21 Crucial Steps III: Corneal Incision, Main and Side
Manually performed incisions into the cornea are associated with many surgical difficulties – radial keratectomy, for instance, in many cases led to a hyperopic shift in the long run. The femtosecond laser offers the option of intrastromal keratotomy to correct astigmatism, particularly since modern laser systems do not cause an applanation of the central cornea and the creation of folds thus can be avoided. These intrastromal incisions have proved to be safe and effective, performed during LCS they do not cause any additional costs. The predictability is considered higher than in manual techniques; the only common complications up to now being reported are are low-grade inflammation and microperforations, which are mostly self-healing. The laser can also safely perform clear cornea main and sideport incisions with microscopic evaluations showing a high cut quality and very few remaining tissue bridges. Descemet membrane detachment, though, is a potential though not frequent complication.
Keywords: Intrastromal astigmatic keratotomy, laser-assisted main and side port incisions
The cornea consists of five layers (the epithelium, Bowman’s layer, the stroma, Descemet’s membrane, and the endothelium) with a central thickness of approximately 0.52 mm and a peripheral thickness of approximately 0.65 mm depending on the age and other individual factors (▶ Fig. 21.1). Since the cornea presents the main refractive surface of the eye with a total refraction power of over 48 D, refractive errors such as astigmatism can result from surface irregularities. The stroma is located between Bowman’s layer and Descemet’s membrane and makes about 90% of the corneal thickness. It consists of collagen fibrils, which are bundled uniformly parallel to the corneal surface. 1 A permanent corneal scar can occur as soon as Bowman’s layer, the second layer of the cornea that is localized under the epithelium, is disrupted. For the treatment of astigmatism, corneal incisions in the corneal periphery are performed. These incisions flatten out the central portion of the cornea and therefore modify the corneal shape. 2 It is of great interest not to affect Bowman’s layer when performing corneal incisions in order to prevent corneal tissue scar formation.
Fig. 21.1 Histology of the cornea. (Distance between the corneal fibris is a preparation artifact)
Astigmatism is a very common refractive error that can occur naturally and also after surgical procedures such as cataract surgery or keratoplasty. 3, 4 Hoffmann and Hütz et al found that in 23,239 eyes, 36.1% had an astigmatism over 0.75 D with a mean astigmatism of 0.98 D. 5
Similar findings were made in patients prior to cataract surgery. Here, 34% of the cataract patients had an astigmatism of 1.0 D and more. 6
The extent of astigmatism after cataract surgery can vary depending on the surgical procedure. The decreased visual acuity postoperatively can cause a distinct dissatisfaction that can be very frustrating not only for the patient, but also for the surgeon. Also, astigmatism can cause certain halo effects, which are even described to be worse in eyes with a high surgical-induced astigmatism. 7
In order to reduce visual impairment and photic phenomena due to clinically significant astigmatism, several surgical approaches have been performed in the past decades. All the treatments mainly aim to perform corneal incisions in the steep meridian of the astigmatism. Corneal incisions can be performed manually such as radial keratectomy (RK) and arcuate keratotomy. During the 1960s, RK was first introduced into ophthalmology to treat myopia and astigmatism (▶ Fig. 21.2). 8 Since then, many other studies followed where RK was regarded as a successful treatment option for refractive errors. 9 However, the long-term results revealed an unstable refraction due to a progressive hyperopic shift. 10, 11 Manual astigmatic keratotomy (AK) is described to have a poor reliability and predictability apart from other reported complications such as infections and corneal perforation. 12
Fig. 21.2 Intraoperative image throughout the operating microscope of an eye after radial keratotomy with six radial corneal incisions.
Also, manually performed incisions are associated with many surgical difficulties, which lead to the widespread opinion that they should be only performed by experienced surgeons. Additionally, the incisions can also be performed with laser-assisted femtosecond energy.
21.2 Femtosecond Laser–Assisted Intrastromal Astigmatic Keratotomy
Besides manually performing corneal incisions in eyes with significant astigmatism, astigmatic incisions can also be completed with laser-assisted keratotomy. In 2009, femtosecond laser surgery was introduced into ophthalmology for cataract surgery. Since then, image-guided incisions can be performed, where the location and extent of the incisions can be navigated very precisely. Here, two different methods were developed to visualize the cornea intraoperatively. Rotating Scheimpflug imaging or spectral domain optical coherence tomography (OCT) can provide image-guided corneal incisions. Femtosecond laser surgery can complete either a penetrating or an intrastromal AK, where one or two curved corneal incisions are performed at the steepest meridian of the astigmatism (▶ Fig. 21.3). 13, 14 It is important to mention that the former femtosecond laser systems had an applanating effect on the corneal curvature through the docking system. The docking provided a flat surface but induced the formation of corneal folds at the same time. Nowadays, the established laser systems do not cause an applanation of the central corneal surface, which avoids a folding of the central cornea. Hence, with the liquid interfaces, the corneal curvature remains more in its natural shape during laser treatment, which potentially presents an advantage of the femtosecond laser systems used nowadays with liquid interfaces. Studies could even show that these corneal folds can lead to an incomplete capsulotomy during laser treatment. 15
Fig. 21.3 Planning screen for penetrating arcuate incisions (screenshot).
