Ординатура / Офтальмология / Английские материалы / The Art of Phacoemulsification_Mehta, Alpar_2001

Subsequent aspiration is best done with the use of viscoelastic (dry aspiration). Fill the chamber with viscoelastic and with the left-hand; enter via a side port, using the bimanual aspirator (no irrigation). Gradually aspirate out the fragments, refilling the bag every time the chamber shallows. The big advantage of using viscoelastic is that it compresses the capsular tear opening preventing vitreous from coming out, and at the same time opening up the bag permitting easy access to the cortical remnants.

In case the capsular tear has extended, as the surgeon, inattentively, did not recognize the tear in time, it is important to do a shallow anterior vitrectomy by placing the vitrector behind the posterior level (or to put it another way, deeper than the posterior capsule) and then do a little more vitrectomy to remove the vitreous at this location. Try not swirling with vitrector to the sides, as that would then damage whatever capsule is left behind. Subsequently either under air or with the use of Healon which acts as a tamponade onto the vitreous, holding it back, the implant loops can be moved into the bag to achieve a good fixation or alternatively, achieve a sulcus fixation by placing the IOL in front of the anterior capsule.

CONCLUSION

Irrigation/aspiration is an important step of cataract surgery and needs to be given the full, undivided attention of the surgeon. Caution and careful evaluation of the posterior capsule will go a long way in preventing complications from developing. Most phacoemulsifiers possess positive venting abilities and good chamber maintenance with excellent fluidic ability so as to permit this procedure to be done in a rapid, controlled fashion that is efficacious, yet completely safe.

FURTHER READING

1.Mehta KR: Pitfalls encountered in 1500 consecutive posterior chamber implant. All India Ophthl Soc Proc 165-66, 1986.

2.Mehta KR: Phacoemulsification cataract extraction with foldable IOLS—first 50 cases. All India Ophthl Soc Proc 56-60,1989.

3.Mehta KR: Clear corneal phaco with injectable silicone IOL proc. All India Ophthl Soc Proc (Mumbai) 1995.

4.Mehta KR: Phaco-levitation—a peaceful way. All India Ophthl Soc Proc (Chandigarh) 1996.

5.Mehta KR: Lollipop phaco cleavage—a new technique for hard cataracts. All India Ophthl Soc Proc (Bangalore) 1991.

6.Mehta KR: Phaco with flexible IOL—is it a step forward? All India Ophthl Soc Proc (Bangalore) 1991.

7.Mehta KR: SICS nonphaco—hydroexpression with an irrigating vectis. Proc of SAARC Conference, Nepal, 1994.

8.Mehta KR: Management of subincisional cortex in small incision cataract surgery (SICS). Proc of SAARC Conference, Nepal, 1994.

9.Mehta KR: Methylcellulose induced sterile endophthalmitis following phacoemulsification. Proc of SAARC Conference, Nepal, 1994.

10.Mehta KR: The new multiport phaco tip for safer, more effective phacoemulsification, with virtually zero capsular damage. Proc of SAARC Conference, Nepal, 1994.

In a few months we will celebrate the 50th anniversary of the first intraocular lens (IOL) implantation performed by Harold Ridley on November 29, 1949, at St. Thomas Hospital in London.1

In 1984 Thomas Mazzocco implanted the first foldable IOL made of a silicone elastomer.2 Foldable IOLs are the most preferred introcular lenses. According to the 1997 ASCRS survey, 79% of the respondents said they were interested in putting foldable intraocular implants.3

The growing number of foldable IOLs can be confusing. If the main chemical components are analyzed, IOL materials can be divided into two groups; acrylate/ methacrylate polymers (Table 24.1) and silicone elastomers (Table 24.2). The first group contains rigid PMMA IOLs and the so-called soft acrylic and hydrogel lenses. The second group of IOLs are made of foldable polysiloxanes.The obvious use of small incisions for cataract surgery—low induced astigmatism, fewer postoperative complications, possible less inflammation, and faster rehabilitation of the patient—have encouraged the surgeons to use foldable IOLs.

