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In principle, it would be preferable not to employ scleral indentation since it distorts the anatomy and the surgeon is forced to perform his maneuvers in an artificially altered environment. In reality, the only way to avoid scleral indentation as the surgeon works in the periphery is to use the endoscope for viewing (EAV, see Sect. 17.3). In traditional PPV, however, scleral indentation is a necessity if the surgeon needs to access the anterior part of the vitreous cavity.
28.1The Advantages of Scleral Indentation
•Normal structures, which would remain hidden from view otherwise, become accessible and their pathologies treatable. The structures include the peripheral retina, the vitreous base, and the ciliary body.
•A bullous or highly mobile retina, if indented, will have reduced mobility. The safety margin of PPV increases both on the “hilltop” and on the “slopes” (see below and Fig. 28.1).
28.2The Mechanics of Vitrectomy with Scleral Indentation
The contour of the eyeball dramatically changes with scleral indentation. If the retina was attached, what had a concave profile before becomes a convex one; if the retina was detached, it is now reunited with its foundation.
The angle between the probe’s shaft and the retina changes; consequently the port’s plane relative to the retinal surface also changes. The person performing the
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Fig. 28.1 Cross-sectional view of performing vitrectomy during scleral indentation. The internal elevation created by the indentation of the sclera is not cone-shaped. The effect is a radially oriented “mountain” or a circumferentially oriented “hill” with typically symmetric slopes on either side. The more peripheral the indentation, the less radial the direction of the ridge. If the retina was attached before, it will remain so during the indentation (shown here). If the retina was detached before and the detachment is not too high, the indentation results in a (temporary) retinal reattachment on the hilltop, but the RD persists on the slopes. Vitrectomy should be performed not only over the hilltop but also on the two slopes. The angle of the probe’s shaft relative to the retina changes according to the location the indentation and the site of the sclerotomy (see the text for more details). The surgeon must decide, based on the visual feedback of his actions, whether to turn the port toward, parallel with, or away from the retina. He must also change the settings of the vitrectomy machine according to tissue behavior
indentation must understand its mechanics as well as the risks involved (see below and Sect. 24.1). Retinal injury can result in multiple scenarios.
•Lack of coordination between the external vs the internal movements1 (see below).
•Unexpected changes in the characteristics of the indentation while the intraocular tissues are being manipulated are very dangerous.
–A sudden increase in the height or location of the indentation.2 A common cause of slippage is a lid speculum with a long blade (see Fig. 19.3). When the area of contact between speculum and lid is small, the depressor can be inserted on either side of it and there is no slippage. When moving the depressor along the long blade in the darkness, however, the nurse has no feedback that she reached the blade’s edge until it is too late and the depressor suddenly “jumps.”
1The depressor’s and the intraocular tool’s (probe, scissors etc.).
2Accidental slippage or a nurse who is not paying proper attention.
28.2 The Mechanics of Vitrectomy with Scleral Indentation |
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Pearl
Four variables should be taken into account to accurately describe the characteristics of the indentation: height (depression of the eyewall into the vitreous), location (according to clock hours in the frontal plane), depth (how far posterior from the ora serrata), and angle (this is the variable that changes the least; typically, the direction is radial).
•A surgeon too focused on the port of the probe. If the vitrectomy is done in a central location, the shaft may rub against the peripheral retina and cause an erosion (break) or bleeding.
•Incorrectly set PPV parameters. The aspiration/flow is too high, not adjusted to the peculiar demands of the periphery (see Table 12.2); a retinal break or detachment may result.
•A surgeon who does not take into consideration the implications of the change in the eyewall’s contour (see below).
•Phakic eye. The lens is at risk if the surgeon does not switch hands at the midline (see Fig. 21.2f). Inferiorly, it is impossible to perform peripheral vitrectomy if the working sclerotomies are not properly placed (see Sect. 21.2.2).
In a pseudophakic/aphakic eye, the surgeon must decide whether to switch hands to complete the process. Table 28.1 shows the consequences if the probe remains in the same sclerotomy.
Table 28.1 The implications for the execution of peripheral vitrectomy according to the location of the scleral indentation*
Indentation |
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location |
Consequences |
9 o’clock |
The probe has access to the hilltop and both slopes |
|
The probe’s port can be turned toward or away from the retinal surface as well |
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as sideways; this is true both on the hilltop and on the two slopes |
6 and 12 |
The probe has access to the hilltop and the proximal slope; the area of the distal |
o’clock |
slope will become accessible as the indentation is relocated and what was distal |
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before becomes proximal |
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The port on the hilltop can be turned toward or away from the retinal surface as |
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well as sideways; on the slope its only available position is sideways, but here it |
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can be turned 360° |
3 o’clock |
The probe has access to the hilltop and both slopes |
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The port’s only position on the hilltop is sideways, but here it can be turned |
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360º; on the two slopes it can be turned toward or away from the retinal surface |
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as well as sideways |
*The indentation causes the formation of a “hilltop” and two “slopes” that are radially oriented (i.e., not very peripheral); if the indentation is more peripheral, the consequences change accordingly. In the example used here, the probe is inserted at the 9 o’clock location.
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28.3Internal vs External Illumination
External illumination (BIOM swung out, see Fig. 28.2) has certain advantages.
•The surgeon can choose whether to perform the indentation himself or by the nurse; in the latter case, bimanual surgery is possible, allowing the use of two working tools (see below).
•A larger area of the surgical field can be visualized.
The major disadvantage is that the resolution afforded by the microscope’s light is inferior to that of the light pipe (see Fig. 28.3); the vitreous is more difficult to visualize.
Fig. 28.2 Self-indentation under external illumination. See the text for more details
Fig. 28.3 Indentation with internal illumination. See the text for more details