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Comparison of visual acuity of the patients on the
first day after sub-Bowman keratomileusis or laser in situ keratomileusis
Wei Zhao1, Ting Wu2,
Ze-Hong
Dong1, Jie Feng1, Yu-Feng
Ren1, Yu-Sheng Wang1
1Department of Ophthalmology, Xijing Hospital, the
Fourth Military Medical University, Xi’an 710032,
Shaanxi Province, China
2Out-patient Department,
the Second Artillery Engineering University, Xi’an 710025, Shaanxi Province,
China
Co-first authors: Wei Zhao and Ting Wu
Correspondence to: Yu-Sheng Wang. Department of Ophthalmology, Xijing Hospital, the Fourth
Military Medical University, Xi’an 710032, Shaanxi Province, China. wangyushengsci@126.com
Received: 2015-02-13
Accepted: 2015-06-08
Abstract
AIM: To compare recovery of
the visual acuity in patients one day after sub-Bowman
keratomileusis (SBK) or laser in
situ keratomileusis (LASIK).
METHODS: Data from 5923 eyes in
2968 patients that received LASIK (2755 eyes) or SBK (3168 eyes) were
retrospectively analyzed. The eyes were divided into 4 groups according to
preoperative spherical equivalent: between -12.00 to -9.00 D, extremely high
myopia (n=396, including 192 and 204
in SBK and LASIK groups, respectively); -9.00 to -6.00 D, high myopia (n=1822, including 991 and 831 in SBK and
LASIK groups, respectively), -6.00 to -3.00 D, moderate myopia (n=3071, including 1658 and 1413 in SBK
and LASIK groups, respectively), and -3.00 to 0.00 D, low myopia (n=634, including 327 and 307 in SBK and
LASIK groups, respectively). Uncorrected logMAR visual acuity values of patients
were assessed under standard natural light. Analysis of variance was used for
comparisons among different groups.
RESULTS: Uncorrected visual acuity values
were 0.0115±0.1051 and 0.0466±0.1477 at day 1 after operation for patients
receiving SBK and LASIK, respectively (P<0.01); visual acuity values
of 0.1854±0.1842, 0.0615±0.1326, -0.0033±0.0978, and -0.0164±0.0972 were
obtained for patients in the extremely high, high, moderate, and low myopia
groups, respectively (P<0.01). In addition, significant differences
in visual acuity at day 1 after operation were found between patients receiving
SBK and LASIK in each myopia subgroup.
CONCLUSION: Compared with LASIK, SBK is safer and
more effective, with faster recovery. Therefore, SBK is more likely to be
accepted by patients than LASIK for better uncorrected visual acuity the day
following operation.
KEYWORDS: sub-Bowman
keratomileusis; laser in
situ keratomileusis; visual acuity; myopia; recovery
DOI:10.18240/ijo.2016.03.22
Citation: Zhao W, Wu T, Dong ZH,
Feng J, Ren YF, Wang YS. Comparison of visual acuity of the patients on the
first day after sub-Bowman keratomileusis or laser in situ keratomileusis. Int
J Ophthalmol 2016;9(3):448-452
INTRODUCTION
Previous
studies[1-2] have reported several advantages for laser in situ keratomileusis (LASIK),
including fast vision recovery, less pain, and no corneal haze. Several
surgical methods have been developed in recent years such as full-laser and
full-femtosecond techniques. However, LASIK remains the most popular method for
the correction of myopia, hyperopia, and astigmatism. Preparation of the
corneal flap is critical in LASIK. Indeed, the thickness of the corneal flap is
tightly associated with that of residual corneal stromal bed, which is a
critical factor in the occurrence of posterior corneal ectasia following LASIK.
In addition, thicker corneal flap could also restrict the range of refractive
surgery. In conventional LASIK, the thickness of the corneal flap is about 130-180
μm and depends on the keratome used[3]. The progress in femtosecond laser
and microkeratome allowed the reduction of corneal flap thickness to 90-110 μm;
this includes the Bowman's membrane and small portion of inferior corneal
stroma, and has thus been named sub-Bowman keratomileusis (SBK)[4].
