Ординатура / Офтальмология / Английские материалы / Mastering theTechniques of Lens Based Refractive Surgery (Phakic IOLs)_Garg, Alio, Dementiev_2005

Figure 20.5: Humanoptics accommodative 1CU IOL (Humanoptics, Germany)

contraction of the ciliary muscle and relaxation of the zonules. The resultant mechanical energy stored in the capsular bag is transmitted to the implant during accommodation. This leads to a specific posterior deformation of the implant that increases its optical power, allowing near objects to focus on the retina. When accommodation ends, the ciliary muscle relaxes and zonular tension increases. The Capsular bag is stretched and the Accommodative IOL returns to its unaccommodative state. This results in distant objects being focused on the retina. The accommodative potential will be achieved after complete capsular bag shrinkage which takes about 4 weeks after surgery.

LASER ADJUSTABLE IOL

Calhoun Vision (Sanfrancisco, USA) are developing a new IOL whose power can be adjusted postoperatively with laser energy. It is a foldable silicone IOL embedded with photosensitive silicone macromers. These macromers polymerize after being hit by a low energy laser. The power adjustments can be made by causing a polymerization of these macromers in different regions of the lens. If the laser is fired in the center of the lens then polymerization at the center leads to an increase

in the IOL power. Polymerization in the periphery decreases the power of the lens while polymerization in one meridian can be used to treat residual astigmatism after cataract surgery. After the surgeon has achieved a satisfactory refractive result, the remaining photosensitive polymers are destroyed, thereby “locking” the lens at the desired refraction.21,22

TORIC INTRAOCULAR LENSES

The plate haptic IOL (Fig. 20.6) design has been modified to produce a toric IOL with the correction cylinder added along the long axis of the IOL. This is marked on the surface of the lens optic. For the toric design to be effective, the lens should not rotate within the eye after implantation. Rotation is relatively unusual but can occur during the first 4 to 6 weeks after implantation, prior to fibrosis around the lens and through the large positioning holes. After this 4 to 6 week period the lens fixates in the capsular bag via the fibrosis through the positioning holes and fusion of the anterior and posterior capsules. This helps to prevent long-term rotation as well as decentration and dislocation of the IOL.23-26 One needs a proper capsulorhexis for this lens to have its effect and implantation should not be done if there is a breach in the capsulorhexis.

PIGGYBACK INTRAOCULAR LENSES

This concept involves the use of two intraocular lenses placed one on top of the other (piggyback). This may be done as a primary procedure to obtain an optimal refractive result in highly ametropic eyes (e.g. High Hypermetropia) where sufficiently high power in a single IOL may not be available.27-30 The second IOL can also be implanted at a later date as a secondary procedure to correct for a poor refractive result of the previous cataract surgery. Both the lenses can be placed in the capsular bag or one can be placed in the bag and the second IOL in the ciliary sulcus. The main complication with use of piggyback lenses is Interlenticular opacification or interpseudophakic opacification of polypseudophakia (opacification between the two IOLs) which may require both IOLs to be explanted.27-30 This methodology is now rarely used as high hyperopic powers are available.

NEW IOLs FOR MICROINCISION

CATARACT SURGERY

For Microincision cataract surgery (MICS), new IOLs are being developed for insertion through an incision of 1.2 to 1.4 mm. Two such IOLs are the dehydrated acrylic IOL manufactured by AcriTec (Germany), and the hydrophilic acrylic IOL manufactured by ThinOptx, Inc (USA). However, it remains to be seen whether these new ultrasmall incision IOLs will provide satisfactory visual performance, and whether they will maintain a satisfactory long-term stability within the eye. Various IOLs for microincision cataract surgery are:2,12,13

ThinOptx Rollable IOL

The ThinOptx Ultra Choice 1.0 is an innovative IOL material (Figs 20.7 and 20.8) that can be inserted through a 0.9 mm incision. It is the latest advancement in small incision surgery that allows for complete phaco surgery via the two side ports. The optic diameter is 5.5 mm and the total diameter is 11.2 mm. The material is an 18 percent hydrophilic acrylic with a 50 um to 400 um optic and 50 um plate haptics. It can be manufactured

Figures 20.7 and 20.8: ThinOptx Rollable IOL

in powers ranging from +30D to –30D. The front surface of the lens is a curve that resembles a radius and the back surface has a series of 50 um steps with concentric rings. The back surface can be concave, convex or plano. The lens is taken out of its container and placed in a bowl of BSS to make it pliable. It is then taken into the surgeons hand between the index finger and the thumb. The lens is then rolled in a rubbing motion and held with a forceps for insertion through the side port incision. Once inside the bag the lens open up gradually as it warms up to the body temperature.3 Alternatively this lens can be implanted with the help of an injector.

