Image-based Evaluation of Cutting Forces During Ultrasonic Cutting of Bone.

Background: Ultrasonic surgical cutting tools are an appealing alternative to traditional devices. The measurement of time-resolved ultrasonic cutting forces is important to understand the material removal process but a challenge because of inertial effects affecting the results of traditional load cells. Objective: The present work details an attempt to resolve the cutting force of an ultrasonically powered surgical tool at the time scale of the 25 kHz ultrasonic excitation, by using the tool tip itself as the load cell. Methods: Ultra-high speed imaging is used in conjunction with Digital Image Correlation to measure strain and acceleration in the cutting blade. Dynamic equilibrium is then leveraged to identify the cutting force at the tool tip within a single cutting cycle of the tool. The methodology is first verified on simulated data before being applied to experiments on cortical bovine bone (free vibrations and cutting). Results: The force detection threshold was evaluated on the order of 5 N, recorded at 1 Mfps with a Shimadzu HPV-X camera. A particular difficulty was that the blade exhibited some out-of-plane bending which created a bias on the identified cutting force. Finally, it was possible to detect that when the blade engaged in the workpiece, a compressive force acted on the tool while during the disengagement part of the cycle, the force was null. Conclusions: It was shown that it was possible to use the thin flat blade of an in-house developed ultrasonic surgical tool to measure time-resolved cutting forces.