If a contributor after five years of service as a city employe and before reaching superannuation retirement age is disabled while in service and is unable to continue as a city employe as shown by medical examination, he shall be paid a retirement allowance consisting of the city annuity to which he is entitled as of the date of retirement and an additional annuity equal to twenty-five per centum of his final salary which shall include the member's annuity calculated in accordance with section 13 of this act.
53 P.S. § 30568