Michael A. K. Liebschner

Effects of bone cement volume and distribution on vertebral stiffness after vertebroplasty.

Study Design. The biomechanical behavior of a single lumbar vertebral body after various surgical treatments with acrylic vertebroplasty was parametrically studied using finite-element analysis. Objectives. To provide a theoretical framework for understanding and optimizing the biomechanics of vertebroplasty. Specifically, to investigate the effects of volume and distribution of bone cement on stiffness recovery of the vertebral body. Summary of Background Data. Vertebroplasty is a treatment that stabilizes a fractured vertebra by addition of bone cement. However, there is currently no information available on the optimal volume and distribution of the filler material in terms of stiffness recovery of the damaged vertebral body. Methods. An experimentally calibrated, anatomically accurate finite-element model of an elderly L1 vertebral body was developed. Damage was simulated in each element based on empirical measurements in response to a uniform compressive load. After virtual vertebroplasty (bone cement filling range of 1–7 cm3) on the damaged model, the resulting compressive stiffness of the vertebral body was computed for various spatial distributions of the filling material and different loading conditions. Results. Vertebral stiffness recovery after vertebroplasty was strongly influenced by the volume fraction of the implanted cement. Only a small amount of bone cement (14% fill or 3.5 cm3) was necessary to restore stiffness of the damaged vertebral body to the predamaged value. Use of a 30% fill increased stiffness by more than 50% compared with the predamaged value. Whereas the unipedicular distributions exhibited a comparative stiffness to the bipedicular or posterolateral cases, it showed a medial–lateral bending motion (“toggle”) toward the untreated side when a uniform compressive pressure load was applied. Conclusion. Only a small amount of bone cement (∼15% volume fraction) is needed to restore stiffness to predamage levels, and greater filling can result in substantial increase in stiffness well beyond the intact level. Such overfilling also renders the system more sensitive to the placement of the cement because asymmetric distributions with large fills can promote single-sided load transfer and thus toggle. These results suggest that large fill volumes may not be the most biomechanically optimal configuration, and an improvement might be achieved by use of lower cement volume with symmetric placement.

Biomechanical considerations of animal models used in tissue engineering of bone.

Tissue engineering combines the aspects of cell biology, engineering, material science, and surgery to generate new functional tissue, and provides an important approach to the repair of segmental defects and in restoring biomechanical function. The development of tissue-engineering strategies into clinical therapeutic protocols requires extensive, preclinical experimentation in appropriate animal models. The ultimate success of any treatment strategy must be established in these animal models before clinical application. It is clear that the demands of the biological and mechanical environment in the clinical repair of critical size defects with tissue-engineered materials is significantly different from those existing in experimental animals. The major considerations facing any tissue-engineering testing logic include the choice of the defect, the animal, the age of the animal, the anatomic site, the size of the lesion, and most importantly, the micro-mechanical environment. With respect to biomechanical considerations when selecting animals for tissue- engineering of bone, it is evident that no common criteria have been reported. While in smaller animals due to size constraint only structural properties of whole bones can be measured, in larger animals and humans both material properties and structural properties are of interest. Based on reported results, comparison between the tissue-engineered bone across species may be of importance in establishing better model selection criteria. It has already been found that the deformation of long bones is fairly constant across species, and that stress levels during gait are dependent on the weight of the animal and the material properties of the bone tissue. Future research should therefore be geared towards developing better biomechanical testing systems and then finding the right animal model for the existing equipment.

Flow Perfusion Enhances the Calcified Matrix Deposition of Marrow Stromal Cells in Biodegradable Nonwoven Fiber Mesh Scaffolds

In this study, we report on the ability of resorbable poly(L-lactic acid) (PLLA) nonwoven scaffolds to support the attachment, growth, and differentiation of marrow stromal cells (MSCs) under fluid flow. Rat MSCs were isolated from young male Wistar rats and expanded using established methods. The cells were then seeded on PLLA nonwoven fiber meshes. The PLLA nonwoven fiber meshes had 99% porosity, 17 μm fiber diameter, 10 mm scaffold diameter, and 1.7-mm thickness. The nonwoven PLLA meshes were seeded with a cell suspension of 5 × 105 cells in 300 μl, and cultured in a flow perfusion bioreactor and under static conditions. Cell/polymer nonwoven scaffolds cultured under flow perfusion had significantly higher amounts of calcified matrix deposited on them after 16 days of culture. Microcomputed tomography revealed that the in vitro generated extracellular matrix in the scaffolds cultured under static conditions was denser at the periphery of the scaffold while in the scaffolds cultured in the perfusion bioreactor the extracellular matrix demonstrated a more homogeneous distribution. These results show that flow perfusion accelerates the proliferation and differentiation of MSCs, seeded on nonwoven PLLA scaffolds, toward the osteoblastic phenotype, and improves the distribution of the in vitro generated calcified extracellular matrix.

