Principles of vitreoretinal surgery: Techniques and technologies

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CHAPTER 58 Principles of vitreoretinal surgery

Techniques and technologies

Arnd Gandorfer

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Robert Machemer, the inventor of pars plana vitrectomy, wrote his seminal monograph in 1975 ¹, having done more than 500 vitrectomies at that time: ‘Pars plana vitrectomy is not an easy type of surgery. Some formal or informal training with an experienced vitreous surgeon is highly recommended.’ Bearing this in mind, it is clear that this chapter cannot teach vitreoretinal surgeons in practical terms. It should help, however, to deliver a basis to understand the principle techniques of current vitreoretinal surgery. This might be of advice in numerous clinical situations occurring during vitreoretinal surgery – a procedure simply called ‘vitrectomy’.

History and concept of vitreoretinal surgery

Fifteen years after his first publication on vitreous removal in a closed system, Machemer wrote, ‘the next step was treatment of the retina itself. It takes an unconventional mind to break major taboos …².’ It was Machemer himself who previously broke with the greatest taboo. When he left Germany to join the Bascom Palmer Eye Institute, vitreous removal was thought of subsequently resulting in phthisis. In Miami, Machemer came across Kasner whose revolutionary clinical work of the 1960s has proved that an eye can also function without the vitreous³. Machemer and Parel developed the vitreous infusion suction cutter (VISC), a 17-gauge instrument which combined suction, cutting, infusion, and illumination properties in one handpiece⁴. For the first time, it was possible to remove vitreous hemorrhage obscuring the optical axis, which otherwise had to be left untreated^5,⁶. The VISC, however, required a 2.7 mm sclerotomy, and the turbulence the infusion caused at the tip of the vitrectomy port made selective removal of the vitreous difficult. O’Malley and Heintz separated the infusion line from the vitrectomy probe and opened a third sclerotomy for additional instruments, thereby creating the 20-gauge three-port approach which still serves as a standard for vitrectomy almost 40 years later⁷. At the same time, and in collaboration with Oertli instruments (Berneck, Switzerland), Klöti in Zürich developed the ‘Klöti’-stripper for vitreous removal, which is still in daily use in many vitreoretinal services over the world^8,⁹.

The new and spectacular technique of vitrectomy seemed to offer the solution for all problems in the vitreous cavity ². Not only was the optical axis cleared of vitreous opacities, but also the retina itself could now be approached. Epiretinal membranes were removed, and traction was released from the retina¹⁰. In the mid-1970s, however, it gradually became more obvious that the expectations pinned on vitrectomy especially regarding the treatment of complicated forms of retinal detachment could not be fulfilled. At the same time, Scott in Cambridge used silicone oil as a tamponade of otherwise untreatable retinal breaks, a technique initially introduced by Cibis¹¹^–¹⁴. As a logical consequence of development, also knowing Machemer’s approach, Haut in Paris combined both techniques, i.e. vitrectomy and silicone oil as an internal tamponade, and the combination of vitreous removal with traction release and silicone oil tamponade has become the method of choice in the treatment of complicated forms of retinal detachment^2,¹⁵.

Epidemiologic considerations and terminology

Every year, roughly 600 000 vitrectomies are performed worldwide, most of them in industrialized countries. Numbers are expected to increase further, due to the growing incidence of diabetes mellitus and pseudophakic retinal detachment. The term pars plana vitrectomy refers to the surgical approach and is discussed later.

Fundamental principles, indications, and goals of surgery

Vitreous removal is the main goal of vitrectomy. It clears the optical axis and provides access to the retina. Vitreoretinal traction is released by vitreous removal which therefore should be as complete as possible. Vitreoretinal pathology such as epiretinal membranes can then be approached and removed. Subretinal fluid can be drained via the retinal tear if present or through a drainage retinotomy. Endolaser photocoagulation can be applied, and an appropriate tamponade is filled into the vitreous cavity at the end of the procedure. Indications for vitrectomy are summarized in Box 58.1.

Anesthesia

Vitrectomy can easily be performed under local anesthesia. A peri- or retrobulbar block is sufficient if there is no general obstacle against it, such as claustrophobia.

