Q&A

Q Is the probe’s cut rate truly so important in VR surgery?

AYes. In principle, the higher the cut rate, the less likely that the probe will inadvertently bite into the retina, whether detached or attached. (The duty cycle also plays an important role in the fluid “surge” at the aspiration port.) Conversely, higher cut rates also mean that, in the true sense of the word, less “vitrectomy” than “vitreous shaving” occurs: the tissue is released before the actual cut. Just think of performing lensectomy in the vitreous cavity: the cut rate must be low to avoid the fluid surge pushing the material away from the port (see Sect. 38.2.2).

12.1.2.5 Duty Cycle14

The higher, the better; this reduces the risk of drawing retina to the port. Probes with a port that is always open (continuous flow) are becoming available and greatly reduce the risk of iatrogenic retinal injury.

12.1.2.6 Probe Length

In highly myopic eyes, most probes are unable to reach the posterior retina. The surgeon usually compensates for this by indenting the eyewall, which unfortunately distorts the image. This is an especially important issue if a contact lens for highresolution viewing is used.

The ideal probe is long enough to readily reach the posterior pole in the highly myopic eye (see Table 42.1) as if it were an emmetropic one.15

12.1.3 The Light Source/Pipe

The light provided by the vitrectomy machine must be bright enough to allow safe execution of any surgical maneuver. It should be color-adjustable with no harmful UV/IR rays. At least two bulbs should be housed in the console, so if one burns out, the other can be instantly switched on.

Most surgeries are performed with the surgeon holding the light pipe in his nondominant hand. This tool must be:

•Shielded (blocking light on one side so as to prevent blinding the surgeon with direct light).16

•Wide angle (simultaneous illumination of most of the retina).

14Proportion of time when the port is opened vs closed.

15Obviously, this is true for all hand instruments as well.

16Light reflected from instruments or white intraocular surfaces requires adjustment of the angle of illumination.

There are definite benefits for the surgeon if he can use two active hands (bimanual surgery; see Sect. 4.3), which requires a different concept of lighting, even if the “chandelier” type of illumination has its own disadvantages (see Table 12.1). Certain manufacturers provide a 20 g light pipe equipped with a pic, which allows performing surgery with “one-and-a-half” hands.

More is found on endoillumination in Chap. 22.

Table 12.1 Illumination options for the VR surgeon

aIf equipped with a hook (pic), even for sharp dissection.

bThis category includes, regardless of the name of the device, all lights that are fixed externally: bullet, twin etc.

cRequires a trained, attentive nurse.

12.1.4 The Infusion Supply

Gravity-fed systems17 are no longer acceptable. Automatic resupply (infusion compensation) is the optimal solution, in which the vitrectomy machine instantly reestablishes and continually maintains the preset IOP value, irrespective of how much material and how fast a material is removed from the eye.

Pearl

The bottle height is about as specific an indicator of the IOP as the tachometer is about the car’s speed. It is only a rough estimate.

17 If this is the surgeon’s only choice due to the vitrectomy machine’s characteristics, at least he should place the drip chamber of the infusion bottle at the height of the patient’s eye and connect the infusion bottle with the vitrectomy machine’s air pump. The infusion pressure created in the vitreous cavity then equals the air pressure set by the surgeon on the vitrectomy console. This setup eliminates the unreliable guesswork of having the “bottle height” determining the IOP.

12.1.5 The Trocar

•It should require a low piercing force to avoid major IOP elevation during insertion.

•Its shape should be slit-like to allow spontaneous closure of the scleral incision at the conclusion of surgery.

•The one-step system is preferable to the two-step one.

Q&A

Q What are the disadvantages of the two-step entry system?

A Both the transconjunctival and the scleral openings may get lost during the switch from the blade to the trocar. This is frustrating, and if the scleral opening is not found by blindly poking under a conjunctiva that bled or is swollen due to the fluid leaking from the vitreous cavity, the conjunctiva may have to be incised.

12.1.6 The Cannula

•The cannula should be valved to avoid fluid loss when no instrument is inserted in the cannula. Having a plug in the cannula is better than having neither, but the valve is the ideal option.

–Occasionally,18 the surgeon does want free flow through the cannula. Preferably the valve can temporarily be removed (and replaced if necessary)19; otherwise, either the valve is permanently eaten by the probe or a cannula without valve must be inserted (see Fig. 35.9a).

•Even in the OR’s dark environment, it should be easy for the surgeon to find the entrance of the cannula (so that even a presbyopic surgeon does not need the nurse’s help; see Chap. 6).

•The cannula should be color-coded, based on its size (gauge).

12.1.7 System to Inject/Extract Viscous Fluid

•The drainage connection for silicone oil removal should be internal, not external: a blunt needle inserted through the cannula into the vitreous cavity, rather than a silicone sleeve that is held over the cannula’s head.

–The latter can easily aspirate outside air, not silicone oil, if the fit is not watertight.

18For example, passive silicone oil removal.

19DORC (Zuidland, the Netherlands; see Fig. 21.2b).

•The needle should be long enough to reach any remnant silicone oil bubble, even if it is stuck at some distance from the cannula’s internal port. The needle companies provide with their machine is typically too short.

•Ideally, the plunger head of the syringe used for oil extraction does not get stuck (making it impossible for the vitrectomy machine to create vacuum and start oil removal).

12.1.8 The Pedal

•Its switches (buttons) should be programmable so that they can be set according to the individual surgeon’s preferences.

•It should allow for linear, dual linear, and “3D” modes (see Sect. 16.3).

•Having a wireless pedal avoids the accumulation of wires/cables under the surgeon’s feet.

•One of the most crucial functions of the pedal is the “backflush” option, similar to that with the flute needle (see Sect. 13.2.2.1): if the retina is inadvertently caught in the probe’s port, the surgeon must be able, with a readily available button, to immediately reverse the flow and blow the retina away.

12.1.9 Integrated Laser20

It is very important for the laser probe to be curved. There is no location in the eye that cannot be reached with a curved probe; conversely, some areas are risky or impossible to reach with a straight one; a straight probe prevents doing proper endolaser cerclage21 in the phakic eye.

12.1.10 Endodiathermy Probe

•The ideal probe’s tip has a nonsticky surface.

•There should be two tip designs to choose from: one with a sharp, pointed tip and one with a blunt tip that has a large surface.

– It is the latter one that is ideal for chorioretinectomy (see Sect. 33.3).

•The power of the cautery should be highly adjustable.22

20Typically argon. The laser may also be a stand-alone equipment.

21See Sect. 30.3.3.

22At high power (which is also used in chorioretinectomy), the liquid blood, which otherwise hides the exact location of the hemorrhage, will evaporate when the probe is activated just over the blood pool. The source of the bleeding is thus identified and can be treated (at a lower power). This maneuver is often needed in PDR.