Acri Smart (Acri.Tec) IOL

This lens is a sterile foldable intraocular lens made of hydrophilic acrylic material with a hydrophobic coating. (Fig. 20.9). Both the optics and haptics have a square edge design to prevent posterior capsule opacification.3 The method of insertion of this IOL is relatively simple. The injector tip is fitted with a sponge tip which comes with the cartridge and this prevents the injector tip from damaging the lens while inserting it inside the eye. Viscoelastic is injected in the cartridge and once the flanges of the IOL are in the groove of the cartridge the cartridge is closed and then inserted in the injector. The injection of the lens is done by the spongy tip. After the Phakonit procedure is completed, the incision is increased to 1.5 mm. Then the tip of the cartridge is kept at the site of the incision without being inserted into the anterior chamber. The IOL is gradually inserted through the incision and it unfolds inside the capsular bag. One has to be careful as implantation maneuvers can create marks on the delicate surface of the IOL.The lens is available with two

Figure 20.9: The AcriTec (Acri smart) foldable IOL

optic sizes –5 mm and 6 mm, the latter being more appropriate for younger patients. The 5 mm optic lens has now been redesigned to eliminate spherical aberration.

Acri Flex IOL

The Acrimed Acriflex 46 CSE is a 25 percent hydrophilic acrylic IOL with a hydrophobic coating, a 5.5 mm optic and a total diameter of 11 mm. It is designed to be implanted with a special self blocking capsular tension ring. The ring is inserted in the capsular bag and once it is positioned only then the IOL is injected through the same incision. Approximately a 2 mm incision is required, and the ring stabilizes the lens capsule during IOL implantation, decreases capsular folds and improves IOL centration. This IOL can be implanted in eyes with weak zonules.

Hydriol Slim

The Physiol Hydriol Slim IOL is made of a 26 percent water content hydrophilic acrylic material. It has a 6 mm optic and a total diameter of 10.5 mm. The lens can be injected through an incision of 2.2 mm. The IOL has a four point arched haptic design that ensures better centration and long term stability. The new version of the lens is preloaded in the cartridge and thus avoids any IOL manipulation before implantation.

NEGATIVE SPHERICAL IOL

The Tecnis intraocular lens from Pharmacia is the first IOL designed to reduce spherical aberration. Investigators have demonstrated improvement in contrast sensitivity under mesopic and photopic conditions with this technology. Recent advances in wavefront measurement of total ocular aberrations and corneal aberrations have demonstrated that the asphericity of the cornea remains constant throughout life, while the refractive gradient of the lens changes and produces increasing spherical aberration. The decline in contrast sensitivity with age parallels an increase in spherical aberration. The Tecnis IOL is designed to reproduce the

compensatory negative spherical aberration of the youthful crystalline lens and increase pseudophakic contrast sensitivity. The 6-mm optic polysiloxane lens is equiconvex with a prolate anterior surface and a refractive index of 1.46. The posterior and anterior edges are sharp and the polyvinylidine fluoride haptics have a capsular C design. Clinical trials have shown a uniform improvement of contrast sensitivity over a variety of spherical IOLs in intra-and inter-individual studies.31

Bausch and Lomb have introduced a new IOL called the SoFlex® SE IOL which has an advanced optic design of an aspheric IOL with a full 360 degree square edge. An equi-biconvex IOL design combined with a lower refractive index material provides the pseudophakic patient with the least surface-reflected glare and unwanted optical images. It provides a better contrast sensitivity than standard IOLs.

IMPLANTABLE MINIATURIZED

TELESCOPE (IMT)

Vision Care ophthalmic technologies has developed the IMT device (Fig 20.10) which is basically a miniature

Figure 20.10: Implantable Miniaturized Telescope (IMT)

implantable Galilean telescope which can restore central vision in patients with age related macular degeneration (ARMD). The IMT is the first intraocular magnifying system proposed for optical correction in patients with age-related macular degeneration (ARMD). The optical component is embedded in a carrying device designed as an intraocular lens that is implanted after cataract surgery. It contains a number of microlenses which magnify objects in the central visual field and allow the patient to see without external low vision aids. It is implanted in the posterior chamber after routine cataract surgery and held in position by haptic loops similar to ordinary IOLs32 (Figs 20.7 and 20.8). The IMT provides a nominal magnification of 3.0x and a field-of-view of 6.6 degrees (9.2 degrees for the 2.2x magnification version).