Anatomical considerations

The ciliary body is composed of two parts: the pars plicata and the pars plana. In the adult eye the pars plicata extends 2.5 mm posteriorly to the iris root and contains approximately 75 ciliary processes that produce aqueous humor. Posterior to the pars plicata, the pars plana extends 4.5 mm temporally and 3 mm nasally.

Sclerotomies for vitreoretinal surgery in adults are ideally located 4 mm posterior to the limbus in phakic eyes and 3.5 mm in pseudophakic or aphakic eyes. This avoids bleeding from the pars plicata as well as damage to the lens and to the retina. In infancy, the ciliary body and the pars plana do not attain adult proportions until age 7 years. Therefore the incision into the pars plana is made at 2.5 mm posterior to the limbus.

Surgical approach

For 20-gauge vitrectomy, a conjunctival incision is performed at the limbus both nasally and temporally (Fig. 58.1). Episcleral vessels are cauterized. A microvitreoretinal (MVR) blade is kept perpendicular to the sclera at 3.5–4 mm distance from the limbus as mentioned above. Care is taken to aim toward the midvitreous cavity to reduce the risk of hitting the lens or damaging the retina. The infusion cannula is typically placed inferotemorally. Therefore, a 7-0 Vicryl suture on a spatulated needle is placed in a mattress fashion at the sclerotomy site. A slip knot is tied and holds the infusion line in place. At the end of vitrectomy, the slip knot is loosened and the same mattress suture is used to close the sclerotomy.

Fig. 58.1 Surgical approach for three-port pars plana vitrectomy.

Care should be taken to ensure that the infusion line enters the vitreous cavity, and not the suprachoroidal space. The infusion cannula must be visualized through the dilated pupil by rotating the eye away from the surgeon while the infusion line is grasped and gently pushed toward the central vitreous cavity. Once the position of the infusion cannula has been confirmed, the infusion is turned on (Fig. 58.2).

Fig. 58.2 The infusion line is turned on after it had been visualized in the vitreous cavity.

Superonasal and superotemporal sclerotomies are placed approximately 120–160° apart to facilitate manipulation within the vitreous cavity and to allow access to the entire retina. Some surgeons use sew-on contact viewing systems; others fixate the eye by passing a suture beneath the recti. Given modern wide-angle viewing systems such as the BIOM and others, however, there is no need to do so, and the eye can be better rotated and manipulated without fixation.

Fundamental principles of vitreous removal

The vitreous is not an easy tissue to operate on. Given the gel-like structure on the one hand and its firm attachment to the retina at least in some retinal areas, special devices and techniques are required to remove the vitreous without damaging the retina. The vitreous can’t be removed either by suction alone or by simple cutting. However, the combination of both techniques enables the surgeon to perform an effective vitrectomy without the risk of visual impairment.

The vitreous cutter is used to apply both suction and cutting, and the combination of vitreous aspiration and subsequent cutting off the engaged vitreous is called a duty cycle. It is clear that during the time of cutting, when further aspiration is not possible due to the closed port, no aspiration is applied, and vice versa.

Steve Charles developed an engineering approach to vitreoretinal surgery ¹⁶. It is highly recommended to read his articles to get an understanding of some fundamental principles regarding the engineering aspects of vitreoretinal surgery. It is noteworthy, however, that in previous years one had to choose between high cutting rates associated with lower flow rates, and low cutting rates allowing high aspiration volume. Recent developments in pump systems and improved cutter design have made surgery more effective for the surgeon and safer for patient.

In my personal experience, three principles are of utmost importance:

1 Suction causes traction on the retina. To minimize traction, suction must be reduced or divided into smaller vitreous volumes aspirated, and the cutting rate must be increased. The higher the cutting rate, the less traction is generated, so the risk of retinal breaks is reduced. Therefore, to remove the vitreous base or firm attachments of the vitreous at the retina, as in proliferative diabetic retinopathy, high cutting rates are advantageous (Fig. 58.3).

2 Suction should be reduced to the minimum level needed to engage the vitreous. Therefore, a flow-controlled vacuum system is required to increase suction until the vitreous can be engaged and cut off. Low flow is needed to engage the vitreous near the retina, especially when the retina is mobile, and this is more possible using a peristaltic pump system.