Silicone as an optic material came into vogue for small incision surgery because it can be folded, has good memory, is biocompatible and suffers little surface trauma.4,5 Silicone can be folded at 3 to 9 O’ clock meridian which is a twostep implantation or 6 to 12 O’clock meridian which allows the lens to be placed in a single maneuver. Posterior capsular opacification (PCO) occurs much later with silicone as compared to PMMA IOLs since the silicone optic is much thicker than the PMMA optic and therefore allows posterior capsule to be in close contact with the posterior side of the optic.7 Optic thickness is also responsible for an increased amount of pittings on the IOLs during Nd:YAG capsulotomy. However,

is 11.5 mm, which is 1.0 to 1.5 mm longer than the average ciliary sulcus or capsular bag. It fixates in the bag by fibrosis of the anterior capsular rim and posterior capsules around the mini-loops. Misplacement does not appear to cause serious decentration, which would be expected since the haptic-anchor plate is semi-rigid and will flex but not buckle, and the semi-rigid silicone component of the lens is the same length as the space into which it is being implanted. It is suitable for placement in the bag with or without tears in the anterior capsular

rim or posterior capsule and, if necessary, for placement in the sulcus. This eliminates the need for a back-up lens for each implantation and means the lens can be implanted by less skilled surgeons and by those who do not perform phacoemulsification.11

Toric IOLs are also available in plate haptic design. These are cylindrical lenses used for correcting upto 3.5 D of astigmatism.

The Memory Lens is a flexible IOL, the optic of which is made of poly-HEMA, thermoelastic acrylate material and polypropylene loops.12 The Memory Lens copolymer has a high refractive index (1.47), is hydrophilic and has a glass transition temperature of 25°C. The lenses have to be stored in a refrigerator ay 8°C. The prerolled lens slowly unfolds intraoperatively, taking about 10 to 15 minutes to unfold. Hydrogel lens is also available which is folded by the sure fold system (Figs 24.2 and 3) and is not a prerolled lens unlike the Memory Lens.

The AcrySof lens is made of acrylate polymer. An acrylate polymer material has a relative high refractive index of 1.47, higher than that of silicone (1.41 to 1.46) or hydrogel (1.43) and comparable to that of PMMA (1.49). Acrylate has the highest index of refraction of any approved IOL. Thus, these lenses are the thinnest and should have the lowest bulk for a given dioptric power, edge and thickness. Soft acrylic IOL unfolds more slowly than other foldable lenses, so creases in the optic disappear over a longer time than creases in other foldable

258 THE ART OF PHACOEMULSIFICATION

Fig. 24.4: AcrySof IOL

being inserted

Fig. 24.5: AcrySof IOL

being inserted

materials (Figs 24.4 and 5). In addition, cracks may form in the optic after prolonged and repeated folding procedures. Glistenings were associated when these lenses were packaged in Acrypak and not in the original wagon wheel and are highly influenced by temperature changes. Acrylic lenses have flexible polypropylene/ PMMA loops which provide satisfactory fixation and centration in the capsular bag, facilitate the aspiration of the viscoelastic from the bag at the end of surgery, and allows the lens to be implanted where surgery is complicated by a capsular tear or zonular rupture.13 It can also be implanted in the presence of silicone oil in the vitreous cavity. In this situation, implantation of a silicone IOL is contraindicated because a Nd:YAG capsulotomy cannot be done if the posterior capsule is in contact anteriorly with a silicone lens and posteriorly with silicone oil.

Acrylic lenses produce considerably less amount of PCO (Fig. 24.6). This is because acrylic is biocompatible and bioactive material. According to the sandwich theroy,13

260 THE ART OF PHACOEMULSIFICATION

An Array IOL is a multifocal biconvex, 3-piece silicone IOL with polypropylene haptics and 6 mm optic diameter. A multifocal function is produced by five annular aspherical zones of refraction incorporated into a 4.7 mm diameter of the anterior surface.14 Each zone contains continuous curves of power with a 3.5 D range. Distribution of light varies with the pupil size as follows.

