Hip dysplasia is an inherited trait that over time leads to osteoarthritis of the hip. Maine Coon Cats are affected most often. There is also frequently associated with patellar (kneecap) luxation is frequently associated with hip dysplasia. Below are links that provide up to date information about these conditions.
Feline Hip
Dysplasia Awareness Web Site
Hip Dysplasia - Cat Fancy Magazine
Hip Dysplasia and Patellar Luxation
Feline Hip Dysplasia - Where are we now?
Orthopedic Foundation For Animals