24.1Removal of the Vitreous

The basic movements the surgeon makes with the probe are the following:

•Forward/backward (push deeper or withdraw along the same axis): closer to the posterior pole or to the sclerotomy.

•Around, 360° in the vitreous cavity, with the pivot point being at the sclerotomy site (see Fig. 20.3).

•Rotation, 360° around the axis of the probe’s shaft.

•A combination of the 3. This is by far the most common occurrence.

While the positioning of the probe is done in a “semiconscious” way by the surgeon,1 there are some caveats to remember, which help to reduce the risk of iatrogenic damage ththe probe may otherwise cause:

•The probe is not simply a port; its shaft must also be taken into account when maneuvering in the vitreous cavity. By concentrating only on the port (since this is the active [“useful”] part of the probe) the surgeon can easily forget that the shaft is also there, increasing the risk of “lens touch” during anterior PPV (see

Sect. 25.2.3.1).

1 That is, the surgeon focuses on the goal he wants to achieve with the probe, rather than on the placement of the probe so as to achieve that goal.

F. Kuhn, Vitreoretinal Surgery: Strategies and Tactics,

DOI 10.1007/978-3-319-19479-0_24

•The angle between the probe and the retina changes according to where the probe is.2

–The shaft is truly perpendicular to the retinal surface in certain areas on the opposite side of the disc,3 although scleral indentation can also create the 90° angle (see below and Sect. 28.2). In the posterior pole the angle closely approximates being perpendicular. At all other locations, the angle is an acute one.

–The shaft is parallel with the retina in the vicinity of the scleral insertion.

•The relation of the port’s plane4 to the retina varies with the shaft’s angle, which in turn determines how vitreous or membranes are separated from the retina.

–If the shaft is held at or close to a right angle with the retina,5 the port cannot be turned so that it directly faces the retina.6 If the probe is moved along (parallel to) the concave retinal surface, it is able to separate/cut/remove proliferative membranes safely (see below).7

–Even in the posterior pole, however, this ideal situation is true in only a small area. The angle outside this relatively small circle starts to change, more rapidly on the side where the probe is (i.e., a nasally inserted probe maintains the close-to-right angle in a larger area temporally than nasally).

–The smaller the shaft’s is angle to the retina, the higher the risk that the rotation of the port toward the retina can result in iatrogenic injury: the probe can “bite” into the retina even if the vacuum/flow is very low and the duty cycle high. The implication when working in the periphery is an increased risk in the vicinity of probe insertion compared to that on the opposite side (see Fig. 24.1) – hence the suggestion to switch hands in the pseudophakic eye to complete the vitreous removal at the base.

•The surgeon must select the cut rate according to the demands of the actual situation.

–A high cut rate increases the safety of vitreous removal when working close to the retina. However, with most vitrectomy machines it also reduces the speed of vitreous removal by keeping the port closed (no flow) a significant part of the time.

–The increase in the cut rate also reduces the completeness of the removal of vitreous that is adherent to the retinal surface such as at the vitreous base.8

2The angle also depends, to a lesser degree, on the size and contour of the eye (see below).

3That is, on the nasal side if the probe is used through the temporal sclerotomy.

4Understood as a sheet laid over the port, across and perpendicular to the shaft.

5Such as in the posterior pole.

6The exception is a staphyloma in a highly myopic eye (see Chap. 56).

7Assuming the proper parameters have been set on the vitrectomy machine (see Table 12.2).

8Think about an object that is lifted with a rope: if you cut the rope, the object cannot be lifted. If the vitreous that is engaged by the probe via suction is instantly cut by the guillotine, at least some of the gel is left behind.

a

R

V

S

b

Fig. 24.1 The angle of the probe’s shaft and the risk of retinal injury. (a) When the shaft (S) is held (close to) parallel to the retina (R), such as the case in the vicinity of the probe’s insertion site, the port can be turned so that it faces the retina, increasing the risk of biting into it, rather than simply removing the vitreous (V) from the surface. (b) On the opposite side of the eyeball, the angle of the shaft is (close to) perpendicular to the retina. The port’s plane makes it impossible to directly injure the retina (although injury is obviously possible if the retina is dragged into the port by the adherent vitreous). (This, however, completely changes if the sclera is indented; see Fig. 28.1.) In the posterior pole (not shown here) the angle is close enough to the perpendicular to mimic this scenario, allowing retina-parallel membranes to be cut by the probe if certain conditions are also met (see the text for more details)

The details of vitreous removal are given in Chap. 27.

Q&A

QWhat are the ideal parameters on the vitrectomy machine for fast yet safe vitreous removal?

AThis is impossible to determine unless the specific situation is known. There are many variables that need to be known: the distance of the probe’s port from the retina; the position of the retina (attached or detached); the characteristics of the vitreous gel; the strength of adherence between vitreous and retina; the pump in the vitrectomy machine; the aspiration/flow rate; the duty cycle; the size, shape, and location of the port etc. (see Sect. 12.1). The surgeon must carefully experiment with his own machine and tailor his settings accordingly.

•Inexperienced surgeons often move the probe around constantly. This not only makes the operation longer but also potentially less safe.9

–In most eyes and conditions the vitreous will “flow” toward the port from around the imaginary sphere surrounding the tip of the probe.10 This can be called the “vitreous-to-port” technique. In such cases it is irrelevant whether the port is actually turned toward or faces away from the vitreous and whether the port is immersed in the vitreous or be held at some distance from it.

•In some eyes and conditions11 the central vitreous gel has a stronger resistance/ structure and will not allow being drawn into the port unless the probe is actually sunk into it; in such eyes the vitreous-removal time is dramatically longer since the surgeon must literally “go after” the vitreous. This can be called the “port-to-vitreous” technique.

The technique of vitreous removal is discussed in Chap. 27.

24.2Removal of Proliferative Membranes

In 23 but especially in 25 and 27 g vitrectomy, the probe can also be used to cut and remove epiretinal proliferative tissues.12 Low aspiration/flow and the highest possible cut rate should be used, and the shaft be held so that the port can be moved parallel to the retinal surface.

Pearl

In 20 g surgery different tools (scissors, forceps, spatula) and techniques ([visco-] delamination, segmentation) were used to cut, separate, and/or remove proliferative membranes. In small-gauge PPV, the probe can largely substitute for all these while also making the procedure faster, but the surgeon does sacrifice control to a certain extent. Scissors use provides the ultimate control, especially if a bimanual technique is utilized – but this requires greater dexterity and takes more time. Finally, remember that use of the probe does not reduce the risk of bleeding.

More details about dealing with proliferative tissues are provided in Chaps. 32, 52, and 53.

9Unless very high cut rates are used.

10This is true for the central vitreous gel but not for the still-attached cortex.

11Such as VMTS and PDR.

12As long as there is at least some space between the two tissues; this space is necessary even if the port is very close to the probe’s tip (see Sect. 12.1.2.2). An EMP whose entire extent is adherent to the surface of the retina is thus impossible to remove with the probe.