Measuring RGP Bifocal Segment Heights

BY DAVID W. HANSEN, O.D., F.A.A.O.
APR. 1996

MEASURING BIOMICROSCOPY

Use the slit lamp to measure visible iris diameter (VID), pupil size in mesopic and scotopic illumination, and the position of the lid. These measurements can be accurately quantified with a biomicroscope eye-piece reticule. Reduce the slit lamp to an optic section that approximates mesopic room lighting. With 10x magnification, measure the segment height from the lower lid to the inferior pupil margin.

POSITIONING SEG

One-piece crescent lens -- Position the segment just below the pupil. If the fit is not correct, patients may complain of segment jump, flare and glare from the front cut.

Tangent Streak lens -- With the eye in primary gaze, position the bifocal segment approximately 0.3mm above the lower pupil margin in mesopic conditions. Position the inferior edge of the lens on the cornea near the lower lid.

Decentered DeCarle translating lens -- Align the average distance seg diameter (4.0mm) with the pupil. A truncated base aids the lower lid positioning. If the near segment is too high, reduce the vertical segment height by truncating the lens more. If the near segment is too low, use a lens with a larger vertical diameter.

ONE-PIECE CRESCENT WITH SEG BELOW PUPIL.

TANGENT STREAK WITH SEG 0.3MM ABOVE LOWER PUPIL EDGE.

DECENTERED DECARLE WITH DISTANCE SEG IN FRONT OF PUPIL.

Fused crescent translating lens (FluoroPerm ST) -- Place the bifocal segment 0.4mm to 0.7mm below the inferior pupil margin. High-riding lenses can interfere with distance vision and impede driving and other critical distance vision tasks. The amount of prism to prescribe is extremely important. Heed the manufacturers' guidelines for matching prism and lens power to position the distance and near prescriptions for proper translation. Some lid structures may require truncation of the superior edge and a CN bevel that divorces the superior lid from the lens to produce an even glide.

Solitaire lens -- This is basically a prismatic reading lens with a mono-eccentric distance seg cut on the superior lens surface. The near seg extends across the lower half of the lens. This lens is inherently bottom-heavy and requires less prism to achieve proper orientation because of the removal of weight in the distance viewing area. It's an inferior-positioning lens with the segment line at, or encroaching upon, the lower margin of the pupil when the patient is looking at distance.

FUSED CRESCENT (FLUOROPERM ST) WITH SEG 0.4-0.7MM BELOW LOWER PUPIL EDGE.

SOLITAIRE WITH SEG AT LOWER PUPIL EDGE.

CLS

Dr. Hansen, a diplomate of the AAO Cornea & Contact Lens Section, is in private practice in Des Moines, Iowa.