EL-MINIA MED. BULL. VOL. 20, NO. 2, JUNE, 2009
Ali et al
COMPARISON BETWEEN Q-VALUE CUSTOMIZED ABLATION (CUSTOM-Q)
AND WAVEFRONT OPTIMIZED ABLATION FOR PRIMARY MYOPIA AND
MYOPIC ASTIGMATISM
By
Ahmed Tawfik; MD*; Rabie Hassanien; MD**; Abdlaleem Tolba; MD**;
Ahmed Mostafa:MD**; and Ismail Ahmed;**
Department of Ophthalmomgy, *El-zagazig Faculty of Medicine
and **Minia Faculty of Medicine.
ABSTRACT:
Purpose: To compare treatments with wavefront optimized and custome-Q ablations.
Methods: Two groups of 200 eyes each were treated for myopia and myopic astigmatism
with LASIK. One of the two groups was treated with wavefront optimized ablation and the
second group was treated with custom-Q. They were examined preoperatively and
postoperatively to asses the Q-value, image quality, and other classic outcome parameters.
Results: The wavefront optimized ablation group was comprised of 200 eyes with a mean
spherical equivalent refraction (SE) of -5.2188 diopters (D) (range: -1.15 to -10.50 D); mean
Q-value changed from 0.30 preoperatively to 0.06 postoperatively. The custom-Q ablation
group was comprised of 200 eyes with a mean SE of -5.1575 D (range:-1.35 to -9.00 D);
mean Q-value changed from 0.32 preoperatively to 0.03 postoperatively. A statistically
significant difference in postoperative change in Q-values (P=0.02), and in the postoperative
visual acuity (p=0.42) between the two groups was noted.
Conclusion: Regarding refractive correction there was no difference between the two groups.
There was a marginally significant change in BSCVA between the two groups, and less
impairment in the corneal asphericity in the custom Q group.
KEY WORDS:
Asphericity
Custom-Q
Wavefront
Myopia
LASIK
Astigmatism.
INTRODUCTION:
Corneal refractive surgery is based
on the change in corneal curvature to
compensate for refractive errors of the eye.
As a logical consequence, research
was directed toward aspheric ablation
profiles, and the optical aberrations
induced by the operations.4-6
Standard ablation profiles for the
correction of myopia and myopic
astigmatism were based on the removal of
convex–concave tissue lenticules with
sphero-cylindrical surfaces.1 Although
these algorithms proved to be effective to
compensate for refractive error, the quality
of vision deteriorated significantly,
especially under mesopic and low-contrast
conditions.2,3
Therefore, new aspheric non
individualized algorithms were designed to
compensate for the spherical aberration
induced,7-9 which led to an improved
visual outcome.10 On the other hand, it
has been known for many years that any
refractive treatment of the cornea must
respect the preoperative and postoperative
asphericity of the cornea.11–13
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Ali et al
The outer surface of the human
cornea is physiologically not spherical but
rather like a conoid.14 On average, the
central part of the cornea has a stronger
curvature than the periphery or, in other
words, the refractive power of the outer
corneal surface decreases from central
toward peripheral. For this form, the term
prolate cornea has been coined, and the
opposite form is called oblate cornea.
treated
with
custom-Q
representing the second group.
ablation
The physiologic asphericity of the
cornea shows a significant individual
variation ranging from mild oblate to
moderate prolate.14 Therefore, it was
necessary to introduce a shape factor to
characterize the amount of asphericity of
the cornea numerically, the so-called Qfactor.
Exclusion criteria:
Previous history of refractive
surgery, keratoconus or keratoconus
suspect, corneal thickness less than 480
um, and collagen vascular diseases.
Inclusion criteria:
Age more than 18 years, no contact
lens wear for 2 weeks before baseline
examination, stable refractive error for one
year, myopia ranging between -1.00 and
10.00 diopters (D) with up to -4 D
astigmatism.
PREOPERATIVE ASSESSMENT:
 Medical history.
 Full ophthalmological examination:
1. Uncorrected visual acuity (UCVA).
2. Best spectacle corrected visual acuity
(BSCVA).
3. Manifest refraction, both subjective
and objective.
4. Slit-lamp biomicroscopy.
5. Goldmann applanation tonometery.
6. Corneal topography to measure
corneal asphericity from which Qvalue can be acquired using Allegro
Wave Topolyzer.
