New developments in intraocular lens design

New developments in intraocular lens design

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Correspondence:
Mr P Percival MA FRCOphth
Consultant Ophthalmologist
Department of Ophthalmology
Scarborough Hospital
Scarborough
YO12 6QL

Piers Percival
MA FRCOphth

Abstract

The key issues for modern intraocular lenses are biocompatability, centration, dioptric accuracy, unwanted optical imagery, secondary intervention and injectability through an enlarged 3.0 mm wound without stretch of the wound edges. Broadly, there are three materials to consider: siliconewhich is hydrophobic, the hydrophobic foldable acrylics and the hydrophilic hybrid acrylics. Each will be considered in terms of lens design and key issues, with discussion of likes future trends.

Keywords

Intra-ocular lenses, silicones, acrylics, hydrophilics, biocompatability.

Introduction

As phacoemulsification has now become the 'norm' for cataract surgery, this paper will only concern the design of foldable intra-ocular lenses (IOLs) that may be introduced through a small incision. Although it is not possible to comment on all the currently available styles, it is hoped that enough detail can be given from a sufficiently wide spectrum to be representative of the majority that are available and at the forefront of lens design. A classification is shown in the Table 1.

Silicone

Excellent examples come from Allergan Surgical, Bausch & Lomb and Pharmacia & Upjohn, with their newer materials having the higher refractive index of 1.46, thus making the lenses thinner and easier to insert than previous lenses. There is an elegant insertion technique through a 2.9 mm incision available with the Allergan Unfolder™ or the Bausch & Lomb Mport.¹ Foreign body reaction to silicone has been found to be less than for standard PMMA. Ravalico et al.² evaluated the presence of epithelioid cells 30 and 90 days after surgery. These are formed by differentiation of macrophages and are precursors of multi-nucleate giant cells. Epithelioid cells were observed in 80% of PMMA lenses, and 20% of Heparin modified lenses but were absent on silicone lenses.^2,3 Sharp edges to the silicone optics such as those of the CeeOn®911 are now generally preferred because of a lower incidence of posterior capsule opacification (PCO).^3,4,5 Of all the foldable materials, silicone has been in use the longest, but is now proving to be inferior to some of the newer materials. The chief disadvantages of Silicone are:

(i) Greater capsular fibrosis of the lens capsule than with other materials.^3,6,7 An ideal capsularhexis diameter may be said to be 5.5 mm: a diameter smaller than this can lead to capsular phimosis, the rim contracting sufficiently to cause symptoms of glare and blurring (Figure 1). A diameter greater than 5.5 mm may cause asymmetric contraction leading to button holing of the lens out of the bag. The rim may then contact behind the lens optic leading to decentration.

(ii) Decentration is more common than with other lenses, whether of the three-piece or plate haptic variety.^8,9 Plate haptic lenses may decentre because they are too short for the bag and there are reports of frank dislocation.^1,¹⁰ Schwenn et al. Showed that at 5 months, 10% of lenses had decentred by 1.0 mm and over 30% had rotated by 15° or more. They concluded that such lenses, if toric, would be inappropriate for correcting astigmatism, even with enlarged positioning holes.¹¹ Sun et al. recorded good unaided vision in 84% of eyes with plate haptic toric lenses, but at the expense of secondary intervention in 12/106 cases that had been followed for three months. A total of 25% of their lenses had rotated by 20° or more.¹² Plate haptic lenses have also been associated with non-corneal astigmatism.¹³ Aufforth et. al.⁸ and, Apple et al.¹⁴ reported a large series of explanted silicone lenses. While with modern techniques that include a round capsularhexis the chief reason for explantation should be a wrong lens power, (8% of the group studied). In their study 42% of explantations were primarily due to decentration and 28% due to inflammation. There was no statistical difference between three-piece lences with looped hoptics or one-piece with plate haptics. In the 1998 ASCRS Survey,¹⁰ wrong lens power was a cause for explantation in 49% of three-piece silicones and dislocation/decentration in 48% of plate haptic silicones.

