Retina & Vitreous Surgery: A Practical Guide to Theory, Practice and Management Subhendu Kumar Boral, Suber S Huang
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Vitrectomy Machine, Cutter and InstrumentationCHAPTER 1

The basic equipment required for pars plana vitrectomy includes a vitrectomy machine, a visualization system, an illumination source and a microscope.
 
A. VITRECTOMY MACHINE, CUTTER AND INSTRUMENTATION
Since the introduction of pars plana vitrectomy by Machemer in 1971, significant revolutionary changes happened to improve the approaches, machineries and instrumentation. With the advent of closed chamber vitrectomy, vitreoretinal surgeons were able to remove media opacities and treat even complicated retinal detachments.
 
BASIC EQUIPMENT FOR PARS PLANA VITRECTOMY
 
Vitrectomy Machine
The primary aim of vitreous surgery is to remove vitreous without resulting in undue vitreous and collateral retinal damage. This balance is achieved by matching inflow and outflow system in a vitrectomy machine.
A. Inflow system or infusion system: This regulates the entry of fluid into the vitreous cavity. Each machine (Fig. 1.1A) is equipped with an infusion system that allows control of IOP during surgery. Infusion system can use:
  1. Gas-forced infusion (pressurized system): A bottle of balanced salt solution (BSS) is attached to an air pump (set at a pressure of 20–30 mm Hg during normal surgery) that forces the BSS into the eye through infusion tubing (Fig. 1.1B).
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    Fig. 1.1A: Shows the high-end vitrectomy machine (Constellation vision system, Alcon) with a bottle of balanced salt solution (BSS) is attached to an air pump
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    Fig. 1.1B: Modern vitrectomy panel
    Vented gas forced infusion (VGFI) is a pressurized system of controlling the infusion pressure used in Alcon Accurus machine. Here filtered pressurized gas is delivered into the infusion bottle through tubing. Rapid change in 2pressure can be achieved by this. This is particularly helpful to control intraocular bleed by raising the IOP without any help of the assistant.
    Alcon constellation system offers further modification into this system. Here infusion solution is pressurized within the cassette. Constellation system has inbuilt real-time IOP monitoring which track infusion cannula and tubing resistance. Moreover, presence of dual infusion chamber (where if one bottle empties, the other takes over) minimizes IOP fluctuation (Fig. 1.1B).
  2. Gravity dependent infusion: Depends on the height of the bottle above the patient's eye (usually 18 inch above the patient's eye level to maintain a normal IOP) to maintain IOP. It is assistant dependent and does not provide precise IOP control.
B. Outflow system: The vitrectomy machine pumps are the main factor to control egress of fluid from the vitreous cavity. Two types of pumps are available:
  1. Venturi pumps: Here flow is induced by the vacuum changes within the cassette. Hence, here flow occurs as a result of vacuum and it is directly proportional to the vacuum applied for homogeneous low viscosity substances. Venturi system is controlled by surgeon's foot pedal.
  2. Peristaltic pumps: They are positive displacement machines. The fluid contained within a tube is forced to move into the vitreous cavity by rollers that compress it. Here, the vacuum rises only after occlusion. So flow rate can be maintained vey low with peristaltic pumps and can still achieve high vacuum at occlusion.
Vitrectomy machines having both pumps: Alcon constellation, Oertli OS3, DORC Associate and Geuder Megatron S4.
Vitrectomy machines having only venturi pump: Alcon Accrus.
Vitrectomy machines having only peristaltic pump: Oertli Faros.
Other factors, controlling outflow are:
  • Gauge size: Small/Narrow gauge vitrectomy has higher resistance to aspiration as compared to wider gauge for the same vacuum settings.
  • Port size: It is also a limiting factor reducing the outflow. The newer 25G probes are designed with a larger port area to compensate for this deficiency. Compared with the 25-gauge vitrectomy port, the 27-gauge vitrectomy port is larger (0.079 mm2 vs 0.066 mm2) and closer to the cutter tip (0.211 mm vs 0.330 mm). Thus, surgeon can perform complex maneuvers such as vitrectomy-assisted membrane segmentation and delamination.
  • Duty cycle: The duty cycle of a vitrectomy probe indicates the percentage that describes the ratio of time the port is open measured against the total time of the cut cycle. All other existing vitrectomy cutters, whether they are pneumatic or electric, have a duty cycle that cannot be adjusted. Some cutters have duty cycles that favor a more open port, while some favor a more closed port. In Constellation® Vision System, duty cycle can be adjusted to the needs and actions of the surgeon.
Constellation offers three separate duty cycle options:
  1. Core mode: It is the maximum port-open duty cycle control. It is best suited for core vitrectomy where higher flow rates are more efficient and desirable.
  2. Shave mode: It is the minimum port-open duty cycle. It is best suited for delicately removing tissue in situations where lower flow rates are desirable, such as working near periphery or near detached mobile retina.
  3. 50/50 duty cycle: It is the setting between the core and shave. This mode is for those who prefer the cutter to be open and closed for the same amount of time.
    • The added ability to vary duty cycle, as well as cut speed and aspiration rate, has an enormous impact on the safety and efficacy of the new vitrectomy probes and will likely improve our ability to treat vitreoretinal pathologies.
 
