Refractive surgery of myopia

Surgery to correct refractive errors has become very
popular. It should be performed after the error has
stabilized; preferably after 20 years of age.

Refractive surgery of myopia
1. Radial keratotomy (RK) refers to making deep (90 percent of corneal thickness) radial incisions in the peripheral part of cornea leaving the central 4 mm optical zone (Fig 1).

Fig. 1. Radial keratotomy. (A) configuration of radial
incisions; (B) depth of incision.

These incisions on healing; flatten the central cornea thereby reducing its
refractive power. This procedure gives very good correction in low to moderate myopia (2 to 6 D).

Disadvantages. Note: Because of its disadvantages
RK is not recommended presently. (i) Cornea is
weakened, so chances of globe rupture following
trauma are 
more after RK than after PRK. This point is
particularly important for patients who are at high
risk of blunt trauma, e.g., sports persons, athletes
and military personnel. (ii) Rarely, uneven healing may
lead to irregular astigmatism. (iii) Patients may feel
glare at night.

2. Photorefractive keratectomy (PRK). In this technique, to correct myopia a central optical zone of anterior corneal stroma is photoablated using

excimer laser (193-nm UV flash) to cause flattening of the central cornea (Fig. 2).

Fig. 2. Photorefractive keratectomy (PRK) for myopia
as seen (A) from front; (B) in cross section.

Like RK, the PRK
also gives very good correction for –2 to –6 D of myopia.
Disadvantages. Note: Because of its disadvantages
PRK is not recommended presently: (i) Postoperative
recovery is slow. Healing of the epithelial defect may
delay return of good vision and patient may
experience pain or discomfort for several weeks. (ii)
There may occur some residual corneal haze in the
centre affecting vision. (iii) PRK is more expensive
than RK.

3. Laser in-situ keratomileusis (LASIK). In this technique first a flap of 130-160 micron thickness of anterior corneal tissue is raised. After creating a
corneal flap midstromal tissue is ablated directly with an excimer laser beam, ultimately flattening the cornea
(Fig. 3).

Fig. 3.. Procedure of laser in-situ keratomileusis
(LASIK).

Currently this procedure is being
considered the refractive surgery of choice for myopia
of up to – 12 D.

Patient selection criteria are:

•  Patients above 20 years of age,
•  Stable refraction for at least 12 months.
•  Motivated patient.
•  Absence of corneal pathology. Presence of ectasia or any other corneal pathology and a corneal thickness less than 450 mm is an absolute contraindication for LASIK.

Advances in LASIK. Recently many advances have been made in LASIK surgery. Some of the important advances are:

• Customized (C) LASIK. C-LASIK is based on thewave front technology. This technique, in addition to spherical and cylindrical correction, also corrects the aberrations present in the eye and gives vision beyond 6/6 i.e., 6/5 or 6/4

• Epi-(E) LASIK. In this technique instead of corneal stromal flap only the epithelial sheet is separated mechanically with the use of a customized device (Epiedge Epikeratome). Being an advanced surface ablation procedure, it is devoid of complications related to corneal stromal flap.

Advantages of LASIK. (i) Minimal or no postoperative
pain. (ii) Recovery of vision is very early as compared
to PRK. (iii) No risk of perforation during surgery and
later rupture of globe due to trauma unlike RK.
(iv) No residual haze unlike PRK where subepithelial
scarring may occur. (v) LASIK is effective in
correcting myopia of – 12 D.

Disadvantages. 1. LASIK is much more expensive.
2. It requires greater surgical skill than RK and PRK.
3. There is potential risk of flap related complications
which include (i) intraoperative flap amputation,
(ii) wrinkling of the flap on repositioning,
(iii) postoperative flap dislocation/subluxation,
(iv) epithelization of flap-bed interface, and
(v) irregular astigmatism.
4. Extraction of clear crystalline lens (Fucala's
operation) has been advocated for myopia of –16 to
–18 D, especially in unilateral cases. Recently, clear
lens extraction with intraocular lens implantion of
appropriate power is being recommended as the
refractive surgery for myopia of more than 12 D.
5. Phakic intraocular lens or intraocular contact lens
(ICL) implantation is also being considered for
correction of myopia of >12D. In this technique, a
special type of intraocular lens is implanted in the
anterior chamber or posterior chamber anterior to the
natural crystalline lens.
6. Intercorneal ring (ICR) implantation into the
peripheral cornea at approximately 2/3 stromal depth
is being considered. It results in a vaulting effect that
flattens the central cornea, decreasing myopia. The
ICR procedure has the advantage of being reversible.
7. Orthokeratology a non-surgical reversible method
of molding the cornea with overnight wear unique
rigid gas permeable contact lenses, is also being
considered for correction of myopia upto –5D. It can
be used even in the patients below 18 year of age.