Technical field

[0001] The present invention is related to surgical implants comprising a scaffold structure for bone ingrowth. In particular, the present invention is related to surgical implants of the above kind in which the scaffold structure comprises a graded porosity.

Background art

[0002] From the review article Graded/Gradient Porous Biomaterials, by Xigeng Miao and Dan Sun, Materials 2010, 3, 26-47 it is known to use graded porous implants for repairing bone-cartilage complex tissue. The part with larger pore size is implanted into bone for bone ingrowth, whereas the part with smaller pore size is to allow cartilage to grow in. In other words, the graded porous implant can be used to select or promote attachment of specific cell types on and in the implant prior to and/or after implantation. The part for bone ingrowth and the part for cartilage ingrowth can be made of different materials. The gradient of material properties may range from one which is suitable for load bearing to one which is suitable for soft tissue regeneration.

[0003] US 4978355 describes a metal grid embedded in the contact surface of a plastic implant. An additional anchoring surface for ingress of bone tissue is secured to the embedded grid. The anchoring surface is formed of layers of metal wire which are stacked and secured by sintering.

[0004] US 2005/01 12397 describes a porous structure having a plurality of stacked bonded sheets. The sheets have a plurality of at leaast partially overlapping apertures formed therein, produced by perforation. Perforating the sheets to create the apertures allows for obtaining differential porosity within the sheet or from sheet to sheet. Regions of high porosity are separated by regions of lower porosity.

[0005] Research has indicated that different levels of porosity and pore size of the scaffold structure have an impact on the amount of bone ingrowth and the mechanical stability of the implant. Dense scaffold structures have good mechanical properties but poor bone ingrowth properties. On the contrary, more porous structures provide good biological performance but have a low mechanical strength. The rate of tissue ingrowth in the porous structure is also dependent on the availability of a large surface area for cells to attach and grow on. It is known that most bone forming cells grow on a substratum surface rather than grow in a suspended manner in the cell culture medium. In this regard, a large pore surface area means that a large bone-material interfacial bonding area can be provided.

[0006] Furthermore, interconnected porosity promotes the organisation of vascular canals that can ensure the supply of blood and nutrients for the viability of bone.

Summary

[0007] Osseointegration is important for many surgical implants, however it is not easily stimulated and/or controlled. Bone ingrowth and vascularisation strongly depend on macroporosity parameters such as pore size, pore size distribution and pore interconnectivity. In order to optimize mechanical properties and macroporosity, several graded/gradient implants materials have been proposed, particularly using additive manufacturing technologies.

[0008] Despite the advances to date, there is still a need in the art of improved scaffold structures for surgical implants. In particular, there is a need of providing such scaffold structures which enhance the promotion of bone ingrowth into the structure, yet allowing sufficient freedom in designing the scaffold structure for optimal mechanical properties. There is a need of providing such implants having improved tissue anchoring capabilities. There is also a need in the art of manufacturing scaffold structures of the above kind in a cost-effective way.

[0009] According to a first aspect of the invention, there is therefore provided a surgical implant as set out in the appended claims. The surgical implant comprises a porous structure with interconnected pores. The pores have sizes suitable for ingrowth of bone and/or soft tissue into the porous structure. The porous structure comprises an arrangement of fibres which are attached to one another and are arranged in advantageously planar layers, the layers being stacked. According to aspects of the invention, the porous structure comprises a surface comprising different regions having different porosities. Advantageously, the arrangement of fibres extends to the surface and determines the different porosities by different arrangements of the fibres in the different regions. Advantageously, the different porosities are determined by (different) interspaces between adjacent or consecutive fibres. The term interspace can but does not necessarily refer to the inter-fibre distance. Rather, the term refers more generally to (the size of) the interstitial voids delimited by fibres. Different parameters may influence the porosity of the arrangement of fibres, such as fibre diameter, inter-fibre distance, stacking factor, fibre orientation, etc.

