Sclerotomies and the Cannulas |
21 |
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21.1Transconjunctival vs Conjunctiva-Opening Surgery
The traditional option, 20 g PPV with two or three conjunctival incisions, is fast disappearing.1 The trend is understandable since MIVS has many advantages and only a few disadvantages (see Table 21.1). Still, it must be emphasized that the main benefit of MIVS is not a reduced-sized sclerotomy.2
As mentioned in the Preface, all surgery-related issues discussed in this book are based on the 23 g approach.
21.2Location of the Sclerotomies
The placement of the sclerotomies has huge implications for the techniques3 and prognosis of the surgery. A number of issues must be considered.
21.2.1 Distance from the Limbus
My routine for decades has been to place the sclerotomies 3.5 mm from the limbus, even if the eye is phakic. I have not encountered any difficulty or complication anteriorly (lens injury) or posteriorly (retinal injury). There are also exceptions to the routine:
1A few words are included at the end of this chapter on 20 g PPV since it is not completely out yet. It must also be mentioned that transconjunctival surgery is possible with 20 g instrumentation.
2The 20 g incision is 53% larger than a 23 g one, but it is a difference of only 0.35 mm. I therefore dislike the term “small-gauge surgery” to characterize the 23-25-27 g options. Since sutures are occasionally needed in MIVS as well, I do not prefer the term “sutureless surgery” either. I accept the term MIVS, but interpret it as “transconjunctival vitrectomy.”
3Especially in the phakic eye.
© Springer International Publishing Switzerland 2016 |
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F. Kuhn, Vitreoretinal Surgery: Strategies and Tactics,
DOI 10.1007/978-3-319-19479-0_21
Table 21.1 23 g transconjunctival (MIVS) vs 20 g conjunctiva-opening PPV* |
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Variablea |
23 g |
20 g |
Patient comfort/perisurgical Greatly reduced trauma, consequently much greater comfort, for the patient.b It is |
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trauma |
also much less common to have subconjunctival bleeding (cosmesis) |
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Duration of surgery/speed |
The preparation for and conclusion of (“previtrectomy” and “postvitrectomy” |
The removal of the vitreous itself is |
of vitreous removalc |
phases) the actual surgery are shorter |
faster; the preand post-phases last |
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longer |
Sclerotomy site in case of a Irrelevant; the site of the former sclerotomy is unlikely to be found |
Avoid the previous site/s if possible |
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late reoperation |
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Need to suture the |
If the wound architecture is correct (see Sect. 21.3), suturing of the sclerotomy is |
Suturing is always necessary (unless |
sclerotomy/conjunctiva |
only rarely needed. Exceptions include: |
some special technique is used to |
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Reoperations (if the same location as before is used) |
make the wound self-sealing), which |
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Thin sclera (high myopia, scleritis, autoimmune diseases etc.) |
can cause discomfort, astigmatism, |
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Long surgery |
and conjunctival scarring |
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Silicone oil implantation |
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Vitreous incarceration into This is virtually unavoidable (see Fig. 21.1) due to the presence of the cannula, |
The wound is always cleaned of any |
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the sclerotomyd/ |
unless the following measures are taken: |
prolapsed vitreous; the probe must |
vitrectomy at the |
In a pseudophakic eye, it is possible to clean the area underneath the |
actually be pushed inside the incision |
sclerotomy site |
sclerotomies by indenting the eyewall with the cannula itself. |
to prevent any internal incarceration |
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In a phakic eye, the only option is to remove the cannula, reintroduce the probe |
(which would not be apparent on the |
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and perform “blind” PPV underneath the incision. The downside of this |
scleral surface) |
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procedure is its increased chance for the wound to require suturing |
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Tools with long blades |
Any and every intraocular part of the tool must adhere to the limit of 0.65 mm in |
Any tool of any size and shape can be |
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diameter. The solution if an oversized, long-blade instrument is needed is to |
used in 20 g surgery, as long as its |
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remove the cannula or create a 20 g sclerotomy for that instrument |
diameter does not exceed 0.9 mm: the |
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instrument may have parts that exceed |
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this dimension,f although this requires |
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special techniques of tool insertion |
Changing the wound size |
This is impossible. Tools of smaller size (g) can be used, but leakage through the |
The wound can be temporally enlarged |
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cannula is inevitable. If a smaller cannula is inserted as replacement, the |
and then suture-constricted as |
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wound must be suture-constricted first |
required |
182
Cannulas the and Sclerotomies 21
Difficulty with silicone oil |
Significant or minimal, depending on the equipment |
Minimal or none |
of high viscosity |
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Instrument bendingg |
Significant or minimal, depending on the equipment. The tendency to bend is |
Minimal or none |
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roughly and inversely proportional with size |
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Instrument fragility |
Much increased. It is crucial for the nurse to understand this, especially when |
Significant |
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working with forceps in the darkness of the OR (see Chap. 6); those who |
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sterilize the instruments must also be warned about the tools’ fragility |
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Intraoperative difficulties |
There are no major differences between 20 g and MIVS,h although certain maneuvers such as lensectomy may be more |
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difficult with 23 g; phacofragmentation is currently not available with gauges <23 |
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Intraoperative complications |
No hard-proven difference |
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Complications due to |
The problem does not exist since all instruments are introduced through the |
The wound may be damaged and an |
instrument re/entries |
cannula. There is no difference in instrument introduction whether the tool |
iatrogenic retinal break created, as the |
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has a sharp or blunt tip |
tools repeatedly penetrate the vitreous |
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base. It is much more difficult and |
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thus traumatic to push blunt tools |
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through the sclerai |
Postoperative complications |
It appears that now, with the early MIVS-related issues having been worked out, the risk is smaller with 23 g surgery |
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Cost |
Higher in 23 g surgery |
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*Some of the variables are the primary concerns of the surgeon, but all concern the patient, whether directly or indirectly. aObviously, not all represent the same significance.
