PRACTICE
Viewpoints

Emerging Platforms for Better and Safer Vitreoretinal Surgery

By Peter W. Stalmans, MD, PhD, University Hospitals Leuven, Leuven, Belgium

EDITOR’S NOTE: Prof. Stalmans presented on the topic of vitrectomy fluidics during the 2014 annual meeting of the American Academy of Ophthalmology on its Retina Subspecialty Day.

In this article, Prof. Stalmans addresses the merits and drawbacks of peristaltic pump (flow control) and venturi-based (vacuum control) vitreous aspiration systems. He also discusses his 18-month clinical experience using the DORC EVA platform in real-world vitreoretinal surgery practice.

EVA offers potential for greater safety, improved surgical repair

Vitrectomy machines incorporate various methods to create a vacuum and generate controllable aspiration pressure to induce flow and move fluids for effective vitreous removal during vitreoretinal surgery.¹

In a venturi-based surgical pump system, a flow of air is used to generate a vacuum. The speed of fluid displacement is dependent on the resistance in the tubing and the viscosity of the fluid. A viscous substance such as gel-like vitreous will be displaced more slowly than a liquid-like substance such as balanced saline solution (BSS). A venturi pump is an example of a vacuum control system, where surgeons control the vacuum and the flow follows that vacuum.

An alternative method of fluid displacement is to move the fluid directly in the aspiration line, as seen in ophthalmic surgical devices that employ a peristaltic pump using a rotary wheel to displace the fluid. The speed of fluid displacement is independent of the resistance within the tubing or the fluid viscosity. As such, a peristaltic pump represents a flow control system in which the surgeon generates the flow without any requirement for vacuum build-up. The faster the fluid is displaced, the faster the vacuum is created. The vacuum generated is dependent on the resistance in the tubing and, more importantly, in the vitrectome connected to the system.

While vacuum control allows a faster flow of fluids, venturi pumps are unable to provide accurate, viscosity-independent flow control. On the other hand flow control promotes enhanced safety during peripheral vitrectomy by providing a more constant flow. Peristaltic systems however typically produce mild flow fluctuations characteristic of rotary compression of flexible tubing.

In vitrectomy surgery, flow control offers better safety than vacuum control

Current high-performance vitrectomy machines represent responsive and reliable ophthalmic surgical platforms for posterior and anterior segment procedures. The Stellaris PC and Constellation Vision System both incorporate a venturi-based mechanism for vacuum control only, while the OS3 (Oertli) features a venturi/peristaltic dual pump system. The lightweight VersaVit (Synergetics) vitrectomy system has a diaphragm aspiration pump, and provides high-speed cutting (6,000 CPM) and active duty cycle control.

More recently, a Valve Timing intelligence (VTi) aspiration system (EVA, DORC International) for vitrectomy surgery has been introduced, designed specifically to overcome the limitations of both peristaltic and venturi pressure systems.² EVA combines a series of sensitive computer-controlled operating pistons and closure valves working in very small flow chambers, and functions in vacuum control mode or a flow control setting at the touch of a button.

Compared to a venturi-driven vitrectomy system, the EVA VTi machine generates a much faster rise time in vacuum mode. Unlike a conventional rotary peristaltic system, the pressure output that is generated by EVA VTi in flow control vitrectomy does not exhibit oscillations, producing improved fluidic stability overall.

Typically, in the initial stage of a vitrectomy, a surgeon will aim for fast clearance of the central vitreous through efficient core vitrectomy, performed typically in vacuum mode for good fast attraction of the vitreous into the vitrectome.

However, observation of vitreous movement in cases involving asteroid hyalosis illustrates just how much traction may be exerted on the vitreous when operating under vacuum mode during core vitrectomy. A vacuum setting can compromise intraoperative safety when working closer to a detached retina. Indeed, variations in flow at the tip of the vitrectome due to changes in viscosity between BSS and vitreous may drag the retina into the vitreous cutter and create an iatrogenic retinal break.

In contrast, when working in close proximity to the retina under flow control mode, the flow at the vitrectome tip is kept constant, creating far less drag on the retina and reducing the risk of iatrogenic breaks or tears. With an optimal open bias at the port, the degree of resistance in the aspiration line is kept low, with almost no vacuum build-up, allowing precise and stable fluidic control at the tip of the vitrectome. This permits good access in retinal detachment surgery, where the surgeon can confidently remove the operculum of a horseshoe tear.

A large retrospective trial conducted in Europe and involving more than 7,500 cases of vitrectomy for primary retinal detachment found that the use of a venturi vacuum control pump system negatively influenced the success rate of the surgery in comparison to flow based vitrectomy.³

Newer aspiration control system for extended capabilities in range of vitreoretinal pathologies

Advanced options for addressing a range of different pathologies and conditions involving the vitreous are now made possible using EVA’s aspiration flow control system. Surgeons at the University of Leuven in Belgium have successfully performed more than 2,000 surgical cases using the EVA aspiration system, involving retinal detachments, diabetic vitrectomies, macular pucker, macular holes, floater removal, oil removal, other posterior segment surgeries, trauma-induced injuries, as well as cataract removal.

In vitrectomy surgery, flow control provides greater safety than vacuum control, especially when working in close proximity to the retina, as in vitreous base shaving, when there is a heightened risk of endangering vision. Vacuum control can be advantageous for high vitreous removal speed during core vitrectomy. Therefore being able to select either pressure control mechanism using a single machine facilitates improved vitrectomy fluidics to secure sensitive, viscosity-independent flow control.

Advanced cutter performance enables faster cut rates for more complete vitreous removal

In combination with a two-dimensional cutting (TDC) system for vitreous removal, the EVA platform allows physicians to efficiently address simple as well as difficult vitreoretinal surgery cases with consistent reliable outcomes.

The DORC TDC system, for use with smaller-gauge instrumentation (23-, 25- and 27-gauge), provides high cut speeds as a result of an opening in the vitrectome blade, performing two cuts each time the cutter opens and closes. Also, the port is not occluded even when the blade is in a closed position, which further enhances flow through the system. The TDC vitrectome removes significantly more vitreous than a standard vitreous cutter and an enlarged inner tube aperture provides an almost constant aspiration flow into the port, resulting in almost no vacuum build-up in the tubing.

Excellent results achievable

The overall effects of aspiration, distance from the retina and vitreous cut rate are important factors influencing the amount of vitreoretinal traction created during vitreous cutting.⁴ Modern bioengineering enhancements in underlying vitrectomy fluidics, together with improved vitreous cutting technology, decrease pulsatile vitreoretinal traction as well as fluid and gel turbulence, and importantly enable potentially faster vitreous surgery times. Excellent success rates have been achieved in the surgical repair of multiple vitreoretinal pathologies using the EVA platform. ■

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