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Methods of sizing ICLs should be improved, surgeon says
Most complications stem from imperfect fitting, and a very high percentage of implanted lenses are not sized correctly.
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---Optical biometry performed using Orbscan: a pseudo-isochromatic map of the anterior chamber depth.
ROME – Sizing remains the crucial problem of phakic IOL implantation, especially of the Implantable Contact Lens (ICL; STAAR Surgical), and the standardized white-to-white ± 0.5 mm method is far from reliable and effective, according to Carlo Lovisolo, MD, in his presentation on the ICL at the Rome ’99 meeting.
“There are two main reasons that make the accurate sizing of ICL absolutely essential: first, to ensure precise refractive results, and second, to guarantee safety of implantation,” Dr. Lovisolo said. “A lens that is too large may develop excessive apical clearance and push the iris diaphragm forward, causing angle closure glaucoma. On the other hand, a lens that is too small adheres to the crystalline lens, inducing the onset of cataract.”
Only a well-calculated apical clearance (between 300 µm and 600 µm from the crystalline lens for the average anterior chamber size) can protect the eye from such complications.
The white-to-white measurement, ± 0.5 mm for myopes and hyperopes respectively is, according to Dr. Lovisolo, too rudimentary and approximate, producing incorrect sizing in a very high percentage of cases.
“The data collected from more than 100 patients demonstrate that approximately 54% of the calculation of the white-to-white distance did not correspond to the sulcus-to-sulcus distance. There is so much variation in the anatomical conformation of the eyes that the correspondence between external white-to-white and internal sulcus-to-sulcus measurements is often pure chance,” he said.
A more scientific approach
---An ICL that is too short: peripheral contact with the anterior portion of the lens creates a deficiency in the aqueous circulation, which may contribute to lens opacity. Diagram on the left, Scheimpflug camera image on the right.
COURTESY OF M. VEZZOLI, CENTRO OCULISTICO BERGAMASCO
According to Dr. Lovisolo, the sulcus-to-sulcus distance can only be accurately calculated with the aid of sophisticated high-frequency, high-resolution ultrasound technology.
“Our system is the I ³ ABD by Innovative Image Inc. [Sacramento, Calif.]. Two separate images are taken, rotating the eye temporally and nasally, and the two hemi-diameters (from the center of the crystalline to the sulcus) are calculated together,” he explained.
To guarantee a perfect fit, other parameters to be taken into consideration are anterior chamber depth, calculated with Orbscan (Bausch & Lomb; Claremont, Calif.) and the radius of curvature of the anterior surface of the crystalline lens, which is measured with the Scheimpflug camera (Nidek; Fremont, Calif.).
From these data, a trigonometric formula is obtained and, with the aid of a simple software package, the exact overall length of the ICL can be predicted.
“With this approach, our results have improved dramatically. The apical clearance is correctly predicted in 96% of the cases,” Dr. Lovisolo said.
A considerable drawback of this more scientific approach, however, is the extremely high costs involved. “At present,” Dr. Lovisolo said, “such high-performance ultrasound technology is only available on the market at very high prices. However, the manufacturers are beginning to understand the importance and urgency of the problem, and hopefully, in the near future, less costly instruments will become available.”
Improving the empirical method
---Spinnaker effect of an excessively large lens. The over-vaulting of the myopic ICL resulted in 2 D of undercorrection. Diagram on the left, Scheimpflug camera image on the right.
Meanwhile, whenever the traditional empirical sizing methods have to be used, Dr. Lovisolo suggests an improved formula, which he devised in cooperation with Dr. Giovanni Fumagalli, MD, of the Centro Oculistico Bergamasco, directed by Prof. Camillo Paganoni.
The method combines the usual white-to-white measurement with other parameters, such as anterior chamber depth and angle configuration. In this way, the sizing, although still empirical, is more accurate and the margins of error considerably reduced (see nomogram).
“Since ICL sizes progress in 0.5 mm steps,” Dr. Lovisolo said, “whenever intermediate sizes are needed, we must take into consideration the angle configuration. If the angle is > 0.7 mm, we choose the larger size; when the angle is < 0.7="" mm,="" we="" choose="" the="" smaller="" size.”="">
A wider choice
---Measurement of the two hemi-diameters obtained in the two lateral viewing positions. Ciliary sulcus and geometric lens center indicated by crosses. The sum of the two values produces the sulcus to sulcus distance.
COURTESY OF D. BONFANTI, CENTRO OCULISTICO BERGAMASCO
Dr. Lovisolo pointed out that the limited range of ICL sizes available is a problem that should not be underestimated.
“A minimum 0.2 mm step and sizes over 13 mm are necessary features that should be provided by manufacturers,” he pointed out. “This becomes essential for high myopes. Most of the cataract problems that we had were related to the very large bulbs of high myopes, where the lens diameter was inadequate. A lens that is too small does not fluctuate under the pressure of the aqueous humor, the desired degree of apical clearance is not obtained, and the ICL rests on the crystalline. Consequently, due to the particular optic geometry of the myopic ICL, which is thicker around the edges and thinner in the center, the aqueous humor is trapped centrally, the metabolic turnover is reduced and the subcapsular epithelium is not adequately nourished and produces fibrotic changes and overgrowth of epithelial cells.
“We must not forget,” he added, “that cataract surgery in myopic patients frequently causes retinal complications and capsular secondary opacification, with the consequent vitreoretinal problems.”
For Your Information:
- Carlo Lovisolo, MD, can be reached at v. Formentini 1, 20121 Milano, Italy; (39) 02-8057388; fax: (39) 02-86452896; e-mail: loviseye@tin.it. Dr. Lovisolo has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.