Background and purpose: At present, robotic arm port placement for daVinci trade mark robot-assisted laparoscopic radical prostatectomy is based on the umbilicus. However, the robotic arm has a maximum manufactured required working distance of 25 cm. Accordingly, normal variability of patient height, weight, and umbilical location can leave the working arms too short to reach the membranous urethra. We present data to support port placement using the pubis, rather than the umbilicus, as the landmark.
Materials and methods: If we assume the 25-cm working distance of the robot arm (Z) equals the hypotenuse of a triangle and the Y axis is the sum of the distance from the membranous urethra to the skin (Y1) plus the displacement of the skin secondary to CO(2) insufflation (Y2), then the horizontal distance X is from the robot port site to the pubis. To ascertain Y1, we randomly selected the CT scans of 25 men and measured the depth from the skin over the pubis to the membranous urethra. To determine Y2, we measured the change in height from the table of the port site after CO(2) insufflation in 11 robotic laparoscopic prostatectomies.
Results: The average distance of Y1 was 11 cm; Y2 was 6 cm. Using the formula (Z(2) - (Y1 + Y2)(2))(1/2), the maximum distance X from the port site to the pubis, for an average man, should not exceed 18 cm.
Conclusion: The optimal landmark for calculating the placement of ports for the daVinci robotic arm placement should be the pubis and not the umbilicus. Tall men (>72 inches) are at risk for exceeding functional robot arm length, and in these men, port sites should not be more than 18 cm from the pubis.