Introduction

Looking for optimal solutions to wear risks evident in total hip arthroplasty (THA), silicon nitride ceramic bearings (Si₃N₄) are noted for demanding high-temperature applications such as diesel engines and aerospace bearings. As high-strength ceramic for orthopedic applications, Si₃N₄ offers improved fracture toughness and fracture strength over contemporary aluminas (Al₂O₃). Our pilot studies of Si₃N₄ in 28mm diameter THA showed promising results at ISTA meeting of 2007.¹ In this simulator study, we compared the wear resistance of 40mm to 28mm diameter Si₃N₄ bearings.

The 28mm and 40mm bearings (Fig. 1) were fabricated from Si₃N₄ powder (Amedica Inc, Salt Lake City, UT).¹ Wear tests run were run at 3kN peak load in an orbital hip simulator (SWM, Monrovia, CA) and. The lubricant was standard bovine serum (Hyclone: diluted to 17 mg/ml protein concentration). Wear was measured by gravimetric method and wear-rates calculated by linear regression. SEM and interferometic microscopic was performed at 3.5-million cycles (3.5Mc) to 12Mc.

The simulator was run to 3.5Mc duration with no consistent weight-loss trends. The bearings could show either small positive or negative weight fluctuations in an unpredictable manner (Fig. 2). Surface analysis showed protein layers up to 3μm thick, furrowed due to abrasion by small particles (Fig. 3). The low ceramic wear was camouflaged by protein contaminants alternatively forming and shedding. From 3.5 to 12.8Mc duration we experimented with various detergents and wash-procedures, all to no avail. Protein coatings were also more prevalent on 44 mm heads, likely due to frictional heating by the larger diameter effect. Selected heads were washed with a mild acid solution - the cumulative effect appeared to be removal of some protein layers, but not in a predictable manner.

The Si₃N₄ ceramic is used in demanding industrial applications and it is therefore unfortunate that we are yet not able to quantify the actual wear performance of Si₃N₄/ Si₃N₄ bearings (COC). The contaminating protein layers combined with low-wearing silicon nitride obscured the actual wear data. This has also been a problem in prior studies with alumina and zirconia bearings. Considerable challenges still stand in the way of the optimal biomaterials choices that will result in reduced risk of failure while providing extended lifetimes. Thus important issues remain unsolved and call for innovative solutions. Searching for a more effective ‘wear-measurement’ remedy, we noted that abrasive slurries of bone cement (PMMA) used in contemporary simulator studies were effective in promoting adverse wear in polyethylene bearings. These investigations also revealed that PMMA debris did not damage CoCr heads^2,3, alumina heads^4,5 or diffusion-hardened zirconia heads (ZrDH).⁶ We can therefore speculate at this ISTA meeting of 2014 that future ceramic wear tests should incorporate PMMA slurries. Here a new hypothesis can be formulated, that PMMA particulates will provide a continual and beneficial removal of contaminating proteins from the ceramic surfaces (see Fig. 3) and thereby aid definition of low-wearing COC bearings such as Si₃N₄.

The application of non-oxide ceramics such as silicon nitride presented here may become a viable alternative for THA designs of next decade.

It has been seven years since silicon nitride (Si₃N₄) was first proposed as a new bearing material for total hip arthroplasty [1]. Although its introduction into this application has been hampered by regulatory and clinical hurdles, it remains a strong candidate for advancing the state of care in patients undergoing joint replacement. Si₃N₄ has a distinctive set of properties, such as high strength and fracture toughness, inherent phase stability, low wear, scratch resistance, biocompatibility, hydrophilicity, excellent radiographic imaging, and bacterial resistance, many of which are not fully realized with other bioceramics. This combination of properties is desirable for demanding structural implants in the hip, knee and other total joints. Of foremost concern to clinicians is the wear behavior of any new or novel bearing material. Minimization of wear debris and prevention of corresponding osteolytic lesions are essential regardless of whether the artificial implant is articulating against itself, a metallic or polymeric counterpart. In this regard, Si₃N₄ may have a unique advantage. Other bearing couples rely solely on the presence of a biologic lubricating film to minimize erosive wear. However, Si₃N₄ forms a tribochemical film between the articulation surfaces consisting of silicon diimide Si(NH)₂, silicic acid Si(OH)₄, and ammonia groups NH₃, NH₄OH. Depending upon the bearing couple, this tribochemical film generally produces low friction. It is self-replenishing and resorbable, leading to the minimization of wear debris within the joint capsule. In this paper, we will review the essential physical, mechanical, and surface chemistry of Si₃N₄, and contrast these properties with other available bioceramics. Results from hip simulator testing of Si₃N₄ femoral heads on conventional and highly cross-linked polyethylene will be presented and discussed. Data will demonstrate that various Si₃N₄ bearing couples have wear comparable to other bioceramics. Microscopy and spectroscopic examinations of surfaces will provide a view of the surface stoichiometry and chemical stability of Si₃N₄ in comparison to other bioceramics. Laboratory friction tests will be reported, which show that the tribochemistry of the lubricating film generated by Si₃N₄ favors the use of highly cross-linked polyethylene as a counterface material. Overall results will demonstrate that silicon nitride is poised to become a new generation biomaterial for total joint arthroplasty.

Objective

Superior bone ingrowth and resistance to bacterial infection are ideal for orthopaedic implants. We compared new bone formation, strength of bone bonding, and infection rates between silicon nitride ceramic (Si3N4; abbreviated SiN), medical-grade PEEK (PEEK), and titanium (Ti) in rat calvariae. PEEK and Ti are used in spinal and arthroplasty implants respectively, while SiN is a non-oxide ceramic used in spinal implants for the past 4 years.

Oxide ceramics, such as alumina and zirconia have been used extensively in arthroplasty bearings to address bearing wear and mitigate its delayed, undesirable consequences. In contrast to oxide ceramics that are well-known to orthopaedic surgeons, silicon nitride (Si₃N₄) is a non-oxide ceramic that has been investigated extensively in very demanding industrial applications, such as precision bearings, cutting tools, turbo-machinery, and electronics. For the past four years, Si₃N₄ has also been used as a biomaterial in the human body; specifically in spinal fusion surgery. As a implantable biomaterial, Si₃N₄ has unique properties, such as high strength and fracture toughness, inherent chemical and phase stability, low wear, proven biocompatibility, excellent radiographic imaging, antibacterial advantages, and superior osteointegration. This property combination has proven beneficial and desirable in orthopaedic implants made for spinal fusion, spinal disc reconstruction, hip and knee arthroplasty, and other total joints (Fig. 1). The physical properties, shapes, sizes and surface features of Si₃N₄ can be engineered for each application – ranging from dense, finely polished articulation components, to highly porous scaffolds that promote osteointegration. Both porous and polished surfaces can be incorporated in the same implant, opening a number of opportunities for arthroplasty implant design. Crack propagation modes for in situ toughened Si₃N₄ differ favorably from those of conventional ceramics, rendering Si₃N₄ extremely resistant to catastrophic failure in vivo (Fig. 2). Most significantly, our recent work has shown that Si₃N₄ is resistant to bacterial biofilm formation, colonization and growth, when compared to medical-grade PEEK and titanium. These anti-infective characteristics are particularly valuable for in vivo implantation. We will present the unique properties and characteristics of Si₃N₄, and compare these to other ceramic and non-ceramic biomaterials. Si₃N₄ was once used only in industrial applications, but early data show that this novel biomaterial is positively impacting orthopaedic care and will continue to do so into the future.