RGP Parameter Changes How Much Change Is Significant?
BY LORETTA B. SZCZOTKA, OD, MS, FAAO
The skill in RGP fitting comes in adjusting, improving or correcting the fit of a sub-optimal lens. The fine tuning process may come into play during an initial RGP fit, problem solving during the patient's follow-up or correcting the existing RGP fit on a new patient to your practice.
If the lens is too steep, flatten the base curve or decrease the optical zone size. If the lens is flat, steepen the base curve or increase the optical zone size. If the lens is flexing on a toric cornea, increase the center thickness or refit into a stiffer material. Easy, right? The hard part is knowing how much to change a specific parameter to achieve clinical significance.
How Much Is Enough?
For average corneas, the amount of parameter change needed to achieve a clinical effect is fairly consistent and predictable. These relationships do not hold true for extremely steep corneas (where smaller curvature and size changes will create a clinical effect) or flat corneas (where larger curvature and size changes are typically required for a clinical effect). Many of us err on the conservative side and reorder lenses with such a mild parameter change that a clinical effect is not noticed. Table 1 shows my recommendations for minimal changes required when refitting an RGP lens to achieve a significantly different fluorescein pattern or physical fitting relationship on average corneas. Figure 1 shows that optical zone changes do not appear clinically significant until a change of 0.3mm is reached.
|Figure 1a. Lens with 7.6 mm optical zone.||Figure 1b. Lens with 7.7mm optical zone.||Figure 1c. Lens with 7.8mm optical zone.||Figure 1d. Lens with 7.9mm optical zone.|
If the Lens Fits...
A word of caution here. If you desire to change a parameter but not the fluorescein pattern or the physical fit, then certain alternate rules apply. For example, to maintain a flourescein pattern when changing the optical zone size on average corneas, flatten the base curve by 0.25D (approximately 0.05mm) for every 0.5mm increase in optical zone size. To maintain the same power effect, for every 0.1mm in base curve change, change the power by 0.50D.
TABLE 1Clinically Significant Parameter Changes
|PARAMETER||AMOUNT OF CHANGE FOR CLINICAL SIGNIFICANCE|
|Base curve||0.50D (approx. 0.1mm)|
|Optical zone diameter||0.3mm|
|Secondary curve radius||0.3mm|
|Peripheral curve radius||0.5mm|
(for high Dk RGPs)
Dr. Szczotka is an assistant professor at Case Western Reserve University Dept. of Ophthalmology and Director of the Contact Lens Service at University Hospitals of Cleveland.