Alexander Tsouknidas

New Ti-Alloys and Surface Modifications to Improve the Mechanical Properties and the Biological Response to Orthopedic and Dental Implants: A Review

Titanium implants are widely used in the orthopedic and dentistry fields for many decades, for joint arthroplasties, spinal and maxillofacial reconstructions, and dental prostheses. However, despite the quite satisfactory survival rates failures still exist. New Ti-alloys and surface treatments have been developed, in an attempt to overcome those failures. This review provides information about new Ti-alloys that provide better mechanical properties to the implants, such as superelasticity, mechanical strength, and corrosion resistance. Furthermore, in vitro and in vivo studies, which investigate the biocompatibility and cytotoxicity of these new biomaterials, are introduced. In addition, data regarding the bioactivity of new surface treatments and surface topographies on Ti-implants is provided. The aim of this paper is to discuss the current trends, advantages, and disadvantages of new titanium-based biomaterials, fabricated to enhance the quality of life of many patients around the world.

The effect of kyphoplasty parameters on the dynamic load transfer within the lumbar spine considering the response of a bio-realistic spine segment

BACKGROUND With an increasing prevalence of osteoporosis, physicians have to optimize treatment of relevant vertebral compression fractures, which have significant impact on the quality of life in the elder population. Retrospective clinical studies suggest that kyphoplasty, despite being a procedure with promising potential, may be related to an increased fracture risk of the adjacent untreated vertebrae. METHODS A bio-realistic model of a lumbar spine is introduced to determine the morbidity of cemented augmentation. The model was verified and validated for the purpose of the study and subjected to a dynamic finite element analysis. Anisotropic bone properties and solid ligamentous tissue were considered along with α time varying loading scenario. FINDINGS The yielded results merit high clinical interest. Bi-pedicular filling stimulated a symmetrically developing stress field, thus comparing favourably to uni-pedicular augmentation which resulted in a non-uniform loading of the spine segment. An enslavement of the load transfer was also found to both patient bone mineral density and reinforcement-nucleous pulpous superimposition. INTERPRETATION The investigation presented refined insight into the dynamic biomechanical response of a reinforced spine segment. The increase in the calculated occurring stresses was considered as non-critical in most cases, suggesting that prevalent fractures are a symptomatic condition of osteoporosis rather than a sequel of efficiently preformed kyphoplasty.

Influence of Alveolar Bone Loss and Different Alloys on the Biomechanical Behavior of Internal-and External-Connection Implants: A Three-Dimensional Finite Element Analysis

PURPOSE The purpose of this study was to evaluate the stress distribution during application of occlusal loads to maxillary anterior single external- and internal-connection implant-supported restorations with different amounts of bone loss and with the use of different metal alloys for restorations and fixation screws. MATERIALS AND METHODS Models of external- and internal-connection implants, corresponding abutments/crowns, and fixation screws were developed. These models were then imported into finite element analysis software to study the impact of forces on different implant connections and materials. Each prosthesis was subjected to a 200-N compressive shear force applied at 130 degrees relative to the long axis of the implant. The materials were considered linear, isotropic, and homogenous. The parameters changed for each connection type included: bone resorption in relation to the prosthetic platform (no, 2 mm, or 4 mm of resorption); alloys of the restorations (nonprecious vs precious); and alloys of the abutment screws (titanium vs gold). Von Mises stresses were used to display the stress in five models: implant, restoration, screw, cancellous bone, and cortical bone. RESULTS Statistically significant differences in the stresses of all involved structures occurred when the bone level decreased by 2 mm and by 4 mm. The connection type contributed to statistically significant differences in the stresses in both the restoration and the screw. The alloy type resulted in statistically significant differences in the implant, the superstructure, and the cortical bone stresses. CONCLUSION As bone resorbed, the stresses generated within the internal-connection implant were greater than those generated in the external-connection implant. The same findings applied for the restoration and for cancellous and cortical bone. The stresses generated in the fixation screw were greater in the external-connection implant than in the internal-connection implant for all bone resorption scenarios.

Anisotropic post-yield response of cancellous bone simulated by stress-strain curves of bulk equivalent structures.

