Vision Correction with Corneal Inlays
Technique remains a work in progress despite a long history of attempts.
Cheryl Guttman Krader reports
The first implantation of a synthetic corneal inlay for vision correction was in 1972, but all the devices developed since then have failed to deliver satisfactory results. Could allografts do better?
“It is time to try something different,” said Michael C Knorz MD.
The synthetic corneal inlays used for vision correction can be divided into three generations, categorised by material. The first synthetic corneal inlays were polymethylmethacrylate, and both the vision and safety outcomes with these devices were unsatisfactory. Corneal melts developed in some eyes, and although the inlays are removable, the inlay procedure was not reversible because corneal scarring remained.
The next generation of corneal inlays were hydrogel devices. This group included the 5-mm PermaVision inlay (Anamed) introduced in 1999 for the treatment of hyperopia—available for two years—and the 6-mm Cougar inlay from 2000, available for just six months.
Presenting his personal experience using the PermaVision inlay, Dr Knorz said that after an initial overshoot, the refractive outcomes were fairly stable during two-year follow-up.
However, the refractive predictability was not very good. At one and six months postoperatively, the achieved refraction was within 0.5 D of attempted in only about two-thirds of eyes and within 1.0 D in about 80% of eyes.
In addition, the device had an unacceptable safety profile. Fifteen percent of eyes lost two or more lines of BSCVA, and multiple complications developed at relatively high rates. Inlay decentration greater than or equal to 1 mm occurred in 19% of eyes, and haze occurred in 86%. Twenty-nine percent of patients reported moderate halos and glare. Severe halos and glare affected 38% of patients.
“Almost half of the inlays had to be explanted,” Dr Knorz said.
Follow-up at the slit lamp showed the inlay appeared to be well-tolerated early after implantation, but deposits were visible at the edges by 6 months, and the device basically encapsulated by 12 months. The implants were removable, but the topography changed from its preoperative state, and corneal scars were present.
The third generation of synthetic inlays were developed for presbyopia treatment: the polyvinylidene difluoride KAMRA (AcuFocus), the Raindrop hydrogel (ReVision Optics, recalled in 2019), and the hydrophilic acrylic Flexivue (Presbia). Only Flexivue is still on the market, available in a limited number of countries.
Discussing the KAMRA inlay, Dr Knorz said it was implanted in more than 20,000 eyes after receiving US FDA marketing approval. The procedure was associated with satisfactory vision results, and many patients who received it kept it. Dr Knorz and colleagues examined the outcomes of ten patients who underwent KAMRA removal and found the topography was reversible in 60% of cases.
“The flip side is that 40% of eyes had a permanent change in topography, and we saw persisting scars that caused scattering of light,” Dr Knorz said.
Will allografts do better? Clinical trials are investigating several allograft corneal inlays, including the TransForm corneal allograft (Allotex) prepared from sterile human donor corneas and PEARL (PrEsbyopic Allogenic Refractive Lenticule), which uses lenticules obtained from SMILE procedures.
Dr Knorz presented at the 40th ESCRS Congress in Milan.
Michael C Knorz MD is a professor of ophthalmology at the University of Heidelberg, Germany. He is also the Medical Director of the FreeVis LASIK Centre in Mannheim, Germany. email@example.com
Cheryl Guttman Krader
Wednesday, February 1, 2023