Phakic intraocular lenses.

Abbreviations:

AC OCT, anterior chamber optical coherence tomography; BCVA, best spectacle-corrected visual acuity; FDA, Food and Drug Administration; ICL, Implantable Contact Lens; IOL, intraocular lens; LASIK, laser in-situ keratomileusis

Introduction

For the past decade, laser in-situ keratomileusis (LASIK) has been the preferred refractive surgical procedure for most patients seeking spectacle independence. For patients with high degrees of ametropia, however, issues of ectasia and quality of vision make LASIK a less desirable procedure. For these patients, phakic intraocular lenses (IOLs) represent a promising development for vision correction. In 2003, a survey of members of the American Society of Cataract and Refractive Surgeons found that 79% of respondents perform or plan to perform phakic IOL implants. In the same survey, phakic IOLs were listed as the procedure of choice for patients with high myopia and high hyperopia by 33% and 8% of respondents, respectively – both representing significant increases from previous years. With the US Food and Drug Administration (FDA) approving the first phakic IOL in September 2004, the popularity of phakic IOLs is only likely to increase.

Over the past several years, many reports have demonstrated the early safety and efficacy of phakic IOLs and their favorable comparison with LASIK. The long-term implications of these lenses in a relatively young phakic population are less well known, however. In the past year, there has been a greater understanding of the effects of phakic IOLs on long-term endothelial cell density and cataract formation – particularly in relation to in-vivo anatomic considerations. More insight into quality of vision after phakic IOLs is also now available.

This review discusses visual outcomes after phakic IOLs in comparison with LASIK. Additionally, the new data on lens positioning and the significance for long-term complications are addressed. Finally, new and unique applications of these lenses are discussed.

Laser in-situ keratomileusis versus phakic intraocular lenses

The question of when to perform LASIK versus phakic IOL implantation is a perennial issue. In a study of patients with high myopia (-9D to -19.5D), the Artisan iris-claw phakic IOL (Ophtec BV, Groningen, The Netherlands; marketed in the United States as Verisyse, Advanced Medical Optics, Santa Ana, California) provided superior uncorrected visual acuity, best spectacle-corrected visual acuity (BCVA), and contrast sensitivity as compared with LASIK. In patients with moderate myopia, however, the distinction between the outcomes of these two modalities is less. In a fellow eye-controlled study comparing the Artisan phakic IOL with LASIK in eyes with moderate to high (-8D to -12D) myopia, no significant difference in contrast sensitivity was noted between the two procedures, although more patients preferred the phakic IOL to LASIK. In both studies, more eyes gained lines of BCVA with phakic IOLs compared with LASIK. Additionally, no eyes lost two or more lines of BCVA with phakic IOLs, compared with 12% of eyes that did with LASIK. In patients who had different procedures in contralateral eyes, most preferred the phakic IOL because of the better quality of vision.

A similar trend was found in a comparative multicenter study comparing the Implantable Contact Lens (ICL) posterior chamber phakic IOL (Staar Surgical Co., Monrovia, California) with LASIK in eyes with moderate to high (-8D to -12D) myopia. The ICL was favored in uncorrected visual acuity and BCVA. The ICL also had significantly fewer eyes losing two or more lines of BCVA and significantly more eyes gaining two or more lines of BCVA.

Almost all studies of contrast sensitivity after LASIK or photorefractive keratectomy for moderate and high myopia have demonstrated significant decreases in photopic or mesopic contrast sensitivity. Particularly at high levels of correction, excimer laser photorefractive surgery increases spherical aberrations by producing oblate corneas. With phakic refractive lenses, the prolate cornea is maintained, and contrast sensitivity can be preserved or even improved. Interestingly, preoperative and postoperative astigmatism were not correlated to the contrast sensitivity changes.

Although most comparative studies with phakic IOLs are with LASIK, one small study comparing the Artisan and the ICL demonstrated no significant difference in visual outcomes between the two lenses.

Bioptics

For improved visual acuity and refractive outcomes for high ametropia, phakic IOLs can be combined with corneal refractive surgery in an approach called bioptics. Although it is conceptually reasonable, one practical concern about such an approach is whether the phakic IOL poses a risk to the anterior segment structures during the microkeratome pass for LASIK, leading some authors to suggest flap creation prior to phakic IOL implantation. Others authors have avoided the microkeratome by combining photorefractive keratectomy with phakic IOLs.

In a study of the Artisan iris-claw anterior chamber phakic IOLs used in combination with LASIK for patients with high hyperopia (mean manifest refraction spherical equivalent of 7.39D), however, no incremental endothelial cell loss was noted after LASIK. This suggests that there was no endothelial–IOL contact, even in this group with shallower chambers and thicker IOLs. Most likely, applanation of the cornea during the microkeratome pass moves the lens–iris diaphragm posteriorly, thus shifting the phakic IOL away from the corneal endothelium.

Combining the advantages of intraocular and corneal procedures could potentially provide the greatest benefits to patients with high ametropias.

