Tightening instrument for orthopaedic surgery of the spine

The tightening instrument is made up of two parts joined to each other by means which are designed to automatically uncouple the said two parts by shearing rupture of a pin when a predetermined tightening torque is reached, a first of the two parts being provided with a means permitting driving, characterized in that the second of the two parts is a rod whose end opposite its tightening end is adapted to be introduced into a complementary recess formed in the first part, and in that the said end includes an axial reservoir for receiving the broken end section of the pin.