Monday, August 24, 2009

Which Intraocular Lens (IOL)? A confusing array of choices.

Which Intraocular Lens (IOL) is most suitable for my eyes?

People suffering from cataract and planning for cataract surgery are faced with this difficult question & a frustrating list of choices. I will attempt to make it simple, let me start by explaining a few characteristics of the Intraocular Lenses (IOL):

What is Intraocular Lens (IOL)?
In cataract surgery the natural lens of the eye is removed. This leads a significant decrease in the optical convergence power of the eye or hypermetropia. Intraocular lenses are small lenses made of polymers and implanted into the eye in place of natural lens to help focus the light on to retina.

Foldable vs. Non-foldable Lenses
In conventional cataract surgery & in SICS a large cut / incision is made in the eye to remove the cataract. This is followed by insertion of a rigid 6.0/6.5 mm lens through the incision into the eye. These lenses are rarely used these days.
In phacoemulsification the entire operation done through a small hole of 3.0 mm or less. It is not possible to implant a rigid lens. This difficulty is overcome by use of very flexible lenses which can be folded and loaded in special injectors aninserted through the small hole into the eye. These are currently the preferred lens to be used.

Lens Material (Silicone, Hydrophillic & Hydrophobic Acrylic)
Foldable Intraocular Lenses are commonly made of silicone or acrylic polymer (hydrophobic or hydrophillic).
Silicone was used in older lenses & gave good optical results in the immediate post-operative period. But these lenses were somewhat bulky and lead to severe thickening of anterior & posterior capsules (membrane covering of natural lens which is left behind to provide support for artificial intraocular lens). This lead to deterioration of vision and required treatment by YAG Laser Capsulotomy.
Hydrophillic Acrylic lenses are highly flexible and slim, therefore, can be introduced through extremely small incisions. They have good immediate visual results but suffer from tendency to cause thickening of posterior lens capsule.
Hydrophobic Acrylic lenses are slightly less flexible and somewhat thicker than their hydrophillic counterparts. But they are extremely inert and do not lead to lens capsule thickening. Due to these properties they are ideal for use in children, young adults and the eyes predisposed to inflammation. Being hydrophobic in nature it is possible to incorporate a color tint to avoid blue chromatopsia after surgery.

Spheric vs. Aspheric IOL
Spheric are the conventional lenses where both surfaces of the lens are uniformly curved or are part of a sphere. These lenses have inherent problem of spherical aberrations caused by excessive bending of light at the edges.
Aspheric or advanced optics lenses have their curvatures flattening out towards the periphery or edge, thereby, minimizing spehrical aberrations.
Aspheric lenses give much sharper vision and better contrast but due to decreases depth of focus, the need for reading glasses becomes more urgent. On the other hand spheric lenses, though lack the sharpness & contrast to some extent but have better depth of focus leading to slightly better near vision. However, almost all lens manufacturers are replacing their spherical lenses with aspheric.

Monofocal vs. Multifocal IOL
Monofocal lenses have a single focal length and unlike the natural lens do not possess the ability to change the focus. Therefore the patient needs to wear reading glasses in order to focus at near fine print. But these lenses provide much better contrast and image clarity.
Multifocal lenses have 2 foci - a distance and a near focus. This is achieved by having multiple optical zones fashioned in the lens in the form of alternating rings for distance and near. This leads to splitting of light into distance and near foci. As a result both the distant and near objects can be focussed in the eye at the same time. However, the contrast and sharpness of the vision is reduced significantly. There lenses are also prone to more glare and haloes of light during driving at night time.
So for patients who need sharp vision and better contrast and do not hesitate in wearing reading glasses, monofocal lenses are most suitable. Whereas, those who do not want dependence on reading glasses but are willing to compormise on contrast and sharpness of vision, can opt for multifocal lenses.
Please note that both eyes should have the same type of lens - monofocal OR multifocal.

Toric vs. Non-toric
Toric lenses, besides having spherical power, also have cylindrical power which corrects astigmatism present in the eye. Non-toric lenses do not have any cylindrical power. Eyes with no or negligible astigmatism require non-toric lens and the eyes with significant astigmatism would benefit from toric lenses. Toric lenses are especially useful in patients with high degree of against the rule corneal astigmatism which is not amenable to correction but incision placement.

My preferred lens
Currently I prefer to use Foldable Hydrophobic Acrylic Aspheric Monofocal Lens with a natural color tint and toric where required. Multifocal is preferred where cosmetic requirements is more important and there are no critical visual needs.

Dr. Sanjay Dhawan
Senior Eye Surgeon
New Delhi & Gurgaon, India
Click here to learn more about Cataract Surgery


testing said...

My father has to undergo cataract surgery (PHACOEMULSIFICATION)for his second eye. Can it be done during monsoons? Will the moisture during rainy season have any effect on the post-operative eye? Will the healing procedure take too long? Please let me know.

Dyslexicon said...

You may go ahead with your father's eye surgery. Monsoon & rains do not affect the course of surgery or recovery in any way. Just make sure the living environment is clean.

testing said...

Thanks so much for the quick response.

Neera Agrawal said...

nice information.thanks for the updation.