Not deep enough: Modeling the effect of shallow placement of the DaVinci Xi "bariatric" long trocar on the muscular abdominal wall.

Abstract

The DaVinci Xi Robotic Surgical System (Xi) long cannula (Intuitive Surgical Company, Sunnyvale, CA) provides five additional centimeters of distal length compared to the standard Xi trocar. The extra length allows the cannula to traverse prohibitively thick body wall tissue. Our aims are to quantitatively model the consequences of not preserving the rotational centerpoint of motion (RCM) at the muscular abdominal wall. This is an essential tenet in robotic surgery; it is violated with shallow placement of the long trocar. This leads to unchecked, unnoticed blunt widening of port sites by the robotic arm, increasing hernia risk. We begin with an exploration of the schematic of the Xi robotic arm as patented by Intuitive (U.S. Patent #5931832). We trigonometrically model the lateral displacement of the abdominal wall at the trocar site with respect to vertical trocar shallowness, instrument tip depth, and instrument tip lateral motion from neutral midline. The rigid parallelogram movement structure of the Xi preserves the RCM at the thick black marker printed on every Xi cannula. By limitation of design, both long and standard trocars must have this marker at the exact same distance from their proximal end. The value ranges of our model parameters (presuming a reasonable maximum orientation angle of 45° from midline) are: trocar shallowness [1cm, 7cm]; instrument tip depth [0cm, 20cm]; instrument tip lateral movement [0.0cm, 14.1cm]. Abdominal wall displacement increased proportionally as each instrument tip parameter reached its maximum deviation from the orthogonal midline as described in the plot figure. Maximal wall displacement at maximal shallowness was approximately 7.0cm. Robotic surgery revolutionizes modern operation, particularly within bariatrics. However, the current Xi arm design disallows a true long trocar to be used safely without compromising the RCM, thereby risking hernia development.