Posterior IOLs effective in keratoconus

Posterior chamber phakic IOLs can correct refractive errors associated with keratoconus and can also be combined with other surgical techniques when necessary, according to researchers.

The lenses offer an alternative to corneal transplantation in patients who are intolerant to contact lenses, according to Dr José Juan Esteve-Taboada of the University of Valencia, Spain, and colleagues.

They published their review of the literature in the Journal of Cataract and Refractive Surgery.

In keratoconus, the cornea thins progressively, causing myopia and irregular astigmatism, and higher-order aberrations reduce image quality. Corneal collagen crosslinking (CXL) can halt this process.

However, spectacles can improve the refractive error in only middle cases, and most patients instead rely on rigid contact lenses. In the past, when they lost their tolerance for these lenses, they would need corneal transplants.

Improved options

More recent alternatives include excimer laser surgery, intrastromal corneal ring segments (ICRS) and phakic IOLs (pIOLs). ICRS and pIOL procedures do not remove corneal tissue, so they are less likely to weaken the cornea and trigger progression of the disease.

ICRS implantation reshapes the cornea to improve the topographical irregularities, but most patients still require additional optical solutions to correct residual myopia or astigmatism. Implantation of pIOLs may help these patients.

Anterior chamber pIOLs, fixed to the iris by enclavation, can significantly improve visual acuity. But they cannot completely correct astigmatism.

Posterior chamber pIOLs include the Visian Implantable Collamer Lens and the Visian Toric Implantable Collamer Lens (Staar Surgical).

Composed of hydrophilic porcine sclera tissue, their plate-haptic form allows posterior chamber pIOLs to be inserted through a clear corneal incision. They are permeable to gas and metabolites, enabling maintenance of normal crystalline lens metabolism and preventing cataracts.

They lie between the iris and crystalline lens. Although they significantly improve visual acuity, they can correct only spherical and cylindrical errors. They cannot address the vertical coma, primary coma and coma-like aberrations that frequently mar the vision of patients with keratoconus.

Measuring success

To get a perspective on the success of these posterior chamber pIOLs, Dr Esteve-Taboada and colleagues searched for studies. They found 33 of which 22 were clinical reports, 5 were overviews of keratoconus solutions and 2 were brief reviews.

The manufacturer usually performs the power calculation for pIOLs, using the astigmatism decomposition method or an online calculator provided by the manufacturer that can be accessed by the surgeons.

They often select emmetropia as the target refraction to reduce the preoperative refractive errors.

The manufacturer typically chooses the pIOL size based on the horizontal corneal diameter and anterior chamber depth determined with scanning-slit topography.

The posterior chamber pIOL is inserted into the anterior chamber using an injector cartridge. It unfolds slowly and the surgeon places 4 footplates under the iris.

Generally, the best outcomes are thought to be achieved when CXL is performed prior to pIOL implantation. Combining ICRS with pIOLs may offer the benefits of both. Inserting both simultaneously provides a better view for the pIOL insertion. On the other hand, inserting the ICRS insertion first produces keratometry readings to ensure better prediction of the pIOL power.

Potential long-term complications following posterior chamber pIOL implantation include cataract, pigment dispersion syndrome, pupillary block glaucoma, chronic uveitis, posterior dislocation of the pIOL to the vitreous, and iris ovalisation.

Researchers have reported attempts to improve vision in keratoconic patients since 2007, with a wide range of adults.

In the first study, Coskunseven et al. evaluated the results of combined ICRS and toric pIOL implantation in 3 eyes of 2 keratoconic patients with extreme myopia and irregular astigmatism. Both corrected and uncorrected distance visual acuity improved in all patients. The mean manifest refractive spherical equivalent (SE) refractive error decreased from -18.50 D to 0.42 D. The mean improvement in CDVA was 4.33 lines.

In one of the larger studies that followed, Alfonso et al. reported on 30 eyes of 21 patients with a follow-up of 12 months. Preoperatively, the mean SE was -5.38 D and the mean cylinder was -3.48. Postoperatively, the mean SE was -0.08 D and the mean cylinder was 0.41 D.

No consensus has emerged on inclusion criteria for implanting posterior chamber pIOLs, Dr Esteve-Taboada and colleagues report. Some authors have included only patients with a corrected distance visual acuity of 20/50 or better and some with a corrected distance visual acuity of 40/50 or better.

While they agree on the need for a stable refraction, these authors have listed periods of stability from 6 months to 2 years. The time for discontinuation of rigid gas-permeable contact lens ranges from 2 weeks to 6 months before corneal topography is performed. The authors gave minimum patient ages from 20 years to 30 years or older.

However, there is consensus on the minimum anterior chamber depth being 2.8 mm; that there should be no corneal opacification or scars; and that the endothelial cell density should be greater than 1,800 cells/mm² or greater than 2,000 cell/mm².

All the authors agreed that cataract, history of glaucoma or retinal detachment, macular degeneration or retinopathy, neuro-ophthalmic disease, and history of ocular inflammation are the main exclusion criteria.

Dr Esteve-Taboada et al. concluded that controlled studies with larger cohorts and longer-follow-periods are necessary to determine which refractive procedure and which sequence are most suitable in combination with CXL for the various stages of corneal ectasia.