Femtosecond laser-assisted astigmatic keratotomy: a review
Chang Eye and Vision
Femtosecond laser-assisted astigmatic keratotomy: a review
John S. M. Chang 0
0 Department of Ophthalmology, Hong Kong Sanatorium & Hospital , 8/F, Li Shu Pui Block, Phase II, 2 Village Road, Happy Valley , Hong Kong
Background: Astigmatic keratotomy (AK) remains an accessible means to correct surgically induced or naturally occurring astigmatism. The advantages of femtosecond laser-assisted astigmatic keratotomy (FSAK) over conventional methods have been recognized recently. Main text: This review evaluates the efficacy, complications, and different methods of FSAK for correction of astigmatism in native eyes and those that underwent previous penetrating keratoplasty (PKP). The penetrating and intrastromal FSAK (IFSAK) techniques can reduce post-keratoplasty astigmatism by 35.4% to 84.77% and 23.53% to 89.42%, respectively. In native eyes, the penetrating and IFSAK techniques reduce astigmatism by 26.8% to 58.62% and 36.3% to 58% respectively, implying that the magnitude of the astigmatic reduction is comparable between the two FSAK procedures. Nonetheless, IFSAK offers the additional advantages of almost no risk of infection, wound gape, and epithelial ingrowth. The use of nomograms, anterior-segment optical coherence tomography, and consideration of posterior cornea and corneal biomechanics are helpful to enhance the efficacy and safety of FSAK. The complications of FSAK in eyes that underwent PKP include overcorrection, visual loss, microperforations, infectious keratitis, allograft rejection, and endophthalmitis. The reported difficulties in native eyes include overcorrection, anterior gas breakthrough, and suction loss. Conclusions: In eyes that underwent PKP, FSAK effectively reduces high regular or irregular astigmatism, with rare and manageable complications. Nevertheless, the drawbacks of the procedure include the potential loss of visual acuity and low predictability. For native eyes undergoing femtosecond laser-assisted cataract surgery, IFSAK is a good choice to correct low astigmatism (< 1.5 diopters). The refractive effect of astigmatism from the posterior cornea needs to be considered in the nomograms for native eyes undergoing refractive cataract surgery. To further improve the efficacy of FSAK, more large-scale randomized studies with longer follow-up are needed.
Femtosecond laser-assisted astigmatic keratotomy; Post-keratoplasty astigmatism; Astigmatism correction; Refractive surgery
Astigmatic keratotomy (AK), also known as arcuate
keratotomy, has been performed for more than a century
to correct astigmatism. With advances in technology,
AK is performed with higher accuracy using a
femtosecond laser compared with manual cutting with a blade.
Femtosecond laser-assisted cataract surgery is gaining
popularity among surgeons, from 19% in 2014 to 29% in
]. The femtosecond laser can be used not only to
create capsulotomies and fragment the lens, but also to
produce penetrating corneal or intrastromal incisions
with high precision. Femtosecond laser-assisted AK
(FSAK) is well proven to be effective and safe in
reducing corneal astigmatism in highly astigmatic eyes after
penetrating keratoplasty (PKP) [
]. Patients who
underwent PKP or deep anterior lamellar keratoplasty
(DALK) might have substantial anisometropia; the
primary goal of FSAK is the reduction of astigmatism after
PKP to a level that allows the patient to wear contact
lens or spectacles. This concept is important since the
sequel of AK is somewhat unpredictable  and may
require other visual aids. FSAK also can be performed to
treat corneas that are too thin for refractive surgery or
unsuitable for enhancement because of insufficient
corneal tissue or severe dry eye [
FSAK in post-keratoplasty eyes
Summary of techniques
Three significant variables are present in AKs: optical
zone diameter and the AK depth and arc length. The
optical zone diameter usually is set at a fixed distance from
0.4 to 1 mm within the graft-host junction if no
particular nomogram is used [
]. Regarding the depth, most
AKs are penetrating, with the depth set at a fixed
percentage of the thinnest pachymetry at the optical zone,
ranging from 75% to 90%, or set based on the
preexisting corneal astigmatism [
8, 11, 13
AKs are performed 60 to 90 μm from the epithelium
and 10% to 20% from the posterior cornea [
5, 12, 14,
]. The arc lengths of AK have been reported to range
from 15 to 120 degrees. Most AKs are paired
symmetrically along the steep axis. In some reports, single or
asymmetric paired AKs were executed to correct
irregular astigmatism [
9, 11, 16
]. The side-cut angles are
mostly 90 degrees, except in the studies of Cleary et al.
 and Rückl et al. [
] in which 135 degrees and 60
degrees were used, respectively.
