Upon the application of a member in service or of the board of police commissioners, any member who has had ten or more years of creditable service shall be retired by the board of trustees, not less than thirty and not more than ninety days next following the date of filing such application on an ordinary disability retirement allowance; provided, that the medical board after a medical examination of such member shall certify that such member is mentally or physically incapacitated for the further performance of duty, that such incapacity is likely to be permanent and that such member should be retired.