An electrical individual wheel drive has one electric motor the output shaft of which is connected with an inner central gear of a transmission to form one piece. The application of the sensor on the patient may start a timer set to run for a given time interval. The constraint device also modulates loads borne by the spinal segment undergoing fusion or tissue adjacent thereto. Furthermore, it is easy to maintain the cleaning element in position abutting against the connecting end face when the cleaning element is moved for wiping the connecting end face.