Clinical applications of small aperture optics.
Dispensing opticians **
Presbyopia is primarily an inevitable, age-related condition that causes irreversible loss of the accommodative amplitude of the eye. Despite its ubiquity, the exact mechanism behind presbyopia remains unclear. Worldwide in 2005, more than 1.04 billion people were estimated to have presbyopia. (1) By the year 2020, the worldwide prevalence is expected to rise to 1.37 billion.1 The underlying cause for this age-related loss of accommodation has yet to be fully elucidated and continues to remain a topic of controversy. Models for presbyopia are broadly divided into two areas and are referred to as lenticular mechanisms and extralenticular mechanisms. (2) Although the lenticular theories propose age-related changes to the lens, capsule, and zonular fibres, the extralenticular mechanism includes ciliary muscle dysfunction, loss of elasticity in the posterior zonular fibres, and even decreased resistance of the vitreous against the lens capsule. (3,4)
Presbyopia affects quality of life. Indeed, McDonnell et al showed that presbyopia was associated with substantial negative effects on health-related quality of life in a population study. (5) The safest and least invasive method to treat presbyopia consists of corrective glasses as a separate pair of reading glasses, bifocals, or progressive lenses. Several options to treat presbyopia have been pursued; these include monovision with contact lenses or with laser vision correction (LVC), multifocal ablation patterns on the cornea (termed presbyLASIK), and lenticular approaches with refractive lens exchange with multifocal or extended depth of focus intraocular lenses (IOLs).
Small aperture optics is a concept which dates back to Roman times where refractive errors were corrected with a small hole in an olive leaf. In optics, an aperture is the maximum diameter of a light beam that can pass through an optical system, blocking the unfocused peripheral lights rays and focussing the central rays more closely on the fovea; this results in a sharper image and a more flexible depth of focus. The basic principle is one used on a daily basis by most optometrists to determine the potential visual acuity in a patient using the pinhole technique. The principle is similar to that of basic photography, the depth of focus or the range of vision is a function of both the pinhole size and the focal length. Smaller apertures provide a longer depth of focus. The use of the small aperture concept in clinical ophthalmology has led to the development of new devices to allow for a clear picture at distance, intermediate and near. This article will highlight the currently available devices and their applications.
Small aperture corneal inlays
Synthetic corneal inlays were first used by Jose Barraquer in the late 1940s for the treatment of aphakia and myopia. With improved surgical techniques and availability of improved biosynthetic materials the corneal inlay is an attractive option for presbyopes that seek spectacle independence.
The most recent generation of corneal inlays use a newer synthetic material of hydrogel polymers which have improved biocompatibility, avoiding many of the sight threatening side effects which were previously experienced with the older generation of inlays. The KAMRA Inlay (Acufocus, CA, USA) is an intrastromal corneal inlay operating on the principles of small aperture optics, improving near visual acuity and extending the depth of focus and stereopsis while maintaining good distance acuity. The KAMRA Inlay is made of polyvinylidene difluoride (PVDF) and has an overall diameter of 3.8mm with a central aperture of 1.6mm and a thickness of 5pm (see Figure I). It has 8,400 laser-etched micro-perforations ranging from 5 to 11 [micro]m in size which are designed to allow the diffusion of oxygen and aqueous, thus increasing the biocompatibility once implanted within the corneal stroma. At piano, the small aperture optic of the corneal inlay provides about 2.50D of continuous extended depth of focus in the functional range. Small aperture optics can be leveraged to provide additional near benefit by shifting the defocus curve slightly to the minus. When the eye that receives the small aperture corneal inlay or the small aperture intraocular lens (IOL) is targeted for a small amount of residual myopia (-0.75 D) to shift the defocus curve, the result is further enhancement of near vision without degrading distance vision.