Technically, SBK is therefore a variant of LASIK, and can be called super-thin
corneal flap LASIK; it combines the advantages of conventional LASIK and
EPI-LASIK, while avoiding their disadvantages. With no surprise, SBK is
gradually replacing LASIK. SBK could be classified as femtosecond and
mechanical, according to the processes by which corneal flap preparation is
carried out. Both traditional LASIK and SBK display good effectiveness and
predictability, and the visual acuity is rapidly recovered within a day after
operation[5]. Most patients worry seriously about their visual
acuity the day after operation for study or work, especially teenagers; in
addition, other patients from distant provinces desire to go home as soon as
possible. This underscores the need of investigating the difference between
patient treatment with LASIK and SBK, which has not been reported to date. In
the present study, visual acuity at day 1 after operation was retrospectively
analyzed to evaluate the difference between SBK and LASIK treatments.
SUBJECTS AND METHODS
Subjects A total of 5923 eyes from 2968
patients (1367 males and 1601 females) received SBK or LASIK treatment in the
refractive surgery center of Xijing hospital between January 2008 and January
2013. A total of 3168 eyes were treated with SBK between 2011 and 2013, while
2755 eyes underwent LASIK treatment (between 2008 and 2010). The eyes were
divided into 4 groups according to preoperative spherical equivalent : between -12.00
to -9.00 D, extremely high myopia (n=396,
including 192 and 204 in SBK and LASIK groups, respectively); -9.00 to -6.00 D,
high myopia (n=1822, including 991
and 831 in SBK and LASIK groups, respectively), -6.00 to -3.00 D, moderate
myopia (n=3071, including 1658 and
1413 in SBK and LASIK groups, respectively), and -3.00 to 0.00 D, low myopia (n=634, including 327 and 307 in SBK and
LASIK groups, respectively). The inclusion criteria were: 1) more than 18 years
old; 2) spherical power lower than -12.00 D and astigmatism less than -3.00 D;
3) stable spherical power over the last two years; 4) corneal thickness more than
480 μm; 5) best corrected visual acuity ≥ 0.8; 6) no other eye diseases
pre-operation. The exclusion criteria were: 1) intraoperative corneal flap
complications; 2) edema of corneal epithelium, epithelial implantation,
hemorrhage sub-flap or epithelial dot defects affecting the uncorrected visual
acuity (UCVA) the first day post-operation; 3) severe irritation preventing
patients from cooperating with the vision test the first day post-operation. As
a result, 32/3200 (1%) and 44/2799 (1.57%) eyes in the SBK and LASIK groups
were excluded due to complications. This study have been appoved by the Ethics
Committee of Xijing Hospital.
Operation Methods
ALLEGRETTO
WAVE Eye-Q (Germany) was used for the operation, with automatically rotary
microkeratomes (Moria M2, France) or automatic microkeratomes (Moria OUP-SBK,
France). All operations were performed by the same experienced surgeon.
Immediately after operation, a drop each of 0.5% levofloxacin (Santen, Japan)
and 0.1% fluorometholone (Santen, Japan) were administered to each eye. This
was followed by administration of levofloxacin (one drop every 2h) until the
patient fell asleep.
Postoperative examinations An eye mask was put on each eye
immediately after operation and removed at 8:00 a.m. the next day; then, slit
lamp examination was performed to rule out abnormalities in the corneal flap,
edema of the corneal epithelial layer, corneal epithelial punctate defects,
bleeding under flap, and epithelial ingrowth. Afterward, an experienced vision
examiner examined and recorded the UCVA of the patients under standard natural
light, using the international standard logarithmic visual acuity chart and
decimal visual acuity values were recorded.
Statistical Analysis The SPSS 16.0 software was used for
statistical analyses. Data were expressed as mean±standard deviation. UCVA
(using logMAR visual acuity which calculated by decimal visual acuity) values
for patients were compared among different operation methods, spherical power
degrees, and operation methods in each spherical power degrees, with the
Mann-Whitney U test, independent
sample t-test, or analysis of
variance (ANOVA). P<0.05 was
considered statistically significant.
