Investigation of degraded bone substitutes made of magnesium alloy using scanning electron microscope and nanoindentation.

Degradable bone substitutes made of magnesium alloys are an alternative to biological bone grafts. The main advantage is that they can be manufactured location- and patient-specific. To develop and scale appropriate implants using computational models, knowledge about the mechanical properties and especially the change in the properties during the degradation process is essential. Therefore, degraded open-pored implants were investigated using scanning electron microscope and nanoindentation to find their material composition and mechanical properties. Using both techniques the correlation of the material composition and the average modulus was determined. It could be shown that the average modulus of the degradation layer is distinctly lower than that of the base material. The local average modulus of degrading implant highly depends on the magnesium concentration and the accumulation of elements from the environment. A decrease in magnesium concentration leads to a decrease in the average modulus. Thus, the degrading implant had a lower stiffness than the initial structure.

A simulation model for the degradation of magnesium-based bone implants.

The development of degradable bone implants, in particular made of metal materials, is an emerging field. The advantage of degradable implants is that they do not have to be removed later. In order to be able to develop and scale appropriate implants for different applications, it is necessary to know the change in mechanical properties of the implant during the degradation process in general and at different locations. One area of bone implants are bone substitute materials. They are deployed when there is a defect in the bone which cannot be filled autonomously by the body. In this study, a numerical degradation model of magnesium-based bone substitute materials is developed using the finite element method. Computational models are being developed to reduce experimental animal research in future. Magnesium is a naturally occurring material which is needed to build enzymes in the body. Additionally, magnesium has a Young's modulus close to native bone, wherefore it is attractive for medical applications with bone contact. The simulation model is based on the assumption that the degradation is a diffusion-controlled process driven by the dissolution of magnesium. The model is adapted to a 3D open-pored structure made of the magnesium alloy LAE442. Previous studies showed that implants made of LAE442 lose stiffness without a volume reduction. To simulate the change in mechanical properties, a concentration-dependent Young's modulus is assumed. With this model the formation of the degradation layer is computable as well as the change in mechanical properties, as measured by the effective Young's modulus of the structure. The movement of the interface between the not-degraded and degraded material is modelled using the level set method.

Innovative 3D Model of the Human Middle Ear in High Resolution with a Histological Microgrinding Method: A Feasibility Study and Comparison with µCT.

Conclusion. The development of a histological 3D model of the tympanic cavity visualizes the exact microanatomy of the sound conduction organ and is therefore essential for finite elements simulations and surgical training. Objectives. So far, no accurate histological 3D model of the sound conduction system existed in literature. For 3D reconstruction of the very fine structures inside and outside the auditory ossicles, a method based on histological slices allows a more differential analysis of both hard and soft tissues and could thus be superior to µCT. Method. A complete temporal bone was embedded in epoxy resin and microground in distances of about 34 µm. After photodocumentation of every plane, a 3D reconstruction was performed by using the Computer Aided Design (CAD) program Rhinoceros 5®. For comparison, a µCT of the same specimen resulted in a 3D model of the calcified structures in the middle ear. Results. The histological 3D model gives an excellent overview to all anatomical soft and bony tissues of the human auditory ossicles. Specifically the fine blood vessel system and the exact dimension of cartilage areas inside the ossicles can be illustrated much more precisely than with µCT data. The present technique also allows the evaluation of the fine connecting ligaments inside the tympanic cavity.

Three-Dimensional Nonlinear Finite Element Analysis and Microcomputed Tomography Evaluation of Microgap Formation in a Dental Implant Under Oblique Loading.

PURPOSE: Since bacterial leakage along the implant-abutment interface may be responsible for peri-implant infections, a realistic estimation of the interface gap width during function is important for risk assessment. The purpose of this study was to compare two methods for investigating microgap formation in a loaded dental implant, namely, microcomputed tomography (micro-CT) and three-dimensional (3D) nonlinear finite element analysis (FEA); additionally, stresses to be expected during loading were also evaluated by FEA. MATERIALS AND METHODS: An implant-abutment complex was inspected for microgaps between the abutment and implant in a micro-CT scanner under an oblique load of 200 N. A numerical model of the situation was constructed; boundary conditions and external load were defined according to the experiment. The model was refined stepwise until its load-displacement behavior corresponded sufficiently to data from previous load experiments. FEA of the final, validated model was used to determine microgap widths. These were compared with the widths as measured in micro-CT inspection. Finally, stress distributions were evaluated in selected regions. RESULTS: No microgaps wider than 13 µm could be detected by micro-CT for the loaded implant. FEA revealed gap widths up to 10 µm between the implant and abutment at the side of load application. Furthermore, FEA predicted plastic deformation in a limited area at the implant collar. CONCLUSION: FEA proved to be an adequate method for studying microgap formation in dental implant-abutment complexes. FEA is not limited in gap width resolution as are radiologic techniques and can also provide insight into stress distributions within the loaded complex.

