I was diagnosed with osteoarthritis in my hands/wrist. could 20 years of hard mountain biking caused this to occur? riding on bumpy trails.

Possibly: Causes of OA are varied and relate to your genetics, age, and how hard on your hands and wrists you were. Remember that the jack hammer driver has no more arthritis than a postman, so we will never know fr certain. More important a is how you can live with the situation and continue to ride, or do,what you choose. An orthopoedist or PMR doc can eval,this to determine if you need braces or treatmnt