Among all reviewed studies, most eyes had undergone a
PKP, and a small number had experienced DALK [
]/lamellar keratoplasty [
(i) FSAK vs. manual AK and mechanized AK
It has been suggested that the arc length, depth, and
location precision can be better achieved in FSAK,
compared with manual and mechanized AK [
also is associated with lower risks of wound dehiscence,
epithelial ingrowth, infection, and full-thickness corneal
Bahar et al. [
] reported a trend of better
improvement in the uncorrected visual acuity (UCVA) and
bestcorrected visual acuity (BCVA) in the FSAK group
compared with the manual AK group. However, the
differences were not significant statistically (UCVA p = 0.2;
BCVA p = 0.59), possibly due to the small sample size of
126. However, the improvements in the UCVA and
BCVA were significant only in the FSAK group (Manual
AK UCVA p = 0.09, BCVA p = 0.16; FSAK UCVA p =
0.004, BCVA p = 0.01). Also, the improvements in
defocus equivalent and aberrations were slightly higher in
the FSAK group (p = 0.31 and p = 0.65, respectively).
One surgeon performed all AK procedures but used
different techniques. The incisional depths differed
between the groups, and the nomogram was modified in
the last 10 FSAK subjects.
Hoffart et al. [
] compared the effectiveness between
FSAK and mechanized AK performed by the same
surgeon using the same nomogram. The changes in the
mean UCVA (p = 0.735 and p = 0.194, respectively) and
BCVA (p = 0.168 and p = 0.241, respectively) were not
significant in the FSAK and mechanized AK groups. The
refractive cylinder decreased more in the FSAK group
(p = 0.011). Regarding the angle-of-error analysis, a less
favorable outcome was observed in the mechanized AK
group compared to the FSAK group (p = 0.052).
(ii) Penetrating FSAK
Penetrating FSAK involves cuts performed from the
anterior surface. The wounds are closed, which decreases
the incidence of wound infection. It is believed that the
wound can be opened at a later follow-up examination if
the effect of the astigmatic correction was insufficient.
However, once the wound is opened, differential healing
can cause significant overcorrection [
The preoperative keratometric astigmatic levels ranged
from 4.4 diopters (D) [
] to 9.8 D [
postoperatively they ranged from 0.67 D to 5.2 D, respectively, in
those studies. The keratometric astigmatic changes
varied from 2.38 D [
] to 5.32 D [
] regardless of
underor overcorrection. The percentages of astigmatic
reduction have ranged from 35.4% [
] to 84.77% [
surgically induced astigmatism (SIA) has ranged from 1.577
D to 13.649 D [
]. A summary is shown in Table 1.
Intrastromal FSAK (IFSAK) is performed where the cut
is within the stroma and does not reach Bowman’s layer.
The absence of an open wound can avoid infection,
wound gape, or epithelial ingrowth. Wetterstrand et al.
] suggested that the desired intact posterior corneal
margin should be close to 90 μm by balancing the
measurement accuracy, protection of the endothelium, and
efficacy. This allowed reduction of astigmatism up to
Among the studies of IFSAK, the changes in
keratometric astigmatism have ranged from 0.66 D [
] to 9.28
], with the percentages of astigmatic reduction
ranging from 23.53% [
] to 89.42% [
]. The summary is
presented in Table 1.
(iv) FSAK in eyes after Descemet stripping automated
Yoo et al. [
] reported a case treated with FSAK for
post-Descemet stripping automated endothelial
keratoplasty in which there was an approximate overcorrection
of 7.5 D and refractive astigmatism changed from +
5.25 × 165 preoperatively to + 7.50 × 80 postoperatively.
The authors commented that this massive correction of
about 12.75 D was due to a full-thickness arcuate
incision in the recipient cornea, as the 90% depth was
calculated based on the total corneal thickness (i.e., recipient
cornea + donor cornea). The authors recommended that
the thickness of the donor graft must be excluded to
avoid a full-thickness incision of the recipient cornea.
