This is one of the first studies to characterize 3D-printed orthopaedic acetabular components for THA; commonly used conventionally manufactured components were used as reference in order to highlight the features of these new types of implants. Conventionally manufactured cups with porous backside surfaces have been clinically adopted for roughly 40 years , showing good overall outcomes , thus being a valid benchmark to assess properties of 3D-printed implants that could influence their clinical outcomes. Although the clinical adoption of 3D-printed cups is not widespread yet, several 3D-printed acetabular designs are now available on the market and their use is expected to grow rapidly.
In this study, the use of retrieved cups allowed us to investigate designs that would be otherwise scarcely available, especially 3D-printed implants. A non-destructive method of analysis involving CMM, SEM and micro-CT imaging was used to investigate properties related to the seating of the shell-liner and features influencing osseointegration and implant stability.
As expected, we found differences in the morphometric properties of the porous structures, with twice the porosity, three times the pore size and twice the strut thickness in 3D printed cups. The porous structure was also thicker compared to conventional cups. Although these morphometric features are part of the actual 3D printing protocol, questions remain on what is optimal for bone-implants integration. Interestingly, we identified particles partially attached to the struts of the 3D-printed components; this is an unintended outcome of the 3D-printed process. The preliminary visual analysis of bony attachment revealed a similar percentage for the two groups of implants.
The comparable roundness of the internal surface of 3D-printed and conventional cups suggested that the different manufacturing methods did not influence this dimensional property. A wider range of values was however measured for the 3D-printed cups; it is unclear if this represents a true difference between the two groups, given the low number of implants included in this study. Overall, the internal surface of both 3D-printed and conventional cups is machined to obtain the specific tolerances prescribed by the manufacturer . Moreover, comparable roundness between the two groups suggested that the cups did not deform in situ. The shell can deform upon insertion under certain conditions and an incorrect seating of the liner may cause its fracture (if ceramic) or adversely affect the fluid-film lubrication, which results in increased wear . We found median values of less than 20 μm for both groups; although the dimensional tolerances for this feature were not available, however we could conclude that all cups showed a near-perfect round internal surface.
In terms of morphological properties, both groups exhibited the presence of particles. The key difference is that the beads present on the conventional implants are intended to form the porous structure (Porocoat) on the backside of the cups: titanium beads were sintered on the Ti6Al4V substrate (dense part of the shell) in a multilayered construct via high-temperature cycles in a vacuum furnace. The process induced solid-state diffusion with the substrate and between the beads . Conversely, the partially molten particles on 3D-printed cups are a by-product of the manufacturing process: some of the powder particles that compose the starting feedstock of the 3D-printing process were not completely fused together. These particles, together with the texture lines, were previously observed in EBM-manufactured samples [34,35,36]. The texture lines were created by the layer-by-layer manufacture (‘stairstep effect’) and by small movement of the melt pool during the building process. The energy source (in this case electron beam) melted the metal powder, following the path provided by the computer-aided design (CAD) file and created areas of melted materials (pool) which are subjected to small movement .
In general, these morphological features provide a rough surface which may promote osteoblast cell adhesion and growth (osteoconduction), as previously demonstrated for this specific material construct [23, 24]. This falls under the material ‘contact guidance’ principle, whereby the cell movement and development is affected by the surface morphology of the implant . However, a recent in vitro study by Xie et al.  suggested that the presence of the partially molten particles not only enhanced bacterial adhesion but also inhibited osteogenic activity of human bone mesenchymal stem cells, thus recommending the particles to be removed. The potential release of these particles cannot also be excluded, as suggested by Matouskova et al. ; this may lead to an increased risk of inflammatory reactions or decreased hemocompatibility , or to raised metal (titanium) levels in the blood. The high surface area of exposed metal given by the high porosity of the trabecular structure of 3D-printed components may increase this release process. To date, the only study that investigated the possibility of raised systemic metal (titanium) level in patient with Delta TT cups due to the highly porous architecture did not find any significant outcomes .
To the best of the authors knowledge, the issue of the partially molten particles has not been yet addressed by neither regulatory bodies, such as the Food and Drug Administration (FDA) in the USA, the European Medicines Agency (EMA) in the European Union or the Medicine and Healthcare Products Regulatory Agency (MHRA) in the UK, nor by standardization organizations, such as the American Society for Testing and Material (ASTM), the International Organization for Standardization (ISO) or the British Standards Institution (BSI). The FDA released general guidelines on additive-manufactured medical devices at the end of 2017 , and standards have been published on terminology, design, process, materials and test methods for 3D-printed parts . However, an in-depth regulation is still missing.
