Contact lenses offer a convenient alternative to eyeglasses, providing clear vision and an unobstructed field of view. A crucial measurement for proper fit and comfort is the “Base Curve” (BC). An accurate base curve is fundamental for eye health when wearing contact lenses.
What Base Curve Means
The Base Curve (BC) refers to the curvature of the back surface of a contact lens. This “fitting curve” corresponds with the unique curvature of the wearer’s cornea, the clear, outermost layer at the front of the eye. This measurement, expressed in millimeters (mm), indicates the lens’s steepness or flatness. A smaller BC number, such as 8.2 mm, signifies a steeper curve, while a larger number, like 9.0 mm, indicates a flatter curve.
Contact lens manufacturers offer various BC values to accommodate diverse eye shapes. Soft disposable contact lenses typically range from 8.3 mm to 9.0 mm, with 8.6 mm being a common average. There is no single “normal” base curve, as individual corneal shapes vary significantly. The proper base curve ensures the lens sits correctly on the eye, promoting stability and comfort.
Why Base Curve is Critical
The precise matching of a contact lens’s Base Curve to the eye’s corneal curvature holds significance for comfort and long-term eye health. An improperly fitted lens, even with a slight mismatch, can lead to uncomfortable symptoms and potentially serious eye complications. If a contact lens is too flat, it may move excessively on the eye, causing discomfort, irritation, and blurred vision. This excessive movement can also lead to corneal abrasions (scratches on the surface of the eye) and a persistent foreign body sensation.
Conversely, a contact lens with a base curve that is too steep will fit too tightly on the eye. This tight fit can restrict the natural flow of tears beneath the lens, reducing the oxygen supply to the cornea. Reduced oxygenation, known as corneal hypoxia, can result in corneal edema (swelling of the cornea), redness, and discomfort. Prolonged oxygen deprivation can also trigger corneal neovascularization, where new, abnormal blood vessels grow into the cornea, which can impair vision and potentially lead to permanent damage. An ill-fitting lens can also cause irritation, dryness, and an increased risk of eye infections.
Finding Your Correct Base Curve
Determining the correct Base Curve for contact lenses requires a comprehensive eye examination by a qualified eye care professional. This process involves precise measurements of the eye’s unique shape and curvature. A primary tool for this measurement is a keratometer or corneal topographer, which measures the central curvature of the cornea. These instruments provide detailed readings that help the eye care professional select an appropriate starting Base Curve.
Following these initial measurements, trial lenses are used to assess the fit and movement of the lens on the eye. The eye care professional observes how the lens sits, moves with blinks, and centers on the cornea using a specialized microscope called a slit lamp. The patient’s comfort and clarity of vision with the trial lenses are also evaluated. Patients may wear these trial lenses for a short period to allow the eye’s tear film to stabilize and to confirm comfort before a final prescription is determined. This fitting process ensures the prescribed contact lenses provide optimal comfort, clear vision, and support long-term eye health, emphasizing that self-diagnosis or purchasing lenses without professional guidance is not advisable due to individual variability.