EyeWitness Fall 2010 - 31

Edge good

Edge rolled out

Edge sharp

weight and centration of the lens, the patient’s comfort and blink pattern, and its resistance to on-eye flexure. Center thickness should be minimized whenever possible, as this will also result in reduced edge thickness. Heavier lenses tend to ride low, and, combined with thick edges, often cause patients to inhibit their blink, resulting in three-andnine o’clock staining. Greater center thicknesses or stiffer materials may be required by high astigmats to reduce lens flexure and warpage. Edge design: Lens edges should be smooth, well-tapered, and rolled slightly inward. The apex must be well-centered. Lenses with square or rolled out edges will position erratically or may even be expelled from the eye, and cause excessive lid/edge interaction and excessive lid awareness as well as three-and-nine o’clock staining. Lenses with sharp edges will create a cookie-cutter-like effect, causing discomfort with each blink, on eye movements, or on recentering, as well as a burning sensation.

EMPIRICAL VS. DIAGNOSTIC FITTING
(it takes a lens to fit a lens) Empirical fitting The dictionary defines “empirical” as “derived from or guided by experience or experiment.” In contact lens practice, however, “empirical” refers to lenses that are ordered by calling in keratometry readings and manifest refraction to a laboratory and hoping that the lens they supply will fit the patient well and provide good visual acuity. There are many unknowns when GP lenses are ordered empirically. We are unable to evaluate the patient’s tolerance of a rigid material; unsure of the influence of the patient’s lids and peripheral cornea on the positioning of the lens; we do not know if the lens ordered will decenter and, if so, how much and in which direction; we are unable to determine whether there will be any residual astigmatism; and we are unable to asses whether flare will be a problem as the pupil dilates in dim illumination. Empirical fitting results in more visits for the patient, more chair time for the fitter, and more staff time due to increased lens exchanges, paperwork, and telephone calls. It may also lead to the loss of patient confidence when the ordered lenses do not fit as anticipated.
EyEWitnEss FALL 2010 |

Diagnostic fitting Many GP laboratories are happy to provide diagnostic fitting sets at little or no cost to the fitter. There is often a small initial charge for the set, with a credit for the set issued after the first few fits or with Edge square lenses supplied for the first few patients at no charge. A -3.00D fitting set will work for the majority of low and moderate myopes. As the GP practice grows, a plano set can be added for patients whose refractive error is predominantly astigmatic, then a higher minus set and a low plus set. Base curves should range from 7.00mm to 8.40mm and diameter should be related to the base curve, with smaller diameters (9.0mm-9.2mm) up to 7.50mm; 9.5mm diameters between 7.50mm and 8.0mm; and 9.8mm diameters for flatter corneas. Make sure that the patient’s spectacle prescription is in minus cylinder before selecting the initial trial lens. Compensate for vertex distance if the patient’s prescription is greater than ±4.00D and select the desired base curve in a power that is as close as possible to the patient’s vertexcorrected spectacle prescription. A power that is much different from that needed by the patient may cause the lens to not orient as anticipated due to differences in mass, weight, and center of gravity. A plus lens is thick centrally and has forward center of gravity, which creates a tendency for the lens to ride low. A minus lens, with its thin center and concave profile will generally center better. One should never use a diagnostic minus lens when fitting a hyperope, nor a plus diagnostic when fitting a myope. If the overrefraction is greater than ±4.00D with the closest possible diagnostic lens, make sure to compensate for vertex distance again when calculating the final lens prescription. The diagnostic lens should be of the same material and similar in thickness to the lens being ordered. It should be ordered from the laboratory that supplied the trial set so the secondary curves will be similar on the trial lens and the ordered lens.

FITTING PHILOSOPHIES
Alignment fitting Lenses fit on alignment are designed to ride slightly high and parallel to the corneal curvature. They are fit with lid attachment: ¼ to 1⁄3 of the lens should tuck in under the upper lid and the lens should move downward with each blink as if it were attached to the underside of the upper lid. This eliminates problems with lid-bumping; fosters better, more normal blinking; maximizes comfort, and results in less three-and-nine o’clock staining. The fluorescein pattern should exhibit a thin, even, edge-to-edge tear
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EyeWitness Fall 2010

Table of Contents for the Digital Edition of EyeWitness Fall 2010

EyeWitness Fall 2010 - C1
EyeWitness Fall 2010 - C2
EyeWitness Fall 2010 - 1
EyeWitness Fall 2010 - 2
EyeWitness Fall 2010 - 3
EyeWitness Fall 2010 - 4
EyeWitness Fall 2010 - 5
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EyeWitness Fall 2010 - C3
EyeWitness Fall 2010 - C4
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