Download Influence of stromal surface humidity in intraoperative ultrasound

ARTICLE
Influence of stromal surface humidity in intraoperative
ultrasound pachymetry during LASIK
Rafael Bilbao-Calabuig, MD1; Valentín Jiménez Mateo-Sidrón, MD1;
Fernando Gómez-Sanz, OD1,2
PURPOSE: To investigate the influence of surface stromal humidity in intraoperative
ultrasound pachymetry during LASIK.
SETTING: Clínica Baviera, Instituto Oftalmologico Europeo, Madrid, Spain.
DESIGN: Evaluation of diagnostic test or technology.
METHODS: In this prospective study, 267 consecutive eyes were operated by 2 experienced
surgeons: after the corneal flap was cut with the Moria LSK-One microkeratome, it was
lifted and the degree of stromal surface humidity was evaluated on a subjective scale; we
then measured stromal bed thickness with ultrasonic pachymetry before and after drying the
stromal surface with a Weck-cel sponge. Flap thickness was calculated taking into account
preoperative pachymetry. Statistical analysis was performed using the paired Student t-test
to evaluate the difference between wet and dry stromal bed pachymetry, and a multivariate
regression analysis of this difference with several clinical parameters was made.
RESULTS: Mean flap thickness before drying was 93 ± 15 μm, and after drying 102 ± 15
μm. The mean difference between wet and dried stromal bed pachymetry was 8.8 ± 5.5 μm
(P<0.001). This difference was not associated with the surgeon, preoperative pachymetry
or keratometry, flap thickness, suction ring or right or left eye. The only variable showing a
significant influence to explain the difference found between wet and dry measurements was
stromal bed humidity (P<0.05).
CONCLUSIONS: When performing intraoperative LASIK ultrasonic pachymetry,
the actual stromal thickness should be considered to be around 10 microns less than the
thickness measured without drying. In case of thin preoperative pachymetry, large ablations
or a very wet stromal surface, drying the surface before pachymetry could improve the safety
of the procedure.
J Emmetropia 2013; 4: 9-12
Submitted: 12/02/2012
Revised: 12/22/2012
Accepted: 01/08/2013
Clínica Baviera, Instituto Oftalmológico Europeo, Madrid, Spain
1
Hospital del Henares, Coslada, Madrid, Spain
2
Finantial disclosure: The authors have no commercial or proprietary
interest in the products mentioned herein.
Corresponding Author: Rafael Bilbao-Calabuig, MD
Clínica Baviera, Paseo Castellana, 20, Madrid 28046, Spain
Mail: [email protected]
© 2013 SECOIR
Sociedad Española de Cirugía Ocular Implanto-Refractiva
LASIK is the most popular refractive surgery
procedure performed worldwide, but this technique
permanently distorts corneal biomechanics. In extreme
circumstances this can lead to progressive corneal
ectasia. An insufficient residual stromal bed thickness
left after this surgery is considered an important factor
for this complication1,2,3.
Currently, LASIK surgeons calculate the residual
stromal bed by subtracting the expected flap thickness
and the planned ablation depth from the preoperative
central corneal thickness measured by ultrasound
pachymetry. However, significant differences in flap
thickness have been described between intended and
ISSN: 2171-4703
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INFLUENCE OF STROMAL SURFACE HUMIDITY IN INTRAOPERATIVE US PACHYMETRY
real values with several microkeratome models, and great
variability may occur with a specific microkeratome head,
with standard deviations of around 30 microns having
been published4. Thus, intraoperative assessment of the
corneal flap is important for adjusting the treatment
and ensuring an acceptable postoperative stromal
remnant5. For this reason, since 1999 our group has
been performing intraoperative ultrasonic pachymetry
on every patient as part of our surgical protocol. Some
surgeons prefer drying the stromal surface with a Weckcell® sponge (Beaver-Visitec International Inc. Waltham,
MA, USA) before placing the probe on the corneal bed,
whereas others do not.
In this study we investigated the influence of
surface stromal humidity on intraoperative ultrasonic
pachymetry during LASIK, with the aim of improving
the accuracy of our measurements, thus minimising the
risk of an unexpected or insufficient residual stromal
bed thickness.
PATIENTS AND METHODS
In this prospective study, 267 eyes of 135 patients
(81 women and 54 men) underwent LASIK surgery at
the same centre over a three-month period (JanuaryMarch 2007).
The study followed the tenets of the Declaration
of Helsinki. Informed consent was obtained from
each patient after they received verbal and written
explanation. The protocol received institutional review
board approval.