[0010] According to a second aspect of the invention, there is provided a method of manufacturing a surgical implant of the above kind as set out in the appended claims. The method comprises the steps of forming fibres in advantageously planar layers which are stacked on top of one another, and connecting the fibres of consecutive layers to one another to obtain a porous structure, e.g. a network of fibres. According to aspects of the invention, the method comprises the step of arranging the fibres in proximity of a surface of the porous structure such that the surface comprises different regions having different porosities. Advantageously, the fibres are arranged with different interspaces in different regions of the surface.

Brief description of the drawings

[0011] Aspects of the invention will now be described in more detail with reference to the appended drawings, wherein same reference numerals illustrate same features.

[0012] Figure 1 represents a cross sectional view of an example surgical implant according to aspects of the invention.

[0013] Figure 2 represents a unit pore cell as defined in an orthogonal fibre disposition scheme, with fibres of consecutive layers being orthogonal to one another.

[0014] Figure 3 represents a cross sectional view of an example surgical implant according to an alternative aspect of the invention, wherein different regions have different pore interconnectivity in a direction of build of the porous structure.

[0015] Figure 4 represents a cross sectional view of an example surgical implant as in Fig. 1 , in which additionally a porosity gradient is applied in a direction of build of the layers by changing the inter-fibre distance.

[0016] Figure 5 represents a cross sectional view of an example surgical implant as in Fig. 4, in which additionally the stacking factor is changed through the layers in the direction of the porosity gradient.

[0017] Figure 6 represents a cross sectional view of an example surgical implant which differs from the implant of Fig. 4 in that the porosity gradient is obtained by a change of fibre diameter between layers.

[0018] Figure 7 represents a cross sectional view of an example surgical implant which differs from the implant of Fig. 4 in that the fibres are microporous.

[0019] Figure 8 represents an example fibre arrangement pattern as seen from a direction perpendicular to the plane of the layers in which the fibres are arranged.

Detailed description

[0020] For purposes of illustration, aspects of the invention will be described in relation to a particular example of an acetabular component of a hip implant. It will however be convenient to note that indicated aspects are readily applicable to other kinds of implants, such as spinal implants, cranial implants, maxillofacial implants and dental implants.

[0021] Fig. 1 schematically depicts an implant 10 according to aspects of the invention. The implant 10 is shown as an acetabular component of a hip prosthesis and comprises a scaffold part 1 1. In this particular example, the scaffold part 1 1 is attached to a dense part 12. The dense part 12 forms a shell of hemispherical shape which may be provided with a liner forming a receiving part of a ball joint. A femoral component of the hip prosthesis (not shown) typically comprises a ball which is accepted in the liner. It will be convenient to note that in other types of implants, the dense part may be omitted.

[0022] The scaffold part 1 1 is a porous structure having interconnected pores which are configured for ingrowth of bone. The scaffold part 1 1 comprises an external surface 1 1 1 forming an interface with the surrounding sound bone structure. Advantageously, the external surface 1 1 1 is located opposite surface 1 15 which forms an interface of attachment to the dense part 12. According to an aspect of the present invention, the surface 1 1 1 comprises different regions having different porosities. By way of example, the surface 1 1 1 comprises first regions 1 12 and second regions 1 13. The first regions 1 12 have a higher volume porosity (determined in a volume contiguous to the surface 1 1 1 ) compared to the second regions 1 13. In other words, the second regions 1 13 will have higher density compared to the first regions 1 12.

[0023] According to an aspect, porosity and/or pore size of the first regions

1 12 and of the second regions 1 13 can be selected such that the first regions of higher porosity will promote bone ingrowth, while the second regions of lower porosity will promote the ingress of vascular canals into the scaffold structure 1 1. These vascular canals provide for transport paths for supply of nutrients and cells which further promote bone formation. Therefore, providing on the external surface 1 1 1 adjacent regions of higher porosity and lower porosity will provide for parallel paths for ingrowth of bone and vascular canals, and as a result, will enhance the speed with which bone will grow into the scaffold structure 1 1.