Anatomic considerations

Compared with traditional IOLs for surgical aphakia, phakic IOLs have a relatively small anatomic space in which safely to reside. Proper positioning with adequate stability must be achieved between the corneal endothelium and the anterior lens capsule. Phakic IOLs can be grouped into anterior chamber (angle-fixated and iris-fixated) and posterior chamber lenses. Regardless of the location, accurate sizing and positioning are critical for a successful outcome.

The limbal measurement of the external white-to-white distance, particularly in the horizontal meridian, is a commonly used index for predicting anterior chamber and ciliary sulcus diameters. The correlation of these measurements to anterior chamber and sulcus size is poor.

In a study comparing vertical and horizontal white-to-white measurements with direct anatomic measurements (post-fixation) in post-mortem eyes, positive correlation was found only between the vertical white-to-white distance and the anterior chamber angle diameter.

There was no correlation between the horizontal white-to-white distance and the anterior chamber angle diameter; nor was there correlation between either technique of external measurement and the ciliary sulcus diameter. In this same study, a manual plastic anterior chamber sizer showed good reproducibility of anterior chamber angle diameter, but it is obviously limited to intraoperative use. Clearly, improved clinical methods for assessing anterior chamber dimensions are needed.

One method for assessing anterior segment dimensions is Scheimpflug photography, which gives reproducible slit-lamp images with a high depth of focus. Scheimpflug photographs, along with infrared retroillumination images, were taken of eyes implanted with one of three different phakic IOLs. Over the first 12 months, the NuVita (Bausch and Lomb, Rochester, New York) and the Artisan showed no Z-axis movement whereas the ICL showed significant movement toward the crystalline lens. Rotationally, the Artisan and the ICL demonstrated the least amount of change, and the NuVita lens rotated up to 33.5°.

Partial coherence interferometry is a novel noncontact high-resolution biometry technique using a superluminescent diode with a wavelength of 855 nm. It allows monocular fixation of the eye being measured at the far and the near points and can therefore measure accommodatively induced changes in the anterior segment. In a dynamic study of eyes implanted with the ICL, it was found that during accommodation, the anterior pole of the lens moved forward 36 µm for 3D of accommodation. A concomitant anterior movement of the ICL resulted in no change in the lens–ICL distance. During photopic stimulation alone, however, the distance between the crystalline lens and the ICL decreased with pupil miosis.

Anterior chamber optical coherence tomography (AC OCT) with an infrared (1310 nm) wavelength provides good-quality high-speed two-dimensional images through a noncontact procedure. By defocusing the fixation target of the eye being measured with minus lenses, one can induce and measure physiologic accommodation. In a study of phakic eyes, the anterior pole of the lens moved forward 300 µm with 10D of accommodation, and the radius of curvature decreased concurrently. With age, this movement decreased, and it disappeared by age 65, which represents objective proof of presbyopia. The AC OCT can also be used to study the static and dynamic relationship of phakic IOLs to other anterior chamber structures. In a study of three lenses (an angle-supported anterior chamber IOL, an iris-fixated IOL, and a posterior chamber IOL), lenticular–IOL contact was demonstrated by AC OCT. Interestingly, all the traditional parameters of anterior chamber depth, open angle, and white-to-white were adequate in these cases.

Pupil size

The pupil plays an important role in centration as well as in the limitation of lens edge effects. Refractive surgical treatments are usually centered on the pupil, with the assumption that the pupil center is coincident with the visual axis. In patients with a positive angle kappa, this could present alignment problems.

With regards to glare and haloes, preoperative pupil size does not always correlate with postoperative pupil size. In a study of eyes undergoing cataract surgery, the population-based mean pupil size did not change much after surgery, but individual variations in pupil size were significant. Up to 42% of patients had a postoperative pupil size at least 0.5 mm different than the preoperative size, and up to 14% had a change in pupil size of at least 1 mm. Factors such as sex, iris color, and intraoperative trauma were not predictive for the large changes.

After Artisan implantation, scotopic pupil diameter decreased by 1.1 mm in myopic eyes and by 1.0 mm in hyperopic eyes. No significant difference in horizontal and vertical pupil diameters was noted under photopic conditions, although pharmacologic mydriasis resulted in a greater vertical than horizontal diameter. Implantation of the ICL increased the latency and duration of pupil constriction. The pupil diameter, rate of pupil constriction and redilation, and amplitude of constriction all decreased as well. The cause of these changes is not clear but may be mechanical irritation of uveal tissue.

Complications

Because of their proximity to the corneal endothelium, there is particular concern about the impact of anterior chamber lenses, such as the Artisan iris-claw phakic IOL, on long-term endothelial cell counts. In an analysis of the US FDA study on the Artisan phakic IOL, significant cell loss was not noted up to the 2-year study endpoint. By analyzing the repeatability of three successive counts (3.9–10.7%), this study concluded that current technology cannot reliably discern the 1.5% per year cell loss limit proposed by the FDA.

