Lawrence J. Bonassar Honored for Study of Articular Cartilage Micromechanics

Nearly two decades of research on the microscale mechanics and composition of articular cartilage and their relevance to musculoskeletal disease has helped Lawrence J. Bonassar, Ph.D., snag the 2022 Kappa Delta Anne Doner Vaughn Award.

The Kappa Delta Awards recognize research in musculoskeletal disease or injury with great potential to advance patient care. These discoveries by Dr. Bonassar and his colleagues will not only aid in disease prevention and identifying therapeutic windows for treatment, but will play a crucial role in determining the key components and structures in diseased tissues to be targeted for tissue preservation, repair, or regeneration.

“A striking feature of connective tissues, such as articular cartilage, is their heterogeneity of composition and structure at multiple length scales, which is a concept used in physics to define length or distance determined with the precision of one order of magnitude,” said Dr. Bonassar, the Daljit S. and Elaine Sarkaria professor at the Meinig School of Biomedical Engineering and Sibley School of Mechanical and Aerospace Engineering at Cornell University in New York. “Given the importance of this region, remarkably little is known about the unique mechanical function and biological role in cartilage health and disease. Our research started with a very basic understanding of how cartilage behaves.”
 
For many types of arthritis, such as osteoarthritis (OA), damage begins at the articular cartilage, a very thin surface which covers the ends of bones. This process occurs when inflammatory mediators induce the release of enzymes, resulting in degradation of the extracellular collagen and aggrecan networks,i two of the most important constituents responsible for the mechanical properties of cartilage. Aggrecan is the major proteoglycan in articular cartilage, providing the hydrated gel structure that allows the cartilage to bear loads and dissipate energy. 
 
The Critical Role of the Articular Surface
More than 15 years ago, Dr. Bonassar partnered with Itai Cohen, Ph.D., professor at Cornell University’s Department of Physics, who studies the behaviors of soft materials, including cartilage. He had already developed microrheology techniques to examine the micromechanics of soft tissues. Microrheology is the study of mechanics (e.g., microviscosity) of complex materials at small length scales.
 
Drs. Bonassar and Cohen created a testing device that was small enough to fit on a microscope and could capture images at 10-100 milliseconds to observe how the tissue deforms on the length scale/diameter of a human hair. They discovered the top 100μ (100 microns; a metric unit of measurement where one micron is equivalent to one one-thousandth of a millimeter) of articular cartilage has extremely different mechanical behavior than the rest of the tissue. In fact, it was 10-100 times more compliant (e.g., less stiff or more likely to be deformed).
 
“We learned that 90 percent of the energy dissipation occurred in the top 100μ, so a very small region was doing almost all the work protecting the rest of the tissue,” said Dr. Bonassar. “Having established the important mechanical role of the most superficial region of articular cartilage, we sought to understand how the composition of the tissue (water, collagen, proteoglycan) and mechanics are connected.”
 
To do this, they coupled this state-of-the-art mechanical analysis with compositional analysis using Fourier Transform Infrared (FTIR) microscopy and Raman microspectroscopy to understand how the mechanics and composition of the tissue connect. They discovered that the surface region, which acts as a shock absorber, contains a low concentration of collagen and proteoglycans. They furthered this analysis by altering the composition of the cartilage with an enzyme used to selectively remove proteoglycans from the extracellular matrix (ECM), leaving the collagen network largely intact. ECM are structural support cells that regulate cellular growth.
 
“When you remove proteoglycans from the tissue, you begin a phase transition from a mechanically stiff to a mechanically floppy network,” said Dr. Bonassar. “This local subtle damage to the articular surface is what we believe represents the initial stages of damage in arthritis that starts to cascade into the progression of the disease. Essentially, when the tissue is healthy, it’s stiff and robust enough to carry load, but the top 100μ layer of the articular cartilage is close to a tipping point. With just a little damage, it can cause a huge decrease in its ability to carry load, and once that surface damage happens and function is lost, it’s hard to get back.”
 
Function of Cartilage Implants
The next phase of research was to study implants that are used to replace cartilage utilizing the same techniques to understand how the composition impacts the mechanics of these products. Their work provided a benchmark to understand whether these replacement tissues resembled the function of the native cartilage.
 
Cartilage implants are developed when cells are seeded on a scaffold or a sponge, allowing the cells to grow within the matrix of the scaffold. The team analyzed two cartilage replacement products using the new mechanical evaluation tool. It revealed some behavioral aspects that had never been seen before.
 