Corrosion behavior, biocompatibility and biomechanical stability of a prototype magnesium-based biodegradable intramedullary nailing system.

Implants made of degradable magnesium alloys are a potential alternative to conventional orthopaedic implant materials, e.g. stainless steel or titanium. Intramedullary nails made of the magnesium alloy LAE442 were subjected to cyclic fatigue tests in both distilled water and Hank's Balanced Salt Solution (HBSS) at 37.5°C until implant failure or a limit of 500,000cycles was reached. In distilled water, four of the five nails were still intact after the end of the biomechanical test. In HBSS, a breakage within the first 70,000 bending cycles was observed. Additionally, the degradation rate of this alloy was determined in HBSS according to the weight loss method (0.24±0.12mmyear(-1)) and based on gas release (0.21±0.03mmyear(-1)) with a standard eudiometer. A cytotoxicity test with L929 cells was carried out in accordance with EN ISO 10993-5/12. This test demonstrated sufficient cell viability of the diluted extracts (50%, 25% and 12.5%). The relative metabolic activity of the 100% extract was reduced slightly below 70%, which is classified as a threshold value for cytotoxicity. In conclusion, this in vitro study indicates that intramedullary nails made of LAE442 may not have the required fatigue resistance for load-bearing applications and the development of a corrosion-protective coating may be necessary to prevent early failure of the implant.

Measurement of Ex Vivo Porcine Lens Shape During Simulated Accommodation, Before and After fs-Laser Treatment.

PURPOSE: According to Helmholtz, accommodation is based on the flexibility of the crystalline lens, which decreases with age, causing presbyopia. With femtosecond (fs)-lentotomy treatment, it is possible to restore the flexibility of presbyopic lenses. The efficiency of the treatment can be systematically evaluated using the finite element method based on experimental data. The purpose of this study was to quantify the shape change of ex vivo lenses in different accommodation states according to the fs-lentotomy treatment. METHODS: Five lenses with ciliary body excised from ex vivo porcine eyes (age: approximately 6 months, exact age unknown) were stretched in an accommodation device before and after laser treatment. Depending on the accommodation state, the lens shape, reconstructed from lens thickness, diameter, and anterior and posterior curvature, was measured using optical coherence tomography (OCT). The complete lens shape was parameterized and each measured parameter was compared to the results of a control group (n = 5, age: approximately 6 months, exact age unknown) without treatment. RESULTS: The amplitudes of the parameters thickness (+140%), diameter (+54%), and anterior radius of curvature (+57%) significantly increased after treatment (P < 0.05), and showed no significant change for the control group. By contrast, the amplitude of the posterior radius of curvature showed no change after treatment (P > 0.05). CONCLUSIONS: Measurement of the lens shape in different accommodation states was successful and showed significant changes after the treatment. The resulting data will be utilized as input for a finite element model to systematically evaluate the effect of fs-lentotomy treatment in future work.

In vivo evaluation of a magnesium-based degradable intramedullary nailing system in a sheep model.

The biocompatibility and the degradation behavior of the LAE442 magnesium-based intramedullary interlocked nailing system (IM-NS) was assessed in vivo in a comparative study (stainless austenitic steel 1.4441LA) for the first time. IM-NS was implanted into the right tibia (24-week investigation period; nails/screws diameter: 9 mm/3.5 mm, length: 130 mm/15-40 mm) of 10 adult sheep (LAE442, stainless steel, n=5 each group). Clinical and radiographic examinations, in vivo computed tomography (CT), ex vivo micro-computed tomography (µCT), mechanical and histological examinations and element analyses of alloying elements in inner organs were performed. The mechanical examinations (four-point bending) revealed a significant decrease of LAE442 implant stiffness, force at 0.2% offset yield point and maximum force. Periosteal (new bone formation) and endosteal (bone decline) located bone alterations occurred in both groups (LAE442 alloy more pronounced). Moderate gas formation was observed within the LAE442 alloy group. The CT-measured implant volume decreased slightly (not significant). Histologically a predominantly direct bone-to-implant interface existed within the LAE442 alloy group. Formation of a fibrous tissue capsule around the nail occurred in the steel group. Minor inflammatory infiltration was observed in the LAE442 alloy group. Significantly increased quantities of rare earth elements were detected in the LAE442 alloy group. µCT examination showed the beginning of corrosion in dependence of the surrounding tissue. After 24 weeks the local biocompatibility of LAE442 can be considered as suitable for a degradable implant material. STATEMENT OF SIGNIFICANCE: An application oriented interlocked intramedullary nailing system in a comparative study (degradable magnesium-based LAE442 alloy vs. steel alloy) was examined in a sheep model for the first time. We focused in particular on the examination of implant degradation by means of (µ-)CT, mechanical properties (four-point bending), clinical compatibility, local bone reactions (X-ray and histology) and possible systemic toxicity (histology and element analyses of inner organs). A significant decrease of magnesium (LAE442 alloy) implant stiffness and maximum force occurred. Moderate not clinically relevant gas accumulation was determined. A predominantly direct bone-to-implant contact existed within the magnesium (LAE442 alloy) group compared to an indirect contact in the steel group. Rare earth element accumulation could be observed in inner organs but H&E staining was inconspicuous.