•50-60 % light is allocated to distance focus

•22-38 % at near focus

• 15-18 % at the intermediate focus.

Array IOLs have lower contrast sensitivity and more glare than PMMA IOL, this however may not be clinically significant.

EXPLANTATION OF IOLs

Several possible events which may require explantation of IOLs include broken or bent haptics, linear glistenings on the lens that may give visual disturbances, damaged IOL secondary to folding or insertion problems and wrong IOL power.15 Unfortunately, the small incisions that foldable IOLs are placed through make it impossible to explant them in toto without enlarging the incision. Other clinical situations which may require early removal of foldable IOL include visually significant lens deposits or chronic endophthalmitis not responsive to medical therapy.

Intraocular bisection of silicone IOLs is problematic because once in the eye, the lenses are slippery and resist any manipulation or folding attempts.15 Koch’s bisector can be used which allows easy and controlled cutting.15 Unlike silicone IOL, acrylic surfaces are sticky and can therefore be cut into two halves with scissors or folded and explanted through the initial incision.16

FUTURE OF FOLDABLE IMPLANTS

Future would consist of various modifications in the current foldable lens materials and designs to induce greater biocompatibility, accommodation and inhibition of PCO.17 This would also include the modifications in the existing holders and folders (Fig. 24.8).

The collamer for aphakic lens material that has a great potential and is undergoing investigative study. This is the material that has been used in the implantable contact lens. The collamer elicits very little PCO or anterior capsulorrhexis metaplasia and fibrosis. It is a user-friendly material. Materials for multifocal IOLs most likely will evolve to include the use of polymer comixtures that can give a gradation of refractive indices. Polymer engineering includes creating a central 3 mm of a lens from one polymer with a specified refractive index and the peripheral 3 mm of it with a different polymer and a different refractive index. If the lenses are manufactured with the two polymers that has a determined rate of diffusion into each other, such that a gradation of refractive indices from the periphery to the center of the lens exists, this could produce multifocality.

Other possibilities include “smart molecules” or isomers that might change their refractive index with the application of certain wavelengths of light. A laser

FOLDABLE INTRAOCULAR IMPLANTS 261

Fig. 24.8: Holders and folders for foldable IOLs

at some wavelength on the implant can alter its refractive index and, thereby, its power. Stereoisomers might be another option for aphakic lenses. Stereoisomers, with different refractive indices in equilibrium with each other in a given lens can undergo a shift in equilibrium with delicate application of laser light to alter the equilibrium mixture, and thus the refractive index and achieve multifocality, perhaps astigmatism correction, or just a new refractive power.

The truncated edge of the AcrySof is now largely felt to be responsible for the lower incidence of PCO seen with the implant. Another modification for such a lens might be a matte finish to its equatorial region. The matte finish might help to reduce the undesirable optical images reflected from the AcrySof’s sharp edge. In future, idea is to implant two separate devices. An endocapsular ring may be placed into the equatorial portion of the capsular bag. If this ring is designed with a truncated edge, it is possible to inhibit cell migration, as with the AcrySof lens. Another interesting aphakic design involves a new accommodative IOL. The lens itself is positioned in the most posterior part of the capsular bag space and against the vitreous face. It is fixed into place by instilling atropine for 3 weeks postoperatively. The lens is 11.5 mm in length with polyimide loops for fixation and centration. It has hinges at the optic-haptic junction that allow the optic to freely move backwards and forwards. Only 1 mm of forward movement equals 2 D of refractive changes.

The combination of polymer materials and innovative designs offers almost limitless possibilities in aphakic technology.

R E F E R E N C E S

1. Ridley H: Intraocular acrylic lenses. Trans Ophthalmol Soc UK 71: 617-21, 1952.

2. Mazzocco TR: Early clinical experience with elastic lens implants. Trans Ophthalmol Soc UK 104: 578-79, 1985.

3. Leaming DV: Practice styles and preferences of ASCRS members-1997 survey. J Cataract Refract Surg 22 : 931-39, 1996.