Ming Wang, MD, PhD and Nathan Rock, OD, FAAO
Modern refractive surgeons are challenged by patients who wish to be free of glasses at all distances. All current surgical options for presbyopia present some form of limitation or compromise, such as poor intermediate vision, compromise to distance vision, or part-time need for glasses. No options currently available actually attempt to treat presbyopia as a disease and directly reverse the accommodative loss. Future developments in surgical presbyopia correction are aimed at reducing these compromises.
Since presbyopia is a lens-based problem, it is not surprising that the majority of future developments are focused on intraocular solutions. Many view the ideal intraocular lens (IOL) as the optimal treatment for presbyopia. However, as existing corneal options are optimized and new options are approved, they will continue to have a significant role. As scleral treatment options may reach approval in the United States, interest in these techniques will also likely increase.
ADJUSTABLE INTRAOCULAR LENSES
One of the major barriers associated with current IOL technology is management of residual refractive error. This is particularly challenging for presbyopia-correcting IOLs because small refractive errors can make significant differences in uncorrected vision at distance, near, or both.1 Typically, residual refractive errors are currently surgically corrected with keratorefractive procedures such as LASIK, photorefractive keratectomy (PRK), or astigmatic keratotomy. While effective, corneal procedures may significantly exacerbate dry eye in eyes having recently undergone phacoemulsification in an older population already at risk for poor ocular surface health.
Residual refractive error is more problematic in patients with prior PRK, LASIK, and radial keratotomy, where the corneal power is unnatural. Modern fourth-generation IOL formulas that factor in multiple variables such as Haigis (which factors in anterior chamber depth) and Holladay 2 (using white-to-white, the lens thickness, the refraction, and age) have been helpful for improved outcomes in these groups.2 However, disagreement in predictive IOL power of different formulas may result in residual refractive error in prior refractive surgery patients.3 More post–refractive surgery patients are developing dysfunctional lens syndrome or visually significant cataracts and pursuing lens replacement. Unfortunately, this group often has the most demanding visual expectations. Residual refractive errors treated with additional corneal refractive enhancements may be less predictable. They may also lead to undesirable postoperative complications, such as haze with PRK and epithelial ingrowth or other flap complications after a flap-based treatment.
For this reason, postoperatively adjustable IOLs are appealing. To be successful, several properties are necessary. The lens would need to be biocompatible with ocular tissues. It would need to be safely adjustable to reliably correct myopic, hyperopic, and astigmatic refractive errors to within 0.25 to 0.5 diopters (D) of the target. The ideal adjustable IOL would utilize a noninvasive adjustment that did not require entering the anterior segment or taking the patient to the operating room. Proposed solutions include the use of multiple lens components, magnets, liquid crystals, femtosecond lasers, and ultraviolet light. IOLs have been proposed and patented using these technologies, although most have not reached the stage of human use.4 The first adjustable IOL was approved in the Fall of 2017 by the US Food and Drug Administration (FDA).5
The Acri.Tec AR-1 posterior chamber IOL is a mechanically adjustable IOL that has been tested in human subjects. The IOL contains a piston attached at the optic-haptic junction. The surgeon alters the refractive power of the IOL by moving the piston relative to the cylinder. This adjustment is done through 2 small corneal paracenteses via a manipulator allowing for approximately 2.0 D of power change. The lens has been shown to be safe and adjustments have shown refractive stability without complication for as long as 18 months postoperatively. The disadvantage of this lens type and design is the need for an invasive, though minimal, adjustment in an operating room setting, which limits its future potential. However, it clearly demonstrated the efficacy of an adjustable lens in a clinical setting.6
The light-adjustable IOL (RxLAL [RxSight]; Figure 16-1) has been available in Europe and Mexico since 2008, and the monofocal was approved by the FDA in the Fall of 2017 but has not yet seen widespread use. The 3-piece monofocal IOL is composed of silicone with an optic containing a light-activated photoinitiator and mobile silicone macromers. When exposed to spatially profiled ultraviolet light, the photoinitiator causes a macromer to polymerize. The exposure to treated regions of the lens causes a precise shape change inducing a power change. The adjustment range is up to 2.0 D for hyperopia, myopia, and astigmatism. Adjustments are typically needed in 1 to 2 sessions of less than 2 minutes with a final application to lock in the correction. The adjustment is performed with a slit lamp–mounted light delivery device linked to a computer, which programs the appropriate intensity and duration of light. Several clinical trials have shown effective and safe results of the monofocal RxLAL with precise adjustments.7
For presbyopia, one appealing option of this IOL would be the possibility of “optimized monovision” which does not have to be fully determined preoperatively. For instance, the dominant eye can be targeted plano and the nondominant eye to -1.0 D. After the surgery, if a patient desires to have adjustment to the intended monovision distance, such as closer in, the power of the IOL could be changed as needed. In the future, it is possible that the light adjustment could be used for customization of a multifocal intraocular lens which could help to alleviate the difficulty associated with residual refractive error with these IOLs. A major benefit of this technology versus enhancement with corneal refractive surgery is being able to make the adjustment within days to weeks after the surgery versus waiting 3 months for LASIK or PRK. This lens adjustment could also reduce the additional difficulty of a corneal enhancement for patients with a history of prior refractive surgery.5
MULTIFOCAL INTRAOCULAR LENSES
Current multifocal lenses, including recently introduced toric options, provide impressive distance and reading vision but are not free of compromise. The rings in multifocal lenses can result in undesirable visual effects such as halos, glare, starbursts, and loss of contrast.1 Current diffractive bifocal add powers do result in compromise to reading ability at certain distances. The higher power add models provide excellent near vision but with a loss of intermediate vision. The lower add power models provide great intermediate vision but a loss of clarity at near, with more dependence on part-time readers.
Trifocal diffractive IOLs attempt to address these limitations by adding an additional focal point for intermediate vision. Three major trifocal IOLs are available internationally, which include the AT LISA trifocal (Carl Zeiss Meditec; Figure 16-2), FineVision trifocal (PhysIOL; Figure 16-3), and AcrySof IQ PanOptix (Alcon Laboratories, Inc). These lenses are currently under investigation by the FDA. While very promising, they do have similar limitations to bifocal diffractive multifocal lenses including the need for careful patient selection and visual side effects, such as halos.8,9
The lenses themselves are designed with 2 repeating steps of intermediate and near focal points between flat steps for distance. The AT LISA trifocal and FineVision IOLs distribute light equally between distance, intermediate, and near, each with a third of the share. The intermediate focal point for these IOLs is set at 80 cm or approximately 30 inches. The AcrySof IQ PanOptix dedicates 50% to distance, 25% to intermediate, and 25% to near. The intermediate focal point is 60 cm or about 24 inches.10