Smart Skeletons

Clues hidden deep within healthy bone may reveal the secrets of debilitating skeletal disorders

by Daniel P. Nicolella

Senior Research Engineer Daniel P. Nicolella of the SwRI Materials and Structures Division specializes in biomechanical engineering, particularly in bone mechanics and the performance of orthopedic implants.

Imagine a material with the ability to sense varying loads, adapt its architecture to maintain a constant reliability level in response to those loads, repair any damage occurring as a result of everyday use, and survive tens of millions of load cycles without failure. Sound like science fiction? We walk around with this material every day, and we can't survive without it.

The bone that makes up our skeletons is that self-adapting structural material. It has the ability to sense mechanical and structural requirements and adapt its architecture -- both at the microstructural and, to a lesser extent, the macrostructural levels. Ideally, bones can undergo a lifetime of load cycles (tens of millions of cycles) without failure, which is a goal often sought after in man-made engineering materials, but not always achieved.

Researchers at Southwest Research Institute (SwRI) are studying the mechanisms underlying this "smart" behavior to gain insight into debilitating diseases such as osteoporosis, to design longer lasting orthopedic implants, and to develop high-performance materials for industrial, medical, and military use.

SwRI researchers are using new capabilities to better understand the hierarchical nature of bone and the relationship between applied whole bone forces and local tissue level strains. This image shows that microstructural tissue level strains can be significantly different from global whole bone level strains.The regions of high strain (shades from red to green) are located in areas where the bone cells are present and suggest that the mechanical signals that trigger bone cells to remodel may be much greater than previously thought. Results from these studies will help scientists understand bone remodeling phenomena and may be used to develop tissue-engineered bone replacement materials.

Bone structure and adaptation

Bones are primarily composed of two types of structures characterized by different microstructures: cancellous and cortical bone. Cancellous bone is a highly porous bone material found in vertebrae and in the ends, or epiphyses, of long bones; Pores are often associated with more than half the volume of cancellous bone. The dense material that makes up the outer shells, or cortices, of skeletal bone is called cortical bone. Bone mechanics research at SwRI is primarily focused on the behavior and characterization of cortical bone.

Cortical bone has distinct levels of organization beginning at a nano-scale level continuing to the whole bone geometry. At the nano-scale (one billionth of a meter) level, bone tissue is a composite material comprised of an organic phase, consisting mainly of the protein-based material collagen, and a mineral phase, consisting primarily of hydroxyapatite. The collagen and hydroxyapatite are arranged as long fibrils characterized by a periodicity of 69-70 nanometers resulting from the hydroxyapatite mineral forming at specific sites along the collagen molecules. These fibrils are arranged into bundles, and the fibril bundles, in turn, are arranged into sheets of bone material called lamellae. The lamellae form the outer and inner layers of cortical bone as well as the tube-like structures called osteons found in between. It is this hierarchical organization that gives bone its unique properties.

A high magnification image from an atomic force microscope shows one square micrometer of the basic building blocks of bone: collagen fibrils and hydroxyapatite crystals. As part of an effort to understand the underlying mechanisms that control bone fragility, SwRI researchers are using advanced characterization techniques to identify the physical nature of damage to bone and how this damage affects overall bone strength.

Bone is a dynamically adaptable material -- it adapts its form to meet the physical demands placed on it. Not only does the overall geometry of a bone change, the internal organization of the porous cancellous bone and the micro-architecture of cortical bone are rearranged. The most striking examples of such adaptation are in professional athletes, who gradually increase bone size and density as a result of extreme skeletal loads, and in astronauts, who experience a decrease in bone mass because of an absence of loads in the weightless environment of space. While the phenomenon of self-adaptation is well known, the underlying mechanisms that control this behavior are not well understood.

However, a theory of bone adaptation is beginning to take shape. It is believed that three cells found in bone function in a coordinated manner to maintain healthy bone. Osteoblasts are cells that form new bone and are typically found lining bone surfaces that are undergoing extensive remodeling. Bone-removing cells are large, multinucleated cells called osteoclasts. Osteoclasts break down and remove bone material that is no longer needed or that has been damaged in some way. The third cell type is the osteocyte. Osteocytes are widely believed to be the bone "sensor cells," responsible for sensing the mechanical environment to which the skeleton is subjected. Osteocytes are actually osteoblasts that have become embedded in the newly mineralizing bone tissue.