3 Suction and cutting rate should be controlled independently. A dual linear approach enables the surgeon to apply suction until the vitreous is engaged in the vitrectomy probe, and cut at an adequate rate (Fig. 58.4). Alternatively, the vitreous base can be approached with high cutting rates, and suction can be adopted to engage the vitreous for high speed cutting, without causing undue traction on the retina (Table 58.1).

Fig. 58.3 For vitreous base removal, a high cutting rate and low suction is applied.

Fig. 58.4 For posterior vitreous detachment, high suction and a low cutting rate is necessary.

Table 58.1 Synopsis of cutting rate and suction parameters for dual linear management of the different surgical situations during vitrectomy

This easy and straightforward concept has only become possible by the development of highly effective pump systems and improved cutting characteristics of vitrectomy probes in terms of cutting rate and duty cycle. Today, even at high cutting rates such as 3000 cuts per minute (cpm), very effective removal of the vitreous gel is possible, and vitreous base removal at high cutting rates has significantly reduced surgically induced traction on the retina.

Induction of posterior vitreous detachment

Separation of the posterior hyaloid from the retina is one of the most critical steps during vitrectomy. As most vitreoretinal diseases treated by vitrectomy are caused or at least exacerbated by an abnormal attachment of the posterior vitreous cortex to the retina, removal of the vitreous cortex is mandatory in treating the underlying disease. There are several ways to surgically induce a PVD:

1 A silicone-tipped cannula is attached to suction through the aspiration line and is brought to the peripapillary region. When the vitreous cortex is engaged, the infusion line will stop dripping. Also, the ‘fish-strike’ sign may be seen, which refers to bending of the cannula as it is moved over the retinal surface. A barbed MVR blade or a pick may also be used to separate the posterior hyaloid from the retina.

2 Alternatively, the vitreous cutter is brought to the vitreous cortex at the nasal side of the optic disc, and suction is applied. To engage the vitreous cortex, which may only consist of a thin layer of collagen, a cutter with a small port that is located as far as possible at the instrument’s tip is necessary. Once the vitrectomy port is occluded, the vacuum rises and the cutter is gently pulled toward the center of the eye, thereby separating the vitreous cortex from the retina. For this maneuver, high suction and a very low cutting rate down to single cuts are necessary (see Fig. 58.4).

When the posterior hyaloid is completely detached, it will pop off the optic disc, and a Weiss ring will be seen. Care must be taken not to exert undue traction to the elevated vitreous cortex, as this may result in peripheral retinal tears. The hyaloid is removed from posterior to anterior using the vitreous cutter at high cutting rates to avoid traction on the retina.

Surgery of the vitreoretinal interface

Epiretinal membrane removal

Removal of epiretinal membranes (ERMs) has become a straightforward maneuver. If the membrane is thick and fibrotic, it can be grasped directly with an end-gripping forceps and lifted from the retinal surface. With thinner membranes, a bent 27-gauge needle or a barbed MVR blade may be safer than directly grasping the vitreoretinal interface. Once an edge of the ERM has been elevated, a front of elevated ERM is created. An end-gripping forceps is used to engage the ERM at multiple sites along the edge of residual adhesion, pulling the membrane toward the fovea. Grasping of the membrane should take place as close to the retina as possible.

Internal limiting membrane (ILM) peeling

Removal of the few micrometers thin basement membrane of Müller cells from the retina without impairing macular function is challenging without staining, because of the poor visibility of the ILM and its friable nature. After making an initial flap with a bent needle, or a barbed MVR blade or by using end-gripping forceps, the flap is grasped with the latter one and the ILM is removed from the macula in a circumferential fashion.

There has been some debate on the use of dyes for better visualization of the ILM. There is no doubt that, given a special dye is definitely non-toxic, it helps a lot in removing the ILM completely. However, care should be taken to use any dye available as this may cause permanent functional impairment, and any attempt should be made to minimize both illuminance and surgical time spent at the macula ^17,¹⁸.

Vitreous base shaving

Removal of the peripheral vitreous is of fundamental importance in vitreoretinal surgery. It is generally accepted to be less aggressive in simple macular surgery without peripheral retinal pathology. In cases of retinal detachment, PVR, PDR, giant retinal tears, and penetrating ocular trauma, however, complete vitreous base removal is mandatory and determines re-operation rates as well as the final success rate.

