Application of Laser Engineered Net Shaping (LENS) to manufacture porous and functionally graded structures for load bearing implants
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- Bandyopadhyay, A., Krishna, B.V., Xue, W. et al. J Mater Sci: Mater Med (2009) 20: 29. doi:10.1007/s10856-008-3478-2
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Fabrication of net shape load bearing implants with complex anatomical shapes to meet desired mechanical and biological performance is still a challenge. In this article, an overview of our research activities is discussed focusing on application of Laser Engineered Net Shaping (LENS™) toward load bearing implants to increase in vivo life time. We have demonstrated that LENS™ can fabricate net shape, complex metallic implants with designed porosities up to 70 vol.% to reduce stress-shielding. The effective modulus of Ti, NiTi, and other alloys was tailored to suit the modulus of human cortical bone by introducing 12–42 vol.% porosity. In addition, laser processed porous NiTi alloy samples show a 2–4% recoverable strain, a potentially significant result for load bearing implants. To minimize the wear induced osteolysis, unitized structures with functionally graded Co–Cr–Mo coating on porous Ti6Al4V were also made using LENS™, which showed high hardness with excellent bone cell–materials interactions. Finally, LENS™ is also being used to fabricate porous, net shape implants with a functional gradation in porosity characteristics.
Musculoskeletal disorders are recognized as among the most significant human health problems that exist today. Over 200,000 total hip replacements (THRs) are performed in the United States each year. Shorter implant life is a significant problem especially for the growing number of younger patients because of their active lifestyle. The world wide population of people younger than 40 years of age who receive hip implants is expected to grow significantly in the coming years , which is likely to create a need for implants with longer in vivo lifetime. Even though significant research and development have gone toward understanding musculoskeletal disorders, there is still a lack of bone replacement material that is appropriate for restoring lost structure and function, particularly for load-bearing applications. For example, average life time of hip implant is 7–12 years. Major factors limiting the life of current load bearing implants include (i) mismatch of the Young’s modulus between bone (10–30 GPa) and metallic implant materials (110 GPa for Ti and over 200 GPa for Co–Cr–Mo alloy) leading to stress-shielding; (ii) poor interfacial bond between the host tissue and the implant due to bioinert surface; (iii) wear induced osteolysis and aseptic loosening in metal-on-polymer implants, and (iv) absence of high recoverable strain (∼2%) as well as hysteresis similar to natural bone. Therefore, fabrication of functional load bearing implants with complex anatomical shapes and desired biomechanical performance is still a challenge.
Stress-shielding and weak interfacial bond between the tissue and the implant can be eliminated by the use of porous metals. Use of porous materials can effectively reduce the modulus mismatch  and provide stable long-term anchorage for biological fixation of the implant due to bone tissue ingrowth through the pores [3, 4]. Among various biomedical alloys such as Ti and its alloys, only NiTi alloy (Nitinol) exhibits hysteresis in loading–unloading cycles as well as a 8% recoverable strain similar to natural bone . This similarity in the deformation behavior between Nitinol and bone can improve in vivo lifetime of load bearing implants due to excellent biomechanical compatibility . Conventional powder metallurgical (PM) processing has been used in the past to fabricate surface treated or fully porous metals, including Ti, Ti alloys [6–8], and NiTi alloys [9–11] for biomedical applications. These conventionally sintered metals are often very brittle and pore size, shape, volume fraction, and distribution are difficult to control, which have major influence on mechanical and biological properties. Other fabrication techniques that use foaming agents or molten metal suffer from typical limitations such as contamination, impurity phases, limited, and predetermined part geometries, and limited control over the size, shape, and distribution of porosity. Considerable modification of composition and structure of PM processed NiTi alloys from the feedstock powder also significantly deteriorate the mechanical as well as shape memory properties. For example, embrittling oxide (Ti4Ni2Ox: 0 < x ≤ 1) content of sintered alloys is generally high compared to melt-cast alloys . Moreover, PM NiTi alloys usually contain a large amount of undesirable secondary phases such as Ti2Ni, Ni4Ti3, and Ni3Ti . Therefore, there is significant interest for fabrication methods which can ensure uniform size, shape and distribution of porosity, high levels of purity in porous metals and good shape memory property in NiTi alloys for load bearing applications.
