21.3Inserting the Cannula

The placement of the cannulas also determines the ease with which the surgeon can enter and reenter the vitreous cavity during surgery.10 The trocar used to deliver the cannula is sharp and is almost always11 easily able to insert the cannula. The globe, however, must be immobile to avoid iatrogenic damage to the lens or even the retina. The best tool for securing the globe is the pressure plate (see Fig. 21.4), also allowing the surgeon to:

•Move the conjunctiva away from the spot where the trocar will penetrate the sclera12.

•Identify the required distance of the sclerotomy from the limbus.

Pearl

If the pressure plate is unavailable, I use my other hand’s index finger to stabilize the globe upon trocar entry. Avoiding the use of tooth forceps prevents the tearing of the conjunctiva and/or causing conjunctival bleeding (see

Fig. 39.2 and also Sect. 21.5).

•The trocar is not a round but a slit-like tool; the slit must be held parallel with the course of the collagen fibers in the sclera13 (see Fig. 21.5a).

Fig. 21.4 Fixating the globe with the pressure plate. See the text for details

10It is exceptional that PPV can be performed with a “single in” and “single out.”

11Except if there is extensive scarring on the inside of the sclera.

12Supposedly if the conjunctival opening is not right on top of the scleral opening, the risk of endophthalmitis is reduced.

13The fibers crisscross each other, but run fairly perpendicular to the limbus in its vicinity.

21.4The Order of Cannula Placement

The globe must be pressurized the entire time; hence the general rule that the first one in, and the last one out, is the infusion. If the cannulas are not valved, the order of placement should be the following:

•At the onset of the surgery, the superior cannulas are not placed, and the light pipe and the probe obviously are not inserted, until the infusion line is connected and opened.

•At the conclusion of the surgery, the infusion line/cannula complex is not removed until the superior cannulas have been removed and the wounds are confirmed not to be leaking.

If the cannulas are valved, the three can be placed in any order14, but the light pipe and the probe are not inserted until the infusion line is connected and opened (except when a sample for culturing is taken in [presumed] endophthalmitis). The conclusion of the PPV is identical to that described above.

•If the IOP is very low and the trocar for the infusion cannula is difficult to insert, inject BSS through the pars plana, connect, and open the infusion line before placing the superior cannulas.

•If combined surgery is performed (see Chap. 38), place the cannula for the infusion first. Connection of the line may be delayed until PPV commences so that it is not in the way. The two superior cannulas can be placed prior to the cataract surgery or after the infusion line has been opened.

•If AC manipulations are necessary, place the infusion cannula first but in most cases do not open it to avoid making the AC shallow. An AC maintainer or visco injection may also be needed.

•If the inserted infusion cannula cannot be visualized (see below),15 connect the line but do not open it. If pressurization is needed, use an AC maintainer until the media have been cleared.

Once the infusion line is in place, tape it to the drape (see Fig. 21.6), especially if the eye is phakic. The tape should be close to the eye and leave enough slack so that during eye rotation the cannula is not pulled out: the risk is higher if the loop of the infusion line is too short. The cannula must point toward the midvitreous cavity so that it cannot, even during scleral indentation, puncture the lens capsule.16

14Typically, I place the infusion cannula first but do not connect the line and then the superotemporal one because I do not have to exchange the hand holding the pressure plate; finally comes the superonasal cannula (see below, Sect. 21.5, the exception to this order).

15Significant hyphema, white cataract, severe vitreous hemorrhage etc.

16This is also important when silicone oil is implanted under air: you do not want the oil to coat the posterior lens capsule, but to drip straight down toward the posterior retina.

Fig. 21.6 Taping the infusion line to the drape. See the text for more details

21.5If the Palpebral Opening Is Small

If the palpebral opening is narrow,17 placing the cannulas can be technically difficult. When the superonasal cannula is about to be inserted, the eye cannot be rotated because the already-placed superotemporal cannula hits the eyelid, blocking the eyeball’s movement. There are three possible remedies.

•Press down on the superotemporal cannula so that it slides under the lid.18

•Change the order of cannula placement: insert the superonasal one before the superotemporal one.

•Use the “double-trocar” insertion technique (see Fig. 21.7).

Fig. 21.7 The “doubletrocar” insertion technique. Once the infusion cannula has been placed but before the infusion line is connected, the superonasal cannula is inserted, but the trocar is not withdrawn. The surgeon turns the eye nasally and uses the trocar itself to stabilize the globe as he inserts the superotemporal cannula

17May be compounded by the reduction of the orbital fat so that the eye is deep-seated; this is rather common in elderly patients.

18Later you will have to “fish it out.”