Publications

Please find the newest publications by RMS Foundation collaborators below:

2022

A free and open-source solution for Rietveld refinement of XRD data from the CheMin instrument onboard the Mars rover Curiosity

N. Döbelin, R. Archer, V. Tu

The Rietveld refinement software Profex is an open-source and platform-independent solution for the processing of powder X-ray diffraction datasets. It is based on the BGMN refinement kernel and uses a description of the diffractometer configuration to determine the instrument-related peak profile. In this article we present a Profex configuration file for the Chemistry and Mineralogy (CheMin) X-ray diffractometer (XRD), which is on-board the Mars Science Laboratory rover Curiosity. For the past decade, Curiosity has been on a mission on Mars to find out whether the planet was once habitable for microbial life. The CheMin XRD determines the mineralogical phases and abundances of Martian soil and rocks in Gale Crater, Mars. Since its arrival on Mars in 2012, Curiosity has analyzed powdered soil and rock samples with the CheMin instrument and transmitting the raw XRD data acquired back to Earth.

Adaptations of Profex to work seamlessly with CheMin XRD datasets involved creating a new configuration file for the CheMin instrument, as well as adding the Mars Mineral Compendium, a compilation of structural models specifically selected for the analysis of Mars sedimentary soil and rock samples, to Profex. Using example refinements, we demonstrate that this software solution is well suited for quantitative analysis of CheMin XRD datasets.

Elsevier, Planetary and Space Science, Volume 224, December 2022, 105596; doi.org/10.1016/j.pss.2022.105596

Early failure of a highly cross-linked polyethylene inlay after total hip arthroplasty probably due to insufficient irradiation

P. Wahl, C. Mossu-Haas, C. Dommann-Scherrer, K. Wei, L. Eschbach, P. Gehr, E. Benninger

Highly cross-linked polyethylene (XLPE) is a major advance in total hip arthroplasty (THA), as it suffers from less wear and thus is associated with lower revision rates than standard ultra-high molecular weight polyethylene. Early failures are reported rarely, and associated with specific design or manufacturing issues. We report a case requiring early revision due to adverse reaction to polyethylene particles. Investigations identified insufficient irradiation as the most probable cause of failure. Here are reported the features of a clinical case with determination of the material properties of the retrieved XLPE liner and establishment of the appropriate calibration curves as reference. Periprosthetic joint infection could be ruled out with appropriate sampling as cause for the inflammatory periarticular tumour. Histology identified a massive macrophagic reaction to micrometric polyethylene particles. No component malposition was present, nor any third-body wear. The trans-vinylene index (TVI) indicated insufficient and potentially detrimental irradiation of the polyethylene, while gel content, crystallinity, melting temperature and oxidation index remained within expected ranges. Histologically proven failure of a XLPE THA liner was identifiable despite correct implantation of the components. The cause of failure most probably was an inadequate irradiation, as indicated by determination of the TVI. This case underscores the importance of histologic workup even in aseptic revisions, and of detailed analysis of retrievals. The calibration curves provided are essential for analysis of other retrievals.

SAGE journals, October 25, 2022; doi.org/10.1177/09544119221130673

Effect of minor amounts of β-calcium pyrophosphate and hydroxyapatite on the physico-chemical properties and osteoclastic resorption of β-tricalcium phosphate cylinders

B. Le Gars Santoni, L. Niggli, S. Dolder, O. Loeffel, G.A. Sblendorio, R. Heuberger, Y. Maazouz, C. Stähli, N. Döbelin, P. Bowen, W. Hofstetter, M. Bohner

β-Tricalcium Phosphate (β-TCP), one of the most used bone graft substitutes, may contain up to 5 wt% foreign phase according to standards. Typical foreign phases include β-calcium pyrophosphate (β-CPP) and hydroxyapatite (HA). Currently, the effect of small amounts of impurities on β-TCP resorption is unknown. This is surprising since pyrophosphate is a very potent osteoclast inhibitor. The main aim of this study was to assess the effect of small β-CPP fractions (<1 wt%) on the in vitro osteoclastic resorption of β-TCP. A minor aim was to examine the effect of β-CPP and HA impurities on the physico-chemical properties of β-TCP powders and sintered cylinders. Twenty-six batches of β-TCP powder were produced with a Ca/P molar ratio varying between 1.440 and 1.550. Fifteen were further processed to obtain dense and polished β-TCP cylinders. Finally, six of them, with a Ca/P molar ratio varying between 1.496 (1 wt% β-CPP) and 1.502 (1 wt% HA), were incubated in the presence of osteoclasts. Resorption was quantified by white-light interferometry. Osteoclastic resorption was significantly inhibited by β-CPP fraction in a linear manner. The presence of 1% β-CPP reduced β-TCP resorption by 40%, which underlines the importance of controlling β-CPP content when assessing β-TCP biological performance.

Bioactive Materials, Volume 10, April 2022, Pages 222-235; doi.org/10.1016/j.bioactmat.2021.09.003

Evaluation of imaging setups for quantitative phase contrast nanoCT of mineralized biomaterials

J.-P. Suuronen, B. Hesse, M. Langer, Marc Bohner, J. Villanova

X-ray nano-tomography with phase contrast (nanoCT) using synchrotron radiation is a powerful tool to non-destructively investigate 3D material properties at the nanoscale. In large bone lesions, such as severe bone fractures, bone cancer or other diseases, bone grafts substituting the lost bone might be necessary. Such grafts can be of biological origin or be composed of a synthetic bone substitute. The long-term functioning of artificial bone substitutes depends on many factors. Synchrotron nanoCT imaging has great potential to contribute to further the understanding of integration of implants into bone tissue by imaging the spatial interaction between bone tissue and implant, and by accessing the interface between implant material and bone tissue. With this aim, a methodology for evaluating the image quality is presented for in-line phase contrast nanoCT images of bone scaffold samples. A PMMA-embedded tricalcium phosphate scaffold was used with both a closed and an open porosity structure and bone ingrowths as a representative system of three known materials. Parameters such as spatial resolution and signal-to-noise ratio were extracted and used to explore and quantitatively compare the effects of implementation choices in the imaging setup, such as camera technology and imaging energy, on the resulting image quality. Increasing the X-ray energy from 17.5 keV to 29.6 keV leads to a notable improvement in image quality regardless of the camera technology used, with the two tested camera setups performing at a comparable level when the recorded intensity was kept constant.

J. Synchrotron Rad. (2022). 29, 843-852; doi.org/10.1107/S1600577522003137

Sustained local ionic homeostatic imbalance caused by calcification modulates inflammation to trigger heterotopic ossification

Marc Bohner, Yassine Maazouz, Maria-Pau Ginebra, Pamela Habibovic, Jonathan G. Schoenecker, Howard Seeherman, Jeroen J.J.P. van den Beucken, Frank Witte

Heterotopic ossification (HO) is a condition triggered by an injury leading to the formation of mature lamellar bone in extraskeletal soft tissues. Despite being a frequent complication of orthopedic and trauma surgery, brain and spinal injury, the etiology of HO is poorly understood. The aim of this study is to evaluate the hypothesis that a sustained local ionic homeostatic imbalance (SLIHI) created by mineral formation during tissue calcification modulates inflammation to trigger HO. This evaluation also considers the role SLIHI could play for the design of cell-free, drug-free osteoinductive bone graft substitutes. The evaluation contains five main sections. The first section defines relevant concepts in the context of HO and provides a summary of proposed causes of HO. The second section starts with a detailed analysis of the occurrence and involvement of calcification in HO. It is followed by an explanation of the causes of calcification and its consequences. This allows to speculate on the potential chemical modulators of inflammation and triggers of HO. The end of this second section is devoted to in vitro mineralization tests used to predict the ectopic potential of materials. The third section reviews the biological cascade of events occurring during pathological and material-induced HO, and attempts to propose a quantitative timeline of HO formation. The fourth section looks at potential ways to control HO formation, either acting on SLIHI or on inflammation. Chemical, physical, and drug-based approaches are considered. Finally, the evaluation finishes with a critical assessment of the definition of osteoinduction.

