adaptation, and greater difficulty in fitting. Because of the added time and skill involved in RGP lens fitting, not every contact lens practitioner chooses to offer these lenses.
Schein OD, Glynn RJ, Poggio EC, Seddon JM, Kenyon KR. The relative risk of ulcerative keratitis among users of daily-wear and extended-wear soft contact lenses. A case-control study. Microbial Keratitis Study Group. N Engl J Med. 1989;321(12):773–778.
Contact Lens Fitting
The goals of lens fitting include patient satisfaction (good vision that does not fluctuate with blinking or eye movement) and good fit (the lens is centered and moves slightly with each blink). The details of what constitutes a good fit vary between soft and RGP lenses and involve the “art” of contact lens fitting. For example, a patient who wants contact lenses only for skiing or tennis should probably be fitted with soft contact lenses because of the rapid adaptation that is possible with these lenses. However, a patient with 3 D of astigmatism would probably have the best vision with RGP contact lenses (see Table 4-3).
Soft Contact Lenses
Soft contact lenses are comfortable primarily because the material is soft and the diameter is large, extending beyond the cornea to the sclera. Most manufacturers make a specific style of lens that varies in only 1 parameter, such as a lens that comes in 3 base curves, with all other parameters being the same. The first lens is fit empirically; often, the lens chosen is one that the manufacturer reports “will fit 80% of patients.” Then, on the basis of the patient’s comfort and vision and a slit-lamp evaluation of the fit, the lens may be changed for another base curve and then reevaluated.
A good soft contact lens fit is often described as having a “3-point touch,” which means that the lens touches the surface of the eye at the corneal apex and at the limbus on either side of the cornea (in cross section, the lens would touch the limbus at 2 places). To find a light 3-point touch, one may need to choose a lens with a different sagittal depth. Changing the lens diameter and/or changing the base curve can alter the sagittal depth of a lens. If the base curve is kept constant, as the diameter is increased, the sagittal depth increases and the lens fits more tightly; that is, there is less lens movement. If the diameter is kept constant and the base curve is decreased, the sagittal depth increases, and again, the fit is tightened (Table 4-4).
Table 4-4
In evaluating the soft lens fit, the clinician should observe the lens movement and centration. In a good fit, the lens will move approximately 0.5–1.0 mm with upward gaze or blink, or with gentle pressure on the lower eyelid to move the lens. A tight lens will not move at all, 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 fit is adequate (see Table 4-4).
Once a fit is deemed adequate, an overrefraction is performed to check the contact lens power. The power is changed if necessary, while other parameters are kept the same.
When the initial fitting process is complete, 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 care includes assessment of symptoms and vision and performing a slitlamp examination. The follow-up appointment is usually scheduled for 1 week after the initial fitting (for EW lenses, an additional visit is usually scheduled for 24–48 hours after the first use of the lens); a second office visit is often scheduled for 1–6 months later, depending on the type of lens, the patient’s experience with contact lenses, and the patient’s ocular status.
At the end of the soft contact lens–fitting process, the final lens parameters should be clearly identified (Table 4-5). Also, the medical record should note any signs and symptoms of eye infection, any recommendation for lens wear (eg, DW or EW lenses) and lens care, and any follow-up plans.
Table 4-5
Rigid Gas-Permeable Contact Lenses
RGP lenses, given their small overall diameter, should center over the cornea but move freely with each blink to allow tear exchange. Unlike with soft contact lenses, the parameters of RGP lenses often are not determined by the manufacturer but are individualized for each patient, making RGP lens
fitting more challenging. However, for a normal eye, standard parameters are typically used, and as with soft lenses, a patient is fit from trial lenses. The fit is optimized first; then the vision is optimized by overrefraction (Table 4-6). In the following subsections, some key issues in RGP lens fitting are briefly reviewed; however, a complete coverage of the topic is beyond the scope of this chapter.
Table 4-6
Base curve
Unlike soft contact lenses, an RGP lens maintains its shape when placed on a cornea. As described earlier, a tear layer forms between the cornea and contact lens (in this case, the RGP lens) that varies in shape, depending on the base curve and whether there is corneal astigmatism. The tear layer, usually known as the tear lens, is one of the parameters used to determine the best contact lens fit as well as the required contact lens power.
The type of fit is determined by the relationship between the base curve and the curvature of the cornea (K). For selection of the initial base curve, the following options are available (see Fig 4-4):
Apical alignment (on K). The base curve matches that of the cornea.
Apical clearance (steeper than K). The base curve has a steeper fit (smaller radius of curvature and smaller number in millimeters, and thus more curved) than that of the cornea. Apical bearing (flatter than K). The base curve has a flatter fit (larger radius of curvature and larger number in millimeters, and thus less curved) than that of the cornea.
Position
The most common type of RGP lens fit is the apical alignment fit (see Fig 4-4), in which the upper edge of the lens fits under the upper eyelid (Fig 4-8). This fit allows the lens to move with each blink, enhances tear exchange, and decreases lens sensation because the eyelid does not strike the lens edge with each blink.
