骨科手术前选择合适的内固定物、人工关节及判定截骨的长度十分重要，它不仅可以提高手术的质量，而且可以减少术中临时更换、裁截的麻烦。但通常从普通Ｘ线片上测得的数据往往与实体有一定的差距，是因为Ｘ线片的放大系数随投照的部位、方向、距离等改变而改变。预置标尺... It is very important to choose proper internal fixation, artificial joint and osteotomy length before orthopedic surgery. It can not only improve the quality of operation, but also reduce the inconvenience of temporary intraoperative replacement and cutting. However, the data usually measured on ordinary X-ray films often have a certain gap with the entity because the magnification factor of the X-ray film changes with the location, direction and distance of the projection. Preset ruler