vision correction. These cases can be identified via spherocylinder refraction over the contact lens. However, against-the-rule lenticular astigmatism is probably present when against-the-rule refractive astigmatism (adjusted to reflect the power at the corneal surface) exceeds the keratometric corneal astigmatism. Such eyes may have less residual astigmatism when the refractive error is corrected with soft rather than rigid spherical contact lenses if the corneal astigmatism is compensating for lenticular astigmatism.
For example, consider a patient whose refraction is –3.50 –0.50 × 180 and K measurements of the affected eye are 42.5 D (7.94 mm) horizontal and 44.0 D (7.67 mm) vertical. Would a soft or rigid contact lens provide better vision (ie, less residual astigmatism)? The disparity between the corneal astigmatism of 1.50 D and the refractive astigmatism of 0.50 D reveals 1.00 D of against-the-rule lenticular astigmatism that neutralizes a similar amount of with-the-rule corneal astigmatism. Neutralizing the corneal component of the refractive astigmatism with a rigid contact lens exposes the lenticular residual astigmatism. Therefore, a spherical soft contact lens would provide better vision because the residual astigmatism is 1.00 D for a rigid contact lens.
Correcting Presbyopia
Correcting presbyopia with contact lenses can be done in several different ways:
reading glasses over contact lenses
alternating vision contact lenses (segmented or annular)
simultaneous vision contact lenses (aspheric [multifocal] or diffractive) monovision
From an optical point of view, the use of reading glasses or alternating vision contact lenses is similar to standard spectacle correction for presbyopia. Simultaneous vision contact lenses direct light from 2 points in space—one near, one far—to the retina, resulting in a loss of contrast. Distant targets are “washed out” by light coming in through the near segment(s), and near objects are “washed out” by light coming in through the distance segment(s). Monovision allows one eye to have better distance vision and the other to have better near vision, but this arrangement interferes with binocular function, and the patient then has reduced stereopsis. For these reasons, it is important to fully explain the options to contact lens wearers with presbyopia. As previously demonstrated, it is important to explain to a new contact lens wearer with presbyopia and myopia that he or she may need near correction or one of the other aforementioned options when presbyopic correction with the spectacles had not previously been required. The contact lens correction of presbyopia is discussed in greater detail in the section Contact Lenses for Presbyopia.
Kastl PR, ed. Contact Lenses: The CLAO Guide to Basic Science and Clinical Practice. 4 vols. Dubuque, IA: Kendall-Hunt; 1995.
Contact Lens Materials and Manufacturing
Various materials have been used to make contact lenses. The choice of material can affect contact lens parameters such as wettability, oxygen permeability, and deposits on the lens. In addition, material choice affects the flexibility and comfort of the lens and the stability and quality of vision.
Manufacturing techniques primarily address the ability to make reproducible lenses in a costeffective manner.
Materials
Contact lens materials can be described in terms of flexibility (hard, rigid gas-permeable [RGP], soft, or hybrid). The first popular corneal contact lenses were made of PMMA, a plastic that is durable but not oxygen permeable. Gas-permeable materials are rigid but usually more flexible than PMMA. RGP lenses allow some oxygen permeability (Dk); this factor may vary from Dk 15 to more than Dk 100. This feature has allowed some RGP lenses to be approved for overnight or extended wear. Currently, most RGP lenses are made of silicone acrylate. This material provides the hardness needed for sharp vision, which is associated with PMMA lenses, and the oxygen permeability associated with silicone. Despite advances, wettability still poses a challenge (Fig 4-7).
Figure 4-7 The wettability of a lens surface determines whether a wetting angle will be low (greater wettability, greater
comfort) or high (less wettability, less comfort). (Modified with permission from Stein HA, Freeman MI, Stein RM. CLAO Residents Contact Lens Curriculum Manual. New Orleans: Contact Lens Association of Ophthalmologists; 1996. Redrawn b y Christine Gralapp.)
The newest lenses are made of fluoropolymer, which provides greater oxygen permeability than does PMMA. Disadvantages of fluoropolymer lenses are rigidity and discomfort.
The gas permeability of a material is related to (1) the size of the intermolecular voids that allow the transmission of gas molecules, and (2) the gas solubility of the material. Silicon monomers are the most commonly used materials because their characteristic bulky molecular structure creates a more open polymer architecture. The addition of fluorine increases the gas solubility of polymers and somewhat counteracts the tendency of silicon to bind hydrophobic debris (such as lipid-containing mucus) to the contact lens surface. In general, polymers that incorporate more silicon offer greater gas
permeability at the expense of surface biocompatibility.
Soft contact lenses are typically made of a soft hydrogel polymer, hydroxyethylmethacrylate. The surface characteristics of hydrogels can change instantaneously, depending on their external environment. When hydrogel lenses are exposed to water, their hydrophilic elements are attracted to (and their hydrophobic components are repelled from) the surface, which becomes more wettable. However, drying of the surface repels the hydrophilic elements inward, making the lens surfaces less wettable. The hydrophobic surface elements have a strong affinity for nonpolar lipid tear components through forces known as hydrophobic interactions. Such interactions further reduce surface wettability, accelerate evaporative drying, and compromise the clinical properties of soft lenses.
The oxygen and carbon dioxide permeability of traditional hydrogel polymers is directly related to their water content. Because tear exchange under soft lenses is minimal, corneal respiration depends almost entirely on the transmission of oxygen and carbon dioxide through the polymer matrix. Although the oxygen permeability of hydrogel polymers increases with water content, so does their tendency to dehydrate. To maintain the integrity of the tear compartment and avoid corneal epithelial desiccation in dry environments, these lenses are made thicker, thereby limiting their oxygen transmissibility.
High-oxygen-permeability, low-water-content silicone hydrogels are used for extended wear. The oxygen transmission of these lenses is a function of their silicon (rather than water) content and is sufficient to meet the oxygen needs of most patients’ corneas during sleep. The surfaces of these lenses require special coatings to mask their hydrophobic properties. Other clinically important properties of contact lens hydrogels include light transmission, modulus (resistance to flexure), rate of recovery from deformation, elasticity, tear resistance, dimensional sensitivity to pH and the osmolality of the soaking solution and tears, chemical stability, deposit resistance, and surface waterbinding properties.
Manufacturing
Several methods are used to manufacture contact lenses. Some contact lenses are spin-cast, a technique popularized with the first soft contact lenses. In spin-casting, the liquid plastic polymer is placed in a mold that is spun on a centrifuge; the shape of the mold and the rate of spin determine the final shape of the contact lens. Soft contact lenses can also be made on a lathe, starting with a hard, dry plastic button; this method is similar to the way that RGP lenses are made. Once the soft lens lathe process is complete, the lens is hydrated in saline solution to create the characteristic softness. Lathes may be either manually operated or automated. In either case, manufacturers can create very complex, variable shapes that provide correction for many different types of refractive error; lenses can even be customized to meet individual needs.
Following the introduction of disposable contact lenses—and thus the need to manufacture large quantities of lenses—cast molding was developed. In this technique, different metal dies, or molds, are used for specific refractive corrections. Liquid polymer is injected into the mold and then polymerized to create a soft contact lens of the desired dimensions. This process is completely automated from start to finish, enabling cost-effective production of large quantities of lenses.
Scleral contact lenses have very large diameters and touch the sclera 2–4 mm beyond the limbus. They have been available for years, but because they were originally made of PMMA—and thus were oxygen impermeable—the lenses were not comfortable. With the use of newer RGP materials, interest in these lenses has resurfaced, especially for patients with abnormal corneas. Scleral contact