Ординатура / Офтальмология / Английские материалы / Small Incision Cataract Surgery (Manual Phaco)_Singh_2002

that the cortical lens fibres are arranged radially, and are therefore easiest to aspirate in this direction. When pulling cortex from behind the iris, use gentle to and fro movements in order to loosen the material from the capsule at the equator. Thereby one obtains more material with less suction and so reduces the danger of collapse of anterior chamber.

The posterior capsule must be watched for different lines, (curved or straight). The former indicates that the cortex is still present whereas later indicates that the capsule has been caught in the suction port. The recognition of these lines, particularly the straight ones radiating out from the suction port indicating that the posterior capsule is incorporated in the aspiration port are very important because any further aspiration or movement while it is impacted will lead to the rupture of the posterior capsule. Engagement of the posterior capsule in the port mandates immediate cessation of suction and reflux to disengage it (Fig. 27.3). Otherwise, the capsule will be ruptured and vitreous will be lost. Avoid the build-up of high intraocular pressure, sudden large amplitude movements of the iris, capsule and hazardous intraocular movement of the cannula. Use as little irrigating fluid as possible.

A collapsing bag is a feature of zonule dehiscence and makes removal of the cortical matter very difficult. Continued aspiration of the cortex tends to exacerbate the problem, and a capsular tension ring should be considered.

Cortical aspiration should ideally start from 6 O’clock position and gradually proceed towards 5, 4, 3, 2, O’clock position and 7, 8, 9, 10 O’clock position or vice versa

depending upon the preference of the surgeon or the demands of the situation.

The cortex situated at 11 to 1 O’clock position or the sub incisional cortex is the most difficult to aspirate. There are various ways to aspirate it. Most commonly employed technique is by making one or two side port incisions, about 70 to 90° away from the primary incision and aspirating the subincisional cortex by a Simcoe cannula. Another method, which can be used, is by J shaped or U shaped cannula (Fig. 27.4). In this technique cannula is inserted into the incision sideways, and then rotated to place the tip under the anterior capsule and cortex is aspirated. If capsulorhexis has been done, IOL can be placed in the bag and then rotated by 180°, haptics of the IOL could dislodge the superior cortex, which can then be easily aspirated. This technique works very well specially if there is a lot of cortex left. However, if rhexis has not been performed and IOL is to be placed in the ciliary sulcus, this technique is of no use. A gentle massage of the iris by the irrigation aspiration cannula at the 12 O’clock position can dislodge the subincisional cortex, which then can be aspirated. Disadvantages of this technique include damage to zonules and iris. It can also constrict the pupil thereby making cortical clean-up further more difficult. Small amount of cortical material can be left in the subincisional area, rather than to struggle and cause a posterior capsular rent or zonular dialysis.

Posterior Capsule Polishing

It can be done by gently rubbing the posterior capsule by the Simcoe cannula itself. Apart from this several instruments are available to polish the posterior capsule.

Fig 27.4: Cortical wash through J shaped cannula Courtesy: Alcon (India)

Kratz scratcher is one instrument quite commonly used. It is nothing but a curved irrigating needle roughened up by sand blasting, hydrohoning or coating with particles of diamond dust. It is used by attaching it to cystitome handle. Blunt air injection cannula or an olive tipped needle can also be used for capsular polishing. It should be rubbed gently against the posterior capsule to remove the fine lens matter adherent to the posterior capsule (Fig. 27.5). As the polisher touches the posterior capsule, a halo reflex appears around the scratcher. Pressure applied through the scratcher or polisher must be sufficient enough to produce a halo of about 4 mm. Excessive pressure may result in stress lines.

The effectiveness of the posterior capsule cleaning often depends on the physical characteristics of the capsule. A thin, floppy posterior capsule is the most dangerous. It is difficult to slide the tip of the polisher along it without dragging it into folds that can rupture. A thick, taught capsule is easiest to clean. Not infrequently, a plaque may be present on the posterior capsule. It can sometimes be scratched away by a fine tip of a bent

Fig 27.5: Capsular polishing. Courtesy: Alcon (India)

needle or picked away by a Mcpherson forceps. One should not be too aggressive with residual plaques as an opening in the posterior capsule can be made at a later date with Nd: YAG laser.