Acta Biomaterialia, Available online 6 April 2022; doi.org/10.1016/j.actbio.2022.03.057

A human bone infection organ model for biomaterial research

Theodor Kuehling, Pia Schilling, Anke Bernstein, Hermann O. Mayr, Annerose Serr, Annette Wittmer, Marc Bohner, Michael Seidenstuecker

The aim of this work was to establish an organ model for staphylococcal infection of human bone samples and to investigate the influence and efficacy of a microporous β-tricalcium phosphate ceramic (β-TCP, RMS Foundation) loaded with hydrogels (alginate, alginate-di-aldehyde (ADA)-gelatin) and clindamycin on infected human bone tissue over a period of 28 days. For this purpose, human tibia plateaus, collected during total knee replacement surgery, were used as a source of bone material. Samples were infected with S. aureus ATCC29213 and treated with differently loaded β-TCP composites (alginate +/- clindamycin, ADA-gelatin +/- clindamycin, unloaded). The loading of the composites was carried out by means of a flow chamber. The infection was observed for 28 days, quantifying bacteria in the medium and the osseus material on day 1, 7, 14, 21 and 28. All samples were histologically processed for bone vitality evaluation. Bone infection could be consistently performed within the organ model. In addition, a strong reduction in bacterial counts was recorded in the groups treated with ADA-gelatin + clindamycin and alginate + clindamycin, while the bacterial count in the control groups remained constant. No significant differences between groups could be observed in the number of lacunae filled with osteocytes suggesting no differences in bone vitality among groups. In an ex-vivo human bone infection model, over a period of 28 days bacterial growth could be reduced by treatment with ADA-Gel + CLI and ALG + CLI -releasing β-TCP composites. This could be relevant for its clinical use. Further work will be necessary to improve the loading of β-TCP and the bone infection organ model itself.

Acta Biomaterialia, Volume 144, May 2022, Pages 230-241; doi.org/10.1016/j.actbio.2022.03.020

2021

Bone-on-a-Chip: A Microscale 3D Biomimetic Model to Study Bone Regeneration

Víctor Pablo Galván-Chacón, Athanasia Zampouka, Bernhard Hesse, Marc Bohner, Pamela Habibovic, David Barata

Organ-on-chip models, developed using microengineering and microfluidic technologies, aim to recreate physiological-like microenvironments of organs or tissues as a tool to study (patho)physiological processes in vitro. On-chip models of bone are relevant for the study of bone physiology, diseases and regenerative processes. While a few bone-on-a-chip models exist, recapitulating the cellular components of bone, these models do not incorporate the chemical and structural characteristics of bone tissue. Herein, the development of a bone-on-a-chip platform is reported that comprises a 3D structural model of bone. To build the platform, first, a 3D model of bone is produced in a polymer using two-photon polymerization (2PP) from a 3D nano-computed tomography scan of trabecular bone. This 3D model is then coated with a layer of bone mineral-like calcium phosphate. Finally, the 3D bone model is integrated inside a microfluidic device suitable for cell culture. Human mesenchymal stromal cells, cultured inside the platform for up to 21 days, show high viability and extensive production of extracellular matrix, rich in collagen. This biomimetic bone-on-a-chip platform can contribute to a better understanding of the processes related to bone formation and remodeling, which in turn can be used for the development of bone regeneration strategies.

Adv. Eng. Mater.2022, 2101467; doi.org/10.1002/adem.202101467

Cell-free, quantitative mineralization measurements as a proxy to identify osteoinductive bone graft substitutes

Yassine Maazouz, Giacomo Chizzola, Nicola Döbelin, Marc Bohner

Some synthetic bone graft substitutes (BGS) can trigger ectopic bone formation, which is the hallmark of osteoinduction and the most important prerequisite for the repair of large bone defects. Unfortunately, measuring or predicting BGS osteoinductive potential based on in vitro experiments is currently impossible. A recent study claimed that synthetic BGS can induce bone formation ectopically if they create a local homeostatic imbalance during their in vivo mineralization. This raised the hope that a simple cell free in vitro mineralization experiment would correlate with osteoinduction. The aim of the present study was therefore to assess the ability of a quantitative in vitro mineralization test to predict and rank the osteoinductive potential of BGS. Eight calcium phosphate BGS already tested ectopically in 9 different in vivo studies were used for that purpose. The experiment was able to identify materials that are reliably osteoinductive from those that are not, but was inaccurate in ranking the osteoinductive materials between each other. Chemical contaminants (Ca2+, Mg2+, H+, OH−, PO43−) present in some of the BGS affected the in vitro mineralization experiment results, but not in a direction that could explain the different rankings. In conclusion, this study suggests that an in vitro experiment can be used as a fast and reliable screening tool to identify osteoinductive BGS and underline the need to study ionic contaminants on calcium phosphate BGS.

Biomaterials, Volume 275, August 2021, 120912; doi.org/10.1016/j.biomaterials.2021.120912

Chemically pure β-tricalcium phosphate powders: Evidence of two crystal structures

B. Le Gars Santoni, L. Niggli, G.A. Sblendorio, D.T.L. Alexander, C. Stähli, P. Bowen, N. Döbelin, M. Bohner

The question whether β‐tricalcium phosphate (β‐TCP) can form a solid solution with β-calcium pyrophosphate (β-CPP) and/or hydroxyapatite (HA) has still not been solved. For this reason, wet-chemically synthesized β-TCP powders with only 20 ppm Sr (among 43 tested elements) and with different HA and β-CPP contents, or in other words Ca/P molar ratios, were used. The graphical relationship between these various Ca/P molar ratios determined by X-ray diffraction and by inductively-coupled plasma mass spectrometry showed no discontinuity, indicating the absence of a solid solution between β‐TCP and β-CPP or HA. Analysis of the β‐TCP lattice parameters as a function of the Ca/P molar ratio revealed a discontinuity at a Ca/P molar ratio of 1.500 and a maximum microstrain. These results indicated that at least two β‐TCP structures co-existed, with variable mixing ratios depending on the Ca/P molar ratio, and with a distinct jump at a Ca/P molar ratio of 1.500.

Journal of the European Ceramic Society, Volume 41, Issue 2, February 2021, Pages 1683-1694; doi.org/10.1016/j.jeurceramsoc.2020.09.055

Thermal treatment at 500 °C significantly reduces the reaction to irregular tricalcium phosphate granules as foreign bodies: An in vivo study

Sarah Al-Maawi, Mike Barbeck, Carlos Herrera-Vizcaínoa, Rainer Egli, Robert Sadera, Charles James Kirkpatrick , Marc Bohner, Shahram Ghanaati

Evaporation of phosphate species during thermal treatment (> 400 °C) of calcium phosphates leads to the formation of an alkaline layer on their surface. The aim of this study was to evaluate the hypothesis that the biological response of thermally treated calcium phosphates is modified by the presence of such an alkaline layer on their surface. For this purpose, 0.125–0.180 mm α- and β-tricalcium phosphate (TCP) granules were obtained by crushing and size classification, with some being subjected to thermal treatment at 500 °C. The four types of granules (α-TCP, β-TCP, α-TCP-500 °C, and β-TCP-500 °C) were implanted subcutaneously and orthotopically in rats. Sham operations served as control.

Subcutaneously, α-TCP and β-TCP induced significantly more multinucleated giant cells (MNGCs) than calcined granules. Most of the induced MNGCs were TRAP-negative, CD-68 positive and cathepsin K-negative, reflecting a typical indication of a reaction with a foreign body. The vessel density was significantly higher in the α-TCP and β-TCP groups than it was in the α-TCP-500 °C and β-TCP-500 °C groups. In the femur model, β-TCP-500 °C induced significantly more new bone formation than that induced by β-TCP. The granule size was also significantly larger in the β-TCP-500 °C group, making it more resistant to degradation than β-TCP. The MNGC density was higher in the α-TCP and β-TCP groups than in the α-TCP-500 °C and β-TCP-500 °C groups, including cathepsin-positive, CD-68 positive, TRAP-positive and TRAP-negative MNGCs.

In conclusion, this study confirms that the biological response of calcium phosphates was affected by the presence of an alkaline layer on their surface. Thermally-treated α-TCP and β-TCP granules produced significantly fewer MNGCs and were significantly less degraded than non-thermally-treated α-TCP and β-TCP granules. Thermally treating α-TCP and β-TCP granules shifts the reaction from a foreign body reaction towards a physiological reaction by downregulating the number of induced MNGCs and enhancing degradation resistance.