RESULTS
Preoperative Data
As
shown in Table 1, no statistically significant differences were found between
the SBK and LASIK groups in age (P=0.654),
gender (P=0.782), best spectacle
corrected visual acuity (BSCVA, P=0.498),
spherical equivalent refraction (P=0.719),
and CCT (P=0.523), but a
statistically significant difference was found in mean residual bed thickness (P<0.001). Similar data were obtained
when various subgroups were analyzed, including extremely high, high, moderate,
and low myopia patients (data not shown).
Comparison Between Uncorrected Visual Acuity Values
Obtained After Laser in situ
Keratomileusis and Sub-Bowman Keratomileusis UCVA values were 0.0115±0.1051 and 0.0466±0.1477
the day after operation for patients treated by SBK and LASIK, respectively (P<0.01).
Among the 3168 eyes of the SBK group, visual acuity of 2399 (75.7%) eyes were
≥1.0 (Decimal visual acuity), 512 (16.2%) eyes were 0.8, 157 (5.0%) were 0.6, and
100 (3.1%) were <0.6, respectively. For the 2755 eyes of the LASIK group,
visual acuity of 1771 (64.2%) eyes were ≥1.0, 490 (17.8%) eyes were 0.8, 211
(7.7%) were 0.6, and 283 (10.3%) were <0.6, respectively. Interestingly, the
Mann-Whitney U test showed that the
distribution of visual acuity was significantly different between the two
groups (P<0.05). Indeed, more patients in the SBK group (75.7%) were
found with UCVA ≥1.0 compared with LASIK treated patients (64.2%); in addition,
less patients in the SBK group (3.1%) showed UCVA <0.6 compared with the
LASIK group (10.3%) (Table 2).
Comparison of Uncorrected Visual Acuity Among Patients
with Different Diopters The visual acuity values
were 0.1854±0.1842, 0.0615±0.1326, -0.0033±0.0978, and -0.0164±0.0972 for
extremely high, high, moderate, and low myopia patients, respectively,
indicating a statistically significant difference among these groups (P<0.01).
Comparison of Uncorrected Visual Acuity Between
Sub-Bowman Keratomileusis and Laser in
situ Keratomileusis Subgroups Within Patients with Different Diopters No statistically significant
difference was obtained in UCVA between the SBK and LASIK treatment groups in
low myopia patients (P>0.05). However, the difference in UCVA was
statistically significant between the patients administered SBK and LASIK in
the extremely high, high, and moderate myopia groups (P<0.001) (Table
3).
Table 1 All patients’ pre-operation data
|
Parameters |
SBK n=1585,
3168 eyes |
LASIK n=1383, 2755 eyes |
P |
|
Age (a) |
|
|
0.654 |
|
|
24.54±5.84 |
24.39±5.45 |
|
|
Range |
17-45 |
17-48 |
|
|
Gender, n (%) |
|
|
0.782 |
|
M |
732 (46.18) |
635 (45.91) |
|
|
F |
853 (53.82) |
748 (54.09) |
|
|
BSCVA |
|
|
0.498 |
|
Mean logMAR±SD |
0.004±0.05 |
0.008±0.06 |
|
|
Range logMAR |
-0.30 to
0.22 |
-0.18 to
0.22 |
|
|
Spherical equivalent (D) |
|
|
0.719 |
|
|
-5.46±2.07 |
-5.48±2.20 |
|
|
Range |
-0.25 to
-13.25 |
-0.75 to
-14.00 |
|
|
CCT (μm) |
|
|
0.523 |
|
|
543.59±33.77 |
541.02±27.36 |
|
|
Range |
469 to
660 |
478 to
637 |
|
|
Mean residual bed thickness (μm) |
|
|
<0.001 |
|
|
330.49±33.77 |
308.86±32.65 |
|
|
Range |
259 to
490 |
250 to
451 |
|
BSCVA: Best spectacle corrected visual
acuity; CCT:
Central cornea thickness.
Table 2 Visual acuity of the patients in the SBK and
LASIK groups the day after operation
n (%)
|
Groups |
Decimal visual acuity |
Total |
|||
|
≥1.0 |
0.8 |
0.6 |
<0.6 |
||
|
SBK |
2399
(75.7) |
512 (16.2) |
157 (5.0) |
100 (3.1) |
3168 (100.0) |
|
LASIK |
1771 (64.2) |
490 (17.8) |
211 (7.7) |
283 (10.3) |
2755 (100.0) |
The
Mann-Whitney U test showed P<0.05,
suggesting that visual acuity was significantly different between the 2 groups.