During the last decade, finite element (FE) modelling has become ubiquitous in understanding complex mechanobiological phenomena, e.g. bone–implant interactions. The extensive computational effort required to achieve biorealistic results when modelling the post-yield behaviour of microstructures like cancellous bone is a major limitation of these techniques. This study describes the anisotropic biomechanical response of cancellous bone through stress–strain curves of equivalent bulk geometries. A cancellous bone segment, reverse engineered by micro computed tomography, was subjected to uniaxial compression. The materials constitutive law, obtained by nano-indentations, was considered during the simulation of the experimental process. A homodimensionally bulk geometry was employed to determine equivalent properties, resulting in a similar anisotropic response to the trabecular structure. The experimental verification of our model sustained that the obtained stress–strain curves can adequately reflect the post-yield behaviour of the sample. The introduced approach facilitates the consideration of nonlinearity and anisotropy of the tissue, while reducing the geometrical complexity of the model to a minimum.

A Bio-Realistic Finite Element Model to Evaluate the Effect of Masticatory Loadings on Mouse Mandible-Related Tissues

Mice are arguably the dominant model organisms for studies investigating the effect of genetic traits on the pathways to mammalian skull and teeth development, thus being integral in exploring craniofacial and dental evolution. The aim of this study is to analyse the functional significance of masticatory loads on the mouse mandible and identify critical stress accumulations that could trigger phenotypic and/or growth alterations in mandible-related structures. To achieve this, a 3D model of mouse skulls was reconstructed based on Micro Computed Tomography measurements. Upon segmenting the main hard tissue components of the mandible such as incisors, molars and alveolar bone, boundary conditions were assigned on the basis of the masticatory muscle architecture. The model was subjected to four loading scenarios simulating different feeding ecologies according to the hard or soft type of food and chewing or gnawing biting movement. Chewing and gnawing resulted in varying loading patterns, with biting type exerting a dominant effect on the stress variations experienced by the mandible and loading intensity correlating linearly to the stress increase. The simulation provided refined insight on the mechanobiology of the mouse mandible, indicating that food consistency could influence micro evolutionary divergence patterns in mandible shape of rodents.

Assessment of stress patterns on a spinal motion segment in healthy versus osteoporotic bony models with or without disc degeneration: a finite element analysis.

BACKGROUND CONTEXT With an increasing prevalence of low back pain, physicians strive to optimize the treatment of patients with degenerated motion segments. There exists a consensus in literature that osteoporotic patients exhibit nonphysiologic loading patterns, while degenerated intervertebral discs (IVDs) are also believed to alter spine biomechanics. PURPOSE To evaluate alterations occurring in lumbosacral spine biomechanics of an osteoporotic model, with or without IVD degeneration, when compared with a healthy spine segment. STUDY DESIGN The investigation was based on finite element (FE) analysis of a patient-specific lumbosacral spine model. METHODS A biorealistic model of a lumbosacral spine segment is introduced to determine the morbidity of disc degeneration and osteoporosis. The model was verified and validated for the purpose of the study and subjected to a dynamic FE analysis, considering anisotropic bone properties and solid ligamentous tissue. RESULTS The yielded results merit high clinical interest. Osteoporosis resulted in a nonuniform increase of facet joint loading, which was even more pronounced in the scenario simulating a degenerated disc. The results also revealed an enslavement of intradiscal pressure to the disc state (in the degenerated and superior adjacent level). CONCLUSIONS The investigation presented refined insight into the dynamic biomechanical response of a degenerated spine segment. The increase in the calculated occurring stresses was considered as critical in the motion segment adjacent and superior to the degenerated one. This suggests that prevalent trauma in a motion segment may be a symptomatic condition of a poorly treated formal pathology in the inferior spine level.

The effect of pedicle screw implantation depth and angle on the loading and stiffness of a spinal fusion assembly