“One of the real challenges for companies that make these products is knowing how long to culture them and how much ECM needs to be deposited before the implants can function mechanically,” said Dr. Bonassar. “The maturation threshold is dependent on the type of scaffold and the patient’s cells. However, we were able to provide a clear road map for these companies to define how much matrix needs to be made by the cells. In some cases, once the scaffold is 20 percent full, it’s stiff enough to make the construct fully functional.” 
 
Cell Behavior in Damaged Tissue
Their research continued to focus on the biologic implications or cellular responses of tissue with mechanical injury. Drs. Bonassar and Cohen partnered with cell biologists and veterinarians, Lisa A. Fortier, DVM, Ph.D., James Law professor of surgery, and Michelle L. Delco, DVM, Ph.D., assistant research professor, department of Clinical Sciences, who both work at Cornell University’s College of Veterinary Medicine. This partnership allowed the team to integrate a clinical perspective, as many of drivers of arthritis are similar in humans and animals.
 
They were interested in using the tools previously developed not only to understand pure mechanics, but to determine how the mechanics drove cell behavior, and how the local mechanical environment instructed the chondrocytes (the cells that populate cartilage) to help or harm the tissue’s health.
 
Injuries, such as tears to the meniscus or anterior cruciate ligament (ACL), or a severe ankle sprain can greatly increase a person’s chance of developing arthritis in those joints, in part due to cellular damage that occurs from the impact injury.
 
“We built a device that fits on a fast-imaging microscope that allowed us to deliver controlled amounts of energy to pieces of cartilage––the same impact a person might experience in an ACL or meniscus tear or a car accident,” said Dr. Bonassar. “By capturing images at milliseconds, we observed in real time how the tissue deforms and what happens to the cells in the regions that experienced different amounts of deformation. We discovered that the damage to the cells is directly related to how much strain the tissue experiences and is concentrated in the area of impact. For example, in a matter of minutes following an ACL tear, the chondrocytes, particularly in that top 100μ, are damaged in a very specific way, in that their mitochondria are less efficient at doing their job.”
 
Drs. Fortier and Delco were interested in therapies that target mitochondria to help prevent damage. The team demonstrated that delivering a small peptide to the cartilage stabilized the mitochondria and prevented damage to the cells and tissue. The team is currently looking at these peptides as a potential therapeutic for post-traumatic OA.
 
Key Insights for Other Soft Tissues
The team also applied the approaches they developed for understanding the microscale mechanics, composition and mechanobiology of articular cartilage to answer important questions about the function of other cartilaginous and soft tissues. Key findings include:

• The mechanics of the growth plate arise from the columnar arrangements of cells. Tissue deformations are concentrated between these cellular columns, and these regions of the tissue are the most susceptible to damage. 
• Articular cartilage from the temporomandibular joint is different from all other joints in the body. It contains a layer of fibrous tissue at the articular surface that is highly organized and susceptible to shear loading.  
• The attachment of the meniscus to the tibia has a complex organization designed to create a smooth profile in deformation from the stiff bone to the compliant meniscus.
• Collectively, these studies have had major impact on the cartilage and soft tissue biomechanics communities. 

“Receiving the Kappa Delta Award is incredibly humbling for me and it’s very meaningful for our team to be recognized,” said Dr. Bonassar. “Our interdisciplinary team consisting of an engineer, a physicist and two veterinarians allowed us to look at problems differently from the rest of the field to drive innovation in the clinical setting.”
 
The Kappa Delta Sorority established the Kappa Delta Research Fellowship in Orthopaedics in 1947. The first annual award, a single stipend of $1,000, was made available to the Academy in 1949 and presented at the AAOS meeting in 1950. The Kappa Delta Awards have been presented by the Academy to persons who have performed research in orthopedic surgery that is of high significance and impact. The sorority has since added two more awards and increased the award amounts to $20,000 each. Two awards are named for the sorority national past presidents who were instrumental in the creation of the awards: Elizabeth Winston Lanier, and Ann Doner Vaughn. The third is known as the Young Investigator Award.
 
With more than 39,000 members, the American Academy of Orthopaedic Surgeons is the world’s largest medical association of musculoskeletal specialists. The AAOS provides the highest quality, most comprehensive education to help orthopedic surgeons and allied health professionals at every career level best treat patients in their daily practices. The AAOS is the source for information on bone and joint conditions, treatments, and related musculoskeletal health care issues and it leads the health care discussion on advancing quality.