gms | German Medical Science

Deviation from target refraction is one of the least desirable refractive outcomes after cataract surgery. While careful preoperative biometry and intentional targeting for a small amount of residual myopia can prevent most hyperopic surprises, residual hyperopia has also been more difficult to eliminate in refractive surgery patients who subsequently develop cataracts. Attributable to corneal changes following laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK), corneal power is often overestimated, and these patients are frequently left hyperopic after cataract surgery. A broad array of intraocular lens (IOL) power calculation formulas has been developed to better estimate IOL power in these eyes, but many are wanting, and hyperopia remains a frequent postoperative outcome. A dramatic method to overcome refractive imprecision was recently reported by Mackool and associates in which post-LASIK patients undergoing cataract surgery underwent immediate postoperative aphakic refraction and then, one half-hour later, were returned to the operating room for IOL implantation. An alternative to the difficulties inherent in predicting IOL power is use of a light adjustable lens. The light adjustable lens contains photosensitive silicone molecules that enable postoperative, noninvasive adjustment of refractive power using ultraviolet (UV) light. The light adjustable IOL formulation consists of four basic components: 1) silicone matrix polymer, 2) photoreactive macromer, 3) photoinitiator, and 4) UV absorber. The light adjustable IOL formulation has been described previously and is based upon the principles of photochemistry and diffusion. The application of the appropriate wavelength of light through a defined spatial irradiance profile onto the light adjustable lens polymerizes macromer in the exposed region producing a change in shape and associated predictable power change. By controlling the irradiation dose (i.e., beam intensity and duration) and spatial irradiance profile, the refractive power of the light adjustable lens is modified to add or subtract spherical power, eliminate astigmatic error, and correct higher-order aberrations (HOAs). For example, to correct myopia, irradiation is preferentially applied to the periphery of the lens. This causes photopolymerization and subsequent diffusion of unpolymerized macromer into the irradiated zone. Subsequent swelling of the irradiated zone increases or decreases lens power. To control the amount of refractive error corrected, the treatment duration is varied. One day after light adjustable lens adjustment, the entire lens is irradiated to polymerize the remaining photosensitive macromer and prevent additional change in lens power. This second irradiation procedure is referred to as “lock-in.” To determine whether residual refractive error could be corrected postoperatively using the light adjustable lens technology, we performed a clinical study in patients undergoing cataract surgery with light adjustable IOL implantation. Based on preoperative biometry, light adjustable lenses were implanted that might purposely result in refractive errors up to ±2.0 diopters (D). In this fashion, we were able to adjust lens power and test whether a “refractive surprise” could be corrected in the postoperative period.

A prospective, nonrandomized, single-center clinical study was conducted at Center for Vision Science, RuhrUniversity Eye Hospital, Bochum, Germany. Subjects who required cataract extraction and IOL implantation and volunteered for this study were screened for eligibility. Subjects with significant anterior segment pathology, uncontrolled glaucoma, previous ocular surgery, macular deceases, current use of Flomax, or <7.0 mm dilated pupil diameter were excluded. Twenty patients (one eye per patient) were enrolled, 14 females and six males.

Preoperative biometry was performed using the IOLMaster (Zeiss Meditec, Jena, Germany) and the light adjustable IOL power was selected to result in a slightly postoperative refractive error of up to +0.25 D of hyperopia.

The light adjustable IOL is a foldable posterior chamber, UV absorbing, three-piece photoreactive silicone lens with blue polymethylmethacrylate modified-C haptics, a 6.0mm biconvex optic with squared posterior edge, and an overall length of 13.0 mm. All patients underwent phacoemulsification using a topical anesthetic, clear corneal incision (3.0 to 3.3 mm) and anterior capsulotomy (<5.8 mm). The light adjustable IOL of the appropriate power was implanted in the capsular bag using the Nichamin II Foldable Lens Insertion Forceps (Rhein Medical Inc, Tampa, Florida, USA). The operative eye was patched following surgery. The patch was removed the following day, and patients were instructed to wear Calhoun Vision-supplied UV-blocking photochromic spectacles (7EYE, Pleasanton, California, USA) at all times when indoors and outdoors after surgery until the adjustment and lock-in procedures were completed.

The light adjustable IOL was adjusted and locked in using a digital light device engineered by Carl Zeiss Meditec (Jena, Germany). The digital light device uses a mercury (Hg) arc lamp fitted with a narrow bandpass interference filter producing a beam at 365 nm (±4 nm full width half maximum). The digital light device generates and projects a spatial irradiance pattern onto the light adjustable IOL using a digital mirror device. The digital mirror device is a pixelated, micromechanical spatial light modulator formed monolithically on a silicon substrate. The digital mirror device chip enables customization of the irradiation profile to correct not only spherical and astigmatic errors but also HOAs. The surgeon centers and focuses the treatment beam on the light adjustable lens using an alignment reticle while patient alignment is achieved using a fixation target, paracentral to the delivery beam.