Wedge resection for high astigmatism
Astigmatism after PKP usually ranges between 3 to 5 D
], but some can have up to 20 D of astigmatism [
The wedge resection is a technique performed to correct
high astigmatism, i.e., usually more than 10 D, which is
much higher than that fixed through relaxing incisions,
but the visual rehabilitation is longer. With wedge
resection, the cornea is steepened rather than flattened. The
surgery is performed by making two intersecting arcuate
cuts based on two different arc lengths with varying
angles of cut that intersect each other; a wedge of corneal
tissue is excised from the flatter meridian to steepen the
cornea. The width of the excision varies from 0.2 to
1 mm based on the amount of preoperative astigmatism;
generally, every 0.05 mm of tissue removed corrects 1 D
of astigmatism. Removal of defined widths and depths of
tissue is difficult with manual methods. However, the
femtosecond laser has facilitated such procedure with
higher accuracy [
]. Besides, a trend toward myopic
shift is observed due to a coupling effect. Suture
tightness and removal are essential factors in accuracy or
astigmatism correction with wedge resections.
Stability in post-keratoplasty eyes
Fadlallah et al. [
] reported regression from 1 to 2 years
postoperatively in their long-term study. The SIA
changed from 3.28 D at 6 months to 3.5 D at 1 year to 2.86
D at 2 years postoperatively.
Summary of status of post-keratoplasty eyes
Review of published articles (Table 1) reporting the
results of FSAK performed after PKP/DALK revealed no
significant differences in astigmatism reduction between
procedures with opened penetrating wounds and those
with closed penetrating wounds.
The general belief is that IFSAK has less of an effect
than penetrating FSAK. Although different studies had
different incisional depths, incisional arc lengths, and
optical zone diameters, there is insufficient evidence to
prove that penetrating correction produces a more
significant effect than intrastromal correction. However,
due to the limited number of studies and data that
compared intrastromal AK with penetrating AK, more
extensive studies with higher number of patients and
longer follow-ups are required to prove this.
The advantages of performing an intrastromal
procedure are almost no risk of infection, epithelial ingrowth,
or wound gape. However, after PKP or DALK there is
already an open wound, and, therefore, this advantage is
less than in native eyes.
FSAK in native eyes
The amount of astigmatic correction generally is limited
to 0.5 D to 1.5 D in native eyes, and most of the cuts are
performed at an optical zone of 7.5 mm or more to
prevent dysphotopsia. A summary is shown in Table 2.
(i) Penetrating FSAK
As shown in Table 2, the keratometric astigmatic
changes ranged from 0.352 D [
] to 3.4 D [
], and the
percentages of the astigmatic reductions ranged from
] to 58.62% [
]. Chan et al. [
penetrating FSAK (wound not opened) in 54 eyes that
underwent cataract surgery. The authors set the laser
arc length according to the corneal astigmatic magnitude
to be corrected, based on their nomogram modified
from the Wallace limbal relaxing incision (LRI)
nomogram. The authors concluded that there was a trend
toward undercorrection when target-induced astigmatism
(TIA) was 1 D or more and overcorrection when it was
less than 1 D. This implied that the nomogram might
need further adjustment. Moreover, Wang et al. [
reported that older age, longer incisional length, and
horizontal incisions in eyes with preoperative
against-therule (ATR) corneal astigmatism predicted a greater
postoperative astigmatic correction.
Among the IFSAK studies reviewed in this article, the
keratometric astigmatic changes ranged from 0.45 D
] to 0.87 D [
], and the percentages of astigmatic
reduction ranged from 36.3% [
] to 58% [
Day et al. [
] performed IFSAK in 196 eyes. The
nomogram for the laser arc length was based on the
degree of preoperative corneal astigmatism, age, and type
of astigmatism. The corneal astigmatism decreased by
39% from 1.21 D preoperatively to 0.74 D
postoperatively. Vector analysis showed under-correction of
astigmatism (mean correction index, 0.63 [< 1]; mean
magnitude of error, − 0.47 [< 0]). The angle of error was
small, i.e., 3 degrees. The study did not reveal significant
risk factors for astigmatic under- or overcorrection,
which implied that the nomogram might include other
factors in the future to improve the accuracy.
Day and Stevens [
] performed IFSAK in 87 eyes
during cataract surgery and compared the results to a group
of eyes undergoing cataract surgery without IFSAK in
176 eyes. A personal nomogram for the laser arc length
was used. At 1 and 6 months postoperatively, the IFSAK
group had significantly higher SIA than the non-IFSAK
group (0.78 D vs. 0.43 D, respectively, at 1 month; 0.69
D vs. 0.32 D at 6 months), which indicated that IFSAK
reduced the corneal astigmatism during cataract surgery.