Cortical Clean-up in PC Rent

If the posterior capsular tear occurs at the time of cortical clean-up, cannula should be withdrawn from the anterior chamber immediately. Anterior chamber should be filled with the viscoelastic, so as to push the vitreous face back and distent the capsular bag. If vitreous is not in the anterior chamber, i.e. vitreous face is intact, then the area of posterior capsular tear is left alone and cortex from other areas is removed, preferably with dry aspiration (filling the anterior chamber with viscoelastic repeatedly and aspirating the cortex). Cortex should be aspirated towards the tear and never away from it, otherwise it may pull the vitreous. With dry aspiration almost all the cortex can be removed without disturbing the vitreous. If the cortex has got mixed with the vitreous and vitreous is present in the angle or the wound an anterior vitrectomy should be performed.

IntraocularLenses 28

Tanuj Dada

Harinder Sethi

The evolution of the cataract surgery with the introduction of intraocular lens implantation has been one of the major achievements of modern

medicine. The intraocular lenses provide a precise pseudophakic optical rehabilitation with minimal magnification and excellent optical properties. The advent of small incision surgery made possible by phacoemulsification and foldable IOLs represents another major milestone in cataract surgery. It is important for the ophthalmologists to have an in-depth knowledge of the basic design and optical features of the various IOLs currently in use.

Classification of IOLs

1.Site of implantation

i.Posterior chamber IOLs

ii.Iris plane IOLs

iii.Anterior chamber IOLs

iv.Scleral fixated IOLs.

2.Flexibility of lenses

i.Rigid IOLs–PMMA

ii.Foldable IOLs–silicone, hydrogel, acrylic, collamer.

3.Material of optic

i.Polymethyl methacrylate (PMMA) optic

ii.Silicone optic

iii.Hydrogel or hydrophilic acrylic

iv.Hydrophobic acrylic

v.Thermoset (memory lens)

vi.Collamer.

4.Combination of optic and haptic material

i.Single piece IOL–haptic and optic made of same material, e.g. all PMMA single piece IOL, all acrylic single piece IOL.

ii.Three piece IOL–where optic and haptic are made of different materials, e.g. three piece PMMA IOL (optic made of PMMA and haptics of polypropylene), acrysol IOL (optic made of hydrophobic acrylic and haptics of PMMA.

OPTIC MATERIALS FOR IOLs

An ideal IOL material should have following properties:

•High optical quality

•High index of refraction

•Light weight

•Durable, resistant to mechanical stress

•Easy fabrication

•Non antigenic/non-allergic

•Non carcinogenic

•Sterilization easy

•Lack of inflammatory reaction, foreign body reaction, or tissue chaffing

•Blocking UV radiation

•Implantable through a small incision.

Polymethylmethacrylate (PMMA)

It is an inert polymer of methyl methacrylate monomer. It is manufactured through addition polymerization of methacrylic acid methylester, which is derived from acrylic acid. It was the first material to be used for intraocular implants by Harold Ridley. The idea of this material being used for IOLs came from the fact that it had been noticed that during World War II, intraocular fragments of PMMA in the eyes of the pilots (which came from the shattered canopies of fighter aircrafts), demonstrated inert properties. This material is light, hard and transparent and transmits a broader spectrum of light than the human lens, thereby allowing the transmission of UV rays. Hence UV absorbing materials have been incorporated as covalently bonded or entrapped chromophores to prevent retinal damage. The agents commonly used as UV absorbers are benzotriazoles and benzophenones. The material is hydrophobic and may damage corneal endothelium on contact. The main disadvantage of PMMA lenses is that they are non-flexible and have to be inserted through a larger incision. The specific gravity of PMMA is 1.19 and the refractive index of 1.497. PMMA has the longest track record as an intraocular lens material and has given an excellent optical performance till date.

Surface modification by heparin and other chemicals can be done to reduce the deposits over the IOL and opacification.