bThe smaller the gauge, the less discomfort the patient has. This, however, must be weighed against the increased technical difficulties the surgeon has with various maneuvers if smaller gauge instrumentation is used.
cThere are other, more important factors influencing the speed of surgery; see Sect. 12.1.
dApparently, what was described early in the vitrectomy era as “vitreous wick syndrome” is not such a dangerous condition as long as the incarcerated vitreous only tamponades the sclerotomy (a beneficial effect) but does not cause traction on the peripheral retina (a potentially deleterious effect).
eNo such instrument is currently available. Manufacturing retractable tools of memory material is apparently not feasible. fE.g., a subretinal forceps with long blades.
gMuch of it is relatively easy for the surgeon to overcome.
hThe bending of the instruments and the cleaning of the inside of the sclerotomies in the phakic eye have already been discussed above.
iThe MVR blades come in 20 and 19 g varieties; the incision created with a 20 g blade is often too short to allow the introduction of a blunt tool such as a nonconical light pipe.
Sclerotomies the of Location 2.21
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21 Sclerotomies and the Cannulas |
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•Children. Table 21.2. shows the age-related recommendations.
•Highly myopic eye. I place my sclerotomies at 4 mm if the axial length exceeds 26 mm.
The scleral wound should be perpendicular to the limbus (see below, Sect. 21.3); the distance is measured from the limbus to the proximal endpoint of the wound.
Pearl
Transillumination helps the surgeon identify the lighter pars plana from the darker ora serrata. Endoscopy via direct visual control allows the surgeon to locate the most optimal site of the (remaining 2) sclerotomies (see Sect. 17.3).
Table 21.2 Limbus-to- sclerotomy distance of the pars plana incision in children
Age |
Distance from the limbus, mm |
≤6 months |
1.5 |
6–12 months |
2.0 |
1–2 years |
2.5 |
2–6 years |
3.0 |
≥6 years |
3.5 |
21.2.2 Location in Clock Hours
The placement of the cannulas has huge implications (see Figs. 21.1 and 21.2a, b, c, d) for the ease of performing the surgery as well as the prognosis, especially in the
Fig. 21.1 Schematic representation of the difficulty of vitrectomy underneath the sclerotomy in MIVS. If inserted into the eye through a cannula (C) that sits in the sclera (S), the probe (P) cannot reach the vitreous (V) in the immediate vicinity surrounding the cannula. By changing the angle of the cannula, it is possible to remove a little more vitreous, but the only way to complete the vitrectomy is to reach the area from the opposite side of the eye or remove the cannula and reinsert the probe (neither option is shown here)
21.2 Location of the Sclerotomies |
185 |
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a |
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b |
10:30h |
12h |
1:30h |
T
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4h |
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c |
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d |
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12h |
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T |
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9:15h |
2:45h |
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5h |
e |
f |
Fig. 21.2 The placement of the sclerotomies. (a) A common error is to place the working sclerotomies too close to each other (they are too superior; see the text for more details). T Temporal. (b) A clinical example, surgeon’s view. (c) Schematic representation of the implications of correct cannula placement: the working sclerotomies are just superior to the 3–9 o’clock line. Note that the infusion cannula has been moved closer to the 6 o’clock position. (d) A clinical example, surgeon’s view. (e) Schematic representation of the limitations caused by too-superior cannula placement. The shaded areas represent access to the vitreous base from the respective sclerotomy. The arrow shows the area that remains inaccessible in a phakic eye. (f) Correctly placed cannulas. The shaded areas represent access to the vitreous base from the respective sclerotomy. Even in a phakic eye, there is no area in the vitreous cavity that remains inaccessible to the probe
phakic eye. One of the most common errors a careful observer of VR surgeries4 notices is that the superior (working) sclerotomies are too close to each other. The typical advice is to place them “150° to 160° apart” (in reality they are even closer
4 Just look at the videotapes being shown at scientific meetings.