The amount of space between the lens and the cornea is typically referred to as sag. Sagittal depth actually refers to the entire contact lens from the center of the lens to the plane that the edges circumscribe. A steeper base curve typically results in increased sagittal depth where a flatter base curve reduces it.
Simple Rule for estimating lens base curve on a PLUS POWER lens
- For plus power use the spherical equivalent (SE) and add 4.00 diopters to that. For example, if you have an Rx of + 2.00 sphere, the base curve for the lens will be approximately 6.00.
- Rx +2.00Sph -> [+2.00 +4.00D] = 6.00BC.
Technique 1: Corneal Topographer – use the biometric ruler within the corneal topography software program to measure your patient's horizontal visible iris diameter (HVID). It may be necessary to measure at a 45 degree oblique angle as the topographer may cut off part of the cornea in the horizontal plane.
What happens if the base curve size is wrong? If you realize that you've gotten contact lenses with an incorrect base curve, you may feel it. Lenses with the wrong base curve feel like they won't settle right on your eyes. If the lens is too curved for example, it may just slide around in your eye.
HVID stands for Horizontal Visible Iris Diameter (optometry measurement) Suggest new definition.
The 'BC' or base curve measurement indicates the back curvature of your contact lens, in millimetres. It also means your contact lenses will be less likely to fall out. A lower BC number means a steeper curve of your cornea (the clear front surface of your eye). A higher BC number means your cornea is flatter.
Typical values for a contact lens are from 8.0 to 10.0 mm. The base curve is the radius of the sphere of the back of the lens that the prescription describes (the lower the number, the steeper the curve of the cornea and the lens, the higher the number, the flatter the curve of the cornea and the lens).
Accurate HVID can be measured in a few different ways: 1) manually estimate with hand-held rulers (Figure 1); 2) using the caliper tool on your topographer (Figure 2); and 3) using a contact lens of known diameter (Figure 3).
Lag refers to the resistance of the lens to move with the eye on excursions away from primary gaze. Although some refer to lens sag rather than lag, this describes the distortion or geometry of the shape of the lens, not its movement, although the two are related.
Contact Lens Discomfort
- Eyes stinging, burning, itching (irritation), or other eye pain.
- Comfort is less than when lens was first placed on eye.
- Abnormal feeling of something in the eye (foreign body, scratched area)
- Excessive watering (tearing) of the eyes.
- Unusual eye secretions.
- Redness of the eyes.
Your eye doctor may hold a pupil gauge next to your eye to determine the approximate size of your eye's pupil as part of your contact lens eye examination. Automated instruments that measure pupil size also exist.
“Is there a big difference between 8.6 and 8.7 base curve in contact lenses?” No, the difference is small. The 8.7 curve is . 1mm flatter, but since these are soft lens curvatures, and soft lenses assume some of the shape of the cornea, the fitting value won't be changed dramatically.
It is standard to use the latter. A BC of 8.4 mm of radius is more curved, and therefore a tighter fit, than 8.8 mm base curve. People who have steeper corneas require tighter fitting contacts. If a contact is too loose, it may be uncomfortable, fold, or come right out.
A lens with an 8.5 curvature is identical in curvature to an 8.6 , it's just a marketing ploy to make it look different. The difference between an 8.5 and an 8.6 isn't the curvature , but the fact that it is a different lens. Contacts are the same as shoes in a sense.
A number between +/-2.25 to +/- 5.00 indicates moderate nearsightedness or farsightedness. A number greater than +/- 5.00 indicates severe nearsightedness or farsightedness.
For corneas with no or low levels of corneal astigmatism the suggested initial BOZR is that of the flattest K-reading. As the extent of corneal astigmatism increases the difference in the radius of curvature of the two corneal meridians increases.
Some did not aware that wearing poorly fitted contact lens can cause serious harm to your overall eye health. If you wear lenses that don't fit your eyes there's a good chance you'll develop contact lens intolerance. This condition can cause a person to be forced to wear eyeglasses or undergo laser vision correction.
Wearing poorly-fitting contact lenses can damage the eye and cause problems such as blurred vision, headaches, and eye strain. The diameter and base curve of a contact lens are two measurements that affect how well they fit your eye.
In fact most labs today will not allow you to change or request a base curve at all. This completely overrides any use of the nominal lens formula or simple base curve calculations. 3) Never attempt to change the lens base curve to match a frame base curve.
The base curve of a lens may affect certain aspects of vision, such as distortion and magnification, and wearers may notice perceptual differences between lenses with different base curves.
