Aspheric intraocular lenses

Optical aberrations occur when light from a point object does not form a perfect point after passing through the optical system. The refraction of peripheral rays of light is greater than those passing near the lens center. This is pupil-dependent and may account for the symptoms of dysphotopsia when the Snellen visual acuity is good, but the patient has a poor subjective outcome. The cornea has a positive spherical aberration and the young crystalline lens has negative spherical aberration, resulting in neutralization (Fig. 15.3). However, later in life the aging crystalline lens develops positive spherical aberration and therefore both combine to increase the spherical aberration of the eye. Therefore if one inserts an intraocular lens with a negative spherical aberration to neutralize that of the cornea, visual function will theoretically improve.

Fig. 15.3 (A) Spherical aberrations in young eye. (B) Spherical aberrations in older eye. (C) Using a modified IOL surface to correct for the imbalance in spherical aberrations.

A number of manufacturers have therefore developed aspheric intraocular lenses, which are said to improve contrast sensitivity function and visual performance, particularly in mesopic lighting conditions. In some implants the aspheric surfaces on the anterior, and in some the posterior surface, and the degree of IOL-induced spherical aberration varies.

Another approach has been to produce an intraocular lens with no spherical aberration to allow the small positive spherical aberration from the cornea to provide a degree of pseudoaccommodation.

Although there is a wealth of evidence which suggests that these lens implants are theoretically better, in practical terms the patients may not notice the difference. This may in part be due to the fact that the lens implants are not directly matched to the spherical aberration of the cornea, i.e. they are not customized, and that if there is more than 0.8 mm of decentration all advantage would be lost.

Therefore, there is variable corneal spherical aberration, which is difficult to measure and then match to a standard set of lens implants. Centration and tilt of the intraocular lens can be important determinants of the advantages for these lens implants. The potential to customize intraocular lenses postoperatively may overcome these disadvantages.

Toric intraocular lenses

When planning cataract surgery, the surgeon must take into account the corneal astigmatism, i.e. that which is left following removal of the natural lens. It must also be remembered that a low degree of myopic against the rule astigmatism can be useful in producing pseudoaccommodation. The peroperative options for correcting astigmatism are at the corneal plane, that is placing the incision on the steep meridian, limbal relaxing incisions (LRI), or opposite clear corneal incisions (OCCI). At the lenticular plane one can insert a toric intraocular lens. The lenses must be designed to have no postoperative rotation, otherwise they would induce a cross cylinder effect and have to be stable in both the short and long term. Thirty degrees of rotation will cancel out the effect of the toric correction completely.

The preoperative assessment and lens power calculation have to be accurately measured, and this should ideally be done with the Javal Shiotz keratometer (as the gold standard) but may also be done with an IOL Master or by corneal topography. The surgeon needs to know what astigmatism is induced by the incision(s), and this is influenced by size, location, and position. Many lens manufacturers have on-line calculators to determine the toric intraocular lens power and its orientation.

The horizontal axis of the eye needs to be marked on the limbus immediately pre-operatively, preferably at a slit lamp, prior to any form of anesthesia, to take into account the effect of cyclorotation of the eye (which can be up to 4°) when the patient lies down. After the lens implant is inserted and the viscoelastic removed, the lens then must be aligned with the suggested axis with the aid of marks on the surface of the lens and on the limbus.

The critical success factors are preoperative assessment of the astigmatism, accurate marking of the horizontal axis, and alignment of the intraocular lens.

A 1° axis error results in a 3.3% loss of the intraocular lens cylinder power correction, and 10° off will therefore lose 33% of its cylinder power. If it is 90° off axis, cylinder power will double.

These lenses are very effective if the corneal astigmatism is greater than 1.5 diopters. Less than this, treatment in the corneal plane is a better option due to error in corneal power and axis calculation and axis alignment and stability.

Blue blocking intraocular lenses

The risk factors for progression of age-related macular degeneration are hypertension, hyperopia, smoking, and confluent drusen. Age-related macular degeneration can progress following cataract surgery due to acute (class II photochemical reaction) blue light damage and perioperative inflammation, the latter being associated with compliment activation (Fig. 15.4A). It might also progress in the long term due to increased exposure to blue light and is proportional to the level and duration of exposure and the wavelength. The process is said to occur because energy from blue light is absorbed by the retinal pigment epithelium which interacts with fluorophore (A2E) within lipofuscin to produce free radicals, which subsequently cause oxidative stress to the RPE cells and apoptosis (Fig. 15.4B).

Fig. 15.4 (A) Acute effect of blue light damage. (B) Chronic effect of blue light damage.

The cataractous lens becomes yellow with age to produce a natural filter which is then removed during routine cataract surgery. The rationale for blue filters/yellow intraocular lenses is to maintain a blue light blocking/reducing effect (at peak approx 440 nm, range 400–475 nm) to protect against chronic exposure to blue light. These blockers are said to reduce leakage from the blood–retinal barrier, reduce free radical damage, and therefore reduce the risk of progression of long-term age-related macular degeneration.

Although there is laboratory evidence that this effect does occur, many of the experiments were done in the short-term with acute photic damage and may not represent what happens in the aging human.

There appears to be no detrimental effect on color vision, contrast sensitivity, or visual acuity with these blue-filtering lenses. However, there may be a reduction in mesopic retinal function and an effect on circadian rhythms by reducing the amount of light which reaches non-visual melanopsin containing retinal ganglion cells/blue light photo receptors in the retina, which are responsible for circadian rhythm entrainment. Trials to test this are currently underway.

Therefore, there is a clear rationale for blue light filters, experimental evidence of efficacy, but little clinical proof. It is likely that their total light exposure is a risk factor for progression of macular degeneration, which is dependent on geographical location and lifestyle4.

Multifocal intraocular lenses

There are three types of multifocal/accommodating intraocular lenses. The path of monochromatic light (Fig. 15.5) illustrates the optical effect of the different types of lens.

Fig. 15.5 The light path through different IOLs is visualized by projecting a monochromatic green (550 nm) light through the lens in water.

The lens designs vary in the distance dominance. The amount of reading addition varies between 3 and 4 diopters in the IOL plane, equivalent to 2.25–3.75 in the spectacle plane, and the split between distance and near may be 50/50, or more usually 60/40. The materials now are nearly always acrylic for the diffractive, refractive lenses and silicone for the accommodating lens.

The lens implant technologies may be mixed (mix and match) to enhance the depth of field and use the advantages of each lens i.e. using a diffractive lens in the non-dominant and refractive lens in the dominant eye.

The concept of monovision is important and should always be borne in mind when assessing patients.