Posterior chamber lenses may also be associated with endothelial cell loss. A subgroup study of the US FDA clinical trial for the ICL acquired 4-year endothelial cell density data. Cumulative cell loss over the first 3 years was 8.4–8.9%, but the cohort between 3 and 4 years gained 0.1%, implying stability of the corneal endothelial cell layer. Over the course of the study, the coefficient of variation decreased and the percentage of hexagonal cells increased, suggesting improved stability over time. Clearly, the long-term impact of phakic IOLs on corneal endothelial cell counts remains an area of active research and will require more high-quality, well designed prospective studies.

Due to its location in the posterior chamber, there is particular concern about the ICL and cataract formation. In the US FDA clinic trials on the ICL, the incidence of early and late lens opacities was 2.1% and 0.4%, respectively. All were anterior subcapsular cataracts, and most were thought to be from minor surgical trauma. Mean age in the US FDA cohort was 36.5 years. In an older cohort of patients who received the ICL (V4), there was a 14.5% incidence of lens opacification, with patients over age 50 having the greatest risk of cataract formation.

Interestingly, no correlation between ICL vaulting and the development of lens opacification was found. In fact, midperipheral contact between the ICL and the crystalline lens has been seen both in eyes with and in eyes without subcapsular opacification, suggesting a metabolic rather than a direct mechanical cause of cataract formation. This may correlate with the observation that eyes with lens opacification had a significantly greater decrease in endothelial cell density compared with eyes with clear lenses.

Two cases of nuclear sclerotic cataracts were reported after Artisan lens implantation, but these were noted at least 3 years after surgery, and cataract extraction was not needed until over 12 years after initial implantation.

One mechanism that could account for late lens opacification and long-term endothelial cell loss is chronic inflammation. Laser flare and cell meter measurements show levels of flare within the normal range in eyes after ICL implantation, however. This compares favorably with eyes undergoing cataract extraction, which demonstrate continued inflammation for as long as 3 months. Some studies of the Artisan lens show chronic inflammation continuing up to 24 months (with no trend toward improvement), whereas others revealed no difference between preoperative and postoperative measurements.

Other complications of phakic IOLs have been reported, such as pupillary block glaucoma with posterior chamber phakic lenses. This is typically due to inadequate (or absent) peripheral iridotomies and responds well to additional iridotomies. In one study, pigment deposits were noted on 38% of ICLs but on only 6.6% of Artisan implants. Furthermore, unlike after LASIK, neither anterior chamber nor posterior chamber phakic IOLs change the measured intraocular pressure.

Retinal detachment is a known complication of myopia as well as clear lensectomy. In patients with high axial myopia, the true rate of retinal detachment is not known. For this reason, reports of retinal detachment after angle-supported anterior chamber and posterior chamber (ICL) phakic IOL implantation have taken the location and characteristics of the detachments, as well as their inciting lesions, into consideration in an attempt to determine their cause. Additionally, the long duration between lens implantation and detachment (1.5 years for anterior chamber lenses and 2.5 years for posterior chamber lenses) suggests that most detachments are not directly related to the phakic IOL implantation. Although the anterior chamber IOLs made visualization difficult during retinal detachment repair, the posterior chamber IOLs did not.

Two cases of choroidal neovascularization after implantation of an Artisan and the Phakic Refractive Lens posterior chamber phakic IOL (CIBA Vision, Duluth, Georgia) have been reported. Causation is difficult to establish, however, given that choroidal neovascularization is normally associated with high myopia. Some lens designs have resulted in posterior or anterior dislocation.

Other applications

In addition to the use of phakic IOLs for the correction of naturally occurring ametropia, they have also been used in presbyopia, after penetrating keratoplasty, and in aphakia.

The Newlife bifocal IOL (Ioltech, La Rochelle, France), formerly the Vivarte Presbyopic (CIBA Vision, Duluth, Georgia), is an angle-supported refractive phakic IOL with a concave-convex anterior optic divided into three concentric zones, providing 2.5D of pseudoaccommodation. It is intended for presbyopic patients with low refractive error.

The ability of phakic IOLs to correct high degrees of refractive error has led to their use in patients who have had penetrating keratoplasty. In post-keratoplasty eyes with high myopia and spherocylindrical refractive error, the Artisan lens has shown promise in improving uncorrected visual acuity without loss of BCVA.

Although not technically a ‘phakic IOL’ when used in this capacity, the Artisan iris-claw IOL is an option for aphakic patients who lack adequate capsular support for posterior chamber aphakic IOL implantation. Such cases include following pars plana vitrectomy for retained lens fragments and during penetrating keratoplasty for aphakic bullous keratopathy.

Conclusion

Although they may never become the dominant modality for refractive surgery, phakic IOLs are a valuable development in the refractive surgery armamentarium. By moving the surgery away from the cornea, patients with high ametropias can have surgical correction of their refractive error with excellent visual quality and without the risk of ectasia. Although much development has taken place over the past year in imaging the anterior segment as well as in our understanding of the interactions and potential complications of phakic IOLs, much work has yet to be done. Precise anatomic measurements will be important to properly size lenses. The question of endothelial cell loss will become clearer as long-term data are collected. New lens designs will aim to provide better outcomes with fewer complications. Ultimately, phakic IOLs will become an increasingly viable treatment option for some of these most challenging refractive cases.