Customised approach useful in resolving decentred ablations after LASIK and PRK

Cheryl Guttman in San Francisco, US

THE Custom Contoured Ablation Pattern (C-CAP, VISX) method is proving itself to be an effective modality for addressing eyes with post-laser vision correction surgery decentred ablations which would otherwise be deemed untreatable, asserted Edward E Manche MD at the annual ASCRS Symposium on Cataract, IOL and Refractive Surgery.
Dr Manche reported the outcomes he achieved in a series of 10 eyes which had clinically significant decentred ablations after LASIK in eight cases or PRK in two cases. follow-up was at least three months in seven eyes.

Topographical maps showed all eyes benefited with improved centration and regularisation of the cornea surface. All patients reported reduction or resolution of visually disabling symptoms, which included distortions, diplopia, ghosting, halos and glare.

Functionally, BSCVA improved by at least two lines in eight eyes and remained stable in the other two. Uncorrected visual acuity remained stable or improved in nine of the 10 eyes. The single eye which lost UCVA maintained BSCVA.

"These limited results indicate C-CAP is achieving its clinical goals, which are to regularise the corneal surface and thereby reduce disturbing visual aberrations, enhance overall quality of vision and optimise BSCVA.
"The procedure is not intended necessarily to improve uncorrected vision and patients are informed preoperatively that they might need another procedure as an enhancement to achieve that result," Dr Manche said.

He performed wavefront analyses using the WavePrint system (VISX) in five eyes. These studies showed the C-CAP treatment improved higher order aberrations in all cases. Mean RMS for total higher order aberrations decreased significantly by 45.5% from 0.66 microns pre-C-CAP to 0.36 microns after treatment.

However, Dr Manche noted that C-CAP should be considered an adjunct to fully customised wavefront-guided ablation, not a substitute.
"It is not possible to obtain good wavefront data in an appreciable proportion of these highly aberrated eyes, and without adequate imaging, it would not be possible to treat those cases with wavefront-guided custom ablation.

"However, C-CAP can be complementary to customised wavefront treatment, used first to regularise the corneal surface, allowing a follow-up wavefront-guided treatment or conventional LASIK as a fine-tuning procedure," he explained.
C-CAP is performed using the elevation map produced by the Zeiss-Humphrey Atlas Topography system which allows the surgeon to analyse the decentred ablation and identify the appropriate treatment area.
Then, using the Vision Pro ablation planning software of the topography unit, the surgeon selects the size, shape, depth and location (on or off-axis) for a series of up to 20 sequential ablations.

To perform C-CAP, refractive surgeons need training and certification from VisX. Using the cornea elevation maps and the ablation planning software, they are able to identify the area of decentration, analyse it and select their treatment ablation parameters.
The software will simulate the planned ablation and produce a corresponding postoperative topography map, allowing the surgeon to test the parameters selected and adjust them to achieve a satisfactory result before proceeding.

"The software performs reasonably well in predicting the postoperative topography results, but there is a learning curve for using it. I have now treated about 14 eyes and the planning process takes me 15 to 20 minutes per case. Initially, however, I was spending about an hour planning and revising simulated ablations," Dr Manche said.
He added that his own approach was a conservative one at first in which he tried to minimise the amount of tissue removal. After performing a few cases and learning how the simulated results corresponded with the actual outcomes, he said he has become a little more aggressive.

"I was very careful because I wanted to avoid creating any divots that would be untreatable. My experience showed me the planning software tended to overestimate the amount of tissue removed and so there was less risk for over-ablation and producing a divot.

"Therefore, I became more aggressive with my ablations because initially I was able to regularise the cornea surface but was not completely eliminating the topographical decentrations," he explained.

Edward Manche MD
Stanford University School of Medicine, California, US
edward.manche@stanford.edu