Errors of residual stromal thickness estimation in LASIK.
OBJECTIVE
To investigate inaccuracy and variability in residual stromal thickness estimation in LASIK by pachymetry and measurements of corneal thickness, flap thickness, and ablation depth.


METHODS
In 73 eyes of 37 patients, preoperative and postoperative corneal thicknesses were obtained with slit-scanning elevation topography and the ultrasound pachymeter. LASIK was performed and corneal flaps were created with a microkeratome. Flap thickness and ablation depth (expected and achieved) were calculated. Residual stromal thickness estimation error was analyzed.


RESULTS
The mean preoperative corneal thicknesses were 559.58 +/- 23.47 and 554.92 +/- 29.95 microm for the ultrasound pachymeter and slit-scanning elevation topography, respectively. Measurement differences ranged from -36 to 30 microm. With the pachymeter, calculated mean flap thickness was 139.58 +/- 17.59 microm. With this device, predicted ablation depth differed from achieved depth by 20% or more in approximately one-third (30.14%) of treated patients; ablation differences ranged from 10.0% to 19.99% in 37% of patients and 1.00% to 9.99% in 31.5% of patients.


CONCLUSIONS
Imprecision of microkeratome cuts, preoperative corneal pachymetry, and laser ablation depth have a significant impact on the inaccuracy of residual stromal thickness prediction. Especially in patients with borderline corneal thickness, intraoperative pachymetry measurements and a residual stromal thickness higher than the safety margin of 250 microm are recommended to minimize iatrogenic ectasia.
Cheng
Huey-Chuan
HC
Department of Ophthalmology, Mackay Memorial Hospital, Taipei, Taiwan.
Chen
Yen-Tung
YT
Yeh
Shu-I
SI
Yau
Chi-Wang
CW
D000328:Adult; D001837:Body Weights and Measures; D003319:Corneal Stroma / Q000000981:diagnostic imaging / Q000473:pathology*; D005260:Female; D006801:Humans; D020731:Keratomileusis, Laser In Situ / Q000379:methods*; D008297:Male; D033401:Microscopy, Acoustic; D008875:Middle Aged; D009216:Myopia / Q000601:surgery*; D011446:Prospective Studies; D015203:Reproducibility of Results; D013524:Surgical Flaps / Q000473:pathology*