At one to two weeks post implantation, best-corrected and uncorrected visual acuity (VA) and residual refractive error were measured. VA assessment and manifest refraction were performed by an optometrist who was masked to the patient’s targeted refractive outcome and light adjustable IOL power. The digital light device was then used to adjust the light adjustable IOL power to correct the refractive error. Refractive power adjustments are defined at the spectacle plane. A second irradiation treatment was given a minimum of 20 hours after adjustment to lock-in the lens. The following endpoints were evaluated:

1.

Attempted versus achieved lens power change (manifest refraction).

2.

VA (best-corrected and uncorrected distance VA).

3.

Stability of adjusted lens power (manifest refraction).

4.

Complications and adverse events.

A total of 20 eyes were treated for refractive adjustments in the range of -2.25 to +2.0 D and were followed for one to six months postoperatively. Of the 20 patients, 14 were female and six were male. The average age of the study participants was 71±8 years. The power of implanted light adjustable IOL as well as preoperative refractive errors and axial length will be presented. The distribution of refractive adjustments attempted varied markedly. All eyes were accounted for at the one, three, and six month postoperative visits.

There were no missed visits or any patients lost to follow-up during the study period. An additional 18 patients were implanted with the light adjustable lens and treated for refractive astigmatic adjustments in the range of ±2.0 D; these will be detailed in a separate report.

• Target vs. achieved corrections: The accuracy of the achieved sphere to the targeted refractive adjustment was impressively good. Nineteen of the 20 eyes (95%) treated were within 0.25 D of the target refraction at one day post lock-in follow up visit with 100% of the eyes achieving 0.50 D of the targeted refractive adjustment at one day post lock-in, three and six months postoperative visits. Similar results were demonstrated in the analysis of accuracy of the achieved manifest refraction spherical equivalent. Interestingly, no significant change was induced on cylinder.

• Stability ability of refractive adjustment: One hundred percent (20/20) of the eyes were stable within ±0.25 D with 18 eyes showing no change at one to six months follow-up visits as compared with refraction at one day after lock-in treatment . The mean rate of change was 0.01 D per month, which is several times more stable than that of refractive procedures.

• Stability ability of uncorrected visual acuity: All eyes demonstrated stability of uncorrected visual acuity (UCVA) after lock-in. The data demonstrated significant improvement in UCVA for 85% of the 20 eyes and the UCVA is stable for a follow-up period up to six months. Eighty percent (16/20) of eyes achieved UCVA of 20/25 or better. All eyes were 20/30 or better. The four eyes that did not achieve UCVA of 20/25 or better had residual astigmatism that was not treated.

• Stability of best-corrected visual acuity: All 20 eyes were stable and able to maintain their preadjustment BCVA following refractive adjustment and lock-in treatments with follow-up time up to six months.

• Complications: Any adverse events or complications were recorded. In addition to typical adverse events associated with IOLs, we looked carefully for evidence of uveitis, UV keratopathy, elevated intraocular pressure, posterior capsular opacity, and photic phenomena. There were no adverse events or complications reported.

Refractive error after cataract surgery is particularly undesirable since affected patients have no clear uncorrected far point. Despite a wide array of empirical formulas, IOL power remains difficult to predict in patients who develop cataracts, especially in very long and short eyes, after refractive or corneal surgery. We found the light adjustable lens technology feasible to correct residual refractive error after cataract surgery. Correction of hyperopia as well as myopia between 0.25 to ±2.0 D was achieved with >90% of patients corrected to within 0.25 D of the intended refraction. While this was a feasibility study and adjustments were performed in patients who had not had previous refractive surgery, it is likely that correction of refractive error could be performed in a similar fashion in these eyes as well.

Uncorrected VA improved in all patients, while BCVAs was maintained after lock-in in all patients. The uncorrected and best-corrected VAs were stable for all patients up to six months follow-up visit. One hundred percent of the eyes were stable within ±0.25 D. An important aspect of the light adjustable lens is the need to wear UV protective spectacles until lock-in is performed. This is because the photoreactive silicone macromer undergoes photopolymerization when exposed to UV light.

The light adjustable lens has approximately one hour of built in UV protection in the absence of sunglasses (Chang SH, unpublished data, 2006), but after that, there is a risk for photopolymerization and optical changes. Therefore, patients are instructed to wear the UV blocking sunglasses until lock-in is completed. After lock-in, no UV protection is necessary. While this study demonstrates precise adjustment of refractive errors between 0.25 to 2.0 D, the eye would require a second adjustment to correct more than 2.0 D before lock-in. Chemical analysis of light adjustable lenses reveals that photoreactive macromer is available to undergo multiple photo-polymerization leading to additional refractive change as long as the lens is not locked-in. In vitro studies have achieved a +3.5 D power change with secondary adjustments of light adjustable lenses.

The light adjustable lens and digital light device provide surgeons a means to fine tune refractive power in the postoperative period. In this small pilot series of patients treated for residual refractive error, adjustments were precise and stable for up to six months follow-up. This technology is supposed to provide greater confidence in final refractive outcome as increasing numbers of corneal refractive surgery patients age and develop cataracts.