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
The on-K approach means matching the base curve to the flat K reading. This is the most common starting point. On an astigmatic cornea, an on-K lens will show alignment in the flat meridian and slight clearance in the steep meridian (because the lens is flatter than the steep meridian). This creates a tear lens that partially corrects the corneal astigmatism.
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
When ordering empirically, account for the tear lens power. If you fit steeper than K, the tear lens is plus-powered (the gap is thicker centrally, acting like a plus lens). If you fit flatter than K, the tear lens is minus-powered. Every 0.05mm steeper than K adds approximately +0.25 D of tear lens power, which must be compensated in the ordered lens power.
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 |
Ordering empirically for patients with irregular corneas, high astigmatism, or unusual corneal shapes. These cases benefit greatly from diagnostic fitting because the lens-cornea interaction is less predictable. Even experienced fitters may not accurately predict how the lens will sit on an irregular surface without direct observation.
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