What are the Limitations of a Laser Surgery?
When we think of correcting the vision, there is only one thing that comes to our min, LASIK. Day by day, as the technology is advancing, so are the hopes and expectations from LASIK. But even with the latest techniques of LASIK, there are always situations where it cannot be done. Following are some cases in which a LASIK eye surgery cannot be performed:
Minimum Age – 18 Years
By 18 years, the power of glasses tends to become stable in most of the people. However, in a human system, there can be no hard and fast rules. It means that in some people the number may become stable even by 16 years, while in some it may not be stable even at 22 years. Even if the number is stable by 16 years, LASIK should not be done at this stage because the future cannot be predicted, and if the number increases later, it can only lead to regrets. As a general observation, it is seen that low numbers like -1.0 to -3.0 tend to become stable even before 18 years of age. Numbers from 3.0 to 6.0 may become stable by 18 to 20 years. Number from 6.0 to 9.0 may become stable by 22 years, while numbers beyond nine may or may not be absolutely stable even beyond 22 years.
Maximum Age – No Limit
We have done LASIK in people even at the age of 60 years. It is a question of when one decides in life that enough is enough and that one would like to see the world without these ‘optical crutches’. In our personal experience of 20+ years of LASIK, we have seen that Indians usually accept glasses as part of their identity by the age of 40. Rarely have we seen people asking for LASIK beyond this age. However, in the last few years, we have noticed a definite social change. Parents of children, who have got LASIK done successfully, are now asking whether they too can get rid of their glasses at this stage.
Ideal Age – Between 20 and 40 Years
There is always an age band when people feel that this is the ideal age for LASIK. We feel that this band is within the age of 20 to 40 years. During this period, there is a strong desire to look good without glasses. Matrimony is also a major reason in India, while other reasons like lifestyle freedom, sports, personality enhancement and self-confidence booster play an important role as well.
Pregnancy or Breastfeeding Limitations
LASIK is not advisable during pregnancy or breastfeeding. During this period, the hormonal levels in the female body are high, and there is an associated relaxing of tissues and increase in spectacle power. Even though this increase in spectacle power may happen in only 10 % of the cases, still this number is pretty high to caution against LASIK during this period. In those females, where the number increases during pregnancy, it is observed that the change is usually between 0.5 to 1.0 D of spectacle power. It is also observed, that if the spectacle power increases in first pregnancy, there may be a similar increase in the subsequent pregnancy.
Limitations of Corneal Thickness
Cornea should be adequately thick to perform LASIK. Too thin a cornea becomes a limiting factor. A normal Indian cornea has a central thickness of 530 microns or 0.53 mm. In this thickness, one can safely correct power of -8.0 by SBK (thin flap LASIK) or the Bladeless Femto LASIK.
A cornea which is less than 495 microns is considered thin, while a thickness of fewer than 450 microns is a ‘no go’. The natural assumption from these figures is that if the cornea is thicker than 530 microns, then more spectacle number can be corrected. It is not correct as treatment of power more than -8.0 leads to excessive flattening of the cornea and resultant disturbances in a vision called a spherical aberration.
It is therefore recommended that the ideal upper limit is -8.0, while the acceptable upper limit is -10.0. Beyond this number, LASIK should be avoided, and a better way would be to go in for an ICL implant. The upper limit of correction by LASIK for a plus number, i.e. hypermetropia is +5.0.
Limitations of Corneal Curvature
Too flat a cornea or too steep a cornea can also be a limiting factor.
A normal human cornea has a curvature of 44.0 D. A cornea with a curvature of 41.0 D or less is considered a flat cornea. LASIK involves lifting up of cornea flap, in very flat cornea cases, the size of the flap lifted may be too small. It means that the area of cornea available for laser treatment may have to be reduced and it may result in visual problems like halo’s and glare around lights during the night. Femto Second Laser made flaps do better even in the flat cornea.
A cornea which is more than 48.5 D is considered as a steep cornea. In such cases, lifting the flap may result in buttonholing of the flap, or an irregular thickness of the flap. It happens in only a very small percentage of steep corneas. In case of buttonholing of the flap, the flap is re-positioned bask without doing the laser treatment, and allowed to heal. The flap may be re-lifted after three months, preferably with a Femto Second Laser, as the latter gives more dependable flaps in extremes of corneal curvatures.
Limitations in Corneal Pathology
Keratoconus is a corneal condition where the corneal tissue is inherently weak. Due to the normal eye pressure within the eyeball, the cornea starts bulging out. This makes the cornea steep and irregular. Rays of light entering the eye through such corneas get irregularly scattered thereby resulting in loss of quality of focus and decrease in vision.
LASIK involves ablating the cornea to change its curvature. Ablating the cornea makes the cornea thin. Acceptable level of thinning does not weaken a healthy cornea. However, in an inherently weak cornea, the LASIK induced thinning may weaken the cornea to the extent that it will start to bulge causing optical distortions and a decrease of vision. This abnormal bulging of the cornea may then be stopped by ‘C3R’ or what is called Collagen Cross-Linking.
The importance of cornea screening is to detect sub-clinical or hidden Keratoconus, and reject these cases so that the rest of the eyes undergoing LASIK treatment are safe. With modern methods of corneal scanning like Pentacam, it is possible to detect most of these hidden Keratoconus cases, thereby making LASIK very safe. However, very rarely, one may still have to face a tricky situation, and Collagen Cross-Linking may then have to be done to arrest the progress of the disease.