BACKGROUND The increasing prevalence of spine disorders in industrialized environments has impaired the quality of life in the elder population. In an effort to relieve pain, physicians strive to improve treatment through the consideration of patient specific characteristics during preoperative planning of procedures such as spinal fusion. OBJECTIVE This study aims at quantifying aspects of spondylodesis to the loading and mobility of the utilized instrumentation, as the use of rigid vs. motion sparing materials as well as implantation angle and depth of the pedicle screws are still subject to controversy among surgeons. METHODS A fixation assembly was reverse engineered based on µCT measurements of the involved instrumentation. Two pedicle screws were connected with a rod, thus representing a mono-segmental fixation device. The pedicle screws were embedded in hexahedral structures simulated by bone properties. Upon validation and verification, the response of the model to a compressive and a torsional load was simulated in ANSYS 14, while altering the implantation depth and insertion angle of the pedicle screws along with the rod material. RESULTS The mobility of the instrumentation was drastically increased (by up to 390%) when PEEK rods were used in place of traditional Ti ones, a tendency observed at varying extent for all simulated scenarios. Shallow implantation induced a slight stress increase (∼21%) on the implant and a notable distressing of the bony tissue (∼44%), whereas inclined screw positioning was overall beneficial to the developing stress fields in both, with bone profiting a max. stress release of ∼15% during the application of torsion. CONCLUSIONS The investigation presented refined insight into the biomechanical response of a spinal fusion device. As expected, rigid fixation seems preferable in fusion oriented instrumentation whereas semi rigid devices should be employed for non-fusion applications. Shallow implantation resulted in a slight posterior offset of the stabilization device, which could be beneficial in the treatment of osteoporotic patients.

Production, Characterization, and Applications of Porous Materials

1 Physical Metallurgy Laboratory, Mechanical Engineering Department, Aristotle University of Thessaloniki, P.O. Box 490, 54124 Thessaloniki, Greece 2 Department of Mechanical Engineering, Frederick University, 1036 Nicosia, Cyprus 3 National Research Council Canada, 75 de Mortagne, Boucherville, QC, Canada H1Y 2V9 4Department of Light Weight Design, Metal Foam Centre, Fraunhofer Institute for Machine Tools and Forming Technologies, 09126 Chemnitz, Germany 5Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

The Influence of Bone Quality on the Biomechanical Behavior of a Tooth-Implant Fixed Partial Denture: A Three-Dimensional Finite Element Analysis.

PURPOSE The purpose of this study was to evaluate whether or not bone quality has an effect on the biomechanical behavior of a tooth connected to an implant, when a rigid and a nonrigid attachment are used. MATERIALS AND METHODS Models of fixed partial dentures supported by a tooth and an implant were developed. These models were then imported into finite element analysis software to study the impact of forces on different types of attachments (rigid vs nonrigid) and bones (types 1 to 4). Each fixed partial denture was subjected to a vertical load of 200 N on the premolars and 230 N on the molar. The materials were considered linear, isotropic, and homogenous. Eight different scenarios were tested. The von Mises criterion was used to display the stress in five structures: fastening screw, implant, attachment, cortical, and trabecular bone. The displacements of the tooth and the implant were also examined. RESULTS The calculated maximum observed stress values differed among the simulated scenarios. The biggest values of stress concentrations were observed at the lingual cervical areas, the implant-cortical bone interface, the implant-crown interface, the butt-joint contact of the implant-abutment screw, and the apical parts of the tooth and implant. The main difference between the rigid and nonrigid connection was observed between the natural tooth retainer and the pontic. In the rigid connection, the movement of the natural tooth retainer was smooth. In the nonrigid connection, the attachment exhibited a partial buccal displacement. Von Mises stresses among the different tested structures ranged between 24 and 840 MPa. CONCLUSION The quality of the bone and the rigidity of the connection between a natural tooth and an implant influence both the generated stresses and the displacement of the tooth and the implant. The highest stresses for the implant-trabecular bone interface, the neck of the implant, and the fastening screw were observed in type 3 bone when a rigid connection was used. The lowest stresses for the implant-cortical bone interface, the neck of the implant, and the connector were registered in type 1 bone, when a rigid connection was used. The smallest tooth and implant displacement was observed in type 1 bone, when a rigid connection was used, while the biggest tooth and implant displacement was registered in type 4 bone when a nonrigid connection was used.

Medical Imaging as a Bone Quality Determinant and Strength Indicator of the Femoral Neck

Early diagnosis of osteoporosis is a key factor in preventive medicine of this clinically silent bone pathology. The most severe manifestation of osteoporotic bone loss is encountered in hip fractures and therfore, this study represents an effort in associating bone quality of the femoral neck region to fragility fracture risks through FEA supported imaging techniques. The concepts of Magnetic resonance imaging (MRI), Computer Tomography (CT) and Dual-energy X-ray absorptiometry (DXA) are introduced, along with their limitations in defining bone quality and calculating the apparent bone strength. As DXA dominates surgeons’ preference in evaluating bone mineral density in the hip region, in vivo measurements of this method, sustained by ex-vivo uniaxial compression tests and FEA supported calculations are employed to determine a fracture risk indicator of the femoral neck versus bone mineral density (BMD).