Output force and force ratio of laparoscopic graspers: an evaluation of ergonomics

“Surgeon's thumb,” or thenar paresthesia, can result from prolonged or excessive grip force during laparoscopy. This is particularly relevant in gynecology, where procedures involving enlarged uteri or fibroids are common. Though this method of injury is well known, there is a paucity of data to guide surgeons in selecting more efficient, ergonomic instruments. This study compares the ratio of applied tissue force and required surgeon input in a sample of common laparoscopic graspers, with the goal of providing quantitative data applicable to surgical ergonomics and surgeon instrument choice. Laparoscopic graspers with varied ratcheting mechanisms and tip shapes were evaluated. Brands included Snowden-Pencer, Covidien, Aesculap and Ethicon. A kocher was used as an open instrument comparison. Flexiforce A401 thin-film force sensors were used to measure applied forces. Data was collected and calibrated using an Arduino Uno microcontroller board with Arduino and Matlab software. Single handed, complete closure of each device’s ratcheting mechanism was performed three times. Maximum required force in Newtons (N) was recorded and averaged. The average applied force was measured with a bare sensor and the same sensor between two different thicknesses of LifeLike BioTissue. The most ergonomic grasper was identified by the highest ratio of output force compared to required surgeon input. The kocher required an average input force of 33.66N, with its highest output ratio of 3.46 (112N output). The Covidien EndoGrasp was the most ergonomic, with an output ratio of 0.96 on the bare force sensor (31.4N output) and an average input force of 37.9N. The Snowden-Pencer wavy grasper was the least ergonomic, with an output ratio of 0.07 when applied to the bare force sensor (5.9N output) and the highest average input force of 85.9N. All graspers except for the EndoGrasp had improving output ratios as tissue thickness and subsequent grasper contact area increased. Input force in excess of that provided by the ratcheting mechanisms did not increase output force in a clinically significant amount for any of the instruments evaluated. Laparoscopic graspers vary widely in their ability to provide reliable tissue force without requiring excessive input by the surgeon, and a point of diminishing returns exists when surgeon input exceeds that of the capability of ratcheting mechanisms. Output force and force ratio can be used as quantitative measures of the efficiency of laparoscopic instruments. Providing users with this type of data could allow for easier identification of the most ergonomic instrument for the desired application.