THE PRISMATIC IOL

This prototype prismatic IOL developed in conjunction with IOLTECH LA Rochelle, France is fabricated from PMMA and is designed to shift the location of retinal images away from the nonfunctional fovea to enhances visual performance.33 Colin et al from University of Bordeaux, France reported results from a small pilot study in which patients with age-related macular degeneration underwent piggyback implantation with a new prismatic implant. The pilot study included 4 eyes of four patients implanted piggy back fashion, first with a Standard posterior chamber IOL and then with the new prismatic lens. They reported improvement in reading speed and near vision after the surgery. During the piggyback implantation, the PMMA posterior chamber IOL with a 5 mm optic is implanted in the bag through a slightly enlarged 5 mm incision. Then the prismatic IOL with a power of 22 degrees (5 D) is implanted through the same incision either in the same surgery session or later on. The next step is to constrict the pupil which is problematic due to very thick edge oft he this prototype lens.

The preoperative retinal function is evaluated with the scanning laser ophthalmoscope (SLO). This is very

important because it allows us to determine the residual retinal function outside the fovea and to calculate the axis and power of the prismatic IOL which is required to deviate the light towards the still functioning area. Another advantage of this approach is that it is reversible and the prismatic IOL can be removed if the patients finds it to be intolerable. The prismatic IOL used in the French study is still a prototype and that new designs are under development which will increase the angulation of the loops to create more space between the IOL and the iris. The newly designed lens will also be thinner to reduce the problems associated with constricting the pupil.

BLUE LIGHT-FILTERING IOL

This is the new AcrySof single-piece blue-blocking IOL (SB30AL or AcrySof Natural IOL). This IOL has transmission characteristics similar to those of the natural crystalline lens of a 53-year-old human. A yellow chromophore (Figs 20.11 and 20.12) has been added in the IOL which absorbs the blue light. Therefore in addition to blocking ultraviolet light, wavelengths between 400 and 450 nm are only partially transmitted. The high-frequency blue wavelength light may be more likely to damage the retina through release of free radicals. The AcrySof Natural IOL blocks the unnatural transmission of this blue light and thereby may protect the retina better than standard UV-blocking IOLs. The yellow colored lens is also more easily visible inside the cartridge and thus easier to load and implant.34

present. In such cases the mebrane can be repositioned through the injection of air or expanding gas into the anterior chamber.1-3

Posterior Capsular Rupture during IOL Insertion

The posterior capsule can be ruptured during IOL insertion and if there is pre-existing tear in the posterior

capsule, it can extend. Such a complication can occur if adequate quantity of viscoelastic has not been inserted in the bag or there is considerable leakage of viscoelastic substance during IOL insertion. The use of a passport system with plate haptic lenses is more likely to cause this problem. If there is a tear in the posterior capsule prior to IOL insertion, the injector/passport system should not be used at all. If a tear occurs during IOL insertion, the IOL may be left in the bag if the tear is small and the viscoelastic removed manually with a simcoe cannula or using bimanual irrigation-aspiration. However, if there is a large tear then the IOL should be placed on the margin of the capsulorhexis.1-3,35,36

IOL Damage during Insertion

The haptics may be damaged during IOL insertion through a small wound and one of the haptics may be broken. In such cases it is essential to remove and replaced the IOL. During insertion care should be taken to adequately extend the incision so as not to force the IOL through a small and tight wound. Special caution is warranted when a high power IOL is injected. Use of a wrong forceps for holding the IOL may cause compression marks on the IOL optic and may even crack the IOL requiring explantation of the IOL. Special IOL cutters are available (Osher IOL cutter, Utrata foldable lens cutter) to cut the optic and explant the IOL through an unenlarged incision. If these are not available it is better to enlarge the incision and remove the IOL.1-3

Bag Sulcus Fixation

During insertion of the IOL, the lower haptic may be placed in the bag and the upper haptic may lie in the ciliary sulcus. This can lead to IOL decentration and the haptic may also cause a chronic uveitis/pigment dispersion by rubbing on the iris tissue. Asymmetric loop placement can also cause the windshield wiper syndrome with the superior loop and optic shifting position with eye movements and causing damage to the corneal endothelium. To avoid this compli-cation the upper loop

of the IOL should be carefully dialed in the bag and the surgeon should check that both haptics are in the bag before concluding the surgery.1,2,36,37

IOL Decentration

Implanting a small diameter IOL (which is meant to go in the capsular bag) in the ciliary sulcus can lead to a severe decentration of the IOL. This is especially seen when a rent in the posterior capsule occurs and the small diameter IOL is placed over the margin of the capsulorhexis by the surgeon. It is important to remember that Plate haptic lenses which do not have open loops should never be placed over the capsulorhexis. The surgeon should always have a large diameter (6.5 mm optic, overall diameter 13 mm) IOL available for implanting in the ciliary sulcus in the event of a large posterior capsular rupture. In cases of zonular dehiscence, a PMMA endocapsular ring should be implanted within the capsular bag and then the IOL inserted, to prevent decentration of the IOL.1-3,35,36

