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, 50 (3), 215-21

Indirect Measurement of Pinch and Pull Forces at the Shaft of Laparoscopic Graspers

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Indirect Measurement of Pinch and Pull Forces at the Shaft of Laparoscopic Graspers

John J van den Dobbelsteen et al. Med Biol Eng Comput.

Abstract

The grasping instruments used in minimally invasive surgery reduce the ability of the surgeon to feel the forces applied on the tissue, thereby complicating the handling of the tissue and increasing the risk of tissue damage. Force sensors implemented in the forceps of the instruments enable accurate measurements of applied forces, but also complicate the design of the instrument. Alternatively, indirect estimations of tissue interaction forces from measurements of the forces applied on the handle are prone to errors due to friction in the linkages. Further, the force transmission from handle to forceps exhibits large nonlinearities, so that extensive calibration procedures are needed. The kinematic analysis of the grasping mechanism and experimental results presented in this paper show that an intermediate solution, force measurements at the shaft and rod of the grasper, enables accurate measurements of the pinch and pull forces on tissue with only a limited number of calibration measurements. We further show that the force propagation from the shaft and rod to the forceps can be approximated by a linear two-dimensional function of the opening angle of the grasper and the force on the rod.

Figures

Fig. 1
Fig. 1
Schematic representation of a standard laparoscopic grasper. 1 shaft, 2 rod, 3 handle, 4 forceps
Fig. 2
Fig. 2
Schematic representation of the rhombus linkage
Fig. 3
Fig. 3
Kinematics and force propagation of a four-bar mechanism
Fig. 4
Fig. 4
Force transmission coefficients. The dotted line represents a linear fit to sinusoidal relationship between the force on the rod and the effective force on the link that drives the forceps
Fig. 5
Fig. 5
Force transmission coefficients. Coefficients as a function of opening angle of the grasper for different ratios r between the lengths of the links of the four-bar mechanism
Fig. 6
Fig. 6
Linearization errors. Linearization errors for force transmission coefficients for different ratios r between the lengths of the links of the four-bar mechanism. Circles represent values averaged across opening angles. Error bars display standard deviations in the errors
Fig. 7
Fig. 7
Schematic drawing of the setup. Shaft A was placed in a clamp and attached to the table. The rod B was attached to the force sensor C. Mass D pulled on the wires that were attached to the tips of the forceps and guided via three pulleys
Fig. 8
Fig. 8
Forces on the rod. Forces for the different opening angles and forces applied on the forceps. Error bars represent standard deviations around the mean for the three repeated measurements of each combination of opening angle and applied force. Opening angles represent the angle between the two forceps
Fig. 9
Fig. 9
Force transmission values. Transmission values for different opening angles of the grasper and different forces applied on the rod. Each circle represents a single measurement
Fig. 10
Fig. 10
Predicted forces versus applied forces. The forces on the forceps were estimated from the forces on the rod and the opening angle of the forceps using a two-dimensional linear function. The results are plotted against the actual values used in the experiment. Error bars represent standard deviations in the estimated values across opening angles

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