Need to Know: Wavefront Filtering and PPP for Aberropia

Pinhole optics is a fascinating field that applies a small aperture to improve image clarity and focus. It is used for treating aberrated eyes, such as those with corneal ectasia, corneal scars, or post-surgical complications. It works through the pinhole camera by cutting peripheral rays of light, thus reducing scattering of light and decreasing the circle of blur.

Pinhole optics have been used to correct higher-order aberrations by acting as a wavefront filter. It can be applied at various levels—spectacles, contact lenses, the cornea, and the IOL. Dr Agarwal (et al.) described pinhole pupilloplasty (PPP), which uses pinhole optics at the iris level. This helps further customise the effect of pinhole optics and provides some distinct advantages.

It works on a few simple, yet powerful principles.

The effect of light scattering secondary to distorted corneas is mitigated through pinhole optics. It minimises the circle of confusion on the retina and results in a sharper image, as only direct light rays reach the retina. The depth of focus also increases, thus expanding the range of vision even in pseudophakic eyes.

The surgeon centres the PPP on the visual axis, unlike pinhole IOLs, where the IOL centres within the bag and not necessarily on the visual axis. Thus, the PPP can be placed accurately on the visual axis in eyes with distorted optics. Additionally, it can be placed anywhere behind the region of the cornea with good optics in highly aberrated scarred eyes, while an IOL cannot.

The surgeon can likewise adjust the PPP size intraoperatively, customising it to the patient. However, other pinhole technologies, such as IOLs and corneal small-aperture discs (e.g., Kamra inlay, CorneaGen), are made with a fixed aperture size, and the same aperture is implanted for every aberrated cornea, regardless of the number of aberrations. Yet these can only help to an extent in highly aberrated eyes.

With PPP, the diameter of the pupil is adjustable and can be inversely related to the degree of aberrations.

PPP provides this advantage of being able to adjust the pinhole size to the number of aberrations, which is not possible with any other small-aperture optics. Thus, the degree by which the aperture is reduced does not remain uniform as in IOLs and corneal pinholes.

Stiles–Crawford effect

First, let’s understand the anatomy of the fovea. There are 200-fold more cones in the fovea compared to the rest of the retina. Foveal cones are tightly packed, with densities reaching 150,000–300,000 cones/mm² as opposed to peripheral cone density of approximately 5,000–10,000 cones/mm². This dense arrangement is essential for high spatial resolution and sharp central vision.

To accommodate dense packing in the foveal region, foveal cones are taller and thinner than peripheral cones, measuring 1.5–2.0 μm at the fovea versus 4.0–10.0 μm in the peripheral retina. Their shape and high refractive index relative to the surrounding intercellular medium allow them to act as optical waveguides. So, much like an optical waveguide, the waveguide-like property of cones helps them guide and preferentially transmit light entering at angles closer to their optical axis. This behaviour is a critical aspect of the Stiles–Crawford effect, where light rays entering at the centre of the pupil are perceived as brighter and more effective for vision than those entering near the periphery of the pupil, even if their physical intensities are the same.

The sizing and positioning advantages of the PPP harness the waveguide property of cones and the Stiles–Crawford effect by allowing parallel rays of light to accurately hit the optical axis of the cones, thus enhancing and optimising retinal image quality.

Advantages of PPP

The IC-8 Apthera (Bausch + Lomb) needs to be available in different powers, which increases the need for a large inventory of these small-aperture monofocal IOLs. The alternative opaque optic piggyback IOL (XtraFocus IOL, Morcher) with a central clear aperture also has disadvantages, such as requiring placement in the sulcus atop another IOL. Small-aperture corneal inlays need a femtosecond pocket to be created for placement and correction of any refractive error in patients with corneal laser vision correction or cataract surgery with IOL implantation.

PPP, on the other hand, only requires a 10-0 prolene suture and is not dependent on expensive pinhole IOLs, small-aperture corneal inlays, or expensive laser platforms. Cataract extraction with implantation of a monofocal IOL of the surgeon’s choice is followed by PPP.

Pinhole IOLs and inlays can be used only up to 1.5 D of astigmatism. Dr Agarwal’s clinic has been able to demonstrate improvement in even high degrees of astigmatism by adjusting the size of the PPP according to the degree of astigmatism or HOA.

While pinhole IOLs are recommended in the non-dominant eye, the clinic has used the PPP in both eyes. The Kamra corneal inlay has been reported to have issues with corneal healing, fibrosis, and haze development. 

Decentred PPP

In eyes with central corneal scarring close to the visual axis that is causing irregular astigmatism, even a slightly decentred PPP may surprisingly give good visual improvement without the need for high-risk corneal surgeries.

The use of the pinhole IOL is restricted in the presence of an intact posterior capsule and intact zonules. Patients can benefit from PPP irrespective of the lens or capsule status, as it can be combined with any form of secondary IOL fixation, such as glued IOL or the Yamane.

Use in eyes with radial keratotomy

Corneal pinhole inlays are difficult to place in eyes with multiple radial keratotomy (RK) scars or in very thin corneas in keratoconic eyes. As compared to the PPP, synthetic corneal pinhole apertures such as the Kamra inlay also have the disadvantage of interfering with corneal nutrition (predisposing to haze, scarring, or melts) and can result in irreversible damage to the cornea. A corneal transplant is also difficult in corneas with RK incisions. However, a simple cataract surgery with PPP handles these disadvantages while providing good vision.

PPP can be advantageous when combined with DMEK or PDEK, especially in complex cases such as vitrectomised eyes as it makes graft opening much easier. It also helps to maintain a better air tamponade postoperatively by preventing migration of the air bubble. Its use extends to eyes with peripheral anterior synechiae, as the tautening of the iris secondary to PPP prevents these synechiae from reforming, which helps a closed angle become an open angle.

In combination with penetrating keratoplasty

Similarly, it can be advantageous in eyes with failed penetrating keratoplasty for a planned PDEK or DMEK, as it negates higher-order aberrations associated with corneal grafts while also making PDEK/DMEK surgery much easier and faster. In these complex eyes, peripheral anterior synechiae are often seen and result in secondary glaucoma and a higher risk of graft rejection and failure.

As previously explained, PPP has multiple advantages: It opens the closed angle, helps decrease intraocular pressure, and prevents synechiae from reforming—all while providing better optics. PPP may also be performed primarily with a penetrating keratoplasty after removing the cataract and implanting an IOL.

Other uses for PPP

Eyes with mydriatic pupil, e.g., secondary to Urrets-Zavalia syndrome or trauma, can benefit from PPP as well. In patients with corneal scars secondary to corneal pathology or trauma, the PPP helps improve vision without resorting to complex corneal procedures. In the presence of dense corneal neovascularisation where a corneal graft rejection is definite, a PPP can provide good quality functional vision without having to perform a high-risk corneal graft with all its attendant intra- and postoperative complications.

In the next article in this series, I will discuss the technique of evaluating the patient and performing a PPP.

This is the fifth in a multipart tutorial on higher-order aberrations. Previous articles in the series can be found at escrs.org/eurotimes.

Dr Soosan Jacob is Director and Chief of Dr Agarwal’s Refractive and Cornea Foundation at Dr Agarwal’s Eye Hospital, Chennai, India, and can be reached at dr_soosanj@hotmail.com.