A system for drilling a bore in a bone and measuring a depth of the bore during surgery, said system comprising: a drill comprising a drill chuck for holding a drill bit； a drill bit mounted in the drill chuck； at least one sensor arranged to produce signals for determining the depth of the bore, preferably comprising a strain gauge arranged to produce a signal representing a force exerted on the system during drilling, and a distance sensor arranged to produce a signal representing a distance between a reference point on the system and the bone surface during drilling； first communication means arranged to communicate said signals to electronic processing means； electronic processing means arranged to receive said signals from said first communication means and to determine a depth of a bore from said signals； second communication means arranged to communicate said determined depth to output means； output means for outputting information about the determined depth of the bore received from said second communication means； Wherein said drill bit is part of a rotatable drill bit unit, Which may or may not comprise said drill chuck； and said at least one sensor and one of said first and second communication means are mounted in the drill bit unit.
