8.3 Comparison of RK, PRK and ALK and LASIK:
Description
This article is from the Vision and Eye Care FAQ, by grants@research.canon.com.au (Grant Sayer) with numerous contributions by others.
8.3 Comparison of RK, PRK and ALK and LASIK:
PRK: involves less than 5% the depth of the cornea, typically. Some
likelihood of infection due to use of post-op bandage lenses for
post-op pain control. There is a possibility of infectious keratitis
which will almost always resolve, without side-effects, if treated
early.
RK: can involve up to 95% the depth of the cornea, and eye penetration
has occurred (which can cause blindness in the long term). The risk of
infection is about 1/1000 operations and is treatable. Minute
perforations ("microperforation") are relatively common (5%) and
are harmless; larger ("macroperforations") are very rare and require
sutures. Blindness will only occur if there is an associated infection.
PRK: repeatable, so for high corrections (-6 to -10) if the first procedure
doesn't give 100% correction, the second can.
From study of 298 patients. Severe scarring or under-correction
requiring repeated treatment occurs in a small percentage of patients.
Scarring occurred in 1.8% of patients with an original correction of
up to and including -6.0 dioptres and 8.8% of patients with > -6.0 D.
Under-correction occurred in 2.7% of patients with <= -6.0 D and
30 to 40% of > -6.0 D. 30 eyes were redone (11 due to scarring and
27 to under-correction). After 6 months 63% of the re-treated group
(19 eyes) were within +/- 1 D of a zero correction. Only 1 eye still
had scarring.
(Arch Ophthalmol 1992 Sep 110(9) pp 1230-3).
Another study examined 17 eyes retreated due to undercorrection. After
6 months 64.7% (11 eyes) had uncorrected visual acuity at least as good
as 20/40 and 58.8% (10) were within 1.0 dioptre of emmetropia (ie 0 D
correction). (Am. J. Ophthalmol. 1994 Apr 117(4) pp 456-61)
RK: not repeatable in general (but it is possible to do PRK after RK).
91 eyes of 71 patients who had RK but still were myopic were treated
with PRK. Prior to PRK their corrections ranged from -1.5 to -8.0 D.
Twelve months after PRK uncorrected visual acuity was 20/40 or better
in 90% of patients, and 76% of patients were within +/- 1.0 D of
intended correction. (J. Refract. Corneal Surg. 1994 Mar-Apr
10(2 Suppl) pp 235-8).
procedure can be repeated - called enhancement surgery - with rate
of repeating as high as 30% (Werlibin, Archives of Ophthalmology, 1994,
95% achieve 20/40 or better).
PRK: High accuracy.
A study of 98 eyes 6 months after PRK reported the following results.
original correction % within +/- 0.5 D of
attempted correction after PRK
< -3.0 D 100.0%
-3.1 to -6.0 D 92.3%
-6.1 to -9.0 D 77.8%
> -9.0 D 100.0% (attempted correction
not necessarily 0 D in
this group)
2 eyes lost 2 lines, while 4 eyes gained 2 or more lines of best
corrected visual acuity. (J. Refract. Corneal Surg. 1994 Mar-Apr
10(2 Suppl) pp 231-4)
From a study of 18 patients (23 eyes) with high degrees of myopia
(-8.0 D to -19.50 D). After approximately 6 months, 39% (9 eyes)
were within +/- 1 D and 65% (15 eyes) were within +/- 2D.
(Arch. Ophthalmol. 1993 Dec 111(12) pp 1627-34)
RK: Not terribly predictable accuracy.
"undercorrection occurs commonly" and "amount of correction cannot
be predicted accurately for an individual patient"
(Ophthalmology 1993 July 100(7) pp 1103-15)
From the PERK study (prospective evaluation of RK) involving 435
patients in the US. Follow-up after 4 years (91% still involved).
Results:
55% within +/- 1 dioptre of desired result
28% under-corrected by > 1 D
17% over-corrected by > 1 D
90% prediction interval width 4.42 D "indicating lack of
predictability". Refractive error not stable in some eyes. 23%
had change of greater than 1 D between 6 months and 4 years after
surgery. 64% of 323 patients who had both eyes treated no longer
needed glasses or contacts.
(JAMA 1990 Feb 23 263(8) pp 1083-91)
A better indication of predicatability of RK is documented in
Werblin, Archives of Ophthalmology [ full details to follow ]
PRK: Structural integrity of the eye essentially unaltered.
J. Cataract Refract. Surg. 1994 Jan-Feb 10(1) pp 36-7 reports two
cases of PRK patients who sustained blunt trauma to the eye.
"Corneal abrasion following trauma in two patients who had undergone
PRK healed as expected in a normal cornea."
RK: Structural integrity of the eye is compromised. RK incisions may
remain incompletely healed and can re-open years later.
1 patient had incisions re-open 9 years after RK during surgery
on the cornea. (J. Cataract Refract. Surg. 1993 July 19(4) pp 542-3)
1 patient had traumatic rupture of the cornea 4 years after RK due
to an athletic injury. (Vestn. Oftalmol 1990 Mar-Apr 106(2) pp 64-5).
PRK: There is an occasional side effect of a buildup of collagen fibres
while the eye heals - "plaques". They don't manifest themselves
visually, but the doctor can see them examining the eye minutely.
It happens in about 25% of the cases during healing, though
most they go away in time. Halos are also reported with PRK -
references; Dan Epstein and John Marshall [ details to follow ].
RK: Halos around objects and distorted refractions around point sources
of light.
ALK: can cure myopia up to -30.0 dioptres. Although accuracy of within
1D in 20-40% of operations.
ALK: instrument is accurate, and procedures are performed quickly with corneal
surface left intact and unaltered so minimising healing problems, eg
scar tissue, as in PRK and RK
ALK: hyperopia can also be improvied with this procedure
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