Minimally Invasive Vitreoretinal Surgery

10.1 Introduction

For almost three decades, ophthalmic surgeons used a standardized 20-gauge multiple port vitrectomy system to manage all types of vitreoretinal disorders. A broad array of instruments was developed with this approach to facilitate specific surgical techniques and objectives. The 20-gauge format required separate conjunctival and scleral incisions that were closed with sutures at the end of the procedure. For at least 15 years, the capability to create smaller gauge instrumentation was possible [1, 2]. However, it was the introduction and popularization of the transconjunctival, sutureless entry technique that accelerated the adaptation of smaller incision vitreoretinal surgery [3–5]. Currently, more than half of the vitrectomies in the United States and Japan are done utilizing 23and 25-gauge small incision surgery. As surgical techniques and instrumentation advance, these authors believe that 23and 25-gauge surgery will become the standard format for most vitreous surgery.

Endoscopic surgery also has been applied to vitreoretinal surgery. Currently, there seems to be limited application for this approach but as improvements in optical resolution develop, future interest may grow. A section of this chapter will summarize the current status of this technology for vitreoretinal surgery.

for 20-gauge surgery (0.9 mm). Twenty-three gauge requires a 0.65 mm incision and 25 gauge requires 0.5 mm. Clearly, the size of the incision and the thickness of the sclera are the major determinants of whether the wound will be self-sealing or require suture closure. Thus, in general, smaller incision size would be preferable, but the performance of smaller gauge instruments may be less satisfactory depending on the complexity of the clinical scenario. With any unsutured sclerotomy wound, there is a transient period of hypotony that is probably time-dependent on the formation of a fibrin plug or vitreous plug that first seals the entry site. This period of low pressure is reduced and more predictable in a wound that is securely closed with a suture. Clinically, this may be more relevant in cases such as proliferative diabetic retinopathy in which there may be some residual oozing of blood after removal of surface neovascularization. In these cases, a long period of hypotony in the immediate postoperative period would not be desirable.

A single-step or two-step transconjunctival entry approach can be used to create the entry incisions. A one-step entry is only available with 25-gauge systems, and seems to provide a consistent self-sealing wound. A modified solid needle with a round shaft and polyviol cannulae is used (Fig. 10.1). With currently available 23-gauge systems, the one-step entry system uses

a modified round solid needle, and the two-step approach (Fig. 10.2) uses an angled 23-gauge microvitreoretinal blade that is followed by a blunt trocar and metal cannula. A relatively flat entry blade is more

Fig. 10.2 Elements of the two-step 23-gauge entry system with angled MVR blade, fixation plate, forceps, and trocar/cannulae

Fig. 10.3 Two-step entry using a diamond knife. The blade is tapered and inserted approximately half way in an oblique direction. See Video 1

desirable than a round needle since scleral fibers may be torn or stretched as a round needle enters [6]. The flat incision closes more securely, and tends to seal better. Thus, a diamond blade has also been developed to create a flat, self-sealing tunnel entry into the sclera [Fig. 10.3, Video 1, Avi Grinblat, unpublished data]. With vigorous manipulation during vitrectomy, the scleral incisions may stretch, and one must bear in mind that any incision may leak at the end of surgery and thus require suturing.

It is important to displace the conjunctiva by sliding it before inserting the entry blade over the scleral site [Video2] [7]. This maneuver allows the conjunctiva to cover the scleral incision at the end of the procedure and reduces the possibility of direct access of bacteria into the eye from the fornices. The angle of the entry blade is oblique, about 30° to the tangent of the globe (Fig. 10.4). The needle should be inserted at a level parallel to the ora serrata so that the tip of the cannula does not pass into the peripheral retina or into the lens. When the tip of the needle has passed through the sclera, the entry angle is changed and the needle and cannula are further inserted in a radial direction toward the center of the globe [3, 8–12]. With the two-step entry, the microvitreoretinal blade is inserted, the conjunctiva is fixed, and the trocar and cannula are inserted obliquely, and then angled radially [3]. With the diamond blade, the tip of the trocar, has been modified to be a flat spatulated one that enters the scleral incision.

At the end of the procedure, the incision appearance differs between the one-step and two-step entry. The one-step entry with a round needle produces a ^ shaped incision. The two-step incision with microvitreoretinal blade or diamond knife produces a linear incision.

The reported rates of postoperative hypotony (intraocular pressure ≤5mmHg) range from 2 to 21% on the first postoperative day [13–15]. This rate is dependent on various factors including the type of entry wound, and

how frequently the incisions required suture closure at the end of the operation. In one series using 23-gauge vitrectomy, 11.2% of eyes required suturing of a leaky sclerotomy [16]. Usually hypotony resolves within a week of the procedure [8, 9, 15–23]. The use of an oblique two-step entry compared to conventional incision was demonstrated to have a lower rate of postoperative hypotony (2 vs. 18%) on the first postoperative day [9, 12]. Results of a retrospective study investigating the results on risk factors of sclerotomy leakage and postoperative hypotony in 322 eyes of 292 patients found that intraoperative sclerotomy leakage requiring suturing were in eyes with prior vitrectomy, vitreous base dissection, and young age of the patient [16, 24]. In all cases a two-step incision was used, and the incidence of hypotony was found to be 11.3% at 2h, 6.5% at 5h, 3.8% at 1 day, and 0% at 1 week. Longer periods of hypotony (2–3 weeks) may result in choroidal folds and maculopathy, which can affect visual outcome.

