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Laser-aided 3D Printing Porous Coating

Longevity of cementless arthroplasty is determined by the characteristics of the porous structure formed at the surface.
However, currently used artificial joint surface coating technologies have several limitations.
To overcome the limitations of currently used technologies. New artificial joint surface coating that controls porosity of the porous structure formed at the surface of the artificial joint was developed based on laser-aided direct metal tooling (DMT) technology, which is a three-dimensional (3-D) additive manufacturing (AM) technology. DMT exhibited characteristics comparable, if not better, than the existing commercial TPS in terms of mechanical and physical properties. DMT may be useful for cementless artificial joint surface coating required the porosity control of the porous structure formed at the surface of the artificial joint and provides enhanced longevity and patient prognosis compared to the existing surface coating technologies [Ref. 1].

BENCOX Z MIRABO CUP is the world’s first artificial acetabular cup coated with Laser-aided DMT technology. Since 2014, It has been implanted in Korea.

  • Advantages
    • The essential features of an additive manufacturing, namely, direct metal tooling (DMT), is used for depositing a porous surface coating on Ti-6Al-4V ELI alloy substrates. Properties of this coating were compared to those of coatings deposited using plasma spraying in air (TPS), which is a popular method used to deposit surface coatings on parts of implants used in cementless arthroplasties. While the thicknesses of the DMT and TPS coatings were comparable, DMT coatings had larger pores, were more porous, were rougher, and more resistant to abrasion wear than TPS coatings[Ref. 1].

  • Porosity
    • Morphologies of a DMT coating (left) and TPS coating (right) on a Ti-6Al-4V alloy substrate

      Average porosity (65 ± 5%) of DMT specimens was within the range of porosity for human cancellous bone (50~90%),compared to TPS specimen (40 ± 5%) which is less than porosity range for human cancellous bone. Also, in spite of weakness of mechanical and physical characteristics of coating structure that can be induced by approximately 1.6 times higher porosity of DMT specimens compared with that of TPS specimens, DMT specimens exhibited characteristics comparable, if not better, than the existing commercial TPS specimens in terms of mechanical and physical properties. These may indicate that DMT creates a more proper biomimetic porous structure with desirable mechanical and physical characteristics, compared with TPS. Therefore, DMT can be regarded as a more appropriate surface coating technology for cementless arthroplasty than the existing TPS technology. Furthermore, since DMT can control porosity, it may be utilized to create a patient-specific porous structure on cementless arthroplasty.

  • Abrasion test
    • The wear of the DMT specimen (40.6 ± 10.6 mg) was 42.9% lower than the TPS specimen (71.1 ± 4.2 mg). Furthermore, at the end of the tests, surface damage was visually apparent on TPS coatings but not on DMT coatings. These results indicate that using DMT on arthroplasty may decrease the chance of failures at the coating layer itself rather than the adhesive interface between the substrate and coating layer compared to TPS. When fractures occur at the coating layer, debris separated from the coating layer forms, subsequently loosening the implant directly. In addition, the debris can cause inflammation inside the bones loosening the implant indirectly due to bone resorption

      Summary of the abrasion test results: mass loss (a) and state of the surface of DMT-coated (top) and TPS-coated specimens after 100 cycles (b)

  • Roughness
    • Our results indicate that both the Ra (77.1 ± 11.5 μm) and Rz (370.5 ± 56.5 μm) values for DMT specimens were higher than the Ra (48.9 ± 3.9 μm) and Rz (251.5 ± 20.6 μm) values for the TPS specimens, by approximately 57.7% and 47.3%, respectively (p < 0.05). These results indicate that DMT technology may induce osseointegration with cancellous bones more efficiently when implanted in the human body. Furthermore, the efficient induction of osseointegration may increase initial fixation after implantation, thereby increasing the longevity of arthroplasty. Thus, compared to existing TPS, DMT used on arthroplasty may be a more beneficial and an effective approach

      Results of the roughness value for DMT and TPS

  • References
    • 1. TaeJin Shin, Sung-Jae Park, Kwan Su Kang, Jung Sung Kim, Yongsik Kim, Youngwook Lim, Dohyung Lim, A Laser-Aided direct metal tooling technology for artificial joint surface coating, International journal of precision engineering and manufacturing Vol. 18, No. 2, pp. 233-238.

Biomembrane-Mimic Polymer Coating

  • MPC grafted CLPE

    * MPC : 2-Methacryloyloxyethyl phosphorylcholine

    • The 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer is our original biocompatible polymer whose side chain is composed of phosphorylcholine mimicking the neutral phospholipids of biomembranes.
      The MPC grafting onto the surface of medical devices has already been shown to suppress biological reactions even when they are in contact with living organisms, and is now clinically used on the surfaces of intravascular stents, intravascular guide wires, soft contact lenses and the oxygenator(artificial lung) under the authorization of the Food and Drug Administration of the United States.
      A hundred nanometer scaled thickness of a polymer layer by the chemical grafting of the MPC onto the surface of the CLPE increases the lubricity to the same level as natural cartilage.
      MPC is a water soluble, lubricious, and a biologically inert polymer that could be used as an acetabular liner and tibial insert.

  • Highly hydrophilic
    • The static water-contact angle of the untreated CLPE was approximately 90°, and it drastically decreased (approximately 15°) due to MPC-grafting.
      The water molecules adsorbed on the surface of the highly hydrophilic MPC act as lubricants and reduce the interaction between the MPC and the counter-bearing face.
      Therefore, it is thought that the artificial hip joint bearing with the MPC-grafted surfaces exhibited considerably higher lubricity than that with the untreated CLPE.

  • Low friction surface (friction coefficient)
    • As compared to the untreated CLPE specimens, the PMPC-grafted CLPE specimens showed a reduction of approximately 82% in their coefficients of dynamic friction.
      Therefore, it is thought that the artificial hip joint bearing with the MPC-grafted surfaces exhibited considerably higher lubricity than that with the untreated CLPE.

  • Low friction surface (confocal data)
    • The PMPC-grafted CLPE disks were found to wear significantly less than the untreated CLPE disks after the friction test.

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