Development of a non-assembly 3D printed driving mechanism for a vitreous cutter

Abstract

During a vitrectomy procedure vitreous is removed from the eye with a vitreous cutter, called a vitrectome. This instrument is comprised of two needles that move to cut the vitreous into small pieces before aspirating them. Vitreous cutters are complex instruments that are light and small in size to increase their accuracy. Because of their small size, and complexity, they are assembled by trained technicians with the help of a microscope. It is believed that new technologies such as additive manufacturing (AM) might be able to reduce the manufacturing complexity associated with vitreous cutters. The goal of this study was therefore; "To create a non-assembly driving mechanism for a vitreous cutter that is designed for AM". In order to achieve this goal, the functions of a vitreous cutter were identified, as well as the mechanisms that fulfil these functions in current instruments. Furthermore, the general limitations of AM were researched and listed. Extensive research was done to identify examples in the scientific literature to find linear actuators that were made by means of AM as an integrated assembly. A diaphragm actuator was selected to drive the mechanism based on the limitations of AM, the requirements, and examples found in the scientific literature. An in depth analysis of diaphragm driven vitreous cutters revealed that it would be a challenge to seal the air chamber of the instrument. A new concept was created that used one of the strengths of AM to solve this problem, by creating a compliant interface between the moving needle and the air chamber. A proof of principle prototype was created to quickly test if it would be possible to create movement by sealing the air chamber with differently sized diaphragms. Based on the concept of sealing an air chamber with flexible diaphragms, four concept variations were created.
It was decided to make a series of prototypes of one of these concept variations to study its performance. In total, 9 prototypes were manufactured using material jetting, out of Agilus30 and Vero, without the need of post-assembly. The prototypes were tested by applying a range of air pressure pulses, while measuring the force, the displacement, the actuation time, and the spring properties of the mechanism. With the successful manufacturing of the prototypes, it has been shown that it is possible to produce the novel driving mechanism using current AM techniques without any assembly steps other than the sealing of a cleaning hole to remove support material.  The tests revealed that the spring force of the mechanism was not linear for all materials, and that there was hysteresis present in the mechanism.  Furthermore, it was shown that none of the samples were capable of operating at the desired speed of 8000 pulses per minute (PPM). The soft samples were seen to have the best response time during the backward motion of the mechanism. A new driving mechanism was created in this study that utilized one of the strengths associated with AM. To the knowledge of the author this mechanism is the first of its kind, and has not been previously applied in another instrument. Additional knowledge on the material properties of digital materials is needed to be able to design a diaphragm mechanism that has the appropriate performance characteristics. With additional design efforts and research it could be possible to produce vitreous cutters using AM in a non-assembly manner. This could lead to a future where surgical instruments are manufactured on site and on demand.