The regression effect was comparable between the
Rückl et al. [
] performed IFSAK in 16 eyes without
cataract surgery, with a TIA of 1.59 D. At 6 months
postoperatively, corneal astigmatism decreased by 58%
from 1.50 D to 0.63 D. Vector analysis showed a mean
SIA of 1.59 D and correction index of 1.0. However, it is
worth noting that two (13%) eyes had strong
overcorrection (correction index close to 2.0) and four (25%) eyes
had extensive under-correction (correction index close
to 0.5), that is, six (37%) of 16 eyes had an undesirable
correction. However, the authors did not report the
individual preoperative data from these eyes that might help
identify the risk factors for inaccurate correction. The
corneal astigmatism was stable throughout the study
period postoperatively at 1 day, 1 week, and 1, 3, and
Stability in native eyes
Placement of manual LRIs has been shown to be stable
for up to 3 years [
(i) Penetrating FSAK
Chan et al. [
] performed penetrating AK (wound not
opened) in 50 eyes. The mean preoperative TIA was
1.35 ± 0.48 D, which decreased to 0.67 ± 0.54 D at
2 months and 0.74 ± 0.53 D at 2 years postoperatively.
There was no significant difference between the
postoperative corneal astigmatism over 2 years and no
difference in the magnitude of error, absolute angle of error,
and higher order aberrations postoperatively to 2 years.
Rückl et al. [
] reported stable corneal astigmatism
with IFSAK from 1 day (0.61 ± 0.43 D) to 6 months
(0.33 ± 0.42 D) postoperatively.
Day and Steven [
] compared the SIA resulting from
IFSAK during cataract surgery and standard
femtosecond laser-assisted cataract surgery to exclude
astigmatism induced by the main incision and side ports in
cataract surgery. Regression analysis at 1 and 6 months
postoperatively showed small but significant regression
with standard cataract surgery (0.11 D) and cataract
surgery with IFSAK (0.09 D); however, the values were low
and of little clinical relevance.
Summary of status of native eyes
The differences in astigmatic reduction are not
significant among penetrating wound open, penetrating wound
closed, and intrastromal correction for native eyes
(Table 2). Larger randomized controlled trials of IFSAK
with more extended follow-up periods are needed.
FSAK in post-trabeculectomy eyes
Kankariya et al. [
] reported a case of mixed
astigmatism induced after trabeculectomy treated with FSAK. A
penetrating paired incision (wound open) was made at
the 7.0-mm optical zone. Corneal astigmatism decreased
from 4.15 D to 0.81 D, and the UCVA improved from
20/200 to 20/60, which was the same UCVA as before
trabeculectomy. The intraocular pressure was
maintained, and the trabeculectomy bleb morphology was
Efficacy of combined intrastromal AK and laserassisted in situ Keratomileusis
Loriaut et al. [
] and Shalash et al. [
another technique to correct native eyes, or those that
underwent PKP with high astigmatism by performing
IFSAK after the creation of a laser-assisted in situ
keratomileusis (LASIK) flap followed 1 to 3 months
later by excimer laser photoablation. While this
technique permits correction of a broader range of high
astigmatism and can reduce the astigmatism by over
80%, epithelial ingrowth and microperforations are
Improving efficacy and safety
The commonly used MAK nomograms are the
Lindstrom nomogram [
] and the Hanna nomogram [
for correcting astigmatism after PKP. The zone diameter,
incisional depth, arc length, and age are the variables
that determine the incision. More central placement of
the incision, greater depth, longer incision, and older age
have resulted in a higher effect of the astigmatic
A coupling effect [
] must be considered when
planning astigmatism surgery that predicts the impact of
astigmatic incisions on the spherical equivalent
refraction (SE). The coupling ratio is defined as the ratio of
the amount of flattening of the incised meridian to the
amount of steepening of the opposite meridian.
