A longitudinal study of LASIK flap and stromal thickness with high-speed optical coherence tomography.
OBJECTIVE
To assess corneal anatomic changes after LASIK with a high-speed corneal and anterior segment optical coherence tomography (CAS-OCT) system.


METHODS
Cross-sectional observational study.


METHODS
Fifty-one eyes of 26 healthy persons undergoing LASIK.


METHODS
The CAS-OCT prototype operated at a 1.3-mum wavelength and 2000 axial scans/second. The corneas were scanned with a flap profile pattern (horizontal line, 512 axial scans) and a flap map pattern (4 radials, 256 axial scans each). Both patterns are 8 mm long and are centered on the corneal vertex. LASIK flaps were created using either a mechanical microkeratome (Hansatome; Bausch & Lomb, Inc., Rochester, NY) or a femtosecond laser (Pulsion; IntraLase Corp., Irvine, CA). Intraoperative pachymetry was performed using a 50-MHz ultrasound probe. Three OCT scans were obtained on preoperative and post-LASIK visits up to 6 months. An automated algorithm was developed to process the OCT images and to calculate corneal, flap, and stromal bed thickness profiles and maps. The profiles and maps were divided into central (diameter, <2 mm), pericentral (2-5 mm), and transitional (5-7 mm) zones for analysis.


METHODS
Corneal, flap, and stromal bed thicknesses as determined by OCT and ultrasound pachymetry.


RESULTS
The flap interface was best detected in the pericentral zone. One week after surgery, the repeatability of OCT flap and stromal bed thickness measurement was 2 to 7 microm by pooled standard deviation for zones inside a 5-mm diameter. The central flap thickness in 24 Hansatome eyes with a 180-microm setting was 143+/-14 microm by OCT and 131+/-17 microm by ultrasound. In the 8 IntraLase cases with a 120-microm setting, it was 156+/-11 microm by OCT and 160+/-19 microm by ultrasound. Eyes with other settings also were analyzed. There were small systematic changes in flap thickness up to 1 week and bed thickness up to 3 months.


CONCLUSIONS
We have developed a method for using high-speed OCT to measure LASIK flap thickness after surgery. The measurement is noncontact, rapid, and repeatable. Profile and map measurements provide more information than point measurements previously demonstrated. This could be valuable for planning LASIK enhancement and characterizing microkeratome performance.
Li
Yan
Y
Department of Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio, USA.
Netto
Marcelo V
MV
Shekhar
Raj
R
Krueger
Ronald R
RR
Huang
David
D
D000328:Adult; D000465:Algorithms; D003319:Corneal Stroma / Q000000981:diagnostic imaging / Q000473:pathology*; D003430:Cross-Sectional Studies; D003941:Diagnostic Techniques, Ophthalmological; D005260:Female; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D009216:Myopia / Q000601:surgery*; D011184:Postoperative Period; D013524:Surgical Flaps / Q000473:pathology*; D041623:Tomography, Optical Coherence / Q000379:methods*; D014463:Ultrasonography; D014945:Wound Healing