Articular cartilage injury
Credit: J. M. Horrillo / Getty Images

This frictionless cartilage at the ends of your bones is extremely hard to replicate. "Once you injure that we do not have a very good way to put you back the way you were before," says Creighton. An imperfect repair can lead to arthritis, recurrent swelling, and catching-and-locking-type symptoms.

The Fix: Keeping off extra pounds and working out daily is the best – and only reliable – way to lessen your chances of articular cartilage problems. Also, beware of supposed protective supplements like chondroitin, since they often don't work. "My opinion is the marketing has outpaced the science," says McAdams.