Arthritic hip showing joint degeneration. From the American Academy of Orthopedic Surgeons.

Osteoarthritis is basically when your joint cartilage wears out, like brake linings on a car. Cartilage helps your joints rotate smoothly in their sockets. And, like a car, once that lining is gone, your joint’s rough surfaces are grinding directly against one another, bone on bone. This friction damages your body further, leading to pain, loss of mobility, loss of fitness, loss of bone density due to lack of exercise, and general malaise.

Unlike most of the rest of your body, cartilage can’t repair itself. No matter what any health nut might tell you, joints that degenerate, stay that way. Plus, bones develop deformities like bone spurs that can limit range of motion in your body’s attempt to stabilize the area.