Selective laser melting - Wiley Online Library

Selective Laser Melting: A Unit Cell Approach for the Manufacture of Porous, Titanium, Bone In-Growth Constructs, Suitable for Orthopedic Applications. II. Randomized Structures Lewis Mullen,1 Robin C. Stamp,1 Peter Fox,1 Eric Jones,2 Chau Ngo,3 Christopher J. Sutcliffe1 1

Department of Engineering, The University of Liverpool, Liverpool, UK

2

Department of Advanced Technology, Stryker Orthopedics, Limerick, Ireland

3

Department of Advanced Technology, Stryker Orthopedics, Mahwah, New Jersey

Received 11 December 2008; revised 26 May 2009; accepted 16 July 2009 Published online 6 October 2009 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.31504

Abstract: In this study, the unit cell approach, which has previously been demonstrated as a method of manufacturing porous components suitable for use as orthopedic implants, has been further developed to include randomized structures. These random structures may aid the bone in-growth process because of their similarity in appearance to trabecular bone and are shown to carry legacy properties that can be related back to the original unit cell on which they are ultimately based. In addition to this, it has been shown that randomization improves the mechanical properties of regular unit cell structures, resulting in anticipated improvements to both implant functionality and longevity. The study also evaluates the effect that a post process sinter cycle has on the components, outlines the improved mechanical properties that are attainable, and also the changes in both the macro and microstructure that occur. ' 2009 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 92B: 178–188, 2009 Keywords:

bone in-growth; fixation; randomization; mechanical properties; laser

INTRODUCTION There is an increasing demand for both hip and knee arthroplasty resulting from a more active aging population together with an increase in the number of young patients requiring treatment brought about by sports injury.1,2 This increase in activity levels required from both young and old alike has necessitated novel solutions to the complex problem of implant stability and fixation.3 Typically implants can be fixed to the bone in several ways; these include the use of bone cement, the use of mechanical fixation devices such as screws, interference fits, and activated surfaces that result in bone apposition (in-growth or on-growth).4–6 All of these methodologies are successful in practice and are still used; however, recent attention has turned more favorably towards the biological fixation by bone in-growth. This usually involves the use of porous biomaterials which are produced in a number of ways.7–9 Such structures have been shown to successfully encourage and sustain excellent bone inCorrespondence to: L. Mullen (e-mail: [email protected]) Contract grant sponsor: Engineering and Physical Sciences Research Council (EPSRC) ' 2009 Wiley Periodicals, Inc.

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growth and achieve high performance in terms of implant fixation.10 A further advantage of porous biomaterials is that they are inherently less stiff than their non-porous counterparts, and therefore, the mismatch of implant-bone stiffness is less pronounced, thereby reducing the likelihood of stress shielding and the consequential bone loss this causes.11–17 The use of porous bone in-growth materials therefore results in a more physiological approach to joint replacement surgery as within a cellular material, the structures stiffness reduces with the square of its relative density.18 This approach results in the combination of the structural properties of the implant and the bone leading to improved in vivo mechanical performance in comparison to other techniques as new bone infiltrates the pores.8,19 The primary requirements for bone in-growth are well understood.20–23 Research has shown that the optimal pore size for the in-growth of bone lies between 100 and 700 lm, with the most effective porous implants incorporating an average pore size well within this range.20,21 Porosity is also a critical factor in the performance of bone in-growth structures; with a high, fully interconnecting porosity being desired.7,21–23 Porosity levels of between 60 and 80% are preferred as at porosities above this level the material is unlikely to possess sufficient mechanical properties for the

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Figure 1. A schematic of the SLM process as used in the MCP Realizer system. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

intended application. It has been shown that the stresses in the human acetabulum are considerable and can reach 30 MPa. Consequently implanted devices should surpass this value with an acceptable margin of safety.24 A number of the earlier examples of porous prosthetic coatings resulted in failure, as the required strength levels were unattainable at the high porosities required for successful osseointegration.25,26 These design parameters present challenges to conventional manufacturing processes and hence several novel manufacturing routes have been developed. Selective Laser Melting (SLM) is one such method.27 SLM is a layer based manufacturing process that is ordinarily used for the fabrication of intricate solid geometries from any processable powder with the correct rheology to flow through the powder delivery mechanism. Full melting of the powder particles can be achieved due to the high absorptance of the powder bed to the laser light.28,29 It has been shown that by careful selection of the manufacturing methodology, and the use of a high laser energy density, this process can produce both fully dense and highly porous geometries from a variety of bio-compatible materials.30 This is particularly relevant when applied to the use of titanium and its alloys.

