CHAPTER 4
Contact Lenses
Introduction
The first documented use of contact lenses occurred in the 1880s. Those lenses were large and made of glass, and they extended to the sclera. Corneal lenses were introduced in the 1940s and were made of a plastic called polymethylmethacrylate (PMMA). Soft hydrogel lenses were introduced in the United States in the 1950s and led to the widespread use of contact lenses. Current estimates are that 51% of US adults use some kind of vision correction; of those, 25% use contact lenses. Therefore, more than 30 million Americans use contact lenses. While the vast majority of contact lens wearers are younger than 50 years, contact lenses also have important uses in older patients, such as for the correction of aphakia. The large number of contact lens users means that all ophthalmologists will interact with this group of patients—for fitting, follow-up care, and/or the treatment of complications. Some knowledge of contact lenses is thus essential for all practitioners.
Figure 4-1 Contact lens. Note the relationship among the parts. (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.)
Contact Lens Glossary
It is important for ophthalmologists to know the vocabulary related to contact lenses. The 3 most important terms are base curve, diameter, and power (Fig 4-1):
Base curve The curvature of the central posterior surface of the lens, which is adjacent to the cornea; it is measured by its radius of curvature (mm) or may be converted to diopters (D) by taking the reciprocal of the radius.
Diameter (chord diameter) The width of the contact lens, which typically varies with the lens material; the diameter of soft contact lenses, for example, ranges from 13 mm to 15 mm, whereas that of rigid gas-permeable (RGP) lenses ranges from 9 mm to 10 mm.
Power Determined by lens shape and calculated indirectly by Snell’s law: D = [n2 – n1]/r; for measurement of the posterior vertex power (as with spectacles), the lens (convex surface facing the observer) can be placed on a lensmeter.
The following terms are also important to know:
Apical zone The steep part of the cornea, generally including its geometric center; usually 3–4 mm in diameter.
Corneal apex The steepest part of the cornea.
Dk The oxygen permeability of a lens material, where D is the diffusion coefficient for oxygen movement in the material and k is the solubility constant of oxygen in the material.
Dk/L A term describing the oxygen transmissibility of the lens; depends on the lens material and the central thickness (L).
Edge lift Description of the peripheral lens and its position in relation to the underlying cornea; adequate edge lift (as documented during fluorescein evaluation by a ring of fluorescein appearing under the lens periphery) prevents edges from digging into the flatter corneal periphery.
Fluorescein pattern The color intensity of fluorescein dye in the tear lens beneath a rigid contact lens. Areas of contact appear black; green reflects clearance between the lens and the cornea.
K reading Keratometry reading; determined by a manual or automated keratometer.
Lenticular contact lens A lens with a central optical zone and a nonoptical peripheral zone known as the carrier; designed to improve lens comfort.
Optic zone The area of the front surface of the contact lens that has the refractive power of the lens.
Peripheral curves Secondary curves just outside the base curve at the edge of a contact lens. They are typically flatter than the base curve to approximate the normal flattening of the peripheral cornea. Typically, junctions between posterior curves (base curve and peripheral curve, for example) are smoothed or “blended” to enhance lens comfort.
Polymethylmethacrylate (PMMA) The first plastic used in the manufacture of contact lenses.
Radiuscope A device that measures radius of curvature, such as the base curve of an RGP lens. Flatter surfaces have larger radii of curvature, and steeper surfaces have smaller radii of curvature.
Sagittal depth or vault A term describing the depth (or vault) of a lens; measuring the distance between the center of the posterior surface (or the center of the base curve) to the plane connecting the edges of the lens determines sagittal depth. In general, if the diameter is held constant, the sagittal depth decreases as the base curve increases. Although sagittal depth is critical for determining good fit, designation of the base curve for a particular lens type typically ensures the appropriate sagittal depth.
Tear lens The optical lens formed by the tear-film layer between the posterior surface of a contact lens and the anterior surface of the cornea. In general, with soft lenses, the tear lens has plano power; with rigid lenses, the power varies, depending on the shape of the lens and the cornea.
Wetting angle The wettability of a lens surface. A low wetting angle means water will spread over the surface, increasing surface wettability, whereas a high wetting angle means that water will bead up, decreasing surface wettability. A lower wetting angle (greater wettability) generally translates into greater lens comfort and better lens optics.