Lens Dislocation

Complete lens dislocation into the vitreous is a rare complication. It may occur due to the presence of an unrecognized zonular dialysis during surgery or the presence of pre-existing zonular deficiency such as in post-traumatic eyes or eyes that have undergone previous vitreo-retinal surgery. Such a complication has also been reported after YAG capsulotomy, especially with plate haptic lenses and may re-emerge as a problem with ultrathin lenses. A pars plana victrectomy is necessary for removal of lenses dislocated into the vitreous and the intraocular lens may than be repositioned with iris or scleral suturing or substituted with an anterior chamber IOL.1,2,36,37,38

Glare

Use of small optics (< 5.5 mm) can cause an edge glare, especially during conditions of decreased illumination, which cause a pupillary dilatation. This is a serious problem with square edge lenses such as the AcrySof with a 5.5 mm optic. It can cause a significant visual disability to

the patients, especially during night driving. A trial of 0.5-1 percent pilocarpine may be done to decrease the symptoms in such patients, although the IOL may even have to be explanted due to this problem.2,39-41

Capsular Bag Distension Syndrome

This problem arises when a small capsulorhexis completely covers the optic and thereby seals the capsular bag.42,43 There is sequesteration of fluid secreted from the remnant epithelial cells within the capsular bag and a progressive inflation of the capsular bag. Retained viscoelastic material behind the IOL can also lead to this condition by creating an osmotic gradient and drawing more fluid from across the capsule. This creates to an anterior shift of the IOL and progressive myopia. The condition can be prevented by performing a large capsulorhexis. The treatment of this capsular disten-sion syndrome is done by doing a Nd-YAG laser capsulotomy of the anterior capsule. A nick is created at the edge of the capsulorhexis at 2/3 locations which allows fluid trapped within the capsular bag to escape into the anterior chamber.

Capsular Contracture Syndrome

The anterior capsulorhexis can undergo a progressive contracture leading to a capsular phimosis with obscuration of the visual axis and decentration of the IOL. This occurs due to a fibrous metaplasia of the residual lens epithelial cells and is aggravated if the original capsulorhexis is small. This complication is most frequently seen with the silicone plate haptic lenses. The contracture can be relieved by performing a YAG capsulotomy at the margin of the anterior capsule.

Posterior Capsule Opacification

Posterior capsular opacification (PCO) is currently the most important issue in modern day cataract surgery. Residual lens epithelial cells at the equator and the anterior capsule proliferate and cause an opacification of the posterior capsule after cataract surgery. This leads to a decrease in the visual acuity, contrast sensivity and causes

glare. Silicone and PMMA lenses have higher rates of PCO as compared to acrylic lenses.1,2,47

A YAG laser capsulotomy has to be performed in such cases but it can cause damage to the optic of the IOL and opening up of the posterior capsule increases the risk of a subsequent retinal detachment. This can be reduced by the following factors:

1.Adequate multi quadrant hydrodissection.

2.An in-the-bag fixation of the IOL.

3.Diameter of the capsulorhexis slightly smaller than the optic (seals the bag).

4.High biocompatibility of the IOL.

5.Maximal IOL optic-posterior capsule contact.

6.Square truncated edge of the IOL optic.

7.Primary posterior capsulorhexis with optic capture in pediatric cases.

Endophthalmitis

Delayed onset endophthalmitis which has a delayed onset and an indolent course, has been described in eyes with intraocular lenses. The most common responsible organism in Staphylococcus epidermidis. A more idolent from caused by Propionibacterium acnes may present as chronic granulomatous uveitis with white plaques on the posterior capsule. This infection appears to be enhanced by localized entrapment of organisms within the capsule and has been reported only in eyes with intraocular lenses. Adherence of organisms to lenses may play some role. If Propionibacterium is suspected vancomycin is the treatment of choice, although some cases may not respond to medical management and require an IOL explantation with excision of the involved capsule.1-3

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Mark Packer

I Howard Fine

Richard S Hoffman (USA)

INTRODUCTION

While the achievement of 20/20 visual acuity remains a laudable target for any cataract or refractive surgeon, the goal of high quality vision increasingly reflects our understanding of the visual system as a whole. In fact, Snellen acuity represents only a small portion of functional vision. A comparison of vision and hearing highlights the limitations of standard visual acuity tests: the auditory equivalent of a standard high-contrast Snellen eye chart would be a hearing test with only one high level of loudness for all sound frequencies. Today, contrast sensitivity testing is emerging as a more comprehensive measure of vision that will probably replace Snellen letter acuity testing, just as audiometric testing replaced the “click” and spoken-word tests used prior to the 1940s (Fig. 21.1).1

Engineers understand that Fourier analysis allows the representation of any visual object as a composite of sine