Many protoplasmic processes, or dendrites, emanating from the cell body characterize osteocytes. These cell dendrites form a communication network with surrounding cells -- other osteocytes, as well as osteoblasts and possibly osteoclasts -- that passes the signals from the osteocytes that control the action of the osteoblasts and osteoclasts. These three cell populations, and numerous other biological and biochemical factors, coordinate their activities in a continuous process throughout our lives to maintain a strong, healthy skeletal system.

Unanswered questions

A great deal of effort is currently focusing on why, in some people, bone loses density and becomes brittle. By understanding the underlying mechanisms controlling bone remodeling and adaptation, researchers may gain new insight into the cause and possible new treatments for debilitating skeletal disorders such as osteoporosis (see sidebar, page 18). One critical question is: are osteocytes in fact bone sensor cells? And if so, what do they sense? Scientists have put forth several hypotheses to find the answer. Those that have received the most attention involve:

Strain -- Osteocytes may sense strain or deformation within bone tissue.
Fluid flow -- Osteocytes may sense the flow of nutrients and other biological fluids within bone.
Electric potentials -- Osteocytes may sense the electric potential caused by deformation of bone under load.

Determining what and how osteocytes are sensing is critical to understanding not only healthy bone adaptation and remodeling, but also skeletal diseases that are characterized by alterations in the delicate balance of bone adaptation. Once this process is understood, specific, targeted treatments could be developed to treat or reverse such debilitating skeletal disorders.

Institute engineers are focusing on the relationship between the physical loading of bone and the mechanical stimulus applied to osteocytes, the sensors that control bone remodeling. The inset (shown at left, corresponding to area in white rectangle) highlights a microstructural strain field that results from a globally applied structural load on a bone specimen. As shown, peak local bone strain is several times higher than the globally measured strain. This information will help scientists understand how damage forms in bone and how this damage triggers bone to self-repair and remodel.

Mechanotransduction

Mechanotransduction involves the translation of mechanical (physical) information into a biological response. To understand this process as it relates to bone, SwRI engineers are determining the relationship between apparent skeletal loads and the resulting strains potentially sensed by osteocytes. For this, we are using a technology originally developed by SwRI to support micromechanics research funded by the Air Force and Navy to study fatigue and fracture in aerospace materials -- typically metals and composites. This technology, an automated stereoimaging system called DISMAP, allowed us to observe local deformations occurring at the sensor cell locations, called osteocyte lacunae, while applying a load to bone specimens that simulates whole bone skeletal loads.

The results were intriguing. We found that the amount the skeleton is deformed (a very small amount) during normal activities is amplified many times on the bone sensor cells. This is a significant finding -- it had been assumed that the relationship between global skeletal strain and local cellular deformation was one to one. These results may provide researchers with new information regarding the bone remodeling process.

Bone damage

We know that bone constantly repairs itself. Normal, daily activities generate wear and tear in the form of tiny cracks that form throughout bone tissue. More active people and athletes will generally cause more damage to their bone tissue than less active people. It is thought that the damage process begins at the ultrastructural level and evolves through each hierarchical level of bone organization manifesting as cracks observable by light microscopy. If this wear and tear is not repaired, the damage eventually coalesces into a crack that results in a stress fracture or complete fracture of the bone. However, it is thought that osteocytes sense the presence of bone damage and communicate to the osteoclasts to begin removing the damaged bone, initiating the repair process. Following bone removal, osteoblasts are signaled to build new bone in the former's place, completing the repair of the damaged bone. This is not necessarily a detrimental process. As a matter of fact, it is a beneficial process whereby old bone is continually replaced with new bone and areas that consistently experience higher stress may eventually become stronger by increasing bone mass in that area.