The technology has the unique ability that permits the manufacture of structures, based on the unit cell approach that can be specifically designed to possess similar mechanical and physical properties to those of human bone from a variety of locations within the body.27 A structure that incorporates these ‘‘design-enhancement’’ properties for the simultaneous optimization of both bone in-growth and structural integrity offers great promise as an orthopedic biomaterial and is the foundation on which this study based. Additionally, the unit cell approach has been further developed to enable the fabrication of randomized structures that more closely resemble the organic like appearance of trabecular bone. This SLM technique can be used to manufacture bone in-growth structures that are random in appearance but that contain links which have been individually perturbed by specific, pre-defined amounts, such that they are in fact pseudorandom (i.e., they appear random but they are based on an underlying regular structure). Although random structures may be favored for bone in-growth applications this is mainly due to the perceived performance improvements brought about by their random nature and similarity to trabecular bone. Random structures should also, in theory, possess damage limiting features that could result in improvements to the mechanical properties. This article explains in detail the further development of the unit cell approach27 and fully characterizes the physical and mechanical properties of pseudorandom structures produced using the SLM process.

MATERIALS AND METHODS All samples for this study were produced from a single batch of titanium powder with an average particle size of \45 lm (Sumitomo, Japan) using an MCP Realizer 2, 250 SLM system (MCP Tooling Technologies, UK). Figure 1 shows the machine and a schematic of the optical system used to control the movement of the nominal 54 lm diameter focused laser spot on the build area to a positional accuracy of 65 lm. Samples were built in a layer-wise fashion on a substrate plate connected to the elevator that moves vertically downwards allowing the controlled deposition of powder layers at 50 lm intervals. Over 200 samples were built in total over six individual machine runs as outlined in

TABLE I. A Summary of the Test Samples Manufactured During the Study

Test Porosity and compression Surface topography Tensile

Fatigue

Sample Cylinders 15 mm Ø 3 30 mm Cuboids 6 3 6 3 1.2 mm Rectangular dog-bone specimens 7 3 7 3 50 mm gage length Cuboids 15 3 15 3 30 mm

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Quantity per Test Condition

Total Number of Test Samples

5 1 5

100 4 100

40

80

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Table I. During the setup of these runs, care was taken to ensure that parts were distributed across the build plate in such a way as to avoid location effects, as it has been shown by previous researchers in the field of layered manufacturing that position on the build can have some effect on the mechanical properties of the parts.31 A consistent set of laser processing parameters were used throughout the experiments (Laser power 80 W, Exposure 380 ls) that were derived from previous work on the production of porous structures by SLM.27 Individual identification of all parts was built into the structures from data added using a conventional STL file manipulation tool (Magics, Materialise, Belgium). Post build, the test pieces were removed from the substrate plates using wire erosion to minimize mechanical damage to the pore surfaces. Fatigue specimens were additionally wire cut at both ends to ensure parallelism leading to uniform fatigue stress distribution. Half of the samples were post processed using a high temperature vacuum sinter furnace whereas the other half was tested in their as-manufactured state (green state). All green state parts were ultrasonically cleaned in de-ionized water (20 min), dried (1208C for 24 h), and stored prior to testing. Samples for sintering were given additional ultrasonic treatment in 5% Micro 90 solution (Cole-Parmer) for 3 h, to avoid contaminants entering the sinter oven, before being heat treated at 14008C for 3 h and subsequently cooled to room temperature over a period of 2 h. Samples were produced at 108 of randomization (0–45% in steps of 5%) with a 600 lm unit cell size, and a target unrandomized porosity of 65%. The dimensions of the samples for all tests were such that they contained at least seven cells in each direction to avoid edge effects, and the compression and fatigue samples had a height-to-thickness ratio exceeding 1.5.32 The porosity of the samples was determined gravimetrically using a balance (Adam Equipment, UK) capable of measuring to an accuracy of 60.01 g and digital calipers 60.002 cm prior to mechanical testing. Equation (1) was used to determine the porosities of the structures relative to fully dense CpTi (qTi 5 4.507 g/cm3): Porosity ¼ 1

