I used a car as a metaphor for the human body and basically describe it in the following way.
|source: Jr 88 Rules|
RA is the rust. While there will still be a wear and tear type thing going on, there's also another element at play. Rust will infiltrate a car and eat away at the components. RA is sort of like that, the body has an autoimmune response and will eat away at the cartilage.
Ok, so this is a very rudimentary analogy, but I think it gets the basic point across. Obviously there's a lot more going on with OA and RA than my simple explanation.
Do you find this explanation useful?
How do you like to explain injuries to your clients?