HOW THE KNEE WORKS (AND DOESN’T)

The skeletal structure has two primary types of joints—the ball joint, exemplified by the shoulder, which allows free rotation (a freedom that comes with its own set of problems and injuries, by the way); and the hinge joint, illustrated by the knee, which operates primarily in a single plane (bent to straight) with only a slight rotational or pivoting motion. This restriction of movement is what makes the knee so vulnerable to traumatic injury. Additionally, the knee is regularly subjected to the stress of both supporting body weight and absorbing shock from intermittent impacts such as jumping, walking, and running. Over time these stresses cause a loosening of the connective ligaments, the tendons, and the joint capsule that holds the knee together. Along with a wearing away of cushioning cartilage and collagen, this loosening leads to the pain and dysfunction of bone meeting bone. At its worst, this condition manifests as arthritis.