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12 Major Equipment, Their Accessories and Use |
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regardless of whether the probe’s port is submerged in fluid or gel (see below, under Venturi pump).
The amount to be removed has an upper limit, set by the surgeon.4 This amount cannot be exceeded unless the settings are changed on the console. This increases the control the surgeon has over his actions, reducing the risk of the probe inadvertently biting into the retina5 when working in its proximity.
12.1.1.2 Venturi Pump6: Vacuum Control
Due to the much lower resistance to flow of fluid (BSS or aqueous in syneresis lacunae; see Sect. 26.1.2) compared to gel vitreous, the machine’s settings must be calibrated to fluid removal.7 This not only slows down the speed of surgery but also makes vitreous removal in the vicinity of (especially detached) retina riskier.
12.1.1.3 Combination Pump (e.g., VacuFlow8)
Combining the advantages of both pumps, this design virtual eliminates flow pulsation, therefore minimizing the risk of retinal injury while allowing fast gel removal.
12.1.2 The Probe
A fair number of characteristics are important.
12.1.2.1 Size/Gauge9
In principle, the smaller the size, the better for the patient; however, this is different for the surgeon; it is true that “size matters.”
A significant downside of smaller-gauge instrumentation is the bending of the tools as the surgeon rotates the eyeball or maneuvers the instruments into certain intraocular positions. The smaller the diameter, the higher the tendency of the tool to bend, which may be frustrating and divert attention away from the task itself (see Sect. 3.7). With experience, the surgeon is able to overcome this shortcoming (or switch to a larger gauge as a last resort).
Despite claims to the contrary by some in the industry and the profession, smaller instrument size does mean slower surgery, and with today’s technology certain tasks can either not be completed at all or are more difficult (such as phacofragmentation, implantation/removal of 5,000 cst silicone oil).
4The decisive variable is the amount of material to be removed in a given time period (flow, ml/m); the vacuum is used to achieve that flow rate. If the vacuum reaches its maximum but the flow has not, the flow cannot be increased until the surgeon resets the vacuum value.
5Especially if it is detached and highly mobile.
6Its only real advantage is an instant reaction time when activated by the surgeon.
7If set for gel removal, the eye immediately collapses once the probe is submerged in fluid: the infusion supply cannot instantly adjust and replenish the amount of BSS just removed.
8EVA by DORC (Zuidland, the Netherlands).
9See Sect. 4.4.
12.1 The Vitrectomy Machine and Its Components |
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12.1.2.2 Port Location
The more distal the port10, the better (see Fig. 12.1): this makes work in close proximity to the retina safer and more effective (e.g., shaving the vitreous over detached retina). With more distally placed ports, the surgeon relies less on scissors for cutting epiretinal membranes.
Fig. 12.1 Distal, large port on a 23 g vitrectomy probe. See the text for details
12.1.2.3 Port Configuration and Size
A probe with a slit-shaped opening, especially if with a double slit, is able to remove vitreous with increased speed and safety: there is higher fluid flow but decreased fluid/gel acceleration that could drag the retina into the port.
The port size is obviously limited by the diameter (gauge) of the probe, which in turn is one factor in the speed of vitrectomy.11
12.1.2.4 Cut Rate
The probe (vitrectomy machine) should provide for:
•Single cuts: the pedal needs to be activated before the next (single) cut is made.
•High12 cut rate: some machines13 double the cut rate by making the guillotine cut in both the proximal and distal directions.
•Minimal or no drop in the flow rate even when the cut rate is high.
10 That is, the closer the probe’s opening is to its tip. Probes are now available with a port at only 0.43 mm (20 g) and 0.23 mm (23 g) from the tip.
11 Todays’ surgeon can find a 20 g probe with a 0.45 × .66 mm port and a 23 g probe with a 0.33 × 0.47 mm port.
12A cut/minute rate of 600 used to be called normal and 1,500 c/m high only a few years ago. The cutoff value is unspecified, but today a “high” cut rate is generally understood as >5,000 cpm.
13DORC (Zuidland, the Netherlands), Geuder (Heidelberg, Germany).