This effectively extends the range of vision to nearly 3.00D of uninterrupted, continuous extended depth of focus as opposed to that provided by multifocal or trifocal lens designs, which have pronounced peaks and valleys. This helps patients experience a more natural range of vision like they had when they were in their 30s and could see effortlessly from near to far without the frustration of blurry and clear vision zones. This approach to vision correction may be confused with monovision, as the small aperture corneal inlay eye is targeted for slight myopia (-0.75 D) and the fellow eye for piano; however, the difference in the target refractions actually helps patients maximize the benefit of the corneal inlay to extend depth of focus rather than to improve only one focus at the detriment of another. Although the refractive strategy for each eye may be unique, patients still maintain excellent binocular vision and stereopsis.
Patient selection for corneal inlays is very similar to that of LVC. The potential candidates would be expected to have a stable refractive error, normal corneal topography, adequate corneal thickness required for LVC and a normal ocular examination with no pre-existing ocular comorbidity. The KAMRA corneal inlay is offered to patients with refractive error (either myopia/hyperopia with or without astigmatism) in the presbyopic age range. Patients who are deemed suitable for LVC are potentially good candidates for the small aperture corneal inlay. The inlay eye is treated with LVC first typically aiming for slight myopia (-0.75 D) rather than emmetropia with the corneal inlay implantation, undertaken as a second procedure once the refraction is stable.
A femtosecond laser is used dining LVC to create the corneal flap of the required thickness (usually between 90 to 110 [micro]m) and an excimer laser is used to ablate the stromal bed. About two months following LVC a femtosecond laser is used to create a stromal 'pocket' at about 250 to 300 [micro]m deep in the non-dominant eye. It is within this pocket that the corneal inlay is inserted under topical anaesthesia (see Figure 2).
Small aperture intraocular lenses
The IC-8 lens is the first IOL that provides a true depth of focus using the small aperture principle. This IOL is made of a hydrophobic acrylic, and has an overall diameter of 12.5mm with an optic size of 6mm. Embedded within this optic is the diaphragm/mask measuring 3.23mm with a central aperture of 1.36mm (see Figure 3).
The ideal patients are those in their 60s who are deemed suitable for refractive lens exchange or patients with visually significant cataract. Like the small aperture corneal inlay, the small aperture IC-8 IOL is implanted only in the non-dominant eye of the patient with the dominant eye corrected for emmetropia either with a monofocal or a monofocal toric IOL. Current studies have shown that the small aperture IOL is able to compensate for up to 1.50D of pre-existing corneal astigmatism. (6)
The IC-8 IOL implantation is very similar to a conventional cataract or lens replacement surgery. The IOL is implanted within the capsular bag through a 3mm clear corneal, self-sealing corneal wound. Much like the KAMRA inlay, the IC-8 implanted eye is targeted to leave a mild residual myopic prescription of around -0.75D as this has been shown to encourage appreciation for depth of focus and extends the range of vision to nearly 3.00D of uninterrupted, continuous extended depth of focus (see Figure 4). Although the refractive procedure is different between each eye, the monocular nature of this implant is not to be confused with monovision as stereopsis and binocularity are still maintained. The IOL utilises the pinhole technique by using the opaque black aperture in the IOL to cut out the blurred peripheral rays and focus a smaller number of clear central rays onto the retina, almost acting as a smaller second pupil.
Post-operatively the lens has a minimal effect on the view of the posterior segment and does not impede the ability to photograph the fundus or to perform fundus examination, OCT scans or threshold visual field tests. The solid aperture of the IOL allows avoidance of commonly experienced post-operative side effects such as induced astigmatism and the effects of varying pupil size.
Pinhole piggyback intraocular lens
The most recent addition to the market of small aperture optics is the XtraFocus pinhole intraocular implant. This device is a supplementary implant for pseudophakic patients with irregular corneal astigmatism and is implanted in a piggyback style in the ciliary sulcus to compliment a pre-existing IOL in the capsular bag. The common causes of such irregular corneal astigmatism include keratoconus, post-radial keratotomy, post-comeal transplant and also in patients with pupillary abnormality.
