread, increased clinical monitoring and data surrounding their success and endurance are required.
Therefore, continued examination of both unused and retrieved implants, alongside long-term surveillance of implanted devices, will ensure an up-to-date understanding of 3D printing in orthopaedics.
Conclusion
Three-dimensional printing in orthopaedics will continue to grow with the development of off-the-shelf as well as custom orthopaedic implants. By using the example of a 3D-printed acetabular cup, we have determined an extensive list of terminology, which can also be used to describe features observed in other 3D-printed orthopaedic implants. In addition, where certain terms have conflicting meanings in other industries, clarification has been provided. Whilst the aim of this review is to serve as a resource for key terms of features in a new 3D-printed orthopaedic implant, we have also been able to identify significant gaps that require further investigation for a more complete understanding of this advancing technology.
ICMJE Conflict of Interest Statement
The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the study reported.
Funding Statement
This work did not receive any specific grant from any funding agency in the public, commercial, or not-for-profit sector.
Author contribution statement
Conceptualisation: AN, HH, JH, ADL, KS, AH; writing – original draft preparation: AN; writing – review and editing: HH, JH, ADL, KS, AH.
References
- 1↑
Application Spotlight: 3D Printing for Medical Implants - AMFG n.d. Available at: (https://amfg.ai/2019/08/15/application-spotlight-3d-printing-for-medical-implants/). Accessed on June 23, 2023.
- 2↑
Dall’Ava L, Hothi H, Di Laura A, Henckel J, & Hart A. 3D printed acetabular cups for total hip arthroplasty: a review article. Metals 2019 9. (https://doi.org/10.3390/met9070729)
- 3↑
ISO/ASTM 52900:2021 - Additive Manufacturing — General Principles — Fundamentals and Vocabulary n.d. (Available at: https://www.iso.org/standard/74514.html). Accessed on January 17, 2024.
- 4↑
Alexander AE, Wake N, Chepelev L, Brantner P, Ryan J, & Wang KC. A guideline for 3D printing terminology in biomedical research utilizing ISO/ASTM standards. 3D Printing in Medicine 2021 7 1–6. (https://doi.org/10.1186/S41205-021-00098-5)
- 5↑
Ginestra P, Ferraro RM, Zohar-Hauber K, Abeni A, Giliani S, & Ceretti E. Selective laser melting and electron beam melting of Ti6Al4V for orthopedic applications: a comparative study on the applied building direction. Materials 2020 13 5584. (https://doi.org/10.3390/ma13235584)
- 6↑
Popov VV, Muller-Kamskii G, Kovalevsky A, Dzhenzhera G, Strokin E, Kolomiets A, & Ramon J. Design and 3D-printing of titanium bone implants: brief review of approach and clinical cases. Biomedical Engineering Letters 2018 8 337–344. (https://doi.org/10.1007/s13534-018-0080-5)
- 7↑
Mukherjee T, & DebRoy T. Mitigation of lack of fusion defects in powder bed fusion additive manufacturing. Journal of Manufacturing Processes 2018 36 442–449. (https://doi.org/10.1016/j.jmapro.2018.10.028)
- 8↑
Dutta B, & Froes FH. The additive manufacturing (AM) of titanium alloys. Metal Powder Report 2017 72 96–106. (https://doi.org/10.1016/j.mprp.2016.12.062)
- 9↑
Dall’Ava L, Hothi H, Henckel J, Di Laura A, Shearing P, & Hart A. Comparative analysis of current 3D printed acetabular titanium implants. 3D Printing in Medicine 2019 5 15. (https://doi.org/10.1186/s41205-019-0052-0)
