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21 Sclerotomies and the Cannulas |
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21.7The Cannulas in Use
Ideally, the cannulas provide effortless and complication-free access to the vitreous cavity, remain in place until the end of surgery, and leave behind a self-sealing incision upon removal. Some of these are outside the surgeon’s control, but he can help by observing the following:
•Since the cannula is placed at an oblique angle into the sclera:
–The surgeon must insert all his instruments, and each time, at the same oblique angle to make the entry effortless and avoid stretching the sclera.22
–The surgeon must withdraw all his instruments, and each time, at the same oblique angle to avoid pulling out the cannulas with the instrument.23
•With the infusion line, it is crucial that it does not point toward the macula or disc, especially during F-A-X. Especially if the air pressure is high, direct injury may result.
Pearl
A cannula that repeatedly gets pulled out during surgery is a signal that the sclerotomy will have to be sutured at the end.
Intraoperatively it may become unexpectedly difficult to reinsert a tool through the cannula. There are several potential reasons:
•The cannula is halfway out, which is invisible in the dark (see Fig. 21.8).
•The tool itself has some material (such as thread) stuck to it.
•The nurse handed over an instrument that is of larger gauge.
•A material the surgeon wanted to remove through the cannula got stuck (see
Fig. 21.9).
21.8The Removal of the Cannulas
The cannula is grasped with forceps (in the surgeon’s dominant hand), while the upper lip of the wound is pressed down with an instrument (e.g., larger anatomical forceps) or the index finger of the surgeon’s nondominant hand (my preference).
22Which would increase the leakage potential once the cannulas have been removed.
23There are other causes of inadvertently pulling out the cannula. Thin sclera; material stuck to the external surface of the instrument or the internal surface of the cannula (see below); a curvedtipped memory tool (such as laser probe) was not withdrawn into the shaft prior to tool removal; an outward bent hooked needle of the same gauge (see Sect. 13.2.3.1).
21.8 The Removal of the Cannulas |
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Fig. 21.8 Schematic |
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illustration of the difficulty |
a |
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entering the eye through a |
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half-out cannula. (a) If the |
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cannula (C) is fully pushed |
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in, the angle at which the |
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probe (P) approaches the |
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cannula is irrelevant as long |
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as the angle remains within a |
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reasonable range: the probe |
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will easily slide in (black |
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arrows) since the cannula |
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will tilt once the instrument is |
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inserted into it (it realigns |
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itself with the tool). (b) If the |
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cannula is partially pulled |
C |
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out, however, the probe can |
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be inserted only if held |
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exactly coaxial with the |
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cannula. If approached at an |
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angle different from the |
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coaxial (0°), the cannula will |
b |
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keep tipping over (red |
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arrows) until the coaxial |
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direction is found or the |
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cannula is pushed fully in. |
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(I do this with my finger.) |
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(The sketch is simplified, |
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obviously even in the two |
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unsuccessful attempts the |
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surgeon would place the |
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instruments over the |
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cannula.) |
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P
S
Pressing down and gently massaging the site for a few seconds brings the wound lips together, preventing leakage and resulting in a normal IOP.
If the wound is not watertight, the conjunctiva becomes chemotic or air is seen bubbling under it (“emphysema”).
21.8.1 Hypotony: The Causes
In principle, the sclerotomy, being oblique and at a shallow angle, should be selfsealing: its lower (posterior) scleral lip (“flap”) is pressed against the upper lip by
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21 Sclerotomies and the Cannulas |
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Fig. 21.9 Scar tamponading the cannula. (a) A large and hard piece of proliferative tissue is seen at the tip of the cannula. My intention was to remove the scar in one piece. (The probe is not the proper tool to chew it up: the tissue is hard and may not allow the probe biting into it, or does not have an attackable edge to feed into the port. Finally, even if the scar can be fed into the port, the scar may blunt the guillotine blade. The latter is dangerous because of the potential for traction if there is still vitreous left to be removed.) (b) I handed the forceps to the nurse, assuming that the scar was removed. I wanted to continue with the vitrectomy, but was unable to reinsert the probe into the cannula. The cause was the membrane, which was stuck in the tube; I used the probe from the other side to reduce the size of the scar. (c) The remaining stump is grabbed; it is now sufficiently stretched so I could remove it through the other cannula
a
b
c
21.8 The Removal of the Cannulas |
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the IOP, helped by a plug of gel vitreous, possibly air/gas. In reality, leakage can be expected in the following cases:
•Extremely efficient peripheral vitrectomy: there is no vitreous plug underneath.
•Very high IOP at the conclusion of surgery, which wants to normalize and is thus pressing fluid/gas outward. This can be caused by:
–The high infusion pressure set on the vitrectomy machine.
–External pressure on the eye by the surgeon’s fingers or instruments.
•Thin sclera (high myopia, post-scleritis conditions, trauma etc.): the sclera does not have the structural strength to self-close.24
•Sclerotomy placed in the area of previous vitrectomy wounds or scleral scars of other causes.
•Silicone oil tamponade: as the cannula is withdrawn, the IOP forces oil droplets into the scleral channel, delaying the spontaneous closure.
•Long surgery where significant stretching of the sclera has occurred (see
Sect. 21.7 above).
•A sclerotomy that did not follow the course of the collagen fibers (i.e., is parallel with the limbus; see above, Sect. 21.3) increases the chance of insufficient wound closure.
If vitreous is incarcerated in the wound, it prevents leakage but increases the risk of endophthalmitis (see below, Sect. 21.8.2).
21.8.2 Hypotony: The Consequences
Low postoperative IOP can lead to:
•Hypotonic maculopathy.
•Intraocular bleeding (choroidal, retinal etc.).
•Miosis.
•Increased inflammation.
•Corneal edema and the folding of Descemet’s membrane.
Contrary to popular belief (myth), endophthalmitis is not caused by hypotony.25 Endophthalmitis, however, is a risk if vitreous is incarcerated into the wound and links the outside and inside worlds.
21.8.3 Hypotony: Prevention
•Proper wound architecture (see Sect. 21.3). If need be, use a suture (see Sects. 14.6 and 63.5).
24The normal sclera does contain some elastic tissue to assist in its capacity to self-close.
25This would assume a negative IOP, sucking material into the vitreous cavity.