Mass=Volume qTi

ð1Þ

The average pore size and the pore size distribution were measured using Mercury Porosimetry (PsS AutoPore IV), which is a pore analysis technique that is based on the intrusion of mercury into a structure under controlled pressure. Compression testing was carried out using an Instron 4505 test machine at room temperature, at a crosshead speed of 25.4 mm/min, following ASTM 451 and the Ashby method for the compression of porous structures.32 A further sample was used to determine the Young’s Modulus at a lower cross head speed of 1 mm/min to permit the measurement of the required unloading curve at 75% of the compression strength (Figure 2). Samples were tested

Figure 2. An example of the unloading curves used for the evaluation of Young’s Modulus.

between flat, lubricated platens to ensure even compressive stress distribution. Tensile test samples were fabricated in the build direction with thin solid walls built into both gripping sections to act as strengthening ribs. The tensile tests were carried out at room temperature with a crosshead speed of 1 mm/ min until complete failure occurred within the specimens gage length. Strain was initially measured with a strain gage (Instron; 50 mm gage length), but at 5% it was deemed appropriate to assess the strain by crosshead displacement. Fatigue tests were performed at room temperature using an ESH 10 kN Fatigue testing machine (ESH Testing, Brierley Hill, UK) where loads were applied as a haversine wave function with a frequency of 6 Hz to generate a stress versus number of cycles fatigue curve (S–N). The uni-axial compression strength of each structure was used to determine the upper stress levels applied during testing. For the generation of the S–N curve, stress levels were subsequently reduced at intervals of 10% until a sample survived 2 3 106 cycles. Failure was determined at the onset of progressive axial shortening. For the assessment of both the macro and microstructure, samples were mounted in epoxy resin and sectioned using a Buehler Isomet silicon carbide disk prior to polishing using various grits and then applying a final finish using colloidal silica on a silk cloth to give a surface roughness better than 1 lm. Images were obtained using optical microscopy and SEM (JEOL JSM 7001F). THEORY OF RANDOMIZATION The first step in creating three-dimensional cellular geometries for orthopedic devices via the unit cell approach is to populate a CAD model of the device with space filling polyhedra of pre-defined dimensions (the unit cell), which encapsulate the entire geometry without leaving any voids. These hollow cuboids can then be filled with a repeating geometry. This could be any geometry, however, for the purposes of this study an octahedron was used because it Journal of Biomedical Materials Research Part B: Applied Biomaterials

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Figure 3. A depiction of the process steps for the creation of random geometries.

generates a highly interconnecting pore structure, minimizes the amount of solid material per unit volume, and creates a robust unit cell. The octahedral geometry lends itself well to SLM as it can be optimized to posses no low angle links, i.e., those links which subtend the x-y plane at an angle of less than 308. Links at angles less than 308 can be difficult to build because they are unsupported along their length and they are reliant on the thermal properties of the virgin powder bed to conduct heat from the melted weld pool. The software suite ‘‘Manipulator’’ (University of Liverpool, UK) has been specially developed for the design of porous geometries based on this unit cell approach. During the development of Manipulator, several additional features have been added to its functionality to allow investigation of cellular structures. One such feature gives the ability to produce so-called pseudorandom structures. This function allows a regular lattice to be modified to produce a structure that more closely resembles a naturally occurring material, in this instance trabecular bone. The process steps for achieving this are shown in Figure 3. The randomization function independently varies the X, Y, and Z coordinates of each node on the lattice within a specified range. Boundary values for the range of randomization are expressed as a percentage of the unit cell length in a specific plane, for example if a unit cell of length 10 mm is randomized between 0 and 30% in the X plane, a point with X:Y:Z coordinates of [0:0:0] can be transformed to any random location between [23:0:0] and [3:0:0]. Similarly, if both boundary values are set to 30%, the same point can only be transformed to one of two points; either [23:0:0] or [3:0:0]. This procedure can be repeated in all axes giving rise to the possibility of creating controlled levels of randomization each resulting in a unique structure. This is the first time that it has been possible to perform a controlled study of the effect of randomization on the structural properties of this foam like material. It is anticipated that the randomization of unit cell structures may aid their performance in a number of ways. These are postulated below. 1. Randomization imparts an appearance that more closely resembles the latest technologies that are being developed to represent trabecular bone. This can be seen from the structures that have been created to represent a hip cup Figure 4. Journal of Biomedical Materials Research Part B: Applied Biomaterials

