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Spinal Implants

Silicon nitride is an ideal material for medical implants. Its wear rate is extremely low, and the wear particles are soluble and can be cleared from the body. Silicon nitride is chemically resistant, and it has a high dielectric constant, which confers resistance to fretting corrosion.

When used to make spinal fusion implants, silicon nitride has the flexibility of a dense, porous, or combined architecture that can mimic the cortical-cancellous structure of bone1, 2. Cumulative silicon nitride implantations through 2018 total about 35,000. Of these, fewer than 30 FDA-reportable adverse events manifested, with no implant-related infections relative to an industry standard of 3–10%

Why Is Silicon Nitride So Effective for Spinal implants?

Surface Topography

Surface chemistry is a major factor in the success of any implantable device. Compared to polymer polyetheretherketone (PEEK) or titanium, silicon nitride is hydrophilic, i.e., it attracts body fluids containing proteins and bone-forming cells that are critical to bone healing. Simple manufacturing variations, such as glazing or heating in a nitrogen or oxidizing atmosphere, can modify implant surface chemistry, which allows tailoring of implant chemistry to specific biomedical applications.

Bone Healing

Silicon nitride turns on osteoblasts (bone-forming cells) and suppresses osteoclasts (bone resorbing cells). A manufacturing change called “nitrogen-annealing” results in a near 200% increase in bone formation by cells exposed to silicon nitride13. This finding has excellent implications for speeding up bone healing, bone fusion, and implant integration into the skeleton.

Composite Devices

In a human clinical trial, a composite spine interbody device made of solid and porous silicon nitride fused the cervical spine without any autograft bone filler47. Bioactive silicon nitride powder has also been incorporated into PEEK to form a polymer-ceramic composite. This new composite resists bacterial adhesion while promoting bone formation in a similar fashion as monolithic silicon nitride54. Composite devices based on silicon nitride herald a new class of reconstructive implants2, 55.

Bacterial Resistance

Bacterial infection of any biomaterial implant is a serious clinical problem. Silicon nitride is inherently resistant to bacteria and biofilm formation5456. The antibacterial behavior of silicon nitride is probably multifactorial, and relates to surface chemistry, surface pH, texture, and electrical charge7. Optimizing these surface properties for specific implants is a clear advantage of the material.

Superior Imaging

On X-ray images, plastic implants are invisible while metals obscure the visibility of bone. CT scans and MRI images are also distorted by metal implants. Implants made of silicon nitride are visible on X-ray images without obscuring the underlying bone details. Also, silicon nitride implants allow for distortion- and artifact-free MRI and CT images, thus giving a clear assessment of the implant, bone, and surrounding areas.
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Valeo II LL
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Valeo II PL
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Valeo AL
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Valeo VBR
Valeo II C
Valeo II C

References

1 K. Bodišová, M. Kašiarová, M. Domanická, M. Hnatko, Z. Lencéš, Z.V. Nováková, J. Vojtaššák, S. Gromošová, et al., “Porous Silicon Nitride Ceramics Designed for Bone Substitute Applications,” Ceram. Int., 39 [7] 8355–8362 (2013).

2 K.S. Ely, A.C. Khandkar, R. Lakshminarayanan, and A.A. Hofmann, “Hip Prosthesis with Monoblock Ceramic Acetabular Cup,” US Pat. 8,133,284, (2012).

7. R.M. Bock, B.J. McEntire, B.S. Bal, M.N. Rahaman, M. Boffelli, and G. Pezzotti, “Surface Modulation of Silicon Nitride Ceramics for Orthopaedic Applications,” Acta Biomater., 26 318–330 (2015).

13 G. Pezzotti, B.J. McEntire, R. Bock, M. Boffelli, W. Zhu, E. Vitale, L. Puppulin, T. Adachi, et al., “Silicon Nitride: A Synthetic Mineral for Vertebrate Biology,” Sci. Rep., 6 31717 (2016).

47. H.T. Ball, B.J. McEntire, and B.S. Bal, “Accelerated Cervical Fusion of Silicon Nitride versus PEEK Spacers: A Comparative Clinical Study,” J. Spine, 6 [6] 1000396 (2017).

54. G. Pezzotti, E. Marin, T. Adachi, F. Lerussi, A. Rondinella, F. Boschetto, W. Zhu, T. Kitajima, et al., “Integrating the Biologically Friendly Chemistry of Si3N4 Bioceramics to Produce Antibacterial, Osteoconductive, and Radiolucent PEEK Spinal Implants,” Macromol. Biosci., (in press) (2018).

55. R.M. Taylor, J.P. Bernero, A.A. Patel, D.S. Brodke, and A.C. Khandkar, “Silicon Nitride – A New Material for Spinal Implants,” J. Bone Jt. Surg., 92–Br [Supp I] 133 (2010).

56. D.J. Gorth, S. Puckett, B. Ercan, T.J. Webster, M. Rahaman, and B.S. Bal, “Decreased Bacteria Activity on Si3N4 Surfaces Compared with PEEK or Titanium,” Int. J. Nanomedicine, 7 4829–4840 (2012).

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