An implant is provided with an upper portion in which an internal socket extends. The implant can be tightened by a turning instrument which has first lateral surfaces that can cooperate with corresponding second lateral surfaces in the internal socket. One or more of the first and/or second lateral surfaces is/are arranged completely or partially with friction-enhancing means. The implant and the tool are arranged with interacting parts which extend beyond the first and second lateral surfaces and completely or substantially take up bending moments which act in or on said portion or are directed toward said portion and occur in the event of skewing, or a tendency toward skewing, between the implant and the tool. The arrangement counteracts mechanical stresses in said portion, the latter being able to retain its original shape even in the case of implants with small dimensions.