2. Randomization may promote and increase the attachment and subsequent in-growth of bone cells as the structures become more organic in appearance. 3. It may aid the structure’s damage tolerance due to the lack of structural symmetry and the removal of the natural stress planes. 4. Pseudo randomized structures may carry ‘‘legacy’’ structural properties from the original regular lattice allowing the user to prescribe desirable properties from a particular unit cell. This work is part of a series of publications investigating the effect of randomization on the mechanical, biological, and esthetic properties of these structures RESULTS The Manipulator software can be used to generate structures with infinite degrees of randomization depending on the intended application. The ability to translate the octahedral lattice points randomly between two values (i.e., anywhere between 0 and 30%) results in a structure that is ultimately based on the original unit cell, but that inevitably contains links perturbed at a large range of random angles between 0 and 908. When the randomization is defined by a finite value (i.e., 30%) the range of link angles attained is greatly reduced due to each point only being able to move to one of the two points in each plane. This implies that fewer individual link angles are created and the relationships between randomization and the structure’s properties can be more meaningfully defined. As a consequence of this, the second variant of the randomization process was chosen for the purpose of this study, where such properties as the overall pore morphology and the mechanical strength are to be characterized. Pore Size/Pore Size Distribution

The SEM images in Figure 5 clearly show that there is a marked variation in pore size, shape, and distribution that occur between unrandomized octahedral unit cell structures and those that have been randomized by 30%. It is

Figure 4. A randomized hip cup. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

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Figure 5. SEM images: unit structure (left side), 30% randomized structure (right side), unsintered (top), and sintered (bottom).

apparent from the SEM images that the randomized structure contains an increased number of both larger and smaller pores, which can be verified by the mercury porosimetry data (Figure 6). Analysis of this data shows that an increase in the average pore size occurs post randomization (form 275 lm to 335 lm in the unsintered and form 295 lm to 355 lm in the sintered structures), and that there is also an increase in the overall pore size distribution.

that is caused by the initial destabilization of the unit cell on which the structure is based. Between 15 and 35%, the randomization enhances the properties of the structure such that the sintered compressive strength can exceed 60 MPa. The final section of the graph shows that as the degree of randomization approaches 45%, the increase in compressive strength levels off at a maximum of 64 MPa. Stiffness

Porosity and It’s Variation With Randomization

It is clear from Figure 7 that at low levels of randomization (\20%) there is a variation in porosity as the degree of randomization increases but that this variation is within the expected reproducibility range of the process (62.5%). However, it can also be noted that there is a characteristic reduction in porosity at levels exceeding a 20% degree of randomization and that this reduction does eventually fall outside of the reproducibility range.

The stiffness of this porous biomaterial falls well within the range of human trabecular bone (1–10 GPa3), exhibiting a Young’s modulus of between 2 GPa and 4 GPa for the unsintered randomized structures and between 3.5 GPa and 6.5 GPa for the sintered structures. The stiffness of any material is highly dependant on its porosity and as such, with these structures exhibiting Young’s moduli in the order of human trabecular bone at porosities of 65%, they should result in a large reduction in the severity of any stress shielding effects.

Compressive Strength

It is also evident from Figure 7 that the degree of randomization has a profound effect on the compression strength of unit cell based structures. Small levels of randomization (i.e., between 0 and 15%) result in a decrease in strength

Tensile Strength

For the purpose of this study, the tensile strength was assessed for the unrandomized and the 30% randomized structures, both with and without the application of the Journal of Biomedical Materials Research Part B: Applied Biomaterials

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Figure 6. Mercury porosimetry results.

post manufacture sinter cycle. These tests showed that the tensile strength of the structures followed the same trends as the compressive strength, but were marginally lower, as outlined in Table II. This result is expected as layer manufacturing processes can occasionally contain inherent weaknesses between the bonds of each layer, thereby giving rise to reduced tensile strengths.

