A system for lockably engaging bone screws into a fixation plate may include a locking fixation plate including a threaded screw hole defined by an inner surface surrounding the threaded screw hole, the inner surface comprising an upper countersink and a threaded portion, the threaded portion comprising a pair of threads arranged in a double helix configuration, and a screw comprising a generally conical head tapering into a generally cylindrical shaft that ends at a tip and a single thread beginning near the tip and extending along the generally cylindrical shaft and onto the generally conical head, a thread height of the single thread being constant over a majority of the generally cylindrical shaft and tapering as the thread extends onto the head.骨ネジを固定プレートに係止可能に係合するシステム（１００）は、ネジ山付きネジ孔（３０２）を含む係止固定プレート（３００）を含み、ネジ山付きネジ孔は、ネジ山付きネジ孔を取り囲む内表面によって定められ、内表面は、上方皿穴（３１０ａ）と、ネジ山付き部分（３０７）とを含み、ネジ山付き部分（３０７）は、二重螺旋構造に配置される一対のネジ山を含む。システムは、更に、先端部（２０６）で終端する概ね円筒形のシャフト（２０４）に先細る概ね円錐形の頭部（２０２）と、先端部付近で開始し、概ね円筒形のシャフトに沿って、概ね円錐形の頭部に向かって延びる単一のネジ山（２０８）を含み、単一のネジ山のネジ山の高さは、概ね円筒形のシャフトの大部分に亘って一定であり、ネジ山が頭部に向かって延びるに応じて先細る。