More patients were found with visual acuity ≥1.0 in the SBK group in comparison
with the LASIK group (75.7% vs
64.2%); less patients were with visual acuity <0.6 in the SBK group compared
with the LASIK group (3.1% vs 10.3%).
Table 3 LogMAR visual acuity the day after operation
in patients of the SBK and LASIK groups
|
Overall |
Diopter |
||||
|
Low myopia group |
Moderate myopia group |
High myopia group |
Extremely high myopia group |
||
|
SBK |
0.0115±0.1051 |
-0.0097±0.0963 |
-0.0138±0.0807 |
0.0358±0.1067 |
0.1416±0.1589 |
|
LASIK |
0.0466±0.1477 |
-0.0234±0.0978 |
0.0091±0.1135 |
0.0922±0.1525 |
0.2266±0.1968 |
|
t |
-10.638 |
-2.071 |
-6.518 |
-9.252 |
-4.711 |
|
P |
<0.001 |
0.301 |
<0.001 |
<0.001 |
<0.001 |
DISCUSSION
Several new
techniques including femtosecond laser, full-laser, and full-femtosecond
methods developed in recent years could greatly improve the preparation of
corneal flap. However, conventional LASIK, which only prepares corneal flaps
with a thickness of about 130-180 μm, is still often used for the treatment of
myopia, hyperopia, and astigmatism due to its relatively low cost, good
postoperative effects, and fast recovery[6-7]. Another new method, namely SBK,
is increasingly rated as first choice among patients since it preserves more
residual corneal stromal bed, while displaying comparable effectiveness with
LASIK and a better safety profile[8-9]. Postoperative recovery is very
fast for both LASIK and SBK; this is of great importance for patients, among
which some seriously worry as to when their visual acuity would return to
normal. Indeed, visual acuity the day after operation is a great concern for
the patients. Surprisingly, no study has investigated the differences in visual
acuity between patients receiving SBK and LASIK the day after operation. In the
present study, the UCVA values recorded the day after operation for patients
treated with LASIK and SBK in the past 5y was compared. We found that visual
acuity the day after operation was significantly better in patients treated
with SBK than those of the LASIK group (0.0115±0.1051 vs 0.0466±0.1477, P<0.01). The distribution of visual
acuity was also significantly different between the LASIK and SBK groups (P<0.05).
More patients displayed visual acuity ≥1.0 in the SBK group compared with LASIK
treated individuals (75.7% vs 64.2%),
while less patients with visual acuity <0.6 were found in the SBK group
(3.1% vs 10.3%). In addition,
significantly differences were obtained between SBK and LASIK in patients with
different myopia degrees (P<0.01 for extremely high, high, and
moderate myopia groups). These findings are in agreement with previous reports[3].
Furthermore, the visual acuity was significantly different among extremely
high, high, moderate, and low myopia patients (0.1854±0.1842, 0.0615±0.1326,
-0.0033±0.0978 and -0.0164±0.0972, respectively, P<0.01) the day
after operation, suggesting that visual acuity at day 1 after operation is
affected by the degree of myopia. Importantly, the visual quality and
subjective feeling of the patients were also better in the SBK group compared
with the LASIK group. Fewer dry eyes were observed and less drug was needed in
the SBK group at 1-month after the operation, while dry eyes were found in
several patients of the LASIK group, requiring treatment with artificial tears.
However, follow-up was not regular in many patients, making it difficult to
obtain data from all these patients for accurate statistical analysis.
Many factors,
including abnormal corneal flap, edema of the corneal epithelial layer, corneal
epithelial punctate defects, accommodation of the patients, cloudy or bleeding
under flap, and lacrimal secretion affect the recovery of visual acuity[10-11].
