In Mississippi, permanent partial disability payments are allowed for upto 450 weeks (be sure you consult a mississippi work comp. attorney to confirm the laws have not changed) and are calculated as apercentage fo your wages.  Your current doctor can submit an addendum at anytime.  Your first step should be to retain a Mississippi Worker's Comp. attorney and have them deal directly with your medical documentation to insure you do not run afoul of the filing requirements, deadlines, etc.