No employee of the city who shall become disabled mentally or physically, nor the employee’s dependents, shall be eligible for a pension because of such disability unless such employee shall have been on active duty with the city for a period of at least ten years, regardless of the employee’s age, unless the retirement of such employee shall be caused by mental or physical impairment sustained by the employee in the line of duty which totally and permanently disabled the employee. The question of total and permanent disability shall be determined by the board of trustees upon the concurring report of at least two out of three physicians designated by the board to make a complete physical examination of the employee.