70 has a central portion forming a cylindrical accommodation space 71 of which two sides forming lugs 72 extending therefrom to be substantially parallel to each other the lugs 72 form at least one fixing hole 721 and one limiting slot 722 and the two limiting slots 722 are substantially parallel to each other by inserting screws 73 through the fixing holes 721 the retention frame 70 is fixed to the movable plate 53 of the rail 50 a buffering device 80 is selected from a group consisting of a pneumatic device a hydraulic device and a pneumatic hydraulic device in the instant embodiment the buffering device 80 comprises a pneumatic hydraulic device the buffering device 80 is received in the accommodation space 71 of the retention frame 70 and has an end forming an active rod 81 the active rod 81 has an end forming a limiting block 82 that is slidably received in the limiting slots 722 the limiting block 82 is coupled to the pull bar 611 of the movable blade 61 to have the active rod 81 of the buffering device 80 set in a normally contracted condition and set the limiting block 82 at an innermost end of the limiting slots 722 when the movable blade 61 of the hinge 60 is doing reciprocal rotation the active rod 81 of the buffering device 80 is driven to extend contract and thus causing the limiting block 82 to reciprocally slide within the limiting slots 722 so as to control an opening range of the hinge 60 in the instant.