“The Ultrasound in vitrectomy: an alternative approach to traditional vitrectomy techniques”

The goal of all vitreous surgery is to perform the desired intraoperative intervention with minimum collateral damage in the most efficient way possible. The primary purpose of the vitreous cutter is to remove vitreous gel from the eye with minimal retinal traction. The consequences of uncontrolled vitreoretinal traction and inflow/outflow mismatch can be very severe. The advancement of vitrectomy surgery instrumentation has been motivated by smaller tools and greater practicability since the introduction of pars plana vitrectomy approximately 40 years ago. During the past 2 decades, engineering development has focused on evaluating different gauge instrumentation in an attempt to make the vitrectomy surgery less invasive and faster, lessening patient discomfort and postoperative inflammation. Our study has consisted in a preliminary study in order to develop an instrument capable of performing vitrectomy using ultrasound. In the first phase of the study (ex-vivo testing) our aim was to study the behavior of the flow obtainable using the instrument considering some modifiable variables (testing vitrector tips with new geometries, new shapes and new holes). This first phase was essential for obtaining the useful work window for the following developments. Furthermore, the vitreous is a non-newtonian fluid with uneven viscosity and fibril structure, and its removal through small-gauge probes is technically challenging and potentially dangerous because of the presence of unpredictable, localized retinal adhesion. An accurate comprehension of vitreous perturbation in response to suction and ultrasonic exposure was essential for the improvement of vitrectomy probes. In the second phase of the study (invivo testing) we performed the experimental in-vivo vitrectomies using the hypersonic vitrector prototype in order to evaluate the macroscopic and ultrastructural effects of the ultrasound on the vitreous and retina. In the first phase of our study we evaluated vitreous flow rates during experimental vitrectomies using an ultrasoundbased prototype vitreous cutter. Thanks to an experimental setup, developed by engineers of the University of Pisa, we measured vitreous flow rates in different experimental conditions. In particular we tested different tips (Conic hole, Cylindric hole, Eccentric Cylindric hole, End Milled hole). The experimental vitrectomies were performed on vitreous obtained from porcine cadaveric eyes (manually dissected). After our flow rate analysis, we observed that it was possible to liquefy vitreous using ultrasound. Furthermore, we can assume that, in general, vitreous flow rates increased with increasing aspiration levels and with increasing % of US power. In particular, using 20% US power we registered probe occlusions or, when it was possible to register a flow, very low levels of flow rates. The greatest increase in flow rates was registered between 20% and 40/50% of US power. Enhancing US power over 40/50% we noticed a minor increase, or in some cases even a reduction, of flow rates. This phenomenon is probably due to the cavitation phenomenon (which becomes significant at high % of US) and to the physical limits of the small (0,2 mm) hole of the instrument, which cannot carry out flows above a certain limit. These general considerations are valid for all the tips we tested, in particular, the most regular behaviour was obtained using the Conic hole tip. The advantages of an ultrasound-based vitrector may be the following: small size of the probe and the hole; aspiration hole always open (continuous flow, less retinal traction); higher flow rates with lower aspiration levels. In the second phase of the study we have evaluated the in-vivo efficacy and safety of an ultrasound vitrector prototype comparing the fragmentation capacity with a traditional guillotine vitrector and observing the microscopic effect on the retinal surface: we did not observe macroscopic harmful effects during US vitrectomy and the ultrastructural evaluation of the vitreous allowed us to observe an excellent fragmentation using the ultrasound vitrector; more specifically the longitudinal modality resulted more efficient than the torsional: the fibril fragments obtained using a standard guillotine vitrectomy and 50% US power in longitudinal mode were shorter in the examined vitreous (ROI), compared to the other experimental sets. Regarding the microscopic findings, no differences in the retina after the use of both types of vitrectors (guillotine and ultrasound vitrector) and no clear/direct correlation between US vitrector use and retinal damage were detected. Vitreoretinal diseases include a multitude of disorders of varying gravity. Most often the only therapeutic approach is a surgical approach which is the vitrectomy. Recently, vitrectomy has seen remarkable technological advances that have revolutionized the management of vitreo-retinal diseases. However, some limits cannot be overcome with the current technique: the presence of the guillotine and the cutting mechanism. Ultrasound-based vitrectomy represents a possible alternative: a tool capable of liquefying and excising the vitreous body using ultrasound could encompass all limits of the guillotine-based technique. Furthermore, initial studies have shown promising results with no macroscopic and microscopic retinal defects. This technique has enormous potential that, thanks to future developments, could be used to treat ocular pathologies