A contact lens exam and fitting is a specialized medical procedure intended to ensure the safe, comfortable, and effective use of contact lenses for vision correction. This process is distinct from a standard comprehensive eye exam, which primarily focuses on assessing general eye health and determining a glasses prescription. The fitting component involves a series of precise measurements and evaluations necessary to select a contact lens that matches the eye’s unique physical characteristics. Because contact lenses are classified as medical devices that sit directly on the eye’s surface, the primary goal is to correct vision while maintaining the long-term health of the delicate corneal tissue.
Why a Specialized Exam is Necessary
A contact lens prescription (CLRx) must contain specific physical parameters that are entirely absent from a typical glasses prescription. Glasses sit approximately 12 millimeters away from the eye, meaning the corrective lens power is calculated for that specific distance. Because a contact lens rests directly on the cornea, the power must be mathematically adjusted to compensate for this difference in distance. This adjustment is particularly significant for stronger prescriptions, as an unadjusted power could lead to blurry vision or eye strain.
The specialized exam serves as a medical safety check, focusing on the health of the anterior surface of the eye before a lens is introduced. An ill-fitting contact lens can physically rub against the cornea or conjunctiva, potentially causing abrasion, irritation, or infection. A poorly fitted lens can also restrict the flow of oxygen to the cornea, leading to neovascularization, where new blood vessels grow into the normally clear cornea. This medical evaluation is essential for safe wear.
Key Measurements Taken During the Fitting
The fitting process begins with an assessment of the ocular surface to determine the precise physical specifications of the contact lens.
Corneal Curvature and Base Curve
Corneal curvature is measured using an instrument called a keratometer, or more comprehensively with a corneal topographer, which creates a detailed map of the cornea’s dome-like surface. These measurements determine the lens’s base curve (BC). The BC defines the curvature of the back surface of the lens and ensures it matches the eye’s shape for a stable and comfortable fit.
Lens Diameter and Tear Film Assessment
The overall size of the lens is determined by measuring the diameter (DIA), which specifies the width from edge to edge and ensures the lens correctly covers the cornea. The eye care professional also assesses the quality and quantity of the tear film. Adequate tear production is necessary for comfortable and healthy contact lens wear. Dry eyes or poor tear film stability may necessitate a specific lens material designed to retain moisture.
Dynamic Fit Evaluation
A slit lamp examination is performed with a diagnostic lens placed on the eye to visually assess the dynamic fit. The professional checks how the lens moves and centers on the eye with each blink. They look for movement that is neither too tight, which can restrict oxygen flow, nor too loose, which can cause irritation and unstable vision. This step ensures the chosen lens material interacts positively with the eye’s surface and tear film.
The Trial Period and Final Prescription
Following the initial clinical measurements and the selection of a specific lens type, the patient is provided with diagnostic or trial lenses to wear. This trial period, often lasting about one week, allows the eye to adjust to the lens material and fit in real-world conditions. The patient is also given instruction on insertion, removal, and proper cleaning procedures, which is paramount for preventing eye infections.
A strict wearing schedule is established, and the patient is required to return for a follow-up evaluation. The eye care professional re-examines the fit using the slit lamp after the lenses have been worn for an extended period. The lens movement, corneal health, comfort, and vision are re-evaluated to ensure the initial parameters are optimal.
Only after this follow-up assessment confirms a safe and effective fit is the final Contact Lens Prescription (CLRx) officially finalized and released. By law, this prescription must be provided to the patient, enabling them to purchase lenses from any vendor. The CLRx typically expires after one year, mandating an annual renewal to ensure continuous monitoring of eye health and fit accuracy.