Why Your Knees Don’t Wear Out (Until They Do)


“How did you go broke?” a character asks in Hemingway’s The Sun Also Rises. “Two ways: gradually, and then suddenly.” The same can be said of the cartilage in our knees and elbows. For most of our lives, this thin layer of tissue absorbs forces of up to ten times our body weight — every step, every jump, every decade — without failing. And then, sometimes, it does.

How does cartilage do so much for so long, and why does it eventually give way? In this lecture, I will show that the answer lies in a beautiful piece of physics: cartilage is a network of microscopic fibers living right on the edge of a phase transition between soft and stiff. Small changes — a little less collagen, a little less swelling pressure — can produce enormous changes in how the tissue responds to load. The same principle that protects your joints for eighty years can, once crossed, lead to sudden failure. And the same principle, we are beginning to learn, can guide us in designing a new generation of resilient, lifelike materials.