36 so that the physician can ascertain the insertion depth at a glance the instrument 32 functions as a drill bit the drill bit instrument 32 has generally the same physical dimensions as the obturator instrument 28 like the obturator instrument 28 the drill bit instrument 32 is intended in use to fit for sliding and rotational movement within the interior lumen 46 of the cannula instrument 30 the distal end 36 of the drill bit instrument 32 includes machined cutting edges 54 in use the cutting edges 54 are intended to penetrate hard tissue in response to rotation and longitudinal load forces applied at the proximal end 34 of the drill bit instrument 32 the proximal end 34 presents a tapered flange 56 which is substantially identical to the flange 40 on the obturator instrument 28 like the obturator instrument 28 the tapered flange 56 changes from a larger diameter to a smaller diameter in the direction of the proximal end 34 the tapered flange 56 of the drill bit instrument 32 also includes an array of circumferentially spaced teeth 58 the form and orientation of the teeth 58 on the drill bit instrument 32 correspond to the form and orientation of the teeth 42 on the obturator instrument 28 the group includes a handle 60 the handle 60 engages the functional instruments 28 30.