Anterior segment OCT may generate detailed images to evaluate flap-stroma

The use of anterior segment OCT during femtosecond-assisted LASIK may help surgeons measure corneal thickness and create highly reproducible flaps.

Anterior segment optical coherence tomography may be superior in measuring corneal thickness outside of the central 2 mm, aiding in the creation of flaps during femtosecond-assisted LASIK, according to a study.

“Using [anterior segment OCT] with a femtosecond laser provides efficiency and accuracy in the preoperative and early postoperative period in the measurement of central corneal thickness and the creation of highly reproducible flaps that correspond to the preoperative intended thickness,” the study authors wrote.

They said that the Visante anterior segment OCT (AS-OCT, Carl Zeiss Meditec) is the first non-contact device capable of imaging, measuring and documenting corneal flap thickness and residual stromal thickness, before and after LASIK, and producing a full-thickness pachymetry map.

“AS-OCT is a revolutionary anterior segment imaging method; it is easy, fast and patient friendly,” Raciha Beril Kucumen, MD, one of the study authors, said in an e-mail interview with Ocular Surgery News. “I recommend it to every clinic where anterior segment surgery is being performed.”

Dr. Kucumen and colleagues noted that an earlier study also demonstrated AS-OCT’s utility as a pachymetric map, comparing it to ultrasound and suggesting that it be used for planning keratorefractive procedures and diagnosing corneal diseases.

Methods, results

The prospective study analyzed 51 eyes of 26 patients with myopia and myopic astigmatism. Those with previous ocular trauma or surgery and anterior pathology were excluded.

Patients underwent femtosecond-assisted LASIK with the 30-kHz IntraLase laser (Abbott Medical Optics) and experienced an uneventful postoperative course. AS-OCT images were evaluated at 1 week and 1 month postop, and only images along the horizontal meridian were chosen for measurement.

“The horizontal meridian, rather than the vertical meridians, was concentrated on to avoid false measurement and misinterpretation of the flap data that might be caused by interference from the upper eyelid or by superior location of the hinge-pocket complex,” the study authors wrote.

For each cross-section, flap thickness and residual stromal thickness were measured at seven different points. These consisted of one at the corneal vertex, two ±2 mm from the vertex, two ±3 mm from the vertex and two ±4 mm from the vertex, near the flap margins.

According to the study authors, the detailed, high-resolution images produced using AS-OCT are a testament to the device’s utility in evaluating the flap-stroma relationship. However, AS-OCT did not produce precise images for the central 2 mm of the cornea due to that region’s high reflectivity; measurements taken at the vertex and 1 mm from the vertex could only be estimated.

Flaps were thinner in the central cornea than in the periphery, but a paired sample t-test showed this finding to be statistically insignificant. No difference was shown for flap thickness measured at 1 week and 1 month. However, a statistically significant correlation was found between residual stromal thickness observed perioperatively and measured at 1 week and 1 month.

A secondary finding, the AS-OCT flap measurements also demonstrated the high accuracy, predictability and reproducibility of the femtosecond laser compared with measurements reported for mechanical microkeratome, the study authors said.

Potential benefits of AS-OCT

“I think the clinical application of AS-OCT in anterior segment surgery is as revolutionary as OCT for retinal disease,” Dr. Kucumen said. “Ophthalmology departments should use this imaging method before and after anterior segment surgery and especially before corneal surgery.”

Dr. Kucumen has been involved with two studies assessing the use of AS-OCT in conjunction with femtosecond-assisted LASIK, including this one, and is currently working on research investigating AS-OCT’s utility in areas such as penetrating keratoplasty and Keraring (Mediphacos) implantation.

There is no guaranteed way to solve the problem of obtaining precise images in the central 2 mm of the cornea, Dr. Kucumen said, but she suggested altering the polarization of the scan and decentering the images to get rid of excess reflectivity.

Despite AS-OCT’s inaccuracy in the 2-mm zone, its ability to help produce consistently thin flaps may translate to less disruption of the anterior stroma, allowing for greater biomechanical support and strength, the study authors said. Conventional LASIK involves severing more anterior fibers and is thus associated with less corneal stability.– by Michelle Pagnani

• Raciha Beril Kucumen, MD, can be reached at Yeditepe University Medical School, Department of Ophthalmology, Sakir Kesebir Caddesi, Gazi Umur Pasa Sokak No: 28 Balmumcu, Besiktas Istanbul, Turkey; email: bkucumen@yeditepe.edu.tr.
• Disclosure: Dr. Kucumen has no direct financial interest in any of the products discussed in this article, nor is she a paid consultant for any companies mentioned.

This study demonstrates the limits of AS-OCT technology in measuring corneal flap thickness. In clinical practice, I have also found that it is difficult to identify the flap interface centrally due to signal noise. In addition, the study shows the lack of a totally planar shape to flaps created using femtosecond laser technology; these flaps closely resemble those created with modern-day microkeratomes. One of the greatest difficulties of measuring flap thickness with AS-OCT is the non-reproducible manual placement of the cursor on the flap interface. Cursor placement can vary by as much as 5 µm to 8 µm upon repeated measurements by the same observer. It is interesting to note that the single photograph of a high-resolution OCT scan published in this study shows a variation in flap thickness from 112 µm to 143 µm at points only 2 mm apart on a single midperipheral cornea scan, and yet the flap was still referred to as “near-planar.” Moreover, a second figure suggests more of a meniscus shape to flap thickness created using femtosecond laser.

– Richard J. Duffey, MD
OSN Refractive Surgery Board Member
Disclosure: Dr. Duffey has no relevant financial disclosures.