Balancing forces

Abstract

Laparoscopic or minimally invasive surgery (MIS) has seen a lot of development. For the patient, laparoscopic surgery has many benefits. However, for surgeons this is not the case. For example, laparoscopic grasper design exhibit play and hysteresis during surgical tasks such as grasping and palpating. This impedes haptic feedback, resulting in the surgeon being insufficiently able to perceive the amount of tissue forces that he or she is exerting. Therefore, a surgeon might misjudge the amount of force is applied on tissue, resulting in tissue damage. A mechanical solution has been developed to mitigate these issues. The conventional grasper tip has been redesigned to a monolithic and compliant grasper. Also, a compliant balancing mechanism was designed, to cancel out undesired elastic grasper tip forces. However, this compliant balancer was not a viable solution, because it was very temperature sensitive in its functionality. In an earlier internship project by the author of this thesis, a different solution was proposed: a magnetic balancing mechanism.
Therefore, the design goal of this thesis is: to design a laparoscopic instrument with a compliant grasper, with the forces generated by the elastic deformation of the compliant grasper and other parts statically balanced by a magnetic balancing mechanism. Starting from this goal, design requirements were stated. Most importantly, the balancing forces should be between 80%-100% of all internal elastic forces for the whole motion range of the grasper tip, to ensure significant balancing. Also, the design should be ergonomic. The ultimate prototype design consists of a compliant grasper tip, balanced by an axial magnet configuration for balancing. The handle features a tweezers grip.
The novel instrument validation was twofold: first, a technical validation regarding the balancing requirement was performed. This was done by determining the balancing force, using a linear stage with a force sensor. The balancing force was between 80% and 100% for the whole range of motion of the grasper tip. This means that the balancing force requirement is met. Second, the instrument sensitivity was measured. Sensitivity is defined as the lowest force level of a loaded laparoscopic grasper where an increase can be perceived, by using only haptic feedback at the handle. This was measured during a controlled comparison (n=25) between the novel instrument, a low- and a high-quality laparoscopic grasper. It turned out that on average, the novel instrument enables the user to perceive a force difference at a lower force level compared to the low and high quality laparoscopic graspers.
Therefore, it can be concluded that the novel instrument has a higher sensitivity, and a better preservation of haptic feedback. It can be concluded that the design goal and its requirements have been met by the compliant laparoscopic grasper. However, for future prototype versions, improvements should be made to the ergonomics and modularity of the design. Also, validation should be focused more on the clinical context of the instrument.