SOFT TRANSLATING BIFOCAL
A Soft Bifocal Lens That Does Not Compromise Vision
A new contact lens for presbyopes incorporates a traditional RGP design technique into soft lens materials.
By Donald F. Ezekiel, AM, Dip Opt (WA), DCLP, FACLP, FAAO, FCLSA, and Damon J. Ezekiel, BOptom (UNSW), FAAO, FCLSA
By the year 2020, an estimated 13.3 million people in the United Kingdom will be presbyopic. Statistics show that approximately 40 percent of the U.S. population is currently presbyopic, and research conducted among European eyecare practitioners by GfK, a leading research firm, shows that the European market for multifocal/bifocal contact lenses grew more than 169 percent in 2000.
Current Soft Bifocal Systems
There are a number of soft bifocal contact lenses now available, including Rhythmic bifocal (Lunelle), Lifestyle Bifocal (The Lifestyle Company), Focus Progressive bifocal (CIBA Vision), Acuvue Bifocal (Vistakon) and Echelon bifocal (Ocular Sciences, Inc). These soft lens designs all utilize the "simultaneous vision" principle, in which lens performance depends on pupil size. Unfortunately, pupil size varies with light level, patient age and visual tasks. As the pupil constricts, distance vision becomes progressively blurred.
Robert Mandell, OD, from the University of California, Berkeley, has described simultaneous vision as "looking at two things at once without being able to see either one." Simultaneous vision compromises our patients' vision.
Due to the shortcomings of soft bifocal contact lenses currently available, the most widely prescribed method of correction for presbyopic patients today is monovision. Mandell has described monovision, or alternating vision, as "looking backwards and forwards in order to see which is worse." Again, this compromises our patients' vision. Modified monovision and "enhanced monovision," have also attempted to overcome the short- comings of the soft bifocal and multifocal lenses now available.
Desmond Fonn, MOptom, from the University of Waterloo in Toronto, Ontario, Canada, estimated that as many as 10.5 million or 30 percent of contact lens wearers drop out each year. Joseph Barr, OD, commented, "I firmly believe more people in the past dropped out due to vision problems," ("Does Red-eyed and Whiny Mean Drop Out?" April 2001). How many of these drop-out patients were presbyopic?
Soft Translating Bifocal Lenses
At a bifocal contact lens forum at the International Society of Contact Lens Specialists meeting in England, contact lens educator Irving Borish, OD, stated, "the only soft bifocal contact lens design that could provide optimal vision is a lens of translating design."
Soft translating bifocal contact lenses have been tried in the past without success. We have made a successful translating RGP lens for many years. Transferring this design to a soft lens to enable the lens to work correctly on the eye was not straightforward. After extensive research involving the lens designs, manufacturing technology and clinical evaluation, we have developed the Triton soft translating bifocal lens. It is the only soft bifocal lens now available that provides clear vision "without compromise."
Figure 1. Lens design of the Triton soft translating
The Triton soft translating bifocal is available in polyhema (38.6 percent), methafilcon A (55 percent) and hioxifilcon A (59 percent) materials. The lens is manufactured by Geflex (www.gelflex.com).
The lens design incorporates the following features:
- Back surface design that allows the lens to translate easily
- Unique lower area design that facilitates lens/ lid interaction
- Toric back surface if needed
- Bi-prism design to allow for lens stability and location
Figure 1 shows the details of the Triton lens design.
We have recently modified the Triton contact lens design to address practitioner and patient feedback. The problems reported with the Triton lens included patient awareness and instability with small lens diameters. The updated Triton design resolves these reported problems.
The Triton lens features two location dots engraved at the 3 and 9 o'clock positions at the periphery of the lens, which denote the geometrical center of the lens. The reading, or near power, segment is positioned 1.0mm below the geometrical center of the lens. With translating bifocal designs, the lower lid helps position the near segment and allows the lens to translocate.
To fit the Triton lens correctly, you must use a Triton trial fitting lens. The most important aspect of the fitting is the overall size of the lens in the vertical meridian, as this sets the position of the reading portion of the lens correctly for optimal vision for the patient. Once you determine the correct vertical lens size, calculate the base curves and powers empirically.
The lens has an overall size in the horizontal meridian of 15.0mm or 14.5mm. The 14.5mm is the most common size. Use the 15.0mm size when the lower lid is some 2.0mm below the limbus. Determine the initial trial fitting lens from the size of the patient's aperture and the position of the lower lid relative to the limbus.
The Triton fitting set includes six lenses of two base curves and varying vertical sizes: 13.9mm, 13.4mm, 12.9mm, 12.4mm, 11.9mm and 11.4mm.
You've chosen the correct lens vertical size if the lower lid is located at the limbus and the contact lens location dots are central to the patient's pupil (Figure 2).
|Figure 2. Optimal fitting position of the Triton lens.||Figure 3. Marker dots indicate that this lens is too high on the cornea.|
Use a smaller vertical size lens if the lens marker dots are higher than the center of the patient's pupil (Figure 3). Use a larger vertical size lens if the marker dots are lower than the center of the patient's pupil. If the lower lid falls below the limbus, use a larger vertical size lens so the lens will engage the lid and allow translocation (Figure 4).
Figure 4. Marker dots indicate that this lens rests too low on the
The Triton soft bifocal lens can incorporate a patient's full astigmatic correction at the precise axis with any distance power and the full near addition, thus achieving optimal vision for the patient. Cylindrical powers of 0.50D can make a marked difference in visual clarity for the patient.
Patients who are not good candidates for the Triton soft translating bifocal include those with very loose lower lids and those with narrow apertures. Patients who work on computers, apart from laptop computers, may have a posture problem because they need to look through the lower portion of the lens (like with a bifocal spectacle lens) for near vision.
References are available upon request to the editors of Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #83. (Have a fax number ready.)
Acknowledgements: The staff at Gelflex Laboratories, in particular Peter Bryant and Quang Lee; Irving Borish, OD, whose statements at the ISCLS meeting encouraged us to pursue the translating design for a soft bifocal contact lens; patients who have worked with us and allowed us to develop and clinically evolve the Triton Soft Translating Bifocal lens;.Tania Ezekiel for her editorial input.
Dr. Don Ezekiel (left) is a clinical consultant and founder of Gelflex Laboratories in Perth, Australia. He has written numerous papers and has lectured and conducted workshops in Asia, Europe, Israel and the United States.
Dr. Damon Ezekiel (right) works in a specialty contact lens practice and is active in research and development at Gelflex Laboratories. He has lectured and conducted workshops in Australia, Asia and the United States.