As expected, differences were found between 3D-printed and conventional cups for all the morphometric parameters and the thickness of the porous structure. The Porocoat coating represents a traditional solution to obtain a porous backside on cementless acetabular cup, with titanium beads sintered to a Ti6Al4V shell. The morphometric properties of this coating, as defined by the manufacturer, include an average pore size of 250 μm, average volume porosity of 45% and depth of the coating of 0.762 ± 0.254 mm (mean ± sd) ; these are comparable to the values measured in our study. The influence of porosity and pore size on bone ingrowth and implant osseointegration is a controversial subject , but the morphometric values exhibited by the Porocoat coating fall into the range 100–400 μm that has been suggested by both in vitro and in vivo studies to be the optimum pore size [42,43,44]. In fact, the Pinnacle Porocoat acetabular component has been widely used in primary hip procedures performed in the UK during 2017 (28% of all uncemented cups) , and this coating has been applied to different acetabular cup designs for more than 30 years [27, 45], with good clinical outcomes .
The Delta TT acetabular cup has a highly porous backside that aims to achieve fixation in both primary and revision surgeries. The lattice structure of these cups is a regular matrix composed of multi-planar hexagons called Trabecular Titanium (TT), which have been previously characterized using cylindrical and cubic specimens. Marin et al.  considered two TT structure (‘small’ and ‘large’) in order to investigate the suitability of the AM process to produce porous structure for orthopaedic applications. They reported porosity of 63.2 ± 1.3% and 72 ± 0.8% and pore sizes of 0.64 ± 0.11 mm and 1.43 ± 0.07 mm for the former and the latter, respectively. These values are comparable to porosity and pore size measured in our study. Our measures of strut thickness were also found to be similar to another study by Regis et al.  that reported a value of 355 ± 10 μm. Suggested values of tolerance capability of the EBM technology (± 250/300 μm) support these statements [16, 26].
Clinical follow-up studies related to the Delta TT acetabular design showed good early and mid-term outcomes. Steno et al.  reported on the early results (mean follow-up period of 38.14 months) of 81 revision cases where only one re-revision was performed due to instability of the acetabular component. Perticarini et al.  found that 99.3% of the Delta TT cups used in 134 THA and 8 revision cases were radiographically stable at a mean follow-up period of 72.7 months; dislocation occurred in two cases, aseptic loosening in one patient. Munegato et al.  described good clinical and radiographic results at short- to mid-term follow-up (mean period 39.8 months), with three cases of dislocation.
In general, the aim of all cementless acetabular designs is to guarantee (1) good primary stability, which depends on the fitting between the cup and the host bone and on the coefficient of friction of the backside surface with bone  and (2) long-term fixation with bone. This is strongly influenced by morphometric features of the porous structure such as porosity and pore size. The human trabecular bone is made of a three-dimensional, interconnected, open-porous space, exhibiting high porosity (50–90%), big pore size (in the order of 1 mm) and trabecular (strut) thickness of hundreds of microns [47, 48]. The 3D-printed group showed morphometric values similar to those of trabecular bone. Interestingly, we identified a third material, different from Ti6Al4V and background air, in the cross-section slices of the 3D-printed cups obtained from the micro-CT investigation. These preliminary findings suggest that this may be the bone that grew into the porous structure of the cups, because only a material dense enough to attenuate the incoming X-rays would be visible in the CT slices. Further investigations will provide an insight into these observations.