[0024] Advantageously, a plurality of the second regions 1 13 are provided adjacent or in between a plurality of the first regions 1 12. Advantageously, the first regions 1 12 and the second regions 1 13 alternate on the surface 1 1 1. The area of extension of the first regions and of the second regions is not particularly limited. Advantageously the first regions and the second regions each extend over an area of at least 4 mm ² , advantageously at least 5 mm ² , advantageously at least 10 mm ² , advantageously at least 20 mm ² .

[0025] Advantageously, these first and second regions 1 12, 1 13 can further extend into the depth of the scaffold structure 1 1 , e.g. until surface 1 15.

[0026] According to an aspect, such a structure 1 1 with alternating regions of higher porosity and regions of lower porosity is obtained by forming the scaffold structure 1 1 out of an arrangement of fibres 13, 14. The fibres 13, 14 are disposed in layers 151 , 152, 153, 154, etc which are stacked on top of one another, and which are advantageously parallel to one another. This arrangement of fibres extends to the surface 1 1 1 . Fibres of consecutive layers are attached to one another, and thereby advantageously form a construct which is monolithic, being the scaffold structure 1 1 .

[0027] Such monolithic and porous structures can be obtained by well- known additive manufacturing techniques, such as three-dimensional fibre deposition, three-dimensional powder deposition or similar solid free-form fabrication techniques. The fibres or filaments can be extruded as a paste from a nozzle, as is the case with 3D fibre deposition, or can be 3D printed starting from a powder layer which can e.g. be selectively melted (selective laser sintering), or selectively bound with an adhesive (3D printing).

[0028] 3D fibre deposition (3DFD) (also called robocasting) comprises the extrusion of an advantageously highly viscous paste loaded with metallic or ceramic particles through a thin nozzle. In this case, the paste comprises a powder, such as a metallic or ceramic powder, or a combination of both, an organic binder, optionally a rheology modifier and optionally an anorganic binder, such as a colloidal binder. By computer controlled movement in x, y and z-direction, a porous architecture is built layer- by-layer. The x and y directions typically refer to the plane of the layers 151 -154, whereas the z-direction is the direction of stacking of the layers (perpendicular on the plane of the layers). This process can involve multiple nozzles or a single nozzle. The green part which is obtained by the above process can be post-processed in one or two steps: an optional drying step followed by sintering. Sintering may be carried out under vacuum conditions, or in an inert or reducing atmosphere, e.g. to avoid oxidation in case of metals. After sintering, a highly reproducible and periodic porous structure is obtained. The process variables include the nozzle opening (fibre thickness or diameter), the type of nozzle (fibre shape), the inter-fibre distance (pore size) and the stacking of the layers (architecture). The microporosity and surface roughness of the fibres can be controlled. An equipment for 3DFD typically comprises a paste reservoir with nozzle, mounted on an apparatus with numerical control of three or more axes, e.g. an XYZ-table or a CNC machine. Multiple nozzles can be mounted onto the equipment in order to speed up the production of similar pieces.

[0029] Fibres 13, 14 of consecutive layers advantageously extend along transverse directions and the fibres within the same layer are advantageously spaced apart. By way of example, referring to Fig. 1 , fibres 13 of layer 151 are parallel to one another and have longitudinal axes cross to the longitudinal axes of fibres 14 in the below layer 152. Fibres 13 and 14 may extend perpendicular to one another, or oblique, e.g. at an angle different from 0° and different from 90°. As a result, a highly porous structure can be obtained. The fibres are advantageously, though not necessarily arranged in an orderly fashion. By way of example, fibres 13 within the same layer can be parallel, extend radially from a common centre, be concentric in circles or extend spirally.

