coating of the cartridge is effective and clinically does not show any significant difference. In any case, the VES should not be used in excessive quantities.

8.2.6 Techniques of Sub-2 Injection

It is important to note that an injector–implant pair makes a specific set that cannot be compared and requires training even if the surgeon has prior experience [9].

Irrespective of the injection technique (Fig. 8.25), the construction quality of the corneal incision as well as the compatibility between the incision size before injection and the characteristics of the cartridge contribute to the reliability and reproducibility of injection. Therefore, it is necessary to study the characteristics of the cartridge before every injection and follow the manufacturer’s recommendations that consider various parameters and a number of in vitro and in vivo tests. It is always interesting and useful to watch a video demonstration and perform the first injection in the presence of a competent delegate after having handled the injector. For most of the injectors, the loading and preparation can be carried out by a trained assistant.

Reduction in incision size can possibly put mechanical stress on the corneal incision during injection. This stress could have immediate postoperative consequences like fault in sealing, hypotonia, an increased

a

risk of endophthalmitis or astigmatism, damaging the vision quality.

The Wound-Assisted Technique (WAT) is an injection technique that allows the smallest size of post injection incision (Fig. 8.26). The cartridge end is only kept against, and not inserted through, the incision. This technique is used assuming that the dimensions of the injector cartridge and the incision are similar

Fig. 8.26 Principles of injection on the wound assisted technique

b

enough to create a continuous tunnel of implantation through which the implant will pass. Practically, the internal diameter of the cartridge should be less than, or equal to, the incision size. The injection procedure should not widen the incision excessively (measuring the dimensions of incision pre and post injection using the calibrators allows evaluating clinically the widening of incision caused by the implantation). However, the injection should not cause “fish mouth” (apertognathia of incision) in order to assure perfect sealing in the postoperative phase. The study results confirm that the currently available injectors meant for microincision do not significantly modify the size and architecture of the corneal incision if appropriate dimensions are maintained.

8.2.6.1Visco-Injection Using Wound-Assisted Technique

Today, the use of the wound-assisted technique to insert an IOL through an incision size between 1.5 and 2 mm is indispensable.

8.2.6.2 Incision Construction

•The construction of the incision should be precise and its size should be as per the injection system in order to avoid any incision leakage after the injection.

•A wrong initial architecture of the incision can cause a gap that could lead to a post operative ocular hypotony. In addition, it could possibly induce a retrograde flow of extraocular fluids through the incision towards the anterior chamber, a potential cause of endophthalmitis. A square-shaped incision seems more resistant and stable than a rectangular incision. A minimum of 2 mm length of incision could help in achieving self-sealing and stability.

•Theoretically, the best-suited incision could be a square-shaped three-plane incision. The first plane or preincision is perpendicular to the corneal plane and its borders can possibly join in case of ocular hypotony. The second plane is a minimum of 2 mm deep in the stromal plane capable of resisting ocular hypertension. Finally, the third plane is induced by using the knife vertically in order to

minimize the descemetic traumatism by a penetration in the anterior chamber perpendicular to the corneal plane.

•This well-structured incision has better resistance not only to thermal or mechanical stress of phacoemulsification by microincision irrespective of the technique used, but also to injection stress.

8.2.6.3 Pressurization of the Anterior Chamber

•The anterior chamber is pressurized by injecting a viscoelastic substance which should preferably be cohesive. In fact, the main objective during injection is creating spaces and protecting the cornea and tissues, mainly by maintaining spaces during the maneuvers. A good zero-shear viscosity to create a deep anterior chamber and a high elasticity to amortize the traumatisms are the physical properties required. Cohesive VES attain these objectives better.

8.2.6.4 Loading the Cartridge

In the case of hinged cartridges, a moderate quantity of VES is injected into the grooves of the loading chamber and at the beginning of the injection tunnel. Theoretically, methylcellulose (HPMC) is best suited for this purpose. In practice, any VES can be used for visco-injection. The monobloc hydrophilic acrylic implant is taken out of humid conditioning at the last minute and placed under the locks in the middle of the loading chamber (Fig. 8.27a). Generally, these implants should not remain in nonhydrated conditions for a long time. The IOLs should always be placed in the right direction as most of the implants are angular and aspherical. The posterior angle will be directed towards the rear. It is necessary to position the haptics in the grooves and under the locks and then close the hinges without blocking or affecting any part of the implant. The two jaws of the forceps press the borders of the optic simultaneously while the hinges are folded (Fig. 8.27b). Before folding the hinges, the IOL should be fully loaded and the haptics should not get stuck in the hinges. In some cartridge models, a click sound can signal the closing of hinges which also means that the IOL is correctly loaded. For those models, the click is indispensable before the injection.

8.2.6.5 Loading the Injector

•The cartridge is placed in the injector body (Fig. 8.27c) and should be brought into the chamber by pushing downwards. It should also be pushed backwards to be in contact with the posterior wall of the injector body. This operation is necessary for correct positioning of the soft plunger while inserting it in the loading chamber. As both the injector and the cartridge are transparent, the position of the IOL and the cartridge can be easily verified before the insertion of the plunger tip.

8.2.6.6 Insertion of the Plunger Tip

•The plunger tip is made of silicone and, therefore, is flexible. It should be inserted accurately in the loading chamber. This insertion should be carried out slowly and under observation so as to let the plunger

tip deform and acquire a convenient position in the injection’s axis (Fig. Fig. 8.28). An incorrect position of the soft tip can cause the bursting of the cartridge. The implant is pushed towards the central one-third of the injection tunnel under observation. The tip starts stretching as the diameter reduces. Since the injection is hydraulic, insertion becomes easy. An excessive resistance could mean anomaly, and hence the injection should be stopped. The injection should be carried out in one go without any latent period.