Concerning the stability of femtosecond laser–assisted AKs, Day and Stevens recently published a study, where they analyzed the corneal keratometry after cataract surgery with and without AK. The change in the surgically induced astigmatism (SIA) between 1 and 6 months after surgery was similar in both groups and showed a similar magnitude of SIA regression. 16
Also, studies could show that the efficacy of the incisions varies depending on their depth and length. 17 Since these are both parameters that can be adjusted individually with the laser settings, the efficacy can be customized with the magnitude of preoperative astigmatism. Especially in eyes after penetrating keratoplasty (PKP), a variation of the incisional depth can be of great interest, which was reported to vary between 75 and 90% of the corneal thickness. 18, 19 Wetterstrand et al also reported that there is a correlation of the incisional depth with the efficacy where they stated that a deeper incision leads to a more effective astigmatic correction. Here, 20 eyes post keratoplasty were treated with femtosecond laser–assisted arcuate incisions inside the graft stroma. 20 Eliwa et al recently published a study in which 23 eyes underwent limbal relaxing incisions (LRIs) and 22 eyes did not undergo any astigmatic incisions during cataract surgery. 45 In the LRI group, the topographic astigmatism could be reduced to 51.9% of the preoperative astigmatism (p < 0.0001).
Day and Stevens et al could show that intrastromal incisions present a safe treatment to significantly reduce astigmatism. In 196 eyes, the mean astigmatism correction was 63% with patients having a mean preoperative corneal cylinder of 1.21 D (range: 0.75–2.64 D). Day et al also mentioned some advantages of laser-assisted AK, where they described the laser programming as very easy without causing any additional costs through femtosecond laser–assisted cataract surgery (LCS). 21 This inexpensive procedure can subsequently present a profitable treatment especially for patients with contraindications for toric intraocular lenses (IOL). In a study by Yoo et al, the residual astigmatism was analyzed after toric IOL implantation and was compared to femtosecond laser–assisted AK. Here, they even documented that laser-assisted AKs present a viable alternative to toric lens implantation for astigmatism with comparable results. 22 Concerning the long-term results of LRIs, no significant alterations of higher order aberrations (HOA) after 3 years could be detected, which is why Monaco et al concluded that LRIs present a suitable treatment option for astigmatism. 23
Interestingly, these findings stay in contrast to other studies, where it was shown that AK increases HOA. 24
Today, there are different nomograms that can be used for individual calculation of the depth and axis of the LRI. The leading nomograms are the Donnenfeld LRI nomogram, the nomogram by Julian Stevens, and the ASSORT (Alpins Statistical System for Ophthalmic Refractive Surgery Techniques) Femto LRI Calculator. It is important to mention that the nomogram by Stevens et al presents the only available nomogram for intrastromal corneal incisions and is therefore highly preferred by many surgeons.
The Donnenfeld LRI nomogram was originally developed for manual corneal incisions and is not officially recommended for femtosecond laser–assisted incisions by the publishing company.
There are many surgeons who postulate that a future adjustment of the nomograms is needed to improve the effectiveness of laser-assisted AK. 25 Another concern about LRI is the low predictability. 26 However, the predictability is still considered higher than other manual techniques. 27 As mentioned earlier, the refractive outcome of manually performed corneal incisions such as RK is relatively unstable due to a hyperopic shift that occurs years after the treatment. Bouwhuis et al demonstrated that femtosecond laser–assisted intrastromal arcuate keratotomy has an excellent refractive stability. 28 Even with toric IOLs, long-term studies could show that the rotation stability cannot always be guaranteed, which can potentially lead to a reduced visual outcome as well. 29, 30 Kohnen et al concluded that in cases with a higher astigmatism, toric IOLs in combination with LRI present an accurate treatment in order to adequately correct astigmatism. 31
For AKs as well as toric IOLs, it is necessary to find the correct axis intraoperatively. Several ink- and digital-based markers are available. The most commonly used option is the gravity-driven marker. For example, the model produced by Geuder (Heidelberg, Germany) has an adjusted distance between the marker blades to avoid blockage of the laser beam by the ink during the capsulotomy. Digital systems can perform an intraoperative iris registration and match preoperative and intraoperative data to find the correct axis for AKs (▶ Fig. 21.4; ▶ Fig. 21.5).
Fig. 21.4 A common used gravity-driven marker produced by Geuder (Designed by Dr. Schultz). The device is used to mark the correct axis for astigmatic keratotomies or toric intraocular lens implantation.