Flattening is created at the meridian of the incision while
steepening is induced at the meridian 90 degrees away. If the
coupling ratio is 1, the SE will not change. When the
coupling ratio is greater than 1 and less than 1, the
results are, respectively, a hyperopic shift and a myopic
shift. Incisional arc lengths of 30 to 90 degrees result in
a coupling ratio of close to 1; arc lengths less than 20
degrees have a coupling ratio greater than 1, whereas
those greater than 100 degrees have a coupling ratio less
than 1 [
(i) Nomogram of FSAK in post-PKP eyes
Based on published data, the most frequently used
nomogram for FSAK after PKP is the topographic map
6, 7, 9, 10
]. In this nomogram, the lengths of
the relaxing arcuate incisions are ascertained by the
borders of the steep semi-meridians, and the incisions are
placed either 0.5 mm [
] or 1 mm [
] within the
graft-host junction. The other commonly used
nomogram is the Hanna nomogram with or without
4, 13, 18
], which was designed originally for
manual mechanical AK . The accuracy and
predictability varied considerably in eyes after PKP; hence,
surgeons often have to make adjustments based on
experience and surgical technique. Few reports have
been published on the appropriate nomograms to use in
eyes after PKP or in native eyes.
Another nomogram developed by St. Clair et al. [
was tested on 89 eyes, which is currently the most
significant sample reported in similar studies. According to
the nomogram, the incisional depth, arc length, and
optical zone diameter changed concerning the difference
between the steepest and flattest K values. The mean
refractive cylinder decreased significantly from 6.77 ± 2.80
D to 2.85 ± 2.57 D. A trend of under-correction of 3.62
D was reported with a low incidence of overcorrection,
6.7%, which was comparable to the 8% to 10% reported
]. A coefficient of determination of the generated
nomogram was 0.67, that is, 67% of the variation in
accuracy can be explained by preoperative astigmatism
and incisional parameters, and the other 33% is
recognized as unknown variables or inherent variability.
St. Clair et al. [
] postulated that the effect of AK on
astigmatism after PKP differs from that on native
corneas because of the oblique and irregular tension in the
corneal graft, resulting in a less-than-perfect tissue
distribution during PKP. The age of the donor graft also
might affect the result, since older corneas are stiffer
than younger donor corneas.
Another nomogram of beveled FSAK developed by
Cleary et al. [
] used a side-cut angle of 135 degrees
instead of 90 degrees. The authors hypothesized that a
beveled incision allows the anterior cornea to slide
forward, thus reducing astigmatism and preventing wound
gape. Despite the small sample size of six eyes, it
provides a good starting point for surgeons who want to
attempt beveled FSAK.
The accuracy of these nomograms that are explicitly
designed for use during FSAK after PKP is not yet
ascertained. Large-scale randomized studies are needed to
provide evidence to support or refine these nomograms.
(ii) Nomogram of FSAK in native eyes
Abbey et al. [
] reported a case of native eyes treated
with penetrating FSAK based on their modified version
of the Lindstrom nomogram. The manifest astigmatism
decreased from − 3.50/+ 5.25 × 89 preoperatively to −
1.75/+ 2.75 × 90 postoperatively in the right eye and
from − 3.50/+ 5.25 × 83 to − 1.75/+ 2.25 × 85 in the left
eye. Topography showed improved astigmatism with
and unchanged axis. Its efficacy, however, had not been
Consideration of the posterior cornea
In native eyes, ATR astigmatism was present in 86.6% of
the posterior cornea [
]. Thus, overcorrection of the
ATR astigmatism and under-correction of with-the-rule
(WTR) astigmatism by 0.75 D during cataract surgery
was suggested. Mild residual WTR astigmatism is
preferred over ATR, as it allows better distance and near
Löffler et al. [
] analyzed the effect on the anterior,
posterior, and total corneal astigmatism in eyes that
underwent penetrating FSAK and found a significant
reduction in astigmatism in the anterior and total corneal
astigmatism but not in the posterior corneal
astigmatism. These results are consistent with the finding that
the contribution of the posterior cornea was significantly
lower (0.26 ± 0.10 D) compared to the anterior (0.97 ±
0.30 D) and total corneal (0.96 ± 0.26 D) astigmatism.
While the posterior cornea does not affect the “corneal”
astigmatic correction with FSAK, the effect of the
posterior cornea on the total “refractive” astigmatism should
be considered when performing cataract refractive
surgery and FSAK simultaneously. However, when
performing FSAK on patients who underwent previous cataract
surgery, the refractive result is purely on the anterior
Wang et al. [
] reported 14.9% overcorrection 1
month after penetrating FSAK (wound open) in native
eyes; two-thirds of these overcorrected eyes had WTR
corneal astigmatism preoperatively. The authors
assumed that these overcorrections resulted from not
considering the posterior cornea. A new nomogram was
developed to account for the effect of the posterior
], which reduced the overcorrection to 6.7%;
however, further validation of the nomogram is needed.