- 10↑
Jiao J, Hong Q, Zhang D, Wang M, Tang H, Yang J, Qu X, & Yue B. Influence of porosity on osteogenesis, bone growth and osteointegration in trabecular tantalum scaffolds fabricated by additive manufacturing. Frontiers in Bioengineering and Biotechnology 2023 11 1117954. (https://doi.org/10.3389/fbioe.2023.1117954)
- 11↑
Dall’Ava L, Hothi H, Henckel J, Di Laura A, Shearing P, & Hart A. Characterization of dimensional, morphological and morphometric features of retrieved 3D-printed acetabular cups for hip arthroplasty. Journal of Orthopaedic Surgery and Research 2020 15 157. (https://doi.org/10.1186/s13018-020-01665-y)
- 12↑
Blakey-Milner B, Gradl P, Snedden G, Brooks M, Pitot J, Lopez E, Leary M, Berto F, & du Plessis A. Metal additive manufacturing in aerospace: a review. Materials and Design 2021 209 110008. (https://doi.org/10.1016/j.matdes.2021.110008)
- 13↑
Hothi H, Henckel J, Bergiers S, Di Laura A, Schlueter-Brust K, & Hart A. The analysis of defects in custom 3D-printed acetabular cups: a comparative study of commercially available implants from six manufacturers. Journal of Orthopaedic Research 2023 41 1505–1516. (https://doi.org/10.1002/jor.25483)
- 14↑
Dall’ava L, Hothi H, Henckel J, Laura Di A, Tirabosco R, Eskelinen A, Skinner J, & Hart A. Osseointegration of retrieved 3D-printed, off-the-shelf acetabular implants. Bone and Joint Research 2021 10 388–400. (https://doi.org/10.1302/2046-3758.107.bjr-2020-0462.R1)
- 15↑
Hothi H, Henckel J, Bergiers S, Di Laura A, Schlueter-Brust K, & Hart A. Morphometric analysis of patient-specific 3D-printed acetabular cups: a comparative study of commercially available implants from 6 manufacturers. 3D Printing in Medicine 2022 8 33. (https://doi.org/10.1186/s41205-022-00160-w)
- 16↑
Liu H, Liu L-L, Tan J-H, Yan Y-G, & Xue J-B. Definition of pore size in 3D-printed porous implants: a review. ChemBioEng Reviews 2023 10 167–173. (https://doi.org/10.1002/CBEN.202200043)
- 17↑
Additive manufacturing/EPORE® n.d. Available at: (https://www.implantcast.de/en/company/technology/additive-manufacturing-/-eporer/#c7267). Accessed on July 4, 2023.
- 18↑
Acetabular system | Stryker n.d. Available at: (https://www.stryker.com/us/en/joint-replacement/products/trident-ii.html). Accessed on June 30, 2023.
- 19↑
Song P, Hu C, Pei X, Sun J, Sun H, Wu L, Jiang Q, Fan H, Yang B, Zhou C, et al. Dual modulation of crystallinity and macro-/microstructures of 3D printed porous titanium implants to enhance stability and osseointegration. Journal of Materials Chemistry. B 2019 7 2865–2877. (https://doi.org/10.1039/C9TB00093C)
- 20↑
Chen C, Hao Y, Bai X, Ni J, Chung SM, Liu F, & Lee IS. 3D printed porous Ti6Al4V cage: effects of additive angle on surface properties and biocompatibility; bone ingrowth in Beagle tibia model. Materials and Design 2019 175 107824. (https://doi.org/10.1016/j.matdes.2019.107824)
- 21↑
Pyka G, Burakowski A, Kerckhofs G, Moesen M, Van Bael S, Schrooten J, & Wevers M. Surface modification of Ti6Al4V open porous structures produced by additive manufacturing. Advanced Engineering Materials 2012 14 363–370. (https://doi.org/10.1002/adem.201100344)
- 22↑
Hothi H, Dall’Ava L, Henckel J, Di Laura A, Iacoviello F, Shearing P, & Hart A. Evidence of structural cavities in 3D printed acetabular cups for total hip arthroplasty. Journal of Biomedical Materials Research. Part B, Applied Biomaterials 2020 108 1779–1789. (https://doi.org/10.1002/jbm.b.34520)
