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.

Total hip arthroplasty (THA) represents a very spread and effective surgical procedure. Surgeons and technologists make daily efforts in improving the outcomes of THA, with the ultimate goal of creating a prosthesis that reliably lasts at least as long as a human lifetime. While the results of primary hip arthroplasty are generally very good, revision surgeries might score variable success with regards to their clinical outcomes. In addition, they invariably represent an expensive procedure and a severe burden to the patients. Thus, a reduction of the failure rates of only a few percents can, due to the large number of patients involved, have a vast influence on the accumulated costs and patient suffering. In other words, the key issue in hip arthroplasty resides in the improvement of the prostheses with regard to their long-term in vivo reliability. These circumstances amply justify a continuous search for new hip prostheses with improved structural characteristics and elongated lifetimes.

Most recent innovative trends in THA have focused on the improvement of the tribological behavior of hip joints and challenged the achievement of a longer durability, with the potential for a service-life spanning several decades. Such trends have naturally led to an increase in the use of ceramic materials, either as ceramic femoral heads yet coupled with advanced acetabular cups made of polyethylene (i.e., with improved molecular structure and quality), or as ceramic hip components for both acetabular and femoral bearing surfaces. The greater driving force in using ceramic bearings is their potential of systematically reducing periprosthetic osteolysis (i.e., mainly arising from polyethylene wear debris), which could potentially reduce the number of surgical revisions. The high inertness and biocompatibility of ceramic materials may also reduce to a minimum the collateral effects on the human body, as possibly observed with metallic prostheses (e.g., contamination by metal ions, hypersensitivity, etc.). Despite those advantages, chipping and fracturing have severely limited the popularity of ceramic components. As a further issue, it should be noted that ceramic-on-ceramic articulations strongly require high precision in setting the orientation of the components during surgery (in order to avoid excessive impingement on the ceramic surface). Partly fractured ceramic bearings necessarily dictate revision. The main reason is that the ceramic remnants in the articulation would give rise to severe third-body wear, especially in the presence of a softer bearing counterpart. Clearly, ceramic components offer a very high potential for further improving both structural performance and lifetime of hip joints but, being made of fragile materials, they also require significant progress in surgery technique, further advancements in joint design and materials manufacturing processes, as well as a peer non-destructive control of their structural reliability.

In this presentation, we shall first have a brief survey on the main cases of failure in the recent history of hip prostheses. Then, a description of the most advanced and recent technological approaches to material preparation, reliability control and non-destructive analysis of hip components will also be given. The main aim of this presentation is to drive the attention of the international orthopaedic community on the need for a highly interdisciplinary approach to the study of hip joint arthroplasty. In this context, we provide here some vivid examples of how newly developed Raman spectroscopic methods may provide final solutions to historical problems related to the chemical and structural reliability of materials widely employed in total hip arthroplasty.

Multiaxial rotation of femoral component is generated in a wide range against UHMWPE tibial insert during ambulation or deep bending activities. Simultaneously, microscopic oscillation and twisting might accompany with such a wide-range motion.

Such a combined in-vivo kinetics is expected to bring more severe wear to the sliding surface of knee joint prostheses than that in a case of single macro-kinetics (i.e., that commonly reproduced by conventional wear simulators). In order to reproduce clinical surface degradation correctly and quantitatively in simulator tests, we have to consider microscopic motions at the joint bearing surfaces. The purpose of this study is to analyze the influence of the composite knee motion on wear using a non-destructive spectroscopic approach.

The crystalline phase in UHMWPE is pre-oriented in the tibial insert from the manufacturing process, but the orientation of crystalline lamellae is sensitive to mechanical loading. Therefore, the orientation of the crystalline lamellae on the surface of retrieved UHMWPE tibial inserts could reflect the local motions in vivo generated in the joint during ambulation. The visualization of (orthorhombic) crystalline lamellae might ultimately lead to the possibility of tracking back the wear history of the joint. In this study, polarized Raman spectroscopy was employed in order to non-destructively visualize the lamellar orientation in UHMWPE tibial inserts, which were retrieved after exposures in human body elapsing several years.