Silicone

It is composed of repeating chains of cross-linked dimethyl siloxane. The major advantages of this material are the autoclavibility and decreased trauma to the intraocular structures. Silicone has affinity for proteins, which may account for the build up of the surface proteins on to the IOL optic. The material has a low tensile strenth and must be handled carefully to avoid tearing. It is compressible and has an excellent memory (the ability to return to original shape after deformation). The refractive index is 1.41–1.46 and specific gravity is 1.01–1.06. Due to low refractive index, their relative thickness is more for the same dioptric power and hence high power silicone lenses are thick and cumbersome to handle with an uncontrolled opening inside the eye. Discoloration to

a tan brown colour had been reported in the first generation lenses and these lenses cannot be used in the presence of silicone oil within the eye as it chemically adheres to these lenses.

Recently second generation silicone material has been introduced which has a higher refractive index and is increasingly becoming popular. The Pharmacia-Upjohn CeeOn Edge 911 IOL represents the second generation silicone IOLs. The edge of the optic is square or truncated and it uses polyvinylidine fluoride haptic material with a well-designed Cap C haptic configuration design providing an excellent memory.

Hydrogel

The optic of hydrogel lenses is made up poly-hydoxy- ethylmethacrylate (HEMA) with a 38 per cent water content and bonded with an UV absorber. They are lathe cut in the dry state and require polishing. They are rigid in the dehydrated state and become soft and rubbery on

hydration. Since they are hydrophilic, they are less damaging to the corneal endothelium on contact. These lenses have a low tensile strength and can get torn on insertion. This IOL material may accumulate protein deposits and can be deformed by tissue pressures, leading to optical changes and changes in effective power. Their refractive index is 1.47 and specific gravity is 1.19. This

group of polymers resembles the living tissue in their physical properties more than any other class of materials and are the easiest to insert as they are pre-rolled and open up inside the eye gradually with hydration. However, many lenses made from this material are required to be maintained in a cold chain which is a major disadvantage with these lenses.

Acrylic

Flexible acrylic is a co-polymer of phenylethylacrylate and phenylethylmethacrylate. The cross-linking imparts it a good three-dimensional stability and temperture dependent viscoelasticity. Lenses made of this material are softer and more easily foldable at body temperature than room temperature. Unfolding is much slower and more controlled than silicone lenses. It has high refractive index of 1.55 making it the thinnest lens possible. Its has a unique surface characteristic, i.e. tackiness (tendency to adhere to surgical instruments and posterior capsule. These are available as single piece (all acrylic) and 3- piece lenses (PMMA haptics) and are currently the most popular foldable lens materials with the minimum incidence of posterior capsular opacification. The decreased rate of posterior capsular opacification (PCO) has been attributed to the square edge design of the lens optic, which has a barrier effect on proliferating lens epithelial cells.

The Alcon AcrySof is the most popular IOL in this group. It has an optic made up of flexible acrylic available in 2 diameters of 5.5 mm (MA30BA) and 6 mm (MA60BM) and open loop haptics made of PMMA. The overall diameter of the IOL is 12.5 mm (smaller optic) and 13 mm (larger optic). Recently single piece AcrySof lenses have also been introduced in the market which

have both the optic and haptic made up of acrylic. Various studies have documented that PCO rates are least with the Acrysof lenses which has been related to the good biocompatibility of this IOL. This is due to the “sticky” characteristic of the IOL and its close adherence with both the anterior and posterior capsules and the the square truncated optic edge.

HAPTIC MATERIALS FOR IOLs

Nylon (polyamide)

These are fibre polymers with repeating amide (-CONH-) groups. They are named according to the number of carbon atoms in the monomer subunits. The commonly used ones are nylon 6 (Perlon, Supramid) and nylon 66.

It has a tendency to slowly hydrolyse with gradual water absorption and to be broken down at amide sites by proteolytic enzymes. Hence they have gone into disuse. Polyimide material is similar to polyamide, but with greater heat resistance and has been used with glass and silicon optic.

