Online pachymetry system may enhance safety of LASIK
Stefanie Petrou-Binder MD
A NEW ‘online' optical coherence pachymeter (OCP) allows surgeons to monitor flap thickness and ablation depth during LASIK, and make the necessary modifications intra-operatively, according to Michael Knorz MD, University Hospital , Mannheim , who presented his findings at the XXI Congress of the ESCRS. "Evaluation of residual corneal thickness is crucial to prevent corneal ectasia. For better control of the residual stroma after LASIK ablation we need to be able to measure flap and residual stromal thickness, as well as ablation depth progress during LASIK treatment. Online OCP increases safety during LASIK," Dr Knorz told EuroTimes.
The Mannheim team evaluated 87 eyes of 45 patients who underwent LASIK, using the intraoperative pachymetry OCP system (onlineOCP™, by 4optics, Lübeck , Germany ), since February 2003. The OCP operates at a wavelength of 1310 nm and a frequency of 74 HZ. They performed the ablations with a high-resolution H.Eye.Tech. excimer laser ( TechnoVision Munich , Germany ) which uses a new tissue-saving ablation algorithm. They used an Amadeus microkeratome (AMO) with a 140 micron head for creating the LASIK flaps.
The patients' preoperative mean spherical equivalent was -4.1 D (range: –0.75 D to –8.25 D) and the mean corneal thickness was 533 microns. The mean planned stromal ablation was 91 microns. The surgeons calculated the flap thickness by comparing the difference between pachymetric values obtained with the OCP device before flap creation with those obtained directly before starting the stromal ablation. Their results revealed a mean flap thickness of 125 ± 15 microns. None of the flaps were thicker than 160 microns. The difference between the planned ablation depth and measured ablation depth was -8.0 ± 15.0 microns. Based on this finding, the investigators developed a model to correlate the intended ablation depth with the ablation depth observed with OCP during the ablation, which gave a correlation coefficient of 0.85.
"We saw an excellent correlation when comparing the planned ablation progress with the real time measured ablation progress," Dr Knorz said. H Burkhard Dick MD, Mainz University Eye Clinic, strongly agreed that a device that reliably measures residual stromal depth would increase the safety of LASIK procedures. "Being able to monitor results simultaneously online with OCP and alter measurements accordingly is a great advantage." He explained that ultrasound pachymetry does not offer the same advantage as OCP. To use ultrasound pachymetry intra-operatively, the surgeon would need to wet the surface of the flap. As wetting the flap can alter the ablation, Dr Dick said he prefers to avoid it.
"One of the limiting factors in refractive surgery is the huge variability in microkeratomes and consequently the thickness of the flaps that they cut. A thick flap leaves a thin bed, and increases the ‘danger zone'." To reduce the danger zone, Dr Dick makes sure to increase the residual stromal bed by 50 microns, providing a greater safety margin. He said that few microkeratomes allow the surgeon to individualise flaps, and those offering that option, such as the Amadeus microkeratome, are associated with additional costs.
"Online coherence pachymetry is expensive, and although it can certainly provide improved results, it is not the solution that all refractive surgeons will be able to afford. Our efforts should go into developing more predictable microkeratomes," he said.
Prof. Michael C. Knorz, MD
FreeVis LASIK Centre
University Medical Center , Mannheim , Germany
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