Publications

Please find the newest publications by RMS Foundation collaborators below:

Particles and Ions Generated in Total Hip Joint Prostheses: In Vitro Wear Test Results of UHMWPE and XLPE Acetabular Components

H. Zohdi, B. Andreatta, R. Heuberger

The accurate and detailed characterization of wear particles and ions released from total hip joint prostheses is essential to understand the cause and development of osteolysis, aseptic loosening and hypersensitivity. In this in vitro research, the wear particles and ion release of 22 different test liquids from hip simulator studies were investigated. Wear particles generated from acetabular components made of ultra-high-molecular-weight polyethylene (UHMWPE) or cross-linked polyethylene containing vitamin E (XLPE) were compared using scanning electron microscopy (SEM) and laser diffraction. Additionally, the effect of running-in versus steady-state, accelerated ageing, head materials and calcium sulphate third-body particles on the morphology and size of the created debris was investigated. The Fe, Ni, Mn, Nb, Co, Mo and Al ions released from femoral heads made of stainless steel, CoCrMo and alumina ceramic were analysed using inductively coupled plasma mass spectrometry. The combination of SEM and laser diffraction to analyse both the morphology and the particle-size distributions of the polyethylene wear particles was very powerful. The wear particles were predominantly in the submicron range and globular, with occasional fibrils. The size distributions of the UHMWPE and XLPE particles were similar; however, more fibrils were observed among the UHMWPE particles. The average particle size decreased for most samples in the steady-state phase compared to the running-in. The accelerated ageing and the presence of third-body particles generally caused larger UHMWPE wear particles only. Increasing the size of the stainless steel femoral heads led to an increase in the ion level too.

Tribol Lett  (2017) 65:92, DOI: 10.1007/s11249-017-0872-2

Effect of niobium onto the tribological behavior of cathodic arc deposited Nb–Ti–N coatings

D. D. La Grange, N. Goebbels, A. Santana, R. Heuberger, T. Imwinkelried, L. Eschbach, A. Karimi

This investigation addresses a need for higher quality surfaces and wear resistance of coatings used on load bearing medical implants. Multilayered cathodic arc deposited coatings are candidates for such applications and were the subject of this study. Nb–Ti–N coatings were deposited by cathodic arc using TiNb compound cathodes. The microstructure and properties of the coatings were characterized using XRD, TEM/STEM, EDS, nanoindentation, scratch tests and pin-on-disc testing. Throughout the coating, macroparticles consisting of a Nb rich core and a nitrided titanium porous shell were evidenced by STEM and EDS. It was shown that inhomogeneities related with the soft and malleable metallic Nb inclusions alter the tribological behavior of the coatings. The underlying mechanism of wear in hip-simulator test liquid against UHMWPE was investigated. During pin-on-disc tests, the top layers of the coatings were removed and the embedded niobium droplets became visible. The wear of UHMWPE during pin-on-disks tests was increased on Nb–Ti–N counter surfaces compared to the wear of UHMWPE on TiN itself. It is inferred that the wear of the coatings is initiated by the growth discontinuities and pores around the Nb droplets. The debris released from the coating acts as third body wear particles, grinds the remaining coating and causes the wear of UHMWPE pins.

Wear Vol. 368-369, pp 60-69, 2016; DOI: 10.1016/j.wear.2016.09.003

Calcium phosphates in biomedical applications: materials for the future?

Our populations are aging. Some experts predict that 30% of hospital beds will soon be occupied by osteoporosis patients. Statistics show that 20% of patients suffering from an osteoporotic hip fracture do not survive the first year after surgery, all this showing that there is a tremendous need for better therapies for diseased and damaged bone. Human bone consists for about 70% of calcium phosphate (CaP) mineral, therefore CaPs are the materials of choice to repair damaged bone. To do this successfully, the process of CaP biomineralization and the interaction of CaPs and biological environment in the body need to be fully understood. First commercial CaP bone graft substitutes were launched 40 years ago, and they are currently often regarded as ‘old biomaterials’ or even as an ‘obsolete’ research topic. Some even talk about ‘stones’. The aim of this manuscript is to highlight the tremendous improvements achieved in CaP materials research in the past 15 years, in particular in the field of biomineralization, as carrier for gene or ion delivery, as biologically active agent, and as bone graft substitute. Besides an outstanding biological performance, CaPs are easily and inexpensively produced, are safe, and can be relatively easily certified for clinical use. As such, CaP materials have won their spurs, but they also offer a great promise for the future.

