Flap thickness formula aids in decision-making

January 1, 2007
Cheryl Guttman

The regression equation: flap thickness = 0.26 (preop corneal thickness) – 0.19 (age) – 11.30 (blade type; 0 = new, 1 = used) + 0.76 (average K) – 22.05.

"Calculating re-sidual stromal bed thickness preoperatively using information on corneal thickness, flap thickness, and ablation depth is important, especially in eyes with thin corneas, to identify those at risk for ectasia who may be better candidates for surface ablation than for LASIK," said Dr. Bucci, a private practitioner in Wilkes-Barre, PA.

As Dr. Bucci explained, "The number of micrometers of tissue that will be removed by the ablation can be precisely determined based on the planned amount of correction. This formula provides a good estimate of flap thickness, and so its use can aid in the decision-making process of selecting an appropriate refractive procecedure."

The microkeratome performed safely and reliably, creating flaps that were on average close to the footplate nominal value when a new blade was used. Among the 87 patients who had flaps created using a new blade in each eye, the mean ± SD flap thickness was 145.4 ± 19.8 µm in the first eye and 151.6 ± 21.2 µm in the second eye. In the subgroup of patients who had bilateral flap creation with a single blade, mean flap thickness was 150 ± 18.3 µm for the first eye and 139.0 ± 20.6 µm in the second eye.

The 11-µm difference in flap thickness between the first and second eyes when reusing a blade was statistically significant. Similarly, blade reuse was found to result in significantly thinner flaps when a comparison was made using data from all 257 flaps derived from new blades and the 83 flaps created with a used blade. The mean difference in flap thickness in that comparison was 10 µm, Dr. Bucci reported.

Multivariant regression analysis was performed to identify potential relationships between flap thickness and the following variables: age, gender, spherical equivalent (SE), average K value, preoperative corneal thickness, and blade type (new or used). Age, corneal thickness, blade type, and average K value were all significant predictors of the outcome.

The regression equation developed incorporating those variables was: flap thickness = 0.26 (preop corneal thickness) – 0.19 (age) – 11.30 (blade type; 0 = new, 1 = used) + 0.76 (average K) – 22.05. It predicts flap thickness within ±19.7 µm with 95% confidence, said Dr. Bucci.

Corneal thickness

The strongest predictor of flap thickness in the multivariate analysis was preoperative corneal thickness, he observed, and it is the dominant factor in the regression equation.

"Studies I performed previously using the Hansatome (Bausch & Lomb) and Amadeus microkeratomes also demonstrated thicker flaps were produced in thicker corneas and thinner flaps in thinner corneas," Dr. Bucci said.

He also noted the finding that blade reuse results in significantly thinner flaps is valuable information for surgeons evaluating thin corneas for LASIK and in otherwise planning surgery.