For reasons that aren't understood, osteoarthritis occurs when something goes wrong in one of your joints. The cause may be chemical, mechanical, genetic, metabolic, or hormonal.
What happens to your joints?
Your joints are a complex part of your body. They are formed where 2 or more bones meet, and they are surrounded by muscles, tendons, and cartilage. Osteoarthritis most commonly affects the mobile joints - the ones that move quite a bit - such as those in the knee, hip, finger, or big toe. Mobile joints are also called synovial joints. Mobile joints are only one type of joint. You also have fixed joints, such as those in your skull, which don't move at all, and slightly moveable joints, such as the pelvis and spine.
To understand what happens in osteoarthritis, it helps to know about the normal joint.