Preoperative clinical examination involved a
complete medical history including the review of
ocular symptoms and refraction stability, manifest
and cycloplegic refraction, external ocular motility
examination, slit-lamp microscopy, keratometry
(KR30, Topcon Corp., Tokyo, Japan), corneal
topography (Orbscan II, Bausch&Lomb, Rochester,
NY, USA), ultrasound pachymetry and dilated fundus
examination.
All surgeries were performed by two experienced
refractive surgeons (R. B-C and V. J. M-S) using the
same two-step protocol: first, corneal flaps were cut for
both eyes under a surgical microscope in the operating
theatre, and once completed, laser ablation was then
performed on both eyes in the adjacent laser room,
approximately five minutes later.
Preoperative ultrasonic corneal pachymetry was
repeated on the day of surgery, just before the flap cut,
using a DGH pachymeter (DGH 5100E, 50-60 Hz
probe, DGH Technology Inc., Exton, PA, USA). After
administration of one drop of topical anaesthesia, the
patient was prepared in a sterile fashion. A lid speculum
was used to expose the globe, and one more drop of
proparacaine hydrochloride was instilled. Corneal
flaps were created with the MQ Moria LSK-One
microkeratome (Moria S.A., Antony, France) using in
every case the same 100 micron head, with either the
−1 or H suction ring (8.5 or 9 mm in flap diameter)
according to our surgical protocol. Corneal rinsing with
saline solution was performed during flap creation to
minimise epithelial damage.
Flaps were then lifted, 20 to 30 seconds later, using
a 30G dry cannula, trying to avoid interface wetting;
immediately, the degree of superficial humidity on the
stromal bed was graded on a subjective scale (0 was an
apparently dry surface, 2 was a completely moist surface
and 1 was intermediate moistness). We then measured
stromal bed thickness with the same ultrasound
pachymeter, before and after drying the stromal
surface with a Weck-cel® sponge, and thus determined
the difference between wet and dry stromal bed
pachymetry. Finally taking into account preoperative
pachymetry, we calculated flap thickness before and
after drying. Three pachymetric readings were taken
at each time and then averaged. Once finished, the
corneal flap was repositioned and immediately the same
procedure was performed for the other eye. Finally,
approximately five minutes after the flap creation
and flap thickness measurements, laser ablation was
performed for both eyes using the Technolas 217-Z
(Bausch&Lomb, Rochester, NY, USA). The aim was
to maintain a minimum residual stromal thickness
of 300 microns for all eyes. Any eyes with any even
minor complications during flap creation (epithelial
defect, surface irregularities, corneal bleeding, etc.) were
excluded from the study.
Statistical analysis was then performed using the
SPSS for Windows (SPSS Inc., Chicago, IL, USA).
The paired Student t-test was used to evaluate the
difference between wet and dry stromal bed thickness
measurement, and a multivariate regression analysis
of this difference was then performed with several
clinical parameters (surgeon, right or left eye, suction
ring used, preoperative pachymetry, stromal residual
pachymetry and keratometry, and degree of stromal
surface humidity). A p-value of <0.05 was considered
statistically significant.
RESULTS
For the 267 eyes of 135 patients (81 women and
54 men) in the study, patient age ranged from 21
to 62 (mean 36.4). O these,135 were right and 132
left eyes; 211 eyes received a myopic treatment, 52
a hyperopic treatment and 4 received treatment for
mixed astigmatism. A total of 82 eyes were operated
using the H suction ring (9 mm in flap diameter) and
185 with the −1 ring (8.5 mm). Mean preoperative
keratometry was 43.74 ± 1.46 D and mean preoperative
pachymetry ranged from 495 and 630 microns (mean
549.5 ± 30.9).
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INFLUENCE OF STROMAL SURFACE HUMIDITY IN INTRAOPERATIVE US PACHYMETRY
During intraoperative pachymetry, all eyes were
graded according to the degree of superficial humidity
on the stromal bed, 81 being grade 0 (dry surface), 139
grade 1 (intermediate) and 47 grade 2 (moist surface).
For the 267 eyes, mean flap thickness before drying was
93 ± 15 microns, and after drying 102 ± 15 microns.
The mean difference between wet and dried stromal bed
pachymetry was 8.8 ± 5.5 microns, ranging between
−2 and 33 microns; this difference was statistically
significant (p < 0.001)
The multivariate regression analysis revealed that the
only variable showing a significant influence to explain
the difference found between wet and dry corneal bed
thickness measurements was stromal bed humidity
(p < 0.05): a greater degree of surface humidity found
when lifting the created flap would be related with a
greater difference between wet and dry stromal bed
pachymetry. The mean difference between the wet
and dry measurements in grade 0 eyes was 5.77 ± 3.2
microns, in grade 1 eyes 9.47 ± 4.7 microns and in grade
2 eyes 10.55 ± 4.3 microns (Figure 1). The surgeon, right
or left eye, suction ring used, preoperative pachymetry,
stromal residual pachymetry and keratometry showed
no correlation with the difference wet-dry pachymetry.