Figure 4-8 The most common and most comfortable type of rigid gas-permeable lens fit is apical alignment, in which the
upper edge of the lens fits under the upper eyelid. (Modified with permission from Alb ert DM, Jakob iec FA, eds. Principles and Practice of Ophthalmology. Philadelphia: Saunders; 1994;5:3630. Redrawn b y Christine Gralapp.)
A central or interpalpebral fit is achieved when the lens rests between the upper and lower eyelids. To achieve this fit, the lens is given a steeper fit than K (apical clearance; see Fig 4-4) to minimize lens movement and keep the lens centered over the cornea. Typically, with this type of fit, the diameter of the lens is smaller than with an apical alignment fit, the base curve is steeper than K, and the lens has a thin edge. There is also greater lens sensation because the eyelid strikes the lens with each blink. The resulting sensation discourages normal blinking and often leads to an incomplete blinking pattern and a reduced blink rate. Peripheral corneal staining at the 3-o’clock and 9-o’clock positions may arise from poor wetting. This type of fit is best for patients who have any or all of the following: very large interpalpebral opening, astigmatism greater than approximately 1.75 D, and against-the-rule astigmatism. A flatter -than-K fit (apical bearing) is not typically used with normal eyes.
Other lens parameters
With an RGP lens, the diameter should be chosen so that when the lens moves, it does not ride off the cornea. Typically, the diameter is approximately 2 mm shorter than the corneal diameter. Central thickness and peripheral curves can also be selected, but often the lens laboratory assumes standard parameters. The lens edge is important for enhancing tear exchange and maintaining lens position, as well as for providing comfort. A thicker edge helps maintain the lens position under the upper eyelid in apical alignment fitting; a thin edge maintains centration and comfort for an interpalpebral fit.
Power
The tear lens, as previously noted, is the lens formed by the posterior surface of the RGP lens and the anterior surface of the cornea. Its power is determined by the base curve:
On K. The tear lens has plano power.
Steeper than K. The tear lens has plus power.
Flatter than K. The tear lens has minus power.
The rule for calculating the needed contact lens power from the spectacle sphere power and the base curve of the RGP lens is SAM-FAP (steeper add minus; flatter add plus). For example, if the spectacle prescription is –3.25 –0.75 × 180, the keratometry readings (K readings) are 42.25/43.00 at 90°, and the base curve is slightly flatter than K at 41.75 D (ie, 0.50 D flatter), then according to the FAP rule, the contact lens power should be –3.25 + 0.50 = –2.75 D sphere. The tear lens will correct the corneal astigmatism.
The lens power can also be determined empirically. To do so, place a trial lens of known power on the eye, determine the overrefraction, and then add the lens power and the overrefraction power.
Fit
To evaluate the fit of a contact lens, the clinician considers vision quality, lens movement, and the fluorescein evaluation. Overrefraction determines whether a power change is needed. Vision should
be stable before and immediately after a blink. Stable vision ensures that the lens covers the optical axis, even when it moves with normal blinking.
The peripheral zone of the cornea flattens toward the limbus; therefore, the central vault of a contact lens is determined by its base curve and diameter. Steepening the base curve (ie, decreasing its radius of curvature) obviously increases the vault of a contact lens. However, increasing the diameter of a lens also increases its central vault (ie, sagittal depth) (Fig 4-9).
Figure 4-9 A, Changing the base curve of a contact lens changes the sagittal depth. B, Changing diameter with equal base curve also changes sagittal depth.
Lens position in the alignment fitting should be such that the lens rides high; approximately the upper one-third of the contact lens should be under the upper eyelid (see Fig 4-8). The lens should move as the eyelid moves. Insufficient movement suggests that the lens is too tight. To remedy this situation, the clinician may decrease the sagittal depth by either flattening the base curve (increasing the radius of curvature) or decreasing the lens diameter. Excessive movement, however, suggests that the lens is too loose. To tighten the lens, the clinician may increase its sagittal depth by either steepening the base curve (decreasing the radius of curvature) or increasing the diameter of the lens.
Evaluation of the fluorescein pattern with a cobalt blue light at the slit lamp can help in assessing RGP lens fit (Fig 4-10). If there is apical clearing of the cornea, pooling or a bright green area will be observed; if the RGP lens is touching the cornea, dark areas will be observed.
Figure 4-10 Examples of fluorescein patterns in contact lens fitting. (Courtesy of Perry Rosenthal, MD.)
Once the lens parameters are determined, the information is given to a laboratory, which then makes the lens to these specifications, typically on a lathe. When the lens is received, the major parameters must be checked: base curve (by use of an optic spherometer), lens diameter, and lens power (by use of a lensmeter). Although RGP lens fitting can be more challenging than soft lens fitting, the use of trial lenses and consultation with the laboratory that will make the lens can yield a good fit on most patients with a normal anterior segment (Clinical Example 4-5).
Clinical Example 4-5
A patient with a refraction of –2.00 –2.00 × 180 desires contact lens correction. The keratometry measurement is 44.00 sphere. What is the residual refractive error if this eye is fitted with a spherical RGP contact lens? A spherical soft contact lens? A toric soft contact lens?