The external fixing frame is equipped with a bolt swivel rod with extension and shortening function, referred to as extension-shortening bolt. The rotating end of the bolt. The middle is hexagonal, and the L-shaped wrench inserted into it can rotate the extension-shortening bolt to make it lengthen or shorten. By rotating the bolt in the direction of the marked arrow, it can be lengthened; by rotating it against the direction of the arrow, it can be shortened.