Fatigue Strength

The fatigue properties of the 30% randomized structure were evaluated both with and without the application of the post manufacture sinter cycle (Figure 8). It is evident that there is a large spread in this data, which is expected when characterizing porous materials of this nature.33 The conclusion to be drawn from the data is that a significant improvement in the fatigue life properties is achieved by the application of the post process heat treatment cycle. Sintering was shown to be more than double the compressive fatigue endurance limit of the 30% randomized structures at 2 3 106 cycles from 11.1 MPa to 22.5 MPa. This sintered structure compares excellently to porous bone in-growth structures manufactured by current state of the art technologies such as Chemical Vapor Deposition (HedrocelTM).33 Journal of Biomedical Materials Research Part B: Applied Biomaterials

Microstructure

As the microstructure of SLM parts manufactured as either solid or porous geometry is predominantly the same, the main changes occur during the post manufacture sintering process. Before sintering, the SLM components show a fine a titanium microstructure (Figure 9) that is formed by the rapid freezing of the alloy from the b phase, followed by the b to a phase change that occurs on the subsequent cooling to room temperature. The major effects of the sintering process are to increase a grain size and to consolidate the insipiently melted satellite powder particles.

DISCUSSION Because of the fact that the randomized structures in this study have been derived from a regular 600 lm octahedral unit cell, it would be expected that when the structure is randomized, the mean pore size would remain sensibly the same but that there would be an increase in the overall pore size distribution. In reality, this is only partly true as the small interconnecting pores that dramatically lower the average pore size in the regular structures are gradually eliminated as the degree of randomization increases. This results in structures that exhibit a higher mean pore size in

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Figure 7. A graph showing the relationships between randomization, porosity, and compressive strength.

addition to a larger pore size distribution. However, an increased mean pore size of 355 lm still fits well within the optimum range for the purpose of bone in-growth and should, therefore, have no noticeable negative effect on the performance of the structures in vivo.20,21 In contrast to this, structures with high degrees of randomization generate increased numbers of low angle links that are known to attract greater amounts of sintered plaque that would ordinarily lead to a decrease in the average pore size.27 This is not apparent from the graph in Figure 6 because of the substantial increase in pore size attributed to the elimination of the interconnecting pores. This excess plaque does, however, have an effect on the porosity, where it is postulated that the decrease in porosity after 20% randomization is caused by the gradual increase in struts at angles close to the horizontal (less than 308). This assumption does take into account the 62.5% variation in porosity inherent

within this SLM process. The lower angled links of the randomized structures attract far greater amounts of sintered plaque because the laser is required to fire on unmelted, virgin powder. This implies that the laser energy is dissipated through the underlying powder bed, thereby resulting in a larger heat affected zone and an increased number of powder particles being insipiently melted onto the surface of the struts. When a sample is not post processed by heat treatment, this powder plaque can lead to a reduced porosity structure that does not benefit from the corresponding increase in the mechanical properties that follows from the resultant structural consolidation. Because of a direct influence of the porosity on the compressive strength of porous structures, and due to the fact that a porosity in the region of 60–80% is highly desirable for bone in-growth applications, it would be unadvisable to simply evaluate the compression results outlined in

TABLE II. A Comparison of the Compressive and Tensile Properties of Unit and Random Structures

Structure Unit cell (0% random) 30% Random Unit cell (0% random) 30% Random

Heat Treated

Mean Compressive Strength (MPa)

Unsintered Unsintered Sintered Sintered

36.7 41.3 49 56.4

Mean Tensile Strength (MPa) 30.7 33.6 47.3 49.5



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Figure 8. S–N curves for sintered and unsintered, 30% randomized bone in-growth structures.

Figure 7. From previous work on the 600 lm unit cell octahedral structure,27 it was observed that a 1% decrease in porosity resulted in a 3 MPa gain in compressive strength. This characteristic relationship can be taken into account as shown in Figure 10. Here, the change in compressive strength due to any variation in porosity has been applied to the measured compressive strength to produce a porosity normalized curve that shows the predicted behavior of the randomized structures if each sample was to be built with a consistent porosity of 65%. It reveals that the decrease in porosity at the high levels of randomization

was the primary cause of the high strengths previously witnessed if Figure 7. The porosity normalization curve also shows that the optimum degree of randomization lies in the range of 30–40%, and corresponds to a relative unsintered compressive strength of 45 MPa. The main reasons for this increase in compressive strength with randomization are: 1. the elimination of the natural stress planes inherent in the unrandomized structures,

Figure 9. Optical microscope images showing the differences in microstructure and grain size between unsintered (A) and sintered (B) structures. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.] Journal of Biomedical Materials Research Part B: Applied Biomaterials

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Figure 10. Porosity normalization curves.