Polymethylmethacrylate (PMMA)

The advantage of PMMA haptics is the total lack of degradation in vivo. Since they are stiffer than polypropylene,

they can be easily dialed in the bag. Moreover, they have better memory than polypropylene, with better fixation and centration. Three-piece silicone IOL with polypropylene haptics have a higher incidence of decentration, pigment adherence and capsule opacification compared with PMMA haptics. Single piece all-PMMA exhibit the best loop memory. It is currently the most popular haptic material. An angulation of the haptics 10 degrees anterior to the optic is done to minimize pupillary capture and iris chaffing.

Polypropylene (Prolene)

It is polymer derived from propane. Its chief advantage is its hydrophobic nature making it very resistant to hydrolysis. However a full spectrum UV light irradiation leads to loss of tensile strenth. It is commonly used in the three-piece lenses because of its biological compatibility, resistance to biodegradation, flexibility and tensile strength. It is usually dyed blue or purple for better visibility. A tendency to loose memory with time when deformed by tissue is noted, with higher incidence of

decentration. Prolene loops are more flexible making insertion easier. The main disadvantage of these haptics is that they activate the complement pathway leading to neutrophil chemotaxis with more postoperative inflammation. Bacteria also adhere better to polypropylene and this leads to a greater risk of endophthalmitis. Hence this material is no longer used for making IOL haptics.

Polyvinylidene fluoride (PVDF)

This is the latest IOL haptic material that has recently been introduced with some of the new generation silicone lenses. It provides a good memory and very stable fixation.

Haptic Design

There are basically three types of haptic designs currently in use:

1.Open loop (such as modified C loop design).

2.Plate haptic (Chiron C10UB and C11UB)

3.Mini loop plate haptic design (Medevec VS2)

BASIC PRINCIPLES OF IOL MANUFACTURE

Lathe Cutting

The PMMA is taken as large block and a lathe with a tip is used to cut the optic to the predetermined size, shape and radius of curvature. The lens formed has rough edges and surface, requiring polishing by a tumbling process (lens is placed in a vertically rotating drum where in it tumbles repeatedly with polishing compounds) or specialized rotating rods (moved over the lens to make it smooth). The residual polishing compound is thoroughly removed.

Injection Molding

The PMMA block or pellet is heated and forced at high pressure through a steel mould which is designed to provide a proper shape, size, and power of the lens. Later the material softens and takes the shape of the mould. Thereafter the mould is removed and lens is polished. Injection moulded lenses have a tendency to wrap with time and produce optical distortion. A higher susceptibility to damage by Nd: YAG laser is also reported.

Compression Molding

It is a hybrid process of lathe cutting and injection molding. The PMMA is first lathe cut and the rough version is placed in the mold of the specific shape and power. It is then subjected to high temperature and pressure and later polished.

Cast Molding

It is similar to injection molding except that preformed PMMA is not used. The resin that is purified and distilled from the methylmethacrylate monomer is mixed with a catalyst and poured into the injection mold. The actual polymerization of the PMMA occurs inside the mold. The process allows more accuracy and reproducibility. Moreover, the UV absorber can be added prior to polymerisation giving better bonding.

PROFILE OF RIGID IOLs USED FOR

SMALL INCISION CATARACT SURGERY

Conventionally an optic size of 6.5 mm and an overall diameter of 13 mm has been used for cataract surgery with IOL implantation in the ciliary sulcus. With the advent of small incision cataract surgery and capsulorhexis it has become possible to place the lenses within the capsular bag and hence smaller lenses are now being

used. These lenses are essentially one-piece PMMA implants with either of two kinds of optic design, round or oval. The round optic lenses have a diameter ranging from 5.0 to 5.5 mm, while the oval optic lenses have a horizontal diameter of 5.0 mm and a vertical diameter of 6.0 mm. The overall diameter of these lenses ranges from 11.5 to 12.5 mm, approximately the diameter of the empty capsular sac. The oval lens optic is no longer used as they are more prone to decentration and an increased incidence of postoperative glare and diplopia has been reported with these lenses (Fig. 28.1)

One-Piece

Fig. 28.1: Diagrammatic representation of the most commonly used posterior chamber IOls.