DISCUSSION
Several factors may influence the precision of LASIK
intraoperative ultrasonic pachymetry: during the flap
cut the microkeratome can compress stromal tissue
just before the measurements are obtained; there may
be imprecision associated with the manual placement
of the probe in the three measurements; corneal
irrigation to minimize epithelial injury during the
microkeratome pass can increase surface humidity and
excimer laser ablation can dehydrate corneal stroma if
the measurements are taken at the end of the ablation.
The results shown in Table 1 revealed no statistically
significant differences between both surgeons, neither
in their mean flap thickness, nor in the mean stromal
pachymetric reduction after drying the stromal bed.
Besides, in a series of 42 eyes using the same Moria LSKOne microkeratome and the 100-micron head, Duffey
reported a mean flap thickness after drying the stromal
bed during intraoperative ultrasound (US) pachymetry
of 107 ± 14 microns, comparable to that obtained in
our study (102 ± 18 microns)6.
Figure 1. Difference in intraoperative pachymetry between wet and
dry stromal bed surface.
Corneal hydration and surface humidity have
been extensively described in the literature as factors
influencing US corneal pachymetry. Moreover, Duffey6
reported a mean reduction of 9 microns in preoperative
US pachymetry after drying the corneal epithelium.
Similarly, in our study we found a mean decrease of 8.8
μm after drying the stromal bed.
However, to our knowledge this is the first report
studying the influence and quantifying the effect of
surface stromal humidity in LASIK intraoperative
ultrasonic pachymetry.
Multivariate regression analysis revealed that the
only variable that significantly influenced the difference
between wet and dry stromal bed pachymetry was the
degree of superficial stromal bed humidity. Thus, when
a greater surface humidity is observed after lifting the
flap, the greater the risk of underestimating the real flap
thickness calculated by subtraction, if the surface is not
dried before the pachymetric measurement. This would
overestimate the residual bed assessment and, in certain
eyes, could lead to an insufficient residual stromal bed
thickness after laser ablation.
Other pachymetric imaging methods are used
to determine flap thickness and determine stromal
residual bed7,8, among which are confocal microscopy,
very high-frequency ultrasound or corneal and anterior
segment OCT; they all show excellent resolution
and reproducibility, but they are more technologydependent and more difficult to perform than US
Table 1: Intraoperative pachymetric results with different surgeons.
Surgeon
Preoperative
Pachymetry
Flap thickness
with wet surface
Flap thickness
with dried surface
Difference wet/
dried flap thickness
Jimenez (n=59)
545.7 μm ± 30
94.4 μm ± 15
103.4 μm ± 15
9.0 μm (9.5%)
Bilbao-Calabuig
(n=208)
550.2 μm ± 31
93.3 μm ± 15
102.1 μm ± 15
8.8 μm (9.4%)
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INFLUENCE OF STROMAL SURFACE HUMIDITY IN INTRAOPERATIVE US PACHYMETRY
corneal pachymetry, especially if intraoperative
measurements are needed.
In our group, we have been using intraoperative
US pachymetry routinely in our surgical protocol
for more than 10 years now without finding any
evidence of increased risk of infectious or inflammatory
complications over this period. In a retrospective
study recently published by our group with more than
200,000 operated eyes, the incidence of post-LASIK
infectious keratitis was 0.035%9,10; this was similar to
other published reports11-14 in which this intraoperative
measurement was not used.
Intraoperative drying of the stromal surface
significantly improves the accuracy of flap thickness
measurement. Although the mean difference is
statistically significant (9 microns or 9% of flap
thickness), this overestimation of residual stromal
thickness has little clinical relevance. However, in
selected cases with a very wet surface, the pachymetric
difference found after drying was as large as 33 microns.
In conclusion, we would recommend the use of
ultrasonic intraoperative pachymetry. When it is
performed, the real residual stromal bed thickness
should be considered to be around 10 microns less
than measured without drying. In cases with thin
preoperative pachymetry, large ablations or a very
wet stromal surface, drying the surface before the
intraoperative pachymetric measurement, could
improve the safety of the surgery.
REFERENCES
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stromal thickness estimation in LASIK. Ophthalmic Surg
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JOURNAL OF EMMETROPIA - VOL 4, JANUARY-MARCH
First author:
Rafael Bilbao-Calabuig, MD
Clínica Baviera, Instituto Oftalmológico
Europeo, Madrid, Spain