Contact lenses are precision medical devices placed directly onto the surface of the eye to correct various vision problems. Because they interact intimately with the delicate structure of the cornea, they are classified and regulated as medical devices by the Food and Drug Administration (FDA). Obtaining contact lenses requires a comprehensive examination and a specific, detailed prescription from a licensed eye care professional. This process ensures the lenses not only correct vision but also fit safely and comfortably onto the wearer’s unique eye shape.
The Necessary Eye Examination and Fitting
A contact lens examination includes all the elements of a standard eye exam but adds several specialized measurements and health checks. Beyond assessing refractive error, the eye care professional must conduct precise corneal measurements to ensure a proper lens fit. This often involves using a keratometer, which measures the curvature of the central cornea, or a corneal topographer, which generates a detailed map of the entire corneal surface.
These measurements are important because an improperly fitted lens can restrict oxygen flow or cause abrasions on the cornea. The examination also utilizes a slit lamp, a high-magnification microscope, to thoroughly check the health of the cornea, conjunctiva, and eyelids for any existing conditions. The tear film is also evaluated, as dry eyes can complicate or prevent successful contact lens wear.
The fitting process involves placing a trial lens on the eye and assessing its movement, centration, and coverage. The professional observes how the lens interacts with the eye’s surface, ensuring it moves slightly with each blink to allow for tear exchange and oxygen circulation. Multiple follow-up visits may be required to confirm the optimal fit and comfort before the final prescription is issued.
Deciphering the Contact Lens Prescription
A contact lens prescription contains specific parameters that differentiate it significantly from a standard glasses prescription. The Power, or Sphere (PWR/SPH), indicates the corrective strength in diopters, designated with a plus (+) for farsightedness and a minus (-) for nearsightedness. This value is often adjusted slightly from a glasses prescription because the lens sits directly on the eye rather than a short distance away.
Two unique measurements are the Base Curve (BC) and the Diameter (DIA), both expressed in millimeters. The Base Curve defines the curvature of the back surface of the lens and is matched to the curvature of the patient’s cornea for a proper fit. The Diameter determines the overall size of the lens, ensuring it correctly covers the cornea and rests on the white part of the eye.
For individuals with astigmatism, two additional parameters are required: Cylinder (CYL) and Axis (AXIS). The Cylinder specifies the additional power needed to correct the corneal irregularity, while the Axis is an angle between 1 and 180 degrees that indicates the orientation of that corrective power. Multifocal lenses for presbyopia also include an Addition (ADD) power to assist with near vision.
Legal Requirements for Purchase and Dispensing
Contact lenses can only be dispensed with a valid prescription, a requirement that protects the public from the health risks of ill-fitting lenses. Federal law mandates that eye care providers must automatically give the patient a copy of their contact lens prescription after the fitting is complete. This ensures the prescription is portable, allowing consumers to purchase lenses from any seller.
When a consumer orders lenses from a retailer other than their prescriber, the seller must verify the prescription’s accuracy before dispensing the product. The seller submits the patient’s prescription information to the prescriber for verification. The prescriber then has an eight-business-hour window to respond to this request.
If the prescriber does not respond within the mandated time frame, the prescription is considered automatically verified, and the seller is legally permitted to ship the lenses. This process balances consumer choice with the necessity of ensuring that only the correct medical device is dispensed.
Prescription Expiration and Renewal
Contact lens prescriptions typically expire after one to two years, depending on state regulations and the eye care professional’s judgment. This expiration necessitates regular professional evaluation of eye health and vision, as the lens rests directly on the cornea and requires regular monitoring for subtle complications.
An outdated prescription may mean the lens no longer fits the current shape of the eye, potentially causing hypoxia (oxygen deprivation) or contact lens-related conditions like bacterial keratitis. Regular exams also check for conditions like giant papillary conjunctivitis. Renewing the prescription confirms the lens parameters are still accurate and ensures the continued health and integrity of the eye’s surface.