Comparison of abdominal-wall stretching between basic and enhanced laparoscopic instruments

In laparoscopic surgery, access to the patient's abdomen is gained by using an instrument, consisting of a 300–400 mm long stem with attached tool, inserted through a cannula mounted in the patient's abdominal wall. Sliding of the stem relative to the cannula and rotation of the stem about its longitudinal axis are the only motions not constrained by the abdominal wall. These limited‐motion capabilities necessitate abdominal‐wall stretching for full‐spatial tool displacements. Abdominal‐wall stretching is potentially damaging to the patient and fatiguing to the surgeon. Minimization of stretching is shown to be possible by the addition of a single revolute joint to the basic instrument. The motions allowed by the stem and cannula, the additional joint, and the abdominal wall result in a kinematically redundant system; i.e., an infinite number of joint displacements exist to achieve a desired tool position and orientation (desired tool pose). An optimization technique is applied to determine the minimum stretching for desired tool poses. Elimination of stretching is shown to be possible by the addition of two revolute joints to the basic instrument. Displacement models for the basic and enhanced instruments are found using concepts of manipulator kinematics. Forward and inverse displacement solutions for the instruments are found. The inverse displacement solutions are used to compare the amount of stretching required by each instrument. The stretching is highest for the basic instrument. The instrument with one additional joint produces stretching that is always less than or equal to that of the basic instrument. The instrument with two additional joints eliminates the need for stretching. © 2001 John Wiley & Sons, Inc.