Now that you have seen a normal hip, let's look at what happens when things go wrong in the hip.
Arthritis is the most common reason that the hip joint wears out. Arthritis results from chronic inflammation inside the joint, which wears out the protective cartilage and weakens the joint capsule and ligaments. As the protective cartilage is worn away, bone begins to rub on bone, which causes pain.
Some people have infections, injuries, or diseases such as rheumatoid arthritis or lupus that may speed up this wearing-out process. Other patients can be born with or develop an altered hip at an early age. This causes the normal wear and tear process to accelerate faster because the mechanics of the hip are changed.
Regardless of the cause, the end result is a painful hip that limits motion and the ability to perform daily activities.