Advantages of Modern Vitrectomy Machines (Constellation, Alcon, USA)
  1. Provision of auto infusion (infusion bottle/bag pressurization).
  2. IOP control (0–120 mm Hg) set IOP and then forget–stable IOP.
  3. Vacuum control (0–650 mm Hg).
  4. Flow control.
  5. Automated FAX (integrated air lines).
 
Other Advantages
  1. Proportional diathermy.
  2. Auto gas filling.
  3. Viscous Fluid Control (VFC): To inject silicone oil or for extraction of silicone oil.
  4. Advanced Illumination:
    • Dual port xenon (75 W)
    • Long bulb lifetime
    • Xenon light is less blue and it can cause less photochemical damage.
  5. Embeded 532 nm purepoint laser.
 
Vitreous Cutter (Fig. 1.2)
Modern vitreous cutters are available in 23, 25 and 27G systems.
Vitreous cutters consists of a hollow inner tube surrounded by a hollow outer tube. Both tubes are arranged coaxially.3
Vitreous cutter allows high-speed cutting of formed vitreous and controlled removal of the vitreous gel at low suction.
During vitrectomy, vitreous is drawn into a port near the distal end of the outer tube. Thereafter, the inner tube slides forwards, closes the port and shears off the vitreous. Finally, the cut materials are aspirated out through the inner tube.
A vitreous cutter can be of two types—rotatory and guillotine type, depending on the cutting mechanism. In the rotational type, the inner tube moves around the longitudinal axis. But in guillotine style, the inner tube moves along the longitudinal axis.
The cutter can be driven either pneumatically or electrically. Now-a-days, all cutters employ a guillotine-type of mechanism, which is driven pneumatically by vitrectomy machine. The pneumatic guillotine cutters are powered by a pneumatic drive pulse that act upon a diaphragm. When air pulses are exerted, the diaphragm is extended and the cutter tip closes, completing a cut. When air pulse is released, a spring returns the tip to the open position.
As the cut rate is increased, duration of port open time per cut decreases, while port closed time remains constant.
Another type of pneumatic drive mechanism utilizes a dual drive line system. This is used in Ultravit probe, used with the Constellation Vision System. Here, pneumatic drive pulse act upon the diaphragm, but instead of a return spring, a second pneumatic drive pulse acts against the diaphragm in the opposite direction to reopen the cutter. Thus, this unique designs makes the cutter to operate at high rate up to 7,500 cpm.
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Fig. 1.2 : Shows the modern high-speed cutter with two lines
There are two setting options:
  1. Proportional setting: Here suction is variable depending on position of foot switch but cutting rate is fixed.
  2. 3D setting: Both cutting and suction can be independently controlled with foot switch. 3D setting enhances the safety while working closure to the retina.
Modern Ultravit 5,000 cpm cutter with duty cycle has high speed cutting as well as to modify duty cycle for flow control, independent of vacuum and cutting.
Vitreous cutter is attached to the vitrectomy machine by two lines:
  1. From the cutter to the cutting drive of the machine
  2. Aspiration line: This line will plug into a cartridge in the machine. Vitreous cutter has to be primed in ‘priming mode’ before its use. For priming, the cutter tip has to be placed in a solution of fluid and ‘priming command’ on the vitrectomy machine has to be pressed.
Advantages of 25G+ vitrectomy probe:
  1. High speed cutting from 1,500 to 7,500 cpm.
  2. Higher aspiration flow rates.
  3. Larger cutting port area (0.011 >0.013 in).
  4. Improved duty cycle (37% > 51%).
  5. Cutting port closer to end of needle.
  6. Stiffer needle.
  7. Higher aspiration flow rates.
 
Foot Pedal (Figs 1.3A and B)
Foot pedal controls the vitreous cutter, deploying suction only or both suction and cutter together.
Predetermined aspiration setting will be reached as the foot pedal is maximally depressed.
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Figs 1.3A and B: Shows multitasking foot pedal
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Most of the systems feature linear control of suction and sometimes linear control of cutting rate by the surgeon, as determined by the extent to which the pedal is depressed toward the floor.
Some machine allow the surgeon to click the foot pedal either to the right or to the left to switch between cutting and aspiration modes.
 