Recently, Day et al. [
] reported the results of IFSAK
based on a personal nomogram that considered the
posterior cornea. The arc length was increased by 5 degrees
for ATR astigmatism but decreased by 5 degrees for
WTR astigmatism, which resulted in a higher corrective
index of astigmatism of 63% and lower overcorrection of
Another factor that can affect astigmatic correction is
the accurate placement of the astigmatism axis; every
degree of cyclotorsion error can cause under-correction
of 3.3% [
]. Modern femto-cataract lasers already can
match the astigmatism axis to the iris registration
preoperatively and then align the FSAK to the iris
pattern to achieve better accuracy [
Aside from the effects of zone diameter, arc length,
incisional depth, and age on the incisions in traditional and
modified nomograms, the impact of other corneal
parameters on the incisions has been studied.
Day and Stevens [
] studied the preoperative
parameters of 319 eyes undergoing cataract surgery with
intrastromal AK to identify the factors predictive of the
accuracy of FSAK. The corneal biomechanics evaluated
included corneal hysteresis (CH), which reflects the
corneal damping ability, and corneal resistance factor
(CRF), which indicates the overall corneal rigidity.
Multivariable regression analysis of the SIA showed that CH
and CRF were independent predictors of SIA, such that
the average SIA decreased by 0.06 D for every further
diopter increase of CH and increased by 0.04 D for every
additional diopter increase of CRF. Also, the WTR
astigmatism had an average SIA 0.13 D more than ATR
astigmatism. Although the study had a short follow-up
period of 1 month postoperatively, a previous study
found minimal regression associated with FSAK [
Therefore, the findings indicated that the corneal
biomechanical parameters, CH and CRF, might be included
in later nomograms to improve accuracy.
Furthermore, it was not recommended to place the
incisions in the recipient corneas because the corneal
biomechanics might be altered as a result of scarring at the
graft-host junction. The effect of relaxing incisions in
the recipient cornea was supposed to be blocked by the
new limbus formed by the keratoplasty wound [
Anterior-segment OCT (AS-OCT) is useful for both
preoperative planning and postoperative monitoring of
FSAK patients. In FSAK, one parameter that offsets the
amount of astigmatic correction is the incisional depth,
i.e., the deeper the incision, the more significant the
effect. For penetrating incisions, if the cuts are more
anterior than projected, there might not be sufficient depth
to attain the desired astigmatic correction [
displacement of the intrastromal incision can lead to a
higher risk of anterior perforation, significant
overcorrection, irregular astigmatism, and visual loss [
Detailed AS-OCT measurement of the peripheral corneal
thickness enables precise surgical planning of the
incisional depth, which prevents full-thickness corneal
perforation. Ideally, dynamic AS-OCT would be even more
beneficial by allowing real-time measurement and
adjustment of the incision. AS-OCT assessment of the
incisional depth 3 weeks postoperatively might be helpful.
It facilitates comparison and monitoring of any
mismatch between the programmed and achieved incisional
]. Furthermore, structural changes in the
corneal wound can be studied to rule out any effects
from wound healing.
Complications in post-keratoplasty eyes
The rates of overcorrection in patients who underwent
FSAK after PKP have been reported to be 19.4% [
], and 43.5% [
]. Overcorrection after PKP can be
managed by tightening the sutures; however, the effect is
Interestingly, in earlier studies [
10, 18, 21, 49
] in which
shorter arc lengths were used (up to 80 degrees), no
overcorrection was reported. The recent aggressive
approach to maximize the amount of correction appears
unpredictable. Possible long-term (5 to 10 years)
undesirable effects of this extensive weakening of the donor
graft after FSAK remain unknown. The ultimate goal of
AK is to reduce astigmatism to a level that visual aids
are acceptable to patients. Therefore, a balance between
residual astigmatism and risk of visual acuity
loss/complications should be evaluated in each patient.
(ii) Visual loss
Loss of two or more lines of the BCVA was reported in
eyes after PKP when penetrating FSAK was performed,
ranging from 3.2% to 20% [
9, 13, 44
]. No visual loss has
been reported in association with IFSAK.
