Mark Andre, FAAO, and Patrick Caroline, FAAO Portland, Ore.,

Traditionally, fitting rigid gas permeable contact lenses entails on-eye observation of diagnostic lenses. Seeing a lens’s actual, on-eye fitting characteristics can influence the choice of lens parameters.

Unfortunately, it’s often difficult to make this assessment accurately due to initial reflex tearing and blepharospasm, which lenses often cause in an unadapted wearer. The use of topical anesthetics can overcome this foreign body experience.
Another problem is that the selected diagnostic lens is rarely the right power for the patient, so visual acuity often isn’t optimal during the 20-30 minutes of in-office adaptation. When combined with initial lens awareness, patients tend to find the whole process disconcerting.

Additional shortcomings of the diagnostic fitting technique include:

 • the excessive chair time necessary for the initial fitting procedure;

 • the high number of diagnostic lenses re-quired to accommodate a wide range of base curves and diameters; and
 



• the time, effort and space required for the maintenance and storage of diagnostic lenses.

These drawbacks led us to adopt a more empirical approach to the fitting of RGP lenses, which we’ve found to offer an exceptionally high first-fit success rate, while being far more pleasant for the patient.

Empirical Fitting
The ultimate success of a RGP lens lies in the delicate balance between lens design and corneal curvature. On an astigmatic cornea, any RGP lens creates a number of bearing and clearance points, which dictate lens movement, position and comfort. Appropriately choosing the lens parameters, especially overall lens diameter and base curve radius is, therefore, key to success.
Empirical fitting may be broken down into a simple three-step process.
 
• Lens diameter. This is the parameter that practitioners most frequently overlook when fitting rigid lenses. The optimal lens diameter should be based on the size of the cornea on which it rests. Corneal diameters of 11.7 mm or larger require a larger lens diameter (9.5 mm), and smaller corneal diameters of 11.3 mm or less need a smaller lens diameter (8.5 mm).

A variety of techniques measure corneal diameter, such as a slit lamp measuring reticule or a handheld millimeter ruler. When selecting the appropriate lens diameter, a simple rule of thumb is that it should be 2.5-mm smaller than the horizontal visible iris diameter (See Table 1).

 • Base curve selection. The primary curve on the posterior surface of a RGP lens is the base curve. It alone will determine the amount of lens bearing and clearance in the area of the mid-peripheral cornea. We’ve found that optimal lens performance requires a base curve that provides: an area of lens bearing or a “contact point” mid-peripherally along the horizontal meridian approximately 3.0-4.0 mm from the center of the cornea; and unobstructed lens movement along the vertical meridian.


The areas of lens bearing at 3 and 9 o’clock “lock” the lens into position along the horizontal meridian and restrict lateral movement, which allows the lens to remain centered on the cornea. Additionally, the same base curve must permit unobstructed movement of the lens along the vertical meridian with the blink.



It’s easy to see, then, why individuals with 1.00 to 2.00 D of with-the-rule astigmatism are often ideal candidates for spherical RGP lenses.

Step 1. Measure the central corneal curvature and identify the flat K.

 Example: K’s = 43.00 @ 180/44.75 @ 90

Flat K = 43.00 D

Step 2. Calculate the amount of corneal astigmatism, in other words, the difference between the flat and steep keratometric readings.

Example: K’s = 43.00 @ 180/44.75 @ 90

Corneal astigmatism = 1.75 D

Step 3. Select the base curve radius based on the amount of corneal cylinder and the Flat K (See Table 2).

Example: HVID = 11.5 mm

Lens diameter = 9.0 mm

Flat K = 43.00 D

Corneal astigmatism = 1.75 D

Base curve = 0.50-D steeper than K = 43.50 D

 • Power calculation. Lens power may be calculated by an empirical formula based on the post tear lens power and the manifest refraction.

Step 1. Perform a spectacle refraction and place the prescription in minus cylinder form.

Example: Spec Rx plus cylinder = -7.75 + 1.75 x 90

Spec Rx minus cylinder = -6.00 – 1.75 x 180

Step 2. If the spherical component of the minus cylinder Rx is greater than +/-4.00 D, correct for vertex distance.

Example: -6.00 D spectacle power = -5.50 D at the corneal plane

Step 3. Determine the tear lens power by calculating the difference between the base curve and the flat K.