The implant has an overall diameter of 13mm with a central 6mm black opaque diaphragm and a 1.3mm central pinhole and thin smooth haptics to prevent damage to the uveal tissue. The implant is foldable, so it can be injected through a small (3mm) self-sealing clear comeal wound.
The result is similar to that described by the two previous implants with an improved visual acuity and depth of focus. The obvious drawback is that the implant is solidly opaque and so blocks any useful view of the retina with conventional viewing methods such as slit lamp biomicroscopy and condensing lens examination. The hydrophobic black acrylic material blocks visible light but becomes 'transparent' when exposed to infra-red light, so the retina can be viewed with OCT, scanning laser ophthalmoscopy or infra-red slit lamp.
Patient suitability is key when selecting possible candidates who would benefit from the procedures described here. Current spectacle or contact lens wearing presbyopic patients who are looking for a more permanent solution are ideal candidates. Prospective patients can include those who have previously undergone refractive surgery or those who have had a monofocal successfully implanted in one eye and require freedom from the restrictions of presbyopia. These approaches also work well for patients who have longstanding corneal or iris irregularities or suffer from higher order aberrations.
The benefits of the surgery can be demonstrated by using the pinhole in the test room. If a patient is unsure if they could cope with monovision permanently it is helpful to allow a trial of daily disposable contact lenses to allow for adaptation.
These procedures are ideal for contact lens wearers, especially those who have successfully worn monovision correction, but want lens freedom or a more permanent solution. It is advisable that patients may need to cease lens wear until reliable, repeatable biometry measurements are achieved.
Like any referral for routine cataract surgery the macula should be checked for any subtle epiretinal membrane or vitreomacular traction as well as all routine pre-operative measurements. Patients should be checked for dry eye disease and treated to resolution before being considered suitable for either procedure. Tear film quality and stability play a vital role in the quality of vision achieved as minute deficiencies can have a significant effect and may result in fluctuation in quality of vision.
The advancements in technology and surgical techniques are allowing for a wider range of reliable and more efficient methods of correcting presbyopia permanently, increasing the options we as optometrists can offer as permanent solutions to our ever-growing base of presbyopic patients.
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Gail Meehan BSc (Hons), MCOptom and Sathish Srinivasan FRCSEd, FRCOphth, FACS
Gail Meehan graduated from Glasgow Caledonian University in 2012. Since then she has been working in private practice in Ayrshire. Her interests are in clinical ophthalmology and complex contact lens fitting for abnormal and aberrated corneas. She works as a research optometrist at Ayrshire Eye Clinic where she pursues her interest on assessing the visual and refractive outcomes of new intraocular lenses for correcting presbyopia.
Sathish Srinivasan is a consultant ophthalmic surgeon and joint clinical director at University Hospital Ayr, and medical director at Ayrshire Eye Clinic and Laser Centre. He specialises in corneal, cataract and laser refractive surgery.
Course code: C-S9252 Deadline: 8 June 2018
* Be able to explain the clinical applications of small aperture optics to patients (Group 1.2.4)
* Understand the use of small aperture optics for the correction of presbyopia (Group 6.1.11)
* Be able to explain the clinical applications of small aperture optics to patients (Group 1.2.4)
* Understand the use of small aperture optics for the correction of presbyopia (Group 8.1.3)
Caption: Figure 1 KAMRA corneal inlay showing laser-etched micro-perforations
Caption: Figure 2 KAMRA corneal inlay in situ
Caption: Figure 3 The IC-8 IOL
Caption: Figure 4 Defocus curve comparing a presbyopic eye to a patient treated with an IC-8 implant and monocular IOL in each eye, respectively
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|Title Annotation:||Small aperture optics|
|Author:||Meehan, Gail; Srinivasan, Sathish|
|Date:||May 1, 2018|
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