Surgery of the retinal periphery is not an easy job. Wide-angle viewing systems such as the BIOM enable a panoramic view on the retina and may greatly facilitate vitrectomy, but they do not allow for complete removal of the vitreous base. At present, only external depression and bimanual dissection under the coaxial light source from the operating microscope may provide best visualization and treatment of the vitreous base. Chandelier lights are very helpful in retinal detachment surgery as the surgeon can depress the eye and perform cutting or endolaser coagulation in the periphery, but they may be inadequate in the presence of extremely anterior membranes, in particular in anterior PVR cases.

When shaving the vitreous base to the level of the retina, high cutting rates and a peristaltic pump system are required. High rate cutting reduces traction on the retina, and the peristaltic pump allows us to adopt the flow as low as necessary to engage the vitreous only, and not cut into the retina. Extremely anterior membranes and anterior displacement of the vitreous base in case of anterior PVR may require the use of scissors to relief all traction. In this context, all sclerotomies need to be free of vitreous at the end of every vitrectomy to avoid iatrogenic traction emanating from incarcerated collagen fibers.

Reattachment of the retina

Perfluorocarbon liquids (PFCLs) are fluorinated synthetic liquids containing carbon–fluoride bonds. With specific gravities around 2, PFCLs are denser than water, allowing hydrokinetic manipulation of the retina ¹⁹. They are immiscible with water and are optically clear. The surface tension of PFCLs in water is high, providing an effective tamponading effect and preventing the liquid from passing through a retinal tear into the subretinal space, given no residual traction is present at the retina. Assisted by gravity, the PFCLs fill the vitreous cavity in a posterior-to-anterior fashion, tamponading the retina against the retinal pigment epithelium. This forces the subretinal fluid to pass through the peripheral tear into the vitreous cavity.

There are several important issues to remember when PFCLs are employed:

1 PFCLs are a helpful tool to reattach the retina and to hold it back at the posterior segment when the vitreous base is trimmed in case of retinal detachment, as gravity stabilizes the retina, and the retina becomes less mobile. In the case of proliferative vitreoretinopathy (PVR), this allows better visualization of the membrane formation and provides countertraction, facilitating membrane dissection.

2 PFCLs can reattach the retina only if no residual traction is present that hinders the retina from getting into contact with the retinal pigment epithelium. Otherwise, PFCLs will enter the subretinal space through the retinal tear. Therefore, a complete vitrectomy including the vitreous base and release of any traction must be performed before PFCLs can be employed at the level of the retinal tear.

PFCLs must be carefully removed at the end of vitrectomy, as they carry the risk of toxicity and may additionally damage the inferior retina by gravity.

Retinotomy and retinectomy

Relaxing retinotomy is primarily a procedure of last resort when the retina fails to relax sufficiently to permit reattachment, even after apparently complete removal of the vitreous and all epiretinal and subretinal membranes. Retinotomy (incising the retina) and retinectomy (excising the retina) are indicated only when retinal contraction cannot be relieved adequately by membrane dissection ^20,²¹. The main indications for retinotomy are PVR, proliferative diabetic retinopathy, giant retinal tears, and retinal incarceration due to penetrating trauma or surgically induced by previous vitrectomy at the sclerotomy site.

In practice, diathermy is used to cauterize the retina along the intended path of the retinotomy. Vertical or horizontal scissors are used to transect the retina peripheral to the diathermy marks. It is critical to check the lateral ends of the retinotomy for residual traction. In these cases, the retinotomy has to be enlarged until it reaches ‘healthy’ retina and all traction is released. If the retina is deemed to relax sufficiently, a meticulous air–fluid exchange is performed and all subretinal fluid is carefully removed at the edge of the retinotomy. Endolaser coagulation can be applied to the retinotomy edge. If the anterior retina is severely contracted, it may be resected with the vitreous cutter.

Fluid–air exchange and drainage of subretinal fluid

The intravitreal infusion cannula is connected via a three-way stopcock to the air pump of the vitrectomy machine. By rotating the stopcock, the surgeon can rapidly change from balanced salt solution to air. Air pressure within the system is maintained by the air pump at a surgeon’s preset level.

