Introduction

The osteogenic capability of any biomaterial is governed by a number of critical surface properties such as surface energy, surface potential, and topography. Prior work suggested that the Si-Y-O-N phase(s) present in the form of a thin (<150 nm), interrupted film at the surface of an annealed silicon nitride bioceramic may be responsible for an observed upregulation of osteoblastic activity due to passive surface properties and dissolution of chemical species. In this study high- resolution analytical electron microscopy was utilized to identify the Si-Y-O-N phase present on the annealed silicon nitride surface, and dissolution studies were employed to elucidate mechanisms of the material's favorable cell interactions.

Introduction

Support of appositional bone ingrowth and resistance to bacterial adhesion and biofilm formation are preferred properties for biomaterials used in spinal fusion surgery. Although polyetheretherketone (PEEK) is a widely used interbody spacer material, it exhibits poor osteoconductive and bacteriostatic properties. In contrast, monolithic silicon nitride (Si₃N₄) has shown enhanced osteogenic and antimicrobial behavior. Therefore, it was hypothesized that incorporation of Si₃N₄ into a PEEK matrix might improve upon PEEK's inherently poor ability to bond with bone and also impart resistance to biofilm formation.

Silicon nitride spinal fusion cages have been successfully used in the treatment or correction of stenosis, disc herniation, trauma, and other deformities of the spinal column since 2008. To date over 14,000 devices have been implanted with perioperative and postoperative complication rates of less than 0.2%. This remarkable achievement is due in part to the material itself. Silicon nitride is an ideal interbody material, possessing high strength and fracture toughness, inherent phase stability, biocompatibility, hydrophilicity, excellent radiographic imaging, and bacterial resistance. These characteristics can lead to implants that aid in prevention of nosocomial infections and achieve rapid osteointegration. In this paper, we will review the various in vitro and in vivo studies that demonstrate silicon nitride's effective bacteriostatic and osteointegration characteristics, and compare these to the two most common cage materials – titanium and poly-ether-ether-ketone (PEEK). Human case studies will be also reviewed to contrast the clinical performance of these biomaterials. In comparison to the traditional devices, silicon nitride shows lower infection rates, higher bone apposition, and essentially no fibrous tissue growth on or around the implant. To better understand the mechanisms underlying these benefits, surface characterization studies using scanning electron microscopy coupled with XPS chemical analyses, sessile water drop techniques and streaming zeta potential measurements will be reported. Data from these studies will be discussed in relation to the physiochemical reasons for the observed behavior. Silicon nitride is a non-oxide ceramic in its bulk; but possesses a protective Si-N-O transitional layer at its surface. It will be shown that the chemistry and morphology of this layer can be modified in composition, thickness and structure resulting in marked changes in chemical species, surface charge, isoelectric points and wetting behavior. It is postulated that the needle-like grain structure of silicon nitride coupled with its enhanced wettability play important roles in inhibiting biofilm formation, while its surface chemical environment consisting of silicon diimide Si(NH)₂, silicic acid Si(OH)₄, and derivatives of ammonia, NH₃, NH₄OH, lead to improved bone reformation and bacteriostasis, respectively. Few materials have this combination of properties, making silicon nitride a unique biomaterial that provides improved patient care and outcomes with low comorbidities.

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.

Introduction:

Patient-specific cutting guides (PSCG) built from imaging of the extremity can improve the accuracy of bone cuts during total knee replacement (TKR). Some reports have suggested that PSCG offer only marginal improvement in the accuracy of alignment and component positioning in TKA. We compared outcomes between TKRs done with PSCG versus standard, intramedullary-based instrumentation.

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.