(iii) Posterior perforation
The incidence rates of microperforations in eyes after
PKP undergoing penetrating FSAK have been reported
to be 3.2% to 8.7% [
]. The microperforations were
self-sealing, and the anterior chambers were maintained
with no postoperative sequelae. In most cases,
application of a bandage contact lens was adequate. Al Sabaani
et al.  reported that only one (1.9%) case required
resuturing of the AK wound.
A higher prevalence of microperforations (35%) was
reported in eyes that underwent IFSAK with the creation
of a LASIK flap [
]. The intrastromal AK incision was
made at a depth of 95% of the local corneal thickness
(guided by intraoperative pachymetry) after the flap was
created and lifted. There were no intraoperative leaks,
and a contact lens was applied by the end of surgery
with no postoperative sequelae.
Hashemian et al. [
] proposed that the
microperforations could have resulted from mechanical stress
induced by a Sinskey hook used to separate the tissue
bridges within the margins of the cut rather than from
the primary full-thickness femtosecond laser cut. This
literature review did not identify any reports of
macroperforations. If a full-thickness perforation occurs, the
wound should not be opened and allowed to heal; AK
should be performed again later at another optical zone.
(iv) Infectious keratitis
Infections are more likely to develop in eyes that
underwent PKP because the eyes are more
]. The infection rates associated with FSAK after
PKP have ranged from 0% to 4.8% [
8, 9, 13
infections typically were observed between 6 months and 1
year postoperatively, and all resolved with topical
Occasionally, fibrosis does not develop (even over the
long term) and if the epithelium is compromised
infection can occur as late as 15 years later [
]. We are
unaware of any infectious keratitis associated with IFSAK
as there is no open wound. It has been suggested that
closed wounds minimized the infection risk [
postoperative discomfort .
Only one case of endophthalmitis was reported after
FSAK after PKP [
] with no previous clinical evidence of
wound leakage. Endophthalmitis developed 5 days after
FSAK, and the patient was treated with 9 D of cylinder.
The endophthalmitis resolved with intravitreal antibiotic
therapy but the patient lost two lines of BCVA.
(vi) Allograft rejection
St. Clair et al. [
] reported a 2.2% incidence of graft
rejection in eyes that underwent penetrating FSAK.
Fadlallah et al. [
] reported a 4.8% (3/62 eyes) incidence of
graft rejections that occurred 3 months to 1 year
postoperatively; all resolved after treatment with topical
antibiotic steroids with no postoperative sequelae.
Complications in native eyes
Wang et al. [
] reported an incidence rate of
overcorrection of 14.9% at 3 months postoperatively.
Twothirds of the 14.9% overcorrected eyes had WTR corneal
astigmatism preoperatively, and the authors presumed
that the overcorrection might have resulted from
ignoring the effect of the posterior corneal astigmatism.
(ii) Anterior gas breakthrough
Most small amounts of the anterior gas breakthrough do
not cause problems. However, Kankariya et al. [
reported a case of anterior gas breakthrough during
IFSAK, in which irregular astigmatism was induced.
There was also a significant overcorrection of corneal
astigmatism from 0.84 × 176 preoperatively to 4.97 × 70
1 month postoperatively and a decrease in the BCVA
from 20/20 to 20/30.
(iii) Visual loss
Only one report of visual loss in FSAK performed on
native eyes from 20/20 to 20/30 was reported as
mentioned previously [
(iv) Suction loss
An intraoperative suction loss might affect the accuracy
of the incision. Rückl et al. [
] reported a case of
suction loss due to movement of the patient’s head. The
incisional alignment was affected but remained purely
intrastromal, with no subsequent visual loss.
(v) Misaligned position of incisions
During FSAK, since the femtosecond laser system
identifies the ocular structure on OCT scans, good-quality
OCT scans and ocular stability during the laser firing
stage are vital to ensure the correct position of the
incision. During manual AK, surgeons can cut through the
visual axis if the patient inadvertently moves during the
surgery, causing visual loss. Such scenario is unlikely in
FSAK since most machines stop quickly when suction is
(vi) Endothelial cell loss
There is concern that femtosecond laser energy close to
the endothelium can affect the survival of the
endothelial cells. However, Rückl et al. [
] and Hoffart et al.