Acta Biomaterialia, Volume 121, February 2021, Pages 621-636; doi.org/10.1016/j.actbio.2020.11.034

PEEK as a replacement for CoCrMo in knee prostheses: Pin-on-disc wear test of PEEK-on-polyethylene articulations

Roman Heuberger, Christina Stöck, Jeannine Sahin, Lukas Eschbach

The feasibility to use poly-ether-ether-ketone (PEEK) instead of CoCrMo for femoral condyles of knee prostheses was investigated in this pin-on-disc study. This replacement would hinder the release of the toxic elements Co, Cr and Mo and this possibly at lower production costs. In this preliminary pin-on-disc study, the wear of the pairings PEEK vs. ultra-high-molecular-weight polyethylene (UHMWPE) and PEEK vs. vitamin E containing crosslinked polyethylene (XLPE) was investigated in unidirectional and multidirectional wear tests.

The articulating surfaces of the pins and discs were polished after the wear tests, while most of the initial toolmarks were removed. In addition, there were small scratches on the surfaces. Striations and protuberances were found on some of the UHMWPE samples.

All wear factors were relatively small with less than 1 × 10−6 mm3/Nm. Due to the low wear, the resulting weight loss was highly affected by the soaking of the samples. This was especially the case for the discs due to their larger surface. Thus, the wear of the discs was not used for the comparisons. The wear factors for PEEK pins and XLPE pins were in the range of (0.05 to 0.06) × 10−6 mm3/Nm, both for unidirectional and multidirectional motion. The wear of the UHMWPE pins was with (0.07 ± 0.01) × 10−6 mm3/Nm slightly higher under unidirectional motion and with (0.28 ± 0.03) × 10−6 mm3/Nm four times higher under multidirectional motion.

Thus, PEEK should be considered as an alternative to the CoCrMo alloy in knee prostheses, especially when used in combination with XLPE liners.

Biotribology, Volume 27, September 2021; doi.org/10.1016/j.biotri.2021.100189

2020

A Novel Bioreactor System Capable of Simulating the In Vivo Conditions of Synovial Joints

Adel Tekari, Veit Schmid, Joern Justiz, Reto Luginbuehl

Any significant in vitro evaluation of cartilage tissue engineering and cartilage repair strategies has to be performed under the harsh conditions encountered in vivo within synovial joints. To this end, we have developed a novel automated physiological robot reactor system (PRRS) that is capable of recapitulating complex physiological motions and load patterns within an environment similar to that found in the human knee. The PRRS consists of a mechanical stimulation unit (MSU) and an automatic sample changer (ASC) within an environment control box in which the humidity, temperature, and gas composition are tightly regulated. The MSU has three linear (orthogonal) axes and one rotational degree of freedom (around the z-axis). The ASC provides space for up to 24 samples, which can be allocated to individual stimulation patterns. Cell-seeded scaffolds and ex vivo tissue culture systems were established to demonstrate the applicability of the PRRS to the investigation of the effect of load and environmental conditions on engineering and maintenance of articular cartilage in vitro. The bioreactor is a flexible system that has the potential to be applied for culturing connective tissues other than cartilage, such as bone and intervertebral disc tissue, even though the mechanical and environmental parameters are very different.

Tissue Engineering Part C: Methods, Vol. 26, 16 Dec 2020; doi.org/10.1089/ten.tec.2020.0161

Explicit finite element analysis can predict the mechanical response of conical implant press-fit in homogenized trabecular bone

Marzieh Ovesy, Marcel Aeschlimann, Philippe K. Zysset

Prediction of primary stability is a major challenge in the surgical planning of dental and orthopedic implants. Computational methods become attractive to estimate primary stability from clinical CT images, but implicit finite element analysis of implant press-fit faces convergence issues due to contact and highly distorted elements. This study aims to develop and validate an explicit finite element method to simulate the insertion and primary stability of a rigid implant in a deformable bone while accounting for damage occurring at the bone-implant interface. Accordingly, a press-fit experiment of a conical implant into predrilled bovine trabecular bone was designed and realized for six samples. A displacement-driven cyclic protocol was used to quantify the reaction force and stiffness of the bone-implant system. Homogenized finite element analyses of the experiments were performed by modeling contact with friction and converting an existing constitutive model with elasto-plasticity and damage of bone tissue to be applicable to an explicit time integration scheme where highly distorted elements get deleted. The computed reaction forces and unloading stiffnesses showed high correlations (R2 = 0.95 and R2 = 0.94) with the experiment. Friction between bone and implant exhibited a strong influence on both reaction force and stiffness. In conclusion, the developed explicit finite element approach with frictional contact and element deletion accounts properly for bone damage during press-fit and will help optimizing dental or orthopedic implant design towards maximal primary stability.

Journal of Biomechanics, Volume 107, 23 June 2020; doi.org/10.1016/j.jbiomech.2020.109844

Bone grafts and bone replacement material

B. Rahn, Marc Bohner

No abstract available.

Advanced Craniomaxillofacial Surgery, Chapter 9.4.6, ISBN 9783132428393

Validation of XRD phase quantification using semi-synthetic data

Nicola Döbelin

Validating phase quantification procedures of powder X-ray diffraction (XRD) data for an implementation in an ISO/IEC 17025 accredited environment has been challenging due to a general lack of suitable certified reference materials. The preparation of highly pure and crystalline reference materials and mixtures thereof may exceed the costs for a profitable and justifiable implementation. This study presents a method for the validation of XRD phase quantifications based on semi-synthetic datasets that reduces the effort for a full method validation drastically. Datasets of nearly pure reference substances are stripped of impurity signals and rescaled to 100% crystallinity, thus eliminating the need for the preparation of ultra-pure and -crystalline materials. The processed datasets are then combined numerically while preserving all sample- and instrument-characteristic features of the peak profile, thereby creating multi-phase diffraction patterns of precisely known composition. The number of compositions and repetitions is only limited by computational power and storage capacity. These datasets can be used as input files for the phase quantification procedure, in which statistical validation parameters such as precision, accuracy, linearity, and limits of detection and quantification can be determined from a statistically sound number of datasets and compositions.

Published online by Cambridge University Press: 13 October 2020; doi.org/10.1017/S0885715620000573

β-tricalcium phosphate for bone substitution: Synthesis and properties

Marc Bohner, Bastien Le Gars Santoni, Nicola Döbelin

β-tricalcium phosphate (β-TCP) is one the most used and potent synthetic bone graft substitute. It is not only osteoconductive, but also osteoinductive. These properties, combined with its cell-mediated resorption, allow full bone defects regeneration. Its clinical outcome is sometimes considered to be “unpredictable”, possibly due to a poor understanding of β-TCP physico-chemical properties: β-TCP crystallographic structure is not fully uncovered; recent results suggest that sintered β-TCP is coated with a Ca-rich alkaline phase; β-TCP apatite-forming ability and osteoinductivity may be enhanced by a hydrothermal treatment; β-TCP grain size and porosity are strongly modified by the presence of minute amounts of β-calcium pyrophosphate or hydroxyapatite impurities. The aim of the present article is to provide a critical, but still rather comprehensive review of the current state of knowledge on β-TCP, with a strong focus on its synthesis and physico-chemical properties, and their link to the in vivo response.

Statement of significance

The present review documents the richness, breadth, and interest of the research devoted to β-tricalcium phosphate (β-TCP). β-TCP is synthetic, osteoconductive, osteoinductive, and its resorption is cell-mediated, thus making it one of the most potent bone graft substitutes. This comprehensive review reveals that there are a number of aspects, such as surface chemistry, crystallography, or stoichiometry deviations, that are still poorly understood. As such, β-TCP is still an exciting scientific playground despite a 50 year long history and > 200 yearly publications.

Acta Biomaterialia, 19 June 2020; doi.org/10.1016/j.actbio.2020.06.022

A thermodynamic approach to surface modification of calcium phosphate implants by phosphate evaporation and condensation

Nicola Döbelin, Yassine Maazouz, Roman Heuberger, Marc Bohner, Ashley A. Armstrong, Amy J. Wagoner Johnson, Christoph Wanner

It has been reported in the literature that thermal treatment of calcium phosphate ceramics chemically alters the surface composition by phosphate evaporation. To predict the compositional changes, we have developed a thermodynamic model for the evaporation of phosphorous species from CPP, TCP, HA, and TetCP. In an open atmosphere, the model predicts the formation of a surface layer consisting of a sequence of increasingly phosphate-depleted phases. In a closed system, the atmosphere reaches equilibrium with a single-phase surface layer. To verify our model, we performed a series of experiments which confirmed the predicted formation of phosphate-depleted surface layers. These experiments further demonstrated that controlled supersaturation of the atmosphere led to formation of a phosphate-enriched surface layer as a result of phosphate condensation. In conclusion, our thermodynamic model is capable of predicting the surface modification by phosphate evaporation and condensation of calcium phosphate phases during high-temperature processing in different environments.

