Past Bone Seminars

2001-2002

___October 2001___
Speaker: X. Edward Guo PhD, Associate Professor of Biomedical
Engineering, Director of Bone Bioengineering Laboratory, Department
of Biomedical Engineering, Columbia University
Topic: Trabecular Bone Formation by Mechanical and PTH Stimulation

Dr. Guo’s Research Interests: Bone biomechanics, with emphasis on understanding the cellular/molecular mechanisms of bone response to mechanical and hormone stimulation using both in vivo and in vitro models; computational modeling of bone microstructure and constitutive modeling of biological tissues.

Abstract
Trabecular bone adapts in response to its mechanical environment. A quantitative characterization of the relationship between trabecular bone adaptation and mechanical loading is important in the understanding of the etiology of age-related bone fractures, optimal design of total joint replacements, and bone loss due to microgravity. In addition, trabecular bone responds to changes in its hormonal factors such as parathyroid hormone (PTH), which is an important factor that mediates bone response to mechanical stimuli. In this study, the effect of PTH on trabecular bone adaptation to mechanical load was investigated using an in vivo rat tail vertebra model. Both PTH and mechanical loading stimulate bone formation in trabecular bone through the increase in MAR. These results demonstrate the synergistic interaction between PTH and mechanical stimuli. Furthermore, the results suggest that this synergetic effect of PTH on trabecular bone adaptation by mechanical loading is mostly through the recruitment of osteoblasts. For the first time, a direct correlation between bone formation activities and local mechanical parameters in trabecular bone tissue has been established.

Note: Before Dr. Guo’s presentation Professor Emeritus Albert Hirschman of SUNY Downstate Medical Center was honored in appreciation for his commitment for over thirty years to seminars on mineralized tissue research, beginning with the Bone and Tooth Discussion Group.

___November 2001—I___
Speaker: Haviva M. Goldman PhD, Assistant Professor of Anthropology and Archaeology, Brooklyn College, City University of New York
Topic: The Significance of Heterogeneity in the Microstructural and Geometric Properties of Human Bone

Dr. Goldman’s Research Interests: Haviva recently completed her doctorate in Anthropology through the CUNY Graduate Center (as part of the New York Consortium of Evolutionary Primatology (NYCEP) Graduate Program in Anthropology). Her dissertation research focused on intra-population variability in microstructural and geometric properties of the human mid-shaft femur with age and sex. The project stemmed from an interest in applying histological research to studies of functional adaptation of past (archaeological and fossil) human populations and in better understanding the processes of human variability in skeletal aging. She will continue to pursue her interests in bone biology, extending her research to include a variety of modern and archaeological human population samples, as well as addressing issues of growth and development by studying variability in bone microstructure in juvenile bone as well.

Abstract
Despite extensive research into the effects of aging on bone tissue properties, gaps remain in our knowledge of the causes and extent of heterogeneity in the material (i.e. histological composition) and structural (i.e. geometric shape) properties of bone. As such information is important both for elucidating the relationship between bone structure and its functional adaptation, and for understanding the etiology of age-related diseases such as osteoporosis, a detailed study documenting this variability within the mid-shaft femur of a large, well-documented autopsy sample was undertaken.

Collagen fiber orientation and mineralization density are two aspects of a bone's microstructure that are known to influence the mechanical properties of bone. Although their spatial distributions have been hypothesized to reflect loading during life, their variability within an adult sample is relatively unknown. Using circularly polarized light and backscattered electron microscopy it was possible to obtain images of entire femoral cross-sections that could be examined with respect to these two variables. By also calculating measures of cross-sectional geometry, it was possible to provide information about the regularity of bending loads at the femoral mid-shaft that could be examined relative to the microstructural organization.

Extensive variability characterized each of these properties, such that it was not possible to identify a single pattern of microstructural organization for the human mid-shaft femur, even within a single age or sex group. In addition, despite an average coincidence between microstructural organization and predicted bending forces at the mid-shaft, the vast majority of individuals in this sample show no relationship among these variables. These results indicate that these variables act somewhat independently, resulting in different optimal configurations that may reflect an individual's unique life history. The implications of these results for studies of human bone biology are discussed from both an anthropological and biomechanical perspective.

___November 2001—II___
Speaker: Nancy P. Camacho PhD, Associate Scientist, Mineralized Tissue Section, Research Division, Hospital for Special Surgery; Visiting Assistant Professor of Biomedical Engineering, The City College of the City University of New York
Topic: Bisphosphonates in Osteogenesis Imperfecta: Are We Making Brittle Bones More Brittle?