(iii) Silicone is less YAG compatible than other materials, making capsulotomy a difficult procedure in cases where the posterior capsule is thickened through fibrosis rather than epithelium.

(iv) Silicone is incompatible with vitreo-retinal surgery involving the use of silicone oil, as irreversible optical effects will often necessitate implant exchange. Silicone is, therefore, less suitable than other materials where there is potential for retinal pathology, e.g. in juvenile diabetics and high myopes.^1,^14-16

Acrylics

The thermolabile acrylic from Alcon (Acrysof®) has been an exciting advance in foldable lenses. The lens is ultra-thin because of its high refractive index of 1.55. When warmed, the lens becomes flexible, can be folded, and then unfolded within the eye where it will remain unchanged. The optimal folding temperature is 20°C, which is higher than the normal environmental temperature of an air-conditioned theatre. It appears to be more biocompatable than standard PMMA or silicone, since it induces less fibrosis of the anterior capsule and is associated with less PCO. Contact with the Acrysof® optic and lens epithelial cells results in less fibrous metaplasia than with standard PMMA optics or silicone SI40 lenses. This finding combined with bioadhesion of the capsule rim to the optic provided greater stability.¹⁷ They found that the number of silicone SI40 and PMMA lenses displaying a degree of capsule movement was approximately three times the number of Acrysof® lenses. In this later study all lenses had 6.0 mm optics and looped PMMA haptics. In a parallel study comparing the same lenses there was significantly less PCO with Acrysof® than with a silicone lens three years post-operation, with YAG capsulotomy rates of 0% for Acrysof®‚ lenses, 14% for silicone lenses and 26% for PMMA lenses.¹⁸ Another study reported rates of 5%, 42% and 50% respectively at three years.¹⁹ The reason for the low capsulotomy rate is thought to be partly due to the stickiness with which the Acrysof®‚ adheres to the posterior capsule¹⁷ and partly due to the square cut edge in the design of the optic.^3,4 The strong adhesiveness has been verified by Nagata.²⁰ A further study concerned cell deposition on the anterior surface of the implants: the silicone group had a significantly higher small cell count than both PMMA and Acrysof®‚ groups The Acrysof®‚ was also associated with significantly lower giant cell deposits than the other two groups.²¹ In another study Acrysof®‚ was shown to have superior biocompatability over heparin surface modified lenses in eyes with compromised blood aqueous barriers associated with proliferative or pre-proliferative diabetic retinopathy.²²

Clearly, Acrysof®‚ is more biocompatable than silicone and more forgiving where there is a variance in size or shape of the capsularhexis or the presence of radial tears. However, there are disadvantages with this design:

(i) There are reports of ‘glistenings’ or micro vacuoles in the lens substance.^23-25 These are not noticeable immediately after surgery but appear to be progressive after the first week (Figure 2). They may be associated with impaired contrast sensitivity, rarely with intolerable glare or reduced vision.^23,25 Laboratory studies suggest that these micro-vacuoles may be related to a change of the IOL material at body temperature, leading to microvoids which then fill with aqueous.²⁵

(ii) There have been reports of entoptic phenomena (dysphotopsia) causing problems after stable bag encapsulation with this lens.^10,^26,27 Symptoms vary from arcs and halos of light to shimmering that is stimulated by eye movement and is most noticeable in fluorescent lighting or against oncoming headlights at night. Dysphotopsia was reported in 3-5% of patients implanted with the Acrysof® by Almallah.²⁸ Some symptoms, especially arcs in the temporal field, are explained by internal reflection at the square cut edge of the optic.²⁹ Symptoms are exacerbated by the high refractive index and coefficient of reflection which induce bounce back rays off the inside of the front surface of the implant.²⁶

(iii) The lens is one of the more expensive foldables.

(iv) If the Acrysof® does have to be explanted, whether because of incorrect power or for other reasons such as dysphotopsia, it is found to be extremely difficult to isolate from the lens capsule because of the strong adhesiveness. By the same token, YAG laser capsulotomy may also provide unexpected difficulties.