Journal of the European Ceramic Society 40 (2020) 6095-6106; doi.org/10.1016/j.jeurceramsoc.2020.07.028

Shear resistance and composition of polyethylene protuberances from hip-simulating pin-on-disc wear tests

Roman Heuberger, Emely Lea Bortel, Jorge Sague, Pablo Escuder, Jiri Nohava

Protuberances on the surface of ultra-high-molecular-weight polyethylene (UHMWPE) pins were chemically and mechanically investigated in order to better understand the tribology of UHMWPE vs. CoCrMo, which is a typical material pairing for joint replacements.

Pin-on-disc wear tests were performed using pins made of UHMWPE articulating against discs made of a CoCrMo alloy. Wear tests were performed using two different test fluids: a standard test liquid used for hip-simulator tests and a synthetic synovial fluid containing hyaluronic acid, albumin, immunoglobulin G, the phospholipid lecithin and additionally sodium azide to fight bacterial growth.

After the wear tests, the pin surfaces exhibited scratches as well as protuberances with a pitting-like appearance. These protuberances, i.e. elevations protruding from the articulating surface, were 6 ± 3 μm high on the pins lubricated with the standard test liquid and 20 ± 5 μm high under the lubrication with the synthetic synovial fluid. Investigating the protuberances using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) showed that these were composed mainly of UHMWPE, together with amine groups from proteins. To our knowledge, the mechanical properties and namely shear resistance of these protuberances were investigated the first time. The hardness and the elastic modulus of the protuberances were similar to the bulk material, as revealed by nanoindentation. The shear resistance of the protuberances as measured by a nanoscratch test method was comparable or even higher than that of the bulk material.

Biotribology Vol. 23, September 2020, 100139; doi.org/10.1016/j.biotri.2020.100139

Grafting of calcium chelating functionalities onto PLA monofilament fiber surfaces.

Elias Mulky, Giuseppino Fortunato, Dirk Hegemann, Jorge Sague, Roman Heuberger, Martin Frenz

Polymer surface grafting is widely used in the field of bone regeneration to increase calcium phosphate (CaP) adhesion, with the intent of improving mechanical properties of CaP-polymer composite cements. Reinforcement can be achieved using multiple combined functional groups and/or complex surface geometries that, however, concurrently influence multiple effects such as wetting, roughness, and interfacial strengthening. This study focused on the influence of a chelating group, namely aspartic acid, on the adsorption of divalent ions such as Ba2+ or Ca2+ onto poly-l-lactic acid (PLA) films. The films were analyzed using contact angle measurements and X-ray photoelectron spectroscopy. The adsorption of CaP and its interfacial mechanical properties were investigated using functionalized PLA monofilaments whose surface roughness was analyzed using white light interferometry. Mechanical analysis was conducted by performing pull-out tests. The surfaces were analyzed using scanning electron microscopy and energy dispersive X-ray spectroscopy. Using aspartic acid as a chelating group resulted in a 50 % increased adsorption of barium, an almost threefold increase in calcium coverage of the fiber compared to the control group and a twofold increase in interfacial stiffness. No significant increase in interfacial strength was determined, most likely due to the weakness of the CaP matrix, which was partially visible as residues on the monofilaments in the postfracture imaging. This study shows the potential of surfaces functionalized with aspartic acid as a simple alternative to complex polypeptide based functional groups for the adsorption of divalent ions such as calcium on poly-lactic acid in bone regenerating applications.

Biointerphases 15, 011006 (2020); doi.org/10.1116/1.5129989

In vitro measurement of the chemical changes occurring within β-tricalcium phosphate bone graft substitutes

Y. Maazouz, I. Rentsch, B. Lu, B. Le Gars Santoni, N. Döbelin, M. Bohner

Several mechanisms proposed to explain the osteoinductive potential of calcium phosphates involve surface mineralization (“bioactivity”) and mention the occurrence of concentration gradients between the inner and the outer part of the implanted material. Determining the evolution of the local chemical environment occurring inside the pores of an implanted bone graft substitute (BGS) is therefore highly relevant. A quantitative and fast method was developed to measure the chemical changes occurring within the pores of β-Tricalcium Phosphate (β-TCP) granules incubated in a simulated body fluid. A factorial design of experiment was used to test the effect of particle size, specific surface area, microporosity, and purity of the β-TCP granules. Large pH, calcium and phosphate concentration changes were observed inside the BGS and lasted for several days. The kinetics and magnitude of these changes (up to 2 pH units) largely depended on the processing and properties of the granules. Interestingly, processing parameters that increased the kinetics and magnitude of the local chemical changes are parameters considered to favor calcium phosphate osteoinduction, suggesting that the model might be useful to predict the osteoinductive potential of BGSs.

Statement of significance

Recent results suggest that in situ mineralization of biomaterials (polymers, ceramics, metals) might be key in their ability to trigger ectopic bone formation. This is the reason why the effect on in situ mineralization of various synthesis parameters of β-tricalcium phosphate granules was studied (size, microporosity, specific surface area, and Ca/P molar ratio). To the best of our knowledge, this is the first article devoted to the chemical changes occurring within the pores of a bone graft substitute. We believe that the manuscript will prove to be highly important in the design and mechanistic understanding of drug-free osteoinductive biomaterials.

 Acta Biomaterialia, Volume 102, 15 January 2020, Pages 440-457; doi.org/10.1016/j.actbio.2019.11.035

2019

Pre-clinical testing of human size magnesium implants in miniature pigs: Implant degradation and bone fracture healing at multiple implantation sites

T. Imwinkelried, S. Beck, B. Schaller

Two miniature pig models to assess safety and performance of degradable osteosynthesis implants are presented. Both models provide multiple implantation sites with human size implants. In the first model, different types of magnesium plates and screws for fracture fixation were used to study local and systemic safety aspects in 14 Göttingen minipigs. Implant degradation, gas release and accumulation of alloying elements in organs were assessed for non-coated and plasmaelectrolytic coated magnesium implants and compared to the titanium reference. The observed implant degradation was mostly uniform and did not seem to depend on the implantation site and implant condition. The coating was effective in delaying initial gas release and degradation. No rare earth alloying elements could be detected in local lymph nodes, kidneys, livers or spleens.

In the second model with Göttingen und Yucatan minipigs, full osteotomies were inflicted to four different anatomical sites and treated with magnesium plates and screws to assess fracture healing performance. Two Göttingen pilot minipigs showed promising results including a mandible osteosynthesis which healed within 6 weeks. The subsequent study was compromised by the more massive jaws of the used Yucatan minipigs. Three out of seven animals had to be sacrificed within two months as the stability of magnesium and titanium reference implants in the mandible was surpassed.

In conclusion, the resorbable magnesium implants showed promising in vivo properties. For the analysis of human standard sized implants under full chewing load conditions, lighter Göttingen minipigs were more suitable than heavier Yucatan minipigs.

Materials Science and Engineering:C, Volume 108, March 2020; doi.org/10.1016/j.msec.2019.110389

A BMP/activin A chimera is superior to native BMPs and induces bone repair in nonhuman primates when delivered in a composite matrix

H. J. Seeherman, S. P. Berasi, C. T. Brown, R. X. Martinez, S. Z. Juo, S. Jelinsky, M. Cain, J. Grode, K. E. Tumelty, M. Bohner, O. Grinberg, N. Orr, O. Shoseyov, J. Eyckmans, C. Chen, P. R. Morales, C. G. Wilson, E. Vanderploeg, J. M. Wozney

Bone morphogenetic protein (BMP)/carriers approved for orthopedic procedures achieve efficacy superior or equivalent to autograft bone. However, required supraphysiological BMP concentrations have been associated with potential local and systemic adverse events. Suboptimal BMP/receptor binding and rapid BMP release from approved carriers may contribute to these outcomes. To address these issues and improve efficacy, we engineered chimeras with increased receptor binding by substituting BMP-6 and activin A receptor binding domains into BMP-2 and optimized a carrier for chimera retention and tissue ingrowth. BV-265, a BMP-2/BMP-6/activin A chimera, demonstrated increased binding affinity to BMP receptors, including activin-like kinase-2 (ALK2) critical for bone formation in people. BV-265 increased BMP intracellular signaling, osteogenic activity, and expression of bone-related genes in murine and human cells to a greater extent than BMP-2 and was not inhibited by BMP antagonist noggin or gremlin. BV-265 induced larger ectopic bone nodules in rats compared to BMP-2 and was superior to BMP-2, BMP-2/6, and other chimeras in nonhuman primate bone repair models. A composite matrix (CM) containing calcium-deficient hydroxyapatite granules suspended in a macroporous, fenestrated, polymer mesh-reinforced recombinant human type I collagen matrix demonstrated improved BV-265 retention, minimal inflammation, and enhanced handling. BV-265/CM was efficacious in nonhuman primate bone repair models at concentrations ranging from 1/10 to 1/30 of the BMP-2/absorbable collagen sponge (ACS) concentration approved for clinical use. Initial toxicology studies were negative. These results support evaluations of BV-265/CM as an alternative to BMP-2/ACS in clinical trials for orthopedic conditions requiring augmented healing.

