Spinal Implants


Silicon nitride—A ceramic surgical
implant material

SINTX Technologies is focused on medical-grade silicon nitride. 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, Silicon nitride is biocompatible, bioactive, and has shown bacterial resistance and superb bone affinity3. With >35,000 human spine implantations over 10 years, and a very low percentage of reported adverse events, silicon nitride has an excellent safety record4.

SINTX’s medical-grade silicon nitride has the following advantages:

  • Material phase stability 5
  • Strength and fracture toughness 6
  • Hydrophilicity 7
  • Bacterial resistance 5,8
  • Favorable imaging 9
  • Wear resistance 5,8
  • Osteoconductivity 10,11
  • Osteoinductivity 12,13
  • Non-conductive (electrical)
  • Metal-ion Free

Our Technology is Backed by Strong Data

SINTX has published many studies supporting the benefits of silicon nitride material. Although well-proven in industry14, silicon nitride was first used in a spinal implant trial that began in 198615 with follow-ups at 1, 5, 10, and 30 years. Outcomes showed substantial pain reduction, as well as solid interbody spine fusion, demonstrating the favorable properties of silicon nitride15.

The biological response of living tissue to silicon nitride was described in detail in a 1989 study16. By 12 weeks, up to 90% of the surface of porous silicon nitride contained woven trabecular bone and 75% of all pores were occupied by mature lamella bone. These results showed the favorable osteoconductive properties of silicon nitride.

Later studies with silicon nitride demonstrated an osteoinductive effect on the proliferation and differentiation of human bone marrow cells and the formation of a mineralized matrix17,18,19. The Si3N4 materials were hydrophilic, with a negatively-charged surface at homeostatic pH; properties that contributed to rapid protein adsorption and bone formation.

Further work on implanted silicon nitride in rabbits showed that osteoblasts and osteocytes directly contacted the Si3N4 implants along with a matrix of collagen I and III. Bone remodeling around the Si3N4 implants was enhanced compared to titanium controls20,21,22.

In another study, researchers compared silicon nitride, titanium (Ti), and polyether ether ketone (PEEK)23. Three months after aseptic implantation into rats, Si3N4 showed better appositional healing (Si3N4, Ti, and PEEK showed appositional healing of 65%, 19%, and 8%, respectively). While the addition of Staphylococcus epidermidis bacteria reduced bone formation; the Si3N4 implants still proved better, (i.e., 25% appositional healing for Si3N4, versus 9% for Ti, and 5% for PEEK.)

The reason that Si3N4 shows superior osseointegration and osteoconductivity is related to the elutable surface chemistry of silicon nitride7,10,11,12,13,24,25,26,27. Once implanted, silicon nitride’s surface reacts with water to form silicic acid (H4SiO4) and ammonia (NH3) in accordance with the following chemical reaction:

Si3N4 + 12H2O → 3 Si(OH)4 + 4NH3

Bioavailable silicon in the form of silicic acid enhances osteogenic activity28,29,30,31,32,33,34,35,36 while various nitrogen-based moieties can either be mild disinfectants or powerful oxidants that disrupt microbial cellular functions27,37,38,39,40,41,42,43,44,45. In addition, silicon nitride’s surface charge, wettability, and phase chemistry also contribute to enhanced osteoconductivity.

Several human studies support the above data concerning silicon nitride. A 24-month clinical trial46 compared PEEK cages with autograft bone to porous Si3N4 without added bone graft in cervical fusion. Results showed that porous Si3N4 spacers achieved spinal fusion exclusive of autograft bone. Two other clinical trials comparing cervical fusion rates for non-porous silicon nitride and PEEK cages or allograft spacers showed earlier and more effective fusion with silicon nitride47,48. Case studies have shown the effectiveness of silicon nitride in abating in-vivo infections49 and in achieving solid arthrodesis in the lumbar spine50.

Aside from the properties that make silicon nitride attractive in industry, (i.e., superior strength, wear resistance, corrosion resistance, and fracture toughness51) there is a related set of attributes that make silicon nitride attractive as a biomaterial.

Why Silicon Nitride is so effective for Spinal implants:

Surface topography: Silicon nitride’s topography is apparent at the micron and sub-micron scales. The surface of as-fired silicon nitride consists of anisotropic grains that are typically ≤ 1 µm up to 10 µm with individual features (i.e., asperities, sharp corners, points, pits, pockets, and grain intersections) that can range in size from < 100 nm to 1µm. While this structure is morphologically different from surface-functionalized titanium, it has some common features (e.g., sharp corners, points, and pockets). Ishikawa et al. recently demonstrated that this type of surface microstructure is important in resisting bacterial attachment while concurrently promoting mammalian cell adhesion and proliferation52.

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. Living cells adhere preferentially to silicon nitride over polymer or metal53. Cell adhesion promotes tissue development and enhances the bioactivity of materials. Cell adhesion to silicon nitride is a function of pH, chemical, and ionic changes at the material’s surface.

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 polyether ether ketone 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 offers a potential easy solution; it is inherently resistant to bacteria and biofilm formation54,56. In addition, a recent study has shown a direct bactericidal effect against an oral pathogen26. 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: Silicon nitride implants are radiolucent with clearly visible boundaries and produce no artifacts or scattering under CT and no distortion under MRI. This enables an exact view of the implant for precise intraoperative placement and postoperative fusion assessment9.

Conclusion

With an expanding, ageing and more active population, biomaterial innovations will lead to improved biomedical implant safety, high-performance, and lifetime durability. Already well-proven in diverse industrial applications and currently utilized as intervertebral spinal fusion cages, silicon nitride has the foundational evidence to be applied likewise across a range of biomedical applications.

We invite you to contact SINTX Technologies to see how silicon nitride might be beneficial to your patients and practice.

SINTX is the exclusive manufacturer of silicon nitride spinal implants for CTL-Amedica.

References

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