An intraocular lens is a lens implanted in the eye, usuallyas a treatment for cataracts or myopia. Intraocular lenses have been used tocorrect errors such as near-sightedness, far-sightedness, and astigmatic eyessince 1999, with more than 6 million lenses being implanted annually1.First used in 1949, vast improvements have been made in their designsince, which has led to cataract surgery becoming a precise refractive surgicaltechnique, about a thirty minute procedure. We will first look at the different types of IOLs available at present, theseinclude Toric, Multi-focal, Accommodating, and Aspherical Intraocular lenses.
Toric IOLs are used to correct for astigmatism, and are dependent on theaccuracy of placement within the eye. Multi-focal IOLs create multiple focalpoints in the eye, to correct both distance and near vision at once.Accommodating lenses are used to restore the accommodative response, by usingthe optic shift principle.
Lastly, aspherical lenses are used to improve theoptical quality of the image created by the IOL. Similar to toric lenses,aspherical lenses are dependent on their centration and tilt. Corneal astigmatism, which is where the curvature of thecornea is uneven, leading to improper refraction of light rays, is present in22.2% of patients of cataract surgery to a level of 1.
5 D or more.2For these higher levels of astigmatism, toric IOLs are used as they are notdependent on the healing response of the cornea, and they have shown to be aneffective method of correction. They are however, dependent on rotationalstability within the eye and axis misalignment could result in reduction ofastigmatism correction, or if the rotation is past 30 degrees, the postoperative astigmatism will be higher than the level of corneal astigmatism. Toassist in correct placement, toric IOLs have two markings present on the optic,to line up with the axis of astigmatism. When in the eye the IOL is rotateduntil the markings align with the corneal astigmatic axis, by reference markersplaced on the eye prior to surgery. Different designs of IOLs show differentlevels of rotational stability once in the eye, with friction between the lensand the crystalline bag the main factor in keeping the lens stable.
Lensmaterial affects friction, with silicon lenses showing less friction thanacrylic and PMMA lenses. Lens size in relation to capsular bag size is alsoimportant; if the IOL is too small, there is not enough contact, hence notenough friction for stability, and if too big, the IOL can stretch and distortthe bag. Small side struts on the IOL,known as haptics are also used for stability. These can be open or closed loop,with both having their own advantages and disadvantages.
Multifocal IOLs are mainly used for surgical correction ofpresbyopia, the hardening of the lens which occurs naturally with age andresults in light focusing behind the retina. Multifocal IOLs are used as theiruse is independent of ciliary body function. The lenses are currently the mostreliable for recovering both near and distant vision. Multifocal lenses createfocal points in the eye, corresponding to the working distances.
There aredifferent types of multifocal lenses, with two main types: diffractive andrefractive design. Diffractive lenses can then be categorised into fullydiffractive or partially diffractive, and refractive lenses can be divided intoeither concentric or sectorial. Fully diffractive multifocal IOLs have manyconcentric rings on their surface. Each ring creates a diffraction pattern, andby changing the width of the rings, the diffraction pattern can be used tocreate two focal points within the eye. Asymmetrical diffractive IOLs split thelight unequally and can be used to transmit a higher amount of light to eithernear or distance. Partially diffractive IOLs have a diffraction pattern over a specific area ofthe lens. An example of this type of lens is appodised diffractive multifocalIOLs, where only the central 3.6mm of the lens has the diffraction patternpresent.
The central part of the lens distributes the light between distanceand near vision equally, then moving towards the outer part of the lens, the successiverings distribute more light towards the distance. This lens is pupil dependent,as when the size of the pupil increases more light is distributed to thedistance. Concentric refractive multifocal IOLs have concentric zones that differ incurvature, which creates varying refractive powers. The different zones ofrefractive power create multiple focal points within the eye.
These lenses arealso pupil dependent, as the refractive zones are relatively large. Refractive multifocalIOLs do not precisely split the light into two focal points, but into a smallspread of light near the focal point area. This leads to a broader range ofnear visual acuity instead of the precise optical quality seen with a singlefocal point.3 Sectorial refractive multifocal IOLs, also known as rotationally asymmetrical,are the newest type of multifocal lens. The current technique of implantationis to position the lens towards the inferior portion of the eye. This lensgives two precise focal points, independent of the pupil. As the lens innon-concentric, it does not produce the conventional halo effect, glare isinstead restricted to an area matching the position of the segment.Accommodating IOLs are used to restore the ability of the eyeto adjust its focusing power at all distances.
As the eye ages, the lens insidethe eye becomes less flexible, resulting in the inability to focus near vision.This is a sign of presbyopia in people over 40. As the ciliary muscles arestill active, accommodating IOLs were designed to shift forwards with ciliarymuscle contraction. This is known as the optic shift principle. There are twotypes of accommodating IOLs, single and dual optic. Single optic accommodatinglenses are designed to move anteriorly with accommodative effort, increasingthe effective lens power.
Different single optic lenses rely on ciliary musclecontraction producing either an increase in vitreal fluid pressure, which wouldpush the lens forward, or releasing the zonule tension, allowing the capsularbag of the lens to push against the haptics. Dual Optic lenses are designed tofill the capsular bag.they have a high powered positive anterior optic andnegative posterior optic. The haptic attaches to both optics and keeps the twolenses separated. While the ciliary muscles are relaxed the optics are close toeach other, and under accommodation effort, the capsular bag compresses thehaptics, moving the anterior lens forward. The anterior lens of a dual opticsystem is more powerful than that of a single optic system, meaning dual opticlenses have higher accommodating power.Aspheric IOLs are monofocal IOLs, used to correct spherical aberrationin the eye. A lens with spherical aberration will focus rays that pass throughthe peripheries of the lens closer to the lens than the rays that pass throughthe centre.
Other types of IOL may also increase the positive sphericalaberration of the eye, which naturally increases with age. Using an aspheric IOLadds negative spherical aberration to compensate for the positive sphericalaberration of the eye. This would be an aberration control aspherical IOL, whichis depenent on the lens being centrally positioned in relation to the visualaxis. If this centralisation is off by more than 0.5mm, the reduction of sphericalaberration is lost, and if the is also tilt introduced, the effects ofspherical aberration can be compounded.4 Aberration neutral aspherical IOL are aberration free, meaning they areintended not to introduce any aberration to the eye, but will not correct thespherical aberration of the cornea either.
Centration of the lens is lessimportant with aberration neutral lenses. In order to obtain the best resultsafter implantation, corneal spherical aberration would ideally be measuredaccuratley prior to the surgery, and the lens selected accordingly. Power calculation of the IOL is important as it ensures thatthe lens fits the needs and requirements of each patient. There are fourfactors involved in the power calculation; the eyes axial length (AL), the cornealpower (K), the postoperative IOL position within the eye (estimated lensposition, ELP), and the anterior chamber constant