2. the removal of the repeated unit cell, which eliminates the possibility of multiple row collapse during compression It has also been noted that post manufacture sintering has a major effect on the structural properties of these porous titanium constructs. It can be seen from Figure 10 that the application of the post process sinter cycle increases the expected compressive strength of the samples by 20% (from a maximum of 45 MPa to 55 MPa). Figure 11 shows that sintering also significantly decreases the standard deviation from the mean between like samples (R2 values are 78.34% and 52.79% for sintered and unsintered, respectively). This occurs because sintering consolidates any insipiently melted powder plaque into the strands resulting in the formation of a uniform, well developed macrostructure. During the sintering process, any small defects that were present in the strands post manufacture are also likely to have been eliminated, ensuring that there are fewer sites for stress concentrations to initiate and propagate. Consequently, the constructs exhibit improved structural performance and consistency between like parts. Another property that can dictate the overall performance of a structure is its microstructure. The microstructure of a sample manufactured by SLM is directly controlled by

its thermal history and the alloy composition of the original powder lot. In this case, where the material is CpTi, the microstructure is straight-forward to predict. The thermal history of the material is controlled by the laser parameters, the thermal properties of the surroundings, and the processing of materials nearby (contaminants). Initially, the powder is heated by the laser until it melts and enters the melt pool. When the laser is turned off, the alloy starts to freeze and this brings about the re-growth of pre-existing

Figure 11. Box plots of the data collated for compressive strength at various degrees of randomization. Journal of Biomedical Materials Research Part B: Applied Biomaterials

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Figure 12. SEM images showing the macro and microstructure of the post-manufactured (A) and post-sintered (B) struts.

grains within any underlying structure; in this case grains of b titanium. On further cooling, the temperature drops below the a–b transition temperature and a grain start to nucleate on the b grain boundaries. Because of the rapid cooling rate experienced, a large number of nucleation events take place producing the small a titanium grains that make up the microstructure as seen in Figure 9(a). When a sample undergoes the post process sinter cycle at 14008C, the material is reheated to above the a–b transition temperature and causes the b titanium to reform. Upon completion of the sinter cycle, the material is slowly cooled from the b phase, with a titanium grains again nucleating on grain boundaries, but this time, as the cooling rate is reduced, only a few grains are formed before all of the b phase is transformed to a. Ordinarily, these larger grains have a negative effect on the material’s yield strength and fatigue life properties as they offer less resistance to dislocation movement. However, this effect is greatly overshadowed by the increased strength that is attained due to the consolidation of the insipiently melted powder particles, and this is the reason that sintered parts greatly outperform the as-manufactured parts in mechanical tests such as compressive, tensile, and fatigue testing. This sinter cycle is therefore primarily designed to alter the macrostructure of the components with the surfaces becoming smoother and more rounded to reduce free energy, with any adherent powder plaque being incorporated into the strands. However, as the sinter treatment takes place at low pressure, vacuum etching also occurs roughening the surfaces at the microscopic level [Figure 12(b)]. This vacuum etching may occur as a result of the evaporation of material from the surface of the structure, which produces the observed contour lines (facets) that may inadvertently be beneficial to the bone in-growth process.34 This increased micro-roughness in comparison with the unsintered structure, may lead to the implant exhibiting greater contact with host bone thereby resulting in a more stable platform for bone in-growth to take place.35 Journal of Biomedical Materials Research Part B: Applied Biomaterials

CONCLUSIONS A method has been conceived to alter the appearance of components from a structurally ordered regular engineered form to a form, which at first sight appears random. This random appearance together with a somewhat rough outer surface is the preferred structure that has been requested by surgeons. Not only does this randomization process alter the appearance, but also it can be used to enhance the mechanical properties of the structures by eliminating the natural fault planes that commonly occur in ordered structures. Randomization leads to a small increase in both the average pore size and the overall pore size distribution, arising from the gradual elimination of the small interconnecting pores and the resulting generation of both larger and smaller pores throughout the structure. This coupled with an increase in the amount of insipiently melted powder plaque at high degrees of randomization has an effect on the porosity of the structures, which can be easily amended by altering the laser parameters from which they are manufactured. During heat treatment, the originally undesirable insipiently melted powder particles can be consolidated into the strands to significantly enhance the structure’s mechanical properties and stability. These results are promising from the perspective of providing the basis of an ideal structure to maximize bone ingrowth potential and implant stability, but further work is required to form both a biological perspective and also a greater understanding of the process capabilities. The authors thank Stryker Orthopedics for their continued support and financial backing of the project, and Wesley Brooks for the ongoing support and development of manipulator.

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