[0030] In order to account for the sometimes complex geometry of surgical implants, the scaffold structures 1 1 can be made as a block, e.g. by 3DFD as described above, and machined afterwards, e.g. milled, to the correct geometry, e.g. to fit on the dense part 12. The attachment with the dense part 12 can be provided by known techniques, such as sintering, friction welding, laser welding, etc.

[0031] Advantageous porous structures 1 1 may comprise longitudinal channels extending substantially normal to the external surface 1 1 1 , e.g. the longitudinal channels may extend from the surface 1 1 1 in a direction of approach of the interface 1 15 or the dense part 12. These longitudinal channels may be straight or tortuous. The tortuosity may be defined by staggering the fibres as will be described further below.

[0032] According to an aspect, a first porosity gradient may be provided between the first regions 1 12 and the second regions 1 13. That is, along a first direction, referred to as gradient direction, the porosity, and therefore also the density of the structure 1 1 , is made to change. The first gradient direction advantageously lies on the surface 1 1 1 , or may be a direction at least locally tangential to the surface 1 1 1.

[0033] By way of example, a first region 1 12 is provided with a porosity P1 .

A possible adjacent second region 1 13 is provided with porosity P3, which is different from P1 , e.g. P1 >P3. Possibly, an intermediate region (not shown) may be interposed between first region 1 12 and second region 1 13, which may be provided with porosity P2, with P2 different from P1 and P3. According to an aspect, the porosity changes along the first gradient direction from a higher porosity P1 and hence lower density of the structure 1 1 in the first region 1 12 to a lower porosity P3 and hence a higher density of structure 1 1 in the second region 1 13. Advantageously, the porosity gradient is one with a porosity decreasing from the first region, possibly through the intermediate region, towards the second region. In other words, P1 >P2>P3. [0034] According to yet another aspect, a second porosity gradient may be provided in a direction substantially orthogonal to the first gradient direction, e.g. a direction oriented away from or in approach of the surface 1 1 1 .

[0035] The local porosity can be determined based on the geometry of a unit pore cell 20 as shown and defined in Fig. 2. A pore can be regarded as a cell delimited on all sides by fibres 13, 14. The stacking factor c refers to the interpenetration depth between fibres of consecutive layers. The stacking factor is obtained, e.g. during build of a 3DFD structure but is analogous with other additive manufacturing processes, by increasing the (vertical) build height (z) by an amount less than the fibre diameter, when starting a new layer on top of the previous one. The fibre diameter can be determined by optical microscopy or Scanning Electron Microscope imaging of a cross- section of the material and is mainly determined by the nozzle diameter of the 3DFD apparatus, printing conditions and the shrinkage upon sintering. The stacking factor c may be influenced by the paste composition (e.g. viscosity), fibre thickness, inter-fibre distance and printing conditions such as temperature and humidity. The stacking factor has a strong influence on the mechanical strength of the fibres, but also influences the macroporosity and the interconnectivity of the macropores. The stacking factor c can be measured by means of a Scanning Electron Microscope imaging. Further, a = M-n is fibre diameter (mm), n is inter-fibre distance (mm) and M \s axial centre spacing between two fibres (mm). The macroporosity (P, %) of the cell can be calculated as follows, with SSA being the specific surface area (SSA, mm ² /mm ³ ), S _c is the loss of the surface area of two connected fibres (mm ² ), Sf is surface area of the two fibres (mm ² ), V _ce ii is the unit cell volume (mm ³ ) and Vfibre is the fibre volume (mm ³ ):

b = 2^2 ac — c ² (mm)

π a * b

(mm ² )

4

V _cell = 2(a - c)M ² (mm ³ )

πΜα ²

Vfibre = T (rara ³ )

πα (Λί - V2 ac

SSA = with Vc the volume of the intersection of two fibres with same fibre diameters a.