10.2.1 Models of Wound Architecture

Studies of the morphology of 23-gauge and 25-gauge microincisions through the sclera have been done in animal and human cadaver eyes [25–29]. These studies have found that angled incisions demonstrate less wound gape than incisions made with the trocar/cannula directed perpendicularly to the surface as studied by optical coherence tomography and histologic examination [30]. India ink applied to the conjunctiva demonstrated particles across the incisions in some perpendicular wounds [25]. These studies were made in rabbit eyes, and the sclera is considerably thinner compared to human eyes. In other studies of 25-gauge incisions in human cadaver eyes, it was demonstrated that not more than 55% of obliquely directed entry incisions resulted in two-plane wounds [31]. After brief periods of manipulation placing the vitrectomy instruments through the cannula, the inner aspect of the wound appeared to gape.

Studies using optical coherence tomography and ultrasound biomicroscopy in patients after small-gauge vitrectomy have shown that initially conjunctival blebs may be present over the sclerotomy sites early postoperatively, and that frequently the wound may be visualized within the first 2 weeks after surgery [11, 32–34]. By 1 month the wound is usually not visible, except for vitreous incarceration. Endoscopic viewing of the sclerotomy incisions have also reported a high rate of

vitreous incarceration in the sclerotomy incisions [35], but the sequelae of this observation is uncertain, and it remains controversial if small-incision vitrectomy results in a higher rate of postoperative retinal tears or retinal detachment.

10.2.2 Vitrectomy

Small-incision vitrectomy is highly dependent on the availability of an effective high-speed vitreous cutter. Cutting rates of up to 2,500 cuts/min are currently available [36–38], and recently cutters functioning at 5,000 cuts/min are being introduced. The higher speed allows safer trimming of tissue near the retinal surface with less chance of causing iatrogenic retinal breaks (Fig. 10.5). Other improvements in cutter design have been introduced. Compared to 20-gauge cutters, the 23and 25-gauge cutters have a larger port opening and one placed closer to the tip of the probe (Fig. 10.6). A larger port allows greater flow through a smaller needle diameter and reduces the aspiration force necessary [39, 40]. Placing the port closer to the tip of the cutter allows better tissue resection at the surface of the retina, thereby reducing the need for additional instruments such as scissors. The safety and increased control of cutting has been also enhanced by the recent introduction of duty-cycle control. The duty cycle reflects the amount of time that the cutter port remains open as the excursion of the inner needle blade travels

Fig. 10.5 The vitreous cutter should be able to cut and remove tissue close to the surface of mobile retina without causing iatrogenic retinal breaks

23-gauge

20-gauge

23-gauge port is

50% closer to the tip

Fig. 10.6 The port size has been increased and brought closer to the tip in many small-gauge vitrectomy cutters

across the port. By increasing the duty cycle at higher cutting rates, the cutter can remove more vitreous cleanly without increasing the aspiration force. Reducing the duty cycle will allow working more closely to the retina without drawing retina into the port. With the combination of these refinements, vitrectomy cutters have substantially improved and have become more efficient in removing vitreous and membranes without causing iatrogenic retinal breaks [41].

Early implementers of 25-gauge surgery had to adapt to the increased flexibility and fragility of the 25-gauge instruments [42]. As the eye is moved in different directions during the operation, the instruments would sometimes flex as the globe was moved into extreme positions to access peripheral zones of the vitreous. The flexibility would also affect epiretinal membrane peeling

because the instrument tip would sometimes move in the opposite direction at the extreme limits of the eye movement. The flexibility also limits the ability to access the vitreous base effectively in more complex retinal detachments, and this feature in combination with less cutting efficiency has limited the use of 25 gauge by some surgeons to simpler cases such as macular surgery. Increased stiffness of the 25-gauge cutters is being developed. The modifications include shortening the length of the cutting tip, and changes in selection of stainless steel material for the cutters [3, 43].

In many respects the use of 23-gauge vitrectomy reduces many of the issues relating to too much flexibility [3]. The performance of the vitreous cutters is similar to 20 gauge. The ability to cleanly cut and aspirate vitreous is measured by the flow of tissue which is similar to that attained in 20-gauge cutters. The smaller diameter of the tip with a port closer to the end are features that are actually more advantageous than 20-gauge cutters allowing safer access between tissue planes and small openings between membranes that are adherent to the retinal surface, especially encountered in proliferative diabetic retinopathy. The stiffness of instruments allows better ability to move the globe in a controlled fashion without bending them. The greater rigidity of the probe also allows easier ability to work at the vitreous base when scleral depression is used. It is also possible to use silicone oils with higher viscosities such as 5,000 centistoke. With 25-gauge surgery, generally 1,000 centistoke silicone oil has been advocated [44–48].

10.2.3 Adjuncts

A host of instruments made for small-incision vitrectomy have been developed. These include disposable and nondisposable forceps (Fig. 10.7), vitreous