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refractive errors of a degree that previously prevented applicants from obtaining medical certification needed to
work in the aviation environment.
11.4.47 There is, however, rarely any reason for an applicant to submit to refractive surgery in order to
meet the visual requirement, and it is important that applicants understand this.
11.4.48 Details of the surgical techniques are not important for the medical examiner but some background
knowledge may be useful. Refractive surgery is a rapidly changing field in which many different techniques
have been tried. Some of the more widely used techniques are:
a) Clear lens extraction (CLE)
b) Radial keratotomy (RK)
c) Astigmatic keratotomy (AK)
d) Automated lamellar keratoplasty (ALK)
e) Photorefractive keratectomy (PRK)
f) Laser assisted in-situ keratomileusis (LASIK)
g) Laser thermokeratoplasty (LTK)
h) Intrastromal corneal ring (ICR).
5 YAG: yttrium-aluminum- garnate
III-11-32 Manual of Civil Aviation Medicine
Clear lens extraction (CLE)
11.4.49 The improved safety and excellent results from cataract surgery has led to increasing use of clear
lens extraction together with the use of low power intraocular lenses in individuals with high myopia.
Radial keratotomy (RK)
11.4.50 Radial keratotomy is used to correct myopia and astigmatism. A diamond blade is used to make
a series of radial incisions in the cornea. The incisions must be almost the full thickness of the cornea. The
number and orientation of the incisions are determined by the refractive error. The central portion of the cornea
is not treated, leaving an untouched optical zone of about 4 or 5 mm in diameter. The incisions and their
subsequent healing leads to flattening of the cornea with reduction of the myopia and astigmatism.
Astigmatic keratotomy (AK)
11.4.51 This is similar to RK but placement of the incisions may be non-radial. It can be done as a primary
procedure or as a secondary procedure to correct residual or induced astigmatism following other refractive
surgery, cataract surgery or other corneal trauma.
Automated lamellar keratoplasty (ALK)
11.4.52 This procedure can be used to correct low to moderate degrees of hyperopia. A portion of the
central cornea is removed with a microkeratome. The removed portion of cornea is reshaped then sutured back
onto the eye.
Photorefractive keratectomy (PRK)
11.4.53 In this procedure an excimer laser operating in the ultraviolet portion of the electromagnetic
spectrum is used to remove a portion of the central cornea. The size and shape of the disc of tissue to be
removed are calculated from the pre-operative refractive error. Myopia and astigmatism are the most suitable
cases for PRK but hyperopia can be treated successfully.
Laser assisted in-situ keratomileusis (LASIK)
11.4.54 Also called laser assisted intrastromal keratomileusis, this procedure is most useful for the higher
degrees of myopia. A central corneal hinged flap is made with a microkeratome. The flap is raised and the
excimer laser used to reshape the inner layers of the corneal stroma. When this has been done the corneal flap
is replaced. Rapid visual recovery, better predictability and less trouble with glare are advantages of this
procedure compared with PRK. Flap displacement, however, is a well-recognized complication of LASIK. It can
occur months after the procedure, sometimes from the patient rubbing his eyes too vigorously. Often the flap
can be surgically replaced. Bilateral simultaneous flap displacement is unlikely, but would be incapacitating.
After successful laser surgery, corneas will appear normal on ordinary clinical examination, but the reshaping
can be detected by measuring the corneal surface curvatures using keratography (corneal mapping).
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-33
Laser thermokeratoplasty (LTK)
11.4.55 A holmium-YAG laser is used to induce shrinkage of the corneal stroma in a series of (usually eight)
spots in a circle. The circle is placed close to the limbus when treating hyperopia and more towards the centre
of the cornea when treating myopia.
Intrastromal corneal ring (ICR)
11.4.56 A narrow strip of plastic material is threaded into the peripheral corneal stroma to form a ring which
alters the corneal curvature without surgical invasion of the central cornea. This procedure has the advantage
of being reversible.
Problems with refractive surgery
11.4.57 Considerable experience with refractive surgery has been gained worldwide. The success rate is
high, with some series reporting over 95 per cent of patients with low to moderate refractive errors achieving
uncorrected visual acuity of 6/12 (20/40, 0.5) or better.
　
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