Optical Coherence Tomography in Cornea and Refractive Surgery

Volkan Hurmeric; Sonia H Yoo; Fatih Mehmet Mutlu

Disclosures

Expert Rev Ophthalmol. 2012;7(3):241-250. 

In This Article

AS-OCT in Femtosecond Laser-Assisted Surgery

Femtosecond lasers and OCT imaging are among the most rapidly advancing technologies in ophthalmology. OCT has the ability to make reliable thickness measurements. Femtosecond lasers prepare accurate, uniform and predictable incisions in the cornea. Combining OCT data with the accuracy of a femtosecond laser helps us to prevent complications and gives us new treatment options. AS-OCT can be used in presurgical planning, postsurgical evaluation and follow-up of the patients in cornea and refractive surgery. The pachymetry map of an AS-OCT system is a reliable tool in surgical planning. It can be used in femtosecond laser-assisted astigmatic keratotomy, laser-assisted in situ keratomileusis (LASIK) enhancement and intrastromal tunnel preparation for intracorneal ring segments.

AS-OCT in LASIK

The structure of the LASIK flap can be evaluated with AS-OCT in detail (Figure 4). AS-OCT can detect the flap interface in 95% of the patients 6 months after surgery. AS-OCT can be used to measure not only the thickness but also to understand the shape of the LASIK flap. AS-OCT studies demonstrated that some of the microkeratomes and femtosecond lasers had a meniscus-shaped flap.[3,47,48] These nonuniform flaps were found to be thicker in the peripheral part. The planar-shaped flaps have been suggested to have less impact on the biomechanical stability of the cornea and may contribute to refractive and wavefront outcome of the patients.[49] LASIK flaps prepared with Amadeus II™ (Ziemer Ophthalmic Systems AG, Port, Switzerland), Carriazo-Pendular (Schwind eye-tech-solutions, Kleinostheim, Germany), Moria SBK (Moria, Antony, France) microkeratomes and Intralase™ (Abbott Medical Optics, CA, USA), and Visumax® (Carl Zeiss Meditec, Jena, Germany) femtosecond laser systems have been reported to have a planar configuration.

Figure 4.

Visante optical coherence tomography image of a patient 3 months after laser-assisted in situ keratomileusis. Flap tool is used to measure flap thickness and size. Flap size is measured as 8.5 mm femtosecond laser-assisted flap and demonstrates a planar structure. Flap thickness is 124 µm in the center and 118 µm in the periphery. Residual stromal bed is 464 µm in the center 516 µm in the periphery.

Measuring the thickness of the LASIK flap and the residual thickness bed demonstrates whether future enhancements are possible. In LASIK, enhancement of the original LASIK flap can be lifted several years after the primary LASIK. The femtosecond laser can be also be used to re-cut a new flap.[50] In these cases, another option is to perform a lamellar side-cut-only incision, which is deeper and smaller than the previous flap (Figure 5). The new side-cut will intersect with the previous lamellar bed and will allow the surgeon to lift the flap with ease. In both techniques the depth and the diameter of the new side cut must be selected according to the parameters of the previous flap, which can be measured by AS-OCT. Preoperative marking of the flap edge makes the centralization and positioning of the new side cut easier in these patients.

Figure 5.

Demonstration of laser-assisted in situ keratomileusis enhancement technique with a femtosecond laser-assisted side-cut-only incision. The nondashed line corresponds to the previous laser-assisted in situ keratomileusis flap (A & C). The dashed black line corresponds to the side-cut only incision performed with femtosecond laser (A & C). Black arrows demonstrate the appearance of side-cut-only femtosecond cut on the laser screen (B).

AS-OCT in Femtosecond Laser-Assisted Astigmatic Keratotomy & Intracorneal Ring Segment Implantation

Femtosecond lasers and AS-OCT can be used together in preoperative and postoperative evaluation of patients undergoing astigmatic keratotomy (FS–AK) and intracorneal ring segment (FS–ICRS) implantations.[51] Both techniques require accurate pachymetric mapping of the cornea. The depth of the arcuate incisions in FS–AK and the depth of the tunnel in FS–ICRS is determined according to the pachymetric map of the patient and the selected nomogram. Combining femtosecond laser with AS-OCT has been reported to be more accurate, predictable and safer than manual astigmatic keratotomy (AK).[52] FS–AK is mostly preferred in patients with post-penetrating keratoplasty corneal astigmatism.[53] Incisions are made perpendicular to the steep meridian of the corneal astigmatism and produce a flattening effect along that axis in AK. Cut depths of the arcuate incisions are set according to the thinnest corneal point, measured by the AS-OCT. FS–AK can also be used in virgin eyes and in eyes with irregular astigmatism.[54–56]

AS-OCT is very helpful in the postoperative follow-up of patients with FS–AK (Figure 6) and FS–ICRS (Figure 7). AS-OCT can determine the actual depth of the arcuate incisions and can explain the reason behind unexpected postsurgical outcomes (Figure 6).[57] It can be used to compare the intended and achieved position of the ICRS in the cornea (Figure 7).

Figure 6.

Visante optical coherence tomography images after femtosecond laser-assisted astigmatic keratotomy. (A) Visante optical coherence tomography demonstrates the thickness of the corneal incisions as 429 and 458 µm in patient 1. (B) Visante optical coherence tomography demonstrates full thickness corneal incisions (white arrows) in patient 2.

Figure 7.

Visante optical coherence tomography images after femtosecond laser-assisted intracorneal ring segment implantation. The intended intracorneal ring segment implantation depth was 400 µm. Visante optical coherence tomography image demonstrates that the actual depth of intracorneal ring segment implantation is 240 µm.

AS-OCT in Femtosecond Laser-Assisted Lamellar Keratoplasty

Femtosecond laser assisted lamellar keratoplasty is a promising surgical technique that requires OCT data in presurgical planning.[58] These techniques include femtosecond laser-assisted anterior lamellar keratoplasty (FALK) and femtosecond laser-assisted deep anterior lamellar keratoplasty.[59,60] In FALK the first step is to measure the depth of anterior stromal scarring with AS-OCT. According to the depth of scarring the donor and recipient corneas are prepared with the femtosecond laser. The perfect match between the donor and the recipient eliminates the necessity of suturing in FALK.[61] There are also new surgical techniques that can be performed with femtosecond lasers such as corneal biopsies, corneal tattooing and collagen crosslinking.[62–64] In all of these techniques, depth of corneal incisions and preoperative planning can be performed according to the data obtained from the AS-OCT. The major limitation of the femtosecond laser is that the laser cuts the cornea according to the reference plane. This limitation reduces the accuracy of incisions in corneas with significant anterior and posterior irregularities.

AS-OCT in Femtosecond Laser-Assisted Cataract Surgery

The most recent and exciting advancement in refractive surgery is the development of femtosecond laser-assisted cataract surgery.[65] These systems are capable of performing corneal incisions, continuous curvilinear capsulorhexis, nucleus softening and lens fragmentation.[66] Femtosecond laser-assisted cataract surgery will eliminate decentration-related problems and shorten the active phaco time. Most of these platforms have an integrated OCT system to analyze the anterior segment and to focus the laser in a 3D manner.

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