Friction dynamics of trocars

In minimally invasive surgery, force feedback information on tissue manipulation is altered by friction between the instrument and the sealing mechanism of the trocar. It is unknown how the different sealing mechanisms of currently available trocars influence the friction forces. The current study investigated the dynamic changes in friction for various trocars at different instrument velocities.

The friction characteristics for six common types of trocars were determined. A force sensor was attached to the shaft of a standard 5-mm disposable grasper to measure the forces required to move it through the trocars. Movement velocity and direction of the shaft were controlled by a servomotor. In addition, whether moistening the shaft reduced friction was tested.

The friction depended on the type of trocar, the movement velocity, and the movement direction, and varied between 0.25 and 3.0 N. Specifically, trocars with narrow sealing caps (i.e., high normal force onto the shaft) and trocars with thick sealing caps (i.e., large contact area) generate a high amount of friction. Moistening the shaft reduced friction 15% to 45%. For most trocars, large fluctuations in forces occur when the movement starts or when the direction reverses. The magnitude of these fluctuations varied between 0.2 and 2.5 N.

For some trocars, friction can be as great as the forces associated with instrument-tissue interaction. At movement reversals, friction fluctuates due to deformations of the rubber and silicon parts of the sealing mechanism. Such high variance can deteriorate surgical performance during high precision tasks (e.g., tissue manipulation) that typically involve many changes in movement direction. Comparisons of the investigated trocars indicate that the friction magnitude and variance can be reduced easily by changing the properties of the sealing cap or by lubricating the instruments.