Common Additional Instruments Required for Vitreoretinal Procedures
Scleral encirclage/240 band (Fig. 1.4)—for permanent buckle indent and to support vitreous base.
Scleral Buckle (Figs 1.5 and 1.6)
  1. Asymmetric scleral Buckle (275, 276)
    • Increased buckle hight posteriorly
    • Decreased chance of erosion anteriorly.
  2. Symmetric scleral Buckle (277, 278, 279, 280, 281, 286, 287, 289)
Infusion cannula (Fig. 1.7): Infusion line forms the pivot of vitreous surgeries. The infusion is placed first and taken out last.
23G infusion cannula has 0.56 mm diameter.
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Fig. 1.4: Scleral encirclage/240 band
23G infusion cannulas are available with both 4 mm and 6 mm tip. The 27G infusion line has a beveled sharp-tipped metallic tube 4 mm long, designed for single-step perpendicular insertion.
 
Disposable Instruments
Features of disposable instruments:
  • Improved efficiency
  • Better performances
  • Predictable
  • Saves time
  • Avoid cross contamination.
 
Available Grieshaber and DORC Disposable Instruments
  1. End Grasping forceps (Fig. 1.8)
    • Fine tip enhances visualization
    • Multipurpose design
    • Available in 23G, 25G and 27G.
  2. ILM forceps (Fig. 1.9)
    • End grasping design ideal for “maculorhexis” or other fine membrane grasping
    • Tip style improves visualization
    • Multipurpose grasping application
    • Available in 23G, 25G and 27G.
  3. Asymmetrical forceps (Fig. 1.10)
    • Tip angle conforms to the radius curvature of the retina
      zoom view
      Fig. 1.5: Shows asymmetrical 276 scleral buckle
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      Figs 1.6A and B: Shows asymmetrical 277 scleral buckle
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      Fig. 1.7: Shows infusion cannula for intravitreal BSS infusion with 4 mm tip length
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      Fig. 1.8: Shows Grieshaber end grasping forceps
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      Fig. 1.9: Shows Grieshaber ILM forceps
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      Fig. 1.10: Shows Grieshaber asymmetrical forceps
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      Fig. 1.11: Shows Grieshaber serrated forceps
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      Fig. 1.12: Shows Grieshaber MAX Grip forceps
    • Allows better visualization while grasping tissue
    • Available in 23G and 25G.
  4. Serrated forceps (Fig. 1.11)
    • Serrated tip allows for secure grasping of fibrous membranes
    • Blunt (slightly rounded) distal end.
  5. MAX Grip forceps (Fig. 1.12)
    • For maximum gripping of heavy thick membrane.
  6. Intravitreal scissors (Fig. 1.13)
    • Curved scissors
    • Fine curved blades for multipurpose cutting
    • Smooth cutting action for 40 degree curvature6
    • Cannula Compatible
    • Available in 23G and 25G.
  7. Flute needle (Fig. 1.14): For vitreous lavage passively. Pressure Gradient = Intravitreal Pressure – Atmospheric Pressure
  8. Soft tip cannula (Fig. 1.15)
    • High flow through cannula ensure efficient fluid removal
    • Teal Silicone Tip = High visibility at fluid/air meniscus and maximum color contrast to retina
    • Silicone tip length = approx. 1.5 mm
    • Available in 23G and 25G.
  9. Soft tip Backflush (Fig. 1.16)
    • For handling tissues, hemorrhages and bleeding
    • Surgeon controlled immediate aspiration and back flush
      zoom view
      Fig. 1.13: Shows Grieshaber intravitreal scissors
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      Fig. 1.14: Shows flute needle
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      Fig. 1.15: Shows soft tip cannula
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      Fig. 1.16: Shows soft tip Backflush
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      Fig. 1.17: Shows intravitreal Tano scraper
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      Fig. 1.18: Viscous fluid control (VFC)
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      Fig. 1.19: Shows an operating microscope meant for vitreoretinal surgeries (red arrow for surgeon's view) with sideview for the assistant (yellow arrow)
    • Can be used as both active and passive device
    • Available in 20G, 23G and 25G.
  10. Intravitreal Tano diamond-dusted membrane scraper (Fig. 1.17)
    • For peeling ILM, tightly adhered ERM. It is sufficient to remove membranes from the surface of the ILM and layers of the ILM without disruption of the underlying RNFL.
  11. Intravitreal spatula and membrane pic
    • For removing ERM from detached retina.
  12. Silicone oil injector (Fig. 1.18)
    • Both 23G and 25G viscous fluid injector/extractor are available for silicone oil injection/extraction.
B. Viewing system—discussed in Chapter 2
C. Illumination system—discussed in Chapter 3
D. Operating microscope (Fig. 1.19)—Purposes of operating microscope are:
  1. For clear visualization of the structures being manipulated.
  2. Greater depth of field is required for the surgeon to focus finely.
It should be equipped with an assistant's scope.
A good microscope for vitreoretinal surgery should have:
  • X-Y movement with respect to the patient's eye. X refers to nasal temporal movement and Y refers to superior-inferior movement
  • It should have foot switch controlled zoom, on-off and fine focus
  • The microscope should be fitted with the laser filter and image inverting systems.