Modifications have now been made with the SA30Spc (Figure 3). This is a single-piece lens and is said to be injectable through a 3.2 mm incision using the Monarch II™ injector. The square edges have a matt finish to lessen the chance of dysphotopsia. However, as the haptics are of the same bioadhesive material, the lens could be even more difficult to explant should this ever became necessary. The SensarTM is another acrylic with similar properties, but with rounded edges to lessen the chance of dysphotopsia and is not dependent on warming in order to be folded. Micro vacuoles also appear to be eliminated.

Hydrophilics

Several companies have now introduced acrylic lenses which incorporate hydroxy ethyl methacrylate(HEMA) in order to improve flexibility. These lenses are hydrophilic, and are presented in balanced salt solution. They are more biocompatable than silicone and have advantages over the Acrysof®‚ in being one-piece and less expensive. One of the first of these lenses to be marketed was the Hydroview™. The optic has a water content of 18% and is bonded to PMMA looped haptics. Inflammatory reaction is minimal. However, reservations exist on account of a propensity for epithelial cell proliferation which extends from the capsularhexis edge over the anterior surface.³⁰ In one series, this change occurred in 33% of cases, with visual symptoms requiring secondary intervention in 6%.³¹ There is also a high incidence of PCO and occasionally the brittle PMMA haptics may be found to snap.³⁰

Pure HEMA lenses with a water content of 38% have now been in use for over twenty years³² and a twelve-year follow-up study has confirmed excellent bio-compatibility³³ so that its incorporation as a hydrophilic is likely to be widely accepted.

The Memory Lens also includes ethylene glycol dimethacrylate and has a 20% water content. This lens is presented to the surgeon in a pre-rolled (Figure 4) state at temperatures above 25°C, and because of the structure of the molecular network, it slowly recovers its original shape.³⁴ A new version with a tighter role is now available, and this may be inserted through a 3.2 mm wound. The fact that it is pre-folded reduces operating time, but this is offset by the few minutes watching it unfold within the eye. Completion of unfolding and disappearance of central folds usually takes twenty minutes. A further drawback is that the lens requires a critical storage and transport temperature of between 2° and 10°C. In addition, there are the disadvantages of polypropylene loops which may aggravate decentration.

Hydrophilics are naturally more biocompatable than hydrophobic lenses. Besides offering greater protection to the corneal endothelium, there is less intraocular inflammation and induced capsular fibrosis as confirmed by Miyake et al.³⁵ In an in vitro study, Apple et al¹⁴ compared the degree of silicone oil adherence to seven different IOL types as well as to human lens capsule. They showed that the greater the hydrophilicity and the lower the dispersive energy, the less was the silicone oil coverage. Although a hydrophilic acrylic was not included, they found the oil coverage to be 10% with the heparin modified lens and human lens capsule (the least), 20% with PMMA, 34% with Acrysof®‚ and 100% with silicone. Perfluorocarbon gas showed minimal adherence. All hydrophobic IOL’s can be associated with silicone oil bubbles after vitreo-retinal surgery (Figure 5) and the implication is that the hydrophilic should be preferred to the acrylic where there is a potential for vitreo-retinal surgery later in life. However, hydrophilics are not all the same. Reservations concerning the Hydroview™ have already been mentioned. Anterior surface pits were noted in four out of fifteen ACR6D lenses from Corneal,³⁶ and a toxic anterior uveitis necessitated recall of 263,000 Memory Lenses®.^{37, 38}

The author’s personal experience has been predominantly with the Stabibag™ and Centerflex™ hydrophilics, the former having 28% and the latter 26% water content. In an unpublished study of 120 implantations randomised between the Stabibag™, the Acrysof‚ and the Allergan SI30 in eyes with no other pathology, the Stabibag™ was found (at two years) to be the cleanest lens with least cellular precipitation, pigment, debris or other deposition on the surface, but the greatest PCO. Unfortunately, as is possible with all angulated soft lenses, the lens can bow backwards according to the degree of capsular contraction which itself is unpredictable. This shift may result in hyperopia and loss of dioptric accuracy. In the author’s experience, six eyes out of 1,000 implanted with the Stabibag™ have required secondary intervention with piggyback lenses to rectify this problem. Other patients have merely been disappointed in the need to wear distance glasses.

