The assumption for many prescribers is that designing a custom soft contact lens is similar to designing a rigid contact lens. Upon the introduction of soft contact lenses, previous experience was “hard” contact lenses, and that background still influences many prescribers. The most critical specification of a rigid lens is the main radius or base curve. By altering the base curve, the actual resultant power of the lens on the eye is altered. In a rigid lens the lacrimal lens power had to be incorporated in the final prescription. In a custom soft contact lens, the base curve can be significantly altered with little influence on the final lens power prescription.
Upon the introduction of soft contact lenses, the prescriber had choices of base curve and diameter. The prescriber based the initial selection on the “K” readings and then by company fitting guide suggestions would try different lenses until an acceptable fit resulted. Measuring the corneal diameter was not proposed initially. Over time, soft contact lens manufacturers found that certain lens diameters were being ordered more routinely. As the soft contact lenses were made thinner, the influence of the base curve was minimized. Manufacturers reduced their “SKUs” to what in most cases today are one diameter and one base curve – maybe two base curves if you are fortunate. In many cases, as long as the patient is not too critical, this has worked adequately.
As manufacturing became proficient and dependable with excellent molding technology, mass producing soft contact lenses for the average size eye was the goal. However there was the significant group of patients that were not successful soft contact lens patients. The contact lens drop-out rate haunted our profession. More astute prescribers were realizing the error of depending solely on base curve to choose a lens.
Along with others, Graeme Young, M.Phil PhD, F.C.Optom., D.C.L.P., F.A.A.O. (1),(2) published and lectured on the importance of the diameter in relation to the size of the cornea. He questioned why prescribers even take corneal measurements at all when their first lens of choice was not influenced by this information.
A decade ago, sagittal depth or commonly named sag height was evolving as a means for greater accuracy in designing custom soft contact lenses, but there were two problems. The first is that prescribers would attend lectures explaining sag height but then either found it too confusing or too burdensome to use. The second was custom soft contact lenses were not being precisely manufactured.
These problems were diminished with the integration of SpecialEyes Custom Soft Contact Lenses. The Company evolved simply due to the company’s proprietary advancement in lathe manufacturing that was reproducible to micron levels. Having availability of unlimited powers, diameters, curves, thickness and a choice of different water contents in a non-dehydrating and non-ionic material opened up new potentials for the astute prescriber. However, there was still the hesitancy to use sagittal depth in designing custom soft contact lenses – whether that is custom toric contact lenses or custom multifocal contact lenses.
A more understandable approach to designing custom soft contact lenses was needed. Realizing the lens diameter was the most important specification, I wanted to develop a technique that would be easy to incorporate into everyday practice. Also, understanding how the radius of the lens influences the fitting minimally but did affect the in vivo quality of clarity, the base curve relationship needed to be a part of this technique.
A decade ago, I developed the SpecialEyes Arc Length Design, which is based on the concept of simply needing 2 measurements to yield similar design achieved with some sagittal depth techniques. The two measurements needed for the SpecialEyes Arc Length Design are the limbus to limbus measurement of the cornea, otherwise known as H.V.I.D. (horizontal visible iris diameter), along with the corneal radius. These two measurements mathematically yield the arc length of a particular cornea. Trial and error testing of both the custom soft contact lens diameter and base curve resulted in the SpecialEyes Arc Length Calculator. This software allows a highly successful custom soft contact lens design for simple to very complicated prescriptions – for toric custom soft contact lens patients and custom multifocal contact lens patients. Simply insert the spectacle prescription, the central “K” readings, and an accurate H.V.I.D. and the calculator will design a lens that will be 5 mm. larger than the corneal arc length (or 2.5 mm. outside the limbus on each side.)
Sophisticated and dependable designs, such as SpecialEyes’ custom toric contact lenses, which are available to a patient in a few days, make any prescriber a hero to their patients. Once the prescriber is confident in using the SpecialEyes Arc Length Calculator for custom spherical and toric contact lenses, they can graduate and use the SpecialEyes Bifocal Calculator for designing custom bifocal contact lenses with customized distance and near zones. Loyal patients impressed with sophisticated custom sphere, toric, and multifocal contact lens care will build any practice.
The SpecialEyes Arc Length Calculator is available on the SpecialEyes website. The SpecialEyes Bifocal Calculator is available only through SpecialEyes. For more information or assistance with the calculators call 866-404-1060 or submit a request and one of the SpecialEyes representatives will be glad to assist you.
P. Douglas Becherer, O.D., F.A.A.O.
Dr. Becherer is in private practice in Swansea, IL and is a consulting optometrist to SpecialEyes. He is past chair of the AOA Contact Lens and Cornea Section, Past President of the Heart of America Contact Lens Society, Adjunct Assistant Professor of UMSL, and Adjunct faculty at SCO.
References:
Young, G. et al. (2002). Clinical evaluation of factors influencing toric soft contact lens fit. Optom Vis Sci: 79:11-19 2Young, G. (2003). Eye & Contact Lens. Vol 29, No. 1S.