] reported no significant endothelial cell loss after
Wellish et al. [
] reported a case of corneal ectasia after
multiple manual keratotomy procedures. Twelve
enhancement procedures were performed to treat residual
astigmatism after myopic astigmatism treated with
manual AK, which resulted in a double hexagonal
keratotomy. A conically shaped protrusion of the central
cornea, Munson’s sign, diffuse subepithelial scarring, and
central corneal thinning were seen. Therefore, repeated
AK for enhancement should be performed cautiously.
Ectasia has not been reported after FSAK.
Other surgical treatment options for astigmatism after
Other refractive surgeries including LASIK [
laser subepithelial keratomileusis/photorefractive
], toric intraocular lenses (IOLs) [
], and intrastromal corneal ring segments [
sometimes used to correct astigmatism after PKP.
Until now, only one report compared penetrating FSAK
and toric IOLs. In that study, Yoo et al. [
] studied the
clinical efficacy and safety of FSAK (9-mm optical zone,
85% depth, closed wound) performed after cataract
surgery and compared them with toric IOL implantation in
cataract patients with corneal astigmatism. The authors
found no significant difference in the residual refractive
astigmatism between the two treatment methods. These
results indicated that either manual or femtosecond laser
AK could be substituted for toric IOL implantation in
patients with mild corneal astigmatism.
FSAK reduces astigmatism in post-keratoplasty eyes with
high regular or irregular astigmatism. Complications are rare
and manageable. The predictability varies, and improvement
of the BCVA is not guaranteed [
]. VA losses have been
reported. Large-scale, randomized studies using newly
developed nomograms with long-term follow-up are needed.
For native eyes undergoing femtosecond laser-assisted
cataract surgery, IFSAK should be the choice for
astigmatic correction, and until better nomograms become
available, IFSAK should be reserved to treat low
amounts of astigmatism (< 1.5 D).
For patients who have already had their cataracts
removed or those who underwent PKP, the effect of
astigmatic correction is almost completely on the anterior
cornea, and the posterior cornea contributes very little.
However, when performing refractive astigmatic
correction, i.e., FSAK with cataract surgery, the effect of the
posterior cornea on astigmatism should be considered.
Patients should be instructed to avoid rubbing their
eyes to prevent sight-threatening complications. Notably,
in the 6 months postoperative period, the penetrating
incisions can become infected, even when the wound is
closed. It is best not to open the penetrating incision
also though the effect might be greater since it can lead
to late infections (up to 15 years). The patient should be
informed of this risk preoperatively.
The effectiveness of IFSAK seems to be comparable to
that of penetrating AK. Because of the superior safety
profile of IFSAK, more attention should be paid to this
As defined by vector analysis with the Alpins
SIA: surgically induced astigmatism is defined as the
amount of astigmatism the surgery actually induced.
TIA: target induced astigmatism is defined as the
amount of astigmatism the surgeon intended to
induce, it is equal to preoperative measured corneal
astigmatism if the target is to clear all astigmatism.
DV: difference vector is defined as the amount of
astigmatism that has to be postoperatively corrected
to finally reach the intended target astigmatism, it is
equal to the postoperative astigmatism.
The coefficient of determination [
] is the
proportion of the variance in the dependent variable
that is predictable from the independent variable(s).
AK: Astigmatic keratotomy; AS-OCT: Anterior segment OCT; ATR:
Against-therule; BCVA: Best-corrected visual acuity; CH: Corneal hysteresis; CRF: Corneal
resistance factor; D: Diopters; DALK: Deep anterior lamellar keratoplasty;
FSAK: Femtosecond laser-assisted astigmatic keratotomy; IFSAK: Intrastromal
FSAK; IOL: Intraocular lens; LASEK: Laser-assisted subepithelial keratectomy;
LASIK: Laser-assisted in situ keratomileusis; LRI: Limbal relaxing incisions;
PKP: Penetrating keratoplasty; SE: Spherical equivalent refraction; SIA: Surgical
induced astigmatism; TIA: Target induced astigmatism; UCVA: Uncorrected
visual acuity; WTR: With-the-rule
I am immensely grateful to Silvania Y.F. Lau, Maymay S.Y. Cheng, and Jack
C.M. Ng for their help in preparing the manuscript.
Availability of data and materials
John S. M. Chang is the sole contributor to this review. The author read and
approved the final manuscript.
Ethics approval and consent to participate
Consent for publication
The author declares that he has no competing interests.
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