Dr. Camacho’s Research Interests: Ultrastructure and mechanical behavior of bone and cartilage; spectroscopic imaging of mineral and matrix organization in connective tissues; mineralization abnormalities in bone disease; osteogenesis imperfecta; pathologic calcification; effect of therapeutics on fracture healing

Abstract
Recently, bisphosphonates have been proposed as a therapy for children with Osteogenesis Imperfecta (OI), a heritable disease characterized by brittle bones and multiple fractures. There have also been recent reports of potential negative effects of bisphosphonates on bone quality, namely increased microdamage and brittleness. In our current studies, we are investigating the effects of alendronate on bone quality in an animal model of OI, the oim/oim mouse. Femoral three-point bend biomechanical tests combined with geometric analysis, infrared imaging and quantitative backscattered electron imaging (qBEI) measurements of tissue density have been carried out to determine material properties of cortical and metaphyseal bone in growing oim/oim and wildtype (+/+) mice treated with alendronate for short term studies (8 week period), and long term studies (24 weeks). In addition, we have investigated the effects of alendronate when given intermittently versus continuously. The results of these studies support the theory that alendronate treatment is effective in reducing fractures in OI, that continuous treatment is more effective than intermittent, and that increased tissue mineral density is an important determinant of brittleness in both non-treated oim/oim and alendronate-treated wildtype mice bone. Further insights into the effects of bisphosphonates on bone properties will aid in the determination of the best approach for treatment of children such that bone strength and bone quality are maximized.

___December 2001___
Speaker: Clark T. Hung PhD, Assistant Professor of Biomedical Engineering, Cellular Engineering Laboratory, Cardiac Cell Mechanics Laboratory, and Bone Bioengineering Laboratory, Department of Biomedical Engineering, Columbia University
Topic: Fluid Flow Effects on Bone Cells: Influence of Flow-Cell-Substrate; Interactions and Cell Mechanical Properties

Dr. Hung’s Research Interests: Physical effects on cells and orthopaedic cellular and tissue engineering.

Abstract
Bone cell mechanotransduction studies have focused more recently on fluid flow related stimuli. Using parallel-plate or laminar flow chambers, a well-defined stimulus can be applied to cultured cells while permitting optical microscopy, biochemical and molecular assays to be performed. The flow of fluid over the cells gives rise to several potential concomitant stimuli including a hydrostatic pressure gradient, convective transport of agonists, electrokinetic phenomena (e.g., streaming potentials), and fluid-induced shear stress. We have initiated several bioengineering studies to better understand the role of fluid-induced shear stress and related cell deformation in this in vitro model system. Numerical modeling of fluid flow over hemispherical deformable cells on a flat plane demonstrate that the shear levels that are "seen" by the cell are several-fold greater than that described by the macroscopic wall shear stress, the parameter typically used to describe the applied fluid stimulus. However, these calculations are dependent on the material properties of the cell as well as assumptions regarding cell-substrate interactions. Accordingly, studies are underway to include a triphasic model of the cell (treating the cell as a fluid, solid and anion/cation phases). Parallel studies using an atomic force microscope will permit an independent method to assess cell properties on various biological substrates to provide inputs for this triphasic cell model. Lastly, we have also undertaken bone cell adhesion and biochemical studies to gain a further understanding of cell-substrate interactions and signaling that may participate in the bone cell response to fluid flow and correlate these findings to our modeling studies and cell properties obtained by AFM. This is joint work with Kevin D. Costa and X. Edward Guo

___January 2002___
Speaker: Michael Hadjiargyrou PhD, Assistant Professor of Biomedical Engineering, Orthopaedics, and Genetics, Department of Biomedical Engineering, State University of New York, Stony Brook
Topic: Transcriptional Profiling of the Early Fracture Callus: A Key to Bone Bioengineering?

Dr. Hadjiargyrou’s Research Interests: Understanding the molecular mechanisms that underlie the wound healing (i.e., fracture repair) process, as well as normal bone development. Related areas of interest extend to biomaterials, gene therapy and tissue engineering.

Abstract
Bone regeneration occurs as an elaborate series of events that requires temporal and spatial orchestration of numerous cell types and expression of hundreds to thousands of genes. The healing of a fractured bone is, in essence, a recapitulation of embryonic bone development that proceeds through similar processes such as chondrogenesis, ossification and remodeling. In order to be able to influence these biological events and thus the overall bone regeneration process, a more comprehensive molecular understanding is essential. In an effort to identify gene expression patterns that occur during bone regeneration, a cDNA library was constructed. This library consisted of transcriptionally induced genes (pooled from RNA isolated from post fracture (PF) 3, 5, 7 and 10 day callus) that were subtracted following hybridization with RNA derived from intact bone. Following amplification, subtractive hybridization and cloning, 4,183 cDNA clones were identified as up-regulated genes and further characterized. Of these, 3,799 (91%) were successfully sequenced. These genes included 301 (8%) and 60 (1.6%) that showed homology to mitochondrial and ribosomal genes, respectively. In addition, 2,002 (52.7%) had homology to other known genes and represented multiple functional gene families. Further, more than one third of these clones had no functional information in the literature or public databases. Of these, 1,317 (34.7%) showed homology to expressed sequenced tags (EST's) and 119 (3%) were completely novel. To obtain a more comprehensive understanding of temporal gene expression and significance of the genes in the healing process, custom microarrays were constructed that contained all 4,183 clones. PF day 3, 5, 7, 10, 14 and 21 callus RNA samples were used to probe these microarrays and confirm that greater than 80% of cDNAs are up-regulated greater than two fold, on at least one of the PF days, in comparison with intact bone. We are currently investigating the differential expression of these genes as a function of time (i.e. progression of the healing callus), and performing cluster analysis to potentially assign function to the thousands of EST's, novel sequences and known genes that have not as yet been described as involved in the bone regeneration process. Taken together, these data provide a "window" into the molecular events responsible for the early phases of bone regeneration and suggest that many of the genes involved remain uncharacterized.

