Yes, arthritis causes joint deformity. In fact, the main symptom of rheumatoid as well as osteoarthritis is joint deformity. 

In case of rheumatoid arthritis, joint deformity of hand, wrist etc. is very common. And due to this deformity one or more fingers may lose its functional capability. Even in case of osteoarthritis, there is risk of joint deformity. In this case there is wear and tear of cartilage mainly on the weight bearing joints. And so, due to friction of bones and degeneration of cartilage, there are chances of joint deformity.  

The sad part is that, in the later stage, this joint deformity cannot be reversed using any medications or any alternative therapy. Surgery will be required to reverse this effect. But if you are aware of this condition in the start then aggressive treatment can save you from this. In order to keep you away from such a situation you must depend on conventional medical therapy along with occupational therapy. Occupational therapist can help a patient to live a healthy lifestyle and take efforts to reduce the strain on joints and prevent any further damage to the body. You must learn joint protection techniques from an occupational therapist which helps you to live your daily life with ease.