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 MD.

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.

Robert T. Lin, MD
Medical Director, IQ Medical Center
Los Angeles, California, US
www.iqmedicalcenter.com