preventing vitreous loss in the first place. Assuming that this is ultimately impossible in some cases, the next best strategy is to minimize the potential vitreous lost following capsule rupture.

THE INFUSION/ASPIRATION BALANCE

Phacoemulsification is performed, for all practical purposes, in a closed system. There are two incisions, each one approximately the correct size for the instruments being used. There is a small amount of fluid leakage at each of these incisions, but not a lot. Fluid comes into the eye by means of a gravitational feed from the infusion bottle. Flow from the bottle continues until the intraocular pressure equals the gravitational pressure in the bottle. At this point, all flow stops.

When the phacoemulsification system aspirates fluid and material from the eye, the intraocular pressure lowers. A passive flow of fluid from the irrigation bottle begins to restore the original pressure. If the irrigation and the aspiration levels are approximately equal, a constant-state intraocular pressure will be maintained.

The constant-state intraocular pressure reduces the possibility of vitreous coming forward. Theoretically, if the posterior capsule opens, the positive pressure in the anterior chamber will hold the vitre-

ous back. The vitreous can come forward only if the aspiration system continues attracting vitreous up, into the anterior chamber, or if the aspiration reduces the intraocular pressure faster than the irrigation can restore it. In the absence of an adequate anterior chamber intraocular pressure, the vitreous will move forward. If aspiration is stopped when the posterior capsule opens, the intraocular pressure in the anterior chamber will remain stable and equal to the infusion pressure. This will hold the vitreous back, preventing vitreous loss (Fig. 19–1).

CAPSULAR TEARS

Although the above description is the ideal situation, it is frequently unobtainable because the posterior capsule opens invisibly. The capsular tear may be hidden behind the nucleus, or some fluffy cortex, and cannot be seen until further material has been removed, exposing the rent in the capsule. Cortical strands or capsular folds that may have a similar linear appearance to a tear may mask a tear in the capsule when one develops. However, if aspiration is terminated immediately after identification of the capsular tear, the amount of vitreous that can come forward is significantly restricted. This is due to maintenance of the closed chamber. All of the incisions are plugged with instruments! Vitreous can

go no further than the anterior chamber. There is no way for it to escape from the eye. The potential for massive vitreous loss is essentially nonexistent (Fig. 19–2).

Next, when an opening in the posterior capsule is recognized, the phaco tip should not be removed from the eye. The machine should be continued in position 1, irrigation. This maintains a positive pressure in the anterior chamber and restricts vitreous movement. Without aspiration, vitreous cannot be aspirated from its usual location. The second instrument should be removed from the eye and replaced immediately with a viscoelastic cannula tip. The anterior chamber is then generously filled with viscoelastic. This will hold the vitreous back. The phacoemulsification tip can thereupon be removed from the eye. The viscoelastic maintains a powerful anterior chamber pressure that prevents escape of the vitreous through the rent and its tendency to follow the phaco tip out of the eye (see Chapter 29, Fig. 29–3).

The posterior capsule is also of assistance to restrain vitreous movement. Vitreous can come forward only through the breach in the capsule. Therefore, a small opening will permit less vitreous prolapse than a large opening. The capsule is essentially a diaphragm within which is a defect, a site for vitreous transit. If the eye is not pressurized prior to the removal of the phaco tip, anterior chamber and intraocular pressure will drop, and increased amounts of vitreous will come forward. In addition, the violent passage of vitreous through the rent can extend it. This permits the remaining vitreous to pass more easily and may destabilize the remaining capsular support for later intraocular lens fixation.

FIGURE 19–2 Posterior capsule rupture. There is a limited opening in the posterior capsule. The rest of the capsule is intact and holds vitreous back. The phaco incision is sealed by the vitrectomy tip.

VITREOUS AS A SLINKY TOY

The vitreous body may be thought of as a semielastic material. This can be easily visualized by pulling on the vitreous with a cellulose sponge. It stretches. Next cut the strand of the vitreous. Once cut, the anterior portion of the strand retracts into the eye. This elasticity of the vitreous can be imagined to be similar to a Slinky toy. The coils of a Slinky toy are semi-elastic as well. If one pulls on the top few coils of the Slinky, it stretches. The effect is localized only to the coils near the ones being stretched. No tensions are exerted throughout the remainder of the toy. This is basically what takes place during vitreous loss in phacoemulsification. If the amount of anterior vitreous that is disturbed is limited, tensions are not exerted throughout the vitreous cavity. Therefore, traction forces are not exerted on the vitreomacular interface or on the vitreous base. Thus, the likelihood of developing cystoid macular edema or retinal tears and detachments is minimized.

This is in direct contrast to the vitreous loss that occurs during intracapsular surgery or nucleus expression surgery. In these situations, by virtue of the large incision, there is the potential for a tremendous amount of vitreous to emerge from the capsular tear and progress out of the eye. This is analogous to forceful pulling on all of the coils of the Slinky toy, extending tensions all the way down to the end of the toy. In the eye, this would be similar to extensive vitreous loss. The transmission of the movement of the vitreous produces definite traction exerted at the vitreomacular interface and at the vitreous base. This can be considered to be the fundamental cause leading eventually to macular edema and peripheral retinal complications.

Our objective is to mimic, as much as possible, the unstretched Slinky. A few coils of the Slinky will stretch when there is a limited amount of vitreous prolapse, as is seen with vitreous loss during phacoemulsification. But the closed system will permit only that few. We must do everything possible to refrain from taking steps that will convert this limited stretch into a major one.

Therefore, in order to minimize complications within the eye, we should remember one motto: “Don’t stretch the Slinky.”

PERFORMING THE VITRECTOMY WITH

COAXIAL INFUSION

Traditionally, a mechanical vitrectomy following vitreous loss is carried out using a vitrectomy tip that has coaxial infusion. This is similar to the tip used for phacoemulsification. The concept behind this