- 23↑
Xie K, Guo Y, Zhao S, Wang L, Wu J, Tan J, Yang Y, Wu W, Jiang W, & Hao Y. Partially melted Ti6Al4V particles increase bacterial adhesion and inhibit osteogenic activity on 3D-printed implants: an in vitro study. Clinical Orthopaedics and Related Research 2019 477 2772–2782. (https://doi.org/10.1097/CORR.0000000000000954)
- 24↑
Whitehead KA, Colligon J, & Verran J. Retention of microbial cells in substratum surface features of micrometer and sub-micrometer dimensions. Colloids and Surfaces. B 2005 41 129–138. (https://doi.org/10.1016/j.colsurfb.2004.11.010)
- 25↑
McGee OM, Geraghty S, Hughes C, Jamshidi P, Kenny DP, Attallah MM, & Lally C. An investigation into patient-specific 3D printed titanium stents and the use of etching to overcome selective laser melting design constraints. Journal of the Mechanical Behavior of Biomedical Materials 2022 134 105388. (https://doi.org/10.1016/j.jmbbm.2022.105388)
- 26↑
Ye C, Zhang C, Zhao J, & Dong Y. Effects of post-processing on the surface finish, porosity, residual stresses, and fatigue performance of additive manufactured metals: a review. Journal of Materials Engineering and Performance 2021 30 6407–6425. (https://doi.org/10.1007/s11665-021-06021-7)
- 27↑
Uriondo A, Esperon-Miguez M, & Perinpanayagam S. The present and future of additive manufacturing in the aerospace sector: a review of important aspects. Proceedings of the Institution of Mechanical Engineers, Part G 2015 229 2132–2147. (https://doi.org/10.1177/0954410014568797)
- 28↑
Chahal V, & Taylor RM. A review of geometric sensitivities in laser metal 3D printing. Virtual and Physical Prototyping 2020 15 227–241. (https://doi.org/10.1080/17452759.2019.1709255)
- 29↑
Gao W, Zhang Y, Ramanujan D, Ramani K, Chen Y, Williams CB, Wang CCL, Shin YC, Zhang S, & Zavattieri PD. The status, challenges, and future of additive manufacturing in engineering. Computer-Aided Design 2015 69 65–89. (https://doi.org/10.1016/j.cad.2015.04.001)
- 30↑
DebRoy T, Wei HL, Zuback JS, Mukherjee T, Elmer JW, Milewski JO, Beese AM, Wilson-Heid A, De A, & Zhang W. Additive manufacturing of metallic components – process, structure and properties. Progress in Materials Science 2018 92 112–224. (https://doi.org/10.1016/j.pmatsci.2017.10.001)
- 31↑
Leuders S, Thöne M, Riemer A, Niendorf T, Tröster T, Richard HA, & Maier HJ. On the mechanical behaviour of titanium alloy TiAl6V4 manufactured by selective laser melting: fatigue resistance and crack growth performance. International Journal of Fatigue 2013 48 300–307. (https://doi.org/10.1016/j.ijfatigue.2012.11.011)
- 32↑
Kumar GR, Sathishkumar M, Vignesh M, Manikandan M, Rajyalakshmi G, Ramanujam R, et al. Metal additive manufacturing of commercial aerospace components – A comprehensive review. Proceedings of the Institution of Mechanical Engineers, Part E 2022. (https://doi.org/10.1177/09544089221104070)
- 33↑
3D printing of medical devices | FDA n.d. Available at: (https://www.fda.gov/medical-devices/products-and-medical-procedures/3d-printing-medical-devices). Accessed on January 17, 2024.
- 34↑
3D printing (additive manufacturing) of medical devices or component parts during the coronavirus (COVID-19) pandemic - GOV.UK n.d. Available at: (https://www.gov.uk/guidance/3d-printing-additive-manufacturing-of-medical-devices-or-component-parts-during-the-coronavirus-covid-19-pandemic#advice-for-those-using-3d-printing). Accessed on January 17, 2024.