[0036] Vc depends on the stacking factor c. The stacking factor c can be in the range 0 < c < a. While c = a, V _c is a "Steinmetz solid". Therefore, V _c = y ³ . While c is 0 < c < a, a circular cone volume can be assumed for simplifying the calculation of Vc, which is an approximation of the real elliptic cone volume. Assuming a circular cone volume:

[0037] Reference to porosity in the present description relates to macroporosity, e.g. porosity between the fibres disregarding porosity of or within the fibres. Advantageously, macropores have a pore size of at least 10 μηη in diameter, advantageously a pore size of at least 25 μηι, advantageously at least 50 μηι. Absolute (macro)porosity values in structures according to aspects of the invention are not particularly limiting. Advantageous values are between 40% and 95% porosity, advantageously between 50% and 80%. Average (macro)porosity values of porous structures according to present aspects are advantageously between 50% and 90%, advantageously between 55% and 85%, advantageously between 60% and 80%.

[0038] According to aspects of the invention, the difference (i.e. the change) in porosity (expressed as a percentage) between the first regions and the second regions is at least 4%, advantageously at least 5%, advantageously at least 6%, advantageously at least 8%, advantageously at least 10%. In other words, assuming the (volume) porosity is P1 (%) in the first region (evaluated at the surface 1 1 1 ), and P2(%) in the second region (evaluated at the surface 1 1 1 ), the difference in porosity ΔΡ(%) = P1 - P2. The porosity may change between a porosity between 50% and 95%, advantageously between 60% and 90%, advantageously between 70% and 90% in the first region and a porosity between 40% and 80%, advantageously between 50% and 70%, advantageously between 50% and 60% in the second region.

[0039] In porous (scaffold) structures according to aspects of the invention, the fibres advantageously have a diameter a in the range between 20 μηη and 2 mm, advantageously between 40 μηη and 1 mm, advantageously between 60 μηη and 600 μηι, with advantageous values being 80 μηι, 100 μηι, 150 μηι, 200 μηι, 300 μηι, 400 μηι, 500 μηη. All fibres within a same layer of the structure typically have a same diameter, and the fibre diameter may be the same in all layers of the structure or may change between layers, e.g. by using different nozzles with different diameters for extruding the fibres. [0040] The inter fibre distance n, e.g. within a same layer, may vary between 0 μηη and 5 mm, and is advantageously between 10 μηη and 2 mm, advantageously between 25 μηη and 1 mm, advantageously between 50 μηη and 900 μηι, advantageously between 100 μηη and 800 μηι, and advantageously at least 200 μηι, advantageously at least 300 μηη. The inter fibre distance n typically changes within one layer in order to obtain a change in porosity, and advantageously to obtain a porosity gradient. In the scaffold structures described herein, the interfibre distance relates to the size of a pore cell 20.

[0041] The stacking factor c may vary between 0 and the fibre diameter a, advantageously 0.01 a < c < 0.99a, advantageously 0.02a < c < 0.90a, advantageously 0.03a < c < 0.50a, advantageously 0.05a < c < 0.20a. Advantageously, the ratio c/a is at least 0.075, at least 0.1 , at least 0.125, at least 0.15. The stacking factor typically is constant within one layer, and may change between layers. Typical values of the stacking factor c may range between 10 μηη and 200 μηη, advantageously between 20 μηη and 150 μηι, advantageously between 30 μηη and 100 μηι, e.g. 70 μηι.

[0042] Referring to Fig. 2, the fibre diameter a and the stacking factor c define the size of the interconnection between adjacent pores within a same layer, also referred to as pore throat 21 . The size of the pore throat 21 , which may be defined as a- 2c is advantageously at least 20 μηη, advantageously at least 50 μηη. The inter-fibre distance n and disposition of the fibres (e.g. staggered or aligned) mainly defines the size of the interconnection 22 between pores of consecutive layers. The pore interconnections 22 define paths in a direction perpendicular to the plane of the fibre layers, and therefore may define the pore interconnection in a direction of depth of the structure 1 1 , starting from the external surface 1 1 1 . The size of the pore interconnections 21 and/or 22 relates to the pore interconnectivity and may be important for the first regions with higher porosity to promote bone ingrowth, or for the second regions with smaller porosity to promote vascularisation, and may be important for both regions. Typically, the size of the pore interconnections 22, may be larger for the first regions and smaller for the second regions. Possibly, the first regions and the second regions may have different pore interconnection sizes.

