(1) Upon the application of a member in service or of the State, any member in service on or after July 1, 1977, who has five or more years of credited service or any contributing member who is disabled as a result of an injury arising out of and in the course of the performance of his duties regardless of length of membership on or after July 1, 1985, may be retired by the board not less than thirty days nor more than ninety days next following the date of filing the application on a disability retirement allowance if the system, after a medical examination of the member, shall certify that the member is mentally or physically incapacitated for further performance of duty, that the incapacity is likely to be permanent, and that the member should be retired. For purposes of this section, a member is considered to be in service on the date the application is filed if the member is not retired and the last day the member held office as a member of the General Assembly occurred not more than one year before the date of filing.

(2) Upon retirement for disability on or after July 1, 1977, the member shall receive a retirement allowance determined in accordance with § 9-9-60 if he has attained the age of sixty years or completed thirty-five or more years of credited service. Otherwise, he shall receive a disability retirement allowance which shall be computed as the greater of (a) or (b) as follows:

(a) An allowance equal to fifty percent of the retirement allowance which would have been payable in accordance with § 9-9-60 had he continued in service to the earlier of the age of sixty years or the completion of thirty-five years of credited service.

(b) An allowance determined in accordance with § 9-9-60 based on his credited service at the time of discontinuance of active service on account of disability.