Degradation behaviour of LAE442-based plate-screw-systems in an in vitro bone model.

The use of absorbable implant materials for fixation after bone fracture helps to avoid a second surgery for implant removal and the risks and costs involved. Magnesium (Mg) is well known as a potential metallic material for degradable implants. The aim of the present in vitro study was to evaluate if degradable LAE442-based magnesium plate-screw-systems are suitable candidates for osteosynthesis implants in load-bearing bones. The corrosion behaviour was tested concerning the influence of different surface treatments, coatings and screw torques. Steel plates and screws of the same size served as control. Plates without special treatment screwed on up to a specified torque of 15cNm or 7cNm, NaOH treated plates (15cNm), magnesium fluoride coated plates (15cNm) and steel plates as control (15cNm) were examined in pH-buffered, temperature-controlled SBF solution for two weeks. The experimental results indicate that the LAE442 plates and screws coated with magnesium fluoride revealed a lower hydrogen evolution in SBF solution as well as a lower weight loss and volume decrease in µ-computed tomography (µCT). The nanoindentation and SEM/EDX measurements at several plate areas showed no significant differences. Summarized, the different screw torques did not affect the corrosion behaviour differently. Also the NaOH treatment seemed to have no essential influence on the degradation kinetics. The plates coated with magnesium fluoride showed a decreased corrosion rate. Hence, it is recommended to consider this coating for the next in vivo study.

Influence of lubricant on screw preload and stresses in a finite element model for a dental implant.

STATEMENT OF PROBLEM: Loosening or fracture of the abutment screw are frequent complications in implant dentistry and are detrimental to the long-term success of the restorations. However, little is known about the factors influencing the stability of the screw-abutment complex. PURPOSE: The purpose of this study was to investigate the influence of lubricant action during implant assembly on screw preload and stresses in a dental implant-abutment complex. MATERIAL AND METHODS: A dental implant was modeled for finite element stress analysis. Different friction coefficients (µ=0.2 to 0.5) were chosen for the interfaces between implant components to simulate lubricant action or dry conditions. The stress analyses were each divided into 2 load steps. First, the abutment screw was virtually tightened with a torque of 25 Ncm. This was achieved by applying an equivalent preload calculated according to the different friction coefficients chosen. Second, the construction was externally loaded with a force of 200 N inclined by 30 degrees relative to the implant axis. RESULTS: The screw preload increased with the decreasing friction coefficient. In all components, stresses increased with decreasing friction coefficient. Plastic deformation was observed at the implant neck in an area that expanded with decreasing friction coefficient. No plastic deformation occurred in the abutment. CONCLUSIONS: The results of this study indicated that screw preload should be included in the finite element analysis of dental implants for a realistic evaluation of stresses in the implant-abutment complex. The friction coefficient significantly influenced the screw preload value and modified the stresses in the implant-abutment complex.

Replacement, refinement, and reduction: necessity of standardization and computational models for long bone fracture repair in animals.