Dioptric Accuracy and the Centreflex™

Several factors have an impact on the post-operative refractive outcome. These include the biometrist’s skill, the formulae used, the factory accuracy, the surgical method and, arguably most important of all, implant stability. The Centreflex™ (Figure 6) is uniplanar and has a specially patented haptic design to prevent buckling or antero-posterior movement post-operatively. The 12.0 mm haptic diameter also tends to stretch the posterior capsule to maintain clarity. 435 consecutive refractions from first eye Centerflex™ implants were performed by the author and compared to the 'target' refraction computed at the time of surgery using the Hoffer Q formula for axial lengths of less than 22.0 mm, the SRK(T) formula for axial length greater than 24.5 mm and a mean of the two formulae for lengths in between. A total of 421 (97%) refractions were within 1.0D and 348 (80%) were within 0.5D of target. 344 (79%) eyes were seeing 6/12 or better unaided at one month and 387 (89%) 6/9 with correction. 4 eyes had clinical macular oedema at one month and 44 eyes had pre-existing pathology such as macular degeneration. In a separate study, 40 patients inserted with Centreflex™ lenses were compared with 40 patients inserted with silicone SI30, 40 with Acrysof®‚ and 40 with Stabibag™ lenses at one year. Centration was found to be superior and almost exact in all cases and no backward bowing nor loss of refractive power could be documented. Exact centration means that a 5.75 mm optic is sufficient for the wider pupils of younger patients. Since October 1999 1,800 Centreflex™ lenses have been implanted at the Department of Ophthalmology (Scarborough) and although laser capsulotomies are to be expected in the future, to date there has been only one secondary intervention (due to the refraction 1.5D outside the target). A further advantage of this lens is that it is injectable through an unenlarged 3.0 mm wound. It is known that some injectors (notably for silicone lenses) enlarge the wound by around 0.3 mm through tearing:^39,40 the injector for the Centreflex™ is hexagonal and should be put only within the lips of the corneal tunnel so obviating any tear at the wound edge.

Secondary intervention

YAG laser capsulotomies apart, decentration, dislocation and incorrect lens power remain the leading indications for secondary intervention. However, glare and unwanted imagery are increasing indications for secondary intervasion in relation to acrylic and multifocal IOLs. Of all interventions registered by ASCRS and ESCRS members, three-piece silicones were removed in 27% of cases, multifocal silicones in 25%, three-piece acrylics in 22% and plate haptic silicones in 20%.¹⁰ Both toric lenses¹² and multifocals are known to be associated with a relatively high incidence of secondary intervention, despite careful case selection and counselling, and it is for this reason that their details have been omitted from this article. Concerning the high refractive index Acrysof®, there is clearly a trade off between a very low incidence of PCO and the risk of difficult explantation for unwanted imagery.

Collamer Lenses

One new material which has been researched by Staar Surgical is the hydrophilic copolymer of porcine collagen/HEMA with a refractive index of 1.47. This IOL material has now been approved by the FDA in both plate haptic and the three-piece form for use after phacoemulsification but as yet has too short a follow-up for further comment.

Conclusions

Silicone now appears to be inferior to both Acrylic and Hydrophilic materials in terms of the key issues of biocompatability (which includes capsular fibrosis), centration and secondary intervention. The bioadhesive high refractive index acrylic such as the Acrysof®‚ has considerable advantage in reducing the need for YAG capsulotomy, but may be associated with unwanted optical imagery and a larger than 3.0 mm wound for insertion. The uniplanar hydrophilic Centreflex™ IOL has design features for superb centration and dioptric accuracy, but a relatively short follow-up.

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