What is the Difference Between Stiffness and Toughness?

One thing baby boomers and their predecessors have in common is the need for joint replacements. The number of hip and knee replacements globally is expected to exceed two million per year by 2030. Some projections indicate there may not be enough surgeons to handle the rapidly growing demand. In many cases, the need for the replacement comes from overuse, injury, or osteoarthritis that led to destruction of cartilage, the flexible connective tissue in hip, shoulder, and knee joints. The common phrase uttered by those facing the joint replacement is “I am down to bone on bone”. All this may change with the development of artificial cartilage or injections which can stimulate regrowth of cartilage.

Creating artificial replacement tissue which mimics both the strength and suppleness of native cartilage is not a new idea but being able to match the features of the real thing has proven extremely challenging. I have written numerous posts about various approaches to the problem. None have panned out so far. This may be changing in the near future. Researchers at the University of British Columbia in Canada are working on an approach which may be a step toward achieving the vision to create biodegradable implants for knees and hips.

The potential solution is to implant artificial scaffolds with hydrogel and proteins which help the cartilage regenerate itself as the scaffold biodegrades. To make it successful, the scaffold has to mimic the biological properties of cartilage. The struggle in the research has been to combine the incompatible properties of stiffness and toughness. I have introduced a number of questions so far. What is a scaffold? What is hydrogel? What is the difference between stiffness and toughness? Now, I will try to answer the questions.

In medicine, a scaffold is a biomaterial which provides a temporary framework for cells to grow and organize into new tissue. Scaffolds have many uses in medicine but the focus in this post is cartilage regeneration to repair damaged or diseased cartilage. The scaffold provides a framework for new cartilage cells to grow and organize as the scaffold biodegrades.

Hydrogels are a versatile material with a wide range of potential applications. Medical researchers are continuously developing new and innovative applications. A complete definition of a hydrogel is quite technical, so I will spare that. The important thing is the properties of hydrogels which make it so attractive for medical research and patient treatments. Hydrogels can absorb a large amount of water, which makes them useful for wound dressings and other applications where it is important to keep the tissue moist but also to make an injectable solution. Hydrogels are generally biocompatible, meaning they are not toxic to cells and tissues. This makes them suitable for use in medical applications, such as tissue engineering and drug delivery. For the subject of this post, hydrogels can be made to have a variety of mechanical properties, such as stiffness, strength, elasticity, and toughness. This makes them suitable for a wide range of applications, from soft contact lenses to artificial joints.

The researchers at the University of British Columbia found rabbits implanted with the hydrogel showed notable signs of repair of articular cartilage 12 weeks after implantation. The hydrogel had been completely absorbed and there was no rejection of the implant by the animals’ immune system. The researchers observed bone tissue growth similar to existing tissue. Overall, the results were much better than expected.

The tricky part of the research was to get the right balance of stiffness and toughness. Stiffness of knee cartilage is the ability of the cartilage to resist deformation or bending. Stiff cartilage will not bend easily. Mechanically, when something is stiff, it resists being bent or deformed, but it can mean brittle. When you bend it, it breaks, like glass. If something is tough, it resists breaking when you bend it, but it might be too soft to be useful in a joint.

Toughness of knee cartilage is the ability of the cartilage to absorb energy before it breaks. A tough cartilage can withstand a lot of force without breaking, which is important for protecting the bones from injury. Knee cartilage is a soft tissue which covers the ends of the bones in the knee joint. The ingredients of the cartilage include collagen fibers and proteoglycans. Proteoglycans are a class of molecules which can provide strength and resilience to tissues. The collagen fibers provide the stiffness, while the proteoglycans provide the toughness.

In healthy knee cartilage, the stiffness and toughness are balanced. The balance can get out of whack due to age, obesity, or osteoarthritis. Osteoarthritis is a type of arthritis that occurs when the cartilage in a joint breaks down. When stiffness and toughness of knee cartilage diminishes, it can lead to a number of problems including pain, stiffness, joint instability, or cartilage damage, which can lead to more pain, stiffness, and instability. Not a pretty picture.

There are numerous treatments available for knee osteoarthritis including, exercise, physical therapy, weight loss, medication, injections, and surgery. The potential new solution is to implant artificial scaffolds made of proteins which help the cartilage regenerate itself as the scaffold biodegrades. The new research by Canadian and Chinese scientists was just published in Nature. The article outlines a method to marry these properties in a biodegradable gel.

The new research has focused on getting the right balance between stiff and tough like real human cartilage. Further animal testing is needed, and the research is still not ready for human trials. The researchers plan to fine tune the hydrogel composition and add additional biochemical cues to further promote cell regeneration. Dr. Li, leader of the research team, said, “By optimizing both biochemical and biomechanical cues together, we will see in the future whether these new scaffolds can lead to even better outcomes.” You can find more about the research in ScienceDaily. You can also read about regenerative medicine in Health Attitude: Unraveling and Solving the Complexities of Healthcare and more about my other books and articles at johnpatrick.com.