In the field of fracture healing it is essential to know the impacts of new materials. Fracture healing of long bones is studied in various animal models and extrapolated for use in humans, although there are differences between the micro- and macrostructure of human versus animal bone. Unfortunately, recommended standardized models for fracture repair studies do not exist. Many different study designs with various animal models are used. Concerning the general principles of replacement, refinement and reduction in animal experiments (three "Rs"), a standardization would be desirable to facilitate better comparisons between different studies. In addition, standardized methods allow better prediction of bone healing properties and implant requirements with computational models. In this review, the principles of bone fracture healing and differences between osteotomy and artificial fracture models as well as influences of fixation devices are summarized. Fundamental considerations regarding animal model choice are discussed, as it is very important to know the limitations of the chosen model. In addition, a compendium of common animal models is assembled with special focus on rats, rabbits, and sheep as most common fracture models. Fracture healing simulation is a basic tool in reducing the number of experimental animals, so its progress is also presented here. In particular, simulation of different animal models is presented. In conclusion, a standardized fracture model is of utmost importance for the best adaption of simulation to experimental setups and comparison between different studies. One of the basic goals should be to reach a consensus for standardized fracture models.

Solid state NMR investigation of intact human bone quality: balancing issues and insight into the structure at the organic-mineral interface.

Age-related bone fragility fractures present a significant problem for public health. Measures of bone quality are increasingly recognized to complement the conventional bone mineral density (BMD) based assessment of fracture risk. The ability to probe and understand bone quality at the molecular level is desirable in order to unravel how the structure of organic matrix and its association with mineral contribute to the overall mechanical properties. The (13)C{(31)P} REDOR MAS NMR (Rotational Echo Double Resonance Magic Angle Spinning Nuclear Magnetic Resonance) technique is uniquely suited for the study of the structure of the organic-mineral interface in bone. For the first time, we have applied it successfully to analyze the structure of intact (non-powdered) human cortical bone samples, from young healthy and old osteoporotic donors. Loading problems associated with the rapid rotation of intact bone were solved using a Finite Element Analysis (FEA) approach, and a method allowing osteoporotic samples to be balanced and spun reproducibly is described. REDOR NMR parameters were set to allow insight into the arrangement of the amino acids at the mineral interface to be accessed, and SVD (Singular Value Decomposition) was applied to enhance the signal to noise ratio and enable a better analysis of the data. From the REDOR data, it was found that carbon atoms belonging to citrate/glucosaminoglycans (GAGs) are closest to the mineral surface regardless of age or site. In contrast, the arrangement of the collagen backbone at the interface varied with site and age. The relative proximity of two of the main amino acids in bone matrix proteins, hydroxyproline and alanine, with respect to the mineral phase was analyzed in more detail, and discussed in view of glycation measurements which were carried out on the tissues. Overall, this work shows that the (13)C{(31)P} REDOR NMR approach could be used as a complementary technique to assess a novel aspect of bone quality, the organic-mineral interface structure.

Axial forces and bending moments in the loaded rabbit tibia in vivo.

BACKGROUND: Different animal models are used as fracture models in orthopaedic research prior to implant use in humans, although biomechanical forces can differ to a great extend between species due to variable anatomic conditions, particularly with regard to the gait. The rabbit is an often used fracture model, but biomechanical data are very rare. The objective of the present study was to measure axial forces, bending moments, and bending axis directly in the rabbit tibia in vivo. The following hypothesis was tested: Axial forces and bending moments in the mid-diaphysis of rabbit tibia differ from other experimental animals or indirectly calculated data. METHODS: A minifixateur system with 4 force sensors was developed and attached to rabbit tibia (n = 4), which were subsequently ostectomised. Axial forces, bending moments and bending angles were calculated telemetrically during weight bearing in motion between 6 and 42 days post operation. RESULTS: Highest single values were 201% body weight [% bw] for axial forces and 409% bw cm for bending moments. Whereas there was a continous decrease in axial forces over time after day 10 (P = 0.03 on day 15), a decrease in bending moments was inconsistent (P = 0.03 on day 27). High values for bending moments were frequently, but not consistently, associated with high values for axial forces. CONCLUSION: Axial forces in rabbit tibia exceeded axial forces in sheep, and differed from indirectly calculated data. The rabbit is an appropriate fracture model because axial loads and bending moments in rabbit tibia were more closely to human conditions than in sheep tibia as an animal model.

Extended Finite Element models of introcortical porosity and heterogeneity in cortical bone.

Due to changes in the bone quality during ageing the fracture risk increases. The influence of the different parameters affecting bone quality is not well understood. The Finite Element method offers the opportunity to determine the individual contribution of a parameter by changing single parameters. In this study, the ABAQUS extended Finite Elements Method (xFEM) was applied to simulate the crack propagation in compact bone samples using the quadratic nominal stress as crack criterion. Micro computed tomography images of compact-tension samples machined from a 19 and an 81 years old donor were used to generate Finite Element meshes consisting of linear tetrahedrons via Mimics. Cavities were modelled only in the estimated crack area to avoid a high number of degrees of freedom. Crack area was meshed with a higher number of smaller elements. The other areas were meshed with a small number of larger elements. The changes in the material constants due to the simplification of the model were taken into account by using effective material parameters in these partitions. Our results show that age-related loss in bone toughness results from increased porosity and loss in heterogeneity of material level properties.

