  Purpose: Robotic assistance in otologic surgery can reduce the task load of
operating surgeons during the removal of bone around the critical structures in
the lateral skull base. However, safe deployment into the anatomical
passageways necessitates the development of advanced sensing capabilities to
actively limit the interaction forces between the surgical tools and critical
anatomy.
  Methods: We introduce a surgical drill equipped with a force sensor that is
capable of measuring accurate tool-tissue interaction forces to enable force
control and feedback to surgeons. The design, calibration and validation of the
force-sensing surgical drill mounted on a cooperatively controlled surgical
robot are described in this work.
  Results: The force measurements on the tip of the surgical drill are
validated with raw-egg drilling experiments, where a force sensor mounted below
the egg serves as ground truth. The average root mean square error (RMSE) for
points and path drilling experiments are 41.7 (pm 12.2) mN and 48.3 (pm 13.7)
mN respectively.
  Conclusions: The force-sensing prototype measures forces with sub-millinewton
resolution and the results demonstrate that the calibrated force-sensing drill
generates accurate force measurements with minimal error compared to the
measured drill forces. The development of such sensing capabilities is crucial
for the safe use of robotic systems in a clinical context.