Sci Transl Med. 2019 Apr 24;11(489). pii: eaar4953; doi.org/10.1126/scitranslmed.aar4953

Effect of grain orientation and magnesium doping on β-tricalcium phosphate resorption behavior

M. Gallo, B. Le Gars Santoni, Th. Douillard, F. Zhang, L. Gremillard, S. Dolder, W. Hofstetter, S. Meille, M. Bohner, J. Chevalier, S. Tadier

The efficiency of calcium phosphate (CaP) bone substitutes can be improved by tuning their resorption rate. The influence of both crystal orientation and ion doping on resorption is here investigated for beta-tricalcium phosphate (β-TCP). Non-doped and Mg-doped (1 and 6 mol%) sintered β-TCP samples were immersed in acidic solution (pH 4.4) to mimic the environmental conditions found underneath active osteoclasts. The surfaces of β-TCP samples were observed after acid-etching and compared to surfaces after osteoclastic resorption assays. β-TCP grains exhibited similar patterns with characteristic intra-crystalline pillars after acid-etching and after cell-mediated resorption. Electron BackScatter Diffraction analyses, coupled with Scanning Electron Microscopy, Inductively Coupled Plasma–Mass Spectrometry and X-Ray Diffraction, demonstrated the influence of both grain orientation and doping on the process and kinetics of resorption. Grains with c-axis nearly perpendicular to the surface were preferentially etched in non-doped β-TCP samples, whereas all grains with simple axis (a, b or c) nearly normal to the surface were etched in 6 mol% Mg-doped samples. In addition, both the dissolution rate and the percentage of etched surface were lower in Mg-doped specimens. Finally, the alignment direction of the intra-crystalline pillars was correlated with the preferential direction for dissolution.

Acta Biomaterialia 2019;89:391-402; doi.org/10.1016/j.actbio.2019.02.045

A proposed mechanism for material-induced heterotopic ossification

M. Bohner, R. J. Miron

Repairing large bone defects caused by severe trauma or tumor resection remains one of the major challenges in orthopedics and maxillofacial surgery. A promising therapeutic approach is the use of osteoinductive materials, i.e. materials able to drive mesenchymal stem cells into the osteogenic lineage. Even though the mechanism of this so-called intrinsic osteoinduction or material-induced heterotopic ossification has been studied for decades, the process behind it remains unknown, thus preventing any design of highly potent osteoinductive materials. We propose and demonstrate for the first time that intrinsic osteoinduction is the result of calcium and/or phosphate depletion, thus explaining why not only the material (surface) composition but also the material volume and architecture (e.g. porosity, pore size) play a decisive role in this process.

Materials Today: Volume 22, January - February 2019, Pages 132-141; DOI: 10.1016/j.mattod.2018.10.036

2018

Influence of UV Irradiation and Cold Atmospheric Pressure Plasma on Zirconia Surfaces: An In Vitro Study

R. Smeets, A. Henningsen, R. Heuberger, O. Hanisch, F. Schwarz, C. Precht

Purpose: To compare the influence of ultraviolet (UV) irradiation and cold atmospheric pressure plasma (CAP) treatment on surface structure, surface chemistry, cytocompatibility, and cell behavior on zirconia in vitro. Materials and methods: Zirconia samples (TZ-3YSBE) were treated by UV irradiation, oxygen plasma, or argon plasma for 12 minutes each and compared with the nontreated samples. Surface analysis was conducted using scanning electron microscopy, roughness analysis, and x-ray photoelectron spectroscopy. Cell proliferation, viability, and cell attachment as well as cytotoxicity were evaluated using MC3T3-E1 murine osteoblasts cultivated directly on the zirconia samples. Results: Surface structure and roughness were not affected by the surface treatments. CAP and UV irradiation significantly reduced organic material and increased the surface oxidation on the zirconia samples. Furthermore, CAP and UV treatment significantly decreased the contact angle on the zirconia samples, indicating superhydrophilicity. Cell attachment was significantly increased on oxygen plasma-treated zirconia samples compared with the nontreated samples at all times (P < .001). After 24 and 48 hours, cell proliferation and viability (P < .001) were significantly increased on oxygen plasma-treated samples in comparison with the nontreated, UV-treated, and argon plasma-treated samples. Neither UV nor CAP treatment led to cytotoxicity. Conclusion: In vitro, surface treatment by UV irradiation or CAP causes a significant reduction of organic material, increases the hydrophilicity of zirconia, and improves the conditions for osteoblasts. Results stipulate that treatment of zirconia surfaces with oxygen plasma may favor cell proliferation.

 The International journal of oral & maxillofacial implants 34(2) · January 2018; dx.doi.org/10.11607/jomi.7017

In vitro methods for the evaluation of antimicrobial surface designs

J. Sjollema, S. A. J. Zaat, V. Fontaine, M. Ramstedt, R. Luginbuehl, K. Thevissen, J.Li, H. C. van der Mei, H. J. Busscher

Bacterial adhesion and subsequent biofilm formation on biomedical implants and devices are a major cause of their failure. As systemic antibiotic treatment is often ineffective, there is an urgent need for antimicrobial biomaterials and coatings. The term “antimicrobial” can encompass different mechanisms of action (here termed “antimicrobial surface designs”), such as antimicrobial-releasing, contact-killing or non-adhesivity. Biomaterials equipped with antimicrobial surface designs based on different mechanisms of action require different in vitro evaluation methods. Available industrial standard evaluation tests do not address the specific mechanisms of different antimicrobial surface designs and have therefore been modified over the past years, adding to the myriad of methods available in the literature to evaluate antimicrobial surface designs. The aim of this review is to categorize fourteen presently available methods including industrial standard tests for the in vitro evaluation of antimicrobial surface designs according to their suitability with respect to their antimicrobial mechanism of action. There is no single method or industrial test that allows to distinguish antimicrobial designs according to all three mechanisms identified here. However, critical consideration of each method clearly relates the different methods to a specific mechanism of antimicrobial action. It is anticipated that use of the provided table with the fourteen methods will avoid the use of wrong methods for evaluating new antimicrobial designs and therewith facilitate translation of novel antimicrobial biomaterials and coatings to clinical use. The need for more and better updated industrial standard tests is emphasized.

Acta Biomaterialia Volume 70, 1. April 2018, Pages 12-24; doi.org/10.1016/j.actbio.2018.02.001

Fracture Healing and Bone Remodeling With Human Standard-Sized Magnesium Versus Polylactide-Co-Glycolide Plate and Screw Systems Using a Mini-Swine Craniomaxillofacial Osteotomy Fixation Model.

