Using a femtosecond laser, an IOL can be manufactured in a hybrid mechanical-optical procedure. In this method, the laser's pulses are used to modify the refractive index of the plastic IOL material. This new manufacturing process combines a standard lathe-based procedure to generate the IOL blank and an optical manufacturing procedure to customize the IOL according to the needs of a specific patient. Besides optical features such as asphericity, toricity, and multifocality, the correction of specific higher-order aberrations (HOAs) becomes feasible with this process.
Use of an IOL blank of modest refractive power allows the surgeon to use a smaller incision size. In a next-development step, in-vivo IOL adjustment based on the optical IOL-customization procedure seems feasible. Such a laser procedure would allow multiple readjustments in new cataract patients and open the opportunity to treat existing patients by adding toricity, asphericity, or multifocality to a previously implanted IOL.
Patients' growing needs and expectations for good vision in all ranges have created significant new market opportunities for premium IOLs. Demand has expanded for multifocal and accommodating IOLs designed to correct presbyopia, aspheric optics that address spherical aberration and make vision sharper, toric optics that considerably improve visual results in astigmatic eyes, and phakic IOLs that allow treatment of extreme refractive errors.
Conventionally, IOLs are manufactured based on plastic molding or computer-controlled lathing technologies. Premium IOLs require a lot more technology, time, and expertise to manufacture and are, therefore, more expensive than monofocal IOLs; a premium IOL can add several hundred euros to the cost of surgery for each eye. Complicated lens surfaces such as those in multifocal, trifocal, accommodating, and toric IOLs have been created using combinations of three complementary optical technologies—apodization, diffraction, and refraction. However, current manufacturing technologies are limited with regard to the amount of asphericity, toricity, and multifocality that can be incorporated into an IOL (see Current Premium IOLs).
LASER-MATERIAL INTERACTION PROCESSES
An alternative option that has been developed by Perfect Lens is a flexible hybrid mechanical-optical manufacturing process that allows precise and cost-effective customization of an IOL to meet the exact requirements of a specific patient. This method for modifying the refractive index of plastic materials, called refractive index shaping (RIS; Figure 1), uses femtosecond laser pulses directed to a small designated area within the IOL to create a mini-lens inside the IOL, changing the optical properties of the lens without cutting (see The Potential for In Situ Modified IOLs).
The femtosecond laser has several effects on acrylic lens material, the most recognized of which is that the laser's heat causes a change in the material. Additionally, if the proper wavelength is used, exposure to the laser can alter the polarity in the material and change the hydrophilicity of the acrylic material. The change in hydrophilicity drives a large, repeatable, and homogeneous change in refractive characteristics. Because this process does not require the accumulation of heat, it can be used with a fast scan speed, allowing in vivo application.
PHASE WRAPPING METHOD
In a traditional convex IOL, one is limited to a height of 200 µm in the central slab area for optical power adjustment. For example, the power for a 6-mm lens with a height of 200 µm would be 0.44 D (Δn = 0.01).
Phase wrapping is a process that can be used to create an RIS lens with enhanced diopter change without changing the lens' height. Thus, by creating multiple refractive zones, a convex lens can be reduced to a thin layer of approximately 50-µm thickness. For a lens with a diameter of 6 mm, one zone corresponds to 0.10 D (Figure 2).
MAKING CUSTOMIZED IOLs WITH RIS
The process for making customized IOLs with RIS is depicted in Figure 3. Based on the measurement of an IOL blank, the RIS computer program is prepared, the treatment of the IOL blank is customized according to the patient's refractive features,1 and the IOL's performance is verified.2 With this process, a customized lens can be prepared according to a patient's prescription in 0.10 D increments, and asphericity, toricity, and multifocality can be tailored to his or her visual needs. Because the lens does not depend solely upon shape to redirect light, the thickness of the lens will be equivalent to that of a 10.00 D or less conventional IOL, and it can be injected through a 1.4- to 1.8-mm incision.
A schematic representation of a 3.50 D reduction in the refractive power of an IOL from 22.00 D to 18.50 D is shown in Figure 4. A central layer 50 µm thick within the 200-µm slab of the original IOL is modified with the femtosecond laser, creating a phase-wrapped structure with 20 optical zones. Figure 5 depicts a laser-modified IOL: A refractive change of -3.67 D was generated inside an EC-1J IOL (Aaren Scientific); the modulation transfer function (MTF) is measured at 0.73 at 50 lp/mm.
Perfect Lens has successfully manufactured customized IOLs using the hybrid mechanical-optical process described above. The dioptric power of standard hydrophobic lenses (Aaren Scientific) was altered by up to 5.00 D positively and negatively in a permanent and predictable fashion, with minimal loss in MTF as compared with the original lens.
In future developments, application of RIS technology to adjust the refractive parameters of implanted IOLs will be studied. Using a femtosecond laser at energy levels that are well within existing safety limits, the laser should be able to adjust a lens for almost any refractive characteristic in vivo; however, the amount of change will be limited by the time available for the procedure and therefore will not be used to make dioptric changes of more than 2.00 to 3.00 D.
RIS technology can potentially benefit patients undergoing cataract surgery by allowing precise customization of their new IOLs. The technology will also allow cataract surgeons to adjust existing RIS IOLs in previously operated patients as a means to add toricity, asphericity, or multifocality. n
1. Bille JF. System for forming and modifying lenses and lenses formed thereby. US 8,292,952 B2, October 23, 2012.
2. Bille JF. System for characterizing a cornea and obtaining an ophthalmic lens. US 8,152,302 B2, April 10, 2012.
Josef F. Bille, PhD
- Professor of Physics, Heidelberg University, Germany
- Financial disclosure: Consultant, Shareholder (Perfect Lens)