This is a very dangerous area of employment law and I am afraid the best advice I can give you is to talk to an attorney with experience advising employers about discrimination and termination issues.  There is a MA statute that defines whether a person is handicapped for the purposes of determining whether that person is entitled to "reasonable accommodation" by an employer.  (If an employee is handicapped, both Massachusetts and federal law requires the employer to work with the employee to find a reasonable accommodation.)  Massachusetts General Laws Chapter 151B, Section 1(17) states in relevant part: "The term “handicap” means (a) a physical or mental impairment which substantially limits one or more major life activities of a person; (b) a record of having such impairment; or (c) being regarded as having such impairment . . ."
 
But is a person with arthritis handicapped and, if so, what would constitute a reasonable accommodation?  The latter question is constantly litigated because the answer depends on the unique facts of each case.  An attorney with relevant federal and MA employment law experience, however, can review your specific situation and explain how the courts have answered the question in similar cases.  That information will help you fashion a solution that will minimize your legal exposure.  That is a good thing, because damages for employment discrimination can be devastatingly large.  Good luck.