B. Schaller, J. P. M. Burkhard, M. Chagnon, S. Beck, T. Imwinkelried, M. Assad

PURPOSE: This study compared the degradation profile, safety, and efficacy of bioresorbable magnesium alloy and polylactide-co-glycolide (PLGA) polymer osteosynthesis systems for the treatment of fractures in a load-sharing maxillofacial environment using a new mini-swine fracture fixation model. MATERIALS AND METHODS: Two types of clinically relevant situations were evaluated in 5 Yucatan miniature pigs. Defined porcine midface osteotomies of the supraorbital rim and zygoma were created and fixed with either a coated magnesium (test animals) or PLGA plate and screw osteosynthesis system (control animals). After surgery, the mini-pigs were able to recover for either 1 or 9 months with continuous in vivo post-implantation monitoring. Standardized computed tomography (CT) imaging was taken immediately postoperatively and at termination for all animals. The 9-month cohort also underwent CT at 2, 4, and 6 months after surgery. At necropsy, osteotomy sites and bone-implant units were harvested, and healing was evaluated by micro-CT, histopathology, and histomorphometry. RESULTS: After clinical and radiologic follow-up examination, all fracture sites healed well for both the magnesium and polymer groups regardless of time point. Complete bone union and gradually disappearing osteotomy lines were observed across all implantation sites, with no major consistency change in periprosthetic soft tissue or in soft tissue calcification. Macroscopic and microscopic examination showed no negative influence of gas formation observed with magnesium during the healing process. Histopathologic analysis showed similar fracture healing outcomes for both plating systems with good biocompatibility as evidenced by a minimal or mild tissue reaction. CONCLUSIONS: This study confirms that WE43 magnesium alloy exhibited excellent fracture healing properties before its full degradation without causing any substantial inflammatory reactions in a long-term porcine model. Compared with PLGA implants, magnesium represents a promising new biomaterial with reduced implant sizes and improved mechanical properties to support fracture healing in a load-sharing environment.

J Oral Maxillofac Surg. 2018 Oct;76(10):2138-2150; dx.doi.org/10.1016/j.joms.2018.03.039

In vitro response of mesenchymal stem cells to biomimetic hydroxyapatite substrates: A new strategy to assess the effect of ion exchange

J. M. Sadowska, J. Guillem-Marti, M. Espanol, C. Stähli, N. Döbelin, M. -P. Ginebra

Biomaterials can interact with cells directly, that is, by direct contact of the cells with the material surface, or indirectly, through soluble species that can be released to or uptaken from the surrounding fluids. However, it is difficult to characterise the relevance of this fluid-mediated interaction separately from the topography and composition of the substrate, because they are coupled variables. These fluid-mediated interactions are amplified in the case of highly reactive calcium phosphates (CaPs) such as biomimetic calcium deficient hydroxyapatite (CDHA), particularly in static in vitro cultures. The present work proposes a strategy to decouple the effect of ion exchange from topographical features by adjusting the volume ratio between the cell culture medium and biomaterial (VCM/VB). Increasing this ratio allowed mitigating the drastic ionic exchanges associated to the compositional changes experienced by the material exposed to the cell culture medium. This strategy was validated using rat mesenchymal stem cells (rMSCs) cultured on CDHA and beta-tricalcium phosphate (β-TCP) discs using different VCM/VB ratios. Whereas in the case of β-TCP the cell response was not affected by this ratio, a significant effect on cell adhesion and proliferation was found for the more reactive CDHA. The ionic exchange, produced by CDHA at low VCM/VB, altered cell adhesion due to the reduced number of focal adhesions, caused cell shrinkage and further rMCSs apoptosis. This was mitigated when using a high VCM/VB, which attenuated the changes of calcium and phosphate concentrations in the cell culture medium, resulting in rMSCs spreading and a viability over time. Moreover, rMSCs showed an earlier expression of osteogenic genes on CDHA compared to sintered β-TCP when extracellular calcium fluctuations were reduced.

Statement of Significance

Fluid mediated interactions play a significant role in the bioactivity of calcium phosphates. Ionic exchange is amplified in the case of biomimetic hydroxyapatite, which makes the in vitro characterisation of cell-material interactions especially challenging. The present work proposes a novel and simple strategy to explore the mechanisms of interaction of biomimetic and sintered calcium phosphates with mesenchymal stem cells. The effects of topography and ion exchange are analysed separately by modifying the volume ratio between cell culture medium and biomaterial. High ionic fluctuations interfered in the maturation of focal adhesions, hampering cell adhesion and leading to increased apoptosis and reduced proliferation rate.

Acta Biomaterialia Volume 76, August 2018, Pages 319-332; doi.org/10.1016/j.actbio.2018.06.025. Fulltext download.

Extracellular matrix content and WNT/β-catenin levels of cartilage determine the chondrocyte response to compressive load

H. Praxenthaler, E. Krämer, M. Weisser, N. Hecht, J. Fischer, T. Grossner, W. Richter

During osteoarthritis (OA)-development extracellular matrix (ECM) molecules are lost from cartilage, thus changing gene-expression, matrix synthesis and biomechanical competence of the tissue. Mechanical loading is important for the maintenance of articular cartilage; however, the influence of an altered ECM content on the response of chondrocytes to loading is not well understood, but may provide important insights into underlying mechanisms as well as supplying new therapies for OA. Objective here was to explore whether a changing ECM-content of engineered cartilage affects major signaling pathways and how this alters the chondrocyte response to compressive loading.

Activity of canonical WNT-, BMP-, TGF-β- and p38-signaling was determined during maturation of human engineered cartilage and followed after exposure to a single dynamic compression-episode. WNT/β-catenin- and pSmad1/5/9-levels declined with increasing ECM-content of cartilage. While loading significantly suppressed proteoglycan-synthesis and ACAN-expression at low ECM-content this catabolic response then shifted to an anabolic reaction at high ECM-content. A positive correlation was observed between GAG-content and load-induced alteration of proteoglycan-synthesis. Induction of high β-catenin levels by the WNT-agonist CHIR suppressed load-induced SOX9- and GAG-stimulation in mature constructs. In contrast, the WNT-antagonist IWP-2 was capable of attenuating load-induced GAG-suppression in immature constructs.

In conclusion, either ECM accumulation-associated or pharmacologically induced silencing of WNT-levels allowed for a more anabolic reaction of chondrocytes to physiological loading. This is consistent with the role of proteoglycans in sequestering WNT-ligands in the ECM, thus reducing WNT-activity and also provides a novel explanation of why low WNT-activity in cartilage protects from OA-development in mechanically overstressed cartilage.

Biochimica et Biophysica Acta (BBA) - Molecular Basis of DiseaseVolume 1864, Issue 3, March 2018, Pages 851-859; doi.org/10.1016/j.bbadis.2017.12.024

A nonlinear homogenized finite element analysis of the primary stability of the bone–implant interface

M. Ovesy, B. Voumard, P. Zysset

Stability of an implant is defined by its ability to undergo physiological loading–unloading cycles without showing excessive tissue damage and micromotions at the interface. Distinction is usually made between the immediate primary stability and the long-term, secondary stability resulting from the biological healing process. The aim of this research is to numerically investigate the effect of initial implantation press-fit, bone yielding, densification and friction at the interface on the primary stability of a simple bone–implant system subjected to loading–unloading cycles. In order to achieve this goal, human trabecular bone was modeled as a continuous, elasto-plastic tissue with damage and densification, which material constants depend on bone volume fraction and fabric. Implantation press-fit related damage in the bone was simulated by expanding the drilled hole to the outer contour of the implant. The bone–implant interface was then modeled with unilateral contact with friction. The implant was modeled as a rigid body and was subjected to increasing off-axis loading cycles. This modeling approach is able to capture the experimentally observed primary stability in terms of initial stiffness, ultimate force and progression of damage. In addition, it is able to quantify the micromotions around the implant relevant for bone healing and osseointegration. In conclusion, the computationally efficient modeling approach used in this study provides a realistic structural response of the bone–implant interface and represents a powerful tool to explore implant design, implantation press-fit and the resulting risk of implant failure under physiological loading.

Journal Biomechanics and Modelling in Mechanobiology, 17(5), pp. 1471-1480, 2018; dx.doi.org/10.1007/s10237-018-1038-3

Reversible photodoping of TiO2 nanoparticles for photochromic applications

U. Joost, A. Sutka, M. Oja, K. Smits, N. Döbelin, A. Loot, M. Järvekülg, M. Hirsimäki, M. Valden, E. Nõmmiste

Observations on the strong photochromic effect of crystalline TiO2 quantum dots (mean size ≈ 4 nm) are presented. The synthesized quantum dots consist of irregularly shaped anatase TiO2 nanoparticles (NPs) and are dispersed in butanol (8% by mass). Obtained NPs exhibit a dramatic photoresponse to UV light, enabling effective transmittance modulation in a broad wavelength range extending from the visible to near-infrared region, and even the thermal black body radiation regime beyond 10 μm. The exceptional photoresponse is attributed to hole-scavenging by butanol, TiO2 self-reduction, injection of electrons to the conduction band, and consequent localized surface plasmon resonances in NPs. The observed optical effect is reversible, and the initial high transmittance state can be restored simply by exposing the NPs to air. The applied NP synthesis route is economic and can be easily scaled for applications such as smart window technologies.

