Contact Lens Fitting
The goals of lens fitting are good vision that does not fluctuate with blinking or eye movement, comfort for the entire day, and low risk of complications. Terms used to describe lens geometry, illustrated in Figure 5-4, include the following: The apical zone of the cornea (corneal apex) is the steepest part of the cornea, normally including its geometric center. The base curve is the curvature of the central posterior surface of the lens, which is adjacent to the cornea, described by its radius of curvature. The diameter (chord diameter) is the width of the contact lens from edge to edge. The optical zone is the central area of the lens; curvature of its front surface is chosen to achieve the desired power of the lens. Peripheral or secondary curves of the posterior lens surface farther from its center are designed to follow the shape of the cornea, which is normally flatter toward its periphery. Sagittal depth or vault is the distance between the center of the posterior surface to the plane of the edges of the lens.
Figure 5-4 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 by Christine Gralapp.)
Contact lenses are described by their rear vertex power, which can be checked for rigid lenses by placing the back of the lens on the nose cone of a lensmeter (done less frequently for soft lenses, similarly or in a liquid chamber, correcting for index of refraction). The required contact lens power may differ from the power of the appropriate spectacle correction, to adjust for the difference in position of the correcting lens. This disparity is known as the correction for the effectivity of lenses, and is discussed in Chapter 1. The magnitude of the correction depends on the strength of the lenses, and can be readily obtained from standard tables (Table 5-4).
Soft Contact Lenses
Soft contact lenses are comfortable, thanks to the thin edges encountered by the eyelids. The lenses extend beyond the cornea to the conjunctiva overlying the sclera. A good soft contact lens fit is often described as having a “light 3-point touch,” the lens touching the surface of the eye at the corneal apex and at the limbus on either side of the cornea. A soft or rigid lens can be made to fit tighter by either choosing a smaller radius to steepen the base curve or by increasing the lens chord diameter without changing the radius. Either way, we are increasing the “sagittal depth,” the height of the lens’s rear surface, if it were placed edge-down on a table (Table 5-5 and Fig 5-5).
Evaluating the soft lens fit, the clinician should observe the lens movement and centration after the lens has been in the eye for a while, until the fit has stabilized. In a good fit, the lens will move about 1 mm with upward gaze or blink, or with gentle pressure on the lower eyelid. A tight lens resists movement, and a loose lens will move too much. By evaluating a patient’s vision and comfort, slit-lamp findings (eg, lens movement, lens edge, limbal injection), and keratometry mires, the clinician can determine whether the lens fits well (see Table 5-5).
Table 5-4 “Vertexing” Chart for Converting From Spectacle Prescription to Expected Contact Lens Prescription
Table 5-5 Basic Soft Contact Lens Fitting
Figure 5-5 Changing the sagittal depth. A, Changing the base curve of a contact lens changes the sagittal depth. B, Changing the diameter with equal base curve also changes sagittal depth.
Once a fit is deemed adequate, an overrefraction is performed to check whether the power needs to be adjusted.
When the initial fitting process is complete, the final lens parameters should be clearly identified (Table 5-6). The clinician should teach the patient how to insert and remove the contact lenses, how to care for them, and how to recognize the signs and symptoms of eye emergencies. Follow-up appointments are scheduled depending on the lens and patient.
Table 5-6 Soft Contact Lens Parameters
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.