High wear rate of ultrahigh molecular weight polyethylene (UHMWPE) liner used in current hip replacements is another cause of serious concern due to osteolysis and aseptic loosening [12, 13]. Due to these concerns, there is considerable interest in the alternative wear resistant systems such as metal-on-metal (MM) configurations. MM configurations showed 40 times lower linear wear rate and 200 times lower volumetric wear rate than conventional UHMWPE bearings . Wear debris essentially originate from the interface of femoral head-acetabular liner and acetabular liner-shell. A wear resistant alloy coating on acetabular shell not only reduces its wear rate but also eliminate the use of UHMWPE liner. The benefit of this is obvious: elimination of wear induced osteolysis and possible use of large diameter femoral heads. While a wear resistant alloy coating on acetabular shell seems plausible there is only one metallic alloy combination, i.e., Co–Cr–Mo and Ti6Al4V, suitable for surgical implant, which shows metallurgical incompatibility in terms of intermetallic compound formation. In addition to elastic moduli, mismatch in coefficient of thermal expansion and hardness between the coating and the substrate material could lead to excessive residual stresses in the coatings and consequent delamination/cracking and premature failure.
Functionally gradient materials (FGM) are characterized by gradual changes in composition, crystallinity, and/or grain structure from one interface to another. This uniform structural change across the interface provides a unique functionality and performance for biomedical applications [15, 16]. For example, implants with gradients in porosity and pore sizes that can allow on one side of the implant high vascularization and direct osteogenesis, while promoting osteochondral ossification on the other, is appealing in terms of reproducing multiple tissues and tissue interfaces on the same implant. Functional gradation of porosity across the implant section not only reduces the stiffness of the implant but also improves the adhesion to surrounding tissue. In addition, unitized structure with porosity on one side, which will be in contact with bone, can improve cell–material interactions, and the hard coating on the other side can increase the wear resistance of the structure in contact with femoral head. We have used LENS™ to successfully fabricate such functional FGM implants with compositional as well as porosity variations to allow bone tissue ingrowth.
2 Laser engineered net shaping (LENS™)
LENS™ process is characterized by high solidification rates in the range of ∼103–105 K/s leading to several microstructural benefits such as (i) suppression of diffusion controlled solid-state phase transformations, (ii) formation of supersaturated solutions and nonequilibrium phases, (iii) formation of extremely fine, refined microstructures with little elemental segregation, and (iv) formation of very fine second phase particles such as carbides. Since the fabrication is carried out in a protective atmosphere with oxygen content less than 10 ppm, LENS™ processed materials can retain high purity and desired phases of the feedstock powder. Also, LENS™ allows us to tailor microstructure, porosity, shape, and size of the part in one operation by controlling different process parameters. This process also allows the user to fabricate functionally graded materials such as materials with gradient in composition and/or porosity across the section. Gradient in porosity can be achieved by changing the tool path and is useful to replicate bone structure/porosity into load bearing metal implants thus eliminating the stress-shielding effect. Finally, the process is reliable and has the potential for direct low-volume manufacturing.
3 Porous metals for stress-shielding
4 Functionally graded structures for wear resistance
We have demonstrated that application of LENS™ to fabricate novel porous and unitized structures with functional gradation in composition and/or porosity can potentially eliminate the long standing issues such as stress-shielding, poor interfacial bond between the host tissue and the implant, and wear induced bone loss, in load bearing implants to increase in vivo life time. Porosities, pore characteristics and mechanical properties of laser processed structures can be tailored to suite various biomedical applications by changing LENS™ process parameters. Apart from other processed materials, porous NiTi alloy samples having density in the range of 64–88% with 2–18 GPa moduli and 2–4% recoverable strain has significant potential in next generation load bearing implants.
Authors would like to acknowledge financial support from the Office of Naval Research under the grant number N00014-1-05-0583. We also like to acknowledge financial support from the W. M. Keck Foundation for establishing a Biomedical Materials Research Lab at WSU.