Chem. Mater. 30, pp. 8968-74, 2018; doi.org/10.1021/acs.chemmater.8b04813

Changes in surface characteristics of titanium and zirconia after surface treatment with ultraviolet light or non‐thermal plasma

A. Henningsen, R. Smeets, R. Heuberger, O. T. Jung, H. Hanken, M. Heiland, C. Cacaci, C. Precht

Positive effects of irradiation with ultraviolet (UV) light or treatment with non‐thermal plasma on titanium and zirconia surfaces have been described in various studies. The aim of this study was to assess and compare the changes in the physicochemical surface conditions of titanium and zirconia surfaces after a short treatment with UV light or with non‐thermal plasmas of argon or oxygen. Titanium and zirconia samples with moderately rough surfaces were treated for 12 min either in a UV‐light oven or in a non‐thermal plasma reactor that generates non‐thermal plasmas of oxygen or argon. Changes in surface conditions were assessed by confocal microscopy, dynamic contact angle measurement, and X‐ray photoelectron spectroscopy (XPS). No changes in roughness occurred. Ultraviolet irradiation and non‐thermal plasma significantly increased the wettability of the titanium and zirconia surfaces. X‐ray photoelectron spectroscopy showed an increase of oxygen and a significant decrease of carbon after treatment with either method. Thus, ultraviolet light and non‐thermal plasma were found to be able to improve the chemical surface conditions of titanium and zirconia following a short exposure time. However, further in vitro and in vivo studies are needed to determine the relevance of the results.

European Journal of Oral sciences, 126(2), pp. 126-134; doi.org/10.1111/eos.12400

Mechanical Testing of Maximal Shift Scarf Osteotomy with Inside-Out Plating Compared to Classic Scarf Osteotomy With Double Screw Fixation

L. Bohnert, A. Radeideh, G. Bigolin, E. Gautier, M. Lottenbach

The purpose of the present study was to biomechanically compare the primary stability of our formerly described inside-out plate fixation to the classic double screw fixation for scarf osteotomy in the treatment of hallux valgus. We performed 20 scarf osteotomies on first metatarsal composite bone models. One half were fixed using a double screw technique and the other half using a locking plate inside-out technique. Using a testing device to simulate the physiologic load, the specimen was loaded until failure, and the load at failure, displacement at failure, and work at failure were recorded. The results were compared between the 2 groups and against the findings from intact sawbones. Compared with the intact bone models, the energy absorption was low for both types of osteotomy fixation. Between the 2 fixation groups, the load at failure was greater for plate fixation, although the difference was not statistically significant (p = .051). However, a statistically significant difference was found between both groups comparing work and displacement at failure (p < .001). In conclusion, the formerly described inside-out plating technique is a biomechanically reasonable alternative to screw fixation because of its primary stability after scarf osteotomy for hallux valgus.

Journal of Foot and Ankle Surgery, Volume 57, Issue 6, November–December 2018, Pages 1056-1058; doi.org/10.1053/j.jfas.2018.02.010

A proposed mechanism for material-induced heterotopic ossification

Marc Bohner, R. J. Miron

Repairing large bone defects caused by severe trauma or tumor resection remains one of the major challenges in orthopedics and maxillofacial surgery. A promising therapeutic approach is the use of osteoinductive materials, i.e. materials able to drive mesenchymal stem cells into the osteogenic lineage. Even though the mechanism of this so-called intrinsic osteoinduction or material-induced heterotopic ossification has been studied for decades, the process behind it remains unknown, thus preventing any design of highly potent osteoinductive materials. We propose and demonstrate for the first time that intrinsic osteoinduction is the result of calcium and/or phosphate depletion, thus explaining why not only the material (surface) composition but also the material volume and architecture (e.g. porosity, pore size) play a decisive role in this process.

Materials Today, Volume 22, January–February 2019, Pages 132-141; doi.org/10.1016/j.mattod.2018.10.036

Bisphosphonates reduce biomaterial turnover in healing of critical-size rat femoral defects

M. Hauser, M. Siegrist, A. Denzer, N. Saulacic, J. Grosjean, M. Bohner, W. Hofstetter

Treatment of osteoporotic patients with bisphosphonates (BPs) preserves bone mass and microarchitecture. The high prescription rate of the drugs brings about increases in the numbers of fractures and bone defects requiring surgical interventions in these patients. Currently, critical-size defects are filled with biomaterials and healing is supported with bone morphogenetic proteins (BMP). It is hypothesized that BPs interfere with biomaterial turnover during BMP-supported repair of defects filled with β-tricalcium phosphate (βTCP) ceramics. To test this hypothesis, retired breeder rats were ovariectomized (OVX). After 8 weeks, treatment with alendronate (ALN) commenced. Five weeks later, 6 mm diaphyseal femoral defects were applied and stabilized with locking plates. βTCP cylinders loaded with 1 μg and 10 μg BMP2, 10 μg L51P, an inhibitor of BMP antagonists and 1 μg BMP2/10 μg L51P were fitted into the defects. Femora were collected 16 weeks post-implantation. In groups receiving calcium phosphate implants loaded with 10 μg BMP2 and 1 μg BMP2/10 μg L51P, the volume of bone was increased and βTCP was decreased compared to groups receiving implants with 1 μg BMP2 and 10 μg L51P. Treatment of animals with ALN caused a decrease in βTCP turnover. The results corroborate the synergistic effects of BMP2 and L51P on bone augmentation. Administration of ALN caused a reduction in implant turnover, demonstrating the dependence of βTCP removal on osteoclast activity, rather than on chemical solubility. Based on these data, it is suggested that in patients treated with BPs, healing of biomaterial-filled bone defects may be impaired because of the failure to remove the implant and its replacement by authentic bone.

J Orthop Surg 2018;26(3):1-10; doi.org/10.1177%2F2309499018802487

In vitro study of new combinations for local antibiotic therapy with calcium sulphate – Near constant release of ceftriaxone offers new treatment options

P. Wahl, K. Rönn, M. Bohner, L. Decosterd, Ch. Meier, M. Schläppi, S. Festa, E. Gautier

Introduction: Local application of antibiotics provides high concentrations at the site of interest, with minimal systemic toxicity. Carrier materials might help manage dead space. Calcium sulphate (CaSO4) has a dissolution time that only slightly exceeds the usually recommended duration of systemic antibiotic treatments. This in vitro study evaluates compatibility, release kinetics and antibacterial activity of new combinations of antibiotics with CaSO4 as carrier material.

Methods: CaSO4 pellets added with 8% w/w antibiotic powder were exposed once in phosphate-buffered saline (PBS) solution and once in bovine plasma, in an elution experiment run over 6 weeks at 37 °C. Antibiotic elution was examined at various time points. Concentration was measured by liquid chromatography with tandem mass spectrometry. Antimicrobial activity was checked with an agar diffusion test.

Results: Piperacillin-tazobactam, ceftazidime, cefepime, and meropenem showed fast reduction of concentration and activity. Flucloxacillin and cefuroxime remained present in relevant concentrations for 4 weeks. Ciprofloxacin, levofloxacin and clindamycin lasted for 6 weeks, but also at cell toxic concentrations. Ceftriaxone showed a near-constant release with only a small reduction of concentration from 130 to 75 mg/l. Elution profiles from PBS and plasma were comparable.

Conclusion: CaSO4 provides new possibilities in the local treatment of bone and joint infections. Ceftriaxone appears to be of particular interest in combination with CaSO4. Release persists at clinically promising concentrations, and appears to have a depot-like slow release from CaSO4, with only a small reduction in activity and concentration over 6 weeks. To the best of our knowledge, such a particular persistent release never was described before, for any antibiotic in combination with a carrier material for local application.

