Deciphering the Contact Lens Prescription
BY MARY JAMESON, BHS, COA, NCLC, CPOT
What do the numbers on a contact lens prescription mean? How do they relate to a patient's eyes? How important are they? With all of the controversy surrounding the issue of releasing contact lens prescriptions, we should understand what every detail of the prescription means.
Measuring Base Curve
The back of a contact lens features a series of curves. These curves help the contact lens fit the contour of a cornea. The main back surface curve of a lens is the base curve. It represents the central radius of curvature of the back of the lens. The base curve corresponds to the keratometry reading, the measurement of a patient's corneal curvature.
We typically record keratometry measurements in diopters of power and typically measure the base curve reading in millimeters of radius. You can use a conversion chart to transition from diopters to millimeters of radius.
When you select a trial lens, you can choose a base curve that is on K (meaning it matches the corneal curvature reading), steeper than K (meaning the base curve is smaller than the corneal curvature reading) or flatter than K (meaning that the base curve is larger or longer than the corneal curvature reading). An "on K fit" is not always the ideal. You can compare this to shopping for shoes or clothing -- if you are a certain size in one brand, you may not be the same size in another. The same is true for contact lenses. Patients need to try contact lenses on to make sure the fit is the best possible.
The power of a lens represents the prescription that will neutralize the power of the patient's eye. This number is not always the same for a contact lens fitting as it is for a spectacle prescription. You must take the vertex distance (the distance from the surface of the cornea to the back of the spectacle lens) into consideration because the contact lens fits onto the cornea. You can use an effectivity chart to make the appropriate power adjustments. Also, the over-refraction during the lens fit will help finalize the patient's prescription.
When you fit gas permeable lenses, you must consider the relationship of the base curve to the cornea. When you fit a lens steeper or flatter than the keratometry reading, a lacrimal lens forms between the cornea and the back curvature of the lens. If you fit a lens steeper than the K reading, then you must add minus power to the prescription. Likewise, if you fit a lens flatter than the K reading, then you must add plus power to the prescription. Use the acronym SAMFAP (steeper add minus; flatter add plus) to help remember this calculation.
Contact lens diameter influences the fit of the lens. Diameter is less variable for soft lenses. If you reduce the diameter of the lens but the base curve remains the same, then the lens will show more movement. If you increase the diameter of the lens but the base curve remains the same, the lens will show minimal amounts of movement.
Contact lens prescriptions should be filled just like any other valid medical prescription. Make "No Substitutions" the norm for contact lens prescriptions. Any variance from the prescribed contact lenses can result in problems for your patients.
Ms. Jameson is laboratory supervisor for the Department of Clinical Sciences at the Pennsylvania College of Optometry and is a past chair of the AOA Paraoptometric Section.