Finite element modeling of the human thoracolumbar spine.

Study Design. Biomechanical properties within cadaveric vertebral bodies were parametrically studied using finite element analysis after calibration to experimental data. Objectives. To develop and validate three-dimensional finite element models of the human thoracolumbar spine based on quantitative computed tomography scans. Specifically, combine finite element modeling together with in vitro biomechanical testing circumventing problems associated with direct measurements of shell properties. Summary of Background Data. Finite element methods can help to understand injury mechanisms and stress distribution patterns within vertebral bodies as an important part in clinical evaluation of spinal injuries. Because of complications in modeling the vertebral shell, it is not clear if quantitative computed tomography-based finite element models of the spine could accurately predict biomechanical properties. Methods. We developed a novel finite element modeling technique based on quantitative computed tomography scans of 19 radiographically normal human vertebra bodies and mechanical property data from empirical studies on cylindrical trabecular bone specimens. Structural properties of the vertebral shell were recognized as parametric variables and were calibrated to provide agreement in whole vertebral body stiffness between model and experiment. The mean value of the shell properties thus obtained was used in all models to provide predictions of whole vertebral strength and stiffness. Results. Calibration of n = 19 computer models to experimental stiffness yielded a mean effective modulus of the vertebral shell of 457 ± 931 MPa ranging from 9 to 3216 MPa. No significant correlation was found between vertebral shell effective modulus and either the experimentally measured stiffness or the average trabecular modulus. Using the effective vertebral shell modulus for all 19 models, the predicted vertebral body stiffness was an excellent predictor of experimental measurements of both stiffness (r2 = 0.81) and strength (r2 = 0.79). Conclusion. These findings indicate that modeling of the vertebral shell using a constant thickness of 0.35 mm and an effective modulus of 457 MPa, combined with quantitative computed tomography-based modeling of trabecular properties and vertebral geometry, can accurately predict whole vertebral biomechanical properties. Use of this modeling technique, therefore, should produce substantial insight into vertebral body biomechanical behavior and may ultimately improve clinical indications of fracture risk of this cohort.

Creation of a unit block library of architectures for use in assembled scaffold engineering

Guided tissue regeneration is gaining importance in the field of orthopaedic tissue engineering as need and technology permits the development of site-specific engineering approaches. Computer Aided Design (CAD) and Finite Element Analysis (FEA) hybridized with manufacturing techniques such as Solid Freeform Fabrication (SFF), is hypothesized to allow for virtual design, characterization, and production of scaffolds optimized for tissue replacement. However, a design scope this broad is not often realized due to limitations in preparing scaffolds both for biological functionality and mechanical longevity. To aid scientists in fabrication of a successful scaffold, we propose characterization and documentation of a library of micro-architectures, capable of being seamlessly merged according to the mechanical properties (stiffness, strength), flow perfusion characteristics, and porosity, determined by the scientist based on application and anatomic location. The methodology is discussed in the sphere of bone regeneration, and examples of catalogued shapes are presented. Similar principles may apply for other organs as well.

Computer-Aided Tissue Engineering of a Human Vertebral Body

Tissue engineering is developing into a less speculative science involving the careful interplay of numerous design parameters and multidisciplinary professionals. Problem solving abilities and state of the art research tools are required to develop solutions for a wide variety of clinical issues. One area of particular interest is orthopedic biomechanics, a field that is responsible for the treatment of over 700,000 vertebral fractures in the United States alone last year. Engineers are currently lacking the technology and knowledge required to govern the subsistence of cells in vivo, let alone the knowledge to create a functional tissue replacement for a whole organ. Despite this, advances in computer-aided tissue engineering are continually growing. Using a combinatory approach to scaffold design, patient-specific implants may be constructed. Computer-aided design, optimization of geometry using voxel finite element models or other optimization routines, creation of a library of architectures with specific material properties, rapid prototyping, and determination of a defect site using imaging modalities highlight the current availability of design resources. This study proposes a novel methodology from start to finish which could, in the future, be used to design a tissue-engineered construct for the replacement of an entire vertebral body.