___February 2002___
Speaker: Yixian Qin PhD. Assistant Professor of Biomedical Engineering, State University of New York, Stony Brook
Topic: Fluid Flow Stimulates the Formation of Bone as Dependent on Transcortical Fluid Pressure Gradients

Dr. Qin’s Research Interests: Tissue remodeling and non-invasive assessment of bone physiology and quality, with emphasis on understanding fluid flow mechanism in skeletal tissues, and mechanotransduction of physical stimuli. Related areas of interest extend to diagnostics of skeletal tissue quality, including osteoporosis, space osteopenia and fracture healing.

Abstract
Considering the strong anabolic potential of mechanical stimuli, and the devastating consequences of removing these regulatory signals, it becomes critical to determine how the bone cell population perceives subtle changes in their functional environment. Indeed, improving our understanding of the manner in which mechanical signals influence the temporal and spatial dynamics of bone remodeling may help to devise a biomechanically based intervention for treating osteoporosis, accelerating fracture healing or promoting bony ingrowth into prostheses. The motion of interstitial fluid within bone, which arises as a result of functional load bearing, is hypothesized to be a critical mediator in the perception and response of skeletal tissue to mechanical stimuli. However, little is known about the remodeling responses that occur during in vivo fluid flow stimuli in the absence of matrix deformation. In particular, how bone-remodeling response to specific mechanical fluid parameters is unknown. Our recent studies of bone remodeling and formation demonstrate a strong correlation between the fluid pressure gradient and the surface new bone formation. Fluid flow applied at physiological level not only inhibits disuse induced bone resorption, but also, dose-dependently, encourage bone formation while applied in dynamic frequency. These results can also extend to the trabecular region which low magnitude of fluid pressure and/or surface fluid shear stress can initiate sufficient adaptive response in trabeculae without matrix strain. The results suggest that the fluid flow, which arises by functional loading, is an important mediator in retaining bone quality and quantity, and that small fluctuations in fluid flow, achieved via pressure differentials, has potential for therapeutic applications against skeletal disorders
even in the absence of mechanical strain.

___March 2002___
Speaker: Peter S. Walker PhD, Director of Biomedical Engineering, Cooper Union; Honorary/Research Professor, University College, London; University of Nebraska, Omaha; New York University
Topic: Prospects for the Future of Total Knee Replacement

Dr. Walker’s Research Interests: TKR design, joint biomechanics, minimally invasive surgery, joint resurfacing, knee simulating machines

Abstract
The lecture discusses the evolution of TKR designs up to this time. Follow up shows survivorship of 95% and better at ten years. Using this as a solid base, there are now new challenges that translate into design goals. How can the consistency of the surgery be improved? How can higher flexion angles be achieved to accommodate a full lifestyle? How can the time of surgery be minimized? How can the rehabilitation time be reduced? The potential solutions and progress so far to these design goals will be discussed.

___April 2002___
Speaker: Susannah P. Fritton PhD, Associate Professor of Mechanical Engineering, The City College of New York
Topic: Delineating the Pathway of Interstitial Fluid Flow in Bone

Dr. Fritton’s Research Interests: Understanding the adaptive response of bone to mechanical forces; bone's mechanosensory system

Abstract
Although it is well accepted that mechanical signals are critical to maintain an adequate skeleton, the mechanism by which bone cells sense their mechanical environment and initiate the resorption and/or deposition of bone tissue is not known. Load-induced interstitial fluid flow is believed to play a role in bone's mechanosensory system via the shear stresses that it produces on bone cells, stresses that have been shown to produce biochemical responses in bone cells in vitro. Load-induced bone fluid flow has also been proposed to enhance mass transport in bone to ensure the metabolic function of bone cells that is crucial for bone growth, maintenance, and adaptation.
Diffusion of molecules through the porous bone matrix has been studied in animal models using injected tracers, and recently tracer methods have been used to experimentally confirm the existence of load-induced transport within bone tissue. However, because bone tissue has three distinct porosities (vascular, lacunar-canalicular, and collagen-hydroxyapatite), understanding bone fluid flow remains a challenge. A fundamental question remains unanswered: What is the size of the smallest bone pore that is available for interstitial fluid flow? Conflicting reports exist in the literature as to whether bone fluid can flow through the smallest pores in the mineralized matrix (the collagen-hydroxyapatite microporosity) in addition to flowing through the lacunar-canalicular porosity. In this seminar, our recent work documenting where injected tracers of different sizes travel in the bone microporosity will be presented and compared to findings from the literature. Delineating the pathway of bone interstitial fluid flow will help to further delineate bone's microstructure and should contribute to the understanding of bone's mechanosensory system.