The ability to correct myopia with contact lenses in 2010 is achieved via new lens materials and designs that optimize vision, including aberration-control designs, second and third generation siliconehydrogel (Si-Hy) lens materials, and high oxygen permeable (Dk) fluoro-silicone/acrylate (F-S/A) materials. The expected reduction in contact lens use with myopes worldwide as a result of the impact of refractive surgery has yet to occur.

Contact lenses versus spectacles in myopia

There are many benefits for fitting myopic - notably highly myopic - individuals into contact lenses versus maintaining spectacle wear. The overall field of view is greater (100˚ versus 80˚ with spectacles), fewer optical aberrations are present as the patient views through the optical center of the lens at all times, the eyes do not appear as minified to the viewer and contact lenses increase the magnification of the retinal image, and the induced prismatic effects with oblique gaze are eliminated [1, 2]. The only possible compromise would be the pre-presbyopic high myope who loses the Base In effect achieved with spectacles during near work, requiring more convergence and accommodation and possibly necessitating a near correction earlier than with spectacles.

Rigid gas permeable (GP) lens : applications, design and fitting

Lens Materials

The use of silicone-acrylate lens materials have been replaced by fluoro-silicone/acrylate(F-S/A) materials which provide greater oxygen permeability, wettability and stability than their predecessors. They have been divided into low Dk (25-49), high Dk (50-99) and hyper Dk (≥ 100) [3]. A minimum oxygen transmission value (Dk/center thickness x 10) of 24 is recommended for edema-free daily wear [4] and 125 for extended wear [5]. Therefore, the low Dk lens materials should meet this criterion for the daily wear myope, whereas the hyper DK materials - notably Boston X02 (141 Dk from Bausch & Lomb), FluoroPerm 151 (151 Dk from Paragon Vision Sciences), and Menicon Z (163 from Menicon) meet this criterion for the myopic extended wear patient.

Applications

The most important application of GP lenses is for patients desiring – or benefitting from – good quality of vision. Notably moderate- to-high astigmatic, irregular cornea, and presbyopic individuals benefit from both the ability to mold the front surface of the cornea and the optical quality of GP lenses. Special design GPs are not within the scope of this paper but it has recently been found that when highly astigmatic subjects wore both soft toric and GP lenses, almost 75% preferred the vision of the GPs and the majority elected to stay in them [6]. This is consistent with studies comparing spherical GP to spherical soft lenses.[7, 8] GP lenses have also demonstrated an ability to reduce spherical aberration when worn [9] and, in fact, when subjects were corrected with multiple modalities it has been found that correction with GP lenses resulted in significantly better optical quality than soft lenses or spectacle lenses.[10] This was a result of the ability of GP lenses to reduce both the eye’s asymmetric aberrations as well as positive spherical aberration. The low risk of ocular infection and inflammation due to the absence of limbal compression and less risk of adherence have always been benefits of GP lenses.

Design and Fitting

The smaller size of GP lenses, resulting in greater movement on the eye and more initial lid sensation and lens awareness, in addition to their custom lens design, have deterred practitioners from fitting GP lenses to many patients who would benefit from this modality. However, presenting GPs in a positive but realistic manner [11], use of a topical anesthetic immediately prior to initial lens application [12, 13], and lens design improvements can all result in a positive initial experience for the myopic patient. Lens designs in common use today have larger overall diameters (often 9.5 - 10.5mm) resulting in less movement on the eye, ultrathin profiles – reducing lens mass and optimizing centration, and aspheric or pseudo-aspheric peripheries to better align the lens to the cornea [14]. The use of a plus lenticular with high (often ≥ 5D) minus lenses reduces the edge thickness to optimize comfort while allowing a better view of the peripheral fluorescein pattern [15] (Fig.1). In addition, new corneal-scleral designs (13.5 -16mm) are now being advocated for first-time myopic contact lens wearers [16]. All of these factors should optimize initial comfort.

Fig. 1: A pseudo-steep fluorescein pattern resulting from the blocking of fluorescence in an ultraviolet inhibiting GP lens material in a high minus ower. (from Davis LJ, Bennett ES (15)).

Silicone hydrogel applications and fitting

Materials, Designs and Fitting

The introduction of silicone hydrogel (Si-Hy) lens materials in the late 1990s has resulted in lenses that provide several times more oxygen transmission than their hydrogel predecessors. Whereas the highest Dk value obtainable with a HEMA-based hydrogel lens was approximately 40 [17], silicone hydrogel materials have Dk values ranging from 60 - 140. Although representing only 30% of soft lenses prescribed worldwide, their use is rapidly increasing. [18]. The nonwetting or hydrophobic nature of silicone has been neutralized by such factors as plasma treatment, wettable macromers, or moisturizing agents within the material. The first generation of Si- Hy materials had a higher modulus of elasticity than hydrogel lenses which allows for greater rigidity, ease of handling and potentially better visual acuity 19.] However, the stiffer modulus can cause edge fluting as well as superior mechanical chaffing resulting in superior epithelial arcuate lesions (SEALs) [20]. The introduction of second generation Si-Hy lens materials with a modulus simulating hydrogel lenses has reduced these problems. Although hypoxia-related complications are much reduced with Si-Hy materials it is, nevertheless, important for these lenses to exhibit movement with the blink to minimize peripheral corneal complications such as infiltrates and neovascularization [19].

Recently a third generation hybrid (SynergEyes) has been introduced with a 14.5mm overall diameter, with an 8.2mm center GP material (Paragon HDS100 from Paragon Vision Sciences), and a 27% water hydrophilic skirt. Although the current lens designs have been used primarily in cases of irregular cornea and also presbyopia, they have applications for any astigmatic patient who desires good vision as well as initial comfort. This lens is currently limited by the low Dk skirt as well as a tendency to adhere to the cornea. However, modifications of this lens are forthcoming to address these concerns including the introduction of a wavefront-guided custom lens for better quality of vision [21].

Applications

For individuals desiring good initial comfort, occasional lens wear, a change in eye color, or to participate in athletics, soft lenses are a good alternative. Although the optical quality of a Si-Hy lens material is less than a GP, the quality of vision achieved newer lens designs has great potential. With a few designs this is accomplished via introducing a constant level of spherical aberration at all lens powers that is equal in magnitude but opposite in sign to the population mean of human eyes (Biomedics Premier from CooperVision; PureVision from Bausch & Lomb). Si-Hy multifocal and toric designs continue to improve with the latter providing better vision than spherical lenses in low (often 0.75 - 1.00D) astigmatic patients [22, 23, 24, 25] (Fig.2).

Fig. 2: Comparing the performance of a spherical power (Acuvue Advance from Vistakon; left 4 photos) to the same patient with an astigmaticcorrecting lens (Acuvue Advance for Astigmatism: right 4 photos) (from Lebow (25)).

Conclusion

Contact lens correction of myopic has never been more exciting with new generation Si-Hy lens materials, large diameter GP lenses in hyper transmissible materials, and third generation hybrid lenses. With improvements in special design lenses for high astigmatic, presbyopic, and irregular cornea patients, the future looks even better.