After exclusion of these factors, we found improved visual acuity in the SBK
group compared with LASIK patients. The thinner corneal flap used in the SBK
method could be one of the reasons. Indeed, the thickness of the corneal flap
used in SBK patients was about 20-70 μm less than that used in the LASIK group;
the central corneal thickness was about 110 μm, which made it an ideal corneal
flap that could maximize the preservation of the thickness of corneal stromal
bed and stability of corneal biomechanics[12]. Previous studies have shown that
the uniformity and repeatability of the thickness of corneal flaps prepared by
OUP microkeratome are comparable with those obtained by the femtosecond
technique [8-9,13]. In contrast, the thickness of corneal flaps for
conventional LASIK is highly dependent on the times of microkeratome use[14];
thus, either the uniformity of the corneal flap or the thickness of corneal
stromal bed cannot be ensured. The thickness of the corneal stromal bed should
be higher than 250 μm after LASIK to avoid keratectasia and iatrogenic
keratoconus; while in most cases, clinicians believe that the thickness of the
corneal stromal bed should be at least 280 μm to ensure good postoperative
biomechanics[15]. Herein, we have demonstrated
that the difference in visual acuity the day after operation between the SBK
and LASIK subgroups was more pronounced in extremely high myopia patients,
while this difference was relatively reduced within the low myopia group,
suggesting that SBK better preserved the thickness of the corneal stromal bed
in addition to using less corneal tissues in patients with high myopia degree,
and thus provided better biomechanical features and visual quality[16].
In addition,
other advantages could be obtained with thin corneal flaps but not with thick
ones. First, thin corneal flaps allow the extension of ablation diameter, which
reduces the risk of postoperative glare, thus further improving the
postoperative visual quality. Second, thin corneal flaps allow the preservation
of corneal stromal bed thickness, which could increase the laser ablation extent
and correction range of myopia. This could be used for patients with high
myopia, making it possible to perform secondary correction for refractive
regression[9]. Interestingly, we found that the visual acuity values
were comparable in patients receiving SBK and LASIK at 1-month post-operation
(data not shown), in accordance with previous studies[3,17].
As shown
above, visual acuity was significantly different among various myopia groups
the day after operation. Indeed, the visual acuity was poorer in the high and
extremely high myopia groups the day after operation, but better in the
moderate and low myopia groups. These findings demonstrated that for patients
with extremely high or high myopia, overcorrection could generally be found at
the early stage after operation, leading to relatively poor visual acuity;
however, the overcorrection could become an excellent correction within 10d to
1mo, when the visual acuity increases. Therefore, visual acuity was not
statistically different among extremely high, high, and moderate myopia groups
at 1-month post-operation (data not published).
SBK not only
results in better postoperative visual acuity, but also makes patients more
comfortable. In the conventional LASIK, the corneal flaps show relatively poor
uniformity, which could more or less damage the corneal nerve plexus and result
in decreased postoperative corneal perception and prolonged recovery time[18-19].
In SBK, the corneal flap is prepared with higher precision with more
superficial ablation layer, which makes the postoperative recovery faster and
less depending on the artificial tears[11,20].
One of the
limitations of this study is that it was carried out in a single center and
analyses were done retrospectively. Second, the vision acuity was affected by a
lot of factors such as precision of preoperative examination, the compliance of
patient when visual acuity examination and stability of excimer laser machine
every operation days. In addition, the Early
Treatment Diabetic Retinopathy Study (ETDRS) visual chart, which may be more
suitable for high myopia and fundus diseases patients, was not used in the
present work.
Nevertheless,
our data showed that SBK is a safer method with better efficacy and faster
recovery compared with LASIK. In addition, visual acuity the day after
operation was better with SBK, which should have higher patient compliance.
Furthermore, SBK preserved the corneal stromal bed thickness and extended the
correction range of patients, providing an additional opportunity for patients
with high myopia, thin cornea, or in need of secondary correction[3,12].
Overall, SBK is the better choice for corneal flap preparation with either
femtosecond or OUT-SPK microkeratomes, and should be considered a good choice
for patients.
ACKNOWLEDGEMENTS
The abstract
of this paper has been communicated in the XXXIII Congress of the ESCRS.
Foundations: Supported by the National Natural
Science Foundation of China (No.81000391; No.81370020).
Conflicts of Interest: Zhao W, None; Wu T, None; Dong ZH,
None; Feng J, None; Ren
YF, None; Wang YS, None.
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