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Safety, with respect to loss of best spectacle-corrected visual acuity, records better results for phakic IOL implantation than for LASIK with higher corrections
"Phakic IOLs play an important role in managing higher ametropias, can be additive to other types of refractive surgery already being offered, and the implantation technique is very learnable for practitioners already doing intraocular surgery," said Dr Hardten, adjunct associate professor of ophthalmology, University of Minnesota, and director, refractive surgery, Minnesota Eye Consultants, Minneapolis, USA.
What are the advantages?
Dr Hardten reflects specifically on his experience with two approved lenses, Verisyse (AMO/VISX) and Visian ICL (STAAR Surgical). The Verisyse is an iris fixated IOL that is implanted in front of the iris and, in fact, is attached to the peripheral iris. It is available in powers of -5.0 to -20.0 D, making it an option for patients with both moderate and severe degrees of myopia. The posterior chamber Visian ICL is placed behind the iris and in front of the natural lens of the eye and is approved for refractive errors between -3.0 and -20.0 D of myopia.
He illustrated the clinical performance of the two IOLs by summarising results from myopia clinical trials. Among eyes seen at three years, in the FDA study of Verisyse, 92% achieved 20/40 or better uncorrected visual acuity, only 6% lost two or more lines of BSCVA and 49% had a gain in BSCVA. Endothelial cell counts remained stable and were comparable to those in patients who had not undergone surgery. The Visian ICL achieved similar results. For example, among patients with a preoperative BSCVA of 20/25 or better that were targeted for emmetropia, more than 90% achieved an uncorrected visual acuity of 20/40 or better.
Dr Hardten claims that the picture has improved even further since these studies, "Now, even better results may be attained in clinical use since we have access to IOLs in half-diopter power steps rather than just full-diopter increments. Surgeon-adjusted nomograms have also led to improved outcomes for phakic IOLs as well as for LASIK."
Although only recently introduced onto the market, phakic IOLs already account for 5% of Dr Hardten's refractive surgery practice. Patients he considers a good fit for this option include those with pre-presbyopic myopia with refractive errors of –8.0 to –16.0 D. People with lower myopia are also candidates if they have a thin cornea or atypical topography, while those with a higher level of myopia may be considered if they are young and expected to be tolerant of a smaller-diameter IOL (the iris claw phakic IOL) or residual myopia. However, phakic IOLs do not correct presbyopia, in these cases Hardten advises refractive lens exchange with a multifocal or accommodating IOL is likely to be a more suitable option.
IOL sizing and power calculation
It is important to note that one size or style implant does not fit all patients. For the iris-fixated IOL, both 5.0 mm and 6.0 mm optic models are available. The larger optic is generally preferred for minimizing problems with glare and halos, but it is only available in powers up to –15.0 D while the power range is up to –20.0 D for the smaller-optic implant.
There are also a number of anatomic considerations in determining whether a patient is a good candidate. Relative thickness of the iris should be evaluated because the lens vaults over the iris. "If the iris has a very thick collarette, there can be touch and inflammation", points out Dr Hardten.
Anterior chamber dimensions are also critical measurements. The implant may be too close to the endothelium in the periphery if the chamber is shallow in the midperiphery, while the white-to-white measurement is important since small eyes may not tolerate the IOL width. In addition, although anterior chamber depth may seem adequate, it is important to realize there is about 0.5% loss of depth per year of age.
As described by Georges Baikoff, MD of Marseilles, France, crystalline lens rise is another important concept for evaluating long-term safety with the Verisyse IOL. Defined as the distance between the anterior pole of the crystalline lens and the horizontal plane joining the opposite iridocorneal recesses, it seems to be a factor in identifying the potential for the iris to rub the back of the implant. A study found pigment dispersion occurred almost exclusively in eyes with a crystalline lens rise >600 mm and anterior chamber depth <3.2 mm. "Anterior chamber OCT or high-resolution ultrasound are elegant ways of imaging this, although in most patients it is possible to tell at the slit-lamp whether this might be an issue," Dr Hardten said.
In general, the power of the myopic Verisyse IOL is close to the spectacle correction, but power calculation is performed using the manufacturer's commercial program (Vericalc, AMO) that accounts for the manifest refraction/cycloplegic refraction (MR/CR) and the effective lens position through the anterior chamber depth. Astigmatism management is typically achieved through wound construction and with use of limbal relaxing incisions or postimplantation laser correction. "So far, among approximately 300 implants that we have performed in our practice, the refractive results were enhanced using LASIK in three eyes and PRK in three eyes," Dr Hardten reported. "So, the enhancement rate is low, but the need for finetuning is possible."
The Visian ICL's power is calculated in the same way as the Verisyse IOL and is usually greater than the spectacle correction. The ICL is available in sizes from 11 to 13 mm with a size selected that is slightly greater (add 0.5 mm) than the white-to-white for myopia and matching the white-to-white for hyperopia. Dr Hardten notes that, although this works to a degree, a better measure of sulcus dimensions is needed to size the implants more accurately in the future.
Dr Hardten recognises the important role, played by phakic IOLs, in managing higher ametropias but stresses, "Good outcomes depend on a number of issues that cannot be covered in a single talk or short session."