PROBLEM TO BE SOLVED: To provide a lag screw which can be assembled without special expertise in a short time in only one screwing direction and allows stable and easy osteosynthesis and removal without looseness and with a comparatively small force.SOLUTION: A nail and a lag screw are assembled by screwing-in. The lag screw includes a double-start screw with a high level of stillness stability and a double side edge for screwing-in or out at the tip and three or more leaf-formed recessed grooves for screwing tools to prevent crush at the end face, and is provided with a master screw mechanism which provides smooth initial engaging with a synthesized bone at the pushing-in of a collet chuck and connects to the nail and a sleeve with a lag screw, and with a structure which allows stable friction coupling of the sleeve and the collet chuck for fixing to the nail after completed engaging by a combination mechanism of a taper and slitting, and which does not loosen permanently.COPYRIGHT: (C)2014,JPO&INPIT【課題】特殊な専門技術を必要とせずに、短時間にねじ込み方向の一方向だけで組付けでき、容易に比較的小さい力で、緩みなく安定的に骨の接合と脱着ができるラグスクリュウを提供すること。【解決手段】ねじ込みによりネイルとラグスクリュウとを組付ける構造にあって、ラグスクリュウが先端に静止安定性の高い二条ねじとねじ込みと巻き戻しのための両面刃先と後端面にねじ込み工具のためのナメ防止の三つ葉以上の葉形凹面溝を備えると共に、コレットチャックの押し込み時に接合骨への滑らかな初期食付きを提供する、ネイルとスリーブに角形ねじ結合するマスタースクリュウ機構を備え、さらに食付き完了後にネイルに対してスリーブと固定用コレットチャックをテーパーとスリ割りの組合せ機構により固く摩擦締結し、恒久的に緩まない構造を備えることを特徴とする。【選択図】図6