J Bone Jt Infect 2018; 3(4):212-221; doi.org/10.7150%2Fjbji.26218

Joint academic and industrial efforts towards innovative and efficient solutions for clinical needs

A. De Pieri, S. Ribeiro, D. Tsiapalis, D. Eglin, M. Bohner, P. Dubruel, Ph. Procter, D. I. Zeugolis, Y. Bayon

The 4th Translational Research Symposium (TRS) was organised at the annual meeting of the European Society for Biomaterials (ESB) 2017, Athens, Greece, with a focus on ‘Academia—Industry Clusters of Research for Innovation Catalysis’. Collaborations between research institutes and industry can be sustained in several ways such as: European Union (EU) funded consortiums; syndicates of academic institutes, clinicians and industries; funding from national governments; and private collaborations between universities and companies. Invited speakers from industry and research institutions presented examples of these collaborations in the translation of research ideas or concepts into marketable products. The aim of the present article is to summarize the key messages conveyed during these lectures. In particular, emphasis is put on the challenges to appropriately identify and select unmet clinical needs and their translation by ultimately implementing innovative and efficient solutions achieved through joint academic and industrial efforts.

J Mater Sci Mater Med 2018; 29(8):129; doi.org/10.1007/s10856-018-6136-3

Comparison of Fixation Techniques for Acetabular Fractures Involving the Anterior Column with Disruption of the Quadrilateral Plate: A Biomechanical Study

C. May, M. Egloff, A. Butscher, M. J. B. Keel, T. Aebi, K. A. Siebenrock, J. D. Bastian

Background: In elderly patients who have sustained an acetabular fracture involving disruption of the quadrilateral plate (QLP), postoperative loading of the joint beyond the level of partial weight-bearing can result in medial redisplacement of the QLP. The purpose of this biomechanical study was to compare the performances of 4 different fixation constructs intended to prevent medial redisplacement of the QLP.

Methods: Anterior column posterior hemitransverse (ACPHT) fractures with disruption of the QLP were created on synthetic hemipelves (fourth-generation Sawbones models) and subsequently stabilized with (1) a 12-hole plate bridging the QLP (Group 1), (2) the plate with added periarticular screws along the QLP (Group 2), (3) the plate combined with an infrapectineal buttress plate (Group 3), or (4) the plate with the added periarticular screws as well as the buttress plate (Group 4). The point of load application on the acetabulum was defined to be the same as the point of application of maximum vertical hip contact force during normal walking. Loads were applied to simulate either partial weight-bearing (20 cycles, from 35 to 350 N) or inadvertent supraphysiologic loads (linearly increasing loads until the onset of failure, defined as fragment displacement of >3 mm). A universal testing machine was synchronized with a digital image correlation system to optically track redisplacement at the QLP. The level of significance was set at p < 0.05.

Results: During experimental simulation of partial weight-bearing, maximum fracture step openings never exceeded 2 mm. During simulation of inadvertent supraphysiologic load, the median load to failure was higher (p < 0.05) in Group 2 (962 N; range, 798 to 1,000 N) and Group 4 (985 N; range, 887 to 1,000 N) compared with Group 1 (445 N; range, 377 to 583 N) and Group 3 (671 N; range, 447 to 720 N).

Conclusions: All 4 fixation constructs performed in an acceptable manner on testing with simulated partial weight-bearing. Only additional periarticular screws along the QLP increased the fixation strength.

Clinical Relevance: Redisplacement of the QLP resulting in an incongruency of the hip joint has been associated with poor long-term outcomes. Within the constraints of this study, periarticular long screws were superior to infrapectineal buttress plates in preventing medial redisplacement of the QLP.

JBJS: June 20, 2018 - Volume 100 - Issue 12 - p 1047–1054; DOI: 10.2106/jbjs.17.00295

Absorbable mineral nanocomposite for biomedical applications: Influence of homogenous fiber dispersity on mechanical properties

E. Mulky, K. Maniura-Weber, M. Frenz, G. Fortunato, R. Luginbuehl

Electrospun micro- and nanosized fibers are frequently used as reinforcing elements in low temperature ceramic composites for biomedical applications. Electrospinning of fibers yield, however, not individual fibers, but rather fiber-mats that are difficult to separate. Most investigations have been performed on diced mats and highly nonhomogenous composites. We examined the influence of dispersed electrospun single micro- and nanometer fibers on the mechanical properties of calcium phosphate cement composites. Absorbable poly-l-lactic-acid was electrospun yielding fibers with diameters of 244 ± 78 nm, named nanofibers (NF), and 1.0 ± 0.3 μm, named microfibers (MF). These fibers were cut using a particle assisted ultrasonication process and dispersed with hydroxyapatite nanoparticles and composites of low (5%) and high (30%) NF/MF content were engineered. The homogeneity of the fiber distribution was investigated by analyzing fracture areas regarding the number of fibers and Voronoi area size distribution. Variation of fiber distribution was significantly lower in the NF group as compared to the MF group. For composites containing 5% NF (V/V), an eightfold increase in the compressive fracture strength, and for the 30% NF (V/V) a threefold increase compared was measured. The composite containing 5% NF was identified as optimal regarding fiber distribution and strength. Our new method of engineering these composites allows for high volume fractions of NF with low variation in fiber distribution to be incorporated into composites, and shows the importance of using single filaments as reinforcing agents.

J Biomed Mater Res Part A: 106A: 850–857, 2018; DOI: 10.1002/jbm.a.36284

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

S. Scholtes, E. Krämer, M. Weisser, W. Roth, R. Luginbühl, T. Grossner, W. Richter

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

Multimodal analysis of in vivo resorbable CaP bone substitutes by combining histology, SEM, and microcomputed tomography data

A. Sweedy, M. Bohner, G. Baroud

This study introduced and demonstrated a new method to investigate the repair process of bone defects using micro- and macroporous beta-tricalcium phosphate (β-TCP) substitutes. Specifically, the new method combined and aligned histology, SEM, and preimplantation microcomputed tomography (mCT) data to accurately characterize tissue phases found in biopsies, and thus better understand the bone repair process. The results included (a) the exact fraction of ceramic remnants (CR); (b) the fraction of ceramic resorbed and substituted by bone (CSB); and (c) the fraction of ceramic resorbed and not substituted by bone (CNSB). The new method allowed in particular the detection and quantification of mineralized tissues within the 1–10 µm micropores of the ceramic (“micro-bone”). The utility of the new method was demonstrated by applying it on biopsies of two β-tricalcium phosphate bone substitute groups with two differing macropore sizes implanted in an ovine model for 6 weeks. The total bone deposition and ceramic resorption of the two substitute groups, having macropore sizes of 510 and 1220 μm, were 25.1 ± 8.1% and 67.5 ± 3.2%, and 24.4 ± 4.1% and 61.4 ± 6.5% for the group having the larger pore size.

J Biomed Mater Res Part B: Appl Biomater, 106B: 1567–1577, 2018; DOI: 10.1002/jbm.b.33962

Photofunctionalization and non-thermal plasma activation of titanium surfaces

A. Henningsen, R. Smeets, P. Hartjen, O. Heinrich, R. Heuberger, M. Heiland, C. Precht, C. Cacaci

Objective

The aim of this study was to compare UV light and non-thermal plasma (NTP) treatment regarding the improvement of physical material characteristics and cell reaction on titanium surfaces in vitro after short-term functionalization.

Materials and methods

Moderately rough (Ra 1.8–2.0 μm) sandblasted and acid-etched titanium disks were treated by UV light (0.05 mW/cm2 at λ = 360 nm and 2 mW/cm2 at λ = 250 nm) or by NTP (24 W, -0.5 mbar) of argon or oxygen for 12 min each. Surface structure was investigated by scanning electron microscopy, confocal microscopy and X-ray photoelectron spectroscopy (XPS). Hydrophilicity was assessed by dynamic contact angle measurement. Cell attachment, viability, cell proliferation and cytotoxicity were assessed in vitro using murine osteoblast-like cells.

Results

UV irradiation or NTP treatment of titanium surfaces did not alter the surface structure. XPS analysis revealed a significantly increased oxidation of the surface and a decrease of carbon after the use of either method. NTP and UV light led to a significant better cell attachment of murine osteoblasts; significantly more osteoblasts grew on the treated surfaces at each time point (p < 0.001).

Conclusions

UV light as well as NTP modified the surface of titanium and significantly improved the conditions for murine osteoblast cells in vitro. However, results indicate a slight advantage for NTP of argon and oxygen in a short time interval of surface functionalization compared to UV.

Clinical relevance

UV light and NTP are able to improve surface conditions of dental implants made of titanium.

Clinical Oral Investigations: March 2018, Volume 22, Issue 2, pp 1045–1054; DOI: 10.1007/s00784-017-2186-z

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