The theoretically types of incision that can be made in femtosecond astigmatic keratotomy. (a) Full thickness, (b) Intrastromal, (c) Reverse full thickness
Intrastromal FSAK has the benefit of not disrupting the epithelium or Bowman’s. This results in minimal wound healing and no fibroblast growth  that is associated with visual symptoms such as glare and halos. As the epithelium is not disturbed, these patients are comfortable and asymptomatic postoperatively. As the integrity of Bowman’s is kept intact, the intrastromal FSAK has mostly been studied in minor or medium astigmatism—up to 3.5D. There is, however, one case report of it successfully treating a post-penetrating keratoplasty (PKP) patient with reduction in cylinder from 10.4D to 1.12D .
7.2 Surgical Planning
Preoperative evaluation of the patient should include a comprehensive exam and a combination of objective testing that may include keratometry, corneal topography, corneal tomography, pachymetry, and anterior segment optical coherence tomography. Those patients with ectatic disorders or highly irregular astigmatism can be identified as they need to be approached with caution with FSAK .
There are numerous described nomograms that vary the treatment parameters (depth, arc length, optical zone diameter) based on the amount of astigmatism and patient age. Most of these are based on the nomograms available for manual AK that have been adjusted. One such modification is using 70% of the Donnenfeld nomogram. Our current nomogram consists of paired incisions, 85% depth, 80 μm below anterior surface, and 8.5 mm optical zone. The arc length depends on the axis and amount of astigmatism (Table 7.1). There is no augmentation performed, and if the patient needs more correction, then laser vision correction is offered.
Arc length nomogram
There is only one study that has specifically developed a nomogram for FSAK after PKP and DALK . The presence of peripheral blood vessels will interfere with the laser, so the location of the optical zone may need to be adjusted. The plasma shape of the laser and its extension beyond the set depth needs to be considered to avoid accidental perforation.
7.3 Surgical Technique
There are several commercially available femtosecond laser systems available that can perform astigmatic keratotomy. The settings will vary with each machine as per the manufactures guidelines.
To compensate for cyclotorsion, marking the limbus preoperatively at the slit lamp has been advocated. The patient is then prepped and anesthetized topically. In some devices, the axis is required to be marked and the pachymetry measured to be used to calculate the depth prior to engaging the suction ring. The more recent devices have built-in optical coherence tomography that allows for real-time measurement and simultaneous treatment. Once the suction ring is used to secure the eye, the applanation cone is applied and can be adjusted to ensure proper alignment. The inputted treatment parameters are then applied and the incisions created (Video 7.1). After creation, the applanation cone and suction ring are released. In full-thickness incisions, the incisions can be opened using a Sinskey hook. This can be done at the time of surgery or even delayed by several weeks.
Most of the early use of FSAK has been reported in patients following penetrating keratoplasty (PKP) [1, 6–13]. Its use in patients after cataract surgery [14, 15], after previous refractive surgery , after Descemet stripping automated endothelial keratoplasty [17, 18], and in those with naturally occurring high astigmatism  has also been reported. With the advent of femto-assisted cataract surgery, FSAK has been incorporated into treatment at the time of cataract surgery . As shown in Table 7.2, there was a marked reduction in keratometric astigmatism following the treatment. This correction also resulted in improvement in the patients uncorrected visual acuity as well.
Summary of all studies that have evaluated femtosecond astigmatic keratotomy
Mean keratometry (D) ± SD
7.70 ± 3.10 → 4.75 ± 2.86
8.50 → 4.90
7.00 → 4.30
IntraLase 30 kHza
4.40 → 0.67
1.0 mm smaller than graft
7.16 ± 3.07 → 2.33 ± 1.55
IntraLase FS 60 kHza
90° side-cut angle
9.80 ± 1.90 → 5.20 ± 1.50
IntraLase FS 60 kHza
0.5 mm small than graft
7.46 ± 2.70 → 4.77 ± 3.29
7.84 ± 2.35 → 3.58 ± 2.21
You may also need
Full access? Get Clinical TreeGet Clinical Tree app for offline access