Why Do Contact Lenses Drop Down? Decentering of RGP Lenses Explained

Introduction

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The proper fitting of rigid gas permeable (RGP) contact lenses is a meticulous process, influenced by a variety of factors including the lens's base curve, corneal curvature measurements, and the practitioner's fitting technique. One common issue encountered is the decentering of RGP lenses, which can cause the lens to drop down. This article provides insights into why this occurs and what can be done to address it.

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Understanding Base Curvature and Its Impact

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Base Curve Influence on Fit

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The base curve of an RGP lens is a critical factor in ensuring a snug fit on the cornea. A base curve that is too flat can cause the lens to decenter, leading to it dropping down. Base curves are typically measured in fractions of a millimeter, and even a small change in curvature can significantly affect the fit. If the lens sits too low, it may indicate that the base curve is too steep, allowing the lens to become excessively loose, or it could simply be a tight fit.

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Proper Fitting by a Specialist

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The solution to this issue lies in consultation with your fitting eye doctor. RGP lenses are custom-made by specialized labs and are under warranty for a short period, allowing for a seamless replacement process. If a problem with lens fit is identified, the doctor will send the lens back to the lab for a new one to be ordered, which means there is typically no charge to the patient.

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Complexities in RGP Lens Fitting

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Corneal Surface Variability

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The surface of the cornea is not a simple spherical or toroidal shape; it can vary significantly. The tightness of an RGP lens is primarily determined by the saggital depth, which means a larger diameter lens will generally fit tighter than a smaller one with the same base curve. This complexity underscores the importance of a trial fitting to ensure the best possible fit.

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Measurement Challenges

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When determining the appropriate base curve, it's essential to use accurate measurements. Keratometers and topographers are often used to measure the corneal profile, providing crucial information for fitting RGP lenses. However, these instruments measure in millimeters (mm) or diopters, and there can be slight variations in the conversion factors between manufacturers. For instance, the formula for converting the base curve measurement to diopters can differ, but the base curve itself should remain consistent.

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Example Calculation

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Lets consider an example: A measurement of 45.00D at 108 degrees and 45.50D at 18 degrees can be converted to base curves using the Gullstrand model. To convert these diopters to base curvatures, we use the following calculations:

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1.333 - 1/45  0.075m -- conversion for 45.00Dr
1.333 - 1/45.5  0.0731m -- conversion for 45.50D

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Based on these calculations, you have approximately a 1 diopter (DC) against the rule, indicating that you need to adjust your fitting technique to achieve a central and stable fit. If you follow a fitting philosophy that compromises between a tight and loose fit, you may encounter issues like staining or corneal warpage. However, following a more fitting philosophy that aims for a neutral but snug fit will likely yield better results.

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Addressing the Issue

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Given that your lens has a base curve of 7.8mm, it appears to be too flat by 0.3mm. This information can be cross-verified through an overrefraction, which will provide a more accurate power measurement. Alternatively, you can check the power using fluorescein assessment to ensure a snug central fit. Ideally, you should consider ordering a 7.50mm or 7.40mm base curve lens, which would provide a more appropriate fit and likely a more accurate diopter value.

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Conclusion

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Properly fitting RGP lenses requires careful consideration of multiple factors, including corneal curvature measurements and the lens's base curve. If you encounter issues with your lenses dropping down, reaching out to your fitting eye doctor for a professional assessment and necessary adjustments is crucial. By ensuring an accurate and snug fit, you can significantly enhance your wearing experience and maintain optimal ocular health.