Example: Base curve = 43.50 D

Flat K = 43.00 D

Tear Lens Power = 0.50 D

Step 4. Add the spherical component of the vertex-corrected spectacle Rx to the tear lens power to obtain the final contact lens prescription. If the base curve of the lens is fitted steeper than flat K, increase the minus power of the contact lens by the same amount as the tear lens power (SAM or Steeper Add Minus Rule). If the base curve is fitted flatter than flat K, decrease the minus power of the lens by the same amount as the tear lens power (FAP or Flatter Add Plus Rule).

Example: Vertexed Spec Rx = -5.50 D

Base Curve Radius = 43.50 D

Flat K = 43.00 D

Tear Lens Power = 0.50 D

Final Lens Power (SAM) = -6.00 D

Follow-up
The ideal base curve relationship on a cornea with with-the-rule corneal astigmatism exhibits alignment or slight apical clearance across the central cornea and along the flatter horizontal meridian. We most often accomplish this fitting relationship with a base curve radius that’s fitted on flat K or +/- 0.50 D.

The selected base curve that provided central alignment will create an area of lens bearing mid-peripherally at 3 and 9 o’clock, due to the natural flattening of the mid-peripheral cornea. The same base curve radius will result in a radically different fitting relationship along the steeper vertical meridian. It should exhibit a pooling of fluorescein at 12 and 6 o’clock, which signals unobstructed lens movement along the vertical meridian.

The peripheral lens design should demonstrate 360 degrees of peripheral clearance. The sole function of the flatter peripheral curves is to provide clearance as the lens moves across the flatter portions of the cornea with lateral gaze and blinking.

When fitting RGP lenses, it’s helpful to remember two rules as they apply to lens position and movement.

 • Rule 1. A spherical lens that aligns with or vaults the central cornea will have the greatest bearing upon the flattest portion of the mid-peripheral cornea. In other words, the lens will be tightest where the cornea is flattest.

 • Rule 2. A RGP lens will always move in the direction of least mechanical resistance (e.g., along the steeper meridian). More simply, the lens will be loosest where the cornea is steepest.

Patients with > 2.50 D of astigmatism frequently exhibit excessive lens clearance along the steepest corneal meridian. The result can be significant on-eye flexing of the lens and lens awareness. Attempting to correct the edge clearance with a steeper base curve radius can increase lens awareness due to too much impingement along the flatter corneal meridian. Usually, these patients need a back or bi-toric lens design to achieve the desired lens/cornea relationship.

Low to Moderate Against-the-Rule Astigmatism

The best contact lens fittings create a bearing zone mid-peripherally along the horizontal meridian and allow the lens to move without obstruction along the vertical meridian. In the case of against-the-rule astigmatism, a lens fitted with central alignment (e.g., on flat K) will result in an area of maximal lens bearing at 12 and 6 o’clock, which translates into obstructed lens movement along the flatter vertical meridian. The same base curve relationship along the steeper horizontal meridian, meanwhile, doesn’t permit an adequate bearing relationship at 3 and 9 o’clock. The result is rocking of the lens along the horizontal meridian, as well as nasal or temporal lens decentration. Some patients with minimal degrees of against-the-rule astigmatism can be successfully fitted with RGP lenses that have a base curve radius flat enough to permit some vertical lens movement. The strength of the upper lid interaction may occasionally override the lens’s tendency for displacement and maintain the necessary horizontal lens position. Generally, however, the anatomical structure of the low to moderate against-the-rule cornea (2.00 D or less) isn’t amenable to the fitting of a RGP lens. Most of these patients obtain better results with soft toric contact lenses.

Pros and Cons
A major advantage of empirical over diagnostic lens fitting is that, when the lenses are first placed on the patient’s eyes, the fact that he can see clearly without glasses overshadows any lens awareness. This can play a significant role in the patient’s physical and psychological response to the lenses.

Also, our high first-fit success rate has given us the courage  to dispense the initial lenses for one to two weeks of adaptation prior to a critical on-eye assessment. We now feel that greater error is possible with a diagnostic fitting to determine the appropriate lens parameters for an unadapted lens wearer. Still, a detailed diagnostic fitting is best in cases of against-the-rule astigmatism or some type of underlying pathology (See Low to Moderate Against-the-Rule Astigmatism). 

Mr. Andre is director of contact lens services at Oregon Health Sciences University in Portland. Mr. Caroline is an assistant professor of ophthalmology at Oregon Health Sciences University