[0043] Additive manufacturing techniques allow for easily and effectively making monolithic structures with desired porosity gradients. For porous structures built up out of an arrangement of fibres, the easiest way of obtaining a porosity gradient is through changing the spacing between (parallel) fibres within some or all layers, i.e. the inter fibre distance n. One example is shown in Fig. 1 showing the disposition of the fibres 13, 14 as seen from a direction orthogonal to the planes of the (parallel) layers. In Fig. 1 , the fibres 13 within a same layer are disposed parallel to one another and the fibres 13 and 14 of consecutive layers are transverse, e.g. orthogonal to one another. It can be observed that in the first regions 1 12, the spacing (interfibre distance ni) between adjacent fibres is larger as compared to the interfibre distance (/¾) of the structure in the second regions 1 13. This change in the interfibre distance (or, in more general terms, the spacing between adjacent fibres) can be applied to all layers, or alternatively to some but not all the layers, e.g. only layers having fibres parallel to fibres 13, in order to obtain a porosity change or gradient.

[0044] In addition, or alternatively to a porosity difference on the surface 1 1 1 , the first regions and the second regions may have different pore interconnectivity between consecutive layers, as shown in Fig. 3. The scaffold structure 31 of Fig. 3 comprises first regions 312 and second regions 313 having different pore interconnectivity in the direction 16 perpendicular to the fibre layers. This may be obtained by staggering the fibres 13 and 13' in different layers. Even though in this example the size of a unit pore cell remains the same between the first region 312 and the second region 313 due to identical interfibre distance, the pore cells in second regions 313 are staggered, which reduces the pore interconnectivity between adjacent layers. It will be convenient to note that a change in pore interconnectivity such as shown in the example of Fig. 3 can be combined with the change in porosity such as shown in the example of Fig. 1 .

[0045] Referring to Fig. 4, a scaffold structure 41 is shown which differs from the structure 1 1 of Fig. 1 in that in the first regions 412 and in the second regions 413 a porosity gradient is applied in the structure 41 along a direction 16 of approach to, or away from, the surface 1 1 1 , i.e. a direction of approach to the dense part 12. This porosity gradient is applied in addition to the change in porosity between first regions 412 and second regions 413. In the example shown, the porosity gradient is obtained by changing the interfibre distance n, e.g. increasing n towards the external surface 1 1 1 to obtain a higher porosity at or near the surface 1 1 1 and a lower porosity towards the dense part 12. It will be convenient to note that such a porosity gradient can be applied to only one of the first regions 412 and the second regions 413, or alternatively to both.

[0046] Referring to Fig. 5, a scaffold structure 51 is shown which differs from the structure 41 of Fig. 4 in that the porosity gradient along direction 16 is (further) obtained by changing the stacking factor c through the layers in the stack of structure 51 . In the example, c is increased from the external surface 1 1 1 towards the dense part 12. Changing the porosity by changing the stacking factor can be applied in addition to, or in the alternative of changing the interfibre distance. [0047] Referring to Fig. 6, a scaffold structure 61 is shown which differs from the structure 41 of Fig. 4 in that the porosity gradient along direction 16 is obtained by a change of the fibre diameter through different layers. Layers proximal to the external surface 1 1 1 may comprise fibres 63 having a larger diameter compared to fibres 64 arranged in layers remote from the surface 1 1 1. Alternatively, layers proximal to surface 1 1 1 may comprise fibres having a smaller diameter compared to fibres of layers remote from surface 1 1 1 . In the latter case, a porosity gradient may be obtained by appropriate selection of the inter fibre distance n in each layer, e.g. with n decreasing in the direction 16. The change of fibre diameter as described hereinabove may be combined with other ways of obtaining a porosity gradient in direction 16, such as those described in relation to Figs. 4 or 5.