The main disadvantage of using these lenses is that since they are rigid, implantation of these lenses requires an enlargement of the incision size equal to optic diameter. Once the incision is enlarged to 5-5.5 mm it no longer remains astigmatically neutral and there is an increased chance of postoperative leakage from the wound if left unsutured. Even if there is no leakage of fluid after applying pressure on the cornea/sclera at the operating table one should always apply a suture. This is important because there can be a long-term slippage of the wound lip and progressive against the rule astigmatism in such cases. Therefore, one must apply atleast one 10-0 monofilament nylon suture when rigid

IOLs are used for small incision cataract surgery. Another problem with these lenses is that due to the smaller optic size, there can be problems of glare/diplopia in patients with large pupils and during night vision. There is also a higher incidence of posterior capsular opacification and decentration. These lenses should not be used if the capsulorhexis has been torn or if there is a large posterior capsular rupture such that the lens has to be placed over the margin of the capsulorhexis. Currently these are the most popular lenses used for small incision cataract surgery in our country due to the low cost.

FOLDABLE IOLs

These lenses mark a major landmark in history of IOLs and as the name suggests these lenses can be folded and thus inserted from small incisions in cataract surgery.

Advantages

1.Require a small incision (2.5-3.5 mm) for insertion.

2.This decreases postoperative astigmatism, increases wound stability and allows rapid visual rehabilitation.

3.These lenses have a decreased incidence of risk of posterior capsular opacification and cellular precipitates on the lens surface as compared to PMMA lenses.

4.These lenses are relatively easy to explant due to decreased perilenticular fibrosis with these lenses. In addition due to a soft lens material the optic can be cut into two and easily removed.

5.Currently used foldable lenses also have a better NdYAG laser compatibility and put decreased strain on the zonules because of their reduced weight.

Disadvantages

1.Foldable lenses have a shorter track record and their long-term biocompatibility within the ocular tissues has not been fully evaluated.

2.These lenses have a low tensile strength and are thus more prone to damage during implantation. Permanent fold marks and creases from holding, folding and inserting these lenses may produce disturbances in vision. In a cold temperature acrylic lenses may even crack when folded.

3.Some of the foldable IOLs such as the Memory require a cold chain to be maintained in a foldable form.

4.Lens discoloration has been reported with silicone IOLs.

5.It is difficult to use a foldable IOL in the presence of a posterior capsular rent.

6.If vitreoretinal surgery is required in an eye with a silicone implant, silicone oil cannot be used as a vitreous substitute.

7.Currently foldable lenses are also much more expensive than rigid PMMA lenses.

Surgical Considerations in the

Insertion of Foldable IOLs

There are two basic techniques for foldable IOL insertion. These IOLs can be inserted into the capsular bag by using a passport/insertor/injector system or a holder-folder system. The former system requires the smallest incision size for IOL implantation. The holder-folder method is the most popular method for IOL insertion.

The IOL can be folded by using either of the two principles.

1.Horizontal or longitudinal principle which allows for a two-step implantation technique ensuring control and safety. This is folding along the 6 to 12 O’clock meridian such that the haptics form a “moustache”. The anterior haptic goes in first under the capsulorhexis margin while the posterior haptic stays outside the wound. After the IOL is released inside the capsular bag, the posterior optic is dialled into the capsular bag.

2.Vertical or transverse principle which allows the lens to be placed in the bag in one manoeuvre. This is folding along the 3 to 9 O’clock meridian such that both haptics are placed together in the capsular bag and do not need to be dialed after the lens opens up in the capsular bag.

It is important to remember that silicone IOLs should be dry before handled with the holder/folder, while acrylic lenses should be wet when folded.

In the event of a zonular dialysis the IOL should be inserted after putting an endocapsular ring (ECR) made of PMMA to stabilise the capsular bag.

The injector systems use a disposable cartridge wherein the IOL is placed. There is usually drawing of the IOL outlined on the cartridge, which tells the surgeon as to which direction the IOL should be placed. The IOL is placed in the cartridge coated with a viscoelastic, the cartridge is then closed and inserted into the injector. The screw of the injector is slowly turned and one can visualize the haptic of the IOL coming into the nozzle of the cartridge. The nozzle is then placed inside the capsular bag which has already been filled with viscoelastic and the screw of the injector turned further to release the IOL into the capsular bag.

