Vitrectomy
There are four principles to removing vitreous in a controlled fashion. First, do not pull the vitreous too close to the probe. Second, do not pull vitreous too far up the probe. Third, remove the vitreous in tiny pieces. Fourth, use as low a flow as possible. Regardless of the cut rate, vacuum, or duty cycle selected, it is important to maintain these four principles to avoid putting traction on the retina and cutting unintentionally.
Vitrectomy can be discussed under following headings:
- Viewing system
- Machine
- Vitreous cutters
- Accessaries
- Simple vitrectomy
- Advances
Viewing System
Viewing system is essential as we cannot directly view the posterior segment of the eye through the operating microscope. This occurs because of the refraction of the rays of light from the fundus by the refractive elements in the eye.The viewing system can be either:
1. Contact system: They neutralize the refractive power of the eye. These can be either conventional lenses like hand held lenses (fig 1A), sew on lenses, self stabilizing lenses or wide angle lenses. (fig. 1B)
Fig:1A Hand held irrigating contact lens . Fig 1B: Contact Lenses with Fixation Ring
2. Non contact system: Non contact systems are
i. BIOM: Binocluar Indirect Ophthalmo Microscopy. Incorporates the principles of indirect ophthalmoscopy in the operating microscope. (fig 2)
ii. EIBOS: Erected image binocular ophthamomicroscope (fig 3)
Fig:2 BIOM Fig:3 EIBOS
Machine (FIG 4)
The modern Vitrectomy machine has following Components
- Dual illumination module
- High frequency diathermy module
- Air module
- Vitrectomy module
- Viscous fluid injection and extraction
- Fragmentation module
- Extrude mode
Fig 4: Vitrectomy Machine
1. Dual illumination module: - Machine should provide simultaneous use of two or three illumination ports. Light source is generally a Halogen bulb and automatic switch should occur to second lamp when first lamp fails. There should be integrated heat protection filter for cold light and UV/IR filtering.
2. High frequency diathermy module: - Machines should have diathermy for endo and exo diathermy for selective burn placement on retina and sclera. Diathermy function should also be available during priming function to avoid any delays.
3. Air module: - Air module provides an automatic air infusion system. Air is delivered to the tubing set through a 0.22 mm filter to assure sterility.
4. Vitrectomy Mode: - A standard high speed cutter has cutting frequency of upto 2500 cpm. High speed cutting offers reduced traction and increased stability while working close to retina. Machine should support both pneumatic and electric drive for pneumatic and electric vitrectomies. It should be usable in single cut, fixed and linear cutting control. New horizontal cutting probes have a radial reciprocating action to minimize turbulence or fluttering of tissues (cutting blade moves from left to right across the port). Latest new vitrectomy machine by alcon The Constellation ca cut upto a frequency of 5000cpm.Three different vitrectomy modes exist: proportional, 3D(dual dynamic drive) and momentary.
3D Technology: This allows the operator to simultaneously change cut rate and vacuum which provides way to easily change parameters as needed throughout the surgery. Vacuum can be set to start at low level and rise to max at full footpedal depression,while cutting rate can be set to start at its max setting and decreased as footpedal is depressed.It allows more cutting of vitreous while doing core vitrectomy and fine cutting without much pull when working near retinal surface.