According to this Raman analysis and in comparison with an unused insert, the orientation of surface lamellae was found to have been clearly changed due to wear in accordance to the local motion of the femoral component. Additionally, we could obtain information about the origin of delamination from the in-depth profile for lamellae orientation angle. This study not only shows the possibility of optimizing the UHMWPE structure to minimize wear but also gives a hint for the development of knee simulators of the next generation.

Combined techniques of fracture mechanics and confocal Raman microprobe spectroscopy were applied to characterize, after increasing periods of environmental exposure, bulk and surface toughness values in an advanced alumina/zirconia composite. This material is used in joint prostheses (BIOLOX^® delta femoral heads, manufactured by CeramTec AG). Besides conventional fracture mechanics characterizations, including different types of fracture toughness test, Raman and fluorescence microprobe spectroscopy provided a microscopic insight into the effect of environmentally assisted processes of zirconia phase transformation at the surface on the fracture toughness of the material. We have found that the tetragonal-to-monoclinic polymorphic transformation occurs in the studied composite material as a consequence of an environmentally assisted process, although severe exposures are needed for to obtain a substantial increase of the monoclinic content. Such severe exposures in vitro correspond to exposures in human body of several lifetimes. The effect of an exposure of 10 h in autoclave (in vitro accelerated test) was carefully examined, because this span of time corresponds:

to the period of time recommended for testing in vitro by ISO standard; and,

The main experimental outcomes of confocal Raman spectroscopy and fracture mechanics assessments can be summarized as follows:

the crack-tip toughness level measured in the as-received material was comprehensive of a tangible contribution by transformation toughening, thus showing that phase transformation in the zirconia dispersoids plays a positive role in the toughening behavior of the material;

It appears that concerns arising from the brittleness of alumina-based materials and, thus, from their vulnerability to fracture due to unexpected load situation, can be successfully counteracted by properly adding a dispersion of zirconia particles to the alumina matrix. Such an addition enables the obtainment of a composite material, whose fracture resistance is greatly enhanced by a crack-shielding effect due to phase-transformation processes occurring in the zirconia dispersoids.

Alumina and zirconia have been extensively used for orthopedic implants, such as hip and knee joint replacements. In 1982, Dr Hironobu Oonishi and Kyocera Corp. put the world’s first ceramic knee component, KOM, to practical use. This ceramic knee component shows excellent clinical results for long-time use. Now, the ceramic material of the knee component is changed from the alumina to the zirconia, and over 5000 ceramic components have been used in Japan so far.

The 3 mol% yttria stabilized polycrystalline zirconia (3Y-TZP) has been used as surgical grade zirconia. The 3Y-TZP possesses higher fracture strength and toughness as compared to monolithic alumina. However, it is generally known that spontaneous transformation may also occur at relatively low temperatures in hydrothermal environment as in the case of human body for the 3Y-TZP. Therefore, there is a concern of the degradation of mechanical and wear properties as a consequence of the transformation (low temperature degradation).

Our previous studies confirmed that low temperature degradation can be prevented by optimizing sintering temperature and adopting a Hot Isostatic Press process. In this study, we evaluated the effects of surface nature of ceramic material and heat treatment. Since grinding and polishing of the ceramic implants (e.g. femoral heads) might induce phase transformation, residual stress and microcracks, it is needed to examine the validity of the manufacturing process.

At first, the 3Y-TZP samples with grinded (#400) and mirror polished surfaces were prepared and exposed in saturated vapor in an autoclave at 121°C for 150 h (aging test). Some of the samples were subjected to a heat treatment and then to aging test. Before and after aging test, change in crystal structure was evaluated by X-ray diffraction. Then, for evaluation of the aging effects for microscopic area of the ceramic, a Vickers indentation was introduced on the surface before the aging test. Changes in crystal structure and residual stress were evaluated by a Raman spectroscopic technique.

In the results of the aging test, it was found that the degree of the increase in monoclinic fraction of both grinded and polished surfaces was lower in the samples with heat treatment than in those without heat treatment. These results indicate that the phase stability of the 3Y-TZP was improved by the heat treatment after machining.