Bone-derived CAD library for assembly of scaffolds in computer-aided tissue engineering

To aid in the development of scaffolds for tissue engineering, we propose a library of architectures (unit primitives) that may be strategically merged according to various characteristics. In particular, for bone, mechanical characteristics such as the regional stiffness, micro-architectural levels of mechanical surface strain, void fraction amount and orientation, as well as permeability and other parameters will be critical both individually and in concert. As relationships between the aforementioned parameters are elucidated, the potential to successfully engineer scaffolds improves. Here we expound upon previous research of creating assembled scaffolds from derived analytical shapes, extending it to encompass native human trabecular bone architecture, derived from repeated patterns witnessed in the interior portion of human vertebrae. Several results are reported; namely, the description of numerous tissue primitives and interfaces with commentary on their morphological characteristics, the integration of unit-blocks into a global assembly using a regional bone density map, and their assembly.

Evolution of vertebroplasty: a biomechanical perspective.

This paper is a collection of computational, finite element studies on vertebroplasty performed in our laboratory, which attempts to provide new biomechanical evidence and a fresh perspective into how the procedure can be implemented more effectively toward the goal of preventing osteoporosis-related fractures. The percutaneous application of a bone cement to vertebral defects associated with osteoporotic vertebral compression fracture has proven clinical successful in alleviating back pain. When the biomechanical efficacy of the procedure was examined, however, vertebroplasty was found to be limited in its ability to provide sufficient augmentation to prevent further fractures without risking complications arising from cement extravasations. The procedure may instead be more efficient biomechanically as a prophylactic treatment, to mechanically reinforce osteoporotic vertebrae at risk for fracture. Patient selection for such intervention may be reliably achieved with the more accurate fracture risk assessments based on vertebral strength, predicted using geometrically detailed, specimen-specific finite element models, rather than on bone density alone. Optimal cement volume, placement, and material properties were also recommended. The future of vertebroplasty involving biodegradable augmentation material laced with osteogenic agents that upon release will stimulate new bone growth and increase bone mass was proposed.

The Effect of Compressive Axial Preload on the Flexibility of the Thoracolumbar Spine

Study Design: An in vitro flexibility study of the human thoracolumbar spine under compressive preload. Objective. To attain kinematics descriptive of the thoracolumbar spine in vitro by applying a pure bending moment under a range of physiologic compressive preloads. Summary of Background Data. Many studies on the mechanical behavior of the spine under pure moment have been conducted; however, little is known regarding variations in the range of motion of the thoracolumbar spine attributable to simulated body weight and other physiologic load conditions. Methods. Five fresh human cadaveric thoracolumbar spine specimens (T9–L3) were used. Five compressive axial preloads ranging from 75 to 975 N were applied to each specimen along the spinal curvature through four adjustable brackets attached to each vertebral body. Flexibility measurements were taken by applying a maximum of 5 Nm pure bending moment to the specimen in flexion and extension. The flexibilities in flexion and extension for each loading case were compared. Results. The thoracolumbar spine supported compressive preloads as much as 975 N without damage or instability in the sagittal plane when the preload was applied along the natural curvature of the spine through estimated centers of rotation. The flexibility in bending (flexion/extension) of the ligamentous thoracolumbar spine decreased with increasing compressive preload. Conclusion. A higher bending stiffness was reached after the compressive load exceeded 500 N. Such knowledge could be used to establish better testing guidelines for implant evaluation and more realistic loading conditions.

Biomechanical evaluation of a double-threaded pedicle screw in elderly vertebrae.

We sought to test the hypothesis that a pedicle screw that has two parallel threads of different heights throughout the full length of the screw could increase both bone purchase and pullout strength compared with a standard single-threaded screw of similar dimensions. A single-threaded pedicle screw and a double-threaded pedicle screw were respectively placed into the paired pedicles of 21 vertebral bodies. The screws were then pulled out of the pedicles, and output parameters were measured. Although insertional torque was, on average, 14.5% higher (p = 0.039) for the single-threaded screw, maximum pullout strength (p = 0.12), energy-to-failure (p = 0.39), and stiffness (p = 0.54) were not statistically different for the two screw types. It is concluded that a second, smaller inner thread on a double-threaded pedicle screw does not translate into either increased bone purchase or higher pullout strengths.