Telemetric in vivo measurement of compressive forces during consolidation in a rabbit model.

A quantitative device to gain information about bone healing after fracture or distraction osteogenesis would lower the risk of refracture, malunion and pseudarthrosis. To this date, different biomechanical methods have been proposed for this task with limited application. Furthermore, none of these devices allows monitoring of physiological motion. The aim of this study was to develop a telemetric method for in vivo measurement of compressive forces during physiological motion. An innovative method was developed that can be integrated in an external fixator and that transfers the data to the computer via Bluetooth. After ex vivo validation it was applied to rabbit tibia for assessment of consolidation after tibial osteotomy. After development of the innovative method, the technique was validated ex- and in-vivo in a rabbit model. The presented method enables a telemetric measurement of compressive forces during consolidation. It proved that during consolidation the forces decreased over time from 27.6 to 15.7 Newton. This study presents a new technique to quantify bone healing of fracture or distraction osteogenesis by determination of compressive forces. The innovative of this technique compared to existing methods is the fact that it allows monitoring forces during physiological motion.

Determination of dynamically adapting anisotropic material properties of bone under cyclic loading.

Because bone tissue adapts to loading conditions, finite element simulations of remodelling bone require a precise prediction of dynamically changing anisotropic elastic parameters. We present a phenomenological theory that refers to the tissue in terms of the tendency of the structure to align with principal stress directions. We describe the material parameters of remodelling bone. This work follows findings by the same research group and independently by Danilov (1971) in the field of plasticity, where the dependencies of the components of the stiffness tensor in terms of time are based on Hill's anisotropy. We modify such an approach in this novel theory that addresses bone tissue that can regenerate. The computational assumption of the theory is that bone trabeculae have the tendency to orient along one of the principal stress directions but during remodelling the principal stresses change continuously and the resulting orientation of the trabeculae can differ from the principal stress direction at any given time. The novelty of this work consists in the limited number of parameters needed to compute the twenty-one anisotropic material parameters at any given location in the bone tissue. In addition to the theory, we present here two cases of simplified geometry, loading and boundary conditions to show the effect of (1) time on the material properties; and (2) change of loading conditions on the anisotropic parameters. The long term goal is to experimentally verify that the predictions generated by theory provide a reliable simulation of cancellous bone properties.

A new bending stiffness measurement device to monitor the influence of different intramedullar implants during healing period.

To manage fractures in long bones, intramedullar implants, plates or external fixators are often used. In many cases, the implants are removed after bone consolidation. X-ray images are normally used to monitor bone formation and to determine the point of return to full load bearing and removal of the implants. However, plain radiographs give only inaccurate information about the degree of healing progress. Known quantitative methods as QCT, DEXA, etc. provide information about bone density which certainly contributes to the mechanical properties of healing bone, but they do not provide a direct measurement of the stiffness of the healing callus. In this study we present an in vivo 4-point-bending stiffness device for small animals which is designed to directly monitor the progression of the healing process. The device was tested in a bone-defect model with different test-specimens chosen to simulate the stiffness of bone at different stages of healing. To verify the results, it was tested in an animal fracture study in rabbits during the healing period with and without an intramedulary implant. Both the test-specimen and bones of the in vivo study were compared with data in a materials testing system (MTS) in four-point bending. The device was found to have a high precision and significant in vitro and in vivo correlation with the MTS. The results suggest that this measurement device has the ability to monitor the healing process of bone and to analyse the influence of degradable implants on the mechanical behaviour of bone or bone metabolism effecting pharmaceutics.

[Measurement of the elastic properties of the cancellous bone in the femoral head of the dog]. / Messung der elastischen Eigenschaften der Spongiosa im Femurkopf des Hundes.

The degenerative wear and pathologic damage of the joints are reasons for total endoprotheses in man as well as in dogs. The main problem is the aseptic loosening of the protheses. By usig the finite-element-method, the total endoprothesis is designed with new features, with the purpose of preventing loosening and being better adapted to load transmission. In order to simulate the femur of the dog for the numerical analysis, a material law is developed. By taking into account the anisotropy and the local density of the cancellous bone in the femoral head, the young's modules are experimentally determined. The measurements are performed by ultrasonic methods on femoral heads of euthanised dogs. The results show planar isotropic cancellous bone.