arthritis meaning

What does it mean when a horse's joints click?
My friend has a 5 year old that needs joint injections. She has been doing lots of jumps with it and your joints click. I thought it meant the arthritis, but she says that the horse just click, but the veterinarian still want to give injections. So could someone explain this to me?
Joints make noise for a number of reasons, and arthritis is only one of them. Here's a concise explanation of the joints produce noise. Joints are joints. Human or horse, the tissues are the same, and causes of noise are the same. http://www.loc.gov/rr/scitech/mysteries/joint.html
