Silicon nitride ceramics could be positioned as the next platform technology within the orthopaedic devices industry.

FDA-cleared Si3N4 lumbar spacer.

Industrial-grade silicon nitride (Si3N4) has long been the material of choice for applications demanding high strength, low wear and exceptional durability. The earliest reports of synthetic Si3N4 date back to 1859, and evidence exists of a German patent for the production of the compound through carbothermal reduction of SiO2 as early as 1896. Scientists also reviewed the chemical formula and atomic structure of Si3N4 in 1910 and 1926, while reports of its usefulness as a refractory material first appeared in the early 1950s. The Carborundum Co. filed patents for “mechanically strong articles … consisting of and/or bonded by silicon nitride” in 1952.

By the early 1960s, the development of Si3N4 ceramics as a possible high- temperature structural material had become widespread, leading to several large development programs in the U.S. in the 1970s and 1980s. Considerable work on Si3N4 has been conducted over the past 30 years, primarily in regard to bearings, cutting tools, heat-engine ceramics and electronics.

Because it approaches the equivalent hardness of diamond, Si3N4 also provides an ideal wear surface that resists scratching and pitting. Its wear resistance, hardness and other key properties have prompted such industries as automotive, wind power, motorsports, space exploration, metalworking and electronics to develop a variety of products using components made from Si3N4. Products include bearings, metal-cutting tools, insulators, auto engine parts and rocket thruster components.

In the past few years, a new niche for Si3N4 has emerged in the form of orthopaedics. This market is developing rapidly, with constant technological advances finding their way into operating rooms worldwide.

FDA-cleared Si3N4 interbody fusion device used in the cervical spine.

From R&D to the Operating Room

In orthopaedics, clinical experience has shown that the ceramic materials, when articulating against well-known and established metal implants, exhibit low wear characteristics with substantial elimination of wear debris, which significantly reduces the risk for osteolysis (a common joint disease associated with wear debris). Also, the high strength of Si3N4 allows for greater flexibility in implant design and reduced risk of in-vivo fractures or dislocation.

Ceramics first came into use in the orthopaedics arena in the 1960s, with the development of a femoral head using alumina (Al2O3) for hip joint arthroplasty. The development of toughened zirconia for orthopaedic uses followed in the late 1980s and early 1990s. Around that same time, a team of researchers* began experimenting with Si3N4 for orthopaedic applications.

*Research performed at Amedica Corp., Salt Lake City, Utah.

Traditional ceramics, such as alumina, are prone to brittle fracture as a result of their low toughness. Such brittle failure in ceramic materials typically results from the development of microcracks at and just below the surface. Other ceramics-particularly Si3N4, when enhanced with other proprietary compounds-have a higher toughness, and thus they have significantly higher reliability in orthopaedic applications. Using ceramics like Si3N4 can allow significant improvements in wear properties and long-term reliability.

Researchers have found a way to convert a proprietary blend of silicon nitride powder into a strong, lightweight micro- composite ceramic.** This ceramic material can be used to produce interbody fusion devices for the spine and articulating hip and knee replacement joints-areas where high wear resistance and strength are critical. The material exceeds the strength of other popular implant materials and offers additional key benefits.

**Known as MC2® and developed by Amedica Corp.

To initially create this unique material for orthopaedic applications, the research team blended powders of Si3N4 with selected sintering aids to obtain a doped composition. The dopants were optimized to achieve the highest density and mechanical properties. The homogenous powders were then pressed using conventional dry or isostatic pressing; this was followed by sintering in a controlled atmosphere. Parts were subsequently hot isostatically pressed to remove or minimize residual porosity or other flaws. The process was found to yield a uniform, fine-grained microstructure. Several use patents for this materials technology in orthopaedics have been granted, and formal manufacturing operations began in 2004.

Benefits in Orthopaedics

Since 2004, Si3N4 ceramic technologies have proven valuable in joint replacement protocols because the material resists deformation and is both chemically inert and biocompatible. Ceramics, in essence, are part of the body. Bone is a calcium hydrophosphate-based material, which is a ceramic; thus, the body does not treat implants made from ceramics as a foreign material in the same way that it can with polyetheretherketone (PEEK), which is one of the most common non-ceramic materials used in implant technology. With PEEK implants, fibrous tissue layers are often observed growing around the implant device.

In addition, orthopaedic implant devices made from the micro-composite ceramic are 20 times stronger than PEEK, yet they boast a hydrophilic, roughened surface that may provide a better setting for healing within the body than PEEK’s hydrophobic surface. Radiographically, Si3N4 implants are radiolucent and offer clearly visible boundaries. They also produce no MRI or CT imaging artifacts, which is a major advantage for intraoperative implant placement and post-op assessments.

Future Prospects

Silicon nitride has been cleared by the U.S. Food and Drug Administration (FDA) for use as a cage in spinal fusion surgeries. The hip replacement implant is currently undergoing testing in preparation for FDA approval. In the future, Si3N4 will be used as an articulating implant for total disc replacement; knees, shoulders, digits or any other articulating joint are candidates for the technology.

Overseas, the technology has expanded even further to encompass cancellous structured ceramic (CSC). Similar in structure and porosity to natural cancellous bone, CSC may prove effective as an osteoconductive bone scaffold. CSC has potential application as an insert in spinal spacers, as well as a porous backing for total disc replacements and in hip and knee implants. The CSC material, however, has not yet received FDA approval for use in the U.S.

It is apparent that the rapidly growing market for Si3N4 ceramic use in orthopaedics has ample room to expand, both in types of devices developed and in variations on the material itself. Future prospects for using this material for articulation devices (e.g., hip, knee, elbow and shoulder replacements) look positive. When ceramics, and particularly silicon nitride, are used in articulating devices, they help reduce the risk of wear.

For example, consider a total hip arthoplasty (THA): Current technology uses a metal femoral head made typically from cobalt-chromium (CoCr) that articulates against a plastic cup in the acetabular region of the hip. As patients use the motion of the device to walk, the cup generates wear debris that may fall down into the joint area, potentially causing disease and damage to the surrounding tissue.

As a result, most hip replacements have a life span of 10-15 years. If the typical age of a hip replacement patient is 57-58 years old, and if that patient lives until age 90, he or she could conceivably require two additional hip revision surgeries after the initial one.

Conceptually, the advantage of using a ceramic cup made from Si3N4 is that the wear factor is reduced essentially to zero, and it likely will stay in place and functional for the patient’s remaining lifespan. It will probably be at least 2-3 years until this technology is approved, but the precedents are there in the form of alumina ceramic and zirconia-toughened alumina, both of which have been used in THA procedures.

Silicon nitride demonstrates improved mechanical, friction and wear characteristics over both of its ceramic predecessors. It is headed in the right direction as the preferred material for a variety of orthopaedic applications; the research is thorough and the patents are falling into place. Further investigations will help cement this position.

For the past two years, devices using Si3N4 ceramic technology for spine treatments have been implanted with exceptional outcomes in nearly 2000 patients globally and as part of a clinical study for joint surgeries. The results of these treatments, as well as various research study findings, will be used to advance product acceptance in the marketplace and continue to position Si3N4 ceramics as the next platform technology within the orthopaedic devices industry.

For more information on Si3N4 ceramics for orthopaedic applications, visit www.amedicacorp.com.