By Linda Marsa
Whoever described the human body as a marvel of engineering was clearly not talking about the knee. Design-wise, it’s deeply flawed: To function properly, more than a dozen tendons, ligaments, and bones must work in concert. Despite this complexity, the knee rotates in only two directions, forward and backward — great for when our forebears sprinted across African savannas, but not so hot when we pivot and swerve, or whatever it is you’re doing every time you ski or play basketball. What’s more, the joint wears out faster than the rest of the body, all but guaranteeing injury down the line. 
That helps explain the growing queue for the operating room. In the past 20 years ligament reconstructions have risen 40 percent; in the past four years, knee replacements have jumped threefold. And it’s not just aging baby boomers. “The fastest-growing segment for knee replacement is people under the age of 55,” says Craig Della Valle, a professor of orthopedic surgery at Rush University Medical Center in Chicago.