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Orthopaedic Proceedings
Vol. 98-B, Issue SUPP_1 | Pages 86 - 86
1 Jan 2016
Clarke I Pezzotti G Lakshminarayanan A Burgett-Moreno M Donaldson T
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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 heads2,3, alumina heads4,5 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.


Orthopaedic Proceedings
Vol. 98-B, Issue SUPP_1 | Pages 34 - 34
1 Jan 2016
McEntire B Bal BS Lakshminarayanan A Bock R
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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.


Orthopaedic Proceedings
Vol. 95-B, Issue SUPP_15 | Pages 129 - 129
1 Mar 2013
McEntire BJ Lakshminarayanan A Bal BS Webster T Ercan B Gorth D
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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.

Methods

Specimens of 10 mm × 10 mm by 1.75 mm size were implanted into experimental calvarial defects in 2-year old Wistar rats using standard surgical techniques (n's: SiN=48; PEEK=24; Ti=24). One group of animals was immediately inoculated with 1 × 104 Staphylococcus epidermidis; control animals received saline only. After sacrifice at 3 days, 7 days, 14 days, and 3 months post-inoculation (n=4 rats per time period), one calvarial sample each for PEEK and Ti, and two samples for SiN (per bacterial condition and time point) were retrieved for histology; remaining samples were used for sample push-out testing with a Micro Tester 5848 (Instron) with a 1-kN load cell, using published techniques. New bone formation was measured with tetracycline double-labeling at 11 and 4 days before the 14-day and 3-month time periods.


Orthopaedic Proceedings
Vol. 95-B, Issue SUPP_15 | Pages 8 - 8
1 Mar 2013
McEntire BJ Lakshminarayanan A Bal BS Webster T
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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.