A surgical instrument system is described which comprises an alignment guide (6,100) comprising a body section having a through bore (50) defining a longitudinal axis (122) and a rod (40) comprising a cylindrical member (2) for insertion into the through bore and a restraining system (46, 52, 48, 94) for preventing relative movement of the alignment guide and rod along the longitudinal axis, when the cylindrical member is inserted into the through bore, and the alignment guide is located towards the proximal end of the rod. The restraining system may comprise a resilient element (46) and a recess (52) configured to engage the resilient element. In a preferred embodiment, a resilient member is disposed on the surface of the rod. Such a system allows the alignment guide to be restrained towards the proximal end of the rod so that the rod can be inserted to a patient using one hand, rather than requiring two hands (one to secure the alignment guide relative to the rod and one to insert the rod).