- 35↑
ISO 22441:2022. Sterilization of health care products — low temperature vaporized hydrogen peroxide — requirements for the development, validation and routine control of a sterilization process for medical devices n.d. Available at: (https://www.iso.org/standard/73214.html) Accessed on January 17, 2024.
- 36↑
ISO 17664-1:2021. Processing of health care products — information to be provided by the medical device manufacturer for the processing of medical devices — Part 1: Critical and semi-critical medical devices n.d. Available at: (https://www.iso.org/standard/81720.html. Accessed on January 17, 2024.
- 37↑
ISO 14971:2019(en). Medical devices — application of risk management to medical devices n.d. Available at: (https://www.iso.org/obp/ui/#iso:std:iso:14971:ed-3:v1:en. Accessed on January 17, 2024.
- 38↑
ISO 10993-1:2018 - Biological evaluation of medical devices — part 1: evaluation and testing within a risk management process n.d. Available at: (https://www.iso.org/standard/68936.html). Accessed on January 17, 2024.
- 39↑
CDRH. Technical considerations for additive manufactured medical devices guidance for industry and food and drug administration staff preface public comment n.d. Available at: (https://www.fda.gov/regulatory-information/search-fda-guidance-documents/technical-considerations-additive-manufactured-medical-devices). Accessed on January 17, 2024.
- 40↑
Medical Devices Medical Device Coordination Group Document MDCG 2021-3 Questions and Answers on Custom-Made Devices & considerations on Adaptable medical devices and Patient-matched medical devices Medical Devices 2021. Available at: (https://health.ec.europa.eu/medical-devices-sector/new-regulations/guidance-mdcg-endorsed-documents-and-other-guidance_en). Accessed on January 17, 2024.
- 41↑
Carl AK, & Hochmann D. Impact of the new European medical device regulation: a two-year comparison. Biomedizinische Technik. Biomedical Engineering 2024 69 317–326. (https://doi.org/10.1515/bmt-2023-0325)
- 42↑
Committee F42 on Additive Manufacturing Technologies n.d. Available at: (https://www.astm.org/committee-f42). Accessed on January 18, 2024.
- 43↑
Carter LN, Villapún VM, Grover L, & Cox SC. Exploring the duality of powder adhesion and underlying surface roughness in laser powder bed fusion processed Ti-6Al-4V. Journal of Manufacturing Processes 2022 81 14–26. (https://doi.org/10.1016/j.jmapro.2022.06.057)
- 44↑
Dall’Ava L, Hothi H, Henckel J, Di Laura A, Bergiers S, Shearing P, & Hart A. Dimensional analysis of 3D-printed acetabular cups for hip arthroplasty using X-ray microcomputed tomography. Rapid Prototyping Journal 2020 26 567–576. (https://doi.org/10.1108/RPJ-06-2019-0175)
- 45↑
Swiatkowska I, Martin NG, Henckel J, Apthorp H, Hamshere J, & Hart AJ. Blood and plasma titanium levels associated with well-functioning hip implants. Journal of Trace Elements in Medicine and Biology 2020 57 9–17. (https://doi.org/10.1016/j.jtemb.2019.09.005)
- 46↑
Tognini M, Hothi H, Tucker S, Broomfield E, Shafafy M, Gikas P, et al. Blood titanium levels in patients with large and sliding titanium implants. BMC Musculoskeletal Disorders 2022 23 1–10. (https://doi.org/10.1186/s12891-022-05717-8)
- 47↑
Russell MD, & Jur TA. Engineering analysis of failure: a determination of cause method. Journal of Failure Analysis and Prevention 2017 17 8–14. (https://doi.org/10.1007/s11668-016-0224-9)
- 48↑
ODEP is the Orthopaedic Data Evaluation Panel n.d. Available at: (https://www.odep.org.uk/). Accessed on February 5, 2024.
- 49↑
ODEP Rating System - NEC ODEP n.d. Available at: (https://www.odep.org.uk/about/rating-system/). Accessed on January 24, 2024.