Materals Today 19(2): 69-87, 2016; DOI: 10.1016/j.mattod.2015.10.008

Hydrogen-substituted β-tricalcium phosphate synthesized in organic media

β-Tricalcium phosphate (β-TCP) platelets synthesized in ethylene glycol offer interesting geometries for nano-structured composite bone substitutes but were never crystallographically analyzed. In this study, powder X-ray diffraction and Rietveld refinement revealed a discrepancy between the platelet structure and the known β-TCP crystal model. In contrast, a model featuring partial H for Ca substitution and the inversion of P1O4 tetrahedra, adopted from the whitlockite structure, allowed for a refinement with minimal misfits and was corroborated by HPO42− absorptions in Fourier-transform IR spectra. The Ca/P ratio converged to 1.443 ± 0.003 (n = 36), independently of synthesis conditions. As a quantitative verification, the platelets were thermally decomposed into hydrogen-free β-TCP and β-calcium pyrophosphate which resulted in a global Ca/P ratio in close agreement with the initial β-TCP Ca/P ratio (ΔCa/P = 0.003) and with the chemical composition measured by inductively coupled plasma (ΔCa/P = 0.003). These findings thus describe for the first time a hydrogen-substituted β-TCP structure, i.e. a Mg-free whitlockite, represented by the formula Ca21 − x(HPO4)2x(PO4)14 − 2x, where x = 0.80 ± 0.04, and may have implications for resorption properties of bone regenerative materials.

Acta Crystallographica Section B B72:875-884, 2016; DOI: 10.1107/S2052520616015675

Low metallic wear of dynamic intraligamentary stabilization

Abstract Dynamic Intraligamentary Stabilization (DIS) represents a treatment option for acute anterior cruciate ligament ruptures. The device used for DIS consists of a polyethylene braid and a metallic spring system, allowing the remnants of the ligament to recombine in a stabilized position over the self-healing period. This work addresses the metallic wear generated thereby. A cadaveric study was carried out with n=8 knees over 50'000 cycles, along with a control group to validate the cleaning and assembly process. Gravimetric analysis yielded a total wear of (0.28±0.35)mg for the entire implant. 50% of the wear originated from the bush and 46% from the clamping element. In a worst case scenario, a total wear of 1.7 mg would result during the functional lifetime.

Tribology International, Volume 109, May 2017, Pages 217-221; DOI: 10.1016/j.triboint.2016.12.041

Characterization and distribution of mechanically competent mineralized tissue in micropores of β-tricalcium phosphate bone substitutes

Although bone formation around and within implants has been intensively studied, the role of pores and pore geometry is still debated. Notwithstanding studies reporting the formation of bone and bone components within pores as small as a few micrometers (‘micropores’), bone ingrowth is believed to only occur in pores larger than 100 μm (‘macropores’). A thorough analysis of 10 different porous β-tricalcium phosphate cylinders (Ø: 8 mm; L: 13 mm) implanted for 2–24 weeks in an ovine model demonstrates ingrowth of mineralized tissue (MT) in pores as small as 1 μm. This tissue contained calcium phosphate, collagen, and interconnected cells. It formed within the first 3–4 weeks of implantation, extended over several hundred micrometers within the ceramic, and contributed to the majority of the early MT formation (including bone) in the defect. The indentation stiffness of the MT-ceramic composite was significantly higher than that of bone and MT-free ceramic. The presented results substantiate the importance of micropores for optimal bone healing, particularly at early implantation times.

Materals Today, Vol. 20, Issue 3, April 2017, Pages 106-115; DOI: 10.1016/j.mattod.2017.02.002

A novel method for segmentation and aligning the pre- and post-implantation scaffolds of resorbable calcium phosphate bone substitutes

Micro-computed tomography (microCT) is commonly used to characterize the three-dimensional structure of bone graft scaffolds before and after implantation in order to assess changes occurring during implantation. The accurate processing of the microCT datasets of explanted β-tricalcium phosphate (β-TCP) scaffolds poses significant challenges because of (a) the overlap in the grey values distribution of ceramic remnants, bone, and soft tissue, and of (b) the resorption of the bone substitute during the implantation. To address those challenges, this article introduces and rigorously validates a new processing technique to accurately distinguish these three phases found in the explanted β-TCP scaffolds. Specifically, the microCT datasets obtained before and after implantation of β-TCP scaffolds were aligned in 3D, and the characteristic grey value distributions of the three phases were extracted, thus allowing for (i) the accurate differentiation between these three phases (ceramic remnants, bone, soft tissue), and additionally for (ii) the localization of the defect site in the post-implantation microCT dataset. Using the similarity matrix, a 94±1% agreement was found between algorithmic results and the visual assessment of 556,800 pixels. Moreover, the comparison of the segmentation results of the same microCT and histology section further confirmed the validity of the present segmentation algorithm. This new technique could lead to a more common use of microCT in analyzing the complex 3D processes and to a better understanding of the biological processes occurring after the implantation of ceramic bone graft substitutes.

