The Biomechanics Department conducts basic and applied research aimed at applying the principles of mechanics and materials science to understand and treat orthopedic problems. The primary research areas focus on joint reconstruction and joint restoration, mechanobiology of fracture healing and bone adaptation, and joint mechanics. The complexities of the research questions require an integrated team approach among colleagues and across disciplines.
Performance of bone-implant systems
Research in bone-implant systems centers on understanding the performance of joint replacements using several approaches, including analyses of retrieved implants, in vitro laboratory experiments (such as wear simulator studies), and computational stress analyses to examine the impact of the mechanical burdens placed upon these systems when implanted in the skeleton. The Department’s Orthopaedic Implant Registry System contains more than 17,000 implants retrieved at HSS since 1977, the largest such database in the world. Laboratory experiments are conducted to establish mechanical performance measures for bone-implant systems and to provide experimental data for validation of numerical models. Computer modeling has been used to improve both the fixation and wear resistance of implant designs. These improvements have been translated through our Device Development efforts to the surgical community through licensing of innovative designs to orthopaedic device manufacturers. The successes of the improvements are then verified through long term clinical follow-up and implant retrieval analysis.
Mechanical adaptation of bone
Bone adapts to mechanical loads, increasing in mass in response to load, while decreasing in mass when load is removed, such as during prolonged bed rest or spaceflight. The goal of our research is to provide therapies aimed at keeping bones healthy. We can apply these therapies to clinical problems ranging from osteoporosis to fracture healing to orthopaedic implant fixation.
We have developed animal models to study how bone adapts to mechanical loading. Both models are focused on cancellous bone, which plays a vital role in transferring load from the joint surfaces to the shafts of long bones like the femur and tibia. Examining cancellous bone is important because cancellous sites, such as the neck of the femur and the vertebra of the spine, are most prone to fractures in osteoporosis and are important in the fixation of joint replacements for treating arthritis. Our goal is to understand the characteristics of the load (for example, the number of cycles of load or the magnitude of the load) that are most effective for maintaining or improving bone mass. We are using our models to explore adaptation in the presence of osteoporosis as well as exploring synergistic effects of load and an anabolic agent such as parathyroid hormone. We are also exploring the role of mechanical load on fracture healing.
Biomaterials for joint restoration
A major commitment in parallel with our studies on the performance of bone-implant systems has been the understanding and improvement of the wear properties of the materials used to form the bearing surfaces of total joint replacements. Retrieval analyses, wear simulation experiments, and computer modeling have all been used, for example, in understanding the role of material properties in wear performance of ultra high molecular weight polyethylene, as well as metallic and ceramic bearing materials.
The clinical goal for our research in joint restoration is to provide unique biomaterials to replace articular cartilage and meniscus, the important soft tissues that are often the first to be damaged in trauma and at the onset of osteoarthritis in the knee. We are developing both synthetic solutions, using newly developed materials as permanent replacements for damaged tissue, and tissue-engineered solutions, using material scaffolds as carriers for both cells and vital biologic factors necessary for the creation of new replacement tissue. We have developed a hydrogel scaffold intended to act as a permanent implant to stabilize cartilage defects and to prevent the spread of damage to the remainder of the knee joint. This porous, partly-degradable scaffold has mechanical properties similar to those of articular cartilage and has the ability to exude water from its surface similar to cartilage. These unique properties allow the implant to immediately and continuously bear load while maintaining adequate wear properties.
We have continued our multifaceted approach in developing meniscal replacements. We are applying a unique image-based design process to transfer magnetic resonance images into custom-manufactured synthetic replacements for permanent meniscal re-placement or degradable, cell-seeded implants for tissue engineering a regenerated meniscus. Subsequently, we have mixed the implants in culture media to examine cell production of matrix. We recently studied how mixing conditions affect matrix production and the mechanical properties of the resulting matrix. Given the protective role of the meniscus, mechanical properties are very important in optimizing conditions for tissue engineering.
A key component of our efforts at research and development in joint reconstruction and restoration is an understanding of kinematics and loads. This same information is vital to collaborative research with a number of the Orthopaedic Services (Sports Medicine and Shoulder, Trauma, Hand and Upper Extremity, Foot and Ankle, Spine), for which the mechanical burden placed upon the joints can help explain the implications of trauma and disease and help guide the development and the eventual clinical assessment of appropriate treatments. Our research in joint mechanics seeks to combine laboratory experiments, motion analysis studies performed in collaboration with the Leon Root, M.D. Motion Analysis Laboratory, and computer simulations. Current research projects are centered on the elbow, the shoulder, and the knee. For example, in the knee, we are employing an in vitro robotic testing system to determine knee joint kinematics in the presence of ligament injury and reconstruction and in the presence of total knee replacement. The experimental data from the robot can then be used as inputs in calculating forces acting in knee ligaments, for example, after ligament reconstruction or after knee replacement.
Hospital for Special Surgery
535 East 70th Street
Dana Center, Room 202
New York, NY 10021