Study tracks blade influence on flap thickness
Cheryl Guttman in San Francisco
QUALITY flaps can be obtained using blades from different manufacturers
in a given microkeratome, but blade manufacturer as well as production
lot are two of many variables that need to be taken into consideration
for their effects on flap thickness, according to Robert T. Lin
Dr Lin conducted a study comparing the characteristics of flaps
created with the Nidek MK-2000 microkeratome using blades from three
different manufacturers - Nidek, Med-Logic, and Surgin. The study
included 220 myopic eyes of 110 consecutive patients, using both
the 130 micron and 160 micron (Nidek and Surgin only) microkeratome
heads. The study examined six different lots for the Nidek blades.
He reported that no flap complications occurred in the series, including
epithelial defects, irregular flaps, or striae. The flaps created
with the different manufacturer blades and lots were all similar
with respect to diameter and hinge width, he told a session of the
annual ASCRS Symposium on Cataract, IOL, and Refractive Surgery.
However, when comparing the different manufacturers and the different
lots, there were between-group differences in flap thickness that
were clinically important even if not statistically significant.
Quality control issue
He noted that the quality of the microkeratome is one of the most
important determinants of a good quality LASIK flap, and poor quality
control blade production can potentially lead to problems with visual
morbidity in association with variable flap thicknesses, irregular
flaps, irregular stromal beds, epithelial defects, epithelial ingrowth,
and diffuse lamellar keratitis.
"The results of this study show that Med-Logic and Surgin can
be substituted for Nidek blades in the MK-2000 microkeratome without
sacrificing the safety and efficacy of flap creation. However, surgeons
need to know how their microkeratome is cutting in their hands with
different blades and be aware that different lots of blades from
the same or different manufacturers may produce flaps with a different
mean and range of thicknesses," he commented.
Dr. Lin encouraged LASIK surgeons to measure flap thickness intraoperatively
to allow calculation of mean and standard deviation values as well
as upper and lower limits for a particular microkeratome, and he
urged colleagues to develop personal flap thickness nomograms that
in addition to blade lot and blade manufacturer, incorporate central
corneal thickness, microkeratome head, and surgical order (first
vs. second eye).
In the study he reported, Dr. Lin determined flap thickness by performing
intraoperative ultrasound pachymetry (Sonogauge) using the subtraction
technique (central corneal thickness minus stromal bed thickness).
In an attempt to standardise determination of central corneal thickness
and obtain accurate values, all measurements were performed after
inserting the speculum and drying the conjunctival fornices. Three
measurements were obtained and averaged, but a fourth was taken
and the outlier dropped if any of the three values was not within
10 microns of one another.
The keratectomy was performed to avoid fluid accumulation underneath
the flap, and any fluid present in the bed was wiped with a dry
Wecksel sponge prior to measuring stromal bed thickness.
Mean values for flap thickness using the 130 micron head were 105.6±17.7
microns for the Nidek blades and 99.4±15.8 microns for the
Surgin blades. Using the 160 micron head, the means ranged from
119 to 130 microns, with the Med-Logic blades cutting thinnest,
but this time, the Surgin blades cut thicker than the Nidek brand.
Standard deviation for all three brands was very similar and ranged
from 20.2 to 20.5 microns.
The analysis of variability in flap thickness with use of different
blade lots was based on 126 micron flaps. Mean flap thickness values
ranged from 114.6 to 130.0 microns. Standard deviation values ranged
from 14.5 to 24.8 microns. Mean central corneal thickness for the
six blade lot subgroups varied from 535 microns to 559 microns.
Better measurement technology needed
Dr. Lin also pointed out the need for developing more accurate ways
than subtraction pachymetry to measure flap thickness and noted
opportunities may come through devices based on optical coherence
tomography, specular microscopy, or confocal microscopy.
"There are a number of potential problems in using the ultrasound
pachymeter to measure corneal thickness since the value obtained
is influenced by such factors as corneal hydration, probe angle,
and accuracy of the pachymeter, and the problems may be further
compounded using the subtraction technique involving the use of
two different measures."
There is also some new evidence that flap thickness calculated with
the subtraction method may be low as an artefact of the measurement
technique since engaging the microkeratome causes fluid influx into
the cornea and thereby an increased stromal bed thickness measurement,
Dr. Lin added.
T. Lin, MD
Medical Director, IQ Medical Center
Los Angeles, California, US