Introduction

Highly crosslinked, ultra-high molecular weight polyethylene (HXLPE) acetabular liners inherently have a risk of fatigue failure associated with femoral neck impingement. One of the potential reasons for liner failure was reported as crosslinking formulations of polyethylene, increasing the brittleness and structural rigidity. In addition, the acetabular component designs greatly affect the mechanical loading scenario, such as the offset (lateralized) liners with protruded rim above the metal shells, which commonly induce a weak resistance to rim impingement. The purpose of the present study was to compare the influence of the liner offset length on the impingement resistance in the annealed (first generation) and vitamin E-blended (second-generation) HXLPE liners with a commercial design.

In order to improve fast osseointegration, to modulate inflammatory response and to avoid biofilm formation, several attempts of surface modifications of titanium alloy in term of surface topography and chemistry have been performed over years, but this is still an open issue.

In our research work, a patented chemical treatment was developed and tailored to improve fast osseointegration and to allow further surface functionalization in order to get a multifunctional surface.

After the chemical treatment, Ti6Al4V shows a micro and nano-textured surface oxide layer with high density of hydroxyls groups, as summarized Figure 1: it is able to induce apatite precipitation (during soaking in Simulated Body Fluid), high wettability by blood, specific protein adsorption, positive osteoblast response and surface mechanical resistance to implantation friction.

Hydroxyl groups exposed by the treated surface also allow binding natural biomolecules such as polyphenols, which can further improve the rate and quality of osseointegration by adding anti-inflammatory, antibacterial and antitumoral effects suitable for implants in critical situations. Polyphenols have the further added value of being a low cost and eco-sustainable product, extractable from byproducts of wine and food industry.

On the chemically treated and functionalized samples, the surface characterization was performed using Folin&Ciocalteu test, fluorescence microscopy and XPS analysis in order to check the presence and activity of the grafted biomolecules (polyphenols from red grape pomace and green tea leaves). Cell tests were performed with Kusa A-1 cells highlighting the ability of polyphenols to improve osteoblasts differentiation and deposition of mineralized extracellular matrix.

Surface functionalization can also be performed with chitin derived biomolecules to reduce inflammation.

With the purpose of obtaining the antibacterial effect, during the chemical treatment a silver precursor can also be added to obtain in situ reduced silver nanoparticles embedded in the nano-structured oxide layer. The samples containing nanoparticles on the surface were characterized by means of TEM and FESEM observation highlighting the presence of well distributed and small-sized nanoparticles on the surface and through the thickness of the oxide layer. A long-lasting release in water was observed up to 14 days and antibacterial tests on Staphylococcus aureus showed the ability of the surface to reduce bacteria viability avoiding biofilm formation.

The results showed that the patented chemical treatment can improve the response of osteoblasts to titanium alloy implants, but is also a promising way to obtain multifunctional surfaces with antibacterial, antioxidant, anti-inflammatory and antitumoral properties that can be the future of orthopedic implants.

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.

Introduction

Femoral heads made from zirconia-toughened alumina (ZTA) are the most advanced bioceramic available for total hip arthroplasty. ZTA's superior mechanical properties result from the polymorphic transformation of its zirconia (ZrO₂) phase in the presence of a propagating crack. In vitro derived activation energies predict that several human lifetimes are needed to reach a state of significant transformation;¹ but in vivo confirmation of material stability is still lacking. This investigation determined if transition metal ions might be responsible for triggering the tetragonal to monoclinic (t®m-ZrO₂) phase transformation in this bioceramic.

Introduction

The longevity of highly cross-linked polyethylene (XLPE) bearings is primarily determined by its resistance to long-term oxidative degradation. Addition of vitamin E to XLPE is designed to extend in vivo life, although it has unintended consequences of inducing higher frictional torque and increased wear when articulating against metallic femoral heads.^1–3 Conversely, lower friction was observed when oxide ceramic heads were utilized.³ Previous studies suggest that oxide ceramics may contribute to XLPE oxidation, whereas a non-oxide ceramic, silicon nitride (Si₃N₄), might limit XLPE's degradation.⁴ To corroborate this observation, an accelerated hydrothermal ageing experiment was conducted using static hydrothermal contact between XLPE and commercially-available ceramic femoral heads.