To allow the air to enter the eye, fluid volume must leave the eye simultaneously. This can be achieved by using a vacuum-cleaner needle or active aspiration. With the air pump set at 40 mmHg, fluid will readily be extruded through the vacuum-cleaner needle, which is a passive maneuver as no active suction is applied. Subretinal fluid can be removed if the tip of the vacuum-cleaner needle is placed at a retinal break or at a drainage retinotomy. In this case, the subretinal fluid will be drained through the needle to egress from the subretinal space, and is vented to the atmosphere.

Choice of tamponade

Air, longer-acting gas bubbles, and silicone oil are important tools in vitreoretinal surgery. They are all primarily used to provide an internal tamponade for retinal breaks.

In case of air and longer-acting gas such as sulfur hexafluoride (SF₆), perfluoroethan (C₂F₆), and perfluoropropane (C₃F₈), the high surface tension between the gas bubble and fluid prevents vitreous fluid from entering a retinal break ²². This allows the retinal pigment epithelium to absorb the subretinal fluid, reattaching the retina²³. All these gases are inert in the vitreous cavity, but the longer-acting gases in their pure form will expand as dissolved air from the bloodstream will enter the gas-filled vitreous cavity to equilibrate more rapidly than the gas can be absorbed²⁴. As a consequence, and because of the relatively inelastic sclera, the intraocular pressure will increase. If a complete filling of the vitreous cavity is wanted at the end of vitrectomy, a non-expanding mixture of gas has to be chosen to avoid a marked pressure rise (Table 58.2).

Table 58.2 Physical characteristics of gases commonly used in vitreoretinal surgery

Ultimately, however, each gas is absorbed into the bloodstream, at various time intervals. Absorption of an intraocular gas bubble is approximated by a first-order exponential decay equation. This means that the bubble volume decreases by 50% for each half-life ²⁵.

For patients who must travel by air or go into higher altitudes, the use of an intraocular gas bubble is inadvisable. Air travel is contraindicated in eyes with an intraocular gas bubble of more than 5%, as serious pressure increases occur, leading to pain, central retinal artery occlusion, and rupture of the globe.

Silicone oil

Unlike other vitreous substitutes, silicone oil may remain in the eye almost permanently. Silicone oil is a term used for a group of clear inert hydrophobic polymer compounds based on siloxane chemistry ²⁶. The viscosity is determined by the lengths of the polymer, ranging from 1000 to 12 500 cSt. Silicone oil has a density of 0.975 – less than that of water – and thus floats on vitreous fluid. Its refractive index is 1.4035, which is slightly higher than that of the vitreous (1.33). The interfacial tension of silicone oil is high (40 dyn/cm²), but less than that of the interface between gas and water (70 dyn/cm²).

Silicone oil is used when a more permanent tamponade is required which cannot be provided by gas. This may be important when an initially closed retinal break is expected to reopen, particularly in cases of proliferative vitreoretinopathy. In addition, silicone oil is considered for patients who cannot maintain the position required for effective intraocular gas tamponade. A third rational to use silicone oil is to limit postoperative vitreous hemorrhage in diabetic eyes as blood and oil do not mix, and the optical axis is kept clear by the oil, allowing retinal examination and laser photocoagulation if needed.

The complications of the use of silicone oil are mainly related to its migration into the anterior chamber. This may especially occur in pseudophakic patients with zonulolysis and in aphakic eyes, and cause keratopathy if left untreated. Aphakic eyes need an inferior peripheral iridectomy to enable the aqueous to pass from the posterior chamber to the anterior chamber when the silicone oil occludes the pupil ²⁷. Aphakic patients should be advised not to stay in supine position to avoid the oil entering the anterior chamber.

Silicone oil causes refractive changes. Phakic patients become more hyperopic when filled with silicone oil because the refractive index of silicone oil is higher that that of vitreous, and a negative lens effect occurs with the concave silicone oil surface at the back of the lens. Aphakic eyes experience a myopic shift, which is caused by the convex anterior surface of silicone oil.

Silicone oil removal is usually performed 3–12 months after filling, depending on a stable looking retina and other factors such as lens status, elevation of intraocular pressure, and development of cataract or epiretinal membranes.