However, the hypothesis that bone damage begins at the ultrastructural level is only supported by indirect evidence. In a collaborative project with researchers from the Orthopedic Engineering Laboratory at Case Western Reserve University in Cleveland, Ohio, Institute scientists are attempting to characterize the physical nature of bone damage and its relationship to stresses experienced by the skeleton. A greater understanding of the physical nature of damage and how it relates to the microstructure and ultrastructure of bone will not only provide us with a better understanding of its connection to the bone remodeling process, but will assist engineers in developing more sophisticated analytical models describing bone strength. These models will be used to help predict how the introduction of an implant, such as a hip prosthesis, will affect the surrounding and supporting bone structure.

Finding localized zones or domains of damage at the ultrastructural level, however, is like trying to locate a postage stamp on a 24,000-acre ranch. To increase our chances of detecting damage at this level, we have combined two technologies to form a unique characterization tool: stereoimaging using DISMAP and atomic force microscopy (AFM). Stereoimaging allows us to visualize regions of high strain in a material when that material is stressed. Atomic force microscopy uses an extremely sharp probe to "feel" the surface of a material, resulting in a topographical image of the surface approaching molecular-scale resolution. We hypothesize that highly localized regions of ultrastructural damage are occurring in regions of high strain concentrations. Therefore, we use our stereoimaging technique to observe images of bone, both stressed and unstressed, to identify microstructural regions of high strain concentration. This greatly narrows the area we then search for ultrastructural damage with the AFM.

In addition to combining stereoimaging with AFM, we are also using another novel technique that may help us to identify ultrastructural damage. Cracks or fissures that are occurring in the ultrastructure may close and therefore are not easily detectable. We have found that if a load is applied to the bone specimen, fissures or cracks are visible that, in the absence of load, are not detectable. Therefore, we are using a miniaturized loading fixture that applies a slight stress to the bone specimen, allowing the cracks to open for easier detection. More importantly, this technique may enable us to physically determine what effects disease and aging have on the behavior of bone at the molecular level.

Bone is a "smart" material that adapts its form to meet physical demands. As shown in the above cutaway of a bone surface, osteocyte cells sense the physical loading environment to which the skeleton is subjected, while osteoclast cells remove bone material that is no longer needed or that has been damaged, and osteoblast cells form bone. The process is coordinated by a communication network consisting of osteocytes in contact with each other, as well as with osteoblasts and possibly osteoclasts, to maintain strong, healthy bone.

Conclusion

This research is at a basic level, and we are only beginning to understand how and why bone behaves as it does. Because there are still many unanswered questions regarding bone behavior and adaptation, SwRI is applying unique capabilities and methods to one day answer these critical questions. In pursuit of a better understanding of bone behavior and the effects of disease on bone, we have formed a team that combines specialists in experimental mechanics, materials characterization, and computational mechanics, with other experts in composite mechanics and engineering orthopedic mechanics from Case Western Reserve University, and cell biology, molecular biology, and biochemistry researchers from the University of Texas Health Science Center at San Antonio.

The benefits of this research may lead to pharmacological treatments for osteoporosis and osteoarthritis, thus allowing older people to enjoy a more active lifestyle. By better understanding bone remodeling behavior, we may also be able to assist the National Aeronautics and Space Administration in developing countermeasures that reduce or eliminate bone loss resulting from long-duration space flight.

Orthopedic manufacturers may also be able to design longer-lasting implants that take into account the long-term effects of the bone remodeling process. Finally, these advances may be applied to the emerging area of tissue engineering. In this application, man-made materials could be designed to mimic their biological counterparts so that they provide not only an attractive biological and biochemical environment, but, for structural tissues such as bone and cartilage, also provide the correct mechanical environment for viable tissue to grow.

Acknowledgments

This work is supported in part by the SwRI Advisory Committee for Research and National Institutes of Health research grant AR-43785. The author would like to acknowledge Dr. Jim Lankford for his technical guidance, Art Nicholls for his assistance in designing and performing the loading experiments, and Ramsey Railsback, Arlene Siller-Jackson, and Don Moravits for their technical assistance.

Osteoporosis' Reach

Published in the Fall 1998 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.

Technics Fall 1998 Technology Today
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