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A joint part (1) has a porous portion (2) that is defined by a multiplicity of solid regions where material is present and a remaining multiplicity of pore regions where material is absent, the locations of at least most of the multiplicity of solid regions being defined by one or more mathematical

A joint part (1) has a porous portion (2) that is defined by a multiplicity of solid regions where material is present and a remaining multiplicity of pore regions where material is absent, the locations of at least most of the multiplicity of solid regions being defined by one or more mathematical functions. The nature of the porous portion can be varied systematically by changing one or more constants in the mathematical functions and the part is made by a process of solid freeform fabrication.

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1. A joint part comprising a porous portion, the porous portion having: a multiplicity of solid regions where material is present, locations of at least most of the multiplicity of solid regions being defined by one or more mathematical functions; anda remaining multiplicity of pore regions where ma

1. A joint part comprising a porous portion, the porous portion having: a multiplicity of solid regions where material is present, locations of at least most of the multiplicity of solid regions being defined by one or more mathematical functions; anda remaining multiplicity of pore regions where material is absent,wherein the one or more mathematical functions define the solid region locations so that the locations of the remaining multiplicity of pore regions are nonrandom, andwherein the one or more mathematical functions include at least one term that defines a porosity gradient such that porosity of at least part of the porous portion changes continuously and without any discontinuity. 2. A joint part according to claim 1, in which the locations of all the solid regions of the joint part are defined mathematically. 3. A joint part according to claim 1, in which the one or more mathematical functions define a periodic nodal surface as a boundary surface between the solid and the pore regions. 4. A joint part according to claim 3, in which the periodic nodal surface is selected from the group comprising a primitive (P) surface, a diamond (D) surface and a gyroid (G) surface, the primitive (P) surface, the diamond (D) surface, and the gyroid (G) surface having interconnectivity orders equal to 6, 4, and 3, respectively. 5. A joint part according to claim 4, in which the periodic nodal surface is selected from the group consisting of the primitive (P) surface, the diamond (D) surface, and the gyroid (G) surface. 6. A joint part according to claim 1, in which the joint part further includes a solid portion, the porous portion and the solid portion being part of the same single piece. 7. A joint part accordingly to claim 6, in which the porosity of the porous portion in the region bordering the solid portion is substantially equal to the porosity of the solid portion, the porosity of the porous portion increasing away from the solid portion. 8. A joint part according to claim 1, in which the joint part is a ball and socket joint. 9. A joint part according to claim 8, in which the porous portion defines a socket of the ball and socket joint. 10. A joint part according to claim 9, in which the socket has a concave load-bearing cup portion which is solid and the porous portion extends outwardly from the cup portion to a peripheral outer surface. 11. A joint part according to claim 8, in which the porous portion defines a ball of the ball and socket joint. 12. A joint part according to claim 11, in which the ball has a convex load-bearing ball portion which is solid and the porous portion extends away from the ball portion. 13. A joint part according to claim 1, in which the joint part is a load-bearing implant suitable for implanting in a human body. 14. A joint part according to claim 1, in which the one or more mathematical functions define the distance between adjacent solid regions of the multiplicity of solid regions. 15. A joint part according to claim 1, in which each of the solid regions in the multiplicity of solid regions has the same contour as the other solid regions. 16. A joint part according to claim 1, in which the at least one term comprises a linear term. 17. A joint part according to claim 1, in which the at least one term comprises a radial term. 18. A load-bearing implant comprising a porous portion, the porous portion having: a multiplicity of solid regions where material is present, locations of at least most of the multiplicity of solid regions being defined by one or more mathematical functions; anda remaining multiplicity of pore regions where material is absent,wherein the one or more mathematical functions define the solid region locations so that the locations of the remaining multiplicity of pore regions are nonrandom, andwherein the one or more mathematical functions include at least one term that defines a porosity gradient such that porosity of at least part of the porous portion changes continuously and without any discontinuity. 19. A load-bearing implant according to claim 18, in which the locations of all the solid regions of the implant are defined mathematically. 20. A load-bearing implant according to claim 18, in which the one or more mathematical functions define a periodic nodal surface as a boundary surface between the solid and the pore regions. 21. A load-bearing implant according to claim 20, in which the periodic nodal surface is selected from the group comprising a primitive (P) surface, a diamond (D) surface and a gyroid (G) surface, the primitive (P) surface, the diamond (D) surface, and the gyroid (G) surface having interconnectivity orders equal to 6, 4, and 3, respectively. 22. A load-bearing implant according to claim 18, in which the implant is made of metal. 23. A load-bearing implant according to claim 18, in which the implant is made of ceramic material. 24. A load-bearing implant according to claim 18, in which the implant is made of polymeric material. 25. A load-bearing implant according to claim 18, in which the implant further includes a solid portion, the porous portion and the solid portion being part of the same single piece. 26. A load-bearing implant according to claim 25, in which the porosity of the porous portion in the region bordering the solid portion is substantially equal to the porosity of the solid portion, the porosity of the porous portion increasing away from the solid portion. 27. A load-bearing implant according to claim 25, in which the solid portion defines a cup which provides part or all of the socket of a ball and socket joint. 28. A load-bearing implant according to claim 27, in which the cup has a concave load bearing cup portion which is solid and the porous portion extends outwardly from the cup portion to a peripheral outer surface. 29. A load-bearing implant according to claim 27, in which the cup is an acetabular cup. 30. A load-bearing implant according to claim 25, in which the solid portion defines a ball which provides part or all of the ball of a ball and socket joint. 31. A load-bearing implant according to claim 30, in which the ball has a convex load bearing ball portion which is solid and the porous portion extends away from the ball portion.