Complications and postoperative care

Intraoperative complications of vitrectomy include bleeding from retinal vessels or from sclerotomy sites. This mainly occurs in diabetic eyes and can be controlled by endodiathermy. Retinal detachment can occur after vitrectomy in up to 5% of cases, and is caused by peripheral retinal tears. The use of trocar systems helps to decrease this percentage. Endophthalmitis is a rare complication with less than 0.05%, comparable to other intraocular procedures. Postoperative care includes steroids and antibiotic eye drops for the first week.

Current developments

Over the last several years, there has been a major shift in how vitreoretinal surgery is performed. In 2002, 25-gauge vitrectomy was introduced, and in 2003, Eckardt promoted 23-gauge vitrectomy ^28,²⁹. Both approaches allow for transconjunctival sutureless vitrectomy, associated with increased patient comfort and reduced operating times. While early 25 g instrumentation was limited by excessive flexibility, subsequent generations became more rigid. However, when instruments comparable to 20 g systems are needed, 23 g systems clearly show advantages compared with 25 g, facilitating manipulation of the eye during surgery and shaving of the peripheral vitreous gel.

There had been some concern about postoperative hypotony in sutureless vitrectomy, and about endophthalmitis rates. Small gauge instrumentation has significantly improved since its introduction, and still continues to evolve. Switching from 20 g to 23 g vitrectomy also has a learning curve, even for experienced surgeons, with respect to incision design, removal of the vitreous, especially in the periphery, and closure of the eye at the end of surgery by adequate trocar removal.

The most recent achievement in vitreoretinal surgery is the combination of pharmacology and vitrectomy, or even replacing vitrectomy by an intravitreal injection ³⁰. Mainly plasmin and microplasmin (ocriplasmin) have shown their potential to create PVD and to liquefy the vitreous gel³¹. Several clinical studies were undertaken to investigate the concept of pharmacology-assisted vitrectomy³². These studies show that a reasonable number of macular hole patients won’t need vitrectomy any more, as vitreofoveal adhesion can be cleared by a simple intravitreal injection, with subsequent macular hole closure. It remains to be defined in the future which patients are eligible for this type of treatment, and which will still need vitrectomy.

Drug delivery systems will also change our current treatment modalities. Given their minimally invasive nature, vitreoretinal diseases may be treated earlier before advanced stages of disease have developed. This is of special importance in diabetic retinopathy and in proliferative vitreoretinopathy where visual function remains limited, despite anatomical success. Induction of PVD at an early disease stage may prevent further deterioration by relieving traction and by enhancing vitreous oxygen levels, and neuroprotective agents may hold the promise of maintaining or even improving visual function when mechanical manipulation of the retina has reached its limit ³³. The next step in vitrectomy is improving retinal function by introducing pharmacological means.

References

1 Machemer R. Vitrectomy, a pars plana approach. New York, San Francisco, London: Grune & Stratton; 1975.

2 Zivojnovic R. Silicone Oil in Vitreoretinal Surgery. Dordrecht: Martinus Nijhoff/Dr W. Junk Publishers; 1987.

3 Kasner D, Miller GR, Taylor WH, et al. Surgical treatment of amyloidosis of the vitreous. Trans Am Acad Ophthslmol Otolaryngol. 1968;72(3):410-418.

4 Machemer R, Parel JM, Buettner H. A new concept for vitreous surgery. I. Instrumentation. Am J Ophthalmol. 1972;73(1):1-7.

5 Machemer R, Norton EW. Vitrectomy, a pars plana approach. II. Clinical experience. 1972;10:178-185.

6 Machemer R, Norton EW. A new concept for vitreous surgery. Indications and results. 1972;74(6):1034-1056.

7 O’Malley C, Heintz RM. Vitrectomy via the pars plana: a new instrumentation system. Trans Pac Coast Otoophthalmol Soc Annu Meet. 1972;53:121-137.

8 Klöti R. [Vitrectomy. I. A new instrument for posterior vitrectomy]. Albrecht Von Graefes Arch Klin Exp Ophthalmol. 1973;187(2):161-170.

9 Klöti R. [Vitrectomy. II. Surgical technique with the vitreous stripper (author’s transl)]. Albrecht Von Graefes Arch Klin Exp Ophthalmol. 1973;189(2):125-135.

10 Kampik A, Kenyon KR, Michels RG, et al. Epiretinal and vitreous membranes. Comparative study of 56 cases. Arch Ophthalmol. 1981;99(8):1445-1454.