[0048] It will be convenient to note that the fibres themselves may comprise a microporosity, e.g. porosity with pore size smaller than the size of the macropores as indicated above, as shown in Fig. 7. The scaffold structure 71 of Fig. 7 differs from the structure 41 of Fig. 4 in that the fibres comprise a microporosity. The microporosity may extend in a peripheral region, e.g. sheath 72, of the fibres 73. In this case, the fibres 73 may comprise a dense core 74. Alternatively, fibres 73 may be microporous throughout.

[0049] Microporous fibres may be obtained by subjecting the fibres to a phase inversion process as e.g. described in WO 2009/027525, 5 March 2009, which is incorporated herein by reference. Biomedical implants with macro- and microporous structure may stimulate osseointegration and provide sufficient local mechanical strength for fixation/implantation. Due to the macroporosity, the implant materials can be easily coated with conventional coating procedures such as dip-coating or wash-coating, with growth factors. Due to the microporosity, the as such deposited coatings will have a much better adhesion. Advantageously, the (microporous) fibres are otherwise solid fibres, i.e. they are advantageously not hollow.

[0050] The microporous fibre or filament morphology may be induced by phase inversion. A method for producing such morphology may comprise the steps of: a) preparing a suspension comprising particles of a predetermined material, a liquid solvent, one or more binders and optionally one or more dispersants, b) depositing said suspension in the form of fibres or filaments in a layered fashion, e.g. according to a predetermined disposition of fibres or filaments, thereby creating a porous structure,

c) inducing phase inversion, whereby said filaments are transformed from a liquid to a solid state, by exposing said filaments during the deposition of the filaments to a non-solvent vapour and to a liquid non-solvent, d) thermally treating the structure of step c) by calcining and sintering said structure.

In other words, step c) of the present method involves the step of exposing said filaments during the deposition of the filaments to a non-solvent vapour and to a liquid non-solvent, whereby the deposited filaments solidify and obtain surface roughness and microporosity. In a preferred embodiment, step b) is carried out in a non-solvent environment.

[0051] Advantageously, an alternative step c) comprises the step of c1 ) bringing said filaments during the deposition of the filaments into contact with a non- solvent vapour, and the step of c2) immersing the structure of step c1 ) in a liquid non- solvent, thereby creating a filament-based porous structure having suitable filament morphology. Phase inversion can be completed in a next step (step c2) of the present method by immersing the structure in a liquid non-solvent.

[0052] The fibres or filaments in the sintered porous structure obtained after step d) advantageously comprise an average surface roughness (Ra) which is higher than 4μη"ΐ. Moreover, the filaments in the sintered porous structure obtained after step d) also have a microporosity (after sintering) comprised between 1 and 50%, preferably between 5 and 30%. Microporosity refers to a porosity wherein the pores have a size smaller than macropores as indicated above.

[0053] Referring to Fig. 8, a possible arrangement of fibres is shown. This arrangement is particularly suited as scaffold structure for the acetabular component indicated above. The structure 81 comprises a repeating pattern of three consecutive layers. In a first layer, fibres 83 are arranged in concentric circles. A second layer comprises fibres 84 arranged parallel to one another. A third layer comprises fibres 85 arranged parallel to one another. Fibres 85 are perpendicular to the fibres 84 of the second layer. The porosity difference between different regions or areas in structure 81 can be brought about by changing the inter-fibre distance in one, some or all three layers. As shown in Fig. 8, the in the fibre distance is changed in each one of the three layers in a graded way to obtain different regions with different porosity. It will be convenient to note that the first, second and third layer may be arranged in any order.

[0054] The materials of which the porous structures according to aspects of the present invention are made include metals, ceramics, and composite materials, in particular those materials being biocompatible.