7. Central ultrasonic Corneal pachymetry.
This
study
compares
the
WaveLight ALLEGRETTO (WaveLight
AG, Erlangen, Germany) wavefront
optimized treatment and its custom-Q
treatment. The wavefront optimized
ablation has an aspheric profile in which
the amount of asphericity is not adjustable.
Similarly, the custom-Q ablation is also an
aspheric ablation, but it adds the ability for
the surgeon to define the intended Q-shift
(postoperative Q-value minus preoperative
Q-value) by specifying a desired
asphericity target. The only preoperative
data that custom-Q treatment uses in
addition to the refractive data is a value of
the mean corneal asphericity. It aims to
change the mean asphericity by
symmetrically adjusting the number of
mid-peripheral laser pulses.
The
laser
treatments
were
performed with the ALLEGRETTO
excimer laser. An infrared pupillary eye
tracker with latency of 6 ms was used.
Optical zone diameter of 6.5 mm and
transition zone of 1.0 mm were used in all
surgeries in both groups.
PATIENTS AND METHODS:
Four hundred eyes of 200 patients
seeking laser refractive surgery were
included in this study. The first group of
200 eyes were treated with optimized
ablation, and the remaining 200 eyes were
The target refraction in both groups
was emmetropia. The ALLEGRETTO
wavefront optimized ablation was used in
the wavefront optimized ablation group,
whereas the custom-Q ablation was used in
the custom-Q ablation group.
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EL-MINIA MED. BULL. VOL. 20, NO. 2, JUNE, 2009
Ali et al
1st., 2008 to May 2009. Preoperative
demographic and refractive data are
presented in Table 1, along with the
manifest
refraction data,
BSCVA,
asphericity (expressed as Q-factor). No
statistically significant difference between
groups was noted regarding age (p=.69),
sex (p=0.131), and spherical equivalent
(p=.833).
All surgeries were LASIK with
optical zone diameter of 6.5 mm and
transition zone of 1.0 mm in both groups.
The postoperative evaluation was
done on the first postoperative day by slit
lamp examination. At one week, one
month, and three months follow up the
patients were examined by slit lamp
biomicroscopy,
manifest
refraction,
UCVA, and BSCVA.
There was a statistically significant
difference in the postoperative Q value
between the two groups (p=0.02), with
shift of the cases towards the oblate pattern
in all cases, there was also significant
difference in the postoperative visual
acuity (p= 0.42).
No significant difference between the two
groups regarding the spherical equivalent
postoperatively.
Pre- and postoperative parameters
as well as the preoperative versus
postoperative changes were analyzed using
SPSS the paired 2-sided t test. A p value
less than 0.05 was considered statistically
significant.
RESULTS:
The two groups each of 200 eyes
were treated in the period from February
Table 1: Preoperative demographic and refractive data of the two groups
parameter
Age
Sex %
MRSE
Asphericity (Q)
BSCVA
Wavefront optimized
ablation group
30.230±6.4( 18 to 47)
59.50/40.50
-5.218±2.68(-1.15to10.50)
0.3049±0.08(0.06-0.53)
0.9235±0.1445( 0.5-1)
Custom Q ablation group
P value
31.345±7.31(18 to 49)
51.5/48.5
-5.1575±3.092(-1.35
9.50)
0.3282±0.14(.01-0.8)
0.9122±0.209(0.5-1)
0.095
0.137
0.68
0.131
to- 0.833
MRSE = manifest refraction spherical equivalent, BSCVA = best spectacle-corrected visual acuity.
*Values represented as mean standard deviation (range).
Table 2: Postoperative refractive data of the two groups
parameter
MRSE
Asphericity (Q)
BSCVA
Wavefront optimized group
0.09±0.22 (0-1.25)
0.06±0.44
0.925±0.217 (0.4-1.2)
141
Custom Q group
0.07±0.44(0-1)
0.03±0.77
0.982±0.138 (0.5-1.2)
P value
0.72
0.02
0.42
EL-MINIA MED. BULL. VOL. 20, NO. 2, JUNE, 2009
Ali et al
enhances the central ablation depth as
previously described. An intended change
in Q-factor DQ of _0.6 within an optical
zone of 6.5 mm requires 28.5 mm more
central tissue removal. This increased
central keratectomy depth may limit the
application of Q-factor optimized ablation
to corrections of only mild to moderate
myopia. A stronger attempted asphericity
correction, for example, a target Q of _1.0,
might have yielded more prolate
postoperative corneas, but, on the other
hand, such a strong Q-factor correction
would increase the central keratectomy
depth by another 30 mm, which we judged
not to be appropriate with respect to the
already well-preserved low-contrast visual
acuity data in this study.
