Experimental Analysis of Jaw-Tissue Interaction Forces Using a Compliant Surgical Grasper

Abstract

In laparoscopic surgery, long shafted tools are inserted through small incisions in the patient to access the surgery site with the ultimate goal of reducing trauma to the patient and decreasing recovery time. Since surgeons are using these tools to manipulate tissue instead of their hands as they would in open surgery, there is a loss of tactile feedback [1]. With this loss, a surgeon may apply either excessive or insufficient forces to tissues during manipulation. In the case of insufficient force, tissues would slip out of the grasper and surgery time would increase as a result of the repeated actions. When excessive force is applied, inadvertent consequences can arise such as tissue perforations and trauma [2], putting the patient at risk.Efforts have been made to lessen this risk by either modifying or redesigning graspers. Adding a curved edge to the jaw tip and fitting the jaw with a compliant tip has been shown to reduce the peak pinch forces [3,4]. Other examples include integrating electronic hardware with the grasper in a master/slave or embedded approach [5] with the goal of regaining force feedback. The approach of the device described here is to replace the rigid jaw with a fully compliant, monolithic jaw that deforms as it grasps tissue to lessen the peak pinch force applied to the tissue.A simple grasper was fabricated for testing purposes (Fig. 1) since the jaws were the area of focus. Previous work was performed to determine a jaw profile that exhibited the desired closing behavior and distributing the pinching pressure [8]. Extra features that may be found in commercial graspers were not included, such as rotatable jaws and a ratcheting handle. Actuation of the jaws was simplified such that squeezing the trigger would laterally translate a push/pull rod through a pin-slot configuration at the top of the trigger, with the pivot position for the trigger being near the bottom of the handle.This lateral motion directly opens and closes the jaws by either pushing or pulling the inner legs of the jaw, causing the jaws to open or close respectively as seen in Figure 2. The push rod is attached to the inner legs of the jaws with the outer legs fixed to a stationary outer tube that encloses the push rod. This causes the jaws to come together when the rod is pulled backwards. The jaws have a natural spring-back behavior due to the stored energy from deflection, so the jaws open by themselves when the trigger is released (this default-open design is common in laparoscopic tools). Nitinol ribbon with a cross section of 3 × 0.02 mm was heat treated to induce the desired profile in the jaws.Two tests were performed to compare the compliant grasper and a commercially available rigid grasper (AutoSuture™ EndoClinch™ II 5 mm, Mansfield, MA) in terms of pinch and pull force.A thin-film force sensitive resistor (FSR) was used to record pinch force data during testing. The graspers and resistor were fixed spatially (Fig. 3) so the jaws could clamp the sensor in the same location within the sensor's active area to ensure repeatability. A circuit with a +6 V input was used based on the FSR manufacturer's recommendation to convert the pinch force seen by the sensor into an analog voltage signal read with an NI-DAQmx data acquisition device. Voltage data were then converted to force using a calibration curve.Tests were performed to determine the maximum pull force the graspers were capable of producing. Porcine liver samples 2×7×0.5 cm in size were clamped in the grasper on one end, and weight was incrementally added on the other until the tissue slipped out of the jaws. Thirteen and nine trials were done with the compliant and rigid graspers respectively, with new samples used for each trial.Pinch force data for the compliant grasper were recorded as a function of the linear displacement of the push rod. The trigger was squeezed to translate the rod in 1 mm increments. Figure 4 shows the relationship between the push rod displacem