A measurement system and method for determining a depth of penetration of a working portion of a surgical instrument (e.g., a rotating drill bit in a bore). A first sensor outputs a first signal representative of a displacement of the leading edge of the drill bit in the bore. A second sensor outputs a second signal representative of a force applied to the leading edge of the drill bit. A processor outputs a third signal representative of the depth of penetration of the leading edge of the drill bit when the leading edge of the drill bit passes from a first medium having a first density to a second medium having a second density. The third signal is based on the first and second signals.