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ACTUAL VERSUS EXPECTED LASIK FLAP THICKNESS USING
THE MORIA M2 130 MICROKERATOME HEAD
Abd El Monem Mohamed Hamed, MD. * Shereef Abd El Wahab ,MD.** Maha Attia ELFayoumy,MD.***
*
Assistant professor of ophthalmology Benha Faculty Of Medicine
* * Lecturer of ophthalomology Benha Faculty Of Medicine
*** Lecturer of ophthalomology Benha Faculty Of Medicine
AIM: To evaluate actual versus expected laser in situ keratomileusis (LASIK) flap thickness using
the Moria M2,130 microkeratome head.
METHODS: 50 eyes of 25 patients scheduled for LASIK surgery for myopia where enrolled in this
study. Patients were prepared for surgery. On the day of surgery they were admitted to the LASIK
room where the lids were sterilized and draped. A suction lid speculum was applied to one eye after
instilling one drop of Benox into the conjunctival sac, the eye was then washed with balanced salt
solution BSS and the suction of the lid speculum activated to assure there was no excess BSS in the
conjunctival sac then a final preoperative ultrasonic central corneal pachymetry reading using the
sterile probe of the Tomey AL-2000 pachymer was recorded. The suction ring of the Moria M2
mikroketatome was centered on the cornea and immediately after the lasik flap was fashioned and
the suction ring removed the flap was elevated then an intra operative central stromal bed pachmetry
reading was recorded using the sterile Tomey AL-2000 pachymer probe. The actual flap thickness
was calculated by subtracting the intra operative central stromal thickness from the preoperative
central corneal thickness.
SETTING: Out patient Lasik center in the period from January 2004 till beginning of February
2004.
RESULTS: The mean actual flap thickness created by the Moria M2 130 head microkeratome for
the first cut was 153.8 + 17 (range 120µm – 188µm) and for the second cut 148.4 + 15.82 (range
118- 185). The mean difference in flap thickness between the first and second cut was 4.4µm +
1.7µm with the tendency of the second flap to be slightly thinner than the first. This difference was
found to be statistically insignificant (P>0.05, paired t test). The mean difference between the actual
and expected flap thickness was 9.9µm + 1.7 and this was found to be statistically significant ( P
<0.001 ).
CONCLUSION: The difference between the actual and expected flap thickness using the Moria M2
microkeratome was statistically significant and care should be taken when making assumptions
about flap thickness based upon the manufacturers labeling to calculate the residual bed thickness,
having in mind that the flap may be thinner in some cases and thicker in others. The latter must be
thought of well to avoid violating the rule of leaving at least 250µm of stromal bed after laser
ablation, which may lead to postoperative ectasia , keratoconus and even intraoperative stromal
penetration. It is recommended to do an intraoperative pachymetry in all LASIK cases if possible.
INTRODUCTION
Recently there have been multiple reports of ectasia after LASIK in cases which were supposed to
have a residual corneal thickness of more than 410µm after the LASIK procedure1. Most researchers
have agreed that a residual corneal thickness (RCT = residual stromal thickness after ablation +
corneal flap thickness) of 410µm or more should not lead to post operative ectasia, provided that the
postoperative residual stromal thickness (RST) would not be less than 250µm. A lot of surgeons
follow the rule of leaving a postoperative corneal thickness of 410µm to be on the safe side and
avoid the occurrence of postoperative ectasia 2. In spite of following this rule, there still are reports
of cases of postoperative ectasia1. This fact lead researchers to revaluate the methods of preoperative
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calculation of the RST, and thus the final RCT. Before LASIK the best means of estimating RST is
to subtract the theoretical laser correction obtained from the laser computer and the expected flap
thickness normally obtained with a given microkeratome system from the preoperative central
corneal thickness3. Many surgeons make assumptions about flap thickness based upon the
manufacturers’ labeling of the microkeratome when estimating the RST 4. This however may be
unwise as previous reports mentioned that there is a degree of variation in flap thickness, and flaps
may actually be thinner or thicker than expected; the latter is potentially serious if laser ablation
removes tissue to below the accepted standard of 250µm residual stromal bed. Many factors other
than microkeratome model and head number such as mean preoperative pachymetry, size of suction
ring, steepest K reading and order of surgery (first eye or second eye ) may lead to variability in flap
thickness, however age, sex, average K readings, white to white oscillation and translation speed did
not correlate with flap thickness 5. Several studies have been designed to compare the flap thickness
produced by various mikrokeratomes. In our study we used one microkeratome, the Moria M2 ,with
the 130µ head which the manufacturer expects to make a flap thickness of 160µm and we tried to
avoid other factors that may influence the flap thickness by including eyes with a narrow range of
difference in preoperative pachymetry (range 520µm-560µm). The mean preoperative steepest K
reading was 42.95 + 1.12 (range 41.7 – 45) and the Moria Zero suction ring was used in all cases.