Introduction

Due to its remarkable stoichiometric flexibility and surface chemistry, hydroxyapatite (HAp) is the fundamental structural material in all vertebrates. Natural HAp's properties inspired an investigation into silicon nitride (Si₃N₄) to see if similar functionality could be engineered into this bioceramic. Biological and in situ spectroscopic analyses were used to monitor the response of osteosarcoma cells (SaOS-2) to surface-modulated Si₃N₄ and a titanium alloy after long-term in vitro exposure.

Introduction

Periprosthetic infections are leading causes of revision surgery resulting in significant increased patient comorbidities and costs. Considerable research has targeted development of biomaterials that may eliminate implant-related infections.¹ This in vitro study was developed to compare biofilm formation on three materials used in spinal fusion surgery – silicon nitride, PEEK, and titanium – using one gram-positive and one gram-negative bacterial species.

Introduction

Oxide-based alumina (Al2O3) is used to manufacture femoral heads for total hip arthroplasty (THA). Silicon nitride (Si3N4) is a non-oxide ceramic used to make spinal implants. Ceramic materials are believed to be bioinert, (i.e., stable under hydrothermal conditions). Indeed, clinical data have shown 15–20 year longevity of Al2O3 bearings in THA. In this work, we examined the surfaces of Al2O3 and Si3N4 after exposure to physiologic conditions to see if these ceramics are truly inert.

Introduction

In total hip arthroplasty (THA), polyethylene (PE) liner oxidation leads to material degradation and increased wear, with many strategies targeting its delay or prevention. However, the effect of femoral head material composition on PE degradation for ceramic-PE articulation is yet unknown. Therefore, using two different ceramic materials, we compared PE surface alterations occurring during a series of standard ceramic-PE articulation tests.

Introduction

The in vivo evolution of surface material properties is important in determining the longevity of bioceramics. Fracture toughness is particularly relevant because of its role in wear resistance. Some bioceramics, such as zirconia (ZrO2) undergo in vivo phase transformation, resulting in a marked reduction in toughness and commensurate increased wear. Here, we investigated the effect of accelerated aging on the surface toughness of alumina (Al2O3), zirconia-toughened alumina (ZTA), and silicon nitride (Si3N4) femoral heads, in order to identify the optimal ceramic material for in vivo implantation and long-term durability.

Introduction

Silicon nitride (Si3N4) is a ceramic material presently implanted during spine surgery. It has a fortunate combination of material properties such as high strength and fracture toughness, inherent phase stability, scratch resistance, low wear, biocompatibility, hydrophilic behavior, easier radiographic imaging and resistance to bacterial biofilm formation, all of which make it an attractive choice for orthopaedic applications beyond spine surgery. Unlike oxide ceramics, (e.g., alumina or Al2O3) the surface chemistry and topography of Si3N4 can be precisely engineered to address in vivo demands. Si3N4 can be manufactured to have an ultra-smooth, or highly fibrous, or porous morphology. Its chemistry can be varied from that of a silica-like surface composed of silanol moieties to one which is predominately comprised of silicon-amine functional groups.

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.

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.

Introduction

Vitamin-E (VE, dl-α-tocopherol) is a powerful antioxidant for highly cross-linked polyethylene (XLPE). It was previously reported that VE-stabilized XLPE succeeded in retaining no measurable oxidation even after accelerated aging tests combined with cyclic loading or lipid absorption. Thus, VE-stabilized XLPE is nowadays recognized worldwide as one of the new standard materials in total hip arthroplasty (THA). However, the effects of such VE addition on physical behavior of polyethylene remain to be fully elucidated by contrast to the clear statement of its chemical role (i.e., the enhanced oxidation resistance) in the published literature. In this presentation, we shall attempt to provide those missing notations and to explore the microstructural and biomechanical role of VE in XLPE acetabular liner on the molecular scale.