Around the indentation, circular-like deformation zone and cracks extending from the four corners of the indentation were observed. After the aging test, the transformed area gradually spread towards neighborhood regions depending on the aging time. Besides, a distinct progress of phase transformation at around the crack was also observed. On the other hand, in the samples subjected to the heat treatment, no transformed area was observed around the indentation. These results suggest that low temperature degradation could be prevented by the heat treatment after the machining.

The residual stress fields induced in phase-transformed areas increased during aging test, and heat treatment after the machining was also able to prevent phase transformation even if surface damages, such as indentation or machining, were introduced on the samples.

The experimental determination of residual stress fields on the surface of retrieved femoral heads represents a fundamental step in understanding their wear degradation behavior and the tribological mechanisms, which are operative on the femoral joint during its working life time. In this work, the surface of retrieved alumina and zirconia (Al2O3 and ZrO2) femoral heads were investigated by piezo-spectroscopic tecniques based both on photoluminescence and Raman effects. The high spatial resolution of the laser, impinging on the investigated surface (typically about 1 micron of lateral resolution), enabled us estimating patterns and magnitude of residual stress in extremely narrow zones, comparable with the grain size of the material.

Four retrieved ceramic femoral heads were analyzed. Two balls were made of alumina with a typical grain size of from 4 to 10 microns. Both alumina balls were retrieved after only few years from implantation, due to septic and aspetic loosening. The remaining two femoral heads were made of zirconia with a typical grain size of 1 micron. These latter balls were retrieved after 2 and 13 years, respectively (both for loosening problems). With a systematic collection of a large number of data on a microscopic level it was possible to assess the retrieved femoral heads in to to, thus extending the microscopic analysis to the entire joint.

In allumina balls retrieved after short time implantation, a macroscopic stress field was found, which arose from manufacturing, loading history, and the displacements acting on the femoral head during its lifetime. This stress field was completely overcome by a microscopic residual stress field due to local contacts (e.g., local shocks owing to microseparation, impinging and wear contacts). On the other hand, in zirconia femural heads, the major amount of surface deterioration after long-term exposure arose from tetragonal-to-monoclinic transformation in biological environment. These data allowed us to draw interesting considerations about the role of the material microstructure and the peculiar kinematic mechanisms involved with the use of femoral heads made of different materials.

Spectroscopic techniques, which are complementary to in vitro testing procedures and stress analyses based on finite-element methods, can be very useful for improving the design of the femoral head and for optimizing the microstructural characteristics of the ceramic materials employed. Based on this and previous fluorescence and Raman spectroscopic studies, we also propose that a systematic screening of the ceramic implants before implantation can strongly reduce the probability of failure of the implant.

Alumina ceramic has been used in total hip arthoplasty since the 70’s and, in the last 30 years, a considerable evolution has occurred in designing the microstructural features of this material, taking advantage of improved processing techniques, as the hot isostatic pressing. As a result, a high degree of densification (> 99.5) has been achieved in materials with a high degree of purity and, especially, with a fine grain size ( 2 microns). The surface stress field acting on a femoral head inoperation is not only due to working conditions, but also to unexpected factors, as local impacts on the surface as a result of partial dislocations, formation of debris, etc. These additional factors greatly contribute to activate degradation mechanisms which, unfortunately, may lead to failure of the implant.

In this study, five alumina femoral heads were investigated, which were retrieved from patients after different periods of time. Among those investigated femoral heads, two belonged to a first-generation type of alumina material with a relatively coarse grainsize (average value 8 microns) and were retrieved due to surface degradation after long periods of implantation (19 and 17 years, respectively); the remaining three implants analyzed were instead recently manufactured implants with a fine grain size; they were retrieved after relatively short periods because of different causes as, for example, cup or stem loosening.

Surface stress analysis using the luminescence of Cr3+impurity in alumina was performed on the retrieved femoral heads and a statistical comparison was attempted among implants with different microstructural characteristics. The investigation led to estimate average residual stress and statistical stress distributions as a function of the location on the femoral head.

The analysis was performed both on the very surface and in the sub-surface of the head, using the confocal and the through-focus configurations of the optical spectrometer, respectively. Different statistical distributions of residual stress were observed in alumina femoral heads with different grain sizes and models were created to understand their dependence on processing and surface loading.