Moria M2 has two translation speeds; a speed of 3.3 mm/sec was used in all cases. The collected
data was used to see how close the expected flap thickness was to the actual flap thickness.
PATIENTS AND METHODS
Fifty eyes of thirteen females and twelve males were included in this study , all patient were myopes
with a preoperative error of refraction ranging from -1 diopter to - 5 diopters . Preoperatively the
following parameters were noted: uncorrected visual acuity, best corrected visual acuity, and
manifest refraction. Soft contact lenses were discontinued two weeks before surgery; rigid contact
lenses were discontinued four weeks before surgery. All patients had preoperative corneal
topography (Tomey TMS-2N topographic modeling system) and ultrasound pachymetry (Tomey
AL-2000 pachymeter). The preoperative corneal curvature was measured using automated
keratometry (Nidek ARK7000A). All patients had a slit lamp examination and the preoperative intra
ocular pressure IOP was recorded (Haag Streit applanation tonometer), followed by posterior
segment indirect ophthalmoscopy. On the day of surgery the patients were admitted to the LASIK
room where the lids were sterilized and draped. A suction lid speculum was applied to one eye after
instilling one drop of Benoxinate (Oxybuprocaine hydrochloride 0.4%) into the conjunctival sac, the
eye was then washed with balanced salt solution BSS and the suction of the lid speculum activated
to assure there was no excess BSS in the conjunctival sac then a final preoperative ultrasonic central
corneal pachymetry reading using the sterile probe of the Tomey AL-2000 pachymer was recorded.
All LASIK procedures were performed using the Moria M2 microkeratome with the 130 head which
is theoretically supposed to produce a 160 µm thick flap. All flaps were superiorly hinged and
created after centering the properly sized Moria suction ring on the cornea and activating the
vacuum–suction machine until clinical signs of high IOP were noticed ( pupil dilatation and blacking
out of patients’ vision). Moria suction rings are available in five sizes, plus3, plus2, plus1, Zero and
minus 1. The appropriate suction ring size is selected according to the steepest corneal keratometric
(K) reading and to the table supplied by the manufacturer which specifies the size of the ring to be
used and the stop position to be adjusted and the expected flap and hinge size in mm with every ring
and K reading. The different suction rings for successive ranges of keratomertikc values increases
the safety and reliability in different types of corneas, however in our study we had a narrow range
of steepest K reading that allowed us to use one ring (The Zero ring) for all cases. After the flap was
created, the suction was released and the ring removed, the flap was lifted and three accurate
intraoperative pachmetry readings for the residual stromal bed thickness were recorded using the
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sterile probe of the Tomey AL-2000 pachymer . Laser ablation was done using the Nidek EC-5000
machine, the flap was put back in place and the stromal bed was irrigated with BSS. The same steps
were repeated with the other eye of each patient. A new blade was used for every patient. The first
flap of every patient was always created by a brand new blade while the second flap was always
created by a blade than has already been used once. The actual flap thickness was calculated by
subtracting the central intraoperative stromal bed thickness from the central immediate preoperative
corneal thickness. The actual flap thickness measurements of every created flap and the expected
flap thickness measurements were exported to windows Microsoft Excel and statistically analyzed
using the student t test.
RESULTS:
A total of 50 eyes were reviewed. Sex distribution was 52% females and 48% males there was no
statistical significance between the number of males and females (P>0.05), the mean age was 36
years + 8.9 (SD). 50% of the procedures were in the first eyes and 50% in second eyes. The mean
preoperative pachymetry measurement was 542.7 + 13.3µm (range 520 – 560 µm) and the mean
steepest K reading was 42.95 + 1.12 (range 41.75 – 45). The mean white to white measurement was
11.5 + 0.4 and the mean scotopic pupillary diameter was 5.5 + 0.9. The mean actual flap thickness
created by the Moria M2 130 head microkeratome for the first cut was 153.8 + 17 (range 120µm –
188µm) and for the second cut 148.4 + 15.82 (range 118- 185). The mean difference in flap
thickness between the first and second cut was 4.4µm + 1.7µm with the tendency of the second flap
to be slightly thinner than the first. This difference was found to be statistically insignificant
(P>0.05, paired t test). The mean difference between the actual and expected flap thickness was
9.9µm + 1.7 and this was found to be statistically significant ( P <0.001 )
Chart 1 shows the actual first cut flap thickness in respect to the expected flap thickness.
