The base curve radius (BCR) is one of the most important lens parameters to verify because it affects the lens-to-cornea fitting relationship. The most commonly used method of determining the BCR is the radiuscope or radiusgauge. The BCR is determined by the distance between the real and aerial images observed in the radiuscope (Fig. 7.1).

Following are the steps for determination of the BCR.

If the needle of the instrument will not position at zero when the real image is in focus, the nearest whole number should be used. This whole number will be added or subtracted from the final reading. For example, if the real image is in focus at +1.0 and the final reading is 7.0 mm, the BCR is 8.0 mm.

Verification of the BCR of gas-permeable (GP) contact lenses is also possible with several automated keratometers. These devices have been found to accurately measure the base curve radius within tolerance.¹

Verification of the BCR should be performed before dispensing and after 12 to 24 hours of soaking the lenses in an approved disinfecting solution. GP lenses may flatten on hydration, and verification is necessary to determine if the lens is still within tolerance.²,³

In addition to measuring the BCR, it may be necessary to measure the convex radius—for example, with front toric and bitoric lenses. A different lens mount is used to determine the convex radius (Fig. 7.6). The procedure is similar to determining the BCR with the exception that the real image is now the upper image and the aerial image is now the lower image. The needle should be set at the nearest whole number when the real image is in focus. When the aerial image is in focus, the number is read from the scale and subtracted from the whole number. For example, if the real image is in focus at 8.00 mm and the aerial image is in focus at 1.00 mm, the resulting front curve radius would be 7.00 mm.

The steps for determining a toric or warped lens are similar to those for determining the BCR; however, the spokes of the image will not all be in focus at the same time. First, one set of spokes will come in sharp focus; this will represent the steeper curve. The spokes 90 degrees away from the first set of spokes will next come in focus, and this is the flatter curve. The symptoms of a patient wearing a warped lens will vary from no subjective symptoms but slightly reduced visual acuity to subjective symptoms of blur and decreased visual acuity, accompanied by a spherocylindrical over-refraction. These symptoms alone will not indicate a warped lens. Verification of the BCR is necessary as lens flexure may also produce these symptoms. A lens that is flexing may appear warped immediately on removal; however, typically it will return to the original spherical state soon after removal. Patients exhibiting lenses that are warped will require re-education in the proper care and handling of GP lenses because poor care habits, such as cleaning the lenses between the index finger and thumb instead of the palm of the hand, will greatly increase the likelihood of lens warpage.

The peripheral curve radius can be determined by the same method as BCR; however, if the peripheral curve is not ≥1 mm in width, it is unlikely that it can be obtained. Typically, this is not a parameter that the practitioner can verify.

Back vertex power is typically the method by which power is determined for contact lenses. For low-power lenses, there is very little difference in back and front vertex power; however, there can be as much as 1 D difference in back and front vertex lenses in high prescriptions.⁴ If there is doubt about which surface power to verify, it would be best to check with the laboratory that supplies the lenses.

Back vertex power of contact lenses is determined similarly to spectacles. The lensometer should be adjusted for the individual user before the lens power is verified as specified in the instrument manual. The lens is placed concave side against the lens stop of the lensometer, and the power drum is adjusted until a clear image is achieved. The lens should be clean and dry.
It will be necessary to tilt the lensometer upward into a vertical position to allow the lens to rest on the lens stop or for the lens to be gently held on the lens stop with the thumb and forefinger. Caution should be taken not to flex the lens. Most lensometers have a contact lens accessory device that can be placed on the lens stop to aid in more accurate measurements.

To measure prism on a contact lens, the lens must be centered with the concave side against the lens stop. The reticule inside the ocular consists of concentric black rings designating 1 to 5 prism diopters. For example, when the center of the target is on the first concentric ring of the reticule, it corresponds to one prism diopter (Fig. 7.7). Prism is primarily used in front toric lenses to reduce rotation.

Front toric lenses should be placed on the lensometer with the prism in the base-down direction. The resulting power will be recorded in a spherocylindrical notation. For example, if the lens is positioned so the prism indicates one prism diopter base down and the power drum indicates powers of −1.00 D and −3.00 D at 90, the power would be recorded as − 1.00 − 2.00 × 90.

In toric lenses with no prism, such as bitorics and back torics, the power is not recorded in the spherocylindrical form, but as the values found in each meridian. An example of this is as follows:

BCR: 7.50/8.04 mm

Powers: P1/−3.00 D