Starting the GP Fitting Process
Selecting the initial GP contact lens is a critical decision that sets the foundation for the entire fitting process. The goal is to choose a lens with parameters that will produce an acceptable starting fit, which can then be refined based on the on-eye evaluation. Two primary approaches exist: diagnostic fitting (using trial lenses) and empirical fitting (ordering based on measurements alone).
Diagnostic Fitting
Diagnostic fitting involves placing a trial lens from a diagnostic set directly on the patient's eye and evaluating the fit in real time.
The Diagnostic Set
A diagnostic (trial) lens set contains multiple lenses with different base curves and sometimes different diameters and powers. These lenses are designed to cover a range of corneal curvatures, allowing you to find the best starting fit through direct observation.
Procedure
- Review the patient's keratometry (K) readings to identify the starting base curve
- Select a diagnostic lens with a base curve that matches or is slightly steeper than the flat K reading
- Insert the diagnostic lens and allow it to settle for several minutes
- Instill sodium fluorescein and evaluate the fluorescein pattern under cobalt blue illumination
- Assess movement, centration, and overall fit
- If the fit is not optimal, try the next steeper or flatter diagnostic lens
- Once an acceptable fit is found, perform an over-refraction to determine the final power
- Order the final lens based on the diagnostic lens parameters plus over-refraction results
Starting Point: On-K Fitting
The most common starting approach is an on-K fit, where the diagnostic lens base curve matches the patient's flat K reading:
- If flat K = 43.50 D (7.76mm), start with a diagnostic lens of BC 7.75mm or 7.80mm
- Evaluate the fluorescein pattern and adjust steeper or flatter as needed
- An on-K lens typically shows alignment fitting in the flat meridian with slight touch in the steep meridian
Advantages of Diagnostic Fitting
- Direct visualization of the lens-cornea relationship
- Ability to compare multiple fits in a single visit
- Over-refraction provides an accurate final power
- The patient can experience the lens before ordering
- Reduces the likelihood of needing remakes
Empirical Fitting
Empirical fitting involves ordering the lens based solely on clinical measurements without using a diagnostic lens on the eye.
Procedure
- Obtain keratometry readings, refraction, HVID, and other measurements
- Apply fitting rules or formulas to calculate the base curve, diameter, power, and peripheral curves
- Order the lens from the laboratory based on these calculated values
- When the lens arrives, place it on the patient's eye and evaluate the fit
- Adjust parameters as needed based on the on-eye evaluation
Common Empirical Rules
- Base curve: On-K or 0.50 D steeper than flat K
- Overall diameter: Typically 9.2-9.6mm for average corneas
- Optic zone diameter: OAD minus 1.6 to 2.0mm
- Power: Spectacle Rx converted for vertex distance, adjusted for tear lens power created by the base curve selection
Advantages of Empirical Fitting
- Does not require an expensive diagnostic lens set
- The patient receives a lens with their actual prescription power (diagnostic sets usually have plano or limited powers)
- Faster initial fitting visit (no diagnostic lens insertion and evaluation)
- May be the only option for unusual parameters not available in diagnostic sets
Diagnostic vs. Empirical: When to Use Each
| Factor | Diagnostic | Empirical |
|---|---|---|
| Standard corneas | Either approach works | Either approach works |
| Irregular corneas | Preferred (direct observation) | Not recommended |
| First-time fitters | Preferred (hands-on learning) | Requires more experience |
| New practitioners | Preferred (builds pattern recognition) | Less suitable |
| Specialty designs | Often not available | May be only option |
Key Takeaways
- Diagnostic fitting uses trial lenses placed on the eye for real-time evaluation; empirical fitting orders based on measurements alone
- The standard starting point is an on-K fit (base curve matching the flat K reading)
- Diagnostic fitting allows direct fluorescein pattern assessment and over-refraction
- Empirical fitting requires accounting for tear lens power when calculating ordered lens power
- Diagnostic fitting is preferred for irregular corneas, new practitioners, and complex cases
- Both approaches require follow-up evaluation and potential parameter adjustment