The ability to transport to transport a force to the grasping surfaces is broken into two parts. The first is translating user action through the handle to the shaft. The second is transporting that force down the shaft and to the grasping surfaces in a useable fashion. Research has shown that in order to “1-2 N of force would be necessary to effectively grasp/spread loosely connected soft tissue� [4] This 1-2 N of force must also be below the force required (with a safety factor) for the materials chosen to yield. To apply the force to the shaft the handle needs to translate user action into a linear force along the shaft. As indicated by Medtronic two main handle designs are preferred by the surgical
Figure 3
Figure 4
public. They are referred to as pencil grip (figure 3) and scissor grip (Figure 4). The pencil grip is considered the more preferable of the two. The pencil grip is actuated by a squeeze motion, which means a force perpendicular to the shaft is applied. The challenge in making the pencil design work is translating one linear motion into a perpendicular linear motion. In the case of the scissor design the surgeon actuates the forceps by rotating the arms. The technical issue in making this design feasible is translating that rotational motion into linear motion. Both of these designs are made more complicated by the necessity to limit the force the user is allowed to apply. Due to the incredibly small size of some of the components the surgeon has the ability to damage the forceps if they apply too much force. The device that limits force needs to be able to transmit any force exactly as input by the surgeon up until the cut-off force. For any input force over the cut-off force the force limiter will need to output the cut-off force. The specific cut off force is dependent on the final chosen design itself. Failure to fracture occurs at a specific stress for each material, which is the force per area. While the minimum force as specified in the PCR doesn’t change different designs can and will have different stress concentrations that could cause failure at a small part. This complicates the design of the force limiter as very accurate testing or analysis will be needed to ensure a safe cut off force. The design choices for the transport of the forces are limited by the space required by the components used in cautery and temperature dispersion. Also important was ensuring that the force down the shaft could then still be translated back into a useable force of 2N between the grasping surfaces.
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