STATEMENT OF SIGNIFICANCE:

Calcium-phosphate scaffolds are being increasingly used to repair critical bone defects. Methods for the accurate characterization of the repair process are still lacking. The present study introduced and validated a novel image-processing technique, using micro-computed tomography (mCT) datasets, to investigate material phases present in biopsies. Specifically, the new method combined mCT datasets from the scaffold before and after implantation to access the characteristic data of the ceramic for more accurate analysis of bone biopsies, and as such to better understand the interactions of the scaffold design and the bone repair process.

Acta Biomaterialia, May 2017; 54:441-453; DOI: 10.1016/j.actbio.2017.03.001

The Effects of Crystal Phase and Particle Morphology of Calcium Phosphates on Proliferation and Differentiation of Human Mesenchymal Stromal Cells

Calcium phosphate (CaP) ceramics are extensively used for bone regeneration; however, their clinical performance is still considered inferior to that of patient's own bone. To improve the performance of CaP bone graft substitutes, it is important to understand the effects of their individual properties on a biological response. The aim of this study is to investigate the effects of the crystal phase and particle morphology on the behavior of human mesenchymal stromal cells (hMSCs). To study the effect of the crystal phase, brushite, monetite, and octacalcium phosphate (OCP) are produced by controlling the precipitation conditions. Brushite and monetite are produced as plate-shaped and as needle-shaped particles, to further investigate the effect of particle morphology. Proliferation of hMSCs is inhibited on OCP as compared to brushite and monetite in either morphology. Brushite needles consistently show the lowest expression of most osteogenic markers, whereas the expression on OCP is in general high. There is a trend toward a higher expression of the osteogenic markers on plate-shaped than on needle-shaped particles for both brushite and monetite. Within the limits of CaP precipitation, these data indicate the effect of both crystal phase and particle morphology of CaPs on the behavior of hMSCs.

Advanced Healthcare Materials 5(14), pp. 1775-1785, 2016; DOI: 10.1002/adhm.201600184

Full-field Calcium K-Edge X-ray Absorption Near-Edge Spectroscopy on Cortical Bone at the Micron-Scale: Polarization Effects Reveal Mineral Orientation

Here, we show results on X-ray absorption near edge structure spectroscopy in both transmission and X-ray fluorescence full-field mode (FF-XANES) at the calcium K-edge on human bone tissue in healthy and diseased conditions and for different tissue maturation stages. We observe that the dominating spectral differences originating from different tissue regions, which are well pronounced in the white line and postedge structures are associated with polarization effects. These polarization effects dominate the spectral variance and must be well understood and modeled before analyzing the very subtle spectral variations related to the bone tissue variations itself. However, these modulations in the fine structure of the spectra can potentially be of high interest to quantify orientations of the apatite crystals in highly structured tissue matrices such as bone. Due to the extremely short wavelengths of X-rays, FF-XANES overcomes the limited spatial resolution of other optical and spectroscopic techniques exploiting visible light. Since the field of view in FF-XANES is rather large the acquisition times for analyzing the same region are short compared to, for example, X-ray diffraction techniques. Our results on the angular absorption dependence were verified by both site-matched polarized Raman spectroscopy, which has been shown to be sensitive to the orientation of bone building blocks and by mathematical simulations of the angular absorbance dependence. As an outlook we further demonstrate the polarization based assessment of calcium-containing crystal orientation and specification of calcium in a beta-tricalcium phosphate (β-Ca3(PO4)2 scaffold implanted into ovine bone. Regarding the use of XANES to assess chemical properties of Ca in human bone tissue our data suggest that neither the anatomical site (tibia vs jaw) nor pathology (healthy vs necrotic jaw bone tissue) affected the averaged spectral shape of the XANES spectra.

Analytical Chemistry 88(7), pp. 3826–3835, 2016; DOI: 10.1021/acs.analchem.5b04898

In vivo degradation of magnesium plate/screw osteosynthesis implant systems: Soft and hard tissue response in a calvarial model in miniature pigs

