A Longitudinal Study of LASIK Flap and Stromal Thickness with High-speed Optical Coherence Tomography
Received 3 August 2006; accepted 13 September 2006. published online 23 February 2007.
Objective
To assess corneal anatomic changes after LASIK with a high-speed corneal and anterior segment optical coherence tomography (CAS-OCT) system.
Design
Cross-sectional observational study.
Participants
Fifty-one eyes of 26 healthy persons undergoing LASIK.
Methods
The CAS-OCT prototype operated at a 1.3-μm 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.
Main Outcome Measures
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 μm by pooled standard deviation for zones inside a 5-mm diameter. The central flap thickness in 24 Hansatome eyes with a 180-μm setting was 143±14 μm by OCT and 131±17 μm by ultrasound. In the 8 IntraLase cases with a 120-μm setting, it was 156±11 μm by OCT and 160±19 μm 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.
1Department of Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio.
2Cole Eye Institute, Cleveland Clinic Foundation, Cleveland, Ohio.
3Department of Ophthalmology, University of São Paulo, São Paulo, Brazil.
4Department of Biomedical Engineering, Cleveland Clinic Foundation, Cleveland, Ohio.
Correspondence to David Huang, MD, PhD, Doheny Eye Institute, 1450 San Pablo Street, DEI 5702, Los Angeles, CA 99033.
Manuscript no. 2006-866.
Supported by the National Institutes of Health, Bethesda, Maryland (grant no. R24 EY13015), and Carl Zeiss Meditec, Inc., Dublin, California.
Dr Huang has a patent royalty interest in optical coherence tomography technology. Dr Huang and Yan Li receive research grant support from Carl Zeiss Meditec. The other authors do not have proprietary interests related to the article.
ABSTRACT