Activation tool (3, 3') for a bone expansion apparatus, preferably for a plate distractor, the activation tool (3, 3') comprising: - a drive member provided with a drive output, - a first shaft configured to be driven in rotation about a rotation axis via the drive output, - an activation magnetic element configured to interact, preferably coaxially, with a remote magnetic element supported by the bone expansion apparatus, - a second shaft, arranged coaxially to the first shaft, the second shaft supporting the activation magnetic element, characterised in that the activation tool (3, 3') comprises: - a coupling device interposed between the first shaft and the second shaft, which is configured to couple the first shaft and the second shaft in a first mode, referred to as the activation mode, when the first shaft and the second shaft exhibit the same rotational speed, and is configured to angularly decouple the first shaft and the second shaft in a second mode, referred to as the failure mode, when a rotational speed of the second shaft is higher than the rotational speed of the first shaft, and - means for detecting the switch from the first mode to the second mode, the coupling device comprising: - a first coupling member (25, 25') supported by the first shaft, and - a second coupling member (27, 27') supported by the second shaft, one selected from the first coupling member (25, 25') and the second coupling member (27, 27') comprising a first angular stop (29, 29') and a second angular stop (31, 31'), the first angular stop (29, 29') and the second angular stop (31, 31') being angularly offset and delimiting between them an angular housing (33, 33') extending over an angle of at least 90° about the rotation axis, the other selected from the first coupling member (25, 25') and the second coupling member (27, 27') comprising a projecting element (35, 35'), the projecting element (35, 35') being engaged in the angular housing (33, 33') such that the second shaft pivots freely about the rotation axis between a first angular position in which the projecting element (35, 35') is in contact with the first angular stop (29, 29') and a second angular position in which the projecting element (35, 35') is in contact with the second angular stop (31, 31'). - a drive member provided with a drive output, - a first shaft configured to be driven in rotation about a rotation axis via the drive output, - an activation magnetic element configured to interact, preferably coaxially, with a remote magnetic element supported by the bone expansion apparatus, - a second shaft, arranged coaxially to the first shaft, the second shaft supporting the activation magnetic element, characterised in that the activation tool (3, 3') comprises: - a coupling device interposed between the first shaft and the second shaft, which is configured to couple the first shaft and the second shaft in a first mode, referred to as the activation mode, when the first shaft and the second shaft exhibit the same rotational speed, and is configured to angularly decouple the first shaft and the second shaft in a second mode, referred to as the failure mode, when a rotational speed of the second shaft is higher than the rotational speed of the first shaft, and - means for detecting the switch from the first mode to the second mode, - a coupling device interposed between the first shaft and the second shaft, which is configured to couple the first shaft and the second shaft in a first mode, referred to as the activation mode, when the first shaft and the second shaft exhibit the same rotational speed, and is configured to angularly decouple the first shaft and the second shaft in a second mode, referred to as the failure mode, when a rotational speed of the second shaft is higher than the rotational speed of the first shaft, and - means for detecting the switch from the first mode to the second mode, the coupling device comprising: - a first coupling member (25, 25') supported by the first shaft, and - a second coupling member (27, 27') supported by the second shaft, - a first coupling member (25, 25') supported by the first shaft, and - a second coupling member (27, 27') supported by the second shaft, one selected from the first coupling member (25, 25') and the second coupling member (27, 27') comprising a first angular stop (29, 29') and a second angular stop (31, 31'), the first angular stop (29, 29') and the second angular stop (31, 31') being angularly offset and delimiting between them an angular housing (33, 33') extending over an angle of at least 90° about the rotation axis, the other selected from the first coupling member (25, 25') and the second coupling member (27, 27') comprising a projecting element (35, 35'), the projecting element (35, 35') being engaged in the angular housing (33, 33') such that the second shaft pivots freely about the rotation axis between a first angular position in which the projecting element (35, 35') is in contact with the first angular stop (29, 29') and a second angular position in which the projecting element (35, 35') is in contact with the second angular stop (31, 31').