Biodegradable magnesium plate/screw osteosynthesis systems were implanted on the frontal bone of adult miniature pigs. The chosen implant geometries were based on existing titanium systems used for the treatment of facial fractures. The aim of this study was to evaluate the in vivo degradation and tissue response of the magnesium alloy WE43 with and without a plasma electrolytic surface coating. Of 14 animals, 6 received magnesium implants with surface modification (coated), 6 without surface modification (uncoated), and 2 titanium implants. Radiological examination of the skull was performed at 1, 4, and 8 weeks post-implantation. After euthanasia at 12 and 24 weeks, X-ray, computed tomography, and microfocus computed tomography analyses and histological and histomorphological examinations of the bone/implant blocks were performed. The results showed a good tolerance of the plate/screw system without wound healing disturbance. In the radiological examination, gas pocket formation was found mainly around the uncoated plates 4 weeks after surgery. The micro-CT and histological analyses showed significantly lower corrosion rates and increased bone density and bone implant contact area around the coated screws compared to the uncoated screws at both endpoints. This study shows promising results for the further development of coated magnesium implants for the osteosynthesis of the facial skeleton.

Journal of Cranio-Maxillofacial Surgery 44 (3), pp 309-317, 2016; DOI: 10.1016/j.jcms.2015.12.009

In vivo degradation of a new concept of magnesium-based rivet-screws in the minipig mandibular bone

Self-tapping of magnesium screws in hard bone may be a challenge due to the limited torsional strength of magnesium alloys in comparison with titanium. To avoid screw failure upon implantation, the new concept of a rivet-screw was applied to a WE43 magnesium alloy. Hollow cylinders with threads on the outside were expanded inside drill holes of minipig mandibles. During the expansion with a hexagonal mandrel, the threads engaged the surrounding bone and the inside of the screw transformed into a hexagonal screw drive to allow further screwing in or out of the implant. The in vivo degradation of the magnesium implants and the performance of the used coating were studied in a human standard-sized animal model. Four magnesium alloy rivet-screws were implanted in each mandible of 12 minipigs. Six animals received the plasmaelectrolytically coated magnesium alloy implants; another six received the uncoated magnesium alloy rivet-screws. Two further animals received one titanium rivet-screw each as control. In vivo radiologic examination was performed at one, four, and eight weeks. Euthanasia was performed for one group of seven animals (three animals with coated, three with uncoated magnesium alloy implants and one with titanium implant) at 12 weeks and for the remaining seven animals at 24 weeks. After euthanasia, micro-computed tomography and histological examination with histomorphometry were performed. Significantly less void formation as well as higher bone volume density (BV/TV) and bone-implant contact area (BIC) were measured around the coated implants compared to the uncoated ones. The surface coating was effective in delaying degradation despite plastic deformation. The results showed potential for further development of magnesium hollow coated screws for bone fixation.

Materials Science and Engineering C 69, pp 247-254, 2016; DOI: 10.1016/j.msec.2016.06.085

Ag sensitized TiO2 and NiFe2O4 three-component nanoheterostructures: Synthesis, electronic structure and strongly enhanced visible light photocatalytic activity

This study reports on the synthesis and characterisation of two- and three-component visible light active photocatalytic nanoparticle heterostructures, based on TiO2 and NiFe204 and sensitized with Ag. We observe that a Ag content as small as 1 at% in the TiO2/NiFe204 heterostructure increases by more than an order of magnitude the rate constant for the visible light photocatalytic process. We rationalise this in terms of the measured structure and electronic structure data of the binary and ternary combinations of the component materials and focus on details, which show that an optimised deposition sequence is vital for attaining high values of photocatalytic efficiency, because the charge transfer across the interfaces appears to be sensitive to where the Ag is loaded in the heterostructure. The overall higher visible light photocatalytic activity of the TiO2/Ag/NiFe204 heterostructure was observed and is attributed to enhanced charge carrier separation efficiency and migration via vectorial electron transfer.

RSC Advances 6(23):18834-18842, 2016; DOI: 10.1039/C6RA00728G

Influence of Mg-doping, calcium pyrophosphate impurities and cooling rate on the allotropic α↔β-tricalcium phosphate phase transformations

Alpha and beta-tricalcium phosphates are allotropic phases which play a very important role as bone graft substitutes, namely in calcium phosphate cements. Despite extensive research efforts, contradictory reports exist on the importance of quenching for maintaining α-TCP purity. The role of calcium pyrophosphate impurities derived from a certain calcium-deficiency, hydroxyapatite impurities derived from calcium excess, and various ionic substitutions on thermal stability of these phases was not yet fully disclosed. The present work reports on the kinetics of α ↔ β-TCP phase transformations of calcium-deficient TCP powders with different Mg-doping extents (0–5 mol%) prepared by precipitation. Mg clearly enhanced the thermal stability of β-TCP. The effect of cooling rate was more complex and interdependent on the Mg content and the heat treatment schedule. High α-TCP contents were retained upon cooling at 5 °C min−1 for Mg ≤ 1 mol% or upon quenching from 1550 °C for Mg contents ≤2 mol%.

