Design and Testing of a Three Fingered Flexural Laparoscopic Grasper

Laparoscopic surgery requires complex manipulation and movement of internal organs. Current laparoscopic devices succeed in offering surgeons remote access to internal organs, but lack the grasping degrees of freedom achieved by the human hand. Specifically, needle nose end effectors engage organs via pinching and can cause tissue perforation. To enhance surgical capacity, a three fingered laparoscopic device was designed, fabricated and tested. Flexures are used to provide three points of articulation in each finger, while minimizing part count. Flexure joints are modeled as pseudo rigid bodies and designed for manufacture with medical grade plastics. Articulation is achieved by tendon-like control cables. To integrate with current laparoscopic procedures, the device fits through a 12mm trocar port. Furthermore, a handle was designed for this device to offer better control. Testing the device with organ-like objects revealed an increased ability to grasp, move and otherwise engage items. INTRODUCTION Dexterous manipulation of internal organs is central to laparoscopic surgery. These operations are performed through multiple small incisions in the abdomen. Usually between 3 mm and 12 mm wide, these incisions are held open with trocars, which serve as entryways into the body. Typically, one port provides video capability and lighting, while the others are occupied by surgical devices. The surgeon is able to manipulate internal organs using the laparoscopic devices while viewing the procedure on an external monitor. Because only a few small incisions are required, laparoscopic surgery is considered less invasive than open surgery. Minimizing trauma to the patient often results in faster patient recovery. Laparoscopic surgery has many advantages and offers surgeons unparalleled, remote access into a patient’s abdomen. However, current devices can sometimes limit a surgeon’s capability. Many laparoscopic tools are derivatives of the pinching grasper seen below in Figure 1. Figure 1: The pinching grasper. A sliding pin constrains the actuation of the end effector to a scissor-like pinching grip. Operating similarly to needle nose pliers, the end effector engages an organ at a single point. As a result, high stress concentrations often lead to tissue perforation. This issue is compounded by a pinch point created by the non-parallel closing mechanism of the end effector. Furthermore, the limited surface area of the grasper increases the incidence of organs slipping from the device. In a video study of ten laparoscopic colectomies and fifteen cholecystectomies, only 62% of grasping actions resulted in clamping sufficient to perform the desired action (1). Frequent failed attempts both damage tissue and waste valuable time in the operating room. Furthermore, if surgery becomes impossible due to anatomic constraint or excessive bleeding, the laparoscopic procedure must be converted to an open surgery. 1 Reprinted with permission of Harry O’Hanley, Matt Rosario, Yuanyu Chen, John Walton, Audrey Maertens, Jennifer Rosen Pinching issues and tissue perforation occur to a lesser extent in open surgery and hand assisted laparoscopic surgery. However, the large incisions required for open surgical access result in increased patient trauma. Hand assisted laparoscopic surgery is a compromise solution, using an incision large enough for a single hand. The surgeon then performs the procedure using one hand as well as a separate laparoscopic device. In both of these procedures, the surgeon can engage organs manually. Human hands are capable of grasping internal structures with dexterous motion, as well as creating geometric constraints. As such, fingers typically secure objects by curling around them, rather than pinching. Therefore, manual grasping is more efficient and secure than the pinching mechanism of current graspers. To complement the existing advantages of laparoscopic procedures, surgeons require an end effector capable of mimicking manual dexterity. This paper presents the design, fabrication, and testing of a laparoscopic device that manipulates organs by grasping instead of pinching. Fewer failed grasping attempts could lead to more efficient surgeries, replace the need for a hand port, and potentially reduce the need for conversion to open surgery. Furthermore, making the device capable of insertion through standard trocars will allow it to integrate well with current laparoscopic surgical procedures. While this device will not be appropriate for every laparoscopic procedure, the objective of this project is to enhance the laparoscopic device repertoire, always allowing the surgeon access to the most appropriate device. NOMENCLATURE b Vertical component of flexure tip displacement [in] E Young’s Modulus [psi] F Control cable force [lbf] I Area moment of inertia [in] K Flexure stiffness [lbf/in] Kθ Stiffness coefficient [unitless] L Flexure length [in] ρ Curvature coefficient[unitless] θ Final angle of flexure [degrees] θi Initial angle of flexure [degrees] Δθ Degree of actuation [degrees] DESIGN CRITERIA In cooperation with surgeons at Boston Medical Center, requirements were determined for a novel laparoscopic grasper. Drawing upon insight from medical professionals, the conditions and constraints of laparoscopic surgery were defined. The weaknesses of current laparoscopic devices were identified through both surgeon input and instrument testing in a surgical simulation laboratory. From this process, a set of functional requirements was established. 1. Pick up and move organ structures. This is the primary objective of the device. 2. Engage organs in multiple different manners. By offering surgeons more grasping options than just “pinching,” the device will widen surgical capability. Specifically, hand-like grasping will be possible. 3. Fit through a 12mm trocar port. This is a large diameter port available for laparoscopic surgery and was chosen to simplify the proof-of-concept fabrication process. Subsequent designs will be miniaturized for use with 8mm and 5mm ports. 4. Operable by one hand. Surgeons are often required to operate multiple laparoscopic instruments and can thus only dedicate one hand to the operation of a given