Findings from initial experiments indicate the feasibility of using femtosecond laser pulses for in situ creation of customized IOLs.
"The motivation for this novel method of in situ modification of customized IOLs derives from market data showing about 10% of all cataract surgeries result in residual refractive errors of more than 1 D," said Dr. Bille, director, Biomedical Optics Program, University of Heidelberg, Germany. "These data indicate that a method for in situ refinement of IOL optical properties would be invaluable."
The components of the novel experimental system include an infrared femtosecond laser, a newly developed 2-photon microscope for in vivo imaging of the eye, and a scanner (Heidelberg Retina Angiograph II [HRA 2], Heidelberg Engineering). The femtosecond laser is used to modify the refractive index of plastic materials in a technique known as refractive index shaping (RIS).
To increase the amount of dioptric power change that can be achieved, a phase-wrapping technique is employed that essentially compresses the lens and reduces its volume. The 2-photon microscope allows visualization of the refractive index change.
"Using femtosecond laser pulses to target a predetermined 3-D space can typically cause a 1% change in refractive index," Dr. Bille said. "However, to facilitate generation of greater power changes, the phase-wrapping technique is needed.
"Change in refractive index cannot be visualized using conventional microscopy," he added. "However, it is visible with the 2-photon microscope that detects fluorescence differences of the materials, or with differential interference contrast (DIC) microscopy that detects wavelength differences of light passing through two different materials."
Bench testing performed using hydrophobic acrylic IOLs formulated with different levels of ultraviolet light absorbers and yellow dye (provided by Aaren Scientific) and visualization of the refractive index change via DIC microscopy demonstrated success of the RIS technique. Dr. Bille said that using a 500-mW laser, an inscribing speed of 120 mm/sec and higher could be achieved with 0.8-µm resolution of the inscribing pattern.
"By increasing the power of the laser, it is possible for the writing speed to be increased to close to 1 m/sec, and with its micrometer-level of resolution, it would be capable of making very precise modifications in power and wavefront patterns," he said.
"The micrometer-level resolution of this technology represents an important advantage compared with a light adjustable lens [Calhoun Vision], and the power level is comparable with that associated with femtosecond laser capsulotomy," Dr. Bille concluded. "Therefore, we expect this research can readily advance into clinical testing."
Josef F. Bille, PhDE-mail: Josef.firstname.lastname@example.org
The research was supported by Aaren Scientific and Heidelberg Engineering. Dr. Bille is a consultant to both companies.