Refractive Lens Surgery

net change in the total power should be zero.
We know this is true for soft contact lenses
where a –4.0-D soft contact lens provides the
same –4 D of power on a flat or steep cornea,
even though the overall curvature of the lens
is different. The reason is that both surfaces
change proportionately.
Another possibility is that the axial position
of the ICL is much greater than that predicted
preoperatively (it must be deeper than
predicted to reduce the effective power of the
lens). This possibility cannot explain a 15%
difference, because the axial position would
need to be more than 2 mm deeper to explain
a 15% error. Postoperative A-scans and highresolution
B-scans have shown the exact position
of the lens to be close to the anatomic
anterior chamber depth, proving that the axial
position of the lens is not the explanation.
In any case, back-calculated constants for
the ICLs, using the phakic IOL formula above,
result in lens constant ELPs that are 5.47–
13.86 mm, even though the average measured
ELP is 3.6 mm. In the data sets that we have
analyzed, when the optimized back-calculated
ELP is used, the mean absolute error is approximately
0.67 D, indicating that 50% of the
cases are within ±0.67 D. This value is higher
than the ±0.50 D typically found with standard
IOL calculations following cataract surgery.
The ICLs should be better than ACLs,
since the exact location of the lens can be predicted
from the anatomic anterior chamber
depth preoperatively. This difference is puzzling,
not only because of the better prediction
of the ELP, but also because any errors in
the measurement of the axial length are irrelevant
because it is not used in the phakic IOL
formula.
4.7 Bioptics
(LASIK and ACL or ICL)
When patients have greater than 20 D of
myopia, LASIK and ICLs have been used to
achieve these large corrections. Although
Chapter 4 Intraocular Lens Power Calculations 37
only a few cases have been performed by a
few surgeons, the results have been remarkably
good. The surgeon performs the LASIK
first, usually treating 10–12 D of myopia, and
waits for the final stabilized refraction. Once
a postoperative stable refraction is attained,
an ICL is performed to correct the residual
myopia (e.g. 10–20 D). These patients are especially
grateful, since glasses and contact
lenses do not provide adequate correction
and the significant minification of these corrections
causes a significant reduction in preoperative
visual acuity. Changing a 30-D myopic
patient from spectacles to emmetropia
with LASIK and ICL can increase the image
size by approximately 60%. This would improve
the visual acuity by slightly over two
lines due to magnification alone (one line improvement
in visual acuity for each 25%
increase in magnification).
4.8 Conclusions Regarding
Phakic Intraocular Lenses
Phakic IOLs are still in their adolescence.
Power labeling issues, temperature-dependent
index of refractions, changes in the
meniscus shape and actual lens locations are
being experimentally evaluated and are similar
to the evolution of IOLs used following
cataract surgery in the early 1980s. There is
no question that our ability to predict the
necessary phakic IOL power to correct the
ametropia will improve, possibly exceeding
the results with standard IOLs because of the
more accurate prediction of the lens location
axially. Determining the optimal vaulting and
overall diameter to minimize crystalline lens
contact, posterior iris contact and zonular,
ciliary processes or sulcus contact are all being
investigated at this time. These refinements
are no different than the evolution in
location from the iris, to the sulcus and finally
the bag for standard IOLs. Because of our
improved instrumentation with high-resolution
B-scans, confocal microscopes, and ante-