Thickness inmicrons
Actual First Cut Flap Thickness In Respect to
expected flap thickness
200
150
expected for first cut
100
first eye flap
thickness
50
28
25
22
19
16
13
10
7
4
1
0
number of eyes
CHART 1
Chart 2 shows the actual second flap thickness in respect to the expected flap thickness
3
Thickness in microns
Actual Second Flap Thickness In Respect To The
Expected Flap Thickness
200
150
second eye flap
thickness
100
expected for first cut
50
28
25
22
19
16
13
10
7
4
1
0
Number of eyes
CHART 2
Chart 3 shows actual first and second cut flap thickness in comparison to each other
Thickness in microns
Actual First And Second Cut Flap Thickness In
Comparison To Each Other
200
150
first eye flap
thickness
100
second eye flap
thickness
50
28
25
22
19
16
13
10
7
4
1
0
Number of eyes
CHART 3
DISCUSSION:
Laser in situ keratomileusis continues to be the most common corneal refractive surgery performed
worldwide6. It offers rapid visual recovery, low risk for corneal haze, and reliable results for
correction of nearsightedness, farsightedness, and astigmatism. It is performed by first creating an
anterior corneal flap with a microkeratome. The purpose of the microkeratome in LASIK is to create
a corneal flap of a desired thickness, thus exposing the stroma for laser ablation, the lack of precision
in reproducing flap thickness with actual microkeratomes is an important drawback of these
instruments 7.In this study we examined the actual flap thickness produced by the Moria M2
keratome in 50 consecutive eyes and compared it to the manufacturers presumed flap thickness for
the same microkeratome. The Moria M2 is an automated microkeratome designed to produce a
surface-parallel keratectomy of a preset flap diameter and a uniform flap thickness, the 130 head of
the microkeratome which was used in this study is supposed to produce a flap of 160µm thick
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according to the manufacturers manual. The device was evaluated in this study after excluding the
other factors that are known to affect the flap thickness such as variation in preoperative central
corneal pachmetry, size of the suction ring, and steepest K reading by selecting cases with a narrow
range of difference in preoperative pachymetry and K readings, we also used one rig size (The Zero
ring) for all cases. Solomon et al 5 reported a significant difference in flap thickness between the first
and second cuts done by the same micrikeratome blade and thus we recorded the flap thickness
obtained after the first cut and that obtained after the second cut for each case. In this study the mean
actual flap thickness created by the Moria M2 130 head microkeratome for the first cut was 153.8 +
17 and for the second cut 148.4 + 15.82 and the difference in flap thickness between those created
after the first or second cut was found to be statistically insignificant (P>0.05, paired t test).The
mean flap thickness for flaps created after first cut, in this study ranged from 120µm – 188µm, while
those created after the second cut ranged from 118- 185µm. No flaps thinner than 118 µm or thicker
than 188µm were encountered throughout the study. Four out of the fifty (8%) eyes enrolled in the
study had an actual calculated flap thickness of 160µm which was exactly equal to the manufacturers
expected flap thickness, while 46 eyes (92%) had an actual flap thickness that ranged from 118µm to
188µm.This was found to statistically significant (P>0.05).The results of this study are close to the
results of Miranda 8 who studied the flap thickness reproducibility using the Moria Carriazo
Barraquer (CB), the Moria M2 and the Hansatome microkeratome, and found the Moria M2 to be
of the greatest reproducibility with a mean actual flap thickness of 157+ 40µm .The difference in
thickness between the first and second cut flaps was also found to be present but of no significance
when using the Moria M2 microkeratome in contrast to other microkeratomes such as the
Hansatome, Summit Krumeich-Barraquer microkeratome (SKBM),and Amadeus microkeratomes
which showed a significant difference between first and second cut 5 . The results also correlate to
the results of Javaloy 9 who found a difference between the desired and actual flap thickness when
using the Moria M2 microkeratome with a mean flap thickness of 150.13µm + 20.42 ranging from
109µm to 189µm. The actual flap thickness in this study ranged from 118µm to 188µm this differed
from the results of Solomon 5 who reported a range of flap thickness of 84µm – 204µm, this wide
range was not seen in our study may be because only 50 eyes were included, in comparison to the
number of eyes included in the study of Solomon 5, 1634 eyes, in which they used the Moria M2 on
140 eyes the big number of eyes they studied may explain the wide range they encountered.
In conclusion the difference between the actual and expected flap thickness using the Moria M2
microkeratome was statistically significant and care should be taken when making assumptions
about flap thickness based upon the manufacturers labeling to calculate the residual bed thickness,
having in mind that the flap may be thinner in some cases and thicker in others. The latter must be
thought of well to avoid violating the rule of leaving at least 250µm of stromal bed after laser
ablation, which may lead to postoperative ectasia , keratoconus and even intraoperative stromal
penetration. It is recommended to do an intraoperative pachymetry in all LASIK cases if possible.
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Refract Surg. 2004;30:1067-72.
2. Wang Z, Chen J, Yang B. Posterior corneal surface topographic changes after laser in situ
keratomileusis are related to residual corneal bed thickness. Ophthalmology. 1999
Feb;106(2):406-9; discussion 409-10.
3. Flanagan GW, Binder PS. Precision of flap measurements for laser in situ keratomileusis in
4428 eyes. J Refract Surg. 2003 Mar-Apr;19(2):113-23.
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microkeratomes with a second motor for advancement. Ophthalmology. 2003
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