Abstract: The invention relates to a dental treatment instrument for operating a rotational tool. The treatment instrument has a head drive, which is provided for coupling to a shaft of the rotational tool and comprises a sleeve-like guide bushing (1), and a collet (9) for fixing the rotational tool. The guide bushing (1) has surface sections (3) which form parts of a cylindrical shape in at least one first axial region. Recesses (4) are provided which face radially outwards from the surface sections (3) and which impart a shape deviating from a rotationally symmetrical shape to the inner contour of the guide bushing (1). By virtue of the surface sections (3), a circular cylindrical shaft of a rotational tool can be guided or supported in a particularly suitable manner. The recesses (4) are additionally suitable for engaging radial protrusions of a non-circular cylindrical shaft of a rotational tool such that the non-circular cylindrical shaft can be driven in a particularly suitable manner.

Abstract: A bearing for rotatably mounting a rotational part, for example a turbine rotor of a dental handpiece, has a plurality of roll bodies, in particular balls, arranged between an inner bearing ring and an outer bearing ring, a cage surrounding the roll bodies in an axial direction, and a locking element for axially locking the bearing rings with respect to each other, wherein the locking element has a ring-like or ring-segment-like securing element fitted on an inner circumference of the outer bearing ring or on an outer circumference of the inner bearing ring in a region of the cage such that the locking element blocks the roll bodies in the axial direction.