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Answering some critical questions about the
VISX CustomVueÔ Labelling
(and asking some of your own)
The combination of the VISX
Star S4Ô Excimer Laser System and Wavescan WavefrontÒ
system (labeled as CustomVueÔ Treatment) was recently approved
for wavefront-guided LASIK to reduce myopia and myopic
astigmatism. Specifically, the approval was for myopic
astigmatism to –6.00D MRSE, with cylinder between 0.00D and
–3.00D. Surgeons and staff will be asking questions about the
approvals, particularly in relation to Alcon’s CustomCornea
labeling.
Get your facts first, and then you can
distort them as much as you please.
Mark Twain (1835 - 1910)
Why the quote? Well, numbers can be misleading, and
companies are always likely to report their results in the best
light possible. It helps to be able to unravel the confusion by
working your way back to the facts and evaluating them. That
process provides answers that stand up to scrutiny and may
invite questions as to the real meaning of the “reported
figures”.
In that spirit, the following is an attempt to get back to the
facts behind the VISX labeling. Some sample questions you are
likely to hear, and some appropriate answers based on these
facts, are provided below. In addition, there are several
questions that you might want to ask the surgeon or staff.
All information presented here is drawn from the Professional
Use Information manuals for the Alcon CustomCorneaÒ
and VISX CustomVueÔ LASIK procedures.
Doesn’t VISX claim 98% of patients were 20/20 one
year after their surgery in the clinical trial?
Doesn’t VISX claim 79% of patients had postoperative
UCVA the same or better than their preoperative BCVA?
Doesn’t VISX claim improvements in postoperative
symptoms?
Doesn’t VISX have a wider approval range?
Questions worth asking:
Why are wavefront aberrations never discussed in the
VISX materials?
Why does the Wavescan only accept output from a 6.0
mm pupil?
Why does CustomVue not account for registration of
the wavefront and/or patient rotation at the time of surgery?
Questions worth knowing the answer to:
1. Doesn’t
VISX claim 98% of patients were 20/20 one year after their
surgery in the clinical trial?
Wow! That’s impressive. Worth looking into… and worth
questioning. It is important to note at the outset that no
patients in the trial had a best-corrected VA worse than 20/20,
or a PreVue (test lens) acuity of worse than 20/20+3. This is
unlikely to be considered a ‘typical population’, so the
percentage of 20/20 is likely to be higher than typical.
The number of patients evaluated in the time period in question
(one year) was 86, just about 25% of the total number of
patients enrolled in the clinical trial. A total of 248 eyes
were not yet eligible for a one-year visit when the data were
submitted, and 13 were discontinued due to retreatments over the
course of the study. (Eliminating eyes that required retreatment
is a pretty sure way to increase 20/20 percentages.)
The three month visit was the last with no discontinuations, and
the percentage 20/20 was 88.4%. The six month visit was the last
visit for which a reasonable percentage of patients had been
followed (277/351, or about 80%). The percentage of 20/20
reported at that visit was 93.9%, though again 12 eyes were
‘discontinued’ due to retreatment. Presuming they were retreated
because they weren’t 20/20, the percentage 20/20 at 6 months
would drop to under 90% (consistent with the three month
results).
The bottom line? Surgeons hoping for “percentage 20/20” in the
high 90’s should operate on patients with excellent preoperative
acuities, and consider leaving out of their calculations any
patients that they have to later retreat. Neither of these
restrictions seems to serve the interests of fair dealing.
In contrast to the misleading mathematics above, the Alcon
CustomCornea efficacy data is straightforward and easy to
interpret. It includes 139 spherical myopia patients with
preoperative best-spectacle corrected VA of 20/25 or better. At
6 months all patients were accounted for, and 80% (111/139)
enjoyed 20/20 or better UCVA, despite a known tendency in the
FDA trial to undercorrect patients slightly. Recent commercial
data suggest even better results.
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2. Doesn’t
VISX claim 79% of patients had postoperative UCVA the same or
better than their preoperative BCVA?
This again is a statistic based on the elimination of
retreated patients between the three and six month visits
(coming from their 6-month data). Presuming that the 12
retreated patients did not fall into this category, the
percentage would drop from 79% (218/277) to 75% (218/289).
This figure of 75% is respectable, but has been achieved by
conventional LADARVision users for myopic astigmatism
patients, with one surgeon reporting 83% of patients at 3 months
with postoperative UCVA better than or equal to their
preoperative BCVA.1
The CustomCornea data for myopic astigmatism patients is
likely to be available for comparison near year-end.
^ Top
3. Doesn’t
VISX claim improvements in postoperative symptoms?
On the VISX web site, there is a claim made that:
A six-month evaluation of the clinical study participants showed
that four times as many participants were very satisfied with
their night vision after the VISX CustomVue procedure, compared
to their night vision before with glasses or contacts.
The VISX data summarized for postoperative symptoms is
interesting because:
a) 19 eyes did not fill out a survey, and
b) no attempt to measure changes in symptoms
was made.
Since the elimination of 12 eyes because of retreatments has
already been noted, and the number of patients seen at 6 and 12
months is well below the number originally enrolled, there is no
way to make any real sense out of the information available. The
people questioned in the preoperative survey were not the same
people included in the postoperative survey.
