What causes arthritis?

In order to understand what causes arthritis we have to understand what a joint is. A joint is an area where two bones come together to articulate. This makes motion possible. Without joints we would not be able to move. Below is a picture of a joint. It happens to be a knee joint, but that does not matter. It could just as easily be a finger joint or whatever (see figure 1).

The upper bone is called the femur (thigh bone). The lower bone is called the tibia. The two bones do not touch each other because there is something that lines the ends of the two bones called cartilage. Cartilage is an amazing substance. It reduces the friction between two bones to less than that of and ice skate on ice. Imagine that. Wow. That is why you can bend your (nonarthritic) knee effortlessly. Notice figure 2 is an arthritic knee. It demonstrates loss of cartilage, bone spurs and bone on bone contact due to severe arthritis.