8.2.6.7 Injection in the Anterior Chamber

The injection can be carried out in favorable conditions after sufficient pressurization of the anterior chamber and proper loading of the IOL and the cartridge in the injector body. The cartridge bevel is then set under the anterior side of the incision. The injector

is directed almost horizontally. In fact, the incision plane of the tunnel, parallel to the stroma of the cornea, should be followed, as the incision becomes an extension of the bevel. Any anomaly in the cartridge direction (mainly excessively vertical) can result in the blocking of the IOL in the stroma of the cornea, thus leading to an injection outside the globe or lesions on the incision. If the bevel direction is correct, the counter pressure is not always indispensable although it is useful to stabilize the globe during injection. This counter pressure can be obtained using another instrument or from an irrigation catheter through the second incision. The injection can be carried out when the bevel is correctly placed and directed. The pressure on the plunger should be firm and continuous but not excessive (Fig. 8.28b). Any interruption in the insertion has the risk of blocking the IOL in the incision, which is always difficult to extract (the IOL should be gently pulled out using forceps by letting it deform). The pressure on the plunger is released as soon as the IOL goes into the anterior chamber, as the soft plunger continues to deform and advance even after the insertion. If the soft plunger comes out excessively in the anterior chamber, it can get blocked in the incision. If this happens, it should be taken out slowly to let it stretch. Most of the IOLs available for microincision display good stability during injection, especially as the cartridges are well designed, and get unfolded flat in the anterior chamber.

8.2.6.8 Positioning the IOL in the Capsular Bag

The IOL can be positioned at once in the capsular bag during the injection using a second instrument for counter pressure.

It can also be positioned subsequently in the capsular bag. This easy but indispensable operation should be carried out accurately using a micro-manipulator as the sac is filled with viscoelastic substance. The instrument gently presses the optic’s anterior side and, if required, rotates gently in order to place it in the capsular sac. The intra saccular position of all the haptics should be verified before and after removing the viscoelastic substance as these IOLs should necessarily be in an intra saccular position (Fig. 8.28c). Wrong positioning risks subluxation and could require a second intervention to position the IOL correctly.

8.2.6.9 Removing the VES

The VES in the front and the back of the IOL should be removed after implantation in order to restrict the risk of postoperative ocular hypertension and a possible posterior capsular opacification by making a close and premature contact between the posterior side of the IOL and the posterior capsule. The physical characteristics that help attaining these objectives are high pseudoplasticity and cohesion, which help in aspiration. Therefore, cohesive VES are best suited for the purpose. It is possible to use different techniques and care should be taken not to let the IOL move from the capsular sac. We use a direct aspiration of VES in the front and back of the IOL with high values of maximum vacuum and pump output.

8.2.6.10Thin Roller Injector

The thin roller injector is a specific single-use injector. After assembling the single-use injector, the cartridge end is immersed in the irrigation solution. The hydrated implant is then inserted into the given slit. The implant is rolled in the cartridge’s loading chamber by pressing the cap. This compression is maintained for 10 s. No viscoelastic substance is required. Once the implant is rolled, it should be injected quickly through an incision of 1.5–2 mm. After filling and pressurizing the anterior chamber with a viscoelastic substance, the cartridge end is put against the incision border. The implant is injected directly so that it does not unfold in the incision and get blocked. Another instrument is used simultaneously to create a counter pressure (Fig. 8.29a–c). The rolled implant reaches the anterior chamber and unfolds slowly. It should be verified that it unfolds in the correct direction. In fact, this IOL is not symmetrical, and the spurs help in verifying that it is positioned in the correct direction. After verification, the implant is placed in the capsular sac (Fig. 8.30a–c).

8.2.6.11Conclusion

Injecting through a microincision requires the development of intraocular lenses, injectors and appropriate and innovative injection techniques.

Take Home Pearls

ßTo achieve a wound-assisted injection of a sub-2 mm IOL, a rigorous technique is manda-

tory. If the instructions are properly followed, the wound-assisted technique is a reliable and reproducible sub-2 mm injection technique.

ßThe IOL must be compatible with the sub-2 mm micro incision (material and design) with

regard to both the incision size and the postoperative behavior.

ßThe cartridge and the injector must be reliable, adapted to the IOL and compatible with the

micro-incision wound-assisted injection technique (internal diameter less than 1.4 mm).

ßDuring the injection, the injector is angled close to the horizontal plane so that it follows

the incision plane.

ßThe pressure on the plunger must be continuous. Any interruption during the injection has

the risk of blocking the IOL in the incision.

The intraocular lenses need to resist the high mechanical stress and preserve their integrity, optic quality and a postoperative behavior similar to the best lenses available.

The injectors should be reliable, competitive and ergonomic. They should protect the IOL and the incision structure.

The implantation techniques adjusted with every injector and implant should be systematic, reliable and reproducible. Today, the Wound Assisted Technique is the most commonly used technique as it allows making the smallest incision.

The last few years has seen significant progress in these three domains that allowed reducing the incision size considerably, towards the patients’ quality of vision and a phaco refractive approach of crystalline surgery.

Time will surely confirm this trend to reduce the incision size.

Only the progress made in these three domains shall make it possible.