The only true way to evaluate changes due to refractive surgery
is to report the change in symptoms for each
patient. If you want to generate statistics, it must be based on
the statistics regarding the changes in each person, not the
average responses from two different groups of patients. For
instance, after Alcon’s CustomCornea treatment, 19% of patients
reported at 6 months that night driving was better
or significantly better than it had been
preoperatively.^ Top
4. Doesn’t VISX have a wider approval range?
The VISX label includes approval for myopia and myopic
astigmatism up to –6.00D MSRE, with up to 3.00D of cylinder. The
Alcon CustomCornea labeling includes myopia up to –7.00D with
less than –0.50D of astigmatism. There are also requirements
that the wavefront match the manifest refraction of the patient
to some specified degree (within 1.0D in terms of myopia and
cylinder in the case of CustomCornea).
A theoretical patient, then, with a refraction of –0.25 cylinder
and a wavefront-derived cylinder of –1.00 would be a candidate
for Custom Cornea, so the distinction on the basis of cylinder
is not as great as might be presumed, depending on the manifest
refractions of the patients who are being considered for
surgery.
The VISX label further restricts the degree of myopia that can
be treated because the limits are based on the mean refractive
spherical equivalent (MSRE); any cylinder in a patient will
reduce the degree of myopia which can be treated. A patient with
–3.00D of cylinder, for instance, must have an MSRE of –6.00 or
less, so they cannot have myopia in excess of –4.50D.
Alcon’s CustomCornea results for myopic astigmatism in the first
FDA trial were sufficient to meet the minimum efficacy
guidelines for submission, but Alcon was interested in
demonstrating improvement relative to conventional surgery. As
such, a modified algorithm was adopted, necessitating a new
cohort of patients. The results for this myopic astigmatism
trial will be submitted to the FDA later in 2003.
^ Top
Questions worth asking:
5. Why are wavefront aberrations never discussed in the VISX
materials?
The CustomVue material indicates several times that the
surgery is “wavefront-guided”, but no attempt is made to provide
indications of how (or even whether) higher-order aberrations
are affected by surgery. In contrast, the Alcon CustomCornea
labeling includes a label that indicates “slightly improved
optical quality”. In addition, 38% of CustomCornea patients had
a reduction in higher-order aberrations pre- to postop at 6
months (vs only 14% of conventional patients), measured over a
6.5mm pupil.
Not only are similar data not reported for CustomVue, there are
two relevant observations:
a) the WaveScan sensor measures the higher order aberrations
only over the diameter of the patient’s pupil, to a maximum of
6.0mm (a minimum pupil size of 5.0mm is suggested)
b) And, from the labeling: “lthough the Wavescan… measures the
refractive error and wavefront aberrations of the human eyes….,
in the clinical study for this PMA,the average higher order
aberration did not decrease after CustomVue treatment.”
(bold italics added)
The labeling
related to VISX’s wavefront-guided ablation makes no mention of
any change in optical quality or aberration reduction, either
pre- to postoperative or relative to conventional surgery.
^ Top
6. Why does the Wavescan only accept output from a 6.0 mm
pupil?
The WaveScan device calculates the ablation profile on
the basis of data obtained from a 6.0mm (maximum) pupil.
Ablation patterns can be calculated for pupils as small as
5.0mm. Since it is well known that the Shack-Hartmann pattern is
compromised at the very edge of the detection area, it is likely
that useable data are available from a smaller diameter, perhaps
0.5mm smaller than the pupil size over which the wavefront is
measured (5.5mm or 4.5mm respectively, in the cases above).
Given the fact that the majority of important optical
aberrations are in the periphery of the ablation zone, this is a
serious limitation. This is even more critical when considering
the potential for retreatments, though retreatments are not
included in the labeling for either device at this point.
In contrast, the Alcon LADARWave calculates wavefront data from
a dilated pupil, typically at least 7mm dilated. The wavefront
is calculated over this interval, and the ablation pattern is
generated over a 6.5mm zone, appropriately referenced to the
center of the patient’s undilated pupil. This provides the best
of both worlds: an appropriate reference point for the ablation
pattern and an accurately calculated wavefront over a large
pupil diameter.^ Top
7. Why
does CustomVue not account for registration of the wavefront
and/or patient rotation at the time of surgery?
There are numerous studies2,3 that show the
derivation of the wavefront is critically affected by
orientation, sensitive to both X-Y disorientation and
cyclotorsion effects. Recognizing this, the Alcon LADARWave
system was built with the underlying technology to facilitate
proper orientation of the wavefront both at the time of
measurement and the time of surgery. There appears to be no
consideration of this need in the CustomVue system. As such,
calculation of the derived wavefront will be compromised, and
application of the appropriate ablation profile will be less
likely. Both of these factors will limit the results which can
be obtained with wavefront-guided treatment.
- Caster AI, LASIK Results with the LADARVision4000 Excimer Laser System for the Treatment of Myopia and
Myopic Astigmatism, submitted to Journal of Refractive
Surgery, 6/21/2003, MS#2003-060.
- Campin JA, Gray GP, Liedel KK, Pettit GH, Impact of
Pupil Aperture, Wavefront Origin and Higher-Order
Aberrations on the Computation of Sphero-Cylindrical Powers
in Measured Wavefronts, IOVS, 2002; 43:E-Abstract 2047.
- Steinert R, The Importance of Registration in
Wavefront Guided Custom Ablation, ASCRS EyeWorld Daily
Show Supplement: Tracking New Milestones in CustomCornea,
April 2003.
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