Disclosed is a system for treating tissue. The system comprises an introducer assembly (120), a medical instrument, and one or more probes. The introducer comprises a cannula (100), and the cannula defines a lumen. The instrument is for accessing tissue at a treatment site through the lumen of the cannula. The instrument defines one or more indicia, each indicia for defining a probe ablation zone. Suitably the instrument is a bone drill (130). Each probe (140) corresponds to a single indicia of the instrument. Each probe is operable to ablate tissue within a respective probe ablation zone. The instrument defines a marking located so as to indicate that a distal end of the instrument is aligned with a distal end of the cannula when the instrument is inserted through the cannula such that the marking is aligned with a cooperating feature of the cannula.
