Most patients who are Jehovas Witnesses can have artificial knee replacement surgery. However, since blood transfusions are not acceptable, this can present difficulties during or after the operation. Clearly, you will need to discuss these issues with your surgeon in detail when you see him. It is likely that you will be offered various options to reduce the risks as far as possible. These include medications taken for some weeks prior to surgery to help build up the 'blood stock' as much as possible and the use of techniques that salvage the blood spilt within the body cavities for reinfusion into the circulation. You are of course free to exercise your choice and specify the options that you find acceptable. Most NHS hospitals have Liasion Committees who work with Jehovas Witnesses to facilitate their treatment.
It is likely that a detailed assessment of your medical background and state of health will be carried out before your surgeon is able to give you an indication of the safety of proceeding with your operation.