DISCUSSION:
Wavefront-guided
customized
ablation appears to be the gold standard for
ablative treatment of the myopia and
myopic astigmatism regarding the optical
performance of the postoperative eye.15,16
Therefore, it was logical to compare a new
algorithm such as the custom-Q profile
with this standard one .
Regarding refractive outcomes in
the study, however, we could not find any
relevant differences and conclude that the
two treatment strategies appear to be
clinically equivalent.
Although the preoperative and
postoperative overall optical performance
of the eyes treated by the two different
profiles was very similar in the two
groups.
In
summary,
this
study
demonstrated that a Q-factor optimized
ablation profile yielded visual, optical, and
refractive results comparable to those of
the
wavefront
guided
customized
technique for corrections of myopia and
myopic astigmatism.
The Q-factor optimized ablation represents
a customized approach that is much less
time consuming than the wavefrontguided technique since it is based on
preoperative corneal topography, which is
mandatory in any case to detect
keratectatic disorders. The Q-factor
optimized profile has, therefore, the
potential to replace currently used standard
profiles for corrections of myopic
astigmatism.
We found a significant difference
in postoperative Q-factors between the
groups since the aim in the custom- Q
group was to decrease the induced change
in the corneal asphericity and as we
thought this may induce better visual
outcome. Although, there was an oblate
shift in all patient and is logic because of
the myopic correction induced corneal
oblation. But we noticed that this shift was
more obvious as the myopic error increase.
We also found a significant
difference in the visual outcome between
the two groups with more pronounced
visual level correction in the custom Q
group.
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‫‪EL-MINIA MED. BULL. VOL. 20, NO. 2, JUNE, 2009‬‬
‫مقارنة بين كشط القرنية التفصيلي المبني على معامل كيو‬
‫وكشط القرنية المحسن بوجه الموجة في تصحيح الحاالت األولية‬
‫من قصر النظر وقصر النظر المصحوب بالالنقطية‬
‫احمد توفيق علي* – ربيع محمد محمد حسانين** ‪ -‬عبد العليم عبد هللا طلبه**‪-‬‬
‫احمد مصطفي عيد** ‪ -‬اسماعيل احمد نجيب**‪-‬‬
‫قسم طب و جراحة العين‪ -‬كلية طب *جامعة الزقازيق و**جامعة المنيا‬
‫تمثل القرنية ثلثي القوة البصرية للعين بمقدار ‪ 48‬درجة‪ .‬و يعتمد األكسيمر ليزر علي تغيير شكل‬
‫القرنية عن طريق إزالة جزء من نسيج القرنية و بالتالي تغيير القوة البصرية للعين‪.‬‬
‫شملت هذه الدراسة ‪ 400‬عينا يعانون من قصر النظر األولي أو قصر النظر المصحوب باال نقطية‪.‬وتم‬
‫تقسيم المرضي إلي مجموعتين كل مجموعة تشتمل علي ‪ 200‬عينا‪.‬‬
‫ المجموعة األولي ‪:‬‬‫تم إجراء كشط للقرنية بواسطة الليزر المحسن بوجه الموجة‪.‬‬
‫ المجموعة الثانية‪:‬‬‫تم إجراء كشط للقرنية بواسطة الليزر المبني علي معامل كيو‪.‬‬
‫جميع المرضي تم إجراء الليزر لهم بواسطة جهاز األكسيمر ليزر من طراز اليجريتو‪.‬‬
‫وتم فحص المرضي بعد أسبوع و شهر و ثالثة أشهر من تاريخ اجراء العملية‪.‬‬
‫اظهرت النتائج تساوى المجموعتين من حيث الكفاءة فى تصحيح قصر النظر‪ .‬لكن اظهرت النتائج ايضا‬
‫ان المجموعة الثانية اعطت نتائج أفضل من حيث قوة األبصار و المحافظة علي تحدب القرنية عن‬
‫المجموعة األولي‪.‬‬
‫‪144‬‬