By using a pinhole effect to enhance patients’ depth of focus, the AcuFocus™ corneal inlay offers a new approach to correcting presbyopia.

Although many different types of presbyopia correction are currently available, none can give patients a full range of vision without compromise. With progressive lenses, for example, patients gain good vision at a range of distances, but they must also tolerate some degree of marginal astigmatism and deal with the hassle of glasses. Among surgical options for presbyopia, multifocal intraocular lenses (IOLs) impair contrast sensitivity and may cause glare and halos at night, while monovision treatments compromise stereopsis and depth perception.

To minimize such trade-offs, an intracorneal inlay has been developed that improves patients’ near vision by enhancing depth of field. While this approach still has some drawbacks, it gives patients good vision at all distances in most lighting conditions without compromise. The AcuFocus™ inlay, currently in clinical trials with the FDA, could provide an excellent option for treating presbyopia.

Correcting Presbyopia Within the Cornea

As LASIK and surface ablation procedures have demonstrated, correcting refractive error at the level of the cornea offers several advantages, and many of these benefits apply to the use of intracorneal implants for the amelioration of presbyopia.

Because the cornea supplies the majority of the eye’s refractive power, corneal modifications can provide a powerful refractive effect, and modern surgical techniques allow surgeons to modify the cornea with extremely high degrees of precision. As a result, corneal surgery can often achieve more predictable refractive outcomes than IOL implantation, since IOL power calculations and lens positioning cannot be controlled to the same degree as corneal parameters, especially astigmatism.

In addition to providing more predictable results, intracorneal procedures also eliminate some of the surgical risks associated with performing intraocular procedures. Intracorneal procedures entirely avoid complications such as posterior capsule opacification, and other risks are significantly reduced. For example, the risk of infection cannot be entirely eliminated, but corneal infections are typically less serious than endophthalmitis.

Finally, the surgical skills required for intracorneal presbyopia correction are ones surgeons have already honed for performing LASIK and surface ablation. While some training is necessary with any new procedure, most corneal surgeons should be able to perform these new presbyopia treatments quite easily.

Clinical Performance of the AcuFocus™ Inlay   
 
The attractiveness of correcting vision at the level of the cornea has led to the development of intracorneal implants to treat presbyopia. While none of these devices are currently approved for use in the United States, the ACI 7000™ (AcuFocus, Inc.) is currently undergoing FDA clinical trials and could become commercially available in 2009 outside the United States.

Consisting of a thin, opaque 3.8 mm disk with a 1.6 mm open circular aperture in the center, the AcuFocus inlay effectively decreases the size of the pupil to 1.6 mm, creating a pinhole effect. Because this smaller aperture enhances depth of field, the AcuFocus inlay allows patients to see clearly over a wider range of distances. Implanted in one eye, the AcuFocus device lets patients view near, intermediate, and far objects clearly.

Because this device improves near vision without significantly affecting distance vision, it could be an ideal option for correcting presbyopia in emmetropic patients. Although results from the FDA clinical trial are not yet available, a study conducted in Turkey found that emmetropic individuals who had good uncorrected distance vision were able to achieve 20/16 near vision in the eye with the AcuFocus inlay, while retaining 20/16 distance vision in both eyes.

Other Pros and Cons of the AcuFocus™ Implant

In addition to achieving the best presbyopia treatment outcomes I have observed with any modality, the AcuFocus inlay also has few side effects or other disadvantages. The use of a very thin, biocompatible material allows the inlay to be implanted without triggering significant corneal remodeling, and the implant is cosmetically acceptable for most patients.

When implanting anything within the cornea, surgeons must consider how the cornea will react, since changing the shape of the cornea—either by ablating tissue or by inserting an implant within the cornea—can cause corneal tissues to react in such a way as to partially reverse this change. Because the AcuFocus inlay is only 5 microns thick, however, it does not alter the shape of the cornea enough to trigger significant corneal remodeling.

Another advantage of the AcuFocus inlay is that it is essentially invisible to others. While close observation will allow the implant to be seen, it is rarely noticeable to a casual observer. Also, because the implant covers only a small area in the center of the pupil, it does not prevent ophthalmologists from viewing the intraocular structures, so patients can still undergo all routine ocular health checks.

While the AcuFocus implant is generally a good option for correcting presbyopia, it does have one significant limitation that must be considered. Because this implant reduces the amount of light that reaches the retina, patients may have difficulty reading under suboptimal lighting conditions without more light or supplemental readers. In photopic conditions, the 1.6 mm pupil provided by the implant is adequate. In low mesopic or scotopic conditions, however, the eye may not receive enough light to form a clear image of near objects on the retina of the implanted eye.


Other Possible Intracorneal Technologies

While I believe that using a pinhole effect to enhance depth of field is probably the best way of improving a  patient’s near vision without significant complications today, other intracorneal methods for correcting presbyopia are also being considered. Specifically, intracorneal lenses that could provide a small add power in one eye are being developed, including the PRESBYLENS™ Corneal Inlay (ReVision Optics) and the Invue system (BioVision).

In much the same way that a refractive IOL provides different zones for near and distance vision, an intracorneal lenticule could provide some degree of multifocality in the eye in which it is implanted. However, multifocality is always at the expense of contrast sensitivity. By placing a limited amount of add power in the center of the eye, patients should gain functional near vision with only a small compromise in their distance vision. In order for such lenses to prove successful, however, surgeons will need to find a way to account for the corneal restructuring that is likely to occur when a lens with a significant thickness is implanted in the cornea.

The Bottom Line

Corneal inlays hold considerable promise as a new technology for the surgical correction of presbyopia. Currently, the intracorneal technology with the greatest potential appears to be the AcuFocus™ corneal inlay, a thin, 3.8 mm opaque disk with a circular central 1.6 mm pinhole. Implanted in the cornea over the center of the pupil, this inlay enhances patients’ depth of field by decreasing their effective pupil size. Intracorneal refractive lenticules could also be used to correct presbyopia by offering a moderate add power in the center of one eye. However, such a lens would have to overcome various challenges in order to be clinically successful.

Jack Holladay, MD, MSEE, FACS, is a clinical professor at Baylor College of Medicine and president and founder of the Holladay LASIK Institute in Houston, TX. Refractive Eyecare senior editor Kay Downer assisted in the preparation of this manuscript.