Problems Encountered during Foldable IOL Insertion

i.The optic may slip if it is wet, especially if it is a silicone IOL.

ii.Flipping out of the IOL may result from inappropriate instruments and technique.

iii.Breakage of the haptic can occur from incorrect tucking of the haptic during insertion.

iv.Tearing of the optic can occur if the handling is not gentle, especially during the use of hydrogel IOLs.

v.The optic may crack when folded. This is usually seen with acrylic IOLs if they are used in a cold environment.

vi.The IOL may unfold with a sudden jerk within the capsular bag. This can lead to a loss of capsular integrity and a posterior capsular tear.

vii.Delayed unfolding may be seen during the insertion of AcrySof or Memory lens.

viii.The IOL optic may be indented/damaged by pressure from the holding/folding instrument.

ix.Incomplete opening of the holder may occur and the IOL may not be released into the capsular bag. In such cases the IOL needs to be disengaged from the holder with the help of a Sinskey hook introduced from the side port incision.

x.Detachment of the Descemet’s membrane may occur during IOL insertion.

MULTIFOCAL INTRAOCULAR LENSES

The intraocular lenses commonly in use have a fixed focus which can be adjusted by adjusting the IOL power to serve for near, intermediate or distance vision. It is not possible to see near and distant objects clearly with these lenses and thus patients are always dependent on spectacles. Over the past decade, a variety of multifical intraocular lenses (MIOLs) have been introduced and enjoyed a widespread clinical use. Both refractive and diffractive models have been shown to be effective in allowing each eye to achieve quality, uncorrected distance and near acuity after cataract surgery. The major concerns with the use of these lenses are the loss of contrast sensitivity and the inducement of glare and halos from light sources during night vision. All MIOLs require careful attention to IOL power calculations and the creation of a relatively planospherical result after surgery.

REFRACTIVE MIOLs

Target or Centre Surround MIOL

The Chiron NuVueTM is an example of an MIOL with the central near add in the middle of the optic surrounded

by a distance optical power. A new model made of silicone with PMMA haptics has shown surprisingly good clinical results despite the potential for visual blur with pupillary miosis. The NuVueTM is considered to be a “near dominant”, MIOL and some surgeons use it in a monovision capacity for the near eye.

Three-zone MIOL A variety of three-zone MIOLs providing distance and near vision by using a near annulus at various distance from the central distance component have been popular. The Storz True VistaTM and the Domilens Progress ThreeTM are examples of this style. Normal pupil patients do enjoy both near and distance vision but smaller pupils can obstruct the near component with some three-zone MIOLs. One advantage of this lens design is that even though there is pupil dependency, distance vision is always preserved despite the loss of near acuity with miosis.

Spherical Curve MIOL The AMO ArrayTM SA40N MIOL is a lens designed with five zones of near and distance powers on the anterior surface of the optic. These power rings help to reduce pupillary dependency. The ArrayTM is considered a “distance dominant” lens and provides near acuities without correction in the J-3 range or better, offering good midrange and near acuity for most tasks. Some patients will prefer the addition of a bifocal add for finer print and especially under low-light conditions. The AMO ArrayTM is available in a foldable silicone material with PMMA haptics. A new injectable delivery system allows for greater ease of insertion. The AMO ArrayTM lens is currently the most popular multifocal IOL in current use.

DIFFRACTIVE MIOLs

Diffractive optics multifocal technology is slowly gaining wide acceptance. The major advantage of this lens is less pupil dependency and the ability to provide an even distribution of near and distance vision. However, manufacturing techniques are more difficult and critical with these lenses due to difficulties with making of the diffractive plate. Pharmacia has developed a diffractive MIOL, the CeeonTM 811E. Addition of a diffractive component to the popular AcrysofTM acrylic IOL is also under consideration.

ACCOMMODATING INTRAOCULAR LENS

The ability to implant a new lens within the original capsular bag of the crystalline lens and restore the physiologic accommodation is a concept being investigated by