When your patient has decided to go with hard contact lenses, one of the best options is the prescribing of rigid gas-permeable lenses. Prescribing RGP lenses can be cheaper for the patient in the long run as it requires less frequent replacement and is easier to clean. Here's how to assess and compute the details for ordering an RGP contact lens.
Tools needed: 1. Fluorescein strips 2. Slitlamp with Cobalt-blue filter 3. RGP Trial Lens Set Parameters needed: 1. Spectacle refraction 2. K-readings (corneal curvature) 3. Horizontal visible iris diameter
If the patient is wearing soft contact lenses upon entering the clinic, instruct to have them removed and rest for 15 - 30 minutes -- this is to ensure that the corneal curvature is not affected while performing the fitting assessment.
Step 1: Get the initial base curve
To compute for the initial base curve (IBC), you'll need to determine the corneal toricity via the K-readings.
There are two (2) cases:
Case 1: Toricity of the cornea is 0.2 mm or less Get the average K-readings and add 0.1 mm to get the IBC of the trial lens.
K-readings (OD): 7.8/7.8
IBC = (Ave K) + 0.1 mm = 7.8 + 0.1 = 7.9 mm
K-readings (OD): 7.8/8.0
IBC = (Ave K) + 0.1 mm
= (15.8 / 2) + 0.1 mm
= 7.9 + 0.1 mm
= 8.0 mm
Case 2: Toricity of the cornea is more than 0.2 mm Subtract 0.1 mm to the flatter K-reading NOTE! the higher the K-reading, the flatter the radius of curvature
Example 1 K-readings (OD): 7.8/8.1
IBC = 8.1 - 0.1 mm
= 8.0 mm
For every fitting of a trial lens, allow the lens to settle onto the cornea for 15 - 20 minutes.
It is ideal that the contact lens exhibits the following:
- located centrally (or slightly superior)
- has 1-2 mm of vertical movement upon blinking
- fluorescein pattern has 0.5mm edge clearance
Reference Image from Left to right: (A) optimum fit, (B) flat/loose fit, (C) steep/tight fit
If the contact lens shows peripheral pooling, the contact lens fit is flat and loose.
If the contact lens shows a central pooling, the contact lens fit is steep and tight.
Step 2: Determine the diameter of the Lens
Lens Diameter = HVID - 2
Step 3: Determine power of the lens
An RGP trial lens usually has a lens power of -3.00 DS, you can over-refract this by factoring in the spectacle refraction.
Spectacle Rx: -4.00 DS Trial Lens: -3.00 DS
Over-refraction (on phoropter head)
= -4.00 DS - (-3.00 DS)
= -1.00 DS
Step 4: Order lens
Spectacle Rx: -4.00 DS
Toricity: 0.1 mm HVID = 12.0 mm
Lens to Order:
Rx: -3.75 DS (factor in Effective Diameter)
Tint: Gray Base Curve: 7.95 mm
Lens Diameter = 10.0 mm
- Initial base curve doesn't necessarily mean base curve of contact lens, you need to observe the fluorescein pattern prior to over-refraction
- If the spectacle refraction is -4.00 DS and above, factor in the effective power.
- Use gRay tint for Right eye;
bLue tint for Left eye