Effect of laparoscopic grasper force transmission ratio on grasp control. 
Surgeons may cause tissue damage by incorrect laparoscopic pinch force control.  Unpredictable tissue and grasper properties may cause slips or ruptures.  This study investigated how different forms of haptic feedback influence the surgeon's ability to generate a safe laparoscopic grasp while pulling tissues of variable stiffness using graspers with different force transmission ratios.  The results will help define design requirements for training facilities and instruments.  For this study, 10 participants lifted an object barehanded, with tweezers, or with one of two laparoscopic graspers until they where able to complete five consecutive safe lifts under different tissue stiffness conditions.  The participants were presented with indirect visual feedback of pinch force, object location, and target location.  Lifting with instruments (tweezers or graspers) required 4.5 to 14.5 times as many practice trials as barehanded lifting, where no slips were recorded.  Additionally, slips occurred more often with a decreasing force transmission ratio of the graspers and with increasing tissue stiffness.  The maximal pinch force was higher in lifting with instruments than in barehanded lifting (26-60%) irrespective of the stiffness conditions.  Using a grasper, the slip margin often was not high enough in the stiffest condition, resulting in slippage of up to 84%.  Without the direct tactile feedback that occurs with normal skin-tissue contact, subjects using graspers have trouble anticipating slippage when lifting tissue with variable stiffness.  Performance drops with a decreased force transmission ratio of the instrument and increased tissue stiffness.  Furthermore, the pinch forces are not adapted to the variable stiffness conditions.  The same pinch force is applied irrespective of tissue stiffness.  It takes participants longer to learn a safe laparoscopic grasp than to learn barehanded lifts.  Additionally, to perform safe laparoscopic surgery, care should be taken when graspers with a low force transmission ratio are used.