Journal of the European Ceramic Society, 36:817-27, 2016; DOI: 10.1016/j.jeurceramsoc.2015.09.037

Effect of cobalt doping on the mechanical properties of ZnO nanowires

In this work, we investigate the influence of doping on the mechanical properties of ZnO nanowires (NWs) by comparing the mechanical properties of pure and Co-doped ZnO NWs grown in similar conditions and having the same crystallographic orientation [0001]. The mechanical characterization included three-point bending tests made with atomic force microscopy and cantilever beam bending tests performed inside scanning electron microscopy. It was found that the Young's modulus of ZnO NWs containing 5% of Co was approximately a third lower than that of the pure ZnO NWs. Bending strength values were comparable for both materials and in both cases were close to theoretical strength indicating high quality of NWs. Dependence of mechanical properties on NW diameter was found for both doped and undoped ZnO NWs.

Materials Characterization 121, pp. 40-47, 2016; DOI: 10.1016/j.matchar.2016.09.027

Controlled release of NELL-1 protein from chitosan-modified ATCP particles

NEL-like molecule-1 (NELL-1) is a novel osteogenic protein that showing high specificity to osteochondral cells. It was widely used in bone regeneration research by loading onto carriers such as tricalcium phosphate (TCP) particles. However, there has been little research on protein controlled release from this material and its potential application. In this study, TCP was first modified with a hydroxyapatite coating followed by a chitosan coating to prepare chitosan/hydroxyapatite-coated TCP particles (Chi/HA-TCP). The preparation was characterized by SEM, EDX, FTIR, XRD, FM and Zeta potential measurements. The NELL-1 loaded Chi/HA-TCP particles and the release kinetics were investigated in vitro. It was observed that the Chi/HA-TCP particles prepared with the 0.3% (wt/wt) chitosan solution were able to successfully control the release of NELL-1 and maintain a slow, steady release for up to 28 days. Furthermore, more than 78% of the loaded protein's bioactivity was preserved in Chi/HA-TCP particles over the period of the investigation, which was significantly higher than that of the protein released from hydroxyapatite coated TCP (HA-TCP) particles. Collectively, this study suggests that the osteogenic protein NELL-1 showed a sustained release pattern after being encapsulated into the modified Chi/HA-TCP particles, and the NELL-1 integrated composite of Chi/HA-TCP showed a potential to function as a protein delivery carrier and as an improved bone matrix for use in bone regeneration research.

International Journal of Pharmaceutics 511, pp. 79-89, 2016; DOI: 10.1016/j.ijpharm.2016.06.050

Global chondrocyte gene expression after a single anabolic loading period: Time evolution and re-inducibility of mechano-responses

Aim of this study was a genome-wide identification of mechano-regulated genes and candidate pathways in human chondrocytes subjected to a single anabolic loading episode and characterization of time evolution and re-inducibility of the response. Osteochondral constructs consisting of a chondrocyte-seeded collagen-scaffold connected to β-tricalcium-phosphate were pre-cultured for 35 days and subjected to dynamic compression (25% strain, 1 Hz, 9 × 10 min over 3 hr) before microarray-profiling was performed. Proteoglycan synthesis was determined by 35S-sulfate-incorporation over 24 hr. Cell viability and hardness of constructs were unaltered by dynamic compression while proteoglycan synthesis was significantly stimulated (1.45-fold, p = 0.016). Among 115 significantly regulated genes, 114 were up-regulated, 48 of them ≥ twofold. AP-1-relevant transcription factors FOSB and FOS strongly increased in line with elevated ERK1/2-phosphorylation and rising MAP3K4 expression. Expression of proteoglycan-synthesizing enzymes CHSY1 and GALNT4 was load-responsive as were factors associated with the MAPK-, TGF-β-, calcium-, retinoic-acid-, Wnt-, and Notch-signaling pathway which were significantly upregulated SOX9, and BMP6 levels rose significantly also after multiple loading episodes at daily intervals even at the 14th cycle with no indication for desensitation. Canonical pSmad2/3 and pSmad1/5/9-signaling showed no consistent regulation. This study associates novel genes with mechanoregulation in chondrocytes, raising SOX9 protein levels with anabolic loading and suggests that more pathways than so far anticipated apparently work together in a complex network of stimulators and feedback-regulators. Upregulation of mechanosensitive indicators extending differentially into the resting time provides crucial knowledge to maximize cartilage matrix deposition for the generation of high-level cartilage replacement tissue.

Journal of Cellular Physiology, Volume 233, Issue 1, January 2018, Pages 699–711; DOI: 10.1002/jcp.25933

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