Porosity, pore size and pore interconnectivity are key features that affect the interaction between the implant and bone tissue. These parameters are directly connected to the mechanical properties and the biological performance of the implant, influencing the recruitment, adhesion and proliferation of bone cells (contact osteogenesis), as well as the potential for vascularization and perfusion [41, 49]. The rationale for having porous structure on orthopaedic implants is to increase fixation and stability because of the mechanical interlocking created by bone growing into the structure . The size of the pores determines which cells will colonize the material, directly affecting the progression of the osteogenic process. It has been reported that pores of 10–75 μm promote fibrous tissue growth; unmineralized bone tissue penetrates pores of 75–100 μm; and mineralized bone starts forming in pores of ~ 100 μm, with optimal bone infiltration in the range 100–500 μm [42,43,44, 50, 51]. A recent animal study by Tanzer et al.  described consistent bone ingrowth (> 50%) in a 3D-printed porous structure with 50 to 65% porosity and mean pore size of 450 μm. Therefore, it is accepted that lamellar bone formation requires a pore size of at least few hundred microns [49, 52]. However, other studies suggested that pore sizes in the range 500–900 μm facilitate bone ingrowth and can lead to higher infiltration or better cell response compared to structure with smaller pores [17, 19, 21]. It has also been shown that fibrous tissue formation occurs with pores bigger than 1000 μm [43, 48] and an upper limit in terms of pore size should be set also in relation to the mechanical properties. This is the reason why the optimal pore size for bone ingrowth is still a controversial subject in the literature . Overall, optimal porosity and pore size alone do not entail successful implant osseointegration but constitute two of the main parameters within the ‘implant’ factors category. Other factors related to the ‘surgeon’ and ‘patient’ equally affect the final clinical outcome, such as surgical technique (e.g. anterior or posterior approach), implant choice and operation planning or patient age, clinical condition, bone quality and blood supply .
One of the main advantages of 3D-printing technology is the possibility to design and produce in a controlled way porous structure that resembles the properties of bone, as reported by several studies [2, 7, 16, 20, 26, 34, 35, 41]. Both regular (repeated unit cells) or irregular (stochastic) structures can be manufactured; topology optimization, where a mathematical model provides desired properties whilst satisfying prescribed constraints, can also be used [8, 41].
However, the potentially unlimited design freedom guaranteed by 3D printing does not imply that the quality of the final porous structure is suitable for medical application and this depends on the design itself and the fabrication parameters.
Despite the differences (i.e. higher porosity, pore size and thickness of the porous structure for 3D-printed cups), the acetabular designs showed comparable bony attachment, suggesting a similar behaviour with bone in situ. It is of note that 3D-printed cups exhibited a higher median value (85% vs 69%) despite the lower median time to revision (24 vs 49 months), although this difference was not significant. Further studies including more acetabular components would provide better insights on the topic.
The surgical technique applied to remove the cups was not considered to influence this outcome, as assessment of tissue within the porous structures at the surface level was also taken into account. In a retrospective comparative study, Swarts et al.  visually investigated the amount of tissue ongrowth on retrieved conventional acetabular designs, finding a strong association between type of shell porous surface and amount of tissue. Higher values were seen for cups with a backside coating made of beads and pore size around 250 μm, such as the Pinnacle Porocoat. Whilst there are clear differences in the porous structure of the two groups analysed in our study, it is of note that we saw evidence of good bone attachment on both designs.
To date, 3D-printed acetabular components have been mainly used in revision and re-revision surgeries where the patient’s bone quality is not as good as in a standard primary operation whose cause is osteoarthritis [1, 6]. This is probably due to the higher porosity and initial grip provided by the 3D-printed cups. It cannot be excluded that the number of primary operations adopting 3D-printed cups will increase, considering the decreasing age of patient undergoing primary hip surgeries, together with the raising age of the population, which leads to the need of longer-lasting implants.
A strength of this study was the use of micro-CT imaging on orthopaedic implants that are clinically used. Ho et al.  reported in 2006 the capabilities of this investigation method; since then, several studies [19,20,21, 35, 55] have used micro-CT to characterize the morphometric properties of 3D-printed Titanium porous structure (scaffold) for potential medical application, evaluating also both in vitro and in vivo biological outcomes. However, we are the first to combine the analysis of 3D printed implants for clinical application with micro-CT investigations. This method provided non-destructive, full three-dimensional information about shape, size and features of the as-produced components, overcoming obstacles experienced with retrieved samples like the presence of tissue attached on the surface. Other techniques, such as microscopy, can potentially provide similar data but limited to the outer surface of the structures, if the sample is preserved, or involving destructive preparation in order to obtain cross-sections to be analysed [16, 26].
A limitation of this study is the small number of implants. Only EBM-manufactured components were considered for the 3D-printed group, whilst another additive manufacturing technology (selective laser melting, SLM) is also used to produce this type of acetabular cups. The use of 3D-printed orthopaedic implants is not yet widespread, therefore limiting the number of samples available for analysis. All the implants analysed